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

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pipeline. During the time delay of around 0.06 s before the vessels are

able to respond, the ﬁrst component of wave has been travelling

along the rising main shown in Fig. 13.4. The resultant effect is a

short-duration pressure pulse which travels back and forth in the

rising main on top of the main surge controlled by the pressure ves-

sels. Should this short-term pressure ﬂuctuation coincide with the

peak or trough of the primary wave, the effect will be to increase or

decrease the maximum or minimum transient pressure based upon

the main wave only. The predicted and observed head variation at

Riding Mill pumping station following simultaneous trip of six duty

pumps, can be seen in Fig. 13.5. Irregularities in head variation are

present on both traces.

Within the Riding Mill pumping main, this effect was responsible for

operation of an air valve at Penn’s Hill (Fig. 13.6), although a

preliminary analysis which neglected the effect of vessel connection

length had suggested that no such valve operation would occur. Both

the observed and simulated head variations at Penn’s Hill show clearly

the presence of higher-frequency components of wave motion. These

waves travel together with the primary wave which is dominated by

the presence of pressure vessels. Where possible, the pressure vessels

should be placed as close as possible to the pumping main to avoid

unnecessary delay in vessel response and to minimise the development

of these secondary wave components.

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250

200

150

100

Elevation (mAOD)

River Tyne and

Riding Mill PS

Air vessels

Chaina

e (m)

Penn’s Hill

Typical arrangement of

anti-vacuum valve

Anti-vacuum valve

Triple air valve cluster

Letch House tank

Elevation = 223 mAOD

0 930 1400 1760 2160 2455 3050 3815 4230 4865 6207

16.5 m

Varies

Fig. 13.4. Rising main pipeline proﬁle

Pressure transients in water engineering

A good-sized connection to the vessel is also desirable to avoid

excessive head loss during outﬂow thus ensuring the most effective

response from the air chamber. It may be desirable to install a ‘swept’

tee to minimise head loss as ﬂow enters the rising main.

For maximum effect a pressure vessel should be placed as low as

possible. Where static head is low then the difference in level from

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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 15

Elevation (mAOD)

Time (s)

Observed

Predicted

6 pump trip record at Riding Mill PS

n = 1.4

a = 900 m/s

K = 85

320

300

280

260

240

220

200

180

160

140

120

100

Fig. 13.5. Head variations at Riding Mill — 6 pump trip

Air valve/anti-vacuum valve operation

0 10 20 30 40 50 60 70 80 90 100 110 120

Recording failure

Predicted

Observed

?60

?40

?20

?00

?80

?60

Elevation (mAOD)

6 pump trip record at Penn’s Hill

Time (s)

n = 1.4

a = 900 m/s

K = 85

Pipeline elevation

Air valve elevation

Fig. 13.6. Head variations at Penn’s Hill — 6 pump trip

Further aspe cts of pressure vessels

the pipeline to the centreline of the vessel may be quite signiﬁcant. The

smaller the difference in level from the vessel centreline to the down-

stream discharge level the larger will be the required vessel volume.

In systems of low static it is important to employ a realistic assessment

of vessel elevations if a possible underestimation of vessel volume is to

be avoided.

It has been said that the function of the pressure vessel is to limit

changing head or pressure along the rising main. This leaves the rela-

tively short stretch of pipeline between the suction well upstream of

the pump and the connection with the pressure vessel to be considered.

The function of the pressure vessel is to sustain pressure at the vessel

connection and along the rising main. However, on the downstream

side of a failing pump, pressure can fall relatively quickly and a steep

adverse hydraulic gradient can be established within this relatively

short section of pipeline between the pump and the vessel connection

where pressure is being maintained at a relatively high level (Fig. 12.4).

Compared with the situation without a pressure vessel, jdV=dtj

downstream of the vessel is reduced while jdV=dtj upstream of the

vessel is increased.

Flow will tend to reverse more quickly upstream of the vessel connec-

tion than if the vessel were not included. This has important implica-

tions with respect to closure of the check valve downstream of the

pump and will be considered in greater detail when discussing check

valves in Chapters 20 and 21. It may be noted that siting of the

vessel close to the pumps will tend to aggravate check valve closure

conditions and if practical the vessel should be moved further down-

stream rather then being say teed off a manifold directly opposite the

pump branch connections (Fig. 13.7).

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Discharge manifold

Suction well

Pumps

Check valves

To rising main

Pressure vessel

Less desirable vessel locations

Preferred vessel location

Fig. 13.7. Pressure vessel locations

Pressure transients in water engineering

The pressure vessel does not suppress the transient at its source as

does the ﬂywheel, but rather the transient is developed at the pump

and the vessel acts to suppress its effects to an acceptable extent as

the pressure wave enters a downstream pipeline for instance. As

described, the positioning of the vessel and conﬁguration of its con-

nection can inﬂuence the effectiveness of response.

13.5 Installation with air valves

Figure 13.8 shows an hybrid form of vessel incorporating some of

the attributes of a conventional installation but with an air valve

arrangement as a means of replenishing the air charge. In a typical

example the chamber is ﬁtted with a ‘dip-tube’ which extends down-

wards from the air valve connection on top of the vessel, to about

the mid-height of the tank. The lower end of the tube is open. Figure

13.9 illustrates the normal sequence of operation of this protection

device when installed downstream of a pumping station.

(a) At static, the air valve is closed and the tube is ﬁlled with liquid. A

charge of air is enclosed in the upper part of the vessel above the

lower end of the tube and under the static pressure head.

(b) While pumping, the air valve remains closed with the tube liquid-

ﬁlled as before. The air charge is compressed under the pumping

head in the top of the vessel.

to expand, releasing liquid into the rising main thus suppressing the

transient head drop in the main. Thus this vessel acts in the same

manner as a conventional pressure vessel at this stage. The dip-tube

still remains liquid-ﬁlled at this time.

(d) After a time when head has fallen to the air valve operating level,

the valve will open to allow air to ﬂow into the tube and ﬁnally into

the vessel itself. This air ﬂow augments the expanded air volume

already within the vessel. Operation of the air valve largely stabi-

lises head at the vessel and the continued ﬂow of liquid out of

the chamber is essentially balanced by air entering the tank via

the air valve.

(e) After ﬂow reversal has occurred, head starts to rise at the vessel

with air ﬂowing out of the air valve and liquid entering the

vessel. When liquid level reaches the lower end of the dip-tube

the air mass contained in the vessel outside the tube is isolated

while the air in the tube exits via the air valve. The air valve

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Further aspe cts of pressure vessels

shuts when liquid level reaches the top of the tube and the air mass

remaining in the vessel is compressed by the return ﬂow in the

vessel.

As the hydraulic transient decays, conditions are gradually restored

to static as in item (a) above; the air charge has been replenished as

necessary by operation of the air valve. For this type of vessel to

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pipeline

Flanged outlet

Interchangeable pipeline

connection DN 100 to DN 400

Central tube

Protective cap

Pressure gauge

Air regulating valve

Fig. 13.8. Vessel with air valve and central tube

Pressure transients in water engineering

function successfully, the transient piezometric level must fall to allow

air valve operation. The pipeline proﬁle must be suitable to allow this to

happen. A vessel of this form provides protection from the moment of

pumping failure and relies on the air valve at a later stage both to

augment the air mass and to top-up the air charge contained in the

chamber. In principle a vessel of this type could be installed at any

appropriate location along a pipeline where hydraulic conditions are

suitable, as it does not rely on a power supply. Some throttling of

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Steady pumping head

Static head

Air valve (AV) closed

Air charge in vessel

AV closed

Rising main

(a) (b)

(c)

Flow from pump

Head shortly after trip

AV closed

Outflow from vessel

Head rising during reversed flo

Air outflow at AVAir inflow at AV

Head approaching minimum

Inflow to vesselOutflow from vessel

(e)(d)

Fig. 13.9. Sequence of operation of a vessel with air valve

Further aspe cts of pressure vessels

inﬂow to the vessel might also be considered, as in a conventional vessel

connection. Capacity of the vessel should be sufﬁcient to contain the

maximum air volume during the sequence of operation.

The example shown in Fig. 13.8 can occur in different forms as

illustrated in Fig. 13.10. Twin pumping mains are each ﬁtted with two

horizontally aligned vessels. The upper vessel has three sewage air

valves installed along the top of the horizontal vessel. The lower vessel

is connected to the upper vessel at the base of the riser, extending upwards

to the higher of the vessels (Fig. 13.11). During pumping, the hydraulic

gradient ensures that air is purged from the upper vessel with the air

valves closing on completion of air venting. The air charge in the lower

vessel is compressed but with its air mass remaining in the chamber.

228

Centrifugal pumping station

General layout

Elevation through surge suppression tanks

Lower tanks replenished

with air during downsurge

after pump trip

To r i s i n

mains

Vessel connection lines

Concrete tank supports

Air valves Air valves

Upper tanks fitted

with air valves

Mild steel air valve

support frame

Twin GRP rising mains

Surge suppression tanks

Fig. 13.10. Alternative vessel arrangement using air valves

Pressure transients in water engineering

In the event of pump shutdown, pressure falls and the air mass in the

lower chamber expands allowing liquid to ﬂow from the vessel thus

providing a measure of surge protection. Only when hydraulic gradient

has fallen to the operating level of the air valves will these valves open

to allow air inﬂow which augments surge protection.

229

Fig. 13.11. Vessels with air valves

Further aspe cts of pressure vessels

Surge tanks and related

structures

These devices are chambers usually open to the atmosphere, although

pressurised versions have been known. Analysis of a pressurised surge

tank follows the same process as that for a pressure vessel. The simplest

type of surge tank will be a vertical cylinder either above ground

or below ground as at Strathblane on the Loch Lomond Scheme

(Fig. 14.1). There will also be a connection to the pipeline or tunnel

system. Often surge tanks will be of considerable capacity, particularly

where these are used to suppress transients occurring in hydroelectric

power systems. Small surge chambers are also found in water supply

systems where there is a need to control pressure ﬂuctuations.

14.1 Purpose of a surge tank

As with all forms of surge alleviation, the surge chamber is intended to

reduce the rates of change of velocity which would otherwise occur if

the chamber were not present. The presence of a free water surface has

the effect of producing a strong reﬂection of a pressure wave much

like the reﬂection from a reservoir. By providing a source of water and a

means of storing water, the tank is a pressure-relieving mechanism. In

the case of ﬂow deceleration in a pipeline downstream of the surge tank

connection, water from an upstream pipeline will be diverted into the

chamber thus avoiding the need for the same rate of ﬂow deceleration

in the upstream line. Likewise, when ﬂow is accelerating in the down-

stream line the chamber supplies water to the downstream main avoiding

the requirement for the upstream line to supply ﬂow to the downstream

system at the same rate. By reducing the rates of acceleration and

deceleration in the upstream conduit, pressure surging upstream of

the surge tank connection can be correspondingly lessened.

230

There are many instances where a tank or chamber is included,

whose function is not that of a surge tank but whose response to and

effect upon hydraulic transients can be analysed using the same tech-

niques as for a surge chamber.

The suitability of a site for this type of installation depends upon the

level of the maximum piezometric line relative to the ground surface —

that is, maximum water level in the chamber is coincident with piezo-

metric level.

Analysis often has to take account of complexities of shape, especially

where the chamber is below ground, often including galleries or a sur-

face pond in place of an upper gallery as at Clachan HEP station

(Fig. 14.2). Galleries, both upper and lower, effectively concentrate

the chamber volume at these levels where it will be most effective

rather in the manner of an RSJ section where area is concentrated at

a distance from the axis. By having a more modest shaft section, the

water level will rise and fall in the shaft more rapidly than if the area

of chamber was constant with changing water level. This means that

an effective hydraulic gradient necessary for acceleration and decelera-

tion of ﬂow is attained more quickly with a smaller shaft cross-section.

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Pumping flow

Reversed flow in main while

pressure vessels and

feeder tanks refillin

Static level in shaft = 68.58 mAOD

Tunnel water level maintained

by throttle valve

downstream = 67.67 mAOD

Pumping level

1524 j steel pumping main

2134 j tunnel

Surge shaft at north portal

of Balmore tunnel

TWL = 76.81 mAOD

(pumps at Ross Priory tripped)

Cap

Ground level

Fig. 14.1. Simple surge shaft between pumping main and gravity ﬂow tunnel

Surge tanks and related structures