PSV Blowdown

[trainspotter808]

Evening all,
Hopefully a relatively simple question, but struggling to find answers in the relevant standards (API/ISO etc.,)

My question concerns conventional spring loaded pressure relief valves. Typically for gas, relief valve blowdown will be around 7% and for liquid about 20%.

If the safety valve does not fully lift in a relief event and begins to close after the event is over, will the pressure at the relief valve still need to decrease to below the blowdown for the valve to reseat?

For example, if I have a liquid relief valve set at 10 barG, and I have a relief event which results in the safety valve only partially open, and the normal operating pressure in the line is 9 barG - will this mean the safety valve will not reseat and continue to simmer/leak?

Thanks

 

[udyman]

My understanding is that if the PSV lifts you will need to drop below the blowdown for the valve to reseat. Even if the valve does not fully lift.

If you are operating at 90% of set pressure with a conventional valve and the PSV lifts, you are correct that you may not get the PSV to reseat once you return to normal operating pressure. One way around this issue is to replace your conventional PSV with a pilot operated PSV. These PSVs have a smaller blowdown value, which allows them to be operated closer to their set point.

 

[LittleInch]

Your wording is a bit strange to me.

"Blowdown" usually means releasing pressure down to zero as a result of any trip or for maintenance.

What you seem to be talking about is re seat pressure?

Your example has pressures really quite close to each other and you may well be correct. Liquid syste s are much more susceptible to short term spikes or surges so these type of valves may not be the best choice or set correctly.

 

[trainspotter808]

Thanks for the reply.

Blowdown, when we talk about pressure relief valves, is the difference between reseat pressure and set pressure.

My concern with the scenario above, is that because the operating pressure is so close to the set pressure, and actually overlaps the relief valves blowdown pressure. The relief valve will not reseat after a relief event.

 

[The Obturator]

Many factors will ultimately determine how the valve would react in such a situation, such as; design, ratio between calculated orifice area and selected orifice area, ring setting, inlet piping, inlet pressure loss, back pressure etc., etc.

Theoretically, a spring operated conventional Pressure-relief Valve on liquid service, such as the one you describe, would be trying to open in proportion to the increase of inlet pressure.

Realistically, being a spring valve on liquid, the valve will be reacting to the rise of pressure at set pressure point and trying to close straight away against the flow it is seeing at that point, if the flow is not great and/or not sustained. This is also known as chatter and can be quite severe leading to ultimate valve failure. Will it happen at lower flow/pressure? Very likely. Again many factors can influence it. Will it affect the length of blowdown? Maybe, maybe not.

If the scenario you advise is going to be an issue for your installation, it would be best remedied by the use of a modulating design pilot operated pressure relief valve. Such a design provides as close to a proportional lift as possible, if the conditions are optimal for such a selection. There are some spring designs available which claim a similar real proportional action, but not all manufacturers have them. In case, discuss the application with the vendor.

The only way to answer your question is to test at the stated conditions, but this may prove difficult to do (and expensive).

Let me add that API 520 Pt II Section 4.3 has some recommendations if the valve is experiencing pressure fluctuations.

 

[The Obturator]

@LittleInch 'Blowdown' In Pressure-relief Valve speak, is the term in ASME/API which refers to the reseating pressure of a Pressure-relief Valve. Blowdown is expressed as a percentage against the set pressure. eg. A blowdown of 7% means a valve set at 100 would reseat at 93.

I agree though, it should be termed just 'reseating pressure' to avoid confusion with (the other) 'blowdown'.

 

[Latexman]

Yes, "blowdown" is being used correctly by OP. It's been used routinely in my relief vocabulary for decades, and so has "reseating pressure".

 

[trainspotter808]

Thanks for the replies all.

It is just slightly confusing whereby the valves can be quoted as leaktight up to 90% of the set pressure, however after a relief event, when the valve lifts, it will reseat at 80% of the set pressure.

Implying we should limit the operating pressure at the safety valve inlet to around 80% of the set pressure to allow this relief valve to fully reseat.

 

[LittleInch]

You learn something new every day here, so thanks for the information on "blowdown" for relief valves.

The implication I see in the question is that a relief event is then followed by "normal operation". This, for me, is not the way it should work in that the relief event has only occurred due to the system exceeding its design pressure, which should have also resulted in a trip / shutdown or if this is a gas system, a true "blowdown" as the ESD valves close and the system depressurises. Your Pressure / safety Relief valves are there to protect the system from overpressure, not to allow you to operate closer to the design margin than clearly your system can operate without exceeding design pressure. Any opening of a pressure relief valve set at the design pressure, however short, should result in a detailed investigation as to how this happened and put in place controls, procedures, whatever, to prevent it happening again.

Otherwise if you just shrug your shoulders and keep going, you are, IMHO, normalising failure and that's not a good thing.

So good, well managed and controlled systems with a slow acting fluid can operate at 90% of set point, but if you keep alarming and tripping then you're sailing too close to the wind. So there is no blanket "Well we need to operate at 80%" - no one I know thinks like that. Margins for alarms, trips and then relief valve settings means that in reality 85% is probably about as good as you get for low pressure systems. You probably need 8 to 10% or 2 bar, which ever is bigger, between Max operating and Design pressure.

The issue with lower pressures is more one of difficulty controlling to fractions of a bar. So operating at 90 bar with a 100 bar design pressure is probably easier than operating at 9 bar for a 10 bar design pressure system. Your instruments etc just aren't accurate enough to do that.