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

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are involved, one being the downstream part of the East Main supplied

originally with water from the Loch Katrine system and the other a

gravity-driven network supplied from Daer reservoir to the south.

These usually independent systems operate at signiﬁcantly different

piezometric levels, with the Daer gravity system around 100 m higher

than the East Main network. To improve security of supply a link

main has been constructed between the two systems. This link runs

502

610f

457f

546f

546f546f

546f

244f

546f

457f

Link Main DN 700

from Daer reservoir

Cranhill PS

Castlemilk PS

Castlemilk Low

TWL 128.6

East Rogerton

TWL 213.4

Dechmont

TWL 130.4

Udston

TWL 181.4

Distribution main

to Cathkin DN 200

Auchentibber

TWL 208.8

Castlemilk High

TWL 171.5

Burnside PS

East Main

Ruchazie TWL 102.9

Cranhill TWL 111.7

762f

0 1 2 3 4 5

Approximate scale – kilometres

Water tower

Ground tank

Pumping station

Pressure reducing valve

Isolating or fixed throttle valve

Fig. 24.2. East Main and Daer transmission main

Pressure transients in water engineering

from Burnside on the East Main to East Rogerton SR on the Daer

system. Any interruption of supply within the East Main system

would be compensated by inﬂows under gravity from the Daer system

with a pressure-reducing valve in the link main to control the ﬂow

entering the East Main network. Should ﬂows in the Daer network

be disrupted, water would be pumped from the lower East Main into

the higher Daer pipeline system.

24.2.1 Burnside booster pumping station

The link main booster pumping station was located at Burnside as

shown in Fig. 24.2. This new pumping station was ﬁtted with pressure

vessels both upstream and downstream of the pumps. The suction side

vessel had a gross volume of 27.5 m

and contained an air charge of

9.0 m

at a head of 99.0 mAOD. On the downstream side of the

pumps the pressure vessel had a total volume of 16.0 m

and the air

charge was again 9.0 m

under the much higher head of 213.4 mAOD.

24.2.2 Hydraulic transient computations

Prior to introducing the new booster pumping station at Burnside,

computations were undertaken to establish the effects upon existing

installations. Along the section of East Main system shown are a

number of branches connected to local booster pumps which lift treated

water into storage in tanks and water towers. These pumps serve storage

chambers at Cranhill, Castlemilk Low and Castlemilk High and each of

these duty pumps could remain in operation while the link main booster

pumps were running.

24.2.3 Castlemilk Low pumping station

Particularly signiﬁcant was the booster pump serving Castlemilk Low

tank. The performance curve for this pump is shown in Fig. 24.3

together with the normal duty point prior to introduction of the new

booster pumping station at Burnside. The pump performance curve

contains a zone of unstable operation. Unstable curves are those

which contain a relatively ﬂat region of Q—H curve or even where H

increases with Q over part of the performance curve. Considering the

circumstance in which the new link main pump is in operation, the

ﬂow rate in the East Main leading to the link main connection will

be increased, causing head loss in the East Main to rise. The effect

503

Flow instabilities

will be to reduce the suction head at existing booster pumps along the

East Main and in particular at Castlemilk pumping station. The steady

operating head for these pumps will increase. Added to this increased

pumping duty at Castlemilk Low station are transient pressure varia-

tions produced when the new booster pump is operated.

24.2.4 Transient pressures

There is a transient pressure drop as the new pump at Burnside is

started, as shown in Fig. 24.4, causing the temporary operating point

at Castlemilk Low pump to move upwards into the unstable zone of

operation. When the necessary transient pumping head exceeds H

max

(Fig. 24.2), ﬂow falls abruptly towards zero but as it does so, suction

pressure starts to rise because of the deceleration produced. This rise

in pressure will tend to move the operating point back into the stable

zone and allow pumping to recommence, but as soon as this happens

and velocity starts to increase, suction head falls once more and the

operating point moves back into the unstable region of pump operation.

Added to this are transient effects reﬂected from other upstream

parts of the network. The resultant effect is that behaviour remains

504

100

Generated head (m)

0 100 200 300

Flow rate (l/s)

Cut impeller

Castlemilk Low pumps

characteristic curve

WPL SDC 7/8 UNIGLIDE

Duty point –

link main pumps off

max

Fig. 24.3. Castlemilk Low pump performance curve

Pressure transients in water engineering

unstable with an irregular cyclical oscillation remaining as long as

pumps are in operation. Figure 24.5 shows the predicted behaviour

following start of the Burnside link main booster pump. Trip of the

booster pump at Burnside allows the operating point of Castlemilk

Low pump to move back to its original location with stable ﬂow

restored.

505

Castlemilk PS

East Main

Link Main PS

Transient drawdown

Piezometric gradient

without link main pump

Piezometric gradient

with link main pump

Pumping head

without link

main pump

Additional head

with link main

pump running

Fig. 24.4. Hydraulic conditions along ﬁnal part of East Main

Start of link main pumps

Head u/s of Castlemilk PS

0.0 0.5 1.0

Time (min)

Head (mAOD)

Fig. 24.5. Head conditions at Castlemilk after link main pump operated

Flow instabilities

24.2.5 Spread of unstable oscillations and consequences

Unstable behaviour is not conﬁned to the vicinity of the pump affected

but in common with all hydraulic transient effects will spread through-

out the pressurised pipeline system. In Fig. 24.6 is shown the predicted

variation of head upstream of the Burnside link main booster station

with the booster pump initially in operation. The unstable oscillation

present is eliminated when the link main pump at Burnside is tripped.

While the unstable motion predicted did not produce extremes of

pressure outwith allowable limits, undesirable consequences can occur

as a result of such behaviour. Pulsations of ﬂow can cause movement

of check valves with consequent accelerated wear of hinges. Flow

and head ﬂuctuations at a pump will produce a corresponding oscilla-

tion in power to the pump. These effects on pumps and valves are

not conﬁned to the unstable pump itself but affect all pumping stations

of the system.

24.2.6 Possible remedies

It is important when enhancing or modifying an existing scheme to

ensure that the original elements of the system will continue to

behave in an acceptable manner. If this is not predicted to be the

case, necessary measures should be taken to restore stable operation.

In this case, the Castlemilk Low pump was originally ﬁtted with a cut

impeller and so it would be a relatively straightforward matter to install

506

Burnside pumps tripped

Burnside pumps operating

Head upstream of Castlemilk PS

0 1 2 3 4

Time

(

min

)

Head (mAOD)

110

100

Fig. 24.6. Head conditions at Burnside after link main pump is started

Pressure transients in water engineering

a larger-diameter impeller within the existing pump casing. This larger

impeller will allow an increase in duty head while remaining in the

stable zone of operation (Fig. 24.2). The increase in pumping head

will also have implications with regard to power of the motor required

to meet the revised duty when the link main pump is operating.

24.3 Gravity ﬂow system

The second example concerns a pipeline conveying treated water under

gravity from an upstream impounding reservoir with an overﬂow

spillway level at 359.1 mAOD. A treatment works sited 3 km below

the dam has an outlet piezometric level of 283.68 mAOD. An 838 mm

internal diameter mild steel (MS) pipeline of length 26.686 km conveys

water to a downstream storage reservoir having a water level of

183.87 mAOD. Almost midway along the pipeline, 13.175 km from the

treatment works is a break pressure chamber (BPC). The undulating

proﬁle of the main is shown in Fig. 24.7.

507

838.2 mm f steel main

957.6 litres/s

283.68 mAOD

1078.6 litres/s

BPC

838.2 mm f steel main

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Chaina

e (km)

300

260

240

220

200

180

160

140

120

100

Fig. 24.7. Hydraulic conditions along gravity main

Flow instabilities

24.3.1 The break pressure chamber

Upstream of the break pressure chamber (BPC) the 838 mm main bifur-

cates into twin 457 mm diameter lines, each of which terminates at a Glen-

ﬁeld and Kennedy H1 ﬂoat actuated valve. These valves regulate inﬂow to

the BPC. Each valve is connected to a ﬂoat, with each ﬂoat contained

within a small chamber inside the BPC (Fig. 24.8). DN 150 and DN 200

openings link the ﬂoat chambers with t he main tank of the BPC which

hasplandimensionsof3.05m3.66 m over the normal water level range.

24.3.2 Head losses

Measurements of ﬂow and head loss in the section of pipeline upstream

of the BPC gave values of k

from 0.44 mm to 0.46 mm over the ﬂow

range 82.7 Mld to 93.2 Mld. For the section of pipeline downstream

of the BPC k

was evaluated as 0.6 mm at a ﬂow rate 82.25 Mld.

24.3.3 Flow regulation

Inﬂow to the downstream storage reservoir is normally controlled by

ﬂoat actuated valves but with a butterﬂy valve upstream for emergency

508

Float chamber

Overspill weir

Throttled connection

between float

chamber and BPC

Linkage

838 mm f

MS main in

838 mm f

MS main out

2 No. 457 mm f

float operated valves

Float

Fig. 24.8. Break pressure chamber conﬁguration

Pressure transients in water engineering

closure duty. In the event that these downstream valves are closed,

ﬂow into the pipeline below the BPC will fall, causing level in the

chamber to rise. The ﬂoats in the BPC will then move upwards,

progressively closing the twin H1 valves and throttling ﬂow entering

the BPC. Each H1 valve contains a set of four openings or ports

through which ﬂow passes. Shape of the ports can be altered to provide

a range of ﬂow v. head loss characteristics to suit particular require-

ments while providing the necessary maximum design discharge

when fully opened. If discharge required should change over time

the port shape can be altered to suit. Phase I of the project had port

shapes as illustrated in Fig. 24.9, with a total ﬂow area 373.16 cm

when fully opened and with a valve stroke of 85.7 mm. Originally it

was intended that the port shapes would be altered to permit an

increased maximum ﬂow in Phase II through a total ported area of

625.81 cm

over the same stroke length. However, to maximise

discharge a considerably increased ﬂow area totalling 1574.19 cm

was actually created, with stroke length being increased to

103.2 mm. These port forms for Phase II are also shown in Fig. 24.9.

Travel of ﬂoats within the BPC for the ﬁnal Phase II arrangement

was 762 mm — that is, from a minimum elevation of 231.058 mAOD

509

Phase 1 and initial phase 2

valve travel = 85.7

Initial phase 2

port shape

Actual phase 2

port shape

Phase 1 port shape

Phase Total port area – mm

1 37 316 (100%) 11 581 (67%) 1652 (33%) open

2 initial 62 581 (100%) 20 387 (67%)

2 actual 157 419 (100%)

Actual phase 2

valve travel = 103.2

Fig. 24.9. Valve porting arrangements

Flow instabilities

to a maximum of 231.82 mAOD. Crest level of the overﬂow weir

within the BPC was at 231.744 mAOD so that when water level

exceeds crest elevation, spillage over the weir commences and water

runs to waste.

The ported area is dramatically increased for the actual Phase II port

shape. A corresponding drop in head loss coefﬁcient is achieved, as

shown in Fig. 24.10. Loss coefﬁcient for the actual Phase II port con-

ﬁguration remains relatively low up until the ﬁnal 10% or so of closure

so that the ﬂow deceleration will be relatively modest over the ﬁrst

90% of the closure process. Only over the ﬁnal 10% or so does ﬂow

deceleration become important. Flow thus continues to enter the

BPC at a relatively high rate over most of the valve stroke.

510

Phase I porting

Initial Phase II porting

BPC inlet valve loss coefficient (kV)

as a function of valve stroke

Actual Phase II porting

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

Valve stroke (mm)

log

(kV)

Fig. 24.10. Head loss characteristics for valve

Pressure transients in water engineering

24.3.4 H1 valve movement

The relatively rapid ﬂow deceleration during the ﬁnal stages of closure

produces a substantial inertial head rise above the H1 valves and this

head increase propagates into the upstream pipeline as a compression

wave which is eventually reﬂected back to the BPC from the treatment

works after about 30 s (2L=a) as a rarefaction wave, at which time a

relief of pressure occurs above the BPC.

During the interval while the pressure wave has been travelling in the

upstream pipeline, water has been spilling from the BPC over the weir.

This produces a gradual reduction in water level in the chamber

allowing partial reopening of the ﬂoat valves. Due to the poor charac-

teristic of the ﬂoat valves in the actual Phase II port conﬁguration,

even a modest reopening of the valves can produce a substantial ﬂow

increase into the BPC, leading to a falling head in the pipeline upstream

of the valves. This fall in head is reinforced by the relief of pressure

brought about by the rarefaction pressure wave reﬂected back from

the treatment works. The combined effect is to create a more severe

fall in head upstream of the valves than would be produced by either

inﬂuence acting alone. This low head is then propagated into the

upstream pipeline as a rarefaction wave which travels to the treatment

works before being reﬂected back to the BPC as a compression wave,

arriving after a further time interval of around 30 s. During this interval

the water level in the BPC has continued to fall towards the overﬂow

crest. The returning wave causes a head rise upstream of the valves

and a renewed ﬂow into the chamber. Water level in the BPC increases,

the ﬂoat valves close once more and water starts to spill over the weir

crest. The high rate of ﬂow deceleration at the valves produces a further

compression wave which travels into the upstream pipeline.

With the passage of time the combined inﬂuence of the elastic

oscillation in the upstream pipeline and the ﬂuctuation of discharge

through the ﬂoat valves produces a tendency for the amplitude of

oscillation to persist and even increase, as shown in the prediction of

Fig. 24.11. Period of the oscillation in head upstream of the BPC is

about 60 s which corresponds with the fundamental period of the

upstream pipeline 4L=a. Variation of water level in the BPC was also

predicted to have a period of about 1 min. This behaviour is an example

of a ‘self-excited’ oscillation. Figure 24.12 shows the predicted extremes of

head in the gravity main system over a 10 min interval following closure of

the inlet valves at the downstream reservoir. The amplitude of oscillation

is much greater in the section of main upstream of the BPC than in the

lower stretch of pipeline as a consequence of system excitation.

511

Flow instabilities