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The Challenge of Air Valves: A Selective Critical
Literature Review
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Leila Ramezani, S.M.ASCE 1; Bryan Karney, M.ASCE 2; and Ahmad Malekpour, A.M.ASCE 3
Abstract: One key alternative for removing, preventing, and effectively coping with the often vexing presence of air in water pipelines is the
combination air-vacuum valve. Despite their often effective role, such valves can be highly problematic if not well designed and maintained.
This paper critically reviews the current design, application, functionality, and simulation of air valves and the associated shortcomings, with a
primary focus on air/vacuum valves (AVVs). It is argued that the efficient number of air valves along undulating pipelines is yet to be fully
articulated. Air-valve simulations should expressively consider their dynamic behavior, the physical behavior of any air pockets that might
form below an air valve, and the varying character of the water surface at the air valve location. There is a pressing need for a comprehensive
and systematic study on the proper sizing and location of AVVs for the transient protection of systems. Overall, the efficient application of
AVVs requires broad research and development theoretically (i.e., their physical behavior and improved numerical simulations) and experimentally, as well as field studies to document their in situ dynamic behavior and operational efficiency. DOI: 10.1061/(ASCE)WR.19435452.0000530. © 2015 American Society of Civil Engineers.
Author keywords: Air management; Air pocket; Air valve; Head loss; Transient; Water pipe; Review.
Introduction
The control of air in water pipes is performed to decrease or prevent
the potentially devastating consequences of both mobile and entrapped air. Among air management strategies, the most popular is
the application of air release/vacuum valves. Air-release valves
(ARVs) remove any small-scale air that might occur, while air/
vacuum valves (AVVs) generally allow for large-scale removal/
admission of air during filling/draining scenarios. Moreover, AVVs
are often activated during transient conditions, such as pump failure, admitting air into pipes to counter subatmospheric conditions,
and removing the admitted air once pressure increases. Combination air valves (CAVs) integrate the functions of both ARVs and
AVVs. However, air valves inevitably play a complex role and
could expose the system to extreme transients and catastrophic failures, particularly if they fail, cease to function properly, or are
poorly sized and located.
Despite their widespread application, quantitative data regarding air valve efficiency and effectiveness are scant, and too little
attention has been paid to maintenance and operational issues.
Clearly, the proper use of air valves is possible only if such concerns are understood and appropriate measures taken to provide
efficient design criteria intended to eliminate, or at least reduce,
the severity of failures and misbehaviors. This paper argues that
1
Ph.D. Candidate, Civil Engineering Dept., Univ. of Toronto, 35 St.
George St., Toronto, ON, Canada M5S 1A4 (corresponding author).
E-mail: [email protected]
2
Professor, Civil Engineering Dept., Univ. of Toronto, 35 St. George St.,
Toronto, ON, Canada M5S 1A4. E-mail: [email protected]
3
Ph.D. Candidate, Civil Engineering Dept., Univ. of Toronto, 35 St.
George St., Toronto, ON, Canada M5S 1A4. E-mail: a.malekpour@mail
.utoronto.ca
Note. This manuscript was submitted on August 15, 2014; approved on
January 30, 2015; published online on March 18, 2015. Discussion period
open until August 18, 2015; separate discussions must be submitted for
individual papers. This paper is part of the Journal of Water Resources
Planning and Management, © ASCE, ISSN 0733-9496/04015017(11)/
$25.00.
© ASCE
improvement in the design, operation, and maintenance of air
valves can measurably enhance the performance of real-world
water-conveyance systems.
Current Design Practices of Air Valves
The design or selection of AVVs starts with decisions about their
size and positioning along the pipeline. Clearly, too few valves will
inadequately protect the system and too many represents, at the
very least, a waste of capital and operating resources. However,
the choice of appropriate size and location of an air valve is difficult
and problematic. Engineers often refer directly to manufacturers or
to practical recommendations provided in American Water Works
Association (AWWA) M51 guideline for selecting the required size
of air valves and for deciding about their location along the pipes, a
guideline that seeks to provide a basic understanding of the use and
application of air valves (AWWA 2001). Also, manufacturers’ catalogs provide some advice, but it is neither always straightforward
to understand nor clearly justified. In fact, AWWA M51 recommendations are those most often used to select the size, location, and
type of air valves, but even these have problematic features.
Sizing Air Release Valves
The first design parameter of an ARV is orifice size. According to
the current AWWA design guideline, the airflow rate, which determines the size of the ARV, is based on the 2% dissolved air in water
at standard pressure and temperature (AWWA 2001). This accounts
for an airflow rate of 2% of the fluid discharge in the system, assuming that each ARV is to release all the air dissolved in the fluid
at any one of the air valve stations. However, the actual dissolved
air in water varies with pressure and temperature, and this potential
amount is almost never released. That is, no particular section of
water pipes releases this entire amount of dissolved air owing to
the small pressure changes along the pipeline. This implies that
if the first ARV along a pipe releases this amount of air, no air will
be left for release by the second ARV. Therefore, this criterion often
04015017-1
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J. Water Resour. Plann. Manage.
Sizing Air Vacuum Valves for Filling/Draining
Sizing AVVs is a crucial consideration since, for a specified airflow,
valve size controls the pressure differential at which the air is exhausted. A minimum valve size is established by finding the size
for filling, which is usually less than the size for drainage. In the
initial filling of pipelines, the airflow rate through the AVVs should
be the same as the pipe filling rate (i.e., 0.3 m=s). For a draining
scenario, the rate at which air is admitted into the system should be
the same as that at which the pipeline is drained (i.e., 0.3–0.6 m=s).
In practice, pressure differentials of 13.79 and 34.47 kPa (2 and
5 psi) are considered for filling and draining, respectively (AWWA
2001). The 13.79 kPa (2 psi) differential is a threshold pressure that
maintains a low air speed in the valve orifice to prevent turbulence,
valve slam, and substantial pressure rises, which might lead to premature closure. The 34.47 kPa (5 psi) differential pressure for air
inflow is considered a safe pressure threshold to protect pipelines
and gasketed joints from damage due to the effect of vacuum
pressures.
Valves are rated according to the maximum pressure under
which they can operate as well as by orifice size (Thomas 2003).
Manufacturers provide their customers with sizing tables or performance curves with recommended operating pressures. Fig. 1 is a
performance graph, generally provided by a typical air valve manufacturer, for sizing AVVs based on the filling scenario. Evidently,
the orifice size of air valves is designed according to the airflow
rate in the system and the pressure differential across the valve.
Outflow Orifice Diameter (in)
Pressure Differential Across the Air Valve (kPa)
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overestimates the size and number of air valves along a pressurized
main where pressure is changing. McPherson (2009) and McPherson
and Haeckler (2012) proposed considering the reduction in
dissolved-air volume under operating pressures in order to design
smaller air valves. However, the cost effectiveness of this approach
requires a broader study. On the other hand, air in the gaseous phase
may enter the pipe from other sources (e.g., pumps and valves).
Hence, this uncertainty and variability in the amount of air
make the design criterion for sizes of ARVs questionable. Considering the uncertainty in the amount of air intrusion into the system,
the following questions arise: What airflow rates should be used as
design criteria for sizing ARVs? Or, more clearly, what size of ARV
efficiently removes small-scale air in pipes and enhances system
performance?
35.0
1
2
3
4
6
8 10
14
20
30.0
25.0
20.0
15.0
10.0
5.0
0.0
10
100
1000
10000
Air Outflow through the Large Orifice (scfm)
100000
Fig. 1. Performance graph for sizing AVV based on filling scenario
generally provided by a typical air valve manufacturer
© ASCE
Generally, in water distribution systems, the recommended outlet
diameter of AVVs for a filling scenario is 50–200 mm, while diameters of 1–5 mm are suggested for ARVs to release air under pressure during normal operating conditions (Stephenson 1997).
Furthermore, ARVs should be sized such that they prevent destructive transient pressures. Considering the airflow rate under
sonic conditions, Bianchi et al. (2007) studied experimentally
and theoretically the appropriate size of air valves during a filling
scenario that would avoid any transient pressures. The basic considerations for deriving these practical equations are the maximum
allowable overpressure and maximum desired filling velocity. It has
been proposed that the most suitable size for air valves under a
filling scenario is one in which air is released in a shorter time
while generating allowable overpressures. For anyone who has
not directly experienced it, it should be noted that the acoustic impact of a valve approaching sonic conditions can be excruciating.
Hence, sound should be a consideration in air valve design.
Sizing Air Vacuum Valves for Transient Conditions
As illustrated in the previous section, AVVs are largely sized by
considering filling/draining scenarios, both of which are largely
controllable events. Obviously, filling and draining rates can usually be controlled to prevent any consequence of air entrapped in
the system. However, AVVs can have a more crucial role in protecting the system against transient events resulting in vacuum conditions. Such a positive role of AVVs is obtained from published
experimental data confirming that if air is drawn into a pipe during
negative pressures, transient surges will be considerably damped
(Collins et al. 2012). Vacuum conditions occur in pipes as a result
of sudden transient events, such as power failures at pumps. There
can be significant consequences to water pipelines in the presence
of such negative pressures. For instance, the resulting negative
pressures cause flow disruption and they allow contamination to
enter into the pipe. Hence, it is more conservative to size and locate
AVVs by consideration of such events. In other words, the size
of inflow and outflow orifices of AVVs should be selected with
care if vacuum conditions are to be controlled efficiently.
AVVs can protect a system against such negative pressures if
they are well sized and located. Otherwise, they themselves will
induce secondary transient events, as illustrated later in the paper.
For instance, high outflow air valves exacerbate positive transient
pressures in systems with negligible amounts of entrained air (Lee
1999; Stephenson 1997). The importance of AVV characteristics
on transient pressure surges is shown by a case study of a large
twin pump line by Devine and Creasy (1997). A large outlet
may result in the rapid expulsion of air and consequent high pressures. However, at smaller outlets, the air cavity below the air valve
can cushion the deceleration of the reverse flow. This helps to control the secondary transient pressures (Lee 1999). However, if the
outlet size is too small, air compression occurs below the AVV. It
should be mentioned that air under pressure contains a huge
amount of energy and can potentially bring about explosive conditions. Hence, leaving pressurized air in the system should require
that the system be designed according to the strict ASME codes for
pressure vessels. In other words, discharging air too slowly or trying to use a cushioning effect of air will lead to a new set of design
codes, which represents a complication in water pipelines. It is usually advised to properly size AVVs to prevent such explosive conditions, and it is concluded that designing water pipelines based on
air vessel codes is rejected. Generally, AVVs with low outflow and
high inflow capacities have been effective at suppressing peak pressures and valve slamming as well as reducing negative pressures
caused by transient events (Espert et al. 2008; Lee and Leow 1999).
04015017-2
J. Water Resour. Plann. Manage.
J. Water Resour. Plann. Manage.
120
110
100
90
AVV
Pressure Head (m)
70
60
50
Air Chamber
40
Hmax with AVV, air chamber size=25 m3
Hmin with AVV, air chamber size=25 m3
Pipe Elev. (m)
Hmax without AVV,air chamber size=200 m3
Hmin without AVV,air chamber size=200 m3
Hmax without AVV,air chamber size=25 m3
Hmin without AVV,air chamber size=25m3
30
20
10
0
0
1000
2000
3000
4000
Distance (m)
5000
6000
Fig. 2. Effect of presence of AVV at high point on reducing air chamber size (87.5% reduction in chamber size)
© ASCE
AVV
a
-ΔH1
HP
V=0
(a)
2.2
1.0
2.0
DHmax/DH1
QP/Q0
0.9
1.8
0.8
1.6
0.7
1.4
0.6
1.2
0.5
1.0
0.4
0.8
0.6
0.3
0.4
0.2
0.2
0.1
0.0
QP/Q0
Properly sized and located AVVs can potentially serve as a
complementary surge control devices, especially in highly undulating pipelines in which large air vessels must be placed. In such
situations, placing AVVs in the system may dramatically reduce
the size of a large and expensive surge vessel. The effect of AVVs
on reducing the size of the main system surge control devices is
sometimes remarkable, as shown in Fig. 2. In this example, an
air chamber, located near the pump at the upstream end, is designed
for sudden power failure in both the presence and absence of AVVs
at the high point. As shown, the air chamber size, without considering the AVV at the high point, is 200 m3 , while the introduction
of an AVV at the high point reduces the air chamber size to 25 m3 .
That is, the presence of an AVV at the high point reduces the size of
the air chamber by 87.5%. This confirms that AVVs can contribute
greatly to reducing the cost of main surge protection devices in undulating pipelines.
Furthermore, there is a crucial area ratio between the air valve
inlet and outlet if these valves are to play the role of a water hammer
protection device (Zhu et al. 2006; Yang and Shi 2005). Of course,
the magnitude of severe pressure peaks following rapid air valve
closure also depends on the location of the AVV, the pipe configuration, and the flow characteristics. For instance, when a transient
event occurs upstream of the AVV, the severity of the secondary
transient pressure at the AVV location (Fig. 3) depends on the intercepted wave height (i.e., HP ) at the high point. In this figure,
ΔH1 is the reduced transient pressure wave induced upstream
of the AVV. QP =Q0 is the ratio of the reduced discharge after
the wave arrives at the high point. As shown, for higher elevations
of the high point, the intercepted wave height is reduced, QP =Q0 is
increased, and therefore the secondary transient pressure is higher
ΔHmax/ΔH1
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80
0.0
0
(b)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Hp/ΔH1
Fig. 3. Effect of intercepted wave height on secondary transient
pressures at AVV location (data from Ramezani and Karney 2013):
(a) system layout, sudden pressure drop at upstream valve; (b) effect
of intercepted wave height at high point (HP =ΔH 1 ) on secondary
transient pressures (ΔH max =ΔH 1 ) at AVV location
04015017-3
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J. Water Resour. Plann. Manage.
The second design parameter of air valves (i.e., ARVs, AVVs, or
CAVs) is their location and distribution along the pipeline. AWWA
recommends installing air valves at intervals along ascending,
descending, and horizontal lines (i.e., intervals of 1,500–2,500 ft)
and changes in grade as well as all the high points along the pipe.
However, this is at times a conservative approach, and it is seldom
specified how critical each location along a pipe profile is. Elevated
high points are naturally the most common places that ARVs or
AVVs are recommended in both North American (AWWA 2001)
and European guidelines (DVGW 2005). Characterizing critical
high points to install ARVs and AVVs or CAVs requires knowledge
about the potential of air accumulation at the high point and the
transient behavior of the system, respectively. However, there is
little published work regarding the influential parameters on air accumulation along pipelines or at high points in pressurized systems.
Moreover, little work has been done regarding the locations along
pipes where appropriate selection of AVVs is crucial for alleviating
transient induced pressures.
Placing ARVs at improper locations can be problematic rather
than helpful. Among the improper locations of ARV placement
are the high points running under siphon conditions. Obviously,
the existence of a permanent vacuum condition at a high point
(e.g., with a siphon) may restrict the application of ARVs at those
locations. According to catalogs of air valves, ARVs can potentially
operate under vacuum conditions admitting air into the system.
Hence, placing such valves at siphons where permanent vacuum
conditions are actually desirable may lead to flow disruption owing
to breakages of siphon flow. In such cases, the location of the high
point relative to the downstream boundary affects the magnitude of
the discharge reduction.
The consequence of placing ARVs at siphon locations is easily
illustrated. Fig. 4 shows a configuration in which placement of an
ARVat the high point, previously running under vacuum conditions
(i.e., a siphon), breaks the siphon flow. Obviously, siphon flow can
be calculated by writing the Bernoulli equation between Points 1
and 2 as well as 1 and S (Fig. 4), resulting in Eqs. (1) and (2),
respectively. However, at the high point, the existence of an
ARV that could activate during vacuum conditions may introduce
air into the system, increasing the negative pressure in pipeline to
the atmospheric pressure and reducing the design discharge. Here,
the governing equation is Eq. (2). The reduced discharge can be
calculated by considering the atmospheric pressure at the high
point and solving for flow velocity using Eq. (2). The pipe diameter
(m), pipe length (m), design discharge (m3 =s), friction factor, and
K 1 are 0.5, 2,000, 0.328, 0.017, 0.5, and 1, respectively. The
V 22
L V 22
V2
þf
þK 2
2g
D 2g
2g
ð1Þ
PS V 22
L V2
V2
þ
þ f 1−S 2 þ K 1 2
γ
2g
D 2g
2g
ð2Þ
Z1 − Z2 ¼
Z1 − ZS ¼
To prevent such a reduction in the design flow at siphons,
AWWA (2001) recommends placing a vacuum check device at
the outlet of the ARV. Although manufacturers claim to provide
vacuum check valves, their function and practical efficiency in
siphon conditions remains questionable. Even if these devices
are effective at preventing siphon breaks, any malfunction of such
devices could result in a reduction in the design discharge and the
hydraulic efficiency of the pipeline. Hence, placing ARVs at such
locations requires a detailed study of such potential effects and consideration of the associated risks.
1.00
0.95
0.90
Q/Qd
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Location of Air Valves
change in the relative design discharge (Q=Qd ) with the location
of the high point is shown in Fig. 5, in which Q is the reduced
discharge and Qd is the design discharge. As shown, the flow reduction is affected by the location of the high point relative to the
downstream boundary. The closer the high point is to the downstream boundary, the more flow reduction occurs by admitting
air into the system. Such effects are more pronounced in shorter
pipelines. To illustrate, Fig. 6 is obtained by considering a constant
pipe length upstream of the high point in Fig. 4 and altering the
downstream pipe length. As seen in Fig. 6, as the downstream pipe
length decreases, the discharge has lower values, implying more
reduction in design discharge
0.85
0.80
0.75
0.70
0.2
0.3
0.4
LS-2 /L1-2
0.5
0.6
Fig. 5. Reduction in design flow from breaking siphon condition by
ARV at high point (Q = reduced discharge; Qd = design discharge)
1.0
0.9
Q/Qd
(Ramezani and Karney 2013). However, a systematic study of the
effect of the size and location of AVVs on secondary transient pressures has yet to be published. That is, current studies are performed
within a single system configuration, but they have not explored the
locations where the proper sizing of AVVs are most or least crucial
in transient events.
0.8
0.7
1
0.6
Qd
L1-S
0.2
2
S
0.8
1.1
1.4
1.7
LS-2 /L1-S
LS-2
Fig. 4. Breaking siphon condition as a result of admitting air into
system by ARV at high point
© ASCE
0.5
Fig. 6. Ratio of reduced discharge to design flow; design discharge
reduces owing to breaking of siphon condition by ARV at high point
as in Fig. 4 (Q = reduced discharge; Qd = design discharge)
04015017-4
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J. Water Resour. Plann. Manage.
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Type of Air Valve
The third design parameter is the type of air valve. After selecting
the appropriate size and location of air valves, the designer should
select among a variety of types of air valve supplied to the market
by several manufacturers. While applying the same hydraulic and
mechanical principles, these valves are different in physical properties (i.e., material, shape, float arrangement) and performance
criteria with optional add-on features such as surge relief or antislam devices (McPherson 2009). For example, while AWWA
recommends air valves made of cast iron, all manufacturers offer
different material bodies, such as ductile iron, stainless steel, cast
steel, and cast bronze, as well as polypropylene and reinforced nylon. Obviously, the capital cost of these valves differs but the maintenance cost is unknown and the relative merits are often unclear
(Radulj 2007). It is seldom obvious to designers what characteristic
parameter of these valves (e.g., working pressures, antislam features, or fewer reported failures) gives them priority over others
in a specific water main. In other words, the influence of such
characteristics on system performance is not yet fully reported.
Furthermore, there is a gap in the literature regarding whether these
different types of air valve may affect the overall and proper number and size of air valves in a given system. Overall, the priority of
one type of air valve over others is still contested.
Operating Problems of Air/Vacuum Valves during
Air Release
The main function of AVVs is to expel excess air from the system
when it is experiencing high pressure or to admit air into the pipe
under subatmospheric conditions (Wylie and Streeter 1993). However, analysis of the transient behavior of systems protected by
AVVs by developing and applying either numerical models (Lee
and Leow 1999) or simplified water hammer equations (Lingireddy
et al. 2004; Li et al. 2009) reveals that rapid expulsion of air through
air valves causes severe transient pressures referred to as secondary
transient pressures.
Secondary Transient Pressure during Air Release
A secondary transient event is caused by rapid air expulsion and
subsequent rapid valve closure. When water strikes a closed air
valve, it moves at the same rate as the exhaust air, leading to severe
flow deceleration and high pressure surges inside the valve. This is
most common both during filling scenarios and after suppression of
vacuum conditions while the incoming air from the AVVs is being
released from the pipe. Secondary transient pressures have been
studied experimentally during filling scenarios in a simple horizontal (Zhou et al. 2002a, b; Martin and Lee 2012) or a more complex
(De Martino et al. 2008) piping system. Peak pressures up to 2.7
and 15 times the upstream absolute head were observed at values of
d=D < 0.086 and d=D ¼ 0.2 (where d is the air valve diameter and
D is the pipe diameter), respectively (De Martino et al. 2008; Zhou
et al. 2002a, b). In addition, Martin and Lee (2012) observed that
maximum pressures at the air valve orifice occur at d=D ¼ 0.18
and are affected by both the orifice size and the ratio of the absolute
upstream reservoir pressure to the initial absolute air pressure
near the valve in a simple horizontal reservoir-pipe system. They
concluded that initial air volume at the valve is less influential on
peak pressure compared with the two aforementioned factors. Also,
numerical results of secondary transients have been presented
during rapid shutdown and subsequent start-up of a pumping line
for an undulating pipe containing an air valve at the high point
(Lingireddy et al. 2004). A simple predictive formula is also
© ASCE
available for calculating the maximum pressure based on AllieviJoukowski results (De Martino et al. 2008). Secondary transients
can lead to air valve breakage. For instance, such frequent high
pressures have caused breakage in the inlet or outlet sections of
air valves in Pinellas County’s wastewater force main (Li et al.
2009).
Other conditions for creating secondary transients are met when
these valves do not operate well and premature closure occurs
(Lee and Leow 1999; Lingireddy et al. 2004; Li et al. 2009). For
instance, conventional air valves can close prematurely at surprisingly slight pressure changes (Thomas 2003), and it is difficult to
limit pipeline pressures to below-closure thresholds.
Pressure Oscillation Patterns during Air Release
Depending on the size of air release orifice, three types of pressure
oscillation patterns occur in entrapped air pockets during the rapid
filling of a pipeline (Zhou et al. 2002a), and there are two phases
in each oscillation pattern: a low-frequency pressure oscillation
(i.e., rigid column behavior) followed by a sudden pressure peak
(i.e., water hammer behavior) when the water column first reaches
the orifice (De Martino et al. 2008; Zhou et al. 2002a). The duration
of the first phase is reduced with an increase in the driving head and
orifice size and increases with air pocket size (De Martino
et al. 2008).
For small orifice sizes, an air cushioning effect is dominant and
the period of pressure oscillation is long. For large orifice sizes, the
air cushioning effect is negligible and the water hammer effect is
dominant. In this case, there is no long-period pressure oscillation
pattern and just a water hammer impact pressure is present. For
intermediate orifice sizes, the air cushioning effect begins to diminish and the water hammer pressure becomes more pronounced as
the orifice size grows. Therefore, there is both a long-period oscillation followed by a short-period oscillation pattern. Experiments
show that the peak pressure occurs before the final air residual is
removed from the pipe owing to air pocket compression (De
Martino et al. 2008; Martin and Lee 2000).
Influential Parameters on Secondary Transient Pressures
As illustrated by a simplified analytical equation by Lingireddy
et al. (2004), the magnitude of secondary pressures depends on pipe
and air valve characteristics and the pressure inside the valve
immediately before the final air release. Also, experimental data
by Zhou et al. (2002b) and De Martino et al. (2008) show that
the upstream driving head, air pocket volume, and orifice size
of air valves are the most influential factors on maximum pressure.
To illustrate, the pressure peak increases with the upstream driving
head and outlet orifice diameter. The maximum pressure occurs at a
certain ratio of orifice diameter to pipe diameter (i.e., d=D). The
value ascribed to this ratio varies among researchers. Martin and
Lee (2000) reported a value of d=D ¼ 0.14 and later observed peak
pressures at d=D ¼ 0.18 (Marin and Lee 2012), while Zhou et al.
(2002b) reported a value of d=D ¼ 0.2 where the maximum pressure peak occurred. This difference may be due to different conditions under study. For example, different experimental setups,
such as pipe length and diameter, water column length, orifice
diameter, and driving head, are factors that were different in each
study (Zhou et al. 2002b; De Martino et al. 2008). After reaching a
certain orifice size, the peak pressure decreases as the orifice size
increases (Martin and Lee 2000; Zhou et al. 2002a, b, 2004; Martin
and Lee 2012).
Overall, since the d=D criteria for the occurrence of peak pressure are different in each study, these data cannot be generalized.
Furthermore, this phenomenon is given little physical interpretation
in the literature. For example, the literature contains no discussion
as to why an air pocket’s cushioning effect is less influential at
04015017-5
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J. Water Resour. Plann. Manage.
lower driving heads and smaller air release rates, although peak
pressure generally decreases with air pocket volume. In addition,
almost all of these criteria are based on assuming a rigid water column theory without considering friction and elastic effects. Hence,
the effects of water elasticity on such criteria and pressure oscillation patterns are yet to be investigated.
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Real-Time Dynamic Behavior of Air/Vacuum Valves
The dynamic behavior of air valves affects the system’s transient
response during air release from the pipe. To illustrate, the time of
closing and opening of an air valve, the amount of residual air below the air valve before it closes, and the amount of time it takes to
close are defined as the dynamic characteristics of air valves. Such
parameters are difficult to measure in the laboratory and, therefore,
involve some uncertainty. For instance, the discharge coefficient of
air valves is determined through standard published tables (AWWA
2001), and there is considerable uncertainty in these values. Such
dynamic behavior is different for each type of air valve (i.e., its float
shape, weight, and size affect the closing behavior of the valve).
Pipe layout, the severity of transient, and type of air valve installation contribute to this as well.
The dynamic behavior of air valves is rarely systematically
studied or modified. The main reason for this lack of attention
is a poor understanding of the dynamics of air valves owing to scant
large-scale experimental and field data. Also, the complexity of
two-phase flow characterization might explain the scarcity of publications on this topic. Although the effect of such dynamic behavior is not comprehensively understood, some experimental works
have been published on this topic. It is noteworthy that scale effects
have not been explored in experimental studies, and therefore, they
cannot be generalized for all types of air valves. Yet, these explorations have important implications for the dynamic behavior of air
valves.
Recent work attempts to provide technical information and to
understand the behavior of air valves having different geometrical,
mechanical, hydraulic, and dynamic characteristics. Because sizing
charts provided by manufacturers are unreliable, initially the characteristic curves of air valves must be tested and compared with the
characteristics provided by manufacturers (Lucca et al. 2010). This
can lead to more realistic simulations of air valve behavior in pipelines. For example, Bergant et al. (2012) determined the dynamic
performance of float-type air valves (e.g., metal ball or plastic cylinder floats) by measuring their response to flow acceleration and
deceleration in a large-scale pipeline for three scenarios of pump
start-up and shutdown and pipe rupture. They presented the relationship between flow rate and pressure drop across an air valve for
several float positions.
Air Release or Admission through Air Valves Considering
Dynamic Behavior
Generally, air release through AVVs is divided into two stages
(Bergant et al. 2012). In the first stage, water and air accelerate
toward the air valve until almost all the air is removed from the pipe
(i.e., initial event). In the second stage, residual air compresses until
high pressure occurs and the water column stops and reverses
(i.e., air compression event). The flow velocity at the end of the
initial event is referred to as the terminal velocity, which is the
maximum value of the water velocity. However, according to
real-time experiments on different models of air valves, some air
valves close before air is completely exhausted from the pipe
(Bergant et al. 2012), while others close only after a certain amount
of water is discharged. The volume of residual air depends on the
dynamic behavior of the air valves (e.g., closing times), the pipe
layout, the severity of the transient, and the air–water mixing
© ASCE
process. Also, a longer closing time means less residual air.
Furthermore, the higher the impacting water column, the smaller
the residual air (Carlos et al. 2011).
Following valve closure, the residual air continues to compress
until the water column is arrested and then reverses. A maximum
pressure spike occurs at the time of flow reversal (Bergant et al.
2012). The residual air is an influential parameter on transient pressures, especially at start-up events. Therefore, the most dangerous
situations are those in which small uncontrolled volumes of air remain inside the pipe and cannot be removed properly through an air
valve (Bergant et al. 2012; Carlos et al. 2011; Arregui et al. 2003).
Results show that the smaller the residual air below the air valves,
the higher the maximum pressure. Moreover, experimental data
show that pressure peaks are inversely proportional to the closure
time of air valves up to a certain limit, after which peak pressure is
not dependent on closure time (Arregui et al. 2003). However,
Carlos et al. (2011) showed that in real systems, the closing time
of air valves is not always a significant factor for the pressure peak
magnitude. Additionally, peak pressures are inversely proportional
to moving water column inertia and directly proportional to terminal velocity (i.e., velocity of water at the time of air valve closure)
(Arregui et al. 2003).
Overall, the effectiveness of air valves for surge protection depends on several factors, such as system configuration, the physical
properties of the pipeline and fluid, the characteristics of the air
valves, and the air distribution in the system (Lee 1999). However,
the complexity of the characterization of two-phase flows and air
valves has limited the associated theoretical works. Hence, researchers have investigated the dynamic performance of air valves
and their influence on transient pressures by performing large-scale
experiments (Bergant et al. 2012; Carlos et al. 2011; Arregui et al.
2003; Cabrera et al. 2003).
According to such experimental results, air valves do not respond as quickly to vacuum conditions as has traditionally been
expected in the literature (Bergant et al. 2012; Cabrera et al. 2003).
That is, air valve opening occurs with a small delay (on the order of
20 ms). This makes the occurrence of cavitation inevitable. But
after a small delay in the admission of air, the pressure surges are
strongly reduced. There is also a time delay between the air valve
closure and pressure rise due to cavity collapse (Bergant et al.
2012). Such a delay could occur if the valve body stuck to the valve
seat. Yet these valves have proven to be beneficial as surge control
devices if properly sized and located. Experimental results show
that the maximum air pressure at the air valve location depends
strongly on air valve size, and both larger and smaller sizes of air
valves exacerbate transient pressures. Usually, large sizes of air
valves result in high pressures as a result of the high velocity of
the adjacent water column. And smaller air valve sizes give rise
to peak pressures as a result of excessive compression of air pockets
below air valves (Cabrera et al. 2003). Hence, this earlier work confirms that the choice of an appropriate air valve size should be made
if excessive pressure surges are to be avoided. In addition, experimental data imply that the discharge coefficient of air valves plays
an important role in peak pressures. According to experiments simulating the expulsion of air through several commercial air valves,
the smaller the discharge coefficient, the smaller the impact velocity of water arriving at the air valve at the time of air valve closure
and the lower the resulting peak pressure. Peak pressures are
greatly affected by the amount of residual air below the valve.
Small air residuals in the pipe can be dangerous and burst the pipe.
The friction factor and air valve outlet coefficient are other key
parameters since they affect the impact velocity of water. For instance, a smaller air valve outlet coefficient controls the terminal
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velocity of water when it hits the closed air valve and then reduces
the corresponding peak pressure (Carlos et al. 2011).
Therefore, according to the available experimental and theoretical work, the effectiveness of AVVs in surge protection depends on
factors such as the size and discharge coefficient of the air valves.
Water hammer protection of air valves is mostly dependent upon
the optimal value of the outlet to inlet ratio. Certainly this depends
on system configuration (Zhu et al. 2006). A case study of a water
supply in Riyadh, Campbell (Lee and Leow 1999) showed that
nonreturn air valves cushioned the impact of slams from negative
surges, allowing a substantial reduction in the size of surge vessels.
However, what is not obvious is the degree to which the installation
of air valves might protect the system against vacuum conditions if
they are installed in pipelines as the main surge control devices or
supplementary surge protection measures. Furthermore, the uncertainties associated with the design parameters of air valves (e.g., discharge coefficient or even their basic functioning) could affect the
results. But such effects have not been considered in the published
theoretical works.
Air Valve Chattering due to Secondary Transient Pressures
While air is released through the air valve, it compresses and accelerates toward the air valve. Simultaneously, the adjacent water column is accelerating. High pressure peaks can result either from the
compression of air pockets or during air release when the accelerating water column impacts the closed air valve. The reduction of
velocity in a water column can produce secondary transient pressures on the order of 1–10 bar or higher, and reflections of such
pressure waves can potentially cause cavitation in the pipe. Such
transient events can cause air valves to repeatedly open and close
with high frequency, a phenomenon called air valve chattering
(Bergant et al. 2012). The water hammer pressure and the pressure
due to air pocket compression creates high pressure peaks, and air
valve failure can lead to column separation along the pipeline
(Bergant et al. 2004). Obviously, the location of air valves relative
to the hydraulic grade line (HGL) and the downstream boundary
(e.g., a reservoir) affects the frequency of chattering. That is, for
any vertical location of the high point (i.e., air valve location),
if the total pipe length downstream of the air valve is short, then
the wave round trip is small. In these situations, less air enters the
pipe through the air valve to prevent vacuum conditions, and less
time is required to discharge the previously admitted air and to
create the secondary transient. Hence, chattering occurs with more
frequency at the air valve location. Such issues should be considered in the design stage of air valves.
Overall, operational problems are inevitable, and proper pipeline venting can take place only if AVVs are sized and located appropriately throughout a system and if they resume operation
during transient induced pressure spikes, and these are subjects that
require more systematic research. And since water systems are no
better than their weakest component, a more rational approach to
the design and selection of air valves is urgently needed.
Maintenance Problems
Air valves require at least annual maintenance and inspection for
damage. In one European guideline, PED 97/23/EC (European
Commission 1997), AVVs are considered safety valves, and therefore their maintenance is emphasized. Regular maintenance and
function testing are also advised in the German guideline (DVGW
2005). However, due to the relative inaccessibility of most AVV
locations in vaults requiring confined-space-access procedures,
the valves are often left exposed to weathering, physical damage,
and wear, which eventually render them nonoperational. Also,
buried valves, located below manholes to improve accessibility,
have the potential to become inaccessible due to flooding or silting
of the utility vaults below the manholes (Fig. 7). For example, an
assessment of the functionality of air valves existing over 30 years
in the city of Houston water transmission main revealed that half of
the air valves were not functioning (the total number of air valves
was 31) and half of the buried air valves were inaccessible or had
leaky gate valves and could not be tested for damage (Gregory
et al. 2006).
The associated cost of flooded chambers [Fig. 7(a)] adds to the
maintenance cost of air valves. Also, contaminated water might
enter the pipes through leaking air valves during a negative pressure
event, creating health risks (Besner et al. 2010). Consequently, air
valves add to the maintenance cost of a system if they are not well
used and maintained. Fig. 7 shows evidence of poorly maintained
air valves, which has led to nonoperational valves.
Avoiding Operational and Maintenance Problems
The potential of air valves to induce secondary transient events in
pipes has provoked Li et al. (2009) to theoretically investigate a
modern configuration of air valves: installing an air-throttling orifice at the air valve outlet, an antislam device, and an air accumulator that reduce the flow velocity at the valve seat and the resulting
secondary pressures. Air-throttling orifices prevent all the air from
Fig. 7. Flooded and poorly maintained air valves [(a) reprinted with permission from Besner et al. (2010); (b) reprinted with permission from
Escarameia (2005), Copyright © HR Wallingford 2005]
© ASCE
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being expelled, and therefore, a portion is trapped in the valve
chamber. The greater the air accumulation, the more the pressures
are dampened. However, this raises concerns regarding pressure
spikes that can be created by such trapped air pockets during sudden pump start-up. Although this method was tested under two scenarios of shutdown and the following start-up, the start-up period
(t ¼ 300 s) was 60 times longer than the shutdown duration
(t ¼ 5 s), leading to a less severe induced transient. Obviously, care
should be taken to consider sudden start-up cases, resulting in
severe transients, while designing the air accumulator capacity
in the valve. Although the effects were theoretically investigated
through a case study, the real-world practical efficiency of these
valves has not yet been published.
Another approach to minimizing operational problems caused
by secondary transient pressures at AVVs is the use of standpipes
below the AVVs. Here, the main idea is that the pipe diameter is
larger than the standpipe, and consequently, a water hammer wave
will be attenuated when it reaches the main pipe. Properly sized
standpipes can suppress transient pressure caused by air valve
slamming. Generally, parameters affecting a water hammer are
the standpipe length and diameter, the ratio of pipe to standpipe
diameter, and air valve size, as well as the relative residual air
volume with respect to standpipe volume in the standpipe when
the valve is closed (Stephenson 1997). Thus, the available numerical studies indicate that the sizing of standpipes should be performed with care. However, their effectiveness in real systems is
still questionable and has yet to be published.
Additionally, automatic air valves, especially those applied to
wastewater systems, are uncertain and unreliable since they require
regular maintenance, and the possible malfunction of these valves
can create high pressures that endanger pipelines (Tchobanoglous
and Metcalf 1981). However, manufacturers claim that some of
the features of modern automatic air valves allow them to function
efficiently while requiring simple and effective maintenance
(Zloczower 2010). Among such features are conical bodies with
a large midriff that encompasses a larger initial air pocket at the
valve body, causing the air pocket to compress at high pressures
and resulting in a constant water level inside the valve, preventing
the sealing mechanism from clogging and leakage. Also, outwardslanting valve walls and a funnel-shaped bottom prevent the accumulation of grease by redirecting it to the force main. Furthermore,
the previous complex and problematic sealing mechanism with internal levers, hinges, and pins is substituted with a freely moving
float and simple rolling seal mechanism, which provides a larger air
release orifice and works even at low pressures [20.68 kPa (3 psi),
2 m]. More importantly, a light body, nonslam accessories, and a
hydraulically piloted diaphragm instead of a float that can potentially eliminate local surges due to air valves are embedded in these
valves (Zloczower 2010). Of course, the dynamic behavior of these
valves has not yet been studied, and their practical efficiency has
not yet been published. Also, in wastewater systems, such valves
may malfunction as a result of clogging due to floating dirt and
grease (Tukker et al. 2013). Therefore, the idea of little maintenance as claimed by manufacturers can be rejected.
Of course, the most common recommendation in the literature
to avoid secondary pressures is to use smaller outflow sizes of
air valves (Lingireddy et al. 2004; Zhou et al. 2002b). Such findings
in the literature emphasize that each specific system should be
analyzed in order to select an appropriate outflow air valve size.
However, there is a gap in the literature regarding the effect of
AVV location on its sizing. In other words, rules of thumb for selecting an appropriate outflow size of an AVV according to its location are not yet available. Also, there are few studies concerning
either the priority of each of the discussed strategies over the others
© ASCE
or of the best possible or practical approach to avoid operational
problems arising from secondary transient pressures at AVVs.
Therefore, AVVs and ARVs, though designed to reduce the devastating problems created by vacuum conditions and the presence
of air pockets in pipes, also add to the operating and maintenance
problems in water systems. To designers, reducing the related
operating and maintenance problems justifies eliminating even a
single air valve from the system. In addition, the German guideline
(DVGW 2005) encourages restricted use of AVVs in drinking
water pipelines due to the possible intrusion of contamination
and the problems that the sucked-in air, which is possibly transported downstream of the AVV, can create in pipelines. But how
small a number is reasonable is still a key design question.
Certainly, no attention has been paid to such a strategy in the literature, and the great concern is reduced system performance resulting from the removal of a specific air valve along a pipe. This leaves
system owners wondering how effective these air valves are at improving system performance. If an air valve in a specific location of
the pipe profile has little effect on system performance, avoiding it
will have the potential to reduce such operation and maintenance
issues.
Basically, reviewing and evaluating the current design criteria of
air valves (e.g., size, location) could play an important role in highlighting the potential of any possible changes in the design of air
valves along a pipeline. Such analysis would reveal critical locations for installing air valves and allow one to explore how system
performance changes if an air valve, or a series of air valves, fails to
operate. Consequently, the potential to avoid installing a number of
air valves will be realized. This provides a framework for decision
makers to determine the reasonable size, type, and location of air
valves according to the desired system performance. Moreover, to
better assess the performance of air valves and their location and
sizing criteria, supervisory control and data acquisition (SCADA)
systems should monitor air valve open/close positions and differential pressures (McPherson 2009). Such monitoring may facilitate
exploration of the location of the most effective air valves. Further,
it would help in identifying those air valves that require regular
maintenance. Consequently, the aforementioned operational and
maintenance problems are reduced through the elimination of
unnecessary air valves. Overall, if air valves are to be neglected
(Fig. 7) and if it is proven that they have little effect on system
functionality, why not avoid them in the design stage?
Certainly, such an evaluation requires an understanding of the
consequences associated with air intrusion into a system, air accumulation at high points, vacuum conditions, and the absence or
malfunction of air valves, as well as a subsequent consideration
of system performance, all of which still requires in-depth research.
For example, to evaluate current size and location criteria of ARVs,
it is essential to know the amount of air accumulation at locations
along the pipe and the associated consequences (i.e., head loss).
According to the available experimental data, head loss created
by air pockets is a function of airflow rate, pipe slope, and water
velocity (Pothof and Clemens 2010; Lubbers 2007; Lubbers and
Clemens 2005). However, the accumulated air pocket volume
and the relationship between air pocket volume and head loss
are not available for pressurized lines. Also, experimental observations have indicated that air accumulation depends on airflow rate,
but the associated relationship is not reported in the literature.
Therefore, the lack of such data in the literature limits the evaluation of design criteria for ARVs. Obviously, the reported results
could be more effective for such an evaluation if more experimental
data (e.g., volume of accumulated air at high points, relationship
between air pocket volume and head loss) were recorded. On
the other hand, most of these experimental works were performed
04015017-8
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Modeling air valve boundary conditions may be handled within the
usual framework of the characteristics method. Complete equations
used for air valve boundary simulation are presented in Wylie and
Streeter (1993). The mass flow rate of air through a valve depends
on the values of atmospheric absolute pressure, p0 , and absolute
temperature, T 0 , outside the pipe, as well as the absolute temperature, T, and pressure, p, within the pipe. Four conditions—air
inflow and outflow through an air valve at subsonic and sonic
levels—are considered. Also, four assumptions are made in the analytical equations for air valve boundary conditions (Wylie and
Streeter 1993): (1) isentropic flow is assumed for airflow through
the air valve; (2) the air mass within the pipe is assumed to obey
the isothermal law; (3) the air admitted into the pipe is assumed
to stay near the valve; and (4) the water surface elevation is assumed
to remain constant, and the volume of air is considered small compared with the liquid volume of pipeline reach.
However, in practice, these four assumptions for air valve boundary conditions cannot be universally met. For example, if an air
valve is placed along a sloping pipe, the assumption that the air will
remain close to the air valves cannot be satisfied unless a stand pipe is
used and the admitted air volume is small enough to stay in the standpipe. Otherwise, the pipe should be designed so that it contains high
points (Zhu et al. 2006). If none of these situations apply, considering
the air transport mechanism is essential for an accurate numerical
simulation of air valves. Also, the constant water surface at the
air valve location is an assumption whose effect on the transient
analysis of the system and the final reverse flow should be examined.
The effect of assuming a constant water surface at an AVV location on the transient analysis of a system is remarkable. To illustrate, when a power failure occurs, the AVV at the high point starts
to operate, admitting air into the system to prevent column separation. During deceleration of the forward flow downstream of the
AVV, the water level drops at the high point. After forward flow
stoppage, reverse flow accelerates toward the AVV and the water
level at the high point rises. If the effect of the moving boundary at
the AVV location is ignored, the forward flow deceleration rate and
the reverse flow acceleration rate are underestimated, which means
that the computed time of cavity growth and collapse at the high
point will be inaccurate.
Furthermore, in situations where the high point is higher than
the downstream reservoir head (Fig. 8), neglecting the falling water
level at the high point will prevent reverse flow occurrence owing to
the negative reverse driving head. An example is shown in Fig. 8.
The inlet and outlet diameters of the AVV (m) and the inlet and
outlet coefficients of the AVV are 0.2, 0.2, 0.6, and 0.6, respectively. Consequently, the numerical solution based on such a constant water level assumption results in a permanently growing
cavity volume at the high point (Fig. 9). Hence, in such conditions,
obtaining meaningful numerical simulation results depends on consideration of the moving boundary conditions at the AVV location.
© ASCE
Elev. 57 m
85.19 m
L=5000 m
L=3000 m
L=3000 m
3
Q0=1 m /s
D=1 m
f=0.017
Elev. 0 m
Elev. 40 m
Fig. 8. Layout of a pumpline containing a high point higher than the
downstream reservoir head
300
1.2
H
Vair
Qout
250
1
200
0.8
150
0.6
100
0.4
50
0.2
0
0
200
t (s)
Qout (m3/s)
Numerical Modeling of Air Valves: A Critique
AVV @ Elev. 60 m
H (m) Or Vair (m3)
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under the operating conditions of sewer systems (i.e., low ranges of
pressure: 0–3 bar). Hence, the data could be more reliably applied
to water transmission systems if the experiments were performed
under higher pressurized conditions and if scale effects were provided. Consequently, more research should be done on this subject
if evaluation of design parameters of ARVs is desirable. Also, the
evaluation of design criteria for AVVs requires a systematic study
of the transient response of water systems in the presence of AVVs.
Of course, the lack of such studies is an important gap in the
literature.
0
400
Fig. 9. Variations in head (H) and air volume (V air ) at high point and
water discharge (Qout ) at downstream limb of high point in Fig. 8
Overall, in the literature, less attention has been paid to such
shortcomings in the numerical simulation of air valves. Such
numerical issues and the previously discussed dynamic behavior
of air valves imply that further research and improvement are still
required in the field of air valve numerical simulation. Obviously,
real-world data acquisition would be useful in the development of
numerical models and in the validation of the numerical data.
Summary and Conclusions
In the literature, one air management strategy involves the application of air control devices such as air valves (i.e., air release
valves and air/vacuum valves). This paper presents a critical review
of the use and application of air valves. Areas of improvement in
the literature are discussed and the gaps examined. Of course, the
main focus of the paper is on the application of AVVs. Furthermore, design guidelines and operating and maintenance issues,
as well as the current alternatives for preventing such problems,
are discussed. It is concluded that despite the effective role of
air valves in air management, there are very little data about their
frequency and efficiency of functioning, which is necessary information for better sizing and positioning and a more efficient application of air valves.
Although there are several case-specific studies on sizing air
valves based on transient conditions, a comprehensive and systematic study on air valve sizing has yet to be published. There remain
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gaps in the literature concerning proper location and, consequently,
the efficient number of air valves required along an undulating pipeline. On the other hand, the available limited experimental studies on
dynamic behavior of air valves are presented. Further, the need to
modify the numerical modeling of air valves based on such dynamic
behaviors is highlighted. Obviously, such considerations may influence the proper selection of air valve size, number, and location.
Overall, considering the operational and maintenance issues related to air valves, efficient application of such devices requires
more broad-based research and development both in a theoretical
context (e.g., understanding air valve physical behavior and
modifying air valve numerical simulations) and in experimental
or field studies (e.g., understanding air valve dynamic behavior
and operational efficiency). Obviously, the experimental or field data
can serve as a good support for a proper and more accurate simulation of air valve behavior in water pipelines. Such knowledge gaps
and recommendations for further research are discussed in the paper,
with a view toward a more efficient application of air valves.
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