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1.17 Abandonment of Sewers
Disused sewers and drains have the great potential to allow
unwanted flows, such as groundwater to enter the system through
deteriorating faults in the system fabric. They therefore need to
be removed from the system to prevent structural deterioration,
unauthorised use, and ingress of groundwater and infestation by
rodents.
Disused sewers shall be removed or, where this is impracticable,
they shall be filled in accordance with the materials and details
contained on the Standard Drawings in Volume 8.
2 Pumping Stations
2.1 Standards The standards and sources of information to be
used are listed in sections 1.1 and 1.2.
2.2 Hydraulic Design The overall design philosophy of the
pumping system needs to be a balanced design with due consideration
of functional, environmental and economic aspects. For pumping
systems in the vicinity of sensitive receivers, reliability of the
system is of key concern. Bypass or overflow of raw sewage, even in
emergency situations, should be avoided where possible.
Particular attention should be paid to the following issues:
Design flow;
Standby power supply or temporary storage;
Standby pumps;
Overflows and emergency bypass;
Twin rising mains;
Availability of QGWEC power supply;
Land area available and proximity to housing or public
areas;
Access to the proposed site.
Since the pumping station will probably be serving an area of
new development, it is likely that the initial flows to the station
will be much smaller than those expected for the full design. Flows
will increase in the following years to reach the design capacity
of the station. If the inflows are greatly below the pump output,
the result will be excessive periods of inactivity of the station,
with the potential for premature failure of equipment. Such
infrequent operation of pumps will also result in retention of
sewage in the rising main, raising problems with septicity,
corrosion and effects on the receiving STW.
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Consideration should therefore be given to the sizing and
numbers of pumps to match the likely build-up of incoming flows.
Where possible, similar pumps should be installed, on duty and
assist basis, with similar standby pump(s). The use of similar
pumps will avoid any changes in pumping regime due to the rotation
of duty pumps for operational reasons.
Consideration should also be given to installing twin rising
mains. One main would be used in the early years of the scheme to
achieve satisfactory maximum flow velocities and hence minimise
siltation. When flows increase, then the second main would be
brought into use.
Although not strictly required for the early years of a scheme,
it would not be economic to construct one rising main and then
construct the second within a short period, say five years. The
additional costs and disruption of digging a second trench,
together with operational and safety requirements of working
adjacent to a live rising main, would be avoided.
2.2.1 Hydraulic Principles
A pumping system may consist of inlet piping, pumps, valves,
outlet piping, fittings, open channels and/or rising mains. When a
particular system is being analysed for the purpose of selecting a
pump or pumps, the head losses through these various components
must be calculated. The station loss (i.e. the loss on the suction
and delivery pipework from the sump to the common header) should
also be considered. The frictional and minor head losses of these
components are approximately proportional to the square of the
velocity of flow through the system and are called the variable
head.
Friction losses should be determined using the ColebrookWhite
Formula.
Losses in fittings at the station, and outside of it should be
determined using the formula:
H = kv2/2g
Equation 2.2.1
Where H denotes the fitting headloss (m), k is the loss
coefficient, v the velocity (m/s) and g is the gravitational
constant, 9.81m/s2.
Indicative values of k are given in Table 2.2.1below.
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Table 2.2.1 Indicative Minor Loss Coefficients,
k, for Various Fittings
Fitting Coefficient k
Standard 900 bend 0.75
Long Radius 900 bend 0.4
Standard 450 bend 0.3
Tee - line to branch 1.2
Tee flow in line 0.35
Taper up 0.5
Sharp Entry 0.5
Bellmouth Entry 0.1
Sudden Exit 1.0
Non-return valve* 1.0
Gate Valve, fully open* 0.12
*Note that for valves it is advisable to obtain
manufacturers data on headlosses. System head
calculations would normally be carried out using valve
open figures.
It is also necessary to determine the static head required to
raise the liquid from suction level to a higher discharge level.
The pressure at the discharge liquid surface may be higher than
that at the suction liquid surface, a condition that requires more
pumping head. These two heads are fixed system heads, as they do
not vary with rate of flow. Fixed system heads can be negative, if
the discharge level or the pressure above that level is lower than
suction level or pressure. Fixed system heads are called static
heads.
The Total Dynamic Head (TDH) for a system is the sum of the
major and minor friction losses plus the static head. The duty
point for a pump selection will be the required flow at the
TDH.
A system head curve is a plot of total system head, variable
plus fixed, for various flow rates. It may express the system head
in metres and the flow rate in cubic metres per second. Procedures
to plot a system-head curve are:
1. Define the pumping system and its length;
2. Calculate the fixed system head;
3. Calculate the variable system head losses for several flow
rates;
4. Combine the fixed head and variable heads for several flow
rates to obtain a curve of total system head versus flow rate.
The flow delivered by a centrifugal pump varies with system
head. Pump manufacturers provide information on the performance of
their pumps in the form of characteristic curves of head versus
capacity, commonly known as pump curves. By superimposing the
characteristic curve of a centrifugal pump on a system-head curve,
the duty point of a pump can be determined.
The curves will intersect at the flow rate of the pump, as this
is the point at which the pump head is equal to the required system
head for the same flow.
The recommended values for coefficient of ColebrookWhite
Roughness Factor (Section 1.5.1 above) ks for use in rising mains
are contained in Table 2.2.2 below. Note also the values indicated
in Table 1.5.1, which refer to gravity sewers.
Table 2.2.2 Recommended Values of
Colebrook-White Roughness Factors
(ks) for use in Rising Mains
Mean Velocity in m/s ks (mm)
Up to 1.1m/s 0.3mm
Between 1.1m/s and 1.8m/s 0.15mm
The discharge capacity for multiple pumps will not be simply the
sum of the discharge capacity of individual pumps because the
system-head curve for multiple pumps will be different from that of
a single pump.
2.2.2 Pump Arrangements
The number of pumps to be installed depends on the station
capacity and the range of flows. The maximum discharge rate from a
pumping station, when all duty pumps and rising mains are in use
should be slightly greater than or equal to the maximum incoming
flow to the station. Pumps should be selected with head-capacity
characteristics that correspond as closely as possible to the
overall station requirements.
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Standby capacity is required so that should any of the pumps in
the station be inoperable due to routine maintenance or mechanical
failure, the operation of the station can still be maintained. For
instance, in a station where a single duty pump provides the duty
output, a second pump of equal capacity is mounted. Where three
duty pumps of equal capacity are required to meet the maximum
design flow conditions, a fourth pump of similar capacity is
provided as standby.
It is not desirable to have pumps of different sizes for
operation and maintenance reasons, unless the flow ranges vary
widely throughout the day. To cater for slow build-up of flow in
the early years of operation, phased installation of pumps, or the
use of a smaller diameter impeller should be considered.
2.3 Rising Main Design
2.3.1 Rising Main Diameters
The minimum diameter of pumping mains is controlled by the need
to avoid blockage, and therefore should not be less than 100mm.
Where sewage is screened or macerated before pumping the minimum
diameter should not be less than 80mm.
The maximum and minimum diameters are sized to maintain flow
velocities for all stages of pumping within the ranges specified in
Section 2.4.
2.3.2 Twin Rising Mains
The use of twin rising mains should be considered on a case by
case basis. The main factors for consideration include the design
elements, risk assessment and cost benefit analysis.
Considerations for the design elements comprise the rate of
build up of flow, the range of flow conditions, the range of
velocity in the mains, the availability of land for the twin mains
and associated valve chambers as well as the complications in pump
operation.
A thorough risk assessment should be carried out which should
include the likelihood of mains bursting, the consequence of
failure, area affected, sensitive receivers affected (such as
beaches), and
the feasibility of temporary diversion or tankering away.
A cost benefit analysis should include all tangible factors
(such as cost of pipework, land cost, energy cost, etc) and
intangible factors (such as nuisance, closure of beaches, etc).
Twin rising mains should be considered in the following
circumstances:
To accommodate a wide range of flow conditions, such that the
velocity in the mains can be kept within acceptable limits. For
instance, a pumping system serving a new development may have very
low initial flows with a slow build up of flow;
To provide continued operation for a major pumping system when
one of the mains is damaged and where the failure of the system
would have serious consequence;
To minimise adverse environmental impacts to sensitive
areas;
To facilitate future inspection and maintenance of major pumping
systems, while the normal sewage flow can be maintained.
When twin mains are found to be preferred, it is advisable to
use both mains as duty rather than one as duty and the other as
standby, from an economical and operational point of view. Should
one of the duty mains be taken out of operation, the remaining one
would still be able to deliver a higher quantity of flow at a
higher velocity. The occurrence of overflow or bypass can be
minimised or even eliminated. Septicity in the standby mains would
also pose an operational and maintenance problem.
2.3.3 Economic Analysis
As the size of the rising main increases, the velocity and the
system head will decrease, with savings in the cost of pumping. The
increase in the capital cost of rising mains will be offset by the
power cost of pumping. However, it is also important that the
velocity in the mains should be within a suitable range to minimise
the deposition of solids. Excessive hydraulic head losses are to be
avoided.
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The selection of a suitable size for the rising mains should be
based on economic analysis of capital cost and recurrent cost of
the pumping system including the power cost. A trial and error
approach should be adopted in order to arrive at an optimal
solution while maintaining the velocity within acceptable
limits.
Therefore, combinations of different sizes of rising mains and
the system head should be evaluated, taking into account both the
capital cost and the energy cost of pumping.
2.3.4 Rising Main Alignment
The alignment of the rising main should discourage surge in its
flow conditions. Where possible the rising main should be laid with
continuous uphill gradient of not less than 1:500, and with gentle
curves in both horizontal and vertical planes. Long flat lengths of
rising main should be avoided therefore pipes should be laid with
rise and falls of 1:500, rather than flat. Air release valves
should be provided at high points and as the profile of the main
dictates. Washouts should be installed at low points. The
arrangement and locations of valves should be planned together with
the alignment of the rising mains.
2.4 Maximum and Minimum Velocities
The maximum velocity in rising mains should not exceed 2.0 m/s,
The desirable range of velocity should be 1m/s to 2m/s with due
consideration given to the various combinations of number of duty
pumps in operation. (This is because lower velocities cause
siltation, and higher velocities increase surge problems and power
usage).
2.5 Pipe Materials Pipe materials for use in pumping stations
should always be Ductile Iron (DI).
Rising mains outside pumping stations may be ductile iron or
Glass Reinforced Plastic (GRP) with concrete protection, however DI
is preferred.
2.6 Thrust Blocks Thrust blocks are concrete blocks designed to
prevent pipes from being moved by forces exerted within the pipe by
the flow of water hitting bends, tapers, and closed, or partially
closed valves. In the design of pressurised pipelines, thrust
blocks are essential on flexibly jointed pipelines where any pipe
movement would open up the joints in the line and cause water
leakage. Restraint straps may also be required for above-ground
pipework.
Thrust blocks are also necessary near valves where a flexible
joint is located to facilitate removal of the valve for maintenance
purposes. The size of block is dependent upon the angular
deflection, flow, size of pipe and the pressure of water inside the
pipe. The designer should also refer to the pipe manufacturers
literature.
The following design assumptions are to be adopted:
Thrusts developed due to changes in direction of pipeline, dead
end or change in diameter should be considered. Force due to change
in velocity head can normally be assumed as negligible unless there
is a drastic change in pipe diameter;
Thrust blocks should be designed for the condition of no support
being available from the backfill, i.e. to be cast against
undisturbed ground;
For most cases, thrust blocks will be designed to transfer
forces directly onto undisturbed ground using direct bearing, the
acceptable bearing pressure being confirmed by geotechnical
investigation. If the adjacent ground has insufficient bearing
capacity, the block may need to be designed using ground friction
or piling to transfer thrusts to a more competent soil layer.
Consideration should also be given to the presence of adjacent
services and the possibility of future disturbance during
maintenance operations. Complex thrust blocks may be required to
avoid transfer of forces and consequential damage to adjacent
services;
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For pipes with flexible joints such as DI pipes with socket and
spigot joints, all the thrust is assumed to be taken up by the
blocks.
Static thrusts may be calculated using the formulae as
follows:
For blank ends:
Fs = 100 A P
Equation 2.6.1
Where:
Fs = the static thrust (KN)
A = the cross sectional area (m2)
P = the Pressure (bar)
For Bends:
Fs = 100 A P(2 sin /2)
Equation 2.6.2
Where is the angle of deviation at the bend.
Dynamic thrusts for water or sewage may be calculated using
following:
Fd = 2A V 2 sin /2
Equation 2.6.3
Where
Fd = the dynamic thrust (KN)
V = the velocity (m/s)
As stated above, this force is negligible in normal cases, but
if significant, then the total thrust should be taken as the sum of
static and dynamic thrusts.
The above procedures will be satisfactory for most routine
applications. For further guidance, please see CIRIA Report
R128xxxviii. It is recommended that this reference is used for more
complex applications, such as where thrust forces are in excess of
1000KN or loose material is encountered.
2.7 Air Valves and Washout Facilities
These facilities are required to minimise the adverse effects of
surge and to facilitate the operation and maintenance of the rising
main.
2.7.1 Air Valves
Air-relief valves are installed at locations of minimum
pressure. Air is sucked into the air-relief valve when pipeline
internal pressure is below atmospheric. Upon subsequent pressure
rise, the admitted air is then expelled. Air valves should be
installed at all high points., Additional air valves should also be
placed at 800m spacings on long sections of straight grade.
Each air valve will operate independently and therefore several
valves may be required along the pipeline if there are numerous
rises and falls in the vertical profile of the rising main.
2.7.2 Vented Non-return Valves
An air valve combined with a vented non-return valve allows air
enter the pipeline freely on separation of the water column, but
controls the expulsion of air as the column rejoins. This has the
effect of creating an air buffer between the column interfaces,
thus reducing the impact velocity of the rejoining column and the
surge potential of the system.
2.7.3 Wash Outs
The purpose of the washout system is to drain the rising main
for maintenance works. The washout should be installed at low
points of the pipeline profile, and needs to be located carefully,
taking into account that sewage will be discharged. For long rising
mains with few low points, wash-outs should be strategically
located at suitable intervals, generally 800m, to reduce the time
required for emptying the main in an emergency. Location should be
adjacent to a suitably sized gravity sewer for draindown where
possible If a direct connection to a suitably sized sewer is not
available, the washout chamber should be provided with a sump
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so that the drained contents of the rising main may be tankered
away.
2.7.4 Isolating Valves
For long rising mains, isolating valves should be included to
allow sections of the rising main to be isolated and emptied within
a reasonable time. In-line sluice or gate valves are often used as
isolating valves. The isolating valve installation may incorporate
washout facilities.
2.8 Flow Meters
2.8.1 Application and Selection
The variety of choices facing the designer confronted with a
flow measurement application is vast. For example, types of flow
meter using the positive displacement principle include rotary
piston, oval gear, sliding vane, and reciprocating piston. Each
type has advantages and limitations and no single type combines all
the features and all the advantages.
Differential pressure meters have the advantage that they are
the most familiar of any meter type. They are suitable for gas and
liquid, viscous and corrosive fluids. However their usable flow
range is limited and they require a separate transmitter in
addition to the sensor.
Some of the most important parameters for flowmeters are
accuracy, flow range, and whether the medium is sewage or water.
Meter selection should be made in two steps. First by identifying
the meters that are technically capable of performing the required
measurement and are available in acceptable materials of
construction; and second, by selecting the best choice from those
available to cover special measurement features such as reverse
flow, pulsating flow, response time and so on.
2.8.2 Magnetic Flowmeters
Magnetic-type flowmeters use Faradays law of electromagnetic
induction for measurement. When a conductor moves through a
magnetic field of given field strength, a voltage level is produced
in the
conductor that is dependent on the relative velocity between the
conductor and the field. Faraday foresaw the practical application
of the principle to flow measurement, because many liquids are
adequate electrical conductors. So these meters measure the
velocity of an electrically conductive liquid as it cuts the
magnetic field produced across the metering tube. The principal
advantages include no moving components, no pressure loss, and no
wear and tear in components.
Magnetic flowmeters offer the designer the best solution for
pumped sewage flow. With nothing protruding into the flow of
sewage, the chances of a blockage, if installed correctly, are
non-existent. Magnetic flowmeters should always be installed with
full-pipe conditions.
Care should be taken during design to provide sufficient
straight lengths of pipeline up-stream and down-stream of the
flowmeter, in accordance with the manufacturers installation
instructions. As a general guideline, 12 pipe diameters of straight
pipe on the inlet, and 6 pipe diameters on the outlet will ensure
that the flowmeter is able to achieve the specified accuracy. If
the amount of space available is restricted then the minimum length
usually accepted by manufactures is inlet run of 5 pipe diameters
and outlet run of 3 pipe diameters.
The following International and British Standards are a good
source of information on flow meter selection and installation, and
can be quoted in specifications:
BS EN ISO 6817xxxix, 1997: Measurement of Conductive Liquid Flow
in Closed Conduits;
BS 7405xl, 1991: Guide to Selection and Application of
Flowmeters for the Measurement
of Fluid Flow in Closed Conduits.
Flow meters should be pressure tested and calibrated by the
manufacturer, and certified to a traceable international standard.
As a minimum, the overall accuracy should be better than 0.5% of
the flow range. The repeatability of the result should be within
0.2%.
In addition to the calibration certificate, the flow meter
manufacturers should provide the following:
i. Isolated 4-20mA dc and pulse outputs;
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ii. Programmable in-built alarm relays for empty pipe, low and
reverse flows;
iii. In-built digital display for flow rate, totals and
alarms;
iv. Transmitter enclosure shall be protected to IP67;
v. Calibration and programming kit.
The earthing rings should be included according to the
individual manufacturers instructions. The sensor lining should be
neoprene or an equivalent material of similar or improved
properties, suitable for the application of pumped sewage flow. In
below-ground flow meter chamber installations, the installed
equipment should be submersible to the maximum chamber depth.
2.8.3 Ultrasonic Flowmeters
Ultrasonic meters are available in two forms: Doppler and
transit-time. With Doppler meters, an ultrasonic pulse is beamed
into the pipe and reflected by inclusions, such as air or dirt. The
Doppler meter is frequently used as a clamp on device which can be
fitted to existing pipelines. It detects the velocity only in a
small region of the pipe cross section and as such its accuracy is
not good. The single or multi-beam transit-time flow meters project
an ultrasonic beam right across the pipe at an acute angle, first
with the flow, and then opposite to the flow direction. The
difference in transit time is proportional to flow rate. This type
of ultrasonic meter is considerably more expensive but offers
better accuracy. Unlike the Doppler meter, it requires a relatively
clean fluid.
The main use of this type of flow meter in pumped sewage flows
is in retrospective installation where the pumping main cannot be
broken into for operational reasons. A clamp-on ultrasonic flow
meter can be used to give reasonably accurate flow measurement.
For new installations, the lower cost of in-pipe ultrasonic flow
meters could make them a viable alternative to magnetic flow meters
for large diameter pipe installations.
2.9 Surge Protection Measures
Surge (or water hammer) is an oscillating pressure wave
generated in a pipeline during changes in the flow conditions.
There are four common causes of surge in a pipeline:
pump starting;
pump stopping/power failure;
valve action;
improper operation of surge control devices.
The most likely one of these is the sudden stopping of pumps
caused by a power failure.
A surge analysis should usually be carried out unless the system
is simple. This is best carried out using approved software such as
Flowmaster.
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An approximate calculation for a simple pipeline is:
P = a x V g
Equation 2.9.1
Where: P = Pressure change (m)
a = pressure wave velocity (m/s)
V = flow velocity change in 1 cycle (m/s)
g = acceleration of gravity (9.81m/s2)
The above equation can be used for calculation of both negative
and positive pressures
The simple cycle time can be calculated with the formula:
Cycle time = 2 x pipeline length
Wave velocity
Equation 2.9.2
Table 2.9.1 Indicative Surge Wave Velocity
Values for Selected Pipe Materials
Pipe Material Velocity (m/s) Ductile Iron 10001400
Reinforced Concrete 10001200 Plastic 300500
If the surge pressure approaches zero or the pipeline maximum
pressure, a full surge analysis should be carried out. When surge
analysis is complete, suitable surge suppression devices should be
selected by consultation with the manufacturer.
Surge Suppression Methods
Surge suppression could be achieved using one of the following
devices. The most appropriate device will depend on the individual
circumstances of the installation:
Flywheel;
Pressure vessel with bladder;
Dip-tube surge vessel;
Surge tower.
Air valves should not be depended upon as a sole method of surge
control, but their operation under surge conditions should be
carefully considered.
Flywheels
Flywheels absorb energy on start-up, slowing the rate of
velocity change in the pipeline. In reverse, when the pump is
stopping, the flywheel releases energy again, slowing the rate of
velocity change. Together these two actions reduce the peak surge
pressure.
As the flywheel must be located on the drive shaft it is not
suitable for submersible pumps or close-coupled pumps. However,
they are simple devices for wet well/dry well pumps and are
preferred where possible.
If submersible pumps have been chosen, a larger pump running at
a slower speed may have the effect of a flywheel.
Because the flow continues through the pump after the stop
signal, the effect on the stop and start levels should be carefully
considered.
Pressure Vessels
Pressure vessels for surge suppression are tanks partially
filled with a gas (air or nitrogen). Usually the liquid is
contained in a bladder with gas on the outside to prevent the
liquid absorbing the gas or coming into contact with the inside of
the pressure vessel, and this is the preferred type. The bladder
material should be carefully selected for use in the conditions
experienced in Qatar.
Refilling is usually from a high-pressure cylinder and care
should be taken to avoid over pressurisation of the bladder.
Bladders should not lose pressure in normal operation, but they can
fail, leading to absorption of the gas into the liquid, and a drop
in pressure.
Vessels without a bladder are charged with air pressure from an
air compressor, either manually or automatically. There is
therefore additional machinery and an additional maintenance
requirement. This type of surge vessel is not recommended.
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On pump start-up, liquid enters the vessel, compressing the gas
until it equals the liquid pressure. When the pump stops, the gas
pressure forces liquid back out into the pipe system, both actions
slow the rate of pressure change, which reduces the peak surge
pressure.
To dampen oscillations, a non-return valve may be fitted to the
surge vessel outlet pipe, to allow unrestricted flow into the
pipeline, and a bypass around the NRV fitted with an orifice plate
to restrict the flow back into the vessel.
Dip Tube Surge Vessels
A dip tube surge vessel is pressure vessel, the top portion
forming a compression chamber limited by a dipping tube with a shut
off float valve.
This type of vessel is particularly appropriate for use on
rising mains with flat profiles.
Surge Towers
A surge tower is a vertical tank or pipe fitted into the
pipeline, open to atmosphere and the energy storage is by the
static head of the liquid in the tower.
Surge towers are only practical for systems with relatively low
heads and surge pressures, but can pose an odour risk.
Due to the design of a surge tower, there is no routine
maintenance required to ensure the surge tower keeps operating
correctly.
It is unlikely that surge towers would be appropriate for use in
Qatar.
Air Valves
Air valves are required on the pumping mains to release air, but
they should not be used as a surge protection measure.
However, air valves, particularly if fitted with a vented
non-return valve or in-flow check valve, may assist in surge
control, and their operation must be carefully considered.
Air valves require regular maintenance because if the air valve
does not function correctly, large or negative surge pressures
could result, with consequent damage to equipment or personnel.
If air is allowed into the rising main on pump stop/trip through
an air valve, the pump control system should be designed to prevent
a restart until the transient pressures have stabilised.
Control of the pumps is usually by start/stop level signals, but
where surge on start-up may have a significant effect, the use of
soft starters should be considered.
2.10 Screens Screen Selection
Screens should generally be provided for pump protection, unless
they are small (1000l/s;
Fine screening is not required at the pumping station, but is
required at the treatment works to remove debris that may affect
the sewage treatment process.
Screen Installation
The manual duty and standby screen should be installed in the
incoming channel, so that the standby screen can be lowered into
position to
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protect the pumps while the duty screen is removed and
cleaned.
Mechanically raked screens should be installed in a channel or
similar flow-line, which can be completely isolated from the rest
of the system and drained for maintenance. A manually raked bypass
screen shall be provided.
Mechanical screens shall be housed in ventilated and odour
controlled enclosures.
Screens should be provided with actuated penstocks (or valves)
before and after each screen for operational and maintenance
isolation.
All mechanically raked screens should have an automatic cleaning
mechanism, which will clean the screen of accumulated debris and
screenings, depositing them in a collection trough or channel above
the highest possible water level.
Screenings Handling
Manually removed screenings should be placed in a covered
container until removed from site to avoid odour problems.
Mechanically removed screenings should be washed, compacted and
deposited into a covered container to avoid odour problems.
2.11 Pumping Station Selection
Sewage pumping station type selection should be carefully
considered for each scheme. In general, submersible pumping
stations are generally selected for flows up to 100l/s, and wet
well/dry well stations for larger flows. However, each station
should be treated on its own merits and the following
considerations assessed:
Initial and final design flow;
Total head on the pumps;
Rising main profile and the requirements for surge protection
(dry well pumps usually have a greater moment of inertia than
submersibles);
Requirement for Variable Speed Drive (VSD): (submersible motors
are not always adequately rated for use with VSD);
Space available for pumping station (submersible stations
usually require less space);
Proximity of housing or public areas (opening submersible pump
wells may create odour nuisance).
An alternative to wet well submersible pumps and dry well pumps
is the dry well submersible. These should normally be considered
only where an existing dry well installation is being uprated and
there is insufficient space to install a conventional dry well pump
and motor.
Particular attention should be paid to motor cooling and cabling
if dry well submersibles are to be considered.
The designer should present three alternative pump suppliers for
tender purposes.
Submersible pumping stations
Submersible pumping stations should incorporate the following
features:
Minimum of one duty and one standby pump;
Non-return valve and gate valves for isolation of each pump;
Valves to be in a separate, easily accessible chamber adjacent
to the pump sump;
Air reaction operation level controls as follows:
- High level alarm (also float);
- Pump start;
- Pump stop;
- Low level pump protection (also float).
Ultrasonic level controls should not be used for sewage;
Air reaction level equipment should include stainless steel dip
pipe and duty/standby compressors.
Where the available pumps have unsuitable duties for the full
range of flows, the use of variable speed drives should be
considered. However, due to the
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additional heat generated in the motor, the approval of the pump
manufacturer should be obtained before variable speed drives are
used.
Submersible Pump Sump Design
The CIRIA guide The hydraulic design of pump sumps and intakes
by M. J. Prosserxli should be referred to when designing pump
sumps. Some pump manufacturers also provide guidance on the design
of sumps for their pumps. Sump design should be in accordance with
the following criteria:
Sumps should be designed so that the dimensions satisfy the
requirements for the minimum sump volume to ensure the maximum
rated pump starts per hour for the motor and switchgear are not
exceeded;
Sumps should be designed to provide a uniform steady flow of
water into any pump without creating swirl or entraining air.
Unsteady flow can lead to fluctuating loads, vibration, noise and
premature failure. Swirl can affect the flow capacity, power and
efficiency. It can also result in local vortices that introduce air
into the pump, also leading to fluctuating loads, vibration, noise
and premature failure;
Sumps should be designed to prevent the accumulation of
sediment, scum and surface flotsam;
Sump corners should be benched to 45. Minimising the sump floor
area and residual volume will increase the velocity into the pumps
and improve scouring;
The use of flushing devices to improve scour in pump sumps
should be considered;
The velocity in the pump riser pipe at the design duty should be
as high as practicable to reduce the risk of solids deposition.
However, the velocity should not normally exceed 2.5m/s to avoid
significant headloss and risk of pipe erosion;
The water surface in the sump should be as free from waves and
turbulence as possible to provide a strong and reliable echo for
ultrasonic level controls;
At the designed stop level there should still be sufficient
water surface area without obstructions to provide a good echo
return.
Submersible Pump Installation
When submersible pumps are installed, the following should be
considered:
There should be sufficient space between them to prevent
interaction between the pump suctions. This will depend upon the
type of pump being used and the manufacturer should be consulted on
configurations at draft design stage; A rule of thumb is to use an
initial spacing between pump centres of twice the pump diameter.
Further guidance is given in table 2.11.1 below.
Table 2.11.1 Approximate Minimum
PumpSpacingsxlii
Flow (l/s) Spacing (mm)
100 700
200 1000
300 1200
400 1350
500 1500
600 1700
700 1800
800 1900
900 2050
1000 2175
There should also be sufficient space for someone to stand
beside each pump, should work be required in the sump;
Pump mounting stools and duckfoot bends should be securely
bolted to the structural concrete of the sump and not the
benching;
Discharge non-return and isolating valves should be located
outside the sump in a valve chamber;
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Pump guide rails should rise close to the underside of the sump
covers above the pumps;
The covers should have a clear opening large enough to allow the
removal of the pump while on the guide rails;
Support points for the pump power cables and lifting chain
should be provided under the pump covers, which should be easily
accessible from the surface.
Wet/Dry Well Pumping Stations
Wet well/dry well pumping stations should incorporate the
following features:
Normally, two sumps with 2 duty and 1 standby pump for each
sump, for the ultimate flow;
Non-return and two gate valves for each pump isolation;
Where possible, the discharge manifold should be below ground
level to minimise additional pipework and friction losses;
Where wet well/dry well pumping stations are being uprated, dry
well submersible pumps could be considered;
Operation level controls (air reaction) as follows:
- High level alarm (plus float);
- Pump start;
- Pump stop;
- Low level pump protection (plus float).
Air reaction level equipment should include stainless steel dip
pipe and duty/standby compressors.
Where the available pumps have unsuitable duties for the full
range of flows the use of variable speed drives should be
considered. However due to the additional heat generated in the
motor, the approval of the pump manufacturer should be obtained
before variable speed drives are used.
Wet Well Design
The CIRIA guide The hydraulic design of pump sumps and intakes
by M. J. Prosser should be referred to when designing wet wells,
which should incorporate the following features:
Wet wells should be designed to provide a uniform steady flow of
water into any pump without creating swirl or entraining air.
Unsteady flow can lead to fluctuating loads, vibration, noise and
premature failure. Swirl can affect the flow capacity, power and
efficiency, it can also result in local vortices that introduce air
into the pump also leading to fluctuating loads, vibration, noise
and premature failure;
Wet wells should be designed to prevent the accumulation of
sediment, scum and surface flotsam;
Wet well corners should be benched to 45. Minimising the sump
floor area and residual volume will increase the velocity into the
pumps and improve scouring;
The use of flushing devices to improve scour in wet wells should
be considered;
The water surface in the wet well should be as free from waves
and turbulence as possible to provide a strong and reliable echo
for ultrasonic level controls;
At the designed stop level there should still be sufficient
water surface area without obstructions to provide a good echo
return;
Wet wells should be designed so that the dimensions satisfy the
requirements for the minimum sump volume to avoid excessive pump
starts;
The pump suction pipes should be installed through the wet/dry
well dividing wall with a downward bend and bellmouth to position
the pump suction as close to the sump floor as possible to assist
in sediment removal;
There should be sufficient space between the bellmouths to
prevent interaction between the pump suctions.
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Dry Well Design
Dry well design should incorporate the following features:
The pumps should be installed along the wet/dry well dividing
wall with sufficient space between them to allow access for
maintenance and repair;
The pump distance from the dividing wall will be set by the
length of the protruding stub pipe, suction valve and pump inlet
pipe;
Drive shafts should be supported from concrete beams spanning
the dry well;
Consideration should also be given to access around the pumps
and valves. Platforms and walkways should be installed to provide
access to all equipment at a suitable level for safe operation,
maintenance and repair;
The general floor level should be higher than the sump level to
reduce the size of pump plinths and the need for access
platforms;
Careful thought should also be given to the shipping route for
removing equipment;
Access to the dry well and machinery should be by staircase so
that tools and equipment can be carried in and out safely;
Lifting arrangements for the pumps and valves shall be provided
(see also section 2.21 and 2.22);
The dry well floor should slope gently towards the dividing wall
and then to one side where a sump pump should be installed to keep
the floor as dry as possible;
The sump pump should be installed in a small well, large enough
to accommodate the pump and should discharge back through the wall
into the wet well. Consideration should be given to the sump pump
discharge to avoid backflow from the wet well to the dry well;
A high level alarm should be installed in the dry well to give a
warning of flooding before damage to machinery occurs.
Pump Installation
For the most compact arrangement, a close-coupled pump can be
mounted horizontally with the discharge upward, however this
results in the motor being low in the dry well and at risk from
flooding. The most common arrangement is for a vertical pump shaft
with the motor above. This will require a bend between the suction
valve and the pump suction. The bend should be fitted with a
handhole and valve to enable the pump to be drained prior to
maintenance. Further bends may be required to direct the pump or
manifold discharge upwards. Where space allows, installation of the
discharge manifold at the pump level, with the discharge directly
through the side wall should be considered.
Pipes should be sized to achieve sensible velocities, and the
risk of cavitation through insufficient NPSH should be considered
when designing suction pipework. Pumps must be selected to ensure
satisfactory operation when only one pump is operation in a new
rising main.
2.12 Pumps and Motors Centrifugal Pumps
These are the most common type pumps for foul sewage and are
available in a variety of forms. The pump operates by passing the
liquid through a spinning impeller where energy is added to
increase the pressure and velocity of the liquid. Submersible pumps
are centrifugal pumps.
Sewage pumps should have an open type impeller with a minimum
passage of 100mm. Impellers with smaller passages are likely to
suffer from frequent blockage due to the nature of sewage
debris.
Dry well centrifugal pumps should normally have a maximum
running speed of 980rpm. Submersible pumps may run at 1450rpm (4
pole motor), but pumps operating at 2900rpm (2 pole motor) will
suffer excessive wear and premature failure, and should not be
used.
Pump Motors
Motors on submersible pumps should be certified for use in Zone
1 explosive atmospheres unless operating continuously submerged.
Pumps operating in dry conditions should have a casing designed to
provide adequate cooling in the operating conditions.
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Pump motors should normally be fed from 415 volts, 50 hertz,
3-phase power supply. For larger motors 690V or 3.3KV motors can be
used.
Because additional heat is generated in the motor when used with
a variable speed drive, the approval of the pump manufacturer
should be obtained before VSDs are used.
For dry well and screw pumps where the motors are installed
vertically or at a steep angle, they should be specifically
designed for that purpose, with adequately rated end thrust
bearings.
Where flywheels are installed, the motor rating shall be
suitably uprated.
2.13 Sump Design The CIRIA guide The hydraulic design of pump
sumps and intakes by M.J. Prosserxli should be referred to when
designing sumps or wet wells.
Sumps should be designed to provide a uniform steady flow of
water into any pump without creating swirl or entraining air.
Unsteady flow can lead to fluctuating loads, vibration, noise and
premature failure. Swirl can affect the flow capacity, power and
efficiency. It can also result in local vortices that introduce air
into the pump also leading to fluctuating loads, vibration, noise
and premature failure.
Sumps should also be designed to prevent the accumulation of
sediment and surface scum.
Most sumps and wet wells at standard pumping stations will
probably be uniform in section and can be designed to avoid
turbulent flows.
Modelling
For non-standard pumping stations, which may have high flows,
multiple pumps or complex shapes, or where turbulent flows,
vortices, swirl or air entrainment are more likely to occur,
modelling should be considered.
For pumping stations, a physical model built to scale can be
very effective in identifying flow problems and in some cases
modelling by computational fluid dynamics (CFD) methodology may
have benefits. Modelling is the process of replicating the
hydraulic
performance of drainage, pumping and treatment systems by
constructing models of the intended installations. These models
need to be verified before use to provide confidence that they
adequately represent the actual performance of the system.
The verified model is then used to test system performance under
its proposed use. The model must be capable of modification to test
various physical configurations and operating regimes for the
installation, to produce the optimum solution for actual
construction.
Traditionally, physical models were favoured, especially for
coastal/estuary/river systems and complex pumping installations. In
recent years mathematical models have superseded physical models.
Mathematical models are exploiting increased computer hardware and
software capability, and are more efficient than physical models in
time and effort.
Physical Models
Physical modelling consists of constructing a reduced scale,
geometrically similar model of a proposed system, and operating the
model to simulate full-scale flow conditions. Model tests can
provide the designer with the assurance that the proposed scheme
operates satisfactorily, or allows him to improve the flow
conditions and achieve a better design.
Changes in the model can be made by trial and error, and are
usually based on the experience and intuitive understanding of the
engineer conducting the tests. The amount of modification which can
be undertaken on a physical model is limited, and therefore the
initial model should be as accurate as possible.
Factors to be considered in deciding on the need for physical
models include:
The similarity of the proposed scheme to existing satisfactory
designs. As well as the designers own experience, much information
is available from manufacturers published reports and design
guides. However, it should be recognised that most large scale
and/or complex designs will be unique, and hence modelling will be
needed;
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The size and cost of the proposed scheme. Bearing in mind that
physical modelling can take many months with corresponding high
costs, then designers of small schemes should seek to adopt
standard and well-proven designs for small schemes. Large schemes,
such as terminal pumping stations with multiple pumps and complex
inlet arrangements would merit modelling;
The time available for modelling. In some cases the scheme can
be well under way to completion before the possible need for
modelling is realised. Even at such late stages, modelling can save
much time and cost in modifying construction works.
For pumping stations, all of the intake should be modelled,
including the approach works, the inlets and the sump itself.
Upstream pipelines may need to be included.
All hydraulically significant details such as screens,
penstocks, support channels and benching, should be included in the
model. No components above maximum water level need be
modelled.
Model construction should be in durable and waterproof
materials, with clear perspex being the best for viewing purposes.
Model size should be as large as costs allow. Scales can vary from
perhaps 1:4 for very small sumps, up to 1:50 for large intakes to
reservoirs or tanks. For sump models, 1:25 would be the smallest
desirable scale.
Physical testing could typically take between one and six months
for construction, testing and reporting.
Sump Volume
Pump sumps should have a minimum sump volume calculated to
ensure that in the worst flow conditions any pump installed does
not exceed the maximum allowable starts per hour. The CIRIA guide
The hydraulic design of pump sumps and intakes by M.J. Prosserxli
should be referred to when designing sumps or wet wells.
The minimum sump volume is the volume between the start and stop
levels of the duty pump and for a single pump the worst case occurs
when the inflow is exactly half of the pumping rate.
To calculate the minimum sump volume for a specific pump the
formula contained in the above CIRIA guide is:
T = 4V/Qp
Equation 2.13.1
Where:
T is the cycle time for the pump, e.g. if the recommended
maximum starts per hour for a pump is 10, then the cycle time will
be 6 minutes (60/10 = 6)
V is the volume of sump between the start and stop levels in
m3
Qp is the pumping rate in m3/minute
Therefore if Qp is 1.2m3/min (20l/s) and the maximum number of
starts is 10/hour, the volume required will be:
V (m3) = 6(min) x 1.2(m3/min) / 4
V = 1.8m3
For 10 starts per hour this could also be expressed as:
V = 1.5 x Qp
The sump volume when multiple pumps are installed is calculated
as for a single pump, where the minimum sump volume is the capacity
between the start and stop level for each pump. However, additional
capacity is required to allow a vertical distance of 150mm between
the start or stop levels of consecutive pumps.
With sewage there is a possibility of septicity, therefore there
are restraints on the maximum volume of the sump related to the
retention time of the liquid in that sump.
Maximum and minimum start / stop levels
The minimum stop level should be the level at which the pump can
be stopped and restarted without losing suction or as specified by
the pump manufacturer.
To avoid turbulence and odour release at foul sewage pumping
stations, the lowest pump stop
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level is usually set at the invert of the incoming sewer, the
last section of which is laid to a steep fall to avoid the sewer
being used as the sump.
The minimum start level should be the required distance above
the stop level to provide the minimum sump volume.
Allowable pump starts per hour
The maximum allowable starts per hour should be as specified by
the pump or motor manufacturer. In the absence of any specified
figure the following are suitable guidance figures:
Less than 100kW - 15 starts/hour
100kW < 200kw - 10 starts/hour
>200kW - 8 starts/hour
Stop / start levels for single and multiple pump operation
The start and stop levels for single pump operation should be
set within the maximum and minimum start / stop levels defined in
the previous section, provided that the minimum sump volume is
attainable.
The start level for each additional pump should be set a
suitable height above the previous pump to prevent accidental pump
starts caused by surface waves or level sensor errors.
The stop level for each additional pump should be set at the
required distance below the start level to provide the minimum sump
volume for that particular pump. The stop level will normally be
just above the previous duty pump stop level.
The effect of flywheels should be considered in determining
stop/start levels because the flywheel increases the pump start-up
and stop times.
Pump duty level
The pump duty level for a single pump should be the midpoint
between the pump start and stop levels. For multiple pump
installations it should be the midpoint between the top water level
(last duty pump start level) and the bottom water level (first duty
pump stop level).
Pumps should also operate within their performance curve at both
top and bottom water levels under single or multiple pump
operation.
2.14 Suction/Delivery Pipework, and Valves
Pipework
Only superior materials are acceptable for use in pumping
station pipework. The pipework installation should incorporate the
following features:
Sufficient bends and flange adapters to allow easy dismantling
and removal of pumps, non-return valves or other major items of
equipment;
Each dry well pump should be installed with suction and
discharge isolation valves to permit isolation of the pump from the
wet sump and discharge pipework for maintenance;
Each submersible pump should be installed with a discharge
isolation valve to permit isolation of the pump from the discharge
pipework for maintenance;
Each pump should also be fitted with a non-return valve to
prevent reverse flow back through the pump when stopped;
Valves should be positioned to permit the removal of each pump
and non return valve without draining either the wet well or
discharge manifold, and allow the other pumps to continue operating
normally;
Suction isolating valves for dry well pumps should be bolted
directly to a flanged pipe securely fixed through the sump
wall;
Discharge isolation valves should be bolted directly to a flange
on the discharge pipe or manifold;
Discharge non-return valves should be bolted directly to the
discharge isolation valve. They should be installed in horizontal
pipework with a short length of pipe and a flange adapter on the
pump side to allow dismantling;
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Where the pump delivery pipework joins the pumping station
discharge manifold, the entry should be horizontal;
At the opposite end of the pumping station discharge manifold, a
valved connection back to the sump should be provided for draining
the discharge pipework, or flushing the sump;
Consideration should be given to providing an isolating valve on
the pumping main before it leaves the pumping station/chamber and
before any over pumping connection, to allow the pumping station to
be fully isolated and the fixed pipework drained for repair;
All flexible couplings should be restrained on both sides by
securely fixed equipment, thrust blocks or tie straps across the
coupling to prevent displacement of the coupling under
pressure.
Valves
Valves should incorporate the following features:
Isolation valves for sewage should be of the double-flanged
wedge-gate type with a bolt-on bonnet. When fully open, the gate
should be withdrawn completely from the flow. The valve should have
an outside screw rising stem and the handwheel direction of
operation should be clockwise to close. Station valves should have
metal seats;
Valves greater than 350mm diameter should be fitted with
actuators. Where installed in chambers they could be fitted with
non-rising stems to limit the headroom required;
Reflux valves for sewage should be of the double flanged, quick
action single door type, designed to minimise slam on closure by
means of heavy doors, weighted as necessary. The door hinge
pin/shaft should extend through the side of the body and be fitted
with an external lever to permit back flushing;
Reflux valves should be provided with covers for cleaning and
maintenance without the need to remove the valve from the pipeline.
The covers should be large enough so that the flap can be removed
and the valve can be cleaned;
The non-return valves should have proximity switches to prevent
dry running and allow a change of duty (standby on high level will
then start);
All reflux valves should be installed in the horizontal
plane;
Butterfly valves should not be used with sewage.
2.15 Pumping System Characteristics
NPSH, Vibration, Cavitation and Noise
Net Positive Suction Head (NPSH) is used to check the pumping
installation for the risk of cavitation.
Cavitation is the formation and collapse of vapour bubbles in a
liquid. Vapour bubbles are formed when the static pressure at a
point within a liquid falls below the pressure at which the liquid
will vaporise. When the bubbles are subjected to a higher pressure
they collapse causing local shock waves, if this happens near a
surface, erosion can occur.
Cavitation will typically occur in the impeller of a centrifugal
pump, where it can cause noise and vibration as well as affecting
the pump efficiency. If allowed to persist it can lead to damage to
the pump or even breaking away of foundations.
NPSH is the minimum total pressure head required in a pump at a
particular flow/head duty. It is normally shown as a curve on the
pump performance sheet.
NPSH = Pa Vp + Hs Fs
Equation 2.15.1
Where:
Pa = atmospheric pressure at liquid free surface
Vp = vapour pressure of liquid
Hs = height of supply liquid free surface, above eye of pump
impeller
Fs = suction entry and friction losses
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In order to avoid cavitation, the NPSH available should be at
least 1m greater than the NPSH required by the selected pump at all
operating conditions.
When calculating NPSH, absolute values for atmospheric and
liquid vapour pressures are used.
Pump Duty Point
Each pump has a performance curve where the flow is plotted
against head.
Each pipework system has a friction curve where the friction
head is plotted against flow.
The system curve is obtained by adding the static head to the
friction losses and plotting the total head against the flow.
The pump duty point is where the pump performance curve and the
system curve cross. It shows the flow that a particular pump will
deliver through the pipework system at a particular total head at
the pump duty level.
In multiple pump installations, it is essential that the
operating conditions of a single pump running are carefully checked
to ensure that the pump will operate at maximum and minimum static
heads satisfactorily, and without risk of cavitation.
The duty point should be used when considering the suitability
of alternative pumps for a particular duty by comparing the
efficiency and power requirements for each pump at the duty
point.
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Figure 2.15.1 Characteristic Curve for Multiple Pumps
Characteristic curve for new pipe