5 Things to Know Before Buying superimposed relief valve manufacturer

Posted 06 December 2006 - 07:22 AM

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There are two issues. The first is the set pressure and the second is the affect of built up back pressure.

Taking the first issue, we alll agree that the set pressure shall not be any higher than the MAWP of the vessel and this is clearly spelled out in ASME. We also agree that for a conventional PSV, if the superimposed back pressure is greater than what the spring was designed for (probably atmospheric in this case), then the valve won't open at set pressure but at set pressure + superimposed back pressure. And I believe your argument Mvancleave is that the latter is OK as long the allowable over pressure within the vessel is never exceeded before, during and after the valve opens.

After going through API RP520 and 521 again for the umpteenth time, I almost came to agree with you until I went through ASME Section VIII, Div 1 again for the umpteenth time. This is what you missed (by the way, I think you may have an older edition of ASME because my copy is 2006 and the wording you quote is not quite the same).

UG-134 PRESSURE SETTING OF PRESSURE RELIEF DEVICES

(a) When a single pressure relief device is used, the set pressure (60) marked on the device shall not exceed
the maximum allowable working pressure of the vessel. ......

The (60) above is really a superscript that refers to a note. And this note reads:

(60) The set pressure is the value of increasing inlet static pressure at
which a pressure relief device displays one of the operational characteristics
as defined by opening pressure, popping pressure, start-to-leak
pressure, burst pressure, or breaking pressure. (The applicable operating
characteristic for a specific device design is specified by the device
Manufacturer.)

Obviously, this note pertains to many types of relief devices, not just to PSVs.

What we see here is that ASME defines what they mean by "set pressure" and the intent is to define the "set pressure" as an opening pressure. It does not appear that the intent of ASME is to separate the concept of set pressure from opening pressure as you seem to be interpreting the readings. Therefore, if you accept this defiition for set pressure, then you must accept the fact that the PSV on the vessel shall not open or begin to open at a pressure greater than MAWP. And, if you manifold the discharges of your conventional PSVs, you will most certainly have the potential to violate the Code.

I won't argue that API RP520 seems to allow this condition to exist but in my opinion, they are allowing you to violate the spirit of ASME if not the letter and this is unaccpetable. And remember, API RP520 is NOT CODE, just a recommended practice. And another thing, the definitions given for "Accumulation" and "Over Pressure" refer to dynamic states, i.e. the PSV is in relief, not static where it is waiting to open. So again, I don't think that if you want to stay within the spirit of ASME, you can adapt the argument of allowing accumulaiton or over pressure to build up before the valve actually starts to relieve.

One last thing on this particular issue, you quote a section from the Crosby Engineeing Handbook. This was lifted from API RP520 and you really should go to the source standard and not rely on a vendor's hand book.

3.3.2 Effects of Superimposed Back Pressure on Pressure Relief Valve Opening

3.3.2.2 .... Balanced spring-loaded or pilot-operated pressure relief valves should
be considered if the superimposed back pressure is variable.
However, if the amount of variable superimposed back pressure
is small, a conventional valve could be used provided:
a. The set pressure has been compensated for any superimposed
back pressure normally present; and
b. The maximum pressure during relief does not exceed the
Code-allowed limits for accumulation in the equipment being
protected.

API uses the word "small" and Crosby seems to use the word "low". I'll go with API and perhaps it has to do with the version they lifted the text from. But I ask you, who defines "small" or "low"? API certainly doesn't! Crosby certainly doesn't! And I don't see it in ASME.

Bottom line, no, you should not design your system the way you intend. Perhaps you can decouple the vessels so that you won't violate the Code for the vessels in question and only manifold the vessels that won't be affected by this.

I don't have time right now to go into issue two but I think I addressed it in my previous post.

Safety Valves - How They Work

Safety valves are used to protect the system from overpressure. Overpressure occurs when the pressure exceeds the maximum allowable working pressure (MWAP) or the system design pressure. Compared to safety valves, safety valves can open very quickly. The relief valve opens from the set pressure; the valve opens a little first and then opens fully to remove unwanted pressure from the system as quickly as possible.

Safety valves are used to prevent pressure build-up that could lead to malfunction, fire or explosion. Safety valves have only mechanical parts, so they are used when electronic or pneumatic safety devices fail. The safety valve is fully actuated by the system medium, allowing it to remain operational in the event of a power failure.

content

Important terms

selection criteria

application

Important terms

Overpressure: Overpressure that exceeds the set pressure of the safety valve.

Working pressure: The pressure at which the system works under normal working conditions.

Set Pressure: The pressure at which the disc begins to lift and the safety valve opens.

Lift: The distance the disc moves from the closed position to the position required for discharge.

Back pressure: The pressure built up on the outlet of the safety valve during flow. Back pressure = cumulative back pressure + superimposed back pressure.

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Suggested reading:
Understanding Proportional Control Valve Circuits

Accumulated Back Pressure: The pressure at the outlet after the safety valve opens.

Superimposed Back Pressure: The pressure at the outlet of the closed safety valve.

Maximum Allowable Working Pressure (MAWP): The maximum allowable pressure at the specified temperature under normal operating conditions. MAWP is the maximum pressure that the weakest part of the pressure device can handle.

Pressure Relief: The difference between the actual pressure when the disc is lifted and the actual pressure when the valve is closed. Usually expressed as a percentage.

Relief Capability: The rate at which a safety valve releases excess pressure.

selection criteria

To protect your system from overpressure, it is critical to understand and select the following five selection criteria. Read our technical article on selecting safety valves to better understand the five main selection criteria below.

set pressure

back pressure

Discharge capacity

Operating temperature

Valves and Sealing Materials

application

Safety valves are mainly used in industrial applications to prevent overpressure. This overpressure could result in a hazardous situation, fire or explosion. Safety valves are commonly found in:

Oil, gas and petroleum industry. Equipment failure can lead to fire or explosion, in which case the safety valve must discharge hazardous substances.

energy sector. They are typically used to vent steam, air, gas or liquid overpressure in piping or vessels/boilers.

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