WHY USE BREATHER VALVES?
In the packaging of missiles, engines and delicate electronic
gear, it is essential to protect equipment from the effects of
moisture. In order to accomplish this, the shipping and/or storage
containers must be tightly sealed and desiccated. However, these
containers may be exposed to pressure and vacuum differentials
of as much as 5.7 psid (pounds per square inch differential) due
to temperature and/or altitude changes. To resist pressures of
this magnitude, the container would need to be constructed from
a very strong material, which would make it bulky, heavy and costly
to store and ship.
This problem can be overcome by the use of a "controlled
breathing" system, known as Breather Valves or Pressure Relief Valves. These low-pressure,
high-flow Breather Valves automatically adjust the container pressure with
respect to environmental pressure changes and prevent excessive
pressure differentials during air or high altitude truck or rail
transport (Fig. 1). As a result, the use of a Breather Valve can save on the initial cost of building the container and throughout the container’s life by reducing the cost of transport and storage.
At one time, "free-breathing" containers were considered an alternative solution to this problem. The theory, based
on Ficke's Law, was that moisture would not pass freely through
tubes with lengths 10 or more times their diameters. However, this
principle applies only when there is no pressure differential between
the two ends of the tube. The method was found to be unsatisfactory
in actual practice. In fact, in tests conducted by the U.S. Army
Command, the average water gain in three free-breathing containers was over
six times greater than a controlled breathing container with a Breather Valve which
sealed at 0.5 psid pressure and 1.0 psid vacuum (see the McDermit
HOW BREATHER VALVES PROTECT
How will Breather Valves protect the contents of a container from moisture intrusion? The answer to this question depends on five factors:
- The pressure and vacuum settings of the valves.
- The temperature variations to be encountered during storage.
- The temperature and relative humidity of the storage area(s).
- The number of airlifts which the container might experience.
- The amount of desiccant in the container.
1. PRESSURE AND VACUUM SETTINGS
Breather Valves are made in a
variety of settings, ranging from 0.2 psid to 5.0 psid or more.
These settings, which are the points at which the valves seal,
must be at least 1.0 psi to 1.5 psi below the pressure or vacuum
which the container can safely withstand without leaking or deforming
(see Selection below). Generally
speaking, the lower the valve setting, the more often the valve will open, admitting outside
atmosphere and shortening the life of the desiccant.
2. TEMPERATURE VARIATIONS DURING STORAGE
The number of times
a Breather Valve will open during storage depends not only on
the valve setting, but also on the magnitude and frequency of
temperature variations which may occur in a particular storage
area. In sealed containers there is
a pressure change ranging from 1.0 to 1.5 psi for each 30°F
temperature change (Fig. 2).
Long-term tests, which have been run on containers at AGM's
plant in Tucson, Arizona, indicate that valves with sealing pressures
of 0.25 psid will open almost every day, while valves set to
reseal at 0.5 psid may open up to 150 times a year, and valves
set for 1.0 psid rarely open during storage. (It should be noted that
these tests were run on rigid wall containers, and that low-setting
valves on plastic containers with flexible walls will probably
not open as often under the same conditions.)
There are only a few locations in the world other than Tucson where
greater diurnal temperature variations occur. Therefore,
under worldwide storage conditions, valves with a 0.5 psid reseal
in both directions will open no more than 200 times a year, and
valves set for a 1.0 psid reseal in both directions will probably
open less than a dozen times.
3. TEMPERATURE VS. HUMIDITY
In addition to the number of times the Breather Valve opens, the amount of moisture
taken into the container at each opening (or "gulp") will determine
desiccant life, and this is dependent on the climatic conditions of the storage
area. There are places in the world where as much as 0.015 grams of water
per container cubic foot could be taken in at each "gulp."
(Reference NavWeps Report 8374, Table XII). However, high humidity tends
to limit temperature variations (Fig. 3), so that even Breather Valves with
very low settings will probably not open more than 2 or 3 times a year in
4. NUMBER OF AIRLIFTS
For each descent from 10.7 psia (normal pressurization level in an aircraft
cargo compartment) to 14.7 psia (sea level), a Breather Valve set for 0.5
psid reseal in both directions will take in approximately 0.013 grams of
water per cubic foot of container volume. Higher or lower valve settings
will not substantially vary the amount of moisture gain per descent. Therefore,
the amount of desiccant needed will, in part, depend on the number of airlifts
5. AMOUNT OF DESICCANT
It has been noted above that in ground storage, each time a container must
breathe it will take in as much as 0.015 grams of water per cubic foot, and
during each air descent in a pressurized cargo compartment it will take in
as much as 0.013 grams of water per cubic foot. Since MIL-STD-2073-I
requires 1.2 units of desiccant per cubic foot in a sealed rigid metal container
(plus additional amounts for dunnage, if any) and one unit of desiccant will
hold 6.0 grams of water at 40% relative humidity (RH) at 77°F, this amount
of desiccant will protect the container for a total of 480 "gulps" in
ground storage, or a total of 550 airlifts, or some combination of the two.
Keeping the above factors in mind, we see that a Breather Valve, properly selected and used in conjunction with adequate desiccant, can provide years of moisture protection in a lightweight, low cost container.
SELECTING THE RIGHT VALVE
The Breather Valve must perform two functions: limit the amount
of moisture that can enter the container, and protect the container
itself from excessive pressure or vacuum differentials. Therefore,
the ideal valve should remain sealed except for during airlift
or under extreme temperature changes, but when open should have
sufficient flow to relieve air pressure as fast as it builds
up. As noted under "Temperature Variation During
Storage," it has been shown that valves set as low as +0.5
(pressure) and -0.5 (vacuum) psid will protect against excessive
moisture intrusion for years. In order to select a valve which
will adequately protect the container against excessive pressure
or vacuum, we must know the following:
1. HOW MUCH PRESSURE OR VACUUM CAN THE CONTAINER WITHSTAND WITHOUT LEAKING OR DEFORMING? This figure
will establish the pressure at which the valve must achieve its
rated flow, which is measured at 1.5 psi above the reseal setting.
If possible, a safety factor should be utilized by setting the
required flow pressure slightly below the container's deformation
IMPORTANT: Most containers
can withstand more pressure than vacuum. For instance, the container
pictured in Fig. 4 is normally pressurized to 5 psi and can probably
withstand an internal pressure of up to 50 psi without deformation.
However, it took less than 3 psi of vacuum, resulting from a
temperature drop of 90°F during surface transport of this
empty unpressurized container to cause the deformation shown.
For this reason, a pressure setting somewhat higher than the
vacuum setting is often specified to provide a sufficient differential
without overstressing the container. However, too great a differential
(more than 3 psi) between the pressure and vacuum settings can
cause valve design problems and increased cost. If a differential greater than 3 psi is required, 2 one-way valves should
be used. It should be remembered that a total differential in
sealing pressures of 1 to 2 psi will provide more than adequate
moisture protection worldwide.
2. WHAT IS THE EFFECTIVE VOLUME OF THE CONTAINER?
It is essential to know how much air, in cubic feet, will be inside the container. This may be calculated by subtracting the volume of the contents (engine, missile, etc.) from the inside volume of the container.
3. HOW RAPID A CHANGE IN PRESSURE MIGHT BE ENCOUNTERED?
The highest rate of
pressure change will usually occur during the depressurizing and repressurizing
of an aircraft's cargo compartment during air transport. According
to the International Air Transport Association (IATA) Standard
Specification 80/2, "Pressure Equalization Requirements for
Aircraft and Shipping Containers (Par. 3.2)," the cargo compartment
pressure decreases from standard sea level (14.7 lbs./in.²)
to minimum cruise altitude equivalent of 8,500 feet (10.7 lbs./in²)
at a maximum climb rate of 2,500 feet per minute and increases back to sea level at a maximum descent rate
of 1,500 feet per minute. IATA also specifies (Par. 6.2.3) that
Breather Valves shall ensure a minimum air flow of 12% per minute
of the internal container volume. Society of Automotive
Engineers (SAE) Specification AS27166 (which replaces cancelled military specification MIL-V-27166) "Valve,
Pressure Equalizing, Gaseous Products," also specifies 12%
as a minimum flow rate, but subtracts the volume of the material
in the container, resulting in the following formula (Par. 184.108.40.206):
Minimum Flow Rate (ft.³/min.) = (Vc-Vm) 0.12
Where Vc = Volume of container (ft.³)
and Vm = Volume (min.) of material in container (ft.³)
It should be emphasized that this is a maximum, and would tend
to be reduced by other factors, such as temperature change and elasticity
of the container, so no additional safety factors need to be added.
However, where there is a possibility that an empty container could
be transported by air, it might be wise to disregard the displacement
of the contents and use the internal volume of the empty container
as the basis for calculating flow requirements.
ENVIRONMENTAL PROTECTION - CRACKING PRESSURE
Sometimes we are asked to supply a valve that will "crack" (start
to open) at a specified pressure, within a tolerance. While this
requirement can be met, it involves additional testing and, therefore,
increased cost, and - except under extraordinary circumstances - would
appear to be unnecessary, since the settings for seal and design
flow provide the desired protection for both container and contents.
In addition, SAE Specification AS27166 sets maximum cracking
pressure offsets from the reseal pressure.
WHAT ABOUT SAND AND DUST PROTECTION?
SAE Specification AS27166 requires that Breather Valves must
still reseal after being tested for sand and dust protection
per MIL-STD-810. The sand used in the test is similar to talcum
powder, too fine to affect the sealing surfaces of the valve.
However, the specification does not require the valve to keep
sand and dust out of the container if it opens during a dust
storm. This technicality has made it possible for valves with
stamped or wire mesh screens to certify as meeting the specification.
For total sand and dust protection, you need to specify AGM Breather Valve series TA330, TA333-R or TA770-R, as these valves have
dust baffles and covers that will protect the contents of the
container from not only sand and dust, but also wind-driven rain
and water from high-pressure decontamination hoses.
that you have established the design flow rate and the reseal pressure
(which is 1.0 to 1.5 psi below the flow rate pressure), you may
proceed to actual selection of the Breather Valve you will need.
The Breather Valve Selection Chart (Fig. 5) will give you the
flow rates of the various Breather Valves listed on this website,
and indicate maximum net volumes of containers they may be used
Each Breather Valve data sheet includes a Part Number Designation
Chart which shows how to designate the desired resealing pressures
in the part number of the valve. Dimensions, performance characteristics
and optional features, such as a manual release button (essential
for breaking a vacuum seal) and RFI/EMI shielding, are also indicated.
SPECIFICATIONS & REQUIREMENTS
Specification AS27166, entitled "Valve: Pressure Equalizing,
Gaseous Products," details environmental requirements and
the procedures for vibration, temperature, salt, fog, sand and
dust, rough handling, etc., as well as settings and flow. (Note:
Many design activities have found it impractical to use the settings
designated in this specification and have called out other settings
more suitable for their particular container design.) AGM Breather Valve Series TA238, TA240-R, TA330, TA333-R and TA770-R will meet
all requirements of this specification (see Environmental Protection, above).
AS27166 is also referenced in Department of Defense MIL-STD-648C.
In addition, AGM's Breather Valves are specified on more than 300 Army, Navy
and Air Force drawings.
If you require identification of AGM Breather Valves with a part number
other than one shown on this website, please contact AGM for part
number verification. You should also contact AGM if you plan to
put one of AGM's catalog numbers on a document requiring MIL-STD-130
If you have special requirements which cannot be met by using
a standard valve, please contact AGM's design
engineering team regarding possible modifications or a special
valve design to meet your needs.
Unless otherwise specified in AGM's catalog, all valves are supplied
with a nut, washer and gasket for mounting through a hole on a
flat, smooth surface. Valve gaskets may not provide a proper seal
if the mounting surface is curved or rough. Valves can also be
installed in a mounting flange or a threaded boss when a suitable
counter bore has been provided for thread relief.
IMPORTANT: Every AGM Breather Valve is individually tested and
certified for compliance to performance requirements.