What is Rapid Decompression?
Rapid Decompression refers to the sudden loss of cabin pressure in an aircraft that is flying at an altitude where the pressure inside the aircraft is higher than it is outside.
According to MIL-STD-1791C Designing for Internal Aerial Delivery in Fixed Wing Aircraft, the three USAF prime mission cargo aircraft, C-130, C-17, and C-5, all have automatically controlled cargo compartment pressurization systems which maintain the pressure in the cargo area at approximately 8.3 psi differential pressure above the outside air, when at flight altitude. If structural damage occurs, or even if a door or hatch seal is ruptured, the pressure inside the cargo area will begin to equalize with the pressure outside. In the event that the pressure loss is very rapid, it is possible for sealed items, such as shipping containers, to explode due to the sudden drop in pressure outside the container. Parts of these containers could become projectiles, posing a threat to the aircrew as well as the aircraft.
Does Rapid Decompression apply to my shipping container design?
It depends on the specific contractual requirements for the shipping container being designed. A shipping container, or other sealed enclosure, may be required to comply with section 220.127.116.11 of MIL-STD-1791C, which states that:
The item, including critical subcomponents, shall withstand an internal pressure differential of 8.3 psi developed in 0.5 sec or less without any part of the item becoming a hazard. The aircraft system automatically maintains a maximum of 8.3 psi cabin pressure differential with the atmospheric air pressure.
How can I make sure that my container design is Rapid Decompression ready?
One way to comply with the requirement is to demonstrate, either by analysis or testing, that the container structure can withstand 8.3 psi differential pressure. But in most cases, this is not economically feasible because of the amount of reinforcement required. Many containers are made of aluminum or fiberglass to minimize their weight. 8.3 psi may not sound like much, compared to the 40 psi inflation pressure of a car tire, but when 8.3 psi is applied to a large, rectangular box, the forces can be huge. Take a standard 20-foot long ISO shipping container as an example, whose interior dimensions are 19’3″ x 7’8″ x 8’10” (LxWxH). The force applied to the sidewall of the container at 8.3 psi differential pressure is over 203,000 lbs!
When a container structure cannot be designed to withstand an 8.3 psi pressure differential, sufficient venting must be provided on the container to prevent the pressure from exceeding its design pressure. If the 0.5 second time duration is required, it may be necessary to use explosion vents or burst panels, which are designed to rupture at a pressure that will protect the structure. These are delicate by design and must be replaced if they are damaged accidentally, or if their burst pressure is reached.
Can I use a pressure relief valve to satisfy Rapid Decompression requirements?
It is becoming more common to use 15 seconds for the time duration instead of 0.5 seconds, according to data provided in section 2.3.3b of MIL-STD-810G, which is a testing standard for military equipment. 15 seconds represents a more realistic Rapid Decompression scenario in which the aircraft sustains minor damage leading to pressure loss. Where the 15 second duration can be used, a mechanical pressure relief valve such as our PR5002 or PR2001 is an excellent alternative because they are more economical than explosion vents and can open many times during their life, without needing to be replaced. It may also be possible to satisfy the Rapid Decompression requirement using the two-way pressure relief valve (aka breather valve), such as our BV2000 or BV4000, which is required for general pressure equalization of a shipping container designed for air transport.
Contact our Engineers if you need help finding a solution for Rapid Decompression requirements.
A full copy of MIL-STD-1791C can be accessed here. It contains the requirements for Rapid Decompression, the rationale behind the requirement, as well as background calculations. MIL-STD-810G can also be found there.