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Related to my recent question: Why was the Space Shuttle Orbiter's in-orbit time limited?

I was told a limiting factor to time in orbit is that the cryogenic gases used as reactants by the Shuttle's fuel cells would be vented when not used due to pressure buildup (from heating). I was told 1) being a closed-loop system[1] is unrelated 2) the cooling system[1] of the fuel cells would not be considered in normal operation if there's [little to] no draw.

Q: Was there a mechanism to vent fuel cell reactants on the Shuttle?
If yes, what were the conditions required?

1: spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eps/pwrplants.html

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The Power Reactant Supply and Distribution (PRSD) aka cryo tanks on the orbiter had relief valves that fed manifolds that vented overboard, H2 on the starboard side, O2 on the port, both over the wings. This shows the starboard side.

enter image description here

Here is a drawing showing the overboard relief from the Press Manual.

enter image description here

The other answer is talking about the "manifold relief valves" which allowed an isolated, overpressurized cryo manifold to relieve back into the tanks. It was not a tank or overboard pressure relief system.

The H2 tank relief valves cracked at 302 psig, the O2 at 1005 psig. Normal pressure was ~ 250 for H2, 850 for O2.

AFAIK this never happened in the history of the program. EGIL (the flight control position responsible for the electrical system) would have been in big trouble. The PRSD tanks were vacuum insulated (I know it sounds redundant, but the Orbiter did operate in the atmosphere) and normal usage kept the tanks below the crack pressures. In fact normal ops was turning heaters on to keep the pressure up.

See here https://space.stackexchange.com/a/39233/6944 for info on cryo tanks and normal operations

You could also vent reactants through the fuel cells themselves - this was called "purging". The valves that did that were not pressure operated though, they were opened by the crew. This purging was done routinely, every flight.

Source: Diagram- Shuttle Operational Data Book Vol IV aka the Crash Book and personal notes.

Addendum showing what a normal pressure history in the tanks was like:

I've added a plot showing actual cryo tank pressure data from STS-88. The pressure in the 5 O2 cryo tanks is plotted, the graph covers about 30 minutes. At the beginning, one tank's heaters are in AUTO and the heaters are cycling to supply the fuel cell demand. When the crew turned the heaters off for a test, the pressure dropped and all 5 of the tanks supplied the demand. When the crew put the heaters back in AUTO, the one tank picked up all the demand.

enter image description here

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  • $\begingroup$ This is lovely info. With the check valves (the ones with the arrows), how does the manifold relief reroute back to the tanks? Not shown here? $\endgroup$
    – ymb1
    Commented May 19, 2021 at 17:41
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    $\begingroup$ @ymb1 The checkvalve is intended to isolate a leaking tank from the manifold so that one leaking tank doesn't blow down the whole system. Normal flow is from the tank to the manifold is through that checkvalve. For the manifold relief, let's pretend the pressure is high at the place on the diagram marked with the circled 1. The manifold relief valve would open, allowing reactant to flow through it and into the line connecting the tank and the check valve. $\endgroup$
    – Organic Marble
    Commented May 19, 2021 at 17:52
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There were relief valves for the cryogenic tanks:

The reactants flow from the relief valve/filter packages through four reactant valve modules: two hydrogen (hydrogen valve modules 1 and 2) and two oxygen (oxygen modules 1 and 2). Each valve module contains a check valve for each cryogenic tank line to prevent the reactants from flowing from one tank to another tank in the event of a tank leak. This prevents a total loss of reactants. The oxygen valve modules also contain the environmental control and life support system atmosphere pressure control system 1 and 2 oxygen supply. Each module also contains a manifold valve and fuel cell reactant valves.

The manifold relief valves are a built-in safety device in the event a manifold valve and fuel cell reactant valves are closed because of a malfunction. The reactants trapped in the manifold lines would be warmed up by the internal heat of the orbiter and overpressurize. The manifold relief valve will open at 290 psi for hydrogen and 975 psi for oxygen to relieve pressure and allow the trapped reactants to flow back to their tanks.

From https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-eps.html

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  • $\begingroup$ A relief valve can be back to the tank to limit the reactant pressure going to the fuel cells. Or it can be something else. But is it explicitly venting to the outside getting lost? Hence the "what were the conditions required?" My point: as is, no one can be sure it's correct without more details, even if it is. $\endgroup$
    – ymb1
    Commented May 19, 2021 at 16:42
  • $\begingroup$ Thanks for the extra quote: "flow back to their tanks" is as I expected, i.e. not the venting that was alluded to being lost. I think the answer can benefit from making the distinction clearer in the opening line. $\endgroup$
    – ymb1
    Commented May 19, 2021 at 16:47
  • $\begingroup$ If there is a tank over pressure, there is nothing else than to vent the gas to the outside through the relief valve. Bursting a tank should be prevented in any case of over pressure. $\endgroup$
    – Uwe
    Commented May 19, 2021 at 16:49
  • $\begingroup$ Back to the question at hand: And that would explicitly happen simply when the fuel cells are not used? (Thus being a limiting factor to time in orbit?) Nothing so far confirms that. $\endgroup$
    – ymb1
    Commented May 19, 2021 at 16:53
  • $\begingroup$ If there is an over pressure in the tank to be vented through a relief valve, that gas is lost. If gas is lost to prevent an over pressure, time in orbit is limited. Losing a gas for the fuel cells without any effect to maximum time in orbit is impossible. $\endgroup$
    – Uwe
    Commented May 19, 2021 at 18:15

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