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

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The simplest way to reduce rates of deceleration and thus inertial

head is through extending the time of closure of the valve. For the

present example, Fig. 9.8 shows maximum pressure upstream of the

valve as a function of closure time. To reduce maximum pressure to

112

Maximum and minimum head during

3 min butterfly valve closure

835

1671

2506

3342

4177

5012

5848

6401

6968

7818

8669

9235

10 095

10 668

11 529

12 390

13 251

14 112

14 701

15 585

16 469

200

180

160

140

120

100

Chaina

(

)

Head (mAOD)

Maximum head

DN 1000 pipeline profile

Maximum

pressure head

Minimum head

Fig. 9.7. Envelope curves of head for 3 min valve closure

0 2 4 6 8 10 12 14 16 1

Maximum pressure on

float valve closure

Time (min)

Pressure (bar g)

Fig. 9.8. Maximum pressure plotted against valve closure time

Pressure transients in water engineering

similar levels obtained during ﬂoat valve closure requires a valve stroke

time >13 min. Closure intervals can become quite prolonged. This is a

consequence of the need to slow valve movement during the ﬁnal 10—

20% or so of closure in order to reduce the deceleration rate. This same

slow valve movement also takes place over the larger part of the stroke

when little ﬂow change occurs.

9.2 Improving valve operation

An ideal valve would seek to produce a linear variation of velocity with

stroke. In practice this is very difﬁcult to achieve but some steps can be

taken to improve this relationship by reducing the degree of non-

linearity between velocity and stroke. For example, a reduction in

valve diameter can improve this relationship but at a price. The head

loss through the smaller valve is greater than for a line sized valve so

that maximum ﬂow attainable is reduced.

The ﬂow through a large-diameter pipeline can be split into two or

more smaller-diameter branches before entering a downstream reser-

voir. The valves on each branch will also be smaller. These can be

set to close in sequence so that a more gradual deceleration can be

achieved. It is also easier for an operator to close a smaller valve.

One method of reducing the overall time of valve closure is to utilise

an actuator capable of closing the valve at alternative rates over

different parts of the stroke. For example, it is possible to cause the

valve to close relatively quickly during initial stages of movement, say

from 100% open to 20% open, using 50% of the overall closure time

and to move the valve more slowly from 20% open to shut over the

remaining 50% of closure time. Potentially, using a multi-stage

closure, it would be possible to attain an almost linear deceleration

over the entire valve stroke. An example using two-stage closure is

included in section 9.3.

Common ‘off-the-shelf ’ valves such as gate valves, sluice valves and

butterﬂy valves have quite unfavourable characteristics as regards surge

control because of the highly non-linear relationship between K

and

the valve position (Fig. 9.2).

9.3 Two-stage valve closure

Release of water from a high dam may use a valve which discharges

to a downstream watercourse or into a supply pipeline. Since a

considerable amount of energy is involved it is important to include

113

Valve operations

an emergency closing valve upstream of the terminal discharge valve.

In the event of a pipe burst or failure of the terminal valve, ﬂow will

accelerate within the outlet pipework with velocities of the order of

30 m/s being attained. Figure 9.9 illustrates outlet works from a high

dam with emergency closing butterﬂy valves included. These butterﬂy

valves (Fig. 9.10) are ﬁtted with a lever and weight to assist closure

114

Valve house

Emergency

butterfly

valve

Valve

house

Spillway

Penstocks

Draw

off

Dam

crest

Downstream

dam face

120 m

Draw-off well

Flow

regulating valve

Burst site

Section through dam

Fig. 9.9. Reservoir outlet arrangement

Pressure transients in water engineering

but with an oil-ﬁlled piston and cylinder arrangement attached to the

lever limiting rates of movement. The rate of valve closure can be

regulated by altering the oriﬁce through which oil passes while the

valve moves. In this case the oriﬁce arrangement can allow two rates

of movement of the valve. A relatively rapid movement of the valve

was permitted from the 908 position (100% open) down to 98 over

the ﬁrst 50% of the total closure time. Then a much slower closure

rate was used from 98 open to 08 (shut) for the second 50% of stroke

time.

Closure times from 15 s to 60 s were studied during which the valve

angle changed from 908 (fully open) to 98 in half the time and from 98 to

08 (shut) in the remaining time.

Figure 9.11 shows the variation of velocity during a 30 s closure event

and Fig. 9.12 depicts the changing head upstream of the valve during

closure. It will be noted that maximum head exceeds the static level

by <20 mWG.

Calculations can also be used to predict changing forces and

torques acting upon the valve door. In addition to head loss co-

efﬁcient data, manufacturers can also supply force and torque data as

a function of valve position. This information can be used to establish

the force and its direction, acting on the valve door and from this the

frictional resistance moment offered by the valve shaft as it rotates.

Hydrodynamic torque can also be calculated and when added to the

frictional resistance torque the net closure moment can be found as a

115

Piston and cylinder

Weight

Lever

Valve door

Flow

Fig. 9.10. Emergency closing butterﬂy valve

Valve operations

function of valve position during closure. This information allows the

manufacturer to design an appropriate lever and weight which is able

to overcome resistance and allow the valve door to close within the

design period.

116

0.02

1.04

2.06

3.08

4.10

5.12

6.14

7.16

8.18

9.20

10.22

11.24

12.26

13.28

14.30

15.32

16.34

17.36

18.38

19.40

20.42

21.44

22.46

23.48

24.50

25.52

26.54

27.56

28.58

29.60

30.621

31.641

Velocity (m/s)

–0.5

30 s emergency valve closure

At emergency

butterfly valve

At base of

draw-off well

Valve 10% open

Valve shut

Time (s)

Fig. 9.11. Velocity variations during two-stage valve closure

Valve 10% open Valve shut

0.02

1.00

1.98

2.96

3.94

4.92

5.90

6.88

7.86

8.84

9.82

10.80

11.78

12.76

13.74

14.72

15.70

16.68

17.66

18.64

19.62

20.60

21.58

22.56

23.54

24.52

25.50

26.48

27.46

28.44

29.42

140

120

100

Time (s)

Head (m)

Base of

draw-off well

Upstream of butterfly valve

30 s emergency valve closure

Fig. 9.12. Head variations for two-stage valve closure

Pressure transients in water engineering

9.4 Submerged discharge valve

It is also possible to improve the relationship between velocity and valve

position by employing a more sophisticated type of valve. One such is

the submerge discharge valve described by Miller (1969) (Fig. 9.13).

The principle of operation is that a ported sleeve, attached to a shaft

and actuator, is moved vertically to control ﬂow passing into a sump

117

Corner fillets

Spindle

Moving sleeve

Flowmeter

Inlet bend

Actuator

Water surface

Spillway

Discharge sump

Vertical

downpipe

Baseplate

Valve outlet

Fig. 9.13. Submerged discharge valve

Valve operations

where the energy of ﬂow is dissipated. The port shape can be modiﬁed

to suit the circumstances both of the system in which it is installed and

also the range of ﬂows over which the valve is to exercise control. The

port shapes can be altered over time to suit any changes in ﬂow rate as a

scheme is developed. For example, in the early years of a project, ﬂow

may be modest to suit prevailing demands while in later years ﬂow

may have to be increased to meet anticipated increase in demand.

9.5 In-line valves

Two basic purposes can be identiﬁed for in-line valves. The most common

is the isolating valve which is normally fully open and remains so except

during periods of maintenance or possibly when an emergency has

occurred which requires the valve to be closed. The second valve applica-

tion is where an actuated valve is used to control and shut off ﬂow within a

pipeline system but where a terminal valve would be impractical. The

method of analysis is the same for each category.

9.5.1 Isolating valves

While hydraulic transient analyses are commonly associated with

operation of terminal discharge valves for instance, some attention

should be paid to the action of isolating valves installed at intervals

on longer pipelines and also on branch pipelines. These valves are

included for the primary purpose of allowing sectional maintenance

without dewatering the entire system. Should an emergency arise,

such as a pipe burst, ﬂow within the upstream pipeline will accelerate

towards the burst site. Isolating valves in the network adjacent to the

burst and in particular on the supply side of the system, must be able

to close safely and without causing further bursts.

Sluice valves and more recently butterﬂy valves are considered appro-

priate for this application. The butterﬂy valve is easier to operate than a

comparable sluice valve under similar conditions. For ease of manual

operation, handwheel diameter d has been given by Bartlett (1978) as:

d ¼ 6

with D being pipe diameter. The practical continuous handwheel opera-

tion rate is around 20 rpm. Closing a valve against an unbalanced head

may require spur gearing. In one example, McCrone (undated) quotes

3:1 gearing to close a sluice valve against a differential head of

40 mWG. The total number of turns till closure is 144, giving a 7.2 min

118

Pressure transients in water engineering

closure interval. The butterﬂy valve by contrast has a standard 70:1 worm

gear needing only 17

turns to close through 908. Closure time for the

butterﬂy valve is only 0.87 min. Surges produced by these valves were

93 m for the sluice valve and 135 m for the butterﬂy valve. These repre-

sent pressures 232% and 337% higher than the allowable maximum.

The effect of a too rapid isolating valve closure may be to produce

additional pipe bursts upstream due to the allowable peak pressure

having been exceeded. Also on the downstream side of the valve,

pressure will fall, potentially causing vacuum pressures which may

cause a pipe to buckle and collapse (Fig. 9.14).

9.5.2 Actuated valve

Figure 9.15 shows a water transfer system in which ﬂow which has

previously been pumped from a reservoir to a break pressure chamber

(BPC) travels under gravity to a connection with a pumping main.

An in-line valve has been included upstream of the connection to

the second main. Flow passing through the in-line valve enters the

second main and continues to ﬂow under gravity to augment the

volume stored in a second reservoir. The in-line valve is of a needle

119

Compression wave travelling upstream

when isolating valve is closing

Initial flow direction

Initial pipe burst

Possible pipe collapse due to

low pressure after valve closure

Rarefaction wave travelling downstream

when isolatin

valve is closin

Possible pipe burst

due to high pressure

after valve closure

Gradient flattens/reverses post-burst

Gradient steepens

post-burst

Isolating valve

Steady flow hydraulic

gradient pre-burst

Fig. 9.14. Emergency closure of an in-line valve

Valve operations

120

Ground level = 10.8 mAOD

C.L. = 8.75 mAOD

Floor level = 8.15 mAOD

Flow direction

Inlet main

Flowmeter

Needle valve

Isolating valve

Downstream main

Thrust block

Gravity transfer main

Receiving

reservoir

Flow direction

Valve

chamber

Downstream main

(

idle

)

Suction tank

BPC

Fig. 9.15. In-line needle valve

Head upstream of needle valve –

1.8 min closure at a flow rate of 16 Mld

180

160

140

120

100

Time (min)

Head (mAOD)

0 1 2 3

Fig. 9.16. Head upstream of needle valve for 1.8 min closure

Pressure transients in water engineering

type whose head-loss characteristic is shown in Fig. 9.2 and which is

installed in a chamber as shown in Fig. 9.15.

When the valve is closed over a time of 110 s, head upstream of the

valve increases smoothly to a maximum as depicted in Fig. 9.16. After

wave reﬂection from the BPC, head falls to produce an irregular

oscillation. On the downstream side of the valve, head falls during

valve closure (Fig. 9.17). After valve closure and wave reﬂection, head

largely recovers and a fairly regular oscillation is developed. The timescale

of oscillations shown in Figs 9.16 and 9.17 are indicative of the respective

lengths of pipeline upstream and downstream of the closed in-valve.

Overall variations in head through the system can be seen in Fig. 9.18.

121

Head downstream of needle valve –

1.8 min closure at a flow rate of 16 Mld

Head (mAOD)

0 1 2 3

Time (min)

Fig. 9.17. Head downstream of needle valve for 1.8 min closure

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

175

150

125

100

Head (mAOD)

Chaina

e (km)

BPC

Downstream

reservoir

Needle valve chamber

Pipeline profile

Envelope curves of max. and

min. head for 1.8 min closure

Flow rate = 16 Mld

Fig. 9.18. Envelope curves for 1.8 min valve closure

Valve operations