Relief Valve set pressures vs. valve body rating 1

[AlbertaMecchie]

Working on a project, and the request is to move a Fisher H203 downstream of a 4" NPS 300# isolation valve on a natural gas pressure reducing station. I'm not sure yet what the logic behind the request is. Inlet pressure to the series-installed main and monitor regulators (both 600# rated) is 1035 psi. The Fisher H203 is located on a 1" branch just downstream of the main and monitor pressure regulators. Maximum set pressure on a 1" Fisher H203 is 300 psi, H203 valve body is rated for 400 psi. New set pressure on relief valve will be around 72-75 psi. If the working and monitor regulators fail, pressure upstream of the relief valve could start heading up towards maximum line pressure of 1034 psi. My concern is if the regulators fail, with the Fisher H203 moved to this new position downstream of the 4" isolator, if an operator closes that 4" NPS 300# isolator, the pipe flange and inlet valve flange at the isolator could be over-pressured, 300# flanges are good only to 749 psi operating. Any thoughts on this ? I think the H203 should be left where it is with new set pressure of 72-75 psi - or is the 400 psi body rating of the H203 (400 psi) a concern even though it's max spring (relief) setting is 300 psi ? It's tough to find these pop-type small relief valves with body ratings up to 1034 psi and the lower required set pressure of 72 psi. Just looking for 2nd views on this. Thanks.

 

[LittleInch]

It's this bit I don't understand "If the working and monitor regulators fail, pressure upstream of the relief valve could start heading up towards maximum line pressure of 1034 psi." Usually one of the checks you do is to ensure that the maximum flow through a wide open regulator is less than the flow capacity of your pressure relief valve. Hence this situation should not be able to occur.

On your second point I agree. Again this is usually picked up in a HAZOP or MOC procedure whereby you look at all the possible issues and normally in these situations, the downstream system is either rated to the upstream system or relief valves are located and set such that the situation you have is not possible if everything works as it should.

So long as the pressure cannot rise above the design rating of your valves then you don't have an issue.

It's note clear why this change has occurred, but surely there is a MOC procedure or some sort of regulatory oversight? only if your upstream pressure is being reduced below 720 psi can this be seen as acceptable from the data we've been given.

 

[Snickster]

I have been looking at this and there are some things that don't make sense. I assume that this is a new project where the system is under construction and has never been put into operation. Is this correct? I say this because I see some issues if I am understanding the operation of the system as follows:

1) The arrangement shown for the two Moody reducing valves in series appear to be set up for two stage reduction where if the downstream valve fails open the upstream valve takes over operation via the monitoring pilot to reduce the entire pressure reduction required, and if the upstream valve fails wide open then the downstream valve takes over naturally to reduce the entire pressure reduction required. Under normal operating conditions the upstream regulator drops the pressure to 348 psig and the downstream regulator drops the pressure to 50 psig as shown by the set pressures of their respective pilots. The monitor pilot valve is set for 45 psig.

This cannot be correct since the monitoring pilot must be set at a higher pressure than the 2nd stage dropped pressure, say 55 psig. If monitor pilot is set at 45 psig, the valve of this pilot valve will be closed and the outlet control flow line of the upstream regulator pilot will be block if the pressure downstream of the 2nd regulator was 50 psig. This would disenable the operation of the upstream regulator valve. The monitor pilot valve must stay fully open as long as the downstream final pressure is 50 psig set pressure or lower. Therefore the monitor pilot needs to be set at about 55 sig so that if the downstream regulator fails open, the downstream pressure will rise to 55 psig and then the monitor pilot will take over operation of the upstream regulator to control the pressure to the set pressure of the monitor of 55 psig even with the downstream regulator failed wide open.

2) Both regulators, and I assume the pipng is class 600#. This is correct since if the upstream regulator fails open the downstream regulator will see full 1035 psi inlet pressure and will drop pressure to 50 psig as the configuration is set up to do. However the maximum pressure drop capabilities of the regulator is only 800 psi per their manuals, but in emergency is 1000 psi which I assume means valve failure so this seems ok.

3) The piping downstream of the 2nd stage regulator appears to be 300# due to the 300# block valve. This is where things get complicated. According to the Moody manual they state that for the double regulator monitor arrangement set up as this:

"On dead-end systems, a token relief downstream of the second stage regulator is recommended to compensate for slight leaks due to wear and debris in the monitor regulator."

It appears they are saying that with the arrangement since if one regulator fails the other will always take over, you don't need a full capacity relief valve. This is because two regulator valves can't fail at same time (double jeopardy) so you don't need full flow relief valve, only one to handle very minor flow due seat leakages of a system that is blocked (dead-ended) on the downstream side. I don't think I agree with this or if the Codes allow this considering that there are 3 pilot valves also in the system that will not be inspected like a stamped PSV would in addition to the regulator valves themselves, and so it may not be unlikely that one of those 5 items might be able to fail simultaneously with a regulator failed open causing the other regulator to fail open at same time. For instance say the upstream regulator fails wide open. Say the system has been operating for years with very little issues so very little preventative maintenance. So if the upstream valve fails open then the downstream regulator will feel full pressure of 1035 psig and regulate that down to 50 psig. However it could be that over a long period of operation the main valve diaphragm of the downstream regulator has loss its strength so when it gets hit with the higher pressure for the first time it cant take it and fails. Then you will have two wide open failed regulators in series.

If it were me, I would have a relief valve rated for full flow through two regulators in series at set pressure of the relief valve. The fisher relief valve does not appear it is design for full flow of both regulators failing wide open due to the low set pressure and size. The flow through the two regulators in series if failed can be determined by considering both regulator valves wide open so the flow area is the full minimum port area and sonic velocity exists across the minimum area. However I believe the internal port area is greater than the external connection ports so I believe the flow through the valves just becomes as through a 1" pipe with critical flow at the connection of the 1" to the downstream 2" reducer.

That being said. The relief valve should be placed on the upstream side of the 300 # block valve since if the relief valve were on the other side and the block valve was closed, the pressure would exceed the rating of the 300# block valve. Even if the relief valve was not a full rated valve and only sized for seat leakage of upstream valves then the upstream side of the block valve can still be over-pressured with just seat leakage if it was closed. I don't see any reason or benefit of putting the relief valve downstream of the block valve and it appears to be not in accordance with B31 piping codes.

On a final note you indicate that new set differential pressure of pilot valves are to be 420 Pa (61 psig). I think you meant to say new set pressure rather than differential pressure as in this case the 2nd stage reduce pressure would be 913 psig. As I indicated above I believe the monitoring pilot needs to be set at a slightly higher pressure than the 2nd stage reduced set pressure say 65 psig versus 61 to insure pilot of monitoring valve stays fully open under normal conditions.

 

[georgeverghese]

In the modern systems I've seen on similar setups, this token relief is justified only when there is a SIL2 rated overpressure trip system installed ( 2oo3 voting high pressure trip transmitters and at least 1 fail close shutdown valve) working in series combination with the monitor regulator. Else a full flow relief is required. Systems like this are only permitted in ASME B31.8 / 31.4 Code compliant designs such as OSBL gas regulating stations.

Agreed, dont see the logic in moving this PSV downstream of this 4inch isolation valve.
As Snickster says, the HP setting of the monitor regulator makes sense, but not the 310kpag setting.

 

[LittleInch]

I hadn't reviewed the whole thing, but agree, you can't set both regulators to the same pressure or even what looks like some sort of pilot driven override system for the U/S regulator.

Without knowing the details of the capacity of the regulators wide open or the relief valve, we can't say if this is a "weep" relief valve or a full flow valve.

either way it's in the wrong place.