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

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As in all aspects of pressure transient control where air valves are

involved, the remedy is often to control the differential velocity

which exists at the air valve connection just as the valve is about to

close. This requires some regulation of air outﬂow rate through the

valve or alternatively avoiding the abrupt union of water columns.

Possible means of avoiding an unacceptable shock rise in pressure

include the following.

18.1.1 A more restricted air outﬂow device

Using an air valve having more limited air outﬂow capacity will produce

a pressure rise in the air-ﬁlled riser, leading to compression of the air

mass. The pump operating at design speed will experience a greater

delivery pressure, causing it to operate at higher head and lower ﬂow

rate than with the earlier large-oriﬁce valve. With the lower ﬂow rate

and velocity V

u=s

, the shock pressure rise H following air valve closure

and given by equation (18.1), will also be lower, although H

u=s

will be

increased to some extent.

There is a limitation on the extent to which the air valve outﬂow may

be throttled. If the venting rate is restricted to a large extent, head in

the air pocket below the valve may exceed the piezometric level down-

stream of the check valve, causing the valve to open slightly and air to

escape into the downstream system. Depending upon the conﬁguration

of the check valve, some of this air may collect in the upper part of the

332

Delivery

AV connection

d/s NRV

Head (mAOD)

Time (s)

AYR solo pump op/1 s start

200

180

160

140

120

100

–20

8.297

8.326

8.356

8.386

8.416

8.446

8.476

8.506

8.536

8.566

8.595

8.625

8.655

8.685

8.715

8.745

8.775

8.805

8.835

8.864

8.894

8.924

8.954

8.984

9.014

9.044

9.074

9.103

Fig. 18.8. Head in the riser during/following riser ﬁlling (detail)

Pressure transients in water engineering

valve body downstream of the door. When the valve is later opened

fully after the air valve shuts, the door will travel into a region partially

ﬁlled with air. Damping of the door motion can be reduced by this air

pocket, allowing the door to open more rapidly and causing it to

violently strike its stop, possibly resulting in damage.

18.1.2 Soft-start of the pump

The pump may be operated in such a manner that it does not attain its

maximum speed until after all air has been vented through the large-

oriﬁce air valve. Pump speed may be gradually ‘ramped’ so that from

moment to moment the H—Q curve alters (Fig. 18.4). Pump speed

remains at a rate sufﬁciently low that excessive velocity V

u=s

does not

develop. Where H

is the ‘shut valve’ head at the pump at design

speed N

, if the elevation difference between the liquid surface in the

wet well and the air valve is Z then a pump speed N given by:

¼ Z=N

or N ¼ N

ðZ=H

Þð18:2Þ

will be just sufﬁcient to ﬁll the riser and hold the liquid column in place.

A marginally higher speed will ensure a quiet air valve closure but

without opening the check valve.

Computations can be undertaken to establish the relationship

between ‘ramp slope’ dN=dt and pressure rise on air valve closure. In

this case the pump motor is capable of a maximum ramp time of

4 min and Fig. 18.9 shows predicted peak transient head at the pump

delivery, at the riser air valve connection, at the elevated air valve

and at the start of the rising main as functions of the time for the

pump to reach its design speed. A very short ramp time will not be

beneﬁcial as it will allow the pump to achieve its maximum speed

before air valve closure and thus velocity V

u=s

will also be at or close

to the maximum for the low operating head. In this case, ramp times

of 18 s or less are therefore of no beneﬁt.

By extending the time taken for a pump to achieve design speed

till a point beyond the air valve closure time, then the velocities in

the riser can be limited and the corresponding shock pressure rise

when the valve shuts is reduced. In this instance a ramp time of

around 1 min would reduce maximum head to around one-third of

the maximum.

A two-speed motor could be used with the lower speed maintained

until riser ﬁlling has been completed without undue pressure rise.

Thereafter the pump speed can be increased to the higher speed.

333

Air and gas

18.1.3 Use of an accumulator

An alternative surge suppression device which may be found on

borehole riser pipes is a bladder accumulator of the type illustrated in

Fig. 13.2b. The pump is started without any restriction on speed rise

and a large-oriﬁce air valve may be included just upstream of the

check valve on the well head. The shock pressure rise, developed

when the air valve closes, travels as a compression wave to the accumu-

lator from which it is reﬂected as a rarefaction wave and producing rapid

pressure relief. A compression wave also travels downstream from the

air valve, causing a pressure rise which is very short-lived as it is quickly

followed by the rarefaction wave from the accumulator. Prevention of a

shock pressure increase is considered to be better than curing such a

pressure rise after it has developed. The accumulator offers a remedy

if unacceptable pressures are found during commissioning of a scheme

when other remedies are no longer viable.

18.2 Pump start with slow valve clos ure

Where a direct pump start is used then a further option to restrict shock

pressure rise may be to use a slow-closing air-venting valve. Consider

the situation illustrated in Fig. 18.10. A two-stage borehole pump,

Weir Pumps Ltd type SBWM 600, is intended to lift water abstracted

334

Delivery AV connection

Elevation AV Rising main

Maximum head (mAOD)

Time to reach desi

n speed (s)

AYR solo pump op/ramp rate results

108

117

126

135

144

153

162

171

180

189

198

207

216

225

234

180

160

140

120

100

Fig. 18.9. Maximum head during riser ﬁlling plotted against ramp slope

Pressure transients in water engineering

335

Modulo 1

Crucero Clarines

Estanque

Clarines

Rio Unare

Aduccion

El Yai

Toma

Embalse

Santa Clara

Alimentacion

Clarines

Santa Clara

Planta de

Tratamento

6 + 476

1 + 510

360

4 + 875

∆

10" Acero

L = 2840 m

∆

12" Acero

L = 1510 m

∆

10" P.V.C.

∆20"

Fig. 18.10. El Yai water transmission system

Air and gas

from Rio Unare and to transfer this water at a rate of 350 litres/s either

to Santa Clara storage reservoir or directly to the nearby Santa Clara

treatment works. Pressure rating of the existing 20 in (508 mm) steel

rising main is 16 bar(g) and the pump duty head is 110 m.

18.2.1 Air venting through a standard air valve

Direct start of the pump produces a peak velocity of 4.58 m/s in the

initially air-ﬁlled riser (Fig. 18.11). Air is vented at a high rate through

an air valve sited in the horizontal section of pipeline downstream of the

908 bend at the top of the riser (Fig. 18.12). Closure of the air valve

on completion of venting produces a shock pressure rise beyond the

allowable maximum transient pipeline pressure (Fig. 18.13).

18.2.2 A butterﬂy valve for air venting

Installing a small butterﬂy valve in place of the air valve allows a quieter

start-up to be achieved. When the pump is operated, air is expelled via

the open butterﬂy valve connected to a backwash pipe leading back to

the suction well (Fig. 18.14). When all air has been removed, water

starts to ﬂow through the butterﬂy valve back to the wet well. Due

to the greatly increased ﬂow resistance, as liquid rather than air now

ﬂows through the butterﬂy valve and backwash line, and there is a

336

0.003

0.325

0.647

0.968

1.290

1.611

1.933

2.255

2.576

2.898

3.220

3.541

3.863

4.184

4.506

4.828

5.149

5.471

5.793

6.114

6.436

6.758

7.079

7.401

7.723

8.044

8.366

8.688

9.009

9.331

9.652

Time

(

)

El Yai/duty pump start/velocities

Velocity (m/s)

4.5

3.5

2.5

1.5

0.5

–0.5

Ch. –0 km Riser

Fig. 18.11. Velocity at pumping station after pump start

Pressure transients in water engineering

337

10.670 m

3 tonne manual hoist

TPI

400 on

400 on TFA

6.995 m

770 SD

floor opening

6.470 m

8.510 m

min W.L.

0.000 m

Strainer

–3.164 m

6.760 m

–3.430 m

–2.030 m

Inlet pipe

(by others)

400 on

RCV

Fig. 18.12. Elevation through pumping station

2.5

2.540

2.580

2.619

2.659

2.699

2.739

2.779

2.818

2.858

2.898

2.938

2.978

3.017

3.057

3.097

3.137

3.176

3.216

3.256

3.296

3.336

3.375

3.415

3.455

3.495

3.535

3.574

3.614

Time

(

)

El Yai/duty pump start

Head (mAD)

300

250

200

150

100

–50

Ch. 0.0 km Riser

Fig. 18.13. Head at pumpin g station after pump start (detail)

Air and gas

shock rise in pressure at the butterﬂy valve connection (Fig. 18.15).

Sizing the butterﬂy valve appropriately allows the peak transient

pressure at this connection to be limited. Once this initial upsurge

has dissipated, the butterﬂy valve can be closed gradually. The shock

338

Existing 500 DN delivery main

TP1 TP2

New access

walkway

Existing

platform

4400 Approx.

Pumpset

No. 2

Pumpset

No. 1

700 700

750

1900

300 DN

Back flushing pipework

1300 1300

300 DN

TFA

400 DN

300 DN

Fig. 18.14. Plan showing pipework and valves

Pressure transients in water engineering

pressure rise forces the non-return valve open and ﬂow is established in

the downstream rising main.

Following a pump trip, as piezometric level falls, the butterﬂy valve

automatically reopens allowing air to re-enter the riser.

18.3 Air venting through a ‘sparg’ line

Another means whereby an air-ﬁlled riser may be primed after pump

start and without the development of an excessive shock pressure rise

as the riser becomes ﬁlled is through the introduction of a ‘sparg’

line. This may take the form of a pipeline connected at the same

position as an air valve, leading back to the wet well. As the riser ﬁlls

with liquid, air passes through the sparg line to be released into the

wet well. After ﬁlling, the line then returns a relatively small proportion

of the liquid ﬂow to the wet well. The sparg line has to be of sufﬁcient

size that it reduces inertial pressure to an acceptable extent without

having an excessive effect on the efﬁciency of pumping. In the case

of sewage pumping, the sparg line may be at risk of blocking.

18.4 Gas evolution

If no air valve is placed before the check valve in a sewage system or if

an air valve becomes blocked for example, problems may still arise.

339

Time

(

)

El Yai/duty start

Head (mAD)

250

200

150

100

–50

Ch. 0.0 km Riser AV

2.5

2.669

2.838

3.007

3.176

3.346

3.515

3.684

3.853

4.022

4.191

4.360

4.529

4.698

4.868

5.037

5.206

5.375

5.544

5.713

5.882

6.051

6.220

6.390

6.559

6.728

6.897

7.066

7.235

7.404

7.573

Fig. 18.15. Head after pump start (detail)

Air and gas

While the liquid column is static, gas may evolve from the column of

sewage upstream of the check valve both under septic conditions and

also because the column is under vacuum pressure. When the pump

is started and ﬂow commences in the riser, the gas volume is

compressed. As pressure within this volume rises to match the piezo-

metric head downstream of the check valve, gas will be pushed into

the downstream line through this valve. When all gas has been removed

from upstream of the non-return valve then the column of liquid will

impact against the valve door, throwing it open violently into the

downstream ﬂuid which is a mixture of sewage and gas. Damage to

valve doors and stops has occurred as a consequence.

18.5 Gas pockets in a pipeline

A risk may exist in pipelines generally when a pocket of air or gas is

present under transient conditions. This potential problem was

indicated by Martin (1976) with regard to the expulsion of air through

a valve while ﬁlling a pipeline using a pump. In a recently released

design manual edited by Escarameia (2005), studies of air pocket

inﬂuence on hydraulic transients indicated a possible ampliﬁcation of

maximum pressures. Any ampliﬁcation is strongly dependent upon

the conﬁguration of the particular pipeline being studied but ampli-

ﬁcation was predicted by a factor of up to 260% over the equivalent

pressures without the presence of air pockets. Field observations of

Danish uPVC pipelines also showed ampliﬁcation. The dependency

of peak pressure on air pocket volume was also established.

The basic problem can easily be illustrated by considering transient

behaviour in a simple pipeline containing a trapped mass of air or gas

at its upstream limit (Fig. 18.16). Supposing that the liquid-ﬁlled

sloping pipeline has previously been subject to an hydraulic transient

event, say a pumping failure, which has produced a low pressure say

8.0 mWG at the upstream end of the line. Suppose that a volume

of gas has evolved from solution during the low pressure produced by

this initial transient event. For simplicity the gas was not reabsorbed

into the liquid by the inﬂuence of rising pressure.

Under action of the adverse transient, hydraulic gradient ﬂow will

develop from the downstream reservoir towards the gas pocket. The

strength of this reversed ﬂow and its duration will depend upon the

ability of the system to accommodate this ﬂow.

With no gas volume present, the only storage available within the pipe-

line is that resulting from extension of the pipe wall and from compression

340

Pressure transients in water engineering

of the liquid component. The amount of reversed ﬂow and its duration are

modest in these circumstances. A short duration of reversed ﬂow allows

only a modest time for ﬂow acceleration to occur, resulting in a relatively

small reversed velocity. Inertial head rise is a function of this maximum

reversed velocity and so the peak pressure will also be relatively modest.

When a volume of gas is present, an additional storage component

exists, namely the volume available as the gas mass is compressed by

the increasing pressure produced by the reversed ﬂow. This component

can be written by differentiating equation (12.2) as:

dVol=dp

abs

¼1=nVol=p

abs

ð18:3Þ

Allowing reversed ﬂow to develop, the gas volume is compressed with

rising pressure. The hydraulic gradient ﬂattens and the acceleration

towards the gas pocket reduces and ﬁnally ceases as a positive gradient

is established. The peak pressure within the gas pocket will depend on

the maximum reversed velocity and the initial volume of gas present.

The duration of reversed ﬂow and the maximum reversed velocity

will increase as the initial volume of gas is increased. Deceleration of

this reversed velocity will occur when the combined effects of rising

pressure in the gas pocket plus developing pipeline resistance produce

a positive hydraulic gradient within the pipeline. Even a relatively

341

Pump and NRV

1000 m

Riser

Hydraulic gradient at

time of flow reversal

10 m

8 m

Gas pocket

DN 300

Fig. 18.16. Simple system with air pocket

Air and gas