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British Standard
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BRITISH STANDARD BS 8005-2:1987
Sewerage
Part 2: Guide to pumping stations and pumping mains
UDC 628.213
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BS 8005-2:1987
This British Standard, having been prepared under the direction
of the Civil Engineering and Building Structures Standards
Committee, was published under the authority of the Board of BSI
and comes into effect on31 December 1987
BSI 02-2000
First published as CP 2005 December 1968First Part revision as
BS 8005-2 December 1987
The following BSI references relate to the work on this
standard:Committee reference CSB/5Draft for comment 84/11182 DC
ISBN 0 580 15992 2
Committees responsible for this British Standard
The preparation of this British Standard was entrusted by the
Civil Engineering and Building Structures Standards Committee
(CSB/-) to Technical Committee CSB/5, upon which the following
bodies were represented:
Association of Consulting EngineersAssociation of County
CouncilsAssociation of District CouncilsBritish Ceramic Research
Ltd.British CoalBritish Plastics FederationBritish Precast Concrete
Federation Ltd.British Tunnelling SocietyClay Pipe Development
Association LimitedConcrete Pipe AssociationConstruction Industry
Research and Information AssociationConvention of Scottish Local
AuthoritiesCounty Surveyors SocietyDepartment of the Environment
(Property Services Agency)Department of Transport
(Highways)Federation of Civil Engineering ContractorsFibre Cement
Manufacturers Association LimitedHealth and Safety
ExecutiveHydraulics Research Station Ltd.Institute of Water
Pollution ControlInstitution of Civil EngineersInstitution of
Environmental Health OfficersInstitution of Public Health
EngineersInstitution of Structural EngineersInstitution of Water
Engineers and ScientistsRoyal Institute of British
ArchitectsScottish Development DepartmentTrades Union CongressWater
Authorities AssociationWater Research Centre
The following bodies were also represented in the drafting of
the standard, through subcommittees and panels:
British Effluent and Water AssociationBritish Pump Manufacturers
Association
Amendments issued since publication
Amd. No. Date of issue Comments
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Contents
PageCommittees responsible Inside front coverForeword ii
Section 1. General1 Scope 12 Definitions 13 Abbreviations 1
Section 2. Range of components and appliances4 General 25 Pumps
26 Prime movers and drives 37 Controls and electrical equipment 58
Pipework and valves 79 Miscellaneous 8
Section 3. Design of pumping stations10 General 911 Health,
safety and welfare design features 912 Maximum and minimum pumping
rates 913 Pumping heads 914 Number and size of pumpsets 1015 Layout
of pumpsets, pipework, control equipment
and ancillary plant 1116 Substructure design 1217 Wet wells 1218
Ventilation, smell and noise 1319 Lifting facilities 1320
Superstructure 1421 Environment and access 14
Section 4. Design of pumping mains22 Velocities of flow 1523
Diameter 1524 Number of mains 1525 Pressures 1526 Valves 1627
Profiles 1628 Discharge arrangements 1629 Anchorages 1630 Control
of septicity 17
Publications referred to Inside back cover
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BS 8005-2:1987
ii BSI 02-2000
Foreword
This British Standard has been prepared under the direction of
the Civil Engineering and Building Structures Standards Committee
and is directed at general engineering practitioners who may either
be embarking on a career in sewerage or be dealing with a
particular aspect for the first time. It is not intended to be
exhaustive in any field but sets out to present guidance on basic
principles and good practice, indicating where a more detailed and
comprehensive study may be made. BS 8005 supersedes and enhances CP
2005:1968, which is withdrawn, although some of the material
incorporated is a restatement or a revision of the earlier text.BS
8005 gives guidance on the planning, design, construction,
operation and maintenance of works to convey sewage, including
storm sewage, surface water and trade effluents to a sewage
treatment works, tidal waters or other final place of disposal.
Recommendations are given for the repair, renovation and
replacement of sewers.Many end users of this British Standard, such
as governments, public authorities, sewerage authorities and
consultants, issue their own recommendations and specifications for
sewerage which BS 8005 is intended to complement rather than
replace.BS 8005-0 directs the reader to sources of more detailed
information, particularly on important and specialized fields such
as health and safety. It should be regarded as supplying essential
background information for the other Parts of BS 8005.BS 8005 is to
be published in six separate Parts, as follows.
Part 0, Introduction and guide to data sources and
documentation; Part 1, Guide to new sewerage construction; Part 2,
Guide to pumping stations and pumping mains; Part 3, Guide to
sewers in tunnel1); Part 4, Guide to design and construction of
outfalls; Part 5, Guide to rehabilitation of sewers1).
It has been noted that substantial one-part codes and guides
take a long time to revise and if they are reviewed at infrequent
intervals, they tend to become out of date quickly, especially in a
field where technological development is rapid. It is intended
therefore to keep a constant watch on new developments and to
update BS 8005, Part by Part, as soon as the work can be
justified.BS 8301 sets out recommendations for building drainage
and, while it relates generally to smaller pipelines, there is some
overlap between it and BS 8005. BS 6297 gives recommendations for
the design and installation of small sewage treatment works and
cesspools.Apart from Part 0, which is directed more specifically at
the UK sewerage field, BS 8005 is for use both in the UK and, in
appropriate circumstances, overseas.Suggestions for the improvement
of any Part of BS 8005 will be welcomed by the Secretary of CSB/5
at 2 Park Street, London W1A 2BS.A British Standard dose not
purport to include all the necessary provision of a contract. Users
of British Standards are responsible for their correct
application.Compliance with a British Standard does not of itself
confer immunity from legal obligations.
Summary of pages This document comprises a front cover, an
inside front cover, pages i and ii, pages 1 to 18, an inside back
cover and a back cover.This standard has been updated (see
copyright date) and may have had amendments incorporated. This will
be indicated in the amendment table on the inside front cover.
1) In preparation.
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Section 1. General
1 ScopeThis Part of BS 8005 provides guidance on the components,
appliances and design of pumping stations and pumping mains.NOTE
The titles of the publications referred to in this standard are
listed on the inside back cover.The titles of British Standards not
referred to in this Part of BS 8005 but of interest as dealing with
closely associated subjects are listed in Appendix A of BS
8005-1:1987.Other publications that may be of interest are listed
in Appendix B of BS 8005-1:1987.
2 DefinitionsFor the purposes of this Part of BS 8005 the
definitions given in BS 8005-0 apply.
3 AbbreviationsFor the purposes of this Part of BS 8005 the
abbreviations given in BS 8005-1 apply.
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Section 2. Range of components and appliances
4 GeneralThe type and size of sewage pumping stations and
equipment depend upon their duties, location and any special
operational requirements, such as remote or automatic control. This
section covers the main range of components, whilst methods of
installation and housing are dealt with in section 3.Components and
appliances are required to be reliable, robust, easy to maintain
and appropriate for pumping water and other liquids. In addition,
the aggressive nature of sewage, with its variable solid content
and possibility of toxicity and explosive gases, calls for a high
degree of caution and the adoption of the latest safeguards to meet
all possible hazards.
5 Pumps5.1 General
Pumps for handling sewage should be unchokeable and wear
resisting. They may be divided broadly into four groups:
rotodynamic; reciprocating; pneumatic and Archimedean screw. (See
also BS 6297.)
5.2 Rotodynamic pumps
Rotodynamic pumps are relatively cheap to buy, of small overall
dimensions in relation to capacity, light in weight and can be
arranged vertically or horizontally. They may vary from moderate to
high efficiency according to the size of the pump, type of impeller
and the head/quantity characteristic of the duty to be performed.
All types of rotodynamic pumps afford a high degree of flexibility.
Both quantity and head can be varied by changing the speed and/or
diameter of the impeller.When two or more pumps are required to
discharge in parallel to a common rising main thehead/quantity
characteristics should be studied in order to obtain stable
conditions and a good overall efficiency. This important group of
pumps is divided into three types.
a) The centrifugal pump. The capacity of traditional dry well
centrifugal pumps for reasonably economic working may vary from a
minimum of about 7 L/s up to 700 L/s and more, with heads varying
from 3 m to about 45 m. With small to medium capacities the pump
should be of the unchokeable type wherein any solid, up to a
maximum of about a 100 mm sphere, that may enter the pump suction
will be passed through the pump.
The recessed impeller type of centrifugal pump (also called
vortex or torque flow pumps), although of lower efficiency, is less
likely to be affected by fibrous material and can be easier to open
up for maintenance.Submersible centrifugal sewage pumps are
available for a similar range of duties, either as stationary
submerged units or as transportable submersible installations. They
are self-priming with both pump and motor totally submersible and
are accordingly suited for use in wet wells or in dry wells where
there is a flooding risk.Cooling is a problem in a dry well and
special design precautions may be necessary. The discharge
connection of the pump is adaptable for either a flexible hose or
static pipework, and the electric motors are available certified
for use in a hazardous area in accordance with BS 5345.For wet well
installations submersible centrifugal pump units are available
which will slide down guides and seat automatically on the
permanent discharge connection. The weight of the pump forces the
mating flanges into contact thus providing a seal on the discharge
side.Centrifugal disintegrator pumps may be used to assist the
treatment of the sewage. Running and maintenance costs are higher,
especially if they are on a combined sewerage system where there is
a high content of grit in the sewage.b) The mixed flow pump. The
mixed flow pump is more efficient where the volume of sewage to be
pumped is large and where the head lies in the range of 6 m to 18
m.c) The axial flow or propeller-type pump. The axial flow pump is
suitable where large volumes of sewage have to be pumped against
low heads.
The above pump classifications are generalized and, in
particular, the mixed flow design of impeller overlaps the head
ranges of both axial and straight centrifugal pumps. Many modern
design unchokeable pumps have mixed flow impellers but are of
relatively low efficiency. High efficiency centrifugal pumps, mixed
flow pumps and axial flow pumps should only be installed in
association with preliminary screening or the reduction of the
coarser suspended solids.For very small flows a small high
efficiency centrifugal pump can be used as part of a sewage
diverter where the coarser solids are prevented from passing
through the pump.
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5.3 Reciprocating pumps
The reciprocating pump is heavy and of large dimensions in
relation to its capacity. It is reliable, efficient when first
installed, and is capable of operating with a high suction lift and
of discharging against very high heads. It is susceptible to
choking, heavy wear and tear, and loss of efficiency through wear
and valve jamming. The reciprocating pump may be either of
single-acting or double-acting type. Reciprocating pumps are more
expensive in first cost than other types. They are expensive to
maintain and therefore are rarely used for pumping crude sewage.
Preliminary screening of sewage to remove large solids is
advisable.As a safeguard against excessive pressure a bypass is
often provided between delivery and suction. This should be
combined with a pressure relief valve. An alternative is to relieve
the discharge to the suction well.A reciprocating pump requires a
slow speed drive; this is provided through gearing between the
prime mover and the pump. A steady rated discharge is maintained
over a wide variation in head. Discharge is altered by varying the
speed.
5.4 Pneumatic pumps
The pneumatic ejector, whether of the automatically filled
vessel or the air lift type, is suitable where reliability and ease
of maintenance are of greater importance than overall efficiency,
and where a small quantity of sewage is to be pumped against a
relatively small head.The installation usually comprises an ejector
together with an automatic self-starting air compressor, with
provision for air storage. In special situations two ejectors
should be provided to facilitate repairs; where breakdown would
have serious results, a second air compressor should be provided.
It is possible to serve several ejector stations from one central
air compressing station if the distances are not too great.
5.5 Archimedean screw pumps
Archimedean screw pumps are basically screws revolving at a
fixed speed. They provide a steady rate of pumping and high
efficiency over a wide range of flows and are also effective in
pumping varying flows. They are suitable for lifting large volumes
of unscreened sewage or storm water against low heads.The actual
volume lifted for any particular diameter is dependent on the speed
of rotation and on the angle of inclination; the greater the angle
the less the rate of discharge. The angle of inclination varies
from a minimum of 27 to the horizontal to a maximum of 40. The
preferred angle is 38.
Archimedean screw pumps are in two main groups, namely
open-screw and encased-screw. Neither group requires a deep sump.
While the open-screw types are virtually unchokeable, in certain
applications it is advantageous to install a coarse bar screen at
the inlet to prevent large objects, such as baulks of timber, from
entering the screw. A higher degree of protection is required for
the encased-screw pumps.Capacities of Archimedean screw pumps cover
a very wide range varying, depending on diameter and inclination,
from 7 L/s up to about 10 000 L/s.
6 Prime movers and drives6.1 General
The prime movers normally employed for driving sewage pumping
plant are electric motors and internal combustion engines (diesel,
dual fuel or petrol). They should be suitable for the types of
pumps selected and rated for operational conditions. The choice may
depend upon the availability of electricity or a fuel supply. Due
consideration should be given to capital, running and maintenance
costs in selecting either electricity or fuel, together with the
effect of possible interruption of supply from outside influences
such as shortage, mechanical breakdown and supply
difficulties.Where necessary, explosion-proof units should be used.
Fire detection and alarm systems in all buildings should comply
with BS 5839-1.Electricity is normally adopted as the cleanest and
most convenient form of motive power. In the UK 415 V 3-phase
supply is normal for motors up to about 150 kW to 200 kW, whilst
3.3 kV or higher voltage is often used for larger motors. Direct
current (d.c.) drive is occasionally adopted, either by rectifying
from the a.c. grid supply or by local generation from internal
combustion engines.Standby electricity supply in case of breakdown
is frequently provided by a second feeder from a different
substation or by switching to diesel generating plant situated
locally or mounted on a vehicle.
6.2 Electric motors
The electric motor is a convenient, cheap and reliable prime
mover for all types of sewage pumping. Varieties of electric motor
are available to suit the particular conditions of duty to be
performed.As automatic controls have been developed to a high
degree of reliability, an electric motor is particularly suitable
for an unattended automatically operated station.
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The following are the more usual types of electric motor used
for sewage pumping.
a) Squirrel cage induction motor. This type of motor is the
simplest and most robust in design for use on alternating current
(a.c.) supplies. It is commonly used for single speed applications
but can be wound for multi-speed operation, dual speed windings
being fairly common. Other motor speed variations are
available.Starting the motor direct on line demands a high starting
current from the supply, and other methods of starting such as
star-delta, autotransformer, or electronic soft start, may have to
be considered in order to reduce the high starting current from the
supply (and its cost) and satisfy the requirements of the supply
authority.Care should be taken when selecting reduced current
methods of starting that the motor accelerating torque matches the
pump characteristics.This type of motor is suitable for electronic
methods of speed control.Many types of power electronic drive
equipment are available to enable the squirrel cage induction motor
to be considered as an alternative to motors under items b), c)and
d).The main types are as follows:
1) voltage controlled invertors;2) current controlled
invertors;3) pulse width modulated invertors;4) variable voltage at
constant frequency.
b) Wound rotor induction motor. This type of motor has a wound
rotor and can have a lower starting current than the squirrel cage
motor. It is suitable for speed control by means of external
resistors, usually contained in the control equipment. It is
normally only used where the speed control is small and applied for
short periods. The motor efficiency is less at reduced speeds.c)
Synchronous induction motor. This type of motor runs at a fixed
speed independent of the load, the speed being determined by the
frequency of the supply and the number of poles in the
motor.Normally it has good efficiency and power factor and may
attract favourable terms from the supply authority. However, a
separate d.c. supply is required for exciting windings and the
starting performance is poor.
d) Direct current motor. The d.c. motor may be used with
advantage for variable speed applications. Starting methods are
simple and it has a good starting performance and high efficiency
over a wide range of duties. However, maintenance and prime cost
are more expensive than with a.c. motors.
Where a wide range of pumping duties is required, it may be more
economical in respect of capitaland/or energy costs to use motors
with two (or possibly three) speeds or a larger number of constant
speed pumping units controlled in an optimum sequence depending on
flow or level control strategy.
6.3 Internal combustion engine
The following are the more usual types of internal combustion
engine.
a) Diesel engine. The diesel engine is a reliable, efficient
type of prime mover. The medium and slow speed units generally have
longer lives and are heavy; they require heavy foundations and
relatively more space. The high speed units are efficient, compact
and light but generally have a shorter life; they are not so
expensive (in capital costs), nor do they require heavy
foundations. High speed units are, however, often noisy.The slow
and medium speed units can be operated automatically but they
usually need the regular attendance of a skilled staff. High speed
units are suitable for automatic operation, but need more highly
skilled maintenance to ensure reliability.b) Dual fuel engine.
Sewage gas, a by-product of sewage purification, is an economical
fuel for dual fuel engines. These engines can also be operated
efficiently on diesel.c) Petrol engine. The petrol engine is rarely
adopted as a form of prime mover at a permanent sewage pumping
station owing to the comparatively high cost of fuel and
maintenance. Portable pumps are sometimes powered by petrol
engines.
Suitable arrangements should be made for the safe handling of
flammable liquids and for the safe ventilation of combustion
products, particularly where mobile plant is involved.
6.4 Drives
Electric motors (either horizontally or vertically mounted) and
internal combustion engines can be arranged to drive most types of
pumps, by one of the following means.
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a) Direct coupling. Direct coupling of the pump to the prime
mover through a suitable flexible coupling, or occasionally through
a clutch, is normal for horizontal shaft pumps and vertically
mounted pumps.b) Geared drive. A gear box may be inserted to reduce
or increase the pump speed in relation to the prime mover speed, or
to change the direction of the drive from horizontal to vertical.c)
Belt drive. Belt drive, with a flat, V-section or toothed belt, can
be used in place of geared drive, where space permits. This may be
less costly.d) Direct drives. Electro-submersible pumps usually
have the impeller mounted directly on to an extended shaft of the
electric motor. They may be mounted vertically or horizontally.
Very large units may, however, have an intermediate gear box
between the motor shaft and the impeller drive shaft.e)
Intermediate shafting. Shafting, either horizontal or vertical,
connecting the prime mover or gear box to the pump, normally needs
to be supported by intermediate bearings (efficiently lubricated).
Shafting and bearings need to be stiff enough to ensure steady
running without whip, and flexible couplings should be provided to
allow for any misalignment.
The prime mover, the gearbox (if required) and the pump can
sometimes be mounted on a combined base unit. This arrangement
reduces the installation time, and maintenance can be
economical.Control equipment for the drive units should always be
placed above ground. When equipment is placed below ground, the
dampness will shorten its life, make it unreliable in operation,
and in certain stations, especially where flammable gas may be
present, make it a hazard and is likely to make it a serious source
of ignition leading to explosion.
7 Controls and electrical equipment7.1 General
Most electrically driven pumps are controlled automatically;
manual control is now exceptional. Diesel driven pumps can be
controlled automatically but this may be unnecessary if they are at
a station which is always attended. The design policy on control
equipment should be agreed with the user, who may wish to have
similar equipment at several stations in one operating area. The
equipment should, wherever possible, be capable of adjustment after
operational experience.
For all types of automatically controlled pumps it is essential
that an anti-roll-back device is incorporated in the drive
arrangement. The lower fixed bearing is invariably under water and
a return oil lubrication coupled with three seals is recommended to
minimize wear on the lower bearing.
7.2 Controls
Usually the control of pumps is based on a liquid level, the
operation of a pump starter being activated by the closing of
electrical control circuits. Various devices are used, e.g.:
a) floats;b) electrodes;c) air pressure discharge bubblers;d)
ultrasonic beams;e) photoelectric light beams;f) flow rate
detectors (not necessarily related to liquid level);g) pressure
transducers.
The selection of a suitable system for a particular station is a
combination of suitability of proprietary equipment and the
experience and preferences of a designer. It is important to design
for easy accessibility and maintenance of the equipment. Standby
equipment should be considered and also alarms to indicate failure.
The use of telephone and radio alarm and information systems at
remote unattended stations is often justified.A control system for
an installation of electric motor driven pumps usually
automatically operates the motor starter to a pre-determined
sequence. The system should provide for the sequence to be varied
either automatically or manually, e.g. so that one pump can be the
duty pump for a period, and then another. It can be simple, such as
a float which directly closes and opens a switch in a starter, or
complex, such as a series of detectors with relays and a
mini-computer to deal with a range of variable speed pumps. The
alternatives that are available provide variations in flexibility,
together with safeguards and economy of operation. A selector
switch should select automatic or manual operation.Except at small
stations, it may be desirable to include time delay equipment in
the control scheme to ensure sequential starting of pumps after a
power failure. This avoids an excessive momentary electrical load
that might otherwise arise in this exceptional circumstance.
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7.3 Electrical equipment
Each motor starter needs to be suitable for frequent duty and
should be appropriately rated to the motor it controls. It should
incorporate a suitably rated externally operated means of
isolation, mechanically interlocked with its door. It should have
adjustable overload releases that lock-out the equipment, with time
lags, and an inherent no-volt release which should be such that the
motor willre-start automatically on the resumption of voltage
following power failure. In the event of the motor tripping out on
overload, consideration should be given to having a reset button
for manual reset. Automatic reset of the overloads is not
considered desirable. Other features are high rupturing capacity
back-up fuses of suitable rating on each phase, control circuit
fuses and contacts for operation by the control gear. It is usual
to provide indicating lights showing supply on and motor energized.
Emergency stop push buttons should be provided at the starter and
at points of possible danger such as near the motor and near the
pump. Pump starters should be linked to a flow monitor, load
monitor or non-return valve, through a time delay, to shut down a
pump in the event of blockage.Anti-condensation heaters may be
provided in starter enclosures, possibly thermostatically
controlled, and also in the motors. In larger motors thermal
devices may be justified to open the starter controls in the event
of overheating.Individual starters and other electrical items can
be wall mounted in their own enclosures. However, at medium and
large stations it may be more satisfactory to provide a floor
mounted panel for all the electrical gear, including the incoming
supply circuit breakers, electrical meters and distribution
equipment. A composite panel should be arranged so that an
individual unit can be isolated for maintenance while the other
units remain live and in operation. It is essential that each
isolation switch be capable of being padlocked in the off
position.
7.4 Telemetry
The purpose of any telemetry system is to provide operational
and management data to a remote management centre and, in selected
cases, to provide the facility for override control of the plant
from the management centre.Telemetry systems usually cover a sewage
treatment works and any pumping stations within its catchment.
The minimum data required is such that, at the management
centre, decisions can be made, especially during out of normal
hours periods, whether or not to commit limited manpower resources
for immediate attendance at the pumping station to rectify
operational problems or plant breakdowns.Information required from
each pumping station can range from the transmission of a single
high level alarm for very small automatic pumping stations, to full
monitoring and individual alarms such as those detailed for each of
the following items.
a) Pumps: Running, Failed, Stopped, Off auto.b) Electricity
supply: Mains failed, Phase failed.c) Standby generator (where
installed): Running, Failed, Stopped, Low fuel alarm.d) Wet well:
High level alarm.e) Dry well: Flooding alarm.f) Screen: Blockage
alarm, Failure, High differential level.g) Storm overflow:
Operating.h) Intruder/fire: Alarms.
Depending on the application the following may also be
monitored.
1) Works inlet flow/discharge flow rates and associated
integrated totals.2) Other qualitative and quantitative
information.
The means of data transmission can be via public service
telephone lines, own dedicated lines, radio or other media (e.g.
fibre optics, laser links) dependent on system requirements and
availability. Consideration should be given in selecting the
transmission media to system requirements (e.g. update times/scan
rates), operating and maintenance costs, data security and
reliability, expansion capability and the display and archiving of
operational and management information.The telemetry equipment can
be enhanced by local automatic programmable control facilities of
various levels of sophistication, so as to provide data logging and
data processing functions required for system optimization.
Automatic or manual override control through this equipment is
possible from the management centre.Information on telemetry and
computer control of sewerage operations is obtainable from the
Water Authorities Association (WAA) and the Water Research Centre
(WRc).
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8 Pipework and valves8.1 Pumping station pipes and joints
Pipework within pumping stations is usually of ductile iron but
grey iron, steel or plastics pipework can be used. The pipework
should be able to withstand possible distortion due to jointing and
vibration and surge pressures.Pipe joints within pumping stations
are mostly flanged. Detachable flexible couplings should be
interposed where they will facilitate dismantling and accommodate
vibration, but these do not usually hold end thrust and long tie
bolts may be needed for bracing.A closely spaced pair of flexible
joints should be provided on a pumping main immediately outside a
station to accommodate possible differential settlement.Provision
of tappings on suction and delivery pipework for permanent or
temporary pressure gauges should be considered.
8.2 Pumping main pipes and joints
The materials of pipes used for pumping mains include ductile
iron, steel, asbestos cement, GRP, polyethylene, polybutylene,
unplasticized PVC and, occasionally, concrete. The class(es) of
pipework should be selected to withstand the maximum hydraulic
pressures (including surge pressures) and, where applicable,
external loadings. When necessary pipelines should be protected
against corrosion, internally and externally. (See BS 8005-1
regarding materials and BS 1710 regarding colour coding of
services.)Pipe joints for use below ground should preferably be of
the flexible type. If flanges are used on buried pipes the
fastenings should be specially protected. Protection should be
provided by galvanizing, by wrapping with waterproof tape or by
enclosing in bitumen.
8.3 Valves
Pumps normally discharge through their ownnon-return valves. One
or more isolating valves should be included in the installation. A
non-return valve may also be required on a pumping main. For
isolating and maintenance purposes the isolating valve should be
positioned downstream from the non-return valve.
The non-return valve, sometimes called a reflux or a check
valve, complying with BS 5153 prevents backflow when pumping
ceases. It should give a clear flow when the flap is open to avoid
accumulation of rags. For this reason valves with multiple flaps
are not usually satisfactory. Seatings for the flap and the hinge
pin should be renewable as they are subject to severe wear. An
external lever may be provided on the hinge pin so that the flap
can be opened manually, either in attempting to clear a blockage or
for backwashing or drainage. The lever also gives a visual
indication of the extent to which the flap is open during pumping;
this can be adjusted by the addition of a counter-weight to the
lever. Slamming of non-return valves may take place when a reversal
of flow occurs before the flap closes. Partial closing as the
forward velocity of the sewage diminishes and before the flow is
reversed can be assisted by the external lever arm and
counterweight.A non-return valve should be easy to open for
maintenance. The casing should have an arrow cast on to indicate
direction of flow.Isolating valves are normally sluice (or gate)
valves which are also used as washouts. An extensive range is
available. Wedge pattern gates with copper alloy facings, or
resilient seal gates, are usually preferred. Some valves have
inside non-rising screws and some have rising screws which give a
clear indication whether the valve is open or closed. Diaphragm
isolating valves and ball and plug valves are suitable for pumping
installations handling raw sewage but butterfly valves should be
avoided as rags in the sewage may cause blockages.Sluice valves
should preferably be sited with the operating spindle either
vertical or inclined at an angle above the horizontal. If
unavoidable, or if chain operation is to be used, the spindle may
be horizontal but it should never be inclined below the horizontal
as solids can enter the bonnet and interfere with operation.The
most economical operating arrangement is for the handwheel of the
isolating valve to be fixed on the protruding stem of the valve. If
access would then be difficult the spindle can then be extended
(and cranked through gearing or universal joints if necessary) to a
conveniently situated headstock. Valves usually close clockwise but
this is not universal and the direction of opening and closing
should be marked on each handwheel. A designation label is also
useful. Where valves may need to be left in a partly opened state
there should be a position indicator.
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Large valves may have a water jetting attachment to allow for
cleaning the bottom of the seating of accumulated grit. Large
valves on storm water systems which may normally be closed can be
provided with electric, hydraulic or pneumatic actuators for power
opening and closing. A power operated isolating valve can be used
on a large delivery main instead of separate non-return and
isolating valves. In the event of mains failure, power operated
valves will not close to stop backflow unless alternative energy
sources and additional equipment are fitted.Air release valves
should be of a type suitable for sewage, with adequate capacity for
the passing of air and gas produced during operation.Rarely used
valves should be operated at regular (or biannual) intervals over
their full distance of travel.
9 Miscellaneous9.1 Pump protection
Sewage pumps are designed to handle solids and consequently they
are less efficient than pumps for clean water. The solids vary in
character and include unexpected items which get into sewers and
become potential hazards. Rags are a frequent source of trouble;
grit may be a problem after storms. Proprietary refinements in
certain sewage pumps are directed to reducing the risk of pump
blockage by solids.In some circumstances special pumps or plant may
be provided to reduce the risk of failure of the pumps. Special
plant is usually unnecessary at a small pumping station which is
served by small sewers. It should be considered for larger stations
where it is vital to maintain uninterrupted pumping capacity. The
character of the incoming sewage may be a factor if it is known to
have an unusual solids content.Coarse screens can be used to
prevent large objects and some solids from entering pumps. They
always collect rags and this, with the large objects, causes a
build-up which may restrict the sewage flow. The screenings need to
be removed, either manually or by a machine, and either macerated
and returned to the flow or otherwise disposed of. Although the
plant can be automatic it will need attention and maintenance. The
selection of screening plant is a matter of experience and
judgement as there are several basic types and refinements. If
there is doubt as to the need for a coarse screen, provision can be
made for its installation later after a period of operational
experience.
Comminutors or macerators are occasionally provided to protect
pumps.At outlying pumping stations, grit removal plant for pump
protection is an exception. If provided, however, then some
attendance will be required.If grit deposits are likely to occur,
sufficient to cause problems with the pumping system, their build
up may be prevented by using water jets to lift the grit into
suspension to be pumped away with the sewage.With submersible or
submerged pumping units, additional equipment may be provided on
the pump which will allow re-circulation within the sump to take
place prior to the commencement of the pumping cycle. This
re-circulation will also assist in putting the grit into suspension
and will work automatically without additional attendance.In
general, screening and grit removal is best carried out at a sewage
treatment plant and it should be avoided at an outlying pumping
station unless essential. (See CIRIA Technical Note 119: Screens
and Grit in Sewage: Record, treatment and disposal2).)
9.2 Overflow
Every pumping station should have an emergency overflow system
which will operate if there is a complete failure of the pumping
plant. The system can be on the incoming sewer or at the pumping
station.An emergency overflow of sewage could cause nuisance,
pollution, damage or flooding. On no account should the dry well of
a pumping station be liable to flooding. The design of the station
and its overflow system should be such that repairs can always be
made to any plant that has failed.
9.3 Flow measurement
Flow measurement is rarely justified at a pumping station but
its absence may mean that it is not possible to know the actual
discharge rates or quantities of sewage that are handled. The
importance of this information should therefore be considered. All
measurement devices require stable hydraulic flow conditions and
this is not usually possible near pumps. The flow measuring
apparatus may therefore need to be some distance from the pumping
station.Some pump control systems can be associated with incoming
flow measurement (e.g. at a flume). Flow meters can be incorporated
in pumping mains.
2) Obtainable from CIRIA, 6 Storeys Gate, London SW1P 3AU.
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BSI 02-2000 9
Section 3. Design of pumping stations
10 GeneralThe type and size of pumping stations and pumps
depends on the pumping duties, the location, whether the station
will be attended, and the preferences of the user and designer.The
conventional pumping station has a dry well for pumps and other
plant and a separate wet well, usually housing some of the control
equipment. The roof of the dry well, which may extend partly or
wholly over the wet well, should be above ground (and flood) level
and serves as the floor of the superstructure building for motors
and electrical equipment.The building can include facilities for
operators such as a toilet, messroom and store. At large stations
it can also have a workshop and garage. Because of possible smell
and noise problems it is not usually advisable to locate offices or
amenity buildings at pumping stations.It may be possible and
necessary to construct a pumping station partly or wholly
underground, for instance to deter vandalism, but this calls for
special precautions in designing the substructure and in observing
health and safety requirements.A screw pumping station is used to
discharge into a channel or gravity sewer and not into a pumping
main. The motors and control gear should be housed and, if the
screws are in the open, they should be provided with removable
safety covers.Small pumping devices, such as ejectors, may have an
integral reception chamber and can therefore be installed in
basements rather than in separate structures.
11 Health, safety and welfare design featuresIt is essential
when designing sewage pumping stations and pumping mains to
incorporate necessary health, safety and welfare features to comply
with statutory requirements. In addition, relevant recommendations
from authoritative bodies such as the Health and Safety Executive,
the British Standards Institution, the Trades Union Congress and
water supply industry codes of practice should be carefully
studied.The scale of provision will depend on the numbers of staff
and the frequency of visits to the station.Typical hazards are as
follows:
a) falls of persons from heights, and into liquids or on to
moving machinery;b) tripping or slipping on stairways, walkways or
other means of access;c) falling or other travelling objects;
d) inadequate levels of ventilation, particularly in confined
spaces (see clause 18);e) combustion and explosion of flammable
gases;f) electrical shocks and burns;g) faults in the installation
and guarding of machinery (see BS 5304);h) excessive noise,
vibration or fumes (see clause 18).
The following equipment should be provided, as appropriate:
1) first aid and rescue equipment;2) emergency equipment and
alarms;3) telephone and/or radio communication;4) toilet and
washing facilities;5) facilities for the changing and storage of
clothes and for the storage of tools and equipment;6) meal and
office facilities.
For small pumping stations the design could provide for the use
of a specially equipped vehicle to incorporate some of the above
facilities.
12 Maximum and minimum pumping ratesThe maximum discharge rate
from a pumping station, when all the duty pumps and pumping mains
are in use, should be equal to, or preferably greater than, the
maximum design rate of flow to the station. The minimum pumping
rate should achieve a self-cleansing rate of flow in the pumping
main(s). At a large station the minimum pumping rate may be
governed by an assumed minimum flow to the station.For a small
station, with one constant speed duty pump, the pumping rate will
be intermittent and may be unrelated to the rate of flow to the
station.Pumping will also be intermittent at multi-pump stations
whenever the flow to the station is less than the minimum pumping
rate.At medium and large stations, the station discharge can be
kept approximately equal to the rate of flow of the incoming sewage
by the adoption of variable speed pumps. This is not possible at
small stations with constant speed pumps.
13 Pumping headsFor a selected pumping rate the total pumping
head (or pressure) comprises the static lift, the friction in the
pumping main, the friction through the pumps and station pipework
and valves and the entry and exit head losses.
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14 Number and size of pumpsetsThe selection of the type of
pumps, and their sizes and numbers depends, among other things, on
the desired maximum and minimum pumping rates and on the need, or
otherwise, to control the variations in the rate of discharge from
the station.A station with one constant speed duty pump should
normally have a second pump to provide 100 % standby. This may be
the most economical arrangement as far as pumping plant and
electrical power is concerned, but it will result in intermittent
discharge.If the pumping main velocities are satisfactory, a
station can have one variable or one two-speed duty pump and a
similar standby pump. This would reduce the flow fluctuation but
the electrical plant would cost more; it would be less efficient
electrically than a constant speed installation. An alternative is
to have two constant speed duty pumps discharging to the one
pumping main, with a similar pump as standby; this installation can
be further refined by the provision of variable ortwo-speed
motors.To maintain acceptable velocities and reasonable friction
losses, the individual suction and delivery pipe legs are, in many
cases, larger than the pumps. The taper piece required on the
delivery side should be included immediately at the pump branch
before the non-return valve. Tapering on the suction side should be
fitted between the sluice valve and the pump and should be of level
soffit pattern. Tapers should be selected to give good velocity
profiles particularly at inlet, and any bends should, where
possible, be of long radius.If the friction in a pumping main is
significant, no more than two similar pumps should discharge
simultaneously into a single pumping main. The additional output
from a third pump into the same main could be quite small. If
greater flexibility of discharge is desired, two sets of two duty
pumps and one standby, each with its own delivery pumping main,
might be appropriate. When the amount of storm water is significant
one set might use larger pumps than the other.When two pumps
discharge to a pumping main where the friction head is significant
the maximum duty is their combined discharge. When one pump is
operating (at the same speed) it will deliver more than half of
this discharge. Hence head/output calculations (using the pump
characteristic curves) are needed before the duty of a single pump
can be assessed.
Commercially available pumps should be selected, and familiarity
with the range of duties of typical pumps is therefore necessary.
The friction head calculations involve several assumptions and
cannot be precise. The selected installation may have a capacity
that is different (often greater) from that intended. If this is
likely to be a serious impediment to the scheme, arrangements can
be made for adjusting the pump impellers after a trial period of
operation.Even in very small stations it is usually prudent to
provide standby plant to operate automatically when a duty pump
fails. Standby pumps can also be used during maintenance and repair
of other pumps.The number of standby units which should be provided
will depend on the station layout and the possible consequences of
pumps failing at a time of maximum incoming flow. It should not be
overlooked that one pumpset may be undergoing maintenance when this
situation arises.At small stations a portable pump is sometimes
used as a standby. A branch to the pumping main should then be
provided for connection of the portable pump.Provision of an
emergency pumping inlet at any station is always a safeguard
against mains failure, especially if the failure affects a wide
area and there are insufficient mobile generators to serve all
stations. Permanent provision also eliminates the hardest and most
accident prone task, of inserting the suction pipes.The need for
standby electricity supply depends on the importance of continued
operation during a possible period of electricity failure.
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BSI 02-2000 11
15 Layout of pumpsets, pipework, control equipment and ancillary
plantMany small and medium sized wet and dry well pumping stations
with rotodynamic pumps have comparable layouts. The pumps should be
in a line with their vertical spindles passing through the roof
slab to the motors on the floor above. Where universal joints are
provided in the shafts, there should be slight misalignment so that
the bearings do not track on the same path and cause failure.
Although this is referred to as a vertical pumpset, the impeller is
revolving horizontally in its volute. The weight of the pump
assembly should be taken on a support (stool) below the pump casing
and the motor and the drive shaft supported by the floor above. The
pump suction pipeline consists of a vertical 90 bend below the
central inlet of each pump casing, followed by a horizontal
pipeline (with its isolating valve) which passes through the wall
between the wet and dry wells and terminates as a bell-mouth
inlet.Submersible pumps can be used in a dry well situation. It is
essential that only pumps with adequate cooling arrangements be
used and the manufacturers approval should be obtained for the
proposed application.Pumps situated in dry wells need to be primed
before they are started. This is normally achieved by siting the
pumps below the desired start water levels in the wet well and by
providing a small air release pipe from the top of the pump casing.
The level of the suction pipeline should also be coordinated with
the details of the wet well. Siting of a rotodynamic pump above the
start water level should be avoided if possible due to the problems
and additional maintenance which inevitably result. A special
priming method will be required, either using an additional
automatic extractor pump or by using liquid from the pumping main
or by vacuum priming.The outlet from the pump casing of a vertical
pumpset in a dry well is horizontal. The pump delivery pipeline
connects to it and should first include a non-return valve, which
should be in a horizontal attitude, and then an isolating valve to
enable the non-return valve to be readily isolated in the event of
its requiring attention, e.g. to clear a clogged seating. The
suction and delivery sluice valves should preferably not be rotated
through more than 45 to the vertical. The delivery pipelines from
the pumps combine into a header main (bus main or manifold) at the
commencement of the pumping main. Connections should preferably be
horizontal and as short as possible to minimize problems caused by
silt and other debris.
The delivery pipework should be located above the suction
pipework and between the pumps and the wall separating the wet and
dry wells. This leaves the space between the pumps and the opposite
wall clear for access. This arrangement usually means that the
header main is directed to one of the end walls.As the only
reliable dimensions of pipe fittings can be along their
centrelines, the detailed levels and positions of the pipework
should refer to centreline levels and not to invert levels.The pipe
bends in the arrangement described above facilitate possible
dismantling but it is also prudent to introduce flexible couplings
on the suction pipeline for this purpose. A flexible coupling
should be provided on the delivery pipeline if possible; this may
not be practicable due to the anchoring arrangements. It is
important to allow for the whole assembly of suction pipelines,
pumps and delivery pipelines to be erected before the pipelines are
built into the walls. It may be practicable to isolate the whole
assembly for pressure testing and in most cases the delivery
pipework can be tested at pump closed valve head when setting to
work.Other matters which should be considered are as follows.
a) Drainage facilities for emptying isolated pumps and pipework
before they are dismantled, and air/gas release arrangements at
high points (which should be avoided if possible).b) Cross
connections and valves to enable suction lines to be back flushed
either with another pump or by using the contents of the pumping
main.c) Inspection and rodding openings strategically located on
various items in the composite assembly.d) The need to provide
intermediate bearings and flexible couplings on drive spindles
between pumps and motors.e) The need to collaborate with the pump
supplier. The detailed designs should be acceptable to the pump
supplier if he is to be responsible for the operational
efficiencies of the pumpsets.f) The dry well should be adequately
ventilated by extraction from low level to prevent the build-up of
heavier-than-air gases and at high level to prevent the build-up of
methane.
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Some pump control equipment is usually in the wet well and its
function is normally associated with the sewage level. It should be
arranged so that its operation is not impeded by disturbed liquid
surfaces, or by fat, rags or other extraneous matter. Facilities
should be provided for the vertical and positional adjustment after
initial operational experience. The remainder of the control
equipment should be grouped with electrical equipment.If a
floor-mounted cubicle is used, it can also house motor starters and
control equipment (see 6.2 and clause 7). It is important to
provide generous facilities for cables and other connections and to
allow ample working space around the cubicle.
16 Substructure designThe form of substructure should suit the
types and layout of the pumps and other plant. If alternatives are
being considered it will probably be found that submerged pumpsets
require the smallest substructures, vertical pumpsets the next
larger and horizontal pumpsets the largest.The following guidelines
apply to all pumping stations including the very small, and to
ejector stations.
a) Adequate access openings should be provided for all
operational and safety items that will have to be introduced into
the station and which may have to be removed from it.b) There
should be liberal dimensional tolerances in level and location for
all installed items so that they can be conveniently fitted
together and fixed to the structure.c) Pipework is normally
anchored where it is built into the walls of the station and at
these locations some designers provide cast or welded-on puddle
flanges. Elsewhere the pipes and fittings should be supported to
avoid excessive strain on the joints. Large valves should have
individual supports. Vertical pipe runs can be supported at the
base on duckfoot bends and horizontal runs on reinforced supports
with detachable metal straps. The supports and anchorages may need
to withstand both test and surge pressures. They should not impede
dismantling.d) Reasonable access facilities and working space
should be available for operation and maintenance.e) Floor drainage
for a dry well should be generous as it will be needed during
construction and also when pumps and pipework are emptied. It is
usually better to add the floor surfacing after installation of the
major items of plant and pipework. A sump pump should normally be
included.
f) Facilities or provision should be included for emptying a wet
well.g) Adequate lighting should be provided in a wet well, and
electrical apparatus should be certified for use in a hazardous
area in compliance with BS 5345. Provision may be required for
emergency lighting.h) Electrical power points for portable lights
and tools should be provided above ground for use with portable low
voltage output transformers complying with BS 3535.i) Hosing
facilities may be justified for cleaning the wet well and its
control equipment.j) Structural recommendations are as follows.
1) Information about the subsoil of the site, and the
groundwater, should be obtained in advance of detailed design.2)
The dividing wall between wet and dry wells should be considered as
water retaining in accordance with the recommendations of BS
8007.3) As with all buried structures, the substructure should be
designed so as not to suffer movement because of a high external
water table. A risk of subsidence or flotation may also affect the
design and should always be allowed for in the pipelines entering
and leaving the station.4) Protection of the concrete should be
considered if there are aggressive soils or a risk of septic sewage
or corrosive industrial discharges. The surfaces above sewage level
may be vulnerable if hydrogen sulphide is liberated. In exceptional
locations sulphates in the soil and groundwater may be
significant.
k) The environment of a pumping station substructure is
inevitably always humid and steel and ironwork are rapidly corroded
unless effectively protected.
17 Wet wells17.1 Capacity
The size of a wet well should be related to the pumping rates
as, except at large stations, it provides storage for intermittent
pumping. At large stations the incoming sewers can provide some of
the wet well capacity.For small and medium stations the size of the
wet well should be such that the pumps will not start and stop too
frequently (six to 12 starts per hour is a guide).
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BSI 02-2000 13
17.2 Design
The lower part of the wet well is the sump, which should be
shaped to suit the pump suctions. An inefficient arrangement can
result in a significant reduction in pump output due to air
entrainment. It should also be shaped to prevent deposition of grit
and sewage solids which rapidly occurs when the sewage ceases to
move.It is advisable to provide means of stopping the inflow to the
wet well for maintenance purposes.Incoming sewers and sump design
should be arranged to avoid sewage dropping into the wet well, as
this can also cause air entrainment into the pump suctions. The
sewers can backdrop externally into the wet well. If possible, the
pump cut-in levels should be below the level of the incoming
sewers, to prevent backing up except at large stations (see 17.1).
Backdrops cause problems of:
a) turbulence as a result of their discharging below normal
water level introducing air direct to the suction pipe; andb)
blockage in the backdrops themselves.
Sump design should attempt to prevent air entrainment and
subsequent cavitation in a pump.As the efficient operation of a
station will depend on both the pumps and the design of the sump,
the pump supplier should approve the design of the sump and the
suction pipework. At small stations it is usually sufficient if the
pump suctions are not physically restricted and are well submerged
when pumping commences, but for large stations it may be prudent to
have hydraulic model tests to achieve an efficient design for the
composite arrangement. There is considerable divergence of views on
the detailed design of suction pipework.
17.3 Operation
A build-up of scum and grease at the sewage surface in a wet
well can affect the operation of control equipment and access
should be provided for cleaning the control equipment and, if
necessary, for removing the scum. The part of the wet well in which
the pump control equipment is located should have a sewage surface
which is always reasonably tranquil.Adequate ventilation should be
provided as a safeguard against the accumulation of dangerous gases
or vapours.
18 Ventilation, smell and noiseCareful consideration should be
given to the question of adequate and safe ventilation of the
buildings, and of any confined spaces. Toxic and flammable gases
can arise during the handling and processing of sewage, and the
system should be so designed and operated that any air or gas
discharged is vented to a safe place in the open air. Where this is
impracticable, comprehensive tests should be made to ascertain the
nature of any contaminants which might enter the system or be
generated within it, and appropriate precautions should be
incorporated in the design and operating procedures to deal with
them. (See Table 5 of BS 8005-1:1987.)It is difficult to avoid
smell nuisances when pumping sewage particularly in the
circumstances of overflow or pump failure. Care can be taken when
siting buildings to take advantage of prevailing winds and by
covering over outside tanks containing sewage. It is possible to
install filters and odour removal equipment to deal with certain
types of noxious gases.Particular attention should be paid to the
prompt disposal of screenings at pumping stations. The disposal of
screenings may be subject to the statutory requirements of the
Control of Pollution Act 1974. (See BS 8005-0.) Excessive noise can
be damaging as well as unpleasant to operatives and their
neighbours. It can be reduced by the careful design, selection and
installation of machinery. Noise levels within the building can be
reduced by the use of sound absorbent linings. Transmission between
compartments and from the building can be reduced by the use of
heavy imperforate building materials or of discontinuous
construction. The use ofdouble-door vestibules and double glazing
with a large air gap is also effective. Outside the building, the
use of baffles and vegetation will also absorb and disperse
escaping noise.
19 Lifting facilitiesAt every pumping station appropriate and
suitable lifting equipment should be provided, maintained in a
serviceable condition and used. This could take the form of a
simple pulley block, or in a large station an overhead gantry
crane.
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The type, rating and range of operation of cranes and other
lifting equipment will vary widely depending on the pumps and
ancillary equipment which have to be installed and maintained. For
larger installations, permanently installed gantry cranes covering
the whole area of the pumphouse are convenient. Multi-purpose
lifting appliances such as lorry mounted cranes, fork-lift trucks
and small hydraulic excavators are in common use in the vicinity of
pumping stations. Particularly for mobile plant, consideration
should be given to the question of adequate headroom, the proximity
of overhead power cables, turning circle and surface wheel bearing
capacity.Slings, chains, ropes and other lifting gear should be
suitable for the particular lifting operation.The general statutory
standard for the construction and use of all lifting equipment is
contained in Section 2 of the Health and Safety at Work etc. Act
1974, and even if the Factories Act 1961 does not apply, Sections
26 and 27 of the Factories Act 1961 may be taken as guidelines to
detailed testing, inspections and certification. Advice on the
application of the Factories Act to any particular installation may
be obtained from the local office of the Health and Safety
Executive.The high incidence of back injury among pumping station
operatives in particular justifies the provision of suitable
mechanical devices for theoff-loading of plant and materials from
transport vehicles.
20 SuperstructureThe superstructure of a sewage pumping station
will have to suit the substructure in providing accommodation for
pumping units, equipment and operators. The design of the actual
building requires special consideration in respect of size, type
and appearance.Buildings should be substantial, well-proportioned
and with a choice of materials suitable to operational and climatic
conditions. This includes provisions such as damp-proofing,
insulation,air-conditioning and protection against the
weather.Pumping stations should also be protected against vandalism
and unlawful entry by fitting adequate locking devices to windows
and doors. For remote stations and in high-risk areas, alarm
systems can be fitted in addition. If resort is to be made to
underground stations for security purposes, the risk of flooding,
fire and explosion should be seriously taken into account.
21 Environment and accessSewage pumping stations are normally
situated in the outskirts of residential and industrial areas or in
the rural countryside. Good access is essential for vehicles and
plant for maintenance and emergency circumstances, whatever the
weather conditions.Fencing and warning signs are advisable in
hazardous or vulnerable locations.Access roads and parking areas
should be designed with suitability, durability and maintenance
requirements in mind. Similar consideration should be taken in
deciding areas to be grassed and trees and hedges (or fences) to be
provided. Landscaping can be hastened by the use of quick-growing
trees and shrubs, but this involves extra trimming and the risk of
excessive root growth entering into sewer tanks and pipes, and
undermining foundations.Power failures and flooding due to weather
or burst pipes present hazards to be met, particularly in riverside
and remote areas. These can lead to inconvenience to the public
from flooding, pollution and smell unless such emergencies are
taken into account in general environmental considerations.
Licensed Copy: Giorgio Cavalieri, ALSTO
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BSI 02-2000 15
Section 4. Design of pumping mains
22 Velocities of flowTo avoid sedimentation, the minimum
recommended velocity in pumping mains is about 0.75 m/s, but if
there is a velocity of about 1.2 m/s for several hours each day,
the minimum velocity can be as low as 0.5 m/s. The maximum velocity
should normally not be above 3 m/s. Power considerations usually
impose this limit.
23 DiameterThe diameter of a pumping main should be determined
by an economic analysis of the pipeline and pumping costs and by an
assessment of the engineering factors which may sometimes override
the economic analysis. Alternative diameters should be examined
which produce minimum and maximum velocities within the acceptable
limits, and pumping costs should be estimated taking account of the
normal rate of pumping (not necessarily the peak rate). The most
economical scheme will be the one which involves the lowest overall
annual cost, including repayment of capital cost, running and
maintenance.If septicity of the sewage is likely to be a problem
the retention period in the pumping main should be reduced by
adopting a smaller diameter and accepting a higher velocity of
flow, even though this may mean higher power consumption.The
minimum diameter of a main is usually 100 mm but sometimes smaller
mains may be considered to maintain a minimum velocity and avoid
septicity. Mains as small as 50 mm can be used but it is then
necessary to install a macerator in the system to reduce the size
of solids.
24 Number of mainsDuplication of pumping mains should be
considered in the following circumstances.
a) To provide a standby in the event of the other main being
temporarily out of action. Duplication may be provided for the
whole length or for part of the length, e.g. at crossings of
watercourses, canals and railways.b) To accommodate storm sewage
flows which could not be carried in a single main within the
acceptable limits of velocity.c) To permit parallel working of the
centrifugal pumps where their characteristics do not lend
themselves to combined working through a single main.
Arrangements should be made to ensure frequent use of both mains
to prevent septicity. Automatic pump changeover is a convenient
method.In certain circumstances it may be advisable to provide a
connection to a pumping main adjacent to a pumping station for a
mobile pump to enable the station to be bypassed.
25 PressuresThe maximum friction head (pressure) arises at the
maximum velocity. It should be calculated by one of the recognized
hydraulic systems for friction losses in a pipeline flowing full.
It should be remembered that friction factors and viscosity of the
liquid are likely to change when air or oxygen injection is
employed to control septicity (see BS 8005-1).The possibility of
positive and negative pressures due to surge (water hammer) should
be considered. They are more likely to be significant in long mains
or where high velocities arise. Surge analysis is complicated and
is usually only undertaken when surge pressures are expected to be
important. There are now many computer programmes available to
assist in this analysis.Surge pressures can be alleviated by
various means as follows:
a) a suitably designed regulating or non-return valve in the
main;b) a pressure regulating or surge chamber on the main;c)
flywheels on the pumps to avoid sudden shut down;d) a standpipe,
close to the pumping station;e) double-acting surge relief valves
on system.
Other methods can be included such as stageshut-down, or
variable speed drives to reducelong-term fatigue failure of
pipelines due to the effect of surge pressures. Surge pressures
influence the selection of material and class of pipe of a pumping
main. Recommendations are given in CP 312-2 and CP 2010-2, CP
2010-3, CP 2010-4 and CP 2010-53).The pumping main should be so
designed as to be capable of withstanding a hydraulic test pressure
of not less than 1.5 times the maximum working pressure or not less
than 1.5 times the maximum surge pressure, whichever is the
greater, subject to the recommendations in the above codes.
3) Under revision
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26 ValvesThe arrangement and location of isolating, air release,
washout and non-return valves should be planned together.
Preferably a vehicle should be able to reach each location, but
this needs to be reconciled with the importance of a valve and the
possible interference with land usage.On long mains, valves should
be included to allow for sections to be isolated and emptied within
a reasonable time. Special consideration should be given to
crossings of major roads, railways, watercourses and hazardous
locations, but otherwise the sections might be up to about 1 km
long. An isolating valve should be provided just inside or outside
a pumping station so that the station pipework can be dismantled
without emptying the main. Twin mains should be cross connected,
usually at the points selected for isolation and emptying. Where a
section of a main is to be emptied through a washout valve,
provision should be made for the removal of the contents.Air
release valves suitable for sewerage systems should be provided at
summits:
a) to release air when the main is being filled;b) to release
air and gas which arise during normal pumping;c) to mitigate the
effects of surge;d) to permit air to enter when the main is being
emptied,
In the vicinity of an air release valve a main should rise to
the valve at a gradient preferably not flatter than 1 in 500 and
fall away at a gradient not flatter than 1 in 300 for a significant
length each side of the summit.Air release valves can make
considerable noise when operating and they should be regularly
maintained. If a chamber is provided it should be adequately
ventilated to release the volume of air from the main and to
prevent the accumulation of malodorous, toxic or flammable gases.
In some situations air can adequately be released manually through
a vertical pipe with a cock.Care should be taken not to exacerbate
possible surge problems (see clause 25) by the siting or the use of
incorrect types of air release valves. In certain situations it may
be necessary to restrict the rate at which a pumping main is
filled.
Non-return valves are used to prevent backflow after pumping has
stopped and should be provided at the pumps. In special situations
they may have to be sited on a pumping main; they should have
extended spindles and lever arms so that they can be manually
opened for emptying the main but the size of the non-return valve
and static head dictate whether this is practicable. A bypass may
be necessary in some cases.
27 ProfilesWhere possible, a pumping main should be laid with a
continuous uphill grade and with gentle curves on both horizontal
and vertical planes. When a continuous uphill grade is not
possible, air release valves should be incorporated at high points
and as the profile of the main dictates. Washout valves should be
installed at low points.
28 Discharge arrangementsThe discharge of a pumping main should
be arranged to avoid turbulence or splashing. It is preferable to
avoid a vertical drop pipe and to arrange that the end of the
pumping main is always full. If this is not possible, and the
sewage may be septic, the surfaces of the structure at the
discharge should be protected against corrosion. Chambers into
which pumping mains discharge should be well ventilated. (See
clause 11.)
29 AnchoragesPumping mains require anchorages to resist the
thrusts developed at changes of direction, tapers, tees, valves and
blank ends. Anchorages should not impede flexibility or expansion
and, as far as possible, they should allow for possible replacement
of fittings in the pipeline. The maximum thrusts usually occur when
the pipelines are being tested.In situ concrete blocks should be
provided for buried pipelines. For horizontal mains they should
take the form of a cradle wedged against the undisturbed trench
side; the design should be based on the safe bearing pressure of
the ground. Vertical or inclined fittings should be clamped with
metal straps to concrete blocks beneath them. Inclined pipelines,
steeper than 1 in 6 should be anchored by concrete blocks cast
across the pipes and set into undisturbed ground.
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In situations where it may be impractical to provide an anchor
block to resist thrust, the use of flanged joints, self-restraining
flexible couplings, or special harness assemblies across joints,
may be considered. These should transfer the thrust along the
pipeline until either it is possible to provide a concrete anchor
block, or sufficient frictional resistance is developed between the
pipes and the refilled ground to overcome the thrust.
30 Control of septicitySepticity in pumping mains can be
prevented or controlled by the addition of oxidants to the sewage
either in the form of oxygen or oxidizing chemicals such as
hydrogen peroxide. Chlorine injection, with appropriate safeguards,
may also be considered.The injection of gaseous oxygen, either pure
or as air, causes complications which should be taken into account
when designing the pipeline. Automatic air release valves at peaks
may be incompatible with the process and it may be better to have
air release cocks for occasional purging. Materials in the
pipeline, fittings, valves, etc., should be compatible with the
oxygen/chlorine/sewage mixture.The use of air instead of oxygen is
not advised as the quantity needs to be five times greater. Oxygen
absorption from air is less efficient than from undiluted oxygen
and it may be necessary to remove a considerable volume of
nitrogen.
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Publications referred to
BS 1710, Specification for identification of pipelines and
services.BS 3535, Specification for safety isolating transformers
for industrial and domestic purposes.BS 5153, Specification for
cast iron check valves for general purposes.BS 5304, Code of
practice. Safeguarding of machinery.BS 5345, Code of practice for
the selection, installation and maintenance of electrical apparatus
for use in potentially explosive atmospheres (other than mining
applications or explosive processing and manufacture).BS 5839, Fire
detection and alarm systems in buildings.BS 5839-1, Code of
practice for installation and servicing.BS 6297, Code of practice
for design and installation of small sewage treatment works and
cesspools.BS 8005, Sewerage.BS 8005-0, Introduction and guide to
data sources and documentation.BS 8005-1, Guide to new sewerage
construction.BS 8005-3, Guide to sewers in tunnel4).BS 8005-4 Guide
to design and construction of outfalls.BS 8005-5, Guide to
rehabilitation of sewers4).BS 8007, Code of practice for design of
concrete structures for retaining aqueous liquids.BS 8301, Code of
practice for building drainage5).CP 312, Code of practice for
plastics pipework (thermoplastics material).CP 312-2, Unplasticized
PVC pipework for the conveyance of liquids under pressure.CP2010,
Code of practice for pipelines6).CP2010-2, Design and construction
of steel pipelines in land.CP2010-3, Design and construction of
iron pipelines in land.CP2010-4, Design and construction of
asbestos cement pipelines in land.CP2010-5, Design and construction
of prestressed concrete pressure pipelines in land.CIRIA Technical
Note 119: Screens and Grit in Sewage: Record, treatment and
disposal.
4) In preparation.5) Referred to in the foreword only.6) Under
revision as BS 8010.
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