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

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16.5 Air valve operation

To illustrate the possible inﬂuence which discharge conditions may

have upon the upstream pipeline, a sewage pumping system containing

air valves is considered. The sewage transmission system of Sharjah

Main Drainage Pumping Station No. 1 uses parallel DN 450 and DN

700 pumping mains to convey sewage a distance of over 6 km, with

the mains discharging to a modest chamber from which ﬂows enter a

gravity sewer. Air valves were included at around chainage 4.4 km on

each main. While the exit level into the gravity sewer from the dis-

charge chamber was above the air valves’ elevation, the base of the

chamber where the rising mains enter the chamber was below the air

valves. Figure 16.12 shows envelope curves along one of the mains

for a combined pump start/trip analysis with most of the system up

until the air valves’ site subject to vacuum pressures after trip. Maxi-

mum pressures during start-up were based upon the assumption that

the pipelines were entirely primed. Downstream of the air valves,

minimum pressures remained positive.

On ﬂow reversal at the discharge end of the system, the chamber

empties and head falls to the extent that hydraulic levels at the down-

stream chamber are less than at the air valves’ location. This means that

the air which has been admitted to the pipelines through the air valves

cannot entirely be purged from the system during reversed ﬂow. Some

air will remain in the pipeline when pumps are restarted and this will

alter the start-up transient analysis.

292

il (mASL)

max

min

Chaina

e (m)

Elevation (mASL)

–5

301.5

603.0

904.5

1206.0

1507.5

1809.0

2110.5

2411.9

2713.4

3014.9

3316.4

3617.9

3919.4

4220.9

4422.3

4725.2

5028.0

5330.8

5633.6

5936.4

6144.9

Fig. 16.12. Envelope curves for sewage rising main with air valves

Pressure transients in water engineering

There is also the question of alleviating the severe vacuum pressure

conditions which were found to occur after pump trip, between the

pumping station and the air valves. A pressure vessel was included

at the pumping station for this purpose. With the vessel in place,

Fig. 16.13 shows head variations after two pumps were started in

sequence with 30 s between each pump being operated. After around

min. essentially steady ﬂow had been achieved and the pumps

were tripped together. Pump start still produces the maximum system

pressure and air valves continue to operate as before. Head variations

at the pumping station, at the start of the DN 450 main and at the

mid-point of this main, are shown in this ﬁgure. When ﬂow reverses,

the discharge chamber at the downstream end of the system supplies

liquid to remove air which had entered through the air valves. The

air valves in turn were used to allow water to ﬂow to the pumping

station to recharge the pressure vessel. The end result is that the ﬁnal

system head is relatively low as the discharge chamber is almost emptied

and the air valves remain open with air pockets in each pipeline.

16.6 Summary of inﬂuence of discharge arrangements

Generally speaking, head conditions at the downstream end of a system

will not signiﬁcantly inﬂuence the initial rarefaction pressure wave in a

rising main after a pumping failure. At later stages, especially after ﬂow

reversal has occurred, the conﬁguration at the discharge end of a

293

Time (s)

Head (mASL)

–5

d/s vessel Start 450 Mid 450

0.276

19.596

38.916

58.236

77.556

96.876

116.196

135.516

154.836

174.156

193.477

212.797

232.117

251.437

270.757

290.077

309.397

328.717

348.037

367.357

386.677

405.997

425.317

444.637

463.957

483.278

502.598

521.918

541.238

560.558

579.878

599.198

Fig. 16.13. Head variation for combined pump start/trip analysis

Discharge conditions

network may start to inﬂuence events. If ﬂow reversal at the outfall

produces a decrease in piezometric level at this point, the strength of

a compression wave travelling upstream and subsequent upsurge pres-

sures at a pumping station can be signiﬁcantly affected. The reﬁlling

time of vessels will be more prolonged and ﬁlling may be incomplete.

If air valves have operated during the initial fall in head after pump

trip, the time of air venting will be increased by any fall in downstream

head at the outfall. It is even possible for downstream head to decline to

the point at which air expulsion can no longer take place. This creates a

situation in which it is restarting of pumps which will ﬁnally purge any

remaining air from the system. Pumps may also be restarting against a

reduced static head as a consequence of the lowering of downstream

piezometric level.

Detailed consideration of discharge conditions may be especially

important when making comparisons between prediction and ﬁeld

observation. Timings of events and extremes of head and ﬂow may be

signiﬁcantly affected by variations of piezometric level at the down-

stream end of a pipeline.

294

Pressure transients in water engineering

Air valves

Alleviation of sub-atmospheric transient pipeline pressures commonly

requires the introduction of an additional supply of ﬂuid. From a cost

point of view, it is an attractive option to use pipeline ﬁttings which

are already present for other purposes to achieve this result. The air

valve presents this opportunity. The usefulness of these valves in

assisting to alleviate surging action comes from their ability to admit

substantial quantities of air when piezometric level falls below the air

valve elevation. Following a pumping failure for instance, as discharge

from pumps decreases, the air inﬂow augments the diminishing ﬂow

from the pumping station so that downstream of the air valve, ﬂow

deceleration is not as great as that upstream of the valve. This reduced

deceleration implies that subsequent head changes will be correspond-

ingly lower, thus alleviating minimum pressure conditions.

These valves are commonly installed along a pipeline in an arrange-

ment intended primarily to allow easy emptying and ﬁlling of the pipeline

during maintenance and in the case of double-oriﬁce valves (DOVs) to

facilitate venting of pressurised pockets of air. Figure 17.1 illustrates the

main components of an ‘APEX’ DOV manufactured by Glenﬁeld Valves

Ltd. The valve has an oriﬁce at the top which allows for large air ﬂows

through the valve with a modest head drop. Flow ceases when the

ﬂoat rises to seal against the oriﬁce. A second much smaller oriﬁce

with a lever attached is used to control outﬂow of air under pressure.

The smaller oriﬁce is adjustable. Usually these valves are of a self-

acting pattern and so may also operate to admit air during a transient

event if piezometric level at the valve falls below the operating elevation

of the air valve. If this occurs, air will enter the pipeline, producing a

discontinuity in ﬂow past the air valve connection. Figure 17.2 illustrates

the sequence of operation of an air valve.

295

296

Body

Float

Float guide

Seal ring

Seat ring

Cover

Cowl

Orifice cover

Orifice bracket

Sealing face

Fulcrum pin

Float and lever

Body

Adjusting

screw

Fig. 17.1. Double-oriﬁce air valve

1 Venting 2 Filling 3 Closure

4 Sealin

5 Pressurisation 6 Relievin

Fig. 17.2. Operating sequence of sewage air valve

Pressure transients in water engineering

17.1 Normal air valve locations

Air valves are a feature of the majority of water and sewage systems and

the distribution of these valves is largely dictated by the longitudinal

proﬁle of each pipeline. Figure 17.3 illustrates locations in which it

may be desirable to include air valves.

(a) A double-oriﬁce valve may be included downstream of a pumping

station to remove air which may be entrained at a pump’s suction

intake. Following pumping failure, an air valve at this location

297

DOV = double-orifice valve, SOV = sin

le-orifice valve

HGL

DOV

SOV SOV SOV

SOV

800 m

SOV

DOV

SOV

DOV

(c)(b)(a)

(d)

(e)

(f)

800 m

(g) (h)

Fig. 17.3. Typical air valve locations

Air valves

may operate to admit a substantial volume of air depending upon

operating head conditions.

(b) At a deﬁned summit on a pipeline, entrained air may accumulate

and if not removed will eventually lead to a reduction in efﬁciency

of the system. A DOV will allow air to be vented under pressure

through the small oriﬁce. After a pumping failure, if piezometric

level should fall to the valve operating level then air inﬂow will

occur through the large oriﬁce. A valve at this location is important

for emptying and ﬁlling of the adjacent sections of pipeline.

stretch of main, say over a plateau, and the valves serve similar

functions.

(d) This represents long stretches of main of uniform gradient. Air can

migrate along a pipeline of shallow gradient and the ability to vent

this air is useful. Typically an interval of 800 m or so might be

allowed between neighbouring valves.

(e) As for (d).

(f ) As for (d).

(g) In a gravity system an isolating valve may be provided at the

upstream end of the main. A DOV downstream of the isolating

valve is necessary for emptying and ﬁlling of the pipeline and may

also vent any air entrained at the inlet.

(h) A DOV may usefully be included at inlet to a tank or reservoir

where a valve is provided to shut off inﬂow. In the event of ﬂow

reversal with the inlet valve closed, vacuum pressures would

occur in the supply pipeline, were a means of ventilating the pipe-

line not provided.

It should be noted that some pipelines may have no air valves

installed. If, in a pumping main for instance, the pipeline rises continu-

ously to the discharge point, then any air can be expected to migrate

downstream and eventually to escape through the pipe discharge. Simi-

larly, some gravity mains are devoid of air valves by virtue of their

proﬁle. Where ﬂow velocities are high then it may be assumed that

all air will be carried along with the ﬂow and escape at the downstream

end of the system.

17.2 Air valves for surge alleviation

One of the attractions of air valves as a means of surge alleviation arises

from their presence along a pipeline for operational purposes. No extra

298

Pressure transients in water engineering

cost is incurred by their use in hydraulic transient suppression unless

modiﬁcations are required or possibly extra valves.

There are many instances where, knowingly or otherwise, large- or

dual-oriﬁce air valves operate during low transient pressure conditions.

The inﬂow of air which occurs has the effect of preventing substantial

negative pressures from developing in the vicinity of the valve at least.

This may appear to be good practice but it is not without its dangers.

Up until the time when the piezometric level at an air valve con-

nection has fallen to the point at which the valve will operate then

the valve remains shut and has no inﬂuence upon the propagation of

the pressure wave, with ﬂow and rates of deceleration upstream and

downstream of the valve connection being equal.

The rate of air inﬂow is given by the relationship between piezometric

level, valve operating level and ﬂow rate. Suppliers should be able to

provide the necessary data, usually in the form of a chart which can

be digitised for use in a computer model. Figure 17.4 is an example of

the air inﬂow—outﬂow relationship for a large oriﬁce and also for the

small oriﬁce where this adjustable oriﬁce is at its full-open setting.

The air ﬂow rate is expressed in units of free dry air at standard tempera-

ture and pressure (STP). STP are at an ambient temperature and

atmospheric pressure. In computations it is necessary for the computer

model to take cognisance of the effect of compression/expansion of air

in the pipeline. Air volume within the pipeline at the prevailing pres-

sure may be calculated approximately using a polytropic relationship:

abs

Vol

¼ constant ð12:2aÞ

When an air valve commences operation (Fig. 17.5), writing the

equation balancing ﬂows upstream and downstream of the valve

connection then:

u=s

þ Q

air

þ dVol

air

=dt ¼ V

d=s

ð17:1Þ

where Q

air

is air ﬂow rate with inﬂow assumed þve. Air volumes

correspond with prevailing pressure in the pipeline at the valve connec-

tion. As upstream ﬂow rate V

u=s

diminishes following a pumping

failure, the inﬂow of air acts to supplement this upstream ﬂow to sustain

downstream ﬂow rate V

d=s

. Thus the rate of deceleration down-

stream of the valve becomes smaller than that upstream. This reduction

in the rate of deceleration acts to suppress the pressure transient propa-

gating downstream of the air valve, with piezometric level at the valve

itself being maintained at a higher level than otherwise. This is due to

the substantial rate of air inﬂow which can be achieved through the

299

Air valves

large-oriﬁce valve. Completion of the solution is accomplished using the

quasi-invariant relationships along the Cþ and C characteristics:

for the Cþ path:

Jþ¼V

u=s

þ g=aH

and for the C path:

J¼V

d=s

 g=aH

The appearance of the air pocket below the valve connection acts as

an internal boundary from which strong reﬂection of pressure waves will

occur, much as from a reservoir, although with a variable pressure

acting on the water surface rather than a constant atmospheric pressure

as is the case with a large tank or reservoir.

Maintenance of piezometric level at the valve, at a higher level than

otherwise, will act to produce greater rates of deceleration upstream and

300

Q = flow rate (m

/s)

Dp = differential pressure across orifice (bar)

p = absolute pipeline pressure on outflow

and absolute atmospheric pressure on inflow

10 20 30 40 50 60 70 80 90

40 30 20 10

0.2

0.4

0.6

0.8

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

Inflow

Supercritical flow

Vacuum gauge

pressure (bar)

Discharge of free dry air in STD m

/min

Large orifice – inflow and outflow

Outflow

Subsonic flow equations

Small orifice – outflow only

Pipeline gauge

pressure (bar)

Q = 1.02 ¥ ÷(Dpp) Q = 48 ¥ ÷(Dpp)

Fig. 17.4. Air ﬂow characteristics of double-oriﬁce air valve

Pressure transients in water engineering

a more rapid ﬂow reversal in that section of main upstream of the valve.

In a pumping main where air valves have been provided, it is possible

that some of these valves will operate following a pumping failure. This

is particularly likely if no special surge protection has been included at

the pumping station to prevent this from happening. After the pumping

failure, as the negative pressure wave or downsurge propagates into the

rising main, the hydraulic gradient at an air valve may fall to the point

where the valve opens to allow inﬂow of air.

Where a number of air valves along a pipeline operates during the

downsurge following pumping failure then air pockets will be developed

in the pipeline below the connection to each operating valve.

Successive air valves will each act to further suppress the downsurge

so that the minimum piezometric line may appear as shown in

Fig. 17.6. For each valve which functions, deceleration of ﬂow in the

section of line downstream of that valve is < deceleration rate upstream

of the valve connection. Suppression of the pressure transient

downstream of air valves has been achieved by providing alternative

301

Air valve

Pipe cross-section = A

Space << Dx

Dx Dx

C– characteristic

C+ characteristic

u/s

d/s

Air inflow Q

air

Air pocket (Vol )

Fig. 17.5. Schematic of air valve operation

Air valves