Ellis,J. Pressure transients in water engineering, A guide to analysis and interpretation of behaviour

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Control shaft

C.l. of spiral casing

Spiral

extension pipe

Dashpot A

Cam C

Connecting pipe

Piston

(closing

member)

Outer

cylinder

Outer cylinder

Dismantling ring

Bridge piece

Control valve B

Inner

body

Fig. 19.15. Cylindrically balanced relief valve

Pressure transients in water engineering

streamlined passage which allows ﬂow to be deﬂected outwards into a

3 m long ‘hood’ of diameter 1200 mm. Water travels along the hood

in the form of a hollow jet which becomes highly aerated as it exits

from the hood. The valve and hood are angled downwards towards

the tailwater channel at an angle of 98. Energy of the ﬂow is dissipated

by friction between the ﬂowing water and the air and by turbulence

when the ﬂow from the valve enters the tailwater. This downstream

channel had a length of 43 m.

In one application the cylindrically balanced valves were used to

discharge ﬂow passing through a hydropower plant. Operating head

at the plant typically varied in the range 263—274 m depending upon

the number of turbines running. If a machine is shut down its inlet

gates are shut in around 2 s to avoid overspeed. The relief valve for

that turbine is opened in around 3 s to discharge the ﬂow which was

previously passing through the machine. Subsequently, the valve is

closed in a time interval of 70—80 s. At full load, a turbine may consume

almost 20 t of water/s and this ﬂow is diverted through the relief valve

when the turbine is shut down. Exit velocity was 61 m/s.

In common with many hydropower installations around the world,

the turbines were uprated with a consequent increase in the amount

of water passing through the station and during trip, through the

relief valves. Figure 19.16 shows the aerated jet exiting from the

hood and impacting with the tailwater for the uprated situation. At

low tailwater level, part of the aerated ﬂow could be deﬂected upwards

373

Fig. 19.16. Aerated jet leaving relief valve

Relief valves

to pass over the parapet of a road bridge (Fig. 19.17). This situation was

not considered acceptable, as one day a tourist bus stopped on the

bridge with its roof vents open. Two turbines tripped and the resulting

relief valves’ operation soaked the passengers, with water ﬂooding out of

the bus when the door was opened. A longer tailwater channel would

have solved the problem but this was not a practical proposition. The

1200 mm diameter outlet hood was modiﬁed to include a truncated

cone containing oriﬁces. This caused outﬂow to break into a large

number of jets and increased aeration and energy dissipation. While

not completely eliminating the problem of water spilling over the

parapet, some improvement has been achieved.

Other options were tried in an effort to reduce severity of the prob-

lem. One attempt adjusted the relief valve movement so that it could

only open to possibly 70%. A test using a partially opened valve setting,

and without any preliminary analysis of likely effects, produced results

described by the operators as ‘horrifying’. The entire power station

vibrated and shuddered, with penstock pressures reaching 500 psig,

which is above design maximum pipeline pressure. The relief valve

was designed to pass ﬂow without large energy loss within the stream-

lined passages. Part-open operation creates conditions for substantial

head loss within the valve itself. Given the overall amount of energy

involved, around 40 MW/machine, any attempt to dissipate ﬂow

energy within the valve through turbulence will inevitably result in

substantial vibration and must be avoided.

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Fig. 19.17. Spillage over bridge parapet

Pressure transients in water engineering

This example has illustrated that the solution does not just involve

choosing a relief valve but also involves conditions downstream of

the valve to ensure that adequate provision is made for adequate dissi-

pation of energy associated with the outﬂow and safe disposal of

discharging water. In the present instance, a greater length of tailwater

channel would have avoided the problems experienced.

19.10 Bursting disk

The bursting disk may also be viewed as a form of relief valve (Fig. 19.18).

Should internal pressure exceed the set pressure for operation then the

disk will ‘snap through’ and fail around its notched perimeter, allowing a

free discharge of liquid and thus alleviating rising internal pipeline

pressure. Clearly no means of shutting off the escaping ﬂow is provided

by the failed disk and the system has to be shut down by other means.

This form of protection is relatively uncommon in water and sewage

applications.

375

Circular notch

C+ C–

Busting disk

Initial position

Pipeline

u/s

d/s

Dx Dx

Snap-through position

Fig. 19.18. Bursting disk

Relief valves

Check valve dynamics

Check valves have long been a potential source of unacceptable

pressure transients. Within pumping stations the loud bang associated

with check valve closure has been called ‘slam’. This is indicative of a

valve door or doors not closing sufﬁciently quickly and allowing ﬂow

to reverse through the valve. Subsequent elimination of this reversed

ﬂow has caused severe pressure transient effects in many installations.

Proper choice of valve can avoid development of these effects. Avoid-

ance in the ﬁrst instance is much better than attempting to suppress the

effects once they have occurred.

Check valves, also called non-return valves (NRVs) or reﬂux valves,

are used to prevent reversal of ﬂow. In practice with most valve types,

some ﬂow will usually pass through the valve before it is able to close

completely. It is important to understand how a speciﬁc check valve

will response to the reﬂux conditions within a given pipeline system.

Two facets present themselves with regard to the task of establishing

how a check valve will respond. First there is the question of what

rates of ﬂow change, that is accelerations or decelerations, will occur

in a given network and this aspect is discussed in the current chapter.

The second question relates to how different types and sizes of check

valve will perform within a particular system. This second aspect is

considered in Chapter 21.

20.1 Check valve response

Check valve behaviour has received a great deal of attention over the

years, not only with respect to the valves themselves but also their effect

upon pipeline systems. The relative frequency of starting and stopping of

pumps in sewage transmission systems and the resultant accumulation of

376

pressure oscillations over time has inﬂuenced recommendations regarding

allowable amplitudes of pressure transient oscillations for some types of

plastic pipes. A pumping station with a solo duty pump and with a

dischargebranchsizecomparabletothediameterofrisingmainwillbe

more likely to produce higher amplitudes of surging along the main than

a system in which several pumps are operating and which have different

stop levels.

The ability of a check valve door to respond in a way which matches

the changes taking place in the pipeline system is of the utmost impor-

tance. The system will produce a rate of velocity change dV=dt which is

a function of the characteristics of the pipeline network and predicting

this rate of change is a primary objective of transient investigation and

the concern of this chapter.

20.2 Pumping station check valves

One of the most common applications of check valves is within

pumping stations (Fig. 20.1). A check valve is commonly sited down-

stream of each pump within its discharge branch. On occasion, a

larger check valve may also be found at the start of a rising main as

shown in the ﬁgure. This valve will provide protection for the pumping

station in the event of a pipe burst within the station. Check valves are

also found in vessel connections where they form a part of throttle

arrangements.

Within a pump discharge branch, as a check valve closes, say following

a pump failure, ﬂow will decelerate at the valve at a rate which is depen-

dent upon a number of factors including, pumping head, ﬂow, pump

characteristics, speed and moment of inertia. The number of pumps in

operation and the presence or otherwise of surge protection equipment

at or near to the pumping station are also important factors.

An ideal check valve would close at the instant of ﬂow reversal but

this perfect performance is not attainable in practice. There will always

be some reverse ﬂow in the system at the time when the valve ﬁnally

shuts. Within the same installation, alternative patterns of check

valve will respond in different ways, with the time of valve closure

being a function of the speciﬁc valve characteristics.

20.3 Consequences of an unsuitable check valve installation

When a check valve is not suitable for the installation in which it is

placed, serious consequences can result. A graphic illustration of the

377

Check valve dynamics

effect resulting from inappropriate check valve selection is shown in

Fig. 20.2 which depicts the transient pressure variations measured

just downstream of the conventional swing check valve of a booster

pump serving a 30-storey apartment block (Fig. 20.3). The local

water undertaking delivered potable water to a storage facility at the

base of the building with a booster pump providing the necessary

pressure to convey water through a DN 100 wet riser to a storage

tank at the top of the structure, a height of 70 m. The pressure

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Discharge header

Rising main

Isolating valve

Suction

branch

Pump

Suction header

Check valve

Delivery branch

Pressure vessel

Fig. 20.1. Check valves at a pumping station

Pressure transients in water engineering

379

350

300

250

200

150

100

–50

Pressure head (mWG)

Recorded pressure transient in

high-rise building following pump trip

Peak pressure

head = 350 mWG

Record from just downstream

of swing check valve

Steady running head

= 78.83 mWG

Static head

= 75 mWG

Pump trip

Time (s)

High-rise building

0 1 2 3 4 5

Fig. 20.2. Recorded pressure variations downstream of a check valve

Fig. 20.3. High-rise building

Check valve dynamics

record shows steady pumping conditions before pump trip and also the

transient pressure head following pumping failure. Total period of the

record was around 5 s. Static pressure head at the check valve was

70 mWG. Steady running pressure head while pumping rose to

78.83 mWG. Subsequent to pump trip, pressure fell to a minimum of

46.61 mWG as ﬂow reversed. Head then started to increase slightly

prior to check valve closure. After closure, transient pressure rose

abruptly to a head of 350 mWG. Elastic oscillations produced further

severe pressure peaks and after 5 s the maximum head still remained

above the height of the building. The high transient pressures caused

burst pipe connections, vibrations and noise, with the result that

occupants had to be evacuated. This is an instance of an unsuitable

check valve selection resulting in delayed valve closure and allowing

development of substantial reversed ﬂow in the system. The remedy

was to install a valve possessing fast-closing attributes or alternatively

to use a valve with a damped closure.

20.4 Prediction of pumping station hydraulic transients

Pressure transient analysis of a complete system seeks to determine the

extreme head and ﬂow changes which occur in the network as whole.

This will involve predicting the effects of simultaneous trip of the

maximum number of operating pumps, as well as other pump combina-

tions, in an endeavour to ﬁnd maximum rates of velocity change and

peak head variations. Such an analysis would be necessary in the case

of solo pumping for the high-rise building discussed in section 20.3.

In contrast, prediction of reﬂux within many pumping stations requires

studying trip of one pump while the maximum number of remaining

duty pumps continues to operate. This will usually yield the maximum

rate of ﬂow deceleration within the suction and delivery branches of the

failing pump.

Determination of the suitability of a valve for a particular application

requires consideration of two aspects.

First, prediction of the rate of deceleration and time of ﬂow reversal

at a check valve is a matter for hydraulic transient analysis using the

techniques described in earlier chapters. This requires a model to be

set up of the pumping station and possibly part or all of the pipeline

system. It may not be necessary to model an entire network if interest

is solely in the response of the check valve. A ‘local’ model may allow

prediction of check valve closure in many instances where the closure

occurs within a relatively short time and before the entire pipeline

380

Pressure transients in water engineering

has had a chance to respond. Installations with a pressure vessel or

where several pumps are in operation are cases in which such ‘local’

models may be appropriate.

Second, the response of a particular size and pattern of check

valve, to the predicted rates of change of velocity and time of ﬂow

reversal, requires consideration of the valve characteristics. These

characteristics can include such aspects as weight and inertia of

moving parts, travel distances for valve doors, hydrodynamic force

and/or torque acting on the valve door(s), bearing friction, and external

factors such as spring stiffness or dimensions and weights of external

levers. Some consideration will be given to speciﬁc valve types in

Chapter 21.

Some features will tend to produce more severe reﬂux conditions

than others and Fig. 20.1 displays a pumping station arrangement

which will tend to create a high rate of deceleration along the branch

containing a failing pump. The presence of several operating pumps

and the pressure vessel will all tend to maintain higher pressures in

the discharge manifold than otherwise. Figure 20.4 shows the hydraulic

gradient along the failing pump branch during steady pumping and also

a short time after the pump has tripped. Where there are a number of

operating pumps, the failure of one unit will cause a shift in the

operating point of the remaining pumps. The ﬂow will not fall so

dramatically as if only one pump were in operation.

The presence of a local pressure vessel will provide a further source of

liquid in the event that a pump fails. As pressure tends to fall in the

manifold, the gas charge in the vessel expands causing outﬂow from

the vessel into the manifold, thus helping to sustain piezometric level

and promoting a steeper adverse hydraulic gradient along the suction

and discharge branches of the failing pump.

With piezometric level declining only slowly in the manifold, a

steep deceleration gradient is created along the branch of the failing

pump as illustrated in Fig. 20.4. As the failing pump decreases in

speed, its ability to maintain ﬂow also decreases. Within a short

time, which can be as little as 0.2 s, the ﬂow has reversed at the

check valve. Ideally the door of the valve should be close to its seat

at the time of ﬂow reversal so that ﬁnal closure occurs with only a

modest reversed velocity having been established during the ﬁnal

stages of closure. Steepness of the deceleration gradient is also a func-

tion of the pumping head, with a higher head tending to produce a

more rapid ﬂow deceleration for a given pumping station pipework

conﬁguration.

381

Check valve dynamics