Gas Relief Flow Too High 1

[someguy79]

I am trying to configure/size some pressure relief devices in a gas plant. A new subsystem connects a low pressure loop (<10 psig) with a higher pressure (140 psig) set of tanks. In some situations, if valves are put in improper positions, it is possible to send gas from the high pressure tank directly to the low pressure loop without any pressure bleed down.

If I am to maintain 10 psig or less in the low pressure loop, the pressure drop must be achieved in the pipe between the tank and relief device. Such a flow rate is very large and would require an unreasonably large relief valve.

It seems to me, a flow limiting device is needed. I've seen orifice/valves that could do a good job dealing with this situation. Imagine a check valve that uses an orifice to limit flow rather than completely shut it off. I don't know any good suppliers of such objects, or if they would be considered a bad option for reasons I have not yet discovered.

Can anyone recommend solutions, strategies, or suppliers that might be of assistance with this problem?

 

[Latexman]

I have not seen a combination check valve/orifice that is commercially available. Maybe others have.

At our company we are required to assume the check valve fails wide open, so that kind of device would not be any help. You should find out what your company requires or give it some thought. Check valves do fail. It is more common for them to leak past, but occassionally I do hear of some that stick open due to rust, solids, sticky products, etc.

We can take credit for an orifice if we document it as a safety device on the P&IDs and in our maintenance system.

Good luck,
Latexman

 

[dcasto]

I call a combination check valve and orifice a Regulator.

 

[someguy79]

What I'm crossing my fingers and hoping for is different from a regulator. It would be something that limits flow only at high flow rates rather than something that proportions flow to meet desired pressures.

I need to keep the pressure drop fairly low in the low pressure loop during normal operation. A regulator is going to create a need for a more powerful blower or compressor, not to mention the adverse effects on process chemistry.

I suspect Latexman is going to turn out to be correct on both the availability front as well as the question of taking credit for something check valve-like.

 

[insult2injury]

If 1 relief valve is too large....then use 2. Why can't a standard orifice be placed in the high pressure feed line? Are there any scenarios where the pressure drop would be too large for proper operation?

I2I

 

[Latexman]

someguy79,

Sounds like you want an excess flow valve. I think I've also heard them called an excess flow check valve. I have several on my anhydrous ammonia tank. They are required by OSHA on anhydrous ammonia tanks. When I get to work tomorrow I can send the vendor's name.

Good luck,
Latexman

 

[BenThayer]

i would not consider an excess flow valve to be a safety device.

but i would consider a restriction orifice on a P&I-D as being something that would restrict flow.

or consider a rupture disc or rupture pin to get more capacity at a low pressure.

and consider "modeling" the system to take credit for any pressure drop/sonic flow limitations you might have in the rest of the piping that connects the high pressure to the low pressure side.

 

[someguy79]

So far I have four possible options to alliviate my problem.

1 Use more than one relief device.

2 Use burst discs or buckling pin devices rather than conventional spring actuated valves. These should be cheaper than typical relief valves.

3 Take credit for flow going any other possible place.

4 Use some sort of flow limiting device like an excess flow valve. Because they function very similarly to check valves, many people would question using these as safety devices.
Increase the set/relief pressure as much as possible.

The configuration that sounds the best at the moment is a relief valve in parallel with a burst disc. The valve set point would be slightly lower than the disc rupture pressure. This would allow for other overpressure cases to be dealt with by a fairly small valve. The burst disc would be there in case a large flow event happens.

If there were a good way to look at the dynamic response of the case described at the beginning of this thread, that might provide the justification for going with a lesser sized relief device. Unfortunately, all the methods I'm thinking of for it require some software my boss is not likely to purchase.

Oh, well. I'm sure we'll get something reasonable figured out. Thanks for the tips everyone!

 

[Latexman]

Ours are made by Metal Goods Manufacturing Company (MGM). They are required by OSHA on anhydrous ammonia tanks to guard against pipe shear scenarios.

Good luck,
Latexman

 

[insult2injury]

Don't forget to account for the 10% overpressure when determining the burst disc pressure.

I2I

 

[Latexman]

On # 4, a RO would be more robust of a solution.

Good luck,
Latexman