At the end of the last episode the client had run out of 4L-200 liquid cylinders and was wondering if they could put 230psi reliefs on some of the 4L-292 oxygen liquid cylinders they had remaining in stock to get them through for a few days until their regular delivery. This left the questions of:
- Is this OK?
- Why or why not?
- What other considerations might there be?
First, the fact that the vessels are oxygen is irrelevant to the issue of the pressure relief setting. The important issue in all this is the DOT 4L designation.
What matters is the fact that the maximum allowable relief setting on a 4L-200 is 230psi and it is 350psi on a 4L-292. Technically, it is OK to de-rate a vessel by putting a lower pressure relief than is allowed. So “yes” this is OK.
The other consideration, though, is where the rub comes in. If you look at the factory setting for the pressure builder (PB) regulator on a 4L-200 you’ll find that it is 125psi in most cases. The factory setting for the PB regulator on a 4L-292 is more typically about 300 psi. The problem then with putting a 230psi relief on a 4L-292 cryogenic liquid cylinder is that the pressure builder is set at a level higher than the relief. So, the liquid cylinder will try to build pressure to the PB set point, but constantly vent causing the vessel to exhaust itself. If you want to switch out the relief to 230psi on a 4L-292 you need to remember to reset the PB.
It is common for people to put 230 reliefs on 4L-292s without taking the PB setting into consideration. Likewise, checking the PB setting is one of the trouble-shooting steps to do when a liquid cylinder is reported as “bad” (constantly venting). The liquid cylinder might actually just be doing what it is set up to do.
- Is this OK?
- Why or why not?
- What other considerations might there be?
First, the fact that the vessels are oxygen is irrelevant to the issue of the pressure relief setting. The important issue in all this is the DOT 4L designation.
What matters is the fact that the maximum allowable relief setting on a 4L-200 is 230psi and it is 350psi on a 4L-292. Technically, it is OK to de-rate a vessel by putting a lower pressure relief than is allowed. So “yes” this is OK.
The other consideration, though, is where the rub comes in. If you look at the factory setting for the pressure builder (PB) regulator on a 4L-200 you’ll find that it is 125psi in most cases. The factory setting for the PB regulator on a 4L-292 is more typically about 300 psi. The problem then with putting a 230psi relief on a 4L-292 cryogenic liquid cylinder is that the pressure builder is set at a level higher than the relief. So, the liquid cylinder will try to build pressure to the PB set point, but constantly vent causing the vessel to exhaust itself. If you want to switch out the relief to 230psi on a 4L-292 you need to remember to reset the PB.
It is common for people to put 230 reliefs on 4L-292s without taking the PB setting into consideration. Likewise, checking the PB setting is one of the trouble-shooting steps to do when a liquid cylinder is reported as “bad” (constantly venting). The liquid cylinder might actually just be doing what it is set up to do.
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