We are using the McMaster Pressure relief valve this year for the first time. We got the part number that is pre-set to relieve at 125 psi. During practice today it was relieving at 100-105 psi which is before the cutoff switch turns the compressor off, so the compressor keeps running right now.
If I put finger pressure on the pull ring to hold the valve closed while it is pressurizing, we can get the pressure to the point where the switch shuts the compressor off. If I then remove my finger, it will hold pressure. But then when the compressor kicks on again, the pressure relief valve will once again not be able to hold pressure.
We have the pressure valve mounted on a cross fitting with the pressure switch. The cross fitting is mounted to the outlet of the compressor. So the pressure switch is subject to the vibration of the compressor. I don’t know if that is a factor or not.
We have a couple of spare PRVs, so we will probably try one of the spare parts to see if maybe this one valve is improperly calibrated. Just wondering if anyone else has had similar issues and has solved it.
Yeah, it dumps a decent amount of pressure. We have some of the old style valves laying around as well. If we can’t get any of our new ones working, we will switch back. I really like the new valve, but if it won’t hold pressure, that is a problem.
We are also trying this valve for the first time. We got the 125 psi version and it relieves at about 115. Our compressor shuts off reliably at this relief pressure so we are currently keeping it installed. We also have ours hard piped right after the check valve, only in a tee rather than a cross. Our cross and pressure switch is immediately downstream of the relief valve.
We ordered two valves and have not checked the second one to see how it performs.
We had the same issue in 2019, with multiple pressure valves. It worked great until we were forced to over pressurize it for inspection. We switched back to the adjustable one. I’m guessing after it blows once, something doesn’t seat right and just blows early. Very disappointing to see since it helps save a good chunk of weight.
Kinda disappointing. We try to take steps forward in improving your practices every year and look for ways to make things more reliable and less finicky only to find out the “better way” isn’t necessarily fool proof.
We will try our spare valves and see if we can find one that actually relieves at the set point it was designed for. Otherwise we will switch back to the old style adjustable valves and add a dump valve.
Thanks everyone. Back to your regularly scheduled programming.
The McMaster valve has a pull ring on it so that it doubles as a vent.
That is not why we tried it this year. We really liked the idea of not needing to adjust the valve. But the fact that it also served as a system vent was a bonus.
You will encounter inspectors who will disagree with this interpretation (that R78B and D can be combined in this way).
Exercising the valve’s self-test feature as a means of blowing down the system is not best practice, you want to disturb the relief spring as little as possible. This practice can in fact cause the valve to weep at pressures significantly lower than the setpoint, as you and the other posters have experienced.
We remove the pull ring from our relief valves and recommend you do the same.
Interesting. Since this is our first time using the McMaster valve, I have not run into that personally, but several teams that have recommended this valve to us have stated that they use it for both functions.
I was demonstrating this feature to some of our students as they were building our pneumatic system. I wonder if the fact that I exercised the valve a few times has something to do with it blowing off early.
It does stay open as long as you pull the ring. I don’t see anything in the rules that say that it has to vent without human interaction. In Fact, R86 says:
R86. Any pressure vent plug must be:
A. connected to the pneumatic circuit such that, when manually operated, it will vent to the atmosphere to relieve all stored pressure in a reasonable amount of time,
Pulling the ring and holding it open certainly qualifies as “manually operated”.
I happened to be talking to a mentor of one team last year that used this as their relief valve while their team was going through inspection. I watched them demonstrate this to the inspector and he didn’t say anything. He just moved on to the next inspection item.
However, if activating this pull ring valve is causing the pressure relief setting to change as @Nate_Laverdure stated, then we definitely are going to add a separate vent valve.
I see your point. But R78D requires a pressure vent plug. The relief valve is not a vent plug. Since I am only a RI, I will happily send you to LRI for this discussion and abide by the LRI’s decision. Rules aside, my point is a safety point. In a real emergency do you really want to stand stand and wait for the system to vent? An open dump valve is a good indicator that the system is vented. Not that OSHA regulations apply to FRC, but it is an OSHA requirement that pneumatic systems be vented when working on them.
Nate is correct. Inexpensive relief valve like the McCarr ones have issues when reseating. It best to use manual vent on them as little as possible.
According to the description on McMaster, it is. And, if you pull and hold the ring, the system pressure will be fully vented, so it does fulfill the required function.
I do see your point about being able to know that the system is fully vented when working on it and agree that a more standard vent valve does that. However, checking that this valve is open is still the responsibility of the person about to do the work. Since the vent plug must be in an an easily accessible location, it would also easy for that same person to pull the ring on the McMaster PRV to make sure that the system is not pressurized before working on it and as long as the compressor is not running, then you have achieved an equivalent level of safety (you shouldn’t have the compressor running if you are working on the system anyway).
I don’t agree that each of the functions described in R78 must be met by completely separate devices. Except C which calls out a specific part number, there are a number of devices out there that can be used and in some cases, those devices fulfill more than one requirement of R78. There are a number of modular pressure regulators available that include a pressure gauge as part the device. Therefore this one device satisfies R78 E and G. Similarly, if a device has both a pressure relief feature and a vent plug feature and therefore satisfies B and D, it seems to me that this is legal. I have not seen any robot inspectors have a problem with either of these devices in the past, but every region is different and some regions seem to apply these rules a little differently.
This is by far the best reason to not use it as a vent plug. This is excellent advice. Thank you and Nate for passing this along. We will be adding a standard valve to our system for venting.