Oversized RVs - Pressure Drop Issues

[Bill3752]

I have run into several cases that are similar:

1. The relief valve is very much oversized.
2. The inlet dP and / or back pressure is high when analyzed at the RV capacity.

3. The dP's are small when calculated at the scenario rates.

4. Importantly, the highest rate possible to these valves is much smaller than the capacity - for example a compressor that delivers 30% of the RV capacity. I also have a couple of cases where flash tanks are protected by RVs that are so oversized that they could handle three times the entering condensate - assuming it was all flashed.
In these cases, the pressure drops are acceptable at these "worst worst" case conditions.

So is there anyway around the 3%/10% inlet/back pressure rule?

 

[Chance17]

It is always possible to change to a smaller orifice or to install a smaller safety valve.

 

[MortenA]

Use of a RP does not prevent use of engineering skills and judgement - but you will have to argue a lot more than if you just follow the practise.

Best regards Morten

 

[Latexman]

Morten,

RP?

Good luck,
Latexman

 

[MortenA]

api RP 521

RP=Recommended Practise

Im not sufficiently familar with ASME to say if its a requirement there - but not every project in the world is made where ASME is a law. And perhaps you could even get a deviation here?

For Bill: I think the reason for calculating dP upstream for the full flow is that when the PSV crack - then it wont re-seat until the pressure is somewhat lower (blow down). When the pressure has build up - then you could, for a short while, flow more than your max case - simply because the pressure is high. This would cause dP in the inlet pipe and now pressure would drop so low, that the valve would re-seat - and the cycle can repeat.

If its a problem then you should consider replacing the valve.

Best regards
Morten

 

[MortenA]

just checked - i was pretty certain that there were a "RP" thrown in - but maybe there never were - and there isnt one now.

Best regards

Morten

 

[Latexman]

Bill,

The 3% rule on inlet pressure drop is to prevent chattering. Excessive irreversible pressure loss at the inlet of a safety valve can cause rapid opening and closing of the valve. This is known as "chattering". IMO, chattering is a misleading word in this case, because one thinks of someone who is just too talkative, but in reality, chattering in a safety valve will result in an unpredictable reduction in relief capacity, damages to the seating surfaces, and possible destruction of the safety valve and nearby pipe and fittings. If the latter occurs, at best, it could be career damaging/ending. At worst, folks lives may be at stake.

It sounds like you are talking about relief valves that are WAY over-sized, so I'll skip the business about shorten the pipe, reduce the fittings, relocate the PSV closer/to a bigger nozzle, etc.

There can be a little wiggle room, if everyone agrees. Inlet pressure losses up to 5% may be allowed if the pressure relief valve blowdown setting can be increased. This depends on the particular relief valve. Typical blowdown is about 6% of set pressure, and the allowable inlet pressure drop is half this value (3%). If the blowdown ring can be adjusted to increase blowdown to 10%, then the inlet pressure drop can increase to 5% without affecting valve stability. Pressure drops greater than 5% are not acceptable.

It is my experience that getting the approval from management and process safety (to CYA) requires much more resources than buying a smaller PSV and modifying the existing piping just at the PSV to accept the smaller size inlet and outlet. You don't have to replace all the pipe with smaller pipe. Plus, again I get the feeling that some of your relief valves are so oversized the extra 2% won't do any good anyway.

The 10% rule on the outlet pressure drop is to not affect the capacity of the PSV. Discharge piping from a single safety valve must be at least as large as the safety valve outlet flange size nominal area and must meet pressure drop constraints. The size of discharge piping must be checked to ensure that piping discharge pressure drop under any expected condition will not adversely affect set pressure or capacity. If you go to a smaller PSV, it's a no-brainer that you'll meet Code and be good on the outlet pressure drop.

Good luck,
Latexman

 

[dcasto]

Engineering judgement & Risk Managemnet, thats it.

You can have a properly sized relief valve with 400 pages of documentation and process plant operators and marketting will always find ways to have a lower than design throuput/volume/release scerio at any relief valve you install that will cause chatter. Deal with it, which may include living with it.

How many people conven an MOC hazop for a reduction in throughput and redo all calculations for every throughput reduction? How many document absolute minimum pool fire sizes such that your fire case PSV is not chattering?