Page 15: of Marine Technology Magazine (March 2026)

if done with care and a close eye.

Lesson 2. Only use PRVs for their intended purpose:

Purge Systems provide the means to pull a partial vacuum venting internal overpressure.

Which brings us to purge ports. There are two kinds of purge on an undersea housing, remove entrained moisture, vacuum test all the housing seals, and preload the housing o-rings ports: open and self-sealing. The open purge port has a small ori- before deployment.

? ce behind a sealing plug or screw. The small ori? ce controls the

A purge system removes moist entrained air, replacing it with rate of ? ow through a small diameter center hole while the ? nal dry air. A focus on back? lling with dry nitrogen is misplaced. seal screw or sealing plug is installed. The intention is to be quick

It’s not oxygen that needs to be removed, it’s water vapor. It’s about it so not too much vacuum is lost by moist marine air re- entering the housing. Air being drawn into the housing may also the dew point at cold ocean depth we’re worried about. Dry air carry debris across a greased o-ring sealing surface where it may could also come from a SCUBA tank, or CO2 soda maker ? ask.

If an end-user’s housing is constrained to using a PRV as a compromise the ? nal o-ring seal. (See Lander Lab #7, “Manag- purge port, the manufacturer’s PRV’s adapter can easily attach ing Moisture in SubSea Housings”, MTR, March/April 2023.) to the Deck Purge Box.

A self-sealing purge port, such as made by Global Ocean De-

Another danger to avoid: Use of a high-pressure bottle to force sign (San Diego, CA), incorporates an interior check valve. When the purge system adapter is connected, the check valve is held air in carries an obvious danger of blowing off an endcap when open and a free ? ow of air is allowed in two-directions. When the PRV is being used as a purge port. Global Ocean Design’s the purge system adapter is removed from the self-sealing purge Deck Purge Box removes this hazard by utilizing the interior vacuum already made by the vacuum pump to draw air in. Moist port, a spring-loaded check valve closes, and air ? ow is cut-off.

A vacuum leak test using the Global Ocean Design Deck marine air is directed over a desiccant cartridge, which removes

Purge Box (Figure 5) can check all seals, including those of a suspended moisture on contact. It is self-limiting. When interior

PRV, minus the last one, the pressure proof cap. The ? nal seal, and exterior pressures are equal, air ? ow stops. This approach a face seal, can be cleaned, inspected and carefully installed also eliminates the problems of shipping high pressure bottles, then re? lling them in a foreign port. If high pressure bottles are with no rush. There’s a very high probability of getting it right www.marinetechnologynews.com 15

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