Part One, Section 19.
Air Valves WATER DESIGN GUIDELINES
19. Air Valves.
a. General.
1) WSSC defines air valves as referenced in this manual as follows:
a) The 2-inch air valve is a universal type, combination air/vacuum valve and air release valve.
For more information, see Specifications.
b) The 3-inch and larger combination air and vacuum valve incorporates the functions of two (2)
valves. The first valve connects directly to the pipeline and is an air/vacuum valve. The
second is an air release valve that is connected on the side of the air/vacuum valve. For more
information, see Specifications.
2) Generally, air valves or combination air and vacuum valves include two different types of valves;
a large orifice air/vacuum valve and a small orifice air release valve. Air/vacuum valves allow
the escape of large quantities of entrapped air during line filling and permit air to enter during line
draining. In both cases, relatively small pressure differentials are produced across the valves. Air
release valves also allow the escape of accumulated air at relatively high pressures experienced
under normal pipeline operating conditions.
b. Specification Requirements.
1) Size of the air valve and orifice: see Specifications.
2) Combination air and vacuum valve: see Specifications. Show the following on the drawings:
a) Size of both the air/vacuum valve and air release valve, including the orifice size of the air
release valve.
b) Model numbers of the air/vacuum valve and air release valve.
c) Type of flanged joint for the inlet of the air/vacuum valve and the gate valve, either ANSI
B16.1, Class 125 or 250 flanges.
c. Design Requirements for 16-inch and Larger Pipelines.
1) Determine the appropriate location for all air valves or combination air and vacuum valves, see
Location of the Air Valve or Combination Air and Vacuum Valve, in this section.
2) Determine the size air valves or combination air and vacuum valves as follows:
a) Pipelines 16-inch to 24-inch. Use a standard 2-inch air valve, see Standard Detail W/2.0.
Considerations should be given to assessing possible cases or operating conditions that may call
for different valve and orifice sizes.
b) Pipelines larger than 24-inch. Submit design and computations for sizing of combination air
and vacuum valves. Indicate the type and size of valves on the drawings, and refer to Standard
Details W/10.0, W/10.1 and W/10.2. Also see the Specifications and Drawing Requirements in
this section.
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Part One, Section 19. Air Valves WATER DESIGN GUIDELINES
c) Check the type and size of air/vacuum valves and air release valves against the manufacturer's
specifications to ensure consistency between valve type, size, model number and applicable
operating pressure range.
d. Sizing of Air/Vacuum Valves.
1) The size of the air/vacuum valve is based on manufacturer's valve performance curves.
Determine the largest valve size required by considering both normal operating conditions and
catastrophic conditions. Air/vacuum valve sizes need not be the same for all locations.
2) Consider air/vacuum valve sizing for slow venting of air during line filling. The rate of air
venting should equal the rate of line filling to minimize surge effects, which tend to generate high
in-line pressures.
3) Consider air/vacuum valve sizing for admission of air into the pipeline to compensate for negative
pressures generated under normal operating conditions, thereby maintaining a balanced
atmospheric pressure in the line. Evaluate the extent of negative pressure that a pipeline can
withstand under buried conditions using acceptable calculations and structural design. Use
manufacturer's recommendations for appropriate valve sizing.
4) Negative pressures may develop in the pipeline under normal operation or catastrophic
conditions. Under normal operations, the design of air/vacuum valves shall consider the
following situations:
a) During draining of a pipeline, the blowoff connection will be opened to drain the pipeline. Size
the air/vacuum valves to admit air into the pipeline at the maximum, instantaneous rate of water
discharging through the blowoff connection. Depending on the pipeline profile configuration
and the relative locations of air valves and blowoff connections, one air/vacuum valve may
respond to more than one blowoff. In such cases, consider operating multiple blowoffs.
b) During closure of mainline valves, negative pressure may develop in the pipeline, downstream
of the mainline valve. The air/vacuum valves can be sized to admit air at the flow rates under
which closure of mainline valves can occur. For a large transmission pipeline, a transient water
hammer analysis may be required to determine the appropriate size for the air valve or other
methods of transient control.
5) Consider the possibilities of the occurrence of catastrophic conditions, such as water column
separation due to hydraulic transient conditions or line breakage at a low point. Under such
conditions, it is necessary to estimate the rate at which an internal vacuum may occur. Large size
air/vacuum valves may be used as a method to relieve the vacuum pressure. It may be
impractical to size vacuum valves due to an excessively high rate of vacuum occurrence, such as
caused by gravity drain of a downward sloping pipeline under high head. Therefore, make
reasonable assumptions.
e. Sizing of Air Release Valves.
1) Use design charts provided by the manufacturer to determine the orifice size required for an
estimated rate of air release under a certain range of operating pressures.
2) Consider design flow demands, pipe slopes, solubility of air in water as affected by temperature
and air intake rate through air/vacuum valves under normal operating conditions for the
estimation of rate of air release.
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Part One, Section 19. Air Valves WATER DESIGN GUIDELINES
3) Consider various combinations of operating pressures and air release rates to determine the
optimum size of the air release valve. The sizes of air release valves need not be the same for all
locations along the pipeline.
f. Location of the Air Valve or Combination Air and Vacuum Valve.
1) At a minimum, design the air valve or combination air and vacuum valve at each high point of the
pipeline in profile.
2) Minimize the number of air valves or combination air and vacuum valves required since these
valves are susceptible to problems in operation and maintenance.
3) A profile which clearly shows the high points, low points and slope changes shall be prepared for
the pipeline under design, see requirements for profiles in Part One, Section 11 (Vertical
Alignment - Profiles). Upward sloping and downward sloping sections shall be identified in
reference to the predominant flow direction. Select locations of the air valves prior to sizing and
show on profiles.
4) Exercise judgement in selecting the number and location of the air valves or combination air and
vacuum valves. Consider the following guidelines, in selecting the locations of the air valves or
combination air and vacuum valves (The following guidelines discuss the need for air/vacuum
valve and/or air release valves, which are part of the air valve or combination air and vacuum
valve.)
a) The high point is a location where an upward sloping profile changes to a downward sloping
profile. Place air/vacuum valves and air release valves at all high points.
b) For pipelines with a decrease in upward slope, small orifice air release valves shall be placed on
the downstream, less steep side of the slope change. Where there is a drastic decrease in
upward slope, a large orifice air/vacuum valve may be needed.
c) For pipelines with an increase in downward slope, place the air release valves on the
downstream, steeper side of the slope change.
d) For pipelines with long ascending slopes, place the air/vacuum valve at 1/4 to 1/2 mile intervals
along the upward sloping profile.
e) For pipelines with long descending slopes, place the vacuum valves and air release valves at 1/4
to 1/2 mile intervals along the downward sloping profile.
f) For pipelines with horizontal profiles (zero slope), place the air/vacuum valves and air release
valves at 1/4 to 1/2 mile intervals. Ideally long horizontal lines should be avoided in profiling a
water pipeline.
g) At mainline valves on the pipeline, air/vacuum valves may be used to relieve negative pressures
that may develop on the downstream side of the mainline valves when they are closed.
h) For pipelines where a predominant flow direction cannot be defined, place the air/vacuum
valves and air release valves at or near the high points, slope changes and along long pipeline
profiles.
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Part One, Section 19. Air Valves WATER DESIGN GUIDELINES
g. Connecting the Air Valve or Combination Air and Vacuum Valve to the Mainline Pipe.
1) Type of connection for the air valve or combination air and vacuum valve on the pipe section: On
2-inch and smaller valves, use a tapped corporation stop. On 3-inch and larger valves, use a
flanged welded-on connection and coordinate the flanged inlet of the air valve with the design of
the gate valve, see Part One, Section 18 (Pipeline Valves).
2) Locate the air valve or combination air and vacuum valve, centered on a twenty (20) foot length
of pipe with both ends of the pipe section having the same elevation.
h. Piping and Vault Configuration.
1) Pipelines 24-inch and smaller. Use a 2-inch air valve, see Standard Details W/2.0.
2) Pipelines larger than 24-inch.
a) 2-inch air valve on pipelines larger than 24-inch. If the design requires a 2-inch air valve
modify Standard Detail W/2.0. Modifications to the standard detail must be shown on the
drawing, see Part Three, Section 6 (Modifications to Specifications and Standard Details).
b) 3-inch and larger air valve. If the design requires a 3-inch and larger valve, see Standard
Details W/10.0, W/10.1 and W/10.2.
c) The discharge piping for combination air/vacuum valves with air release valve has two (2)
different discharge outlets that are not to be connected together. The discharge piping shall be
designed to discharge freely outside the vault/structure to the atmosphere. See Standard Details
W/2.0, W/2.0a, W/2.0b, W/10.0, W/10.1 and W/10.2.
3) On profile, determine the invert of the pipeline by using the design depth as shown on the details
for the above mentioned vaults and manholes, see the requirements in Part One, Section 16
(Design of Structures).
4) Vent boxes.
a) For 2-inch air valve, show the vent box location on the drawings, see Standard Detail W/2.0a.
b) For 3-inch or larger air valve, show the vent box location on the drawings, see Standard Detail
W/2.0b.
c) Provide the dimension from the centerline of the air valve structure to the centerline of the vent
box on the drawings.
d) If the vent box is located in the 100 year flood plain set the discharge of the vent pipe outlet one
(1) foot above the 100 year flood plain elevation. Indicate the discharge elevation and the 100
year flood plain elevation on the drawings.
e) If the vent box is to be located in a proposed area (grading), indicate the finished grade
elevation at the vent box on the drawings.
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