How to Choose Superimposed Relief Valve?

When selecting a pressure relief valve (PRV) for any application, many factors need to be taken into consideration. One of the most important — and least well understood — is back pressure. This article explains what back pressure is and how it affects the performance of PRVs.

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What is back pressure?

Back pressure is the pressure on the outlet of the pressure relief valve. There are two types of back pressure:

• Superimposed back pressure. Superimposed pressure is the pressure in the discharge header before the pressure relief valve opens. Depending on the system, superimposed back pressure can be constant or variable.
• Built-up back pressure. Built-up back pressure is the pressure that develops as a result of flow after the pressure relief valve opens.

When a PRV discharges, these two elements exist together as combined back pressure.

How does back pressure affect PRV operation?

Back pressure can significantly affect a valve’s performance by reducing both its set pressure and its capacity. Too much back pressure can result in chatter (rapid opening and closing), which can damage the valve.

Different types of PRVs are affected differently by back pressure:

Conventional relief valves

For conventional relief valves, back pressure reduces set pressure directly on a one-to-one basis. For example, a valve with a set point at 100 psig that is subjected to 10 psig of back pressure will not reach set point until the system pressure reaches 110 psig. In this example, if the set point is not adjusted to compensate for the back pressure, this can mean that valves are operating at a level that is higher than their maximum allowable working pressure (MAWP).

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The effect of back pressure on valve capacity is much more significant. Typically, high back pressure can decrease the PRV’s capacity by approximately 50%.

Balanced bellows relief valves

For balanced bellows relief valves, the bellows mitigate the effects of back pressure up to a certain point. These valves are generally not affected unless the back pressure exceeds 30 or 35% of set pressure. The tradeoff is that they can fail at higher pressures.

Pilot-operated relief valves

For pilot-operated relief valves, back pressure is even less of a problem.

How much back pressure is acceptable?

Back pressure needs to be accounted for when sizing a PRV. In general, back pressure should not exceed 10% of the set pressure, especially for conventional relief valves.

How do you compensate for back pressure?

The key to handling back pressure is to take it into consideration when sizing and selecting your valves. If you know that the back pressure in your system will be higher than the recommended limits, you may need to select a larger valve.

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Hi all,

I have come across a conventional relief valve that has challenged my understanding of API 520, so I am here to ask for some insight.

Set Pressure: kPag
Constant Superimposed Back Pressure: kPag
Variable Superimposed Back Pressure: 20 kPag
Built-Up Back Pressure: negligible
Sizing Case: Blocked flow pump discharge

This relief valve already exists in the plant I work at and relieves into the suction side of the pump it protects. The choice of a conventional PSV caught my attention given the high back pressure.

From API 520 Section 5.3.3.1 the clauses strictly refer to built up back pressure not exceeding 10% of set pressure (21% for fire, etc).

Section 5.3.3.2.1 states "Balanced valves can typically be applied where the total backpressure (superimposed plus built-up) does not exceed approximately 50 % of the set pressure.

I guess my questions are, is this a common installation for a conventional relief valve and am I correct in interpreting it as meeting the intent of API 520?

Thanks

You appear to have one of those rare PRV applications involving high constant back pressure.

If you read API-520 5.3.3.1.3. carefully towards the end, it states that for superimposed constant back pressure "...the spring load is reduced to compensate...". This means that for a Conventional Pressure Relief Valve, the shop Spring Setting is = Set Pressure - Constant Back Pressure.

Note that we are both referring to API-520 9th edition (New 10th Edition exists - but I don't yet have).

If the fluid is a compressible gas/vapour then a flow correction factor is necessary if the flow is sub sonic. For Liquids no correction is made.

There is effectively no limit on constant back pressure within reason. The most extreme I have seen is 95% constant back pressure. For liquids the size of the valve is affected in the calculation which takes the pressure differential into account (DeltaP). For balanced bellows PRV's with variable back pressures, gas/vapours and liquids, API-520 states maximum back pressures at 50% but this is a guide only. Many manufacturers have possibilities to go higher and have their own corrections factors, but generally follow the established limitations.

In your case, I would think that the small variable superimposed back pressure was ignored as it represents only 0.8 % of set pressure. Most probably engineered away in 'set pressure tolerances'.

Your PRV nameplate should have a Spring Setting of Set Pressure - Constant Back Pressure = 400 kPag (it may be prefixed CDTP = Cold Differential Test Pressure and have a small temperature correction factor applied for ambient testing).

Your PRV should also have 300LB x 300LB fully rated inlet and outlet flanges. A 300LB outlet is non standard.

Hope that helps with your understanding.


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