3 - Gauge Glasses Safety Valves Blow Down

The following is a list of mountings found on a typical boiler.

Safety valve 2/3

Water gauge glass 2

Feed water check valves 2 sets

Stop valves

Vent cock

Blow down valve/scum valve 1 each

Water sampling connection. Pressure gauge connection.

Boiler MountingsBoiler Mountings

Feed Check ConfigurationFeed Check Configuration

Blow Down Valve ConfigurationBlow Down Valve ConfigurationBlow Down Valve Configuration

Boiler shell

Blow Down Discharge Screw lift

valve

Screw down non return valve

Blow downBlow down

To avoid damage to the valves and the blow down line it is important to open the valves in the sequence 1, 2 & 3.

Gauge GlassesGauge Glasses

All boilers must be fitted with at least two independent means of reading the level of water within the boiler shell or drum.

One of the two devices must be a glass water gauge attached, by means of isolating cocks, to the boiler drum. The other device can be an additional glass water gauge or an approved equivalent device.

The water gauges are to be readily accessible and placed so that the water level is clearly visible.

The lowest visible part of the glass is to be aligned to the point of the lowest safe working water level.

RulesRules

There are two basic glass water gauges tube and plate.

The glass tube type is made from toughened borosilicateround glass tube and is capable of withstanding pressuresup to about 34 bar.

The plate glass type is made from toughened soda-limeglass flat plate and is capable of withstanding pressures upto about 79 bar.

At the high temperatures experienced at these pressures, theboiler water has a corrosive effect on the glass, which hasto be protected from direct contact with the water.

Basic DesignBasic Design

Gauge glasses should always be fitted with isolating cocksrather than valves, the reason being:-

A straight-through passage can be obtained.

Valve lids sometimes become detached from their spindles.

Sediment is more likely to accumulate in a valve chest.

It is easier to see if a cock is open or shut.

Basic DesignBasic Design

Tube Type Gauge GlassTube Type

Gauge GlassNote position of cocks, whichmust always be installed withhandles hanging vertically whensteam & water cocks are openand drain shut.

Position of cock (open/shut) can be easily seen.

Cocks can not vibrate to non-working position.

Gauge Glass ProtectionGauge Glass Protection

High Pressure Plate Gauge

Glass

High Pressure Plate Gauge

Glass

Glass

Joint Mica

Clamping plate

Glass plates protected from the corrosive effect of the boiler water by thin translucent mica sheets.

Lighting box positioned behind the gauge.

Blowing Down a Gauge GlassBlowing Down a Gauge Glass

1 2 3 4 5

Boiler Level Sensor

Boiler Level Sensor High level alarms

Low level alarms

Magnet

Simple float device used to activate magnetic reed switches to indicate high and low level and open/shut feed control valve.

Boiler Level Sensor

Boiler Level Sensor

Weir

Condensingchamber

Fixedleg

Variableleg

D.P.cell

Boiler drum

Isolating &balancevalves

Boiler Level ControlBoiler Level Control

P + IController

Desiredvalue

Diaphragmcontrol valve

I/Pconverter

Leveldetector

Feed water

Single element boiler water level control system

An increase in firing rate will cause a rapid increase of steam bubbles which will displace water and cause the level within the drum to suddenly rise. The controller will thus shut the feed valve at a time when the water supply needs to be increased.The reverse will happen when the steam demand decreases, the water level will suddenly drop and the controller will, therefore, increase the feed water supply.

Swell & ShrinkageSwell & Shrinkage

SecondaryControllerP + I +D

Desiredvalue

Diaphragmcontrol valve

I/Pconverter

Leveldetector

Feed water

Two element boiler water level control system

D/P cell

D/P cell

PrimaryControllerP + I +D

Two Element ControlTwo Element Control

A two element controller receives signals both from the level sensor and also from a steam flow sensor.

As the steam demand increases, the flow sensing secondary controller will over-ride the level sensing primary controller and cause the feed control valve to open.

Two Element ControlTwo Element Control

Three Element ControlThree Element Control

Desired value

Diaphragm control valve

Level detector

Feed water

D/P cell

D/P cell

Primary ControllerP + I +D

D/P cell

RatioRelay

Steam flow detector

Feed flow detector

Secondary ControllerP + I +D

A three element controller receives signals both from the level sensor, a steam flow sensor and also a feed flow sensor.

Under steady steaming conditions, the feed will be supplied to the boiler in direct ratio to the steam flow. As the steam flow increases the feed flow will immediately respond to match.

The level control signal will over-ride the ratio element if the level changes.

Three Element ControlThree Element Control

Safety ValvesSafety Valves

Two safety valves have to be fitted to the main steam drum or shell of the boiler.

Both valves may be fitted to the same chest and mounting but it must be separate from any chest or mounting used for other purposes.

For boilers fitted with superheaters a third safety valve must be fitted to the main steam outlet from the superheater.

Safety valve seat bores must never be less than 38mm in diameter

RulesRules

Standard spring loaded safety valves designs are describedaccording to the amount that the valve will lift against thespring pressure:-

Ordinary spring loaded. High lift spring loaded. Improved high lift spring loaded. Full lift spring loaded.

Full bore relay operated valves are of a different design.

Safety Valve TypesSafety Valve Types

Blow-down: The difference between the set pressure and the pressure at which the valve closes after releasing the excess pressure (Usually about 10%)Easing gear: A means of manually opening the valve to the maximum effective lift.

Safety Valve TerminologySafety Valve Terminology

Flat seats: Greater steam escape edge, easier machining.

Spring protection: Regulations require the loading spring of any safety valve to be protected from steam escaping from the valve.

Valve chest: The bore of the valve chest connecting valve to boiler must be at least equal to the area through the seating of the valve in mm. multiplied by 1.5.

Basic Safety Valve DesignBasic Safety Valve Design

Waste steam pipe: The waste steam pipe and the steam passage from the valves must have a cross-sectional area of at least the area through the seating of the valve in mm. multiplied by 1.1 for ordinary, high lift and improved high lift valves and 2 for full lift valves

Drain pipe: A drain pipe must be fitted to the lowest part of the valve chest on the discharge side of the valve. There must be no isolating valve or cock fitted to the line and there must be a straight fall from the valve to a safe venting position.

Basic Safety Valve DesignBasic Safety Valve Design

An ordinary relief valve (e.g. an air bottle) is restricted in the amount that it can open by the increasing force of the spring as it is compressed.

A boiler safety valve has to lift further and stay open, in order to cope with the greater thermal energy present in the steam.

Safety Valve TheorySafety Valve Theory

Spring force

Pressure

The area of the bore of the valve governs the theoretical maximum amount of steam that can pass through a valve of any design.

Safety Valve TheorySafety Valve TheoryBore area of valve

The bore area A can be calculated from the diameter of the valve seat i.e.

A = piD24

Safety Valve TheorySafety Valve TheoryBore area of valve

As the valve lifts it forms a cylindrical area between the valve and the seat i.e : -

pi x D x lift of valve L

Safety Valve TheorySafety Valve Theory

Lift area

If the lift area and the bore areas of the valve are equated then:-

pi x D x L = pi D24

this can then be resolved to:-

L = pi D2 = Dpi x D x 4 4

If the valve lifts by the bore diameter of the valve seat divided by 4 then maximum steam can escape through the valve.

Safety Valve TheorySafety Valve Theory

Bore area of valve

Lift area

Using the maximum effective lift of D/4 as an ideal, typical lifts for each type of valve are: -

Ordinary spring loaded. D/24High lift spring loaded. D/16Improved high lift spring loaded. D/12Full lift spring loaded. D/5Full bore relay operated. D/4

Safety Valve TheorySafety Valve Theory

Safety Valve TheorySafety Valve Theory

CPHEA =

Safety valve bore must never be less that 38mm

Lift area for specific valve application is derived from:-

Where:-

A = Area through the seating of valve in mm

P = Working pressure of valve in MN/m2

H = Total heating surface of boiler in m2

E = Evaporative rate in kg of steam/m2 of heating surface/hour

C = Discharge co-efficient whose value depends on type of safety valve

Safety Valve TheorySafety Valve Theory

Discharge co-efficient C for each type of valve is: -

Ordinary spring loaded. 50High lift spring loaded. 75Improved high lift spring loaded. 100Full lift spring loaded. 200Full bore relay operated. 320

CPHEA =

Safety Valve TheorySafety Valve Theory

For Superheater Safety Valve the area As is calculated as :-

Where:-A = saturated area through seating of valve in mm.Ts = Degree of superheat temperature in oC.

+=

5561 ss

T

Use can be made of the kinetic energy of the velocity of the steam as it starts to escape from the opening valve lid. If the steam is directed underneath the valve lid it will supply force to increase the compression of the spring and give extra lift. This will increase the effective lift to D/24.

Pressure energy

Velocity energy

Force

Increasing Valve LiftIncreasing Valve Lift

Ordinary Safety ValveOrdinary Safety Valve

Lipped valve lid (to increase lift to D/24)

Valve lid fitted with location wingsto hold lid in alignment on the seat

Clearance between valve lidand spindle guide bush D/4 to allow valve to lift fully on easing gear.

Lid a loose fit on spindle

Ordinary Safety Valve FeaturesOrdinary Safety Valve Features

Design problems Wings on valve lid can restrict steam flow and cause

jamming of the valve. Valve can jam in guide bush. Valve lift limited to D/24.

Waste steam will accumulate above the valve and act downwards on the lips of the valve lid. (Can be an advantage - assists blow-down)

This type of valve is now rarely used.

Ordinary Safety ValveOrdinary Safety Valve

High Lift Safety ValveHigh Lift Safety Valve

High Lift Safety Valve FeaturesHigh Lift Safety Valve Features

Design problemsWings on valve lid can restrict steam flow and cause jamming of the valve.Piston and cylinder ring can jam. Note: Due to the loose nature of the piston on the spindle, valve will still lift to D/24 if jamming occurs.

This type of valve has now been superseded by the Improved High Lift Valve

High Lift Safety ValveHigh Lift Safety Valve

Improved High Lift Safety ValveImproved High Lift Safety Valve

Improved High Lift FeaturesImproved High Lift Features

Normal pressureValve at rest

Boiler pressure 3% above normal

Valve starting to lift

Steam acting on valve lid

and piston lifts valve to D/12

Valve OperationValve Operation

Valve chest Cast steel

Valve body Cast steel

Valve seat Stainless steel, Monel metal

Valve lid Stainless steel, Monel metal

Spindle Stainless steel

Guides/bushes Bronze

Spring Tempered steel

Valve MaterialsValve Materials

Hopkinson HyLifSafety Valve

Hopkinson HyLifSafety Valve

Valve lid, or head, nowforms a piston held within an adjustable valve guide

Valve head now fixed to spindle and thrust carried via a shaped recess in the valve lid

Lipped on valve lid lifts lid into valve guide where steampressure acting on piston increases lift to D/5

Blow-down controlled by height of valve guide

Hopkinson HyLif FeaturesHopkinson HyLif Features

Dewrance Consolidated Safety Valve

Dewrance Consolidated Safety Valve

Steam passing through a valve at high temperature can cause distortion. This can cause leakage, leading to scoring and cutting of the seat.

Thermo-disk Valve HeadThermo-disk Valve Head

Reduced section in way of seat

The thermodisk valve head is designed with the contact area reduced in section to allow the valve head to flex to the profile of the seat. The seat acts as a heat sink, removing thermal stress from the thin sectioned valve head.

Some form of adjustable blow-down ring is a common feature in other designs of high capacity safety valve.

Lowering the ring reduces the reaction forces to give a more rapid blow-down.

Raising the ring increases the reaction forces and and slows the blow down.

Consolidated Blow-down RingConsolidated Blow-down Ring

Full Bore Safety ValveFull Bore Safety Valve

It is important that the relative dimensions of the lid and seat are maintained if the seating faces have to be machined. The seating width (B A & D C ) must remain the same, but will widen as material is removed from the seat faces.

Valve Seat MaintenanceValve Seat Maintenance

Material must also be removed from faces H and G to compensate.

Opening E will also diminish and must be restored in order to maintain the blow-down feature of the valve.

Parts for each valve must be kept separate.

The springs and spindles should be hung-up and sounded.

Check springs for distortion and the free length.

Check that drains are clear and that the easing gear is free.

Check chests and vent pipes for corrosion and cracking.

Check faces of the seats and lids/valve heads for damage.

Following re-assembly, the easing gear should remain disconnected until the valve settings have been adjusted.

Valve Overhaul Valve Overhaul

National legislation and classification society rules require anapproved surveyor to be in attendance when adjusting any safety valves fitted to a boiler which is heated by fuel.

Safety valves fitted to boilers or economisers that are ONLY heated by exhaust gases from the main engines or generators, may be adjusted by a fully qualified engineer, holding a first class certificate, sailing in the rank of chief engineer. In this case, the engineer in question MUST inform an approved surveyor, in writing, at the next port of call following the adjustment of the valves.

Valve Setting Rules Valve Setting Rules

With the easing gear removed, gag one of the valves to prevent it lifting.

Screw down the compression screw on the other valve about one turn tighter than its previous setting and raise the boiler to the required lift pressure. (Approved design pressure plus 3%)

Screw back compression screw until valve lifts and then screw down the compression screw carefully, tapping the valve spindle very lightly until the valve returns to its seat and remains closed.

Valve Setting Valve Setting

The distance between the lower land of the compression screw and the top spindle guide should be carefully measured and the compression split ring adjusted to give a neat fit. The compression screw should then be hardened down onto the compression ring.Note: Some safety valve designs have a locking nut fitted instead of the compression ring.

Repeat for the other valve, gagging the first valve accordingly.

Fit both sets of easing gear, hoods, locking keys and padlocks. Finally check and operate easing gear.

Valve Setting Valve Setting

Securing AdjustmentSecuring Adjustment

Classification societies require that safety valves be subjected to an accumulation of pressure test to ensure the valves are of the correct discharge capacity for the boiler.

With boiler isolated and maximum firing rate applied, the accumulation of pressure within the boiler must not exceed 10 per cent of the approved design pressure. The duration of test (water permitting) is not to exceed 15 minutes for smoke tube boilers and 7 minutes for water tube boilers.

Accumulation of Pressure Accumulation of Pressure