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 Tank Automotive 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 Report).

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:

1. The pressure and vacuum settings of the valves.
2. The temperature variations to be encountered during storage.
3. The temperature and relative humidity of the storage area(s).
4. The number of airlifts which the container might experience.
5. 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 these locations.

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 anticipated.

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 point.

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 rate of pressure change, multiplied by the net volume of air in the container will determine how much air, in cubic feet per minute (cfm), the valve must flow to relieve pressure or vacuum as rapidly as it might build up. Normally, the highest rate of pressure change will 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. 3.6.3.1):

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.

SELECTION
Now 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 on.

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
SAE 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 identification.

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.

MOUNTING
Unless otherwise specified in this 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.