behavior of a PSV for gas when the system protected needs a continuous very low discharge flowrate 3

[gelsi]

Hi,
an auto regulation device of upstream pressure loses its seal while the downstream pipeline is blocked in; so the pressure rises to the set value and the PSV start opening. The leakage flow is a very low fraction of nominal flowrate od PSV, for example 1% or minus. What scenarious you think is true:
1 - the PSV opens to a middle position and than recloses when pressure reaches the blowdown, after a some time reopens, and so on;
2 - the PSV opens and remains opened at a middle position all the time.
I like 2, heven if it is a PSV for gas.
Thanks

 

[The Obturator]

I'll consider the PRV as a spring operated type here.
PRV's are sized according to a known, often worst case, giving an amount of flow to be passed at a pre determined set point.
Anything less than this flow may well result in PRV unstable operation.

Your quoted example is basically saying that the PRV will be seeing a very low flow. In such a case the PRV will be experiencing Chattering - the very rapid opening and closing of the seats. This ultimately leads to PRV and equipment failure since the PRV is not designed to do that.

Effectively you have described this in your scenario 1 except this will be happening very very quickly.

You need to review all relief cases of what the PRV has been sized and selected for. You may even need to consider a pilot operated PRV with modulating action which will open only in proportion to the flow.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[LittleInch]

liquid or gas?

You might need two PSV's, one very samll "thermal", basically 1/2" x 3/4" set at a lower pressure than the bigger valves.

Or better still, install an isolating valve upstream your leaking regulator.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[gelsi]

Thank for your replies!
My intent is more to understand the behavior of the device in that situation rather than to identify the appropriate guidelines for the design.

When the gas-flow that causes the slow overpressure is constant and very low, does the gas-PRV (spring operated)open just as much as necessary, or does it open more than necessary, discharging a greater flow rate and generating a reduction in upstream pressure until it closes (blowdown pressure)?

If re-closing happens, I imagine the time to re-open depends on ratio (volume of gas needed to rise the pressure from blowdown to set) / (constant flowrate), so I think it can not be a quick process.

 

[LittleInch]

It can do both.

You either get"simmering" where the valve chatters and can fatigue after a while or you get pop and slam where the valve lifts, gas flow for a second or two until the pressure drops then it slams shut. Rinse and repeat.

Varies by size, type of valve and volume of gas in the pipe or vessel being relieved.

Neither is a good idea.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

It won’t chatter, but it will cycle open, closed, open, closed, etc. depending on the “low flow rate” and the capacity of the PSV. My guess is it will not be open very long and it will remain closed for quite a while. That’s only half of what chattering is.

Good Luck,
Latexman

 

[georgeverghese]

For a similar application with natural gas, we used pilot operated soft seated regulators - the downstream PSV didnt lift when there was no consumption downstream.

 

[don1980]

Pop-acting PSVs pop open to a position of 40-50% lift height. The pressure must then rise further (overpressure) in order for the PSV to lift to its full-open position. So in the case you describe, the PSV will pop open to 40-50% lift and not rise any further. It will remain in that approximate position until the pressure in the system has dropped to the blowdown pressure, at which point the PSV will close. Then the pressure will start to rise again, and this cycle repeats.

After popping open the PSV could potentially remain open (your point #2) if the leak rate across the regulator is approximately 50% of the rated capacity of the PSV. It's very unlikely that the leakage across a regulator will be that high.

All properly sized PSVs are oversized, due to the fact that one isn't allowed to take credit for the actual capacity of the PSV. For conservatism, codes require the orifice coefficient to be de-rated by multiplying it by 0.9. So, this type of cycling is expected (normal) for every PSV. And this cycling is fundamentally different from chatter instability, which is destructive cycling at a frequency of more than ~15-20 cycles per second.

 

[gelsi]

don 1980, in your opinion or technically if you know, what is the reason because they "open to a position of 40-50% lift height" and not proportionally to the flow rate?

 

[TiCl4]

Gelsi,

Look at a diagram of a pop PSV. As soon as the disc lifts away from the seat, the exposed surface area is suddenly increased due to the geometry of the disc (it “cups” the seat, if you will). The sudden increase in area causes a sharp increase in upward force, popping the valve open. The 40-50% Don mentioned must be a standard value that the valves are designed for.

 

[Latexman]

Look at a diagram of a pop PSV. As soon as the disc lifts away from the seat, the exposed surface area is suddenly increased due to the geometry of the disc (it “cups” the seat, if you will). The sudden increase in area causes a sharp increase in upward force, popping the valve open.

That is the “huddling chamber”.

Good Luck,
Latexman

 

[don1980]

...what is the reason they "open to a position of 40-50% lift height" and not proportionally to the flow rate?

The answer is that ASME, and other codes, incentivize mechanical designs that cause PSVs to reach full lift as quickly as possible. Specifically, codes state that full capacity ("rated capacity") is the flowrate that occurs at 10% above set pressure. So, manufacturers obviously needed to come up with a mechanical design that ensured that PSVs would be fully open at 10% over the set pressure (10% "overpressure"), and this resulted in the pop-valve designs that we have today. It's possible to design a non-pop acting PSV (one that opens in proportion to the system pressure), but those PSVs wouldn't achieve the same lift height at 10% overpressure. So there's not much of a market for such products, which is why those PSVs are not widely manufactured. Also, ASME effectively prohibits the use of proportional (modulating) PSVs by prohibiting mechanical designs that use mechanical (friction) dampening, which is necessary in order for a gas-service PSV to operate in a modulating manner.

So, today's pop valve designs result in the PSV lifting as quickly as possible, but these designs can't cause the PSV to reach full-lift immediately. And that's OK because getting the valve to an immediate 40-50% lift height is good enough - it's high enough for a 10% increase in pressure to get the PSV to full-lift.

 

[saplanti]

I am just guessing that spring design could be the major factor for the height of opening of safety valve in accordance with the code requirements on the pressure.

 

[The Obturator]

saplanti Amongst one of the factors of PRV spring design to consider is the rated lift of the PRV. The scenario presented in the original question talks about a flow as low as 1%. Whether the valve opens partially or whatever, it will not function as it has been designed to do. The spring force will simply be pushing back against a low flow which is incapable of fully opening the PRV. However it has been dressed up, PRV flutter and/or chatter will be the result. We don't even know the system set up, size of PRV etc.,to comment more. The best solution is to have a smaller valve opening first at a lower pressure just as LittleInch suggested earlier. This is an acceptable practice.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.