Hello All,

This may be a very stupid question. But Why is the reason that we set pressure relief valve higher value than the Maximum allowable pressure value?

For example: I have a system which has a maximum pressure 75 PSI (limited by pump) but the pressure relief valve is set to 90 PSI? Shouldn't it be set lower than 90 PSI? or is There any thumb rule?
Not enough details and definitions matter.

If this is a pressure vessel then the relief valve set pressure should be no higher than the MaXimum Allowable Working Pressure (MAWP).

This is stamped on the vessel plate.

Setting a relief valve higher than that is not permitted.

You say "maximum pressure". Be clear about your definition. MAWP or some other pressure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it. Is the relief valve protecting the system or pump? If the pump max pressure is 75 psig and its a 100 psig system, someone may have elected to put a 90 psig relief valve just in case.

If the system design pressure is 75 psig, a relief valve set pressure of 90 psig does not meet code. Relief valves require over-pressure (per Code) so non-fire cases may be 110% set pressure and fire cases may be 121% set pressure but the set pressure itself cannot be higher than MAWP or system design pressure. ThanK @RVAMECHE

So if MAWP 90, for Non fire case My PSV should be 81 PSI and fire case 71 PSI? Is that a right understanding?

Could you please explain more about the "Code". Where could I find one? website?
I really need to read it thoroughly in order to understand this better.
Curious,

FIRST!
You must identify if that 75 psig "maximum pressure" is actually the MAWP of the pump or if it is the maximum deadhead pressure of the pump. I have found that pump manufacturers are often NOT knowledgeable in pressure relief and state the pumps maximum discharge (operating) pressure as just "maximum pressure". Most times the casing/seals/flanges can take a higher pressure (true MAWP), but you have to call the OEM and find the right person to discuss with in order to find out the answer. No offense but you need to talk to a senior engineer. Process safety isn't a good area to have these types of questions.

Code is the piping design code (ASME B31.1, ASME B31.3 typically for piping in the states). This document (which you have to buy) has rules and requirements you have to meet. Ideally somewhere there is a piping service index or something that says something like "XYZ Chemical System - Pipe Spec #6 - ASME B31.3 Normal Fluid Service - Typ Operating Pressure 35-50 psig - Design Pressure 100 psig" etc.

Relief valve set pressures can be <= the MAWP/design pressure of the system. If the system is designed for 100 psig, it can anything below, and including, 100 psig. Typically the design pressure is used as the set pressure to give yourself the biggest cushion between operating pressures and to keep things simple.

Pump max pressure =/= design pressure. Your system should have some sort of pipe spec that defines the design pressure/temperature which you use. These pipe specs may be across multiple processes depending on pressures, temps, materials, etc required. It's not wildy clear and the GRI website isn't great, but as the material is some sort of plastic, it looks to me like the MAWP of the pump body is 75 psi. Therfore having a relief valve set at 90 psi is not correct.

In reality the pump will probably survive, but it isn't correct.

I think RVAs reply is a bit confusing. The set pressure should not exceed the design pressure or the MAWP. The max pressure in the system when the relief valve lifts can be higher,but not the set pressure.

Any system has one element which sets the MAWP. This could be the pump, the pipe or some other component like a flange or valve. That's what matters.

From your data your relief valve is set too high and should be no higher than 75psi, but depends on what the rest of the system looks like.

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

How to Properly Set Relief Valves

By Bob Wojcik

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Stand-alone relief valves are relatively simple to set properly. After installing a relief valve in a circuit, with the pressure port connected to the pressure line and the tank port connected directly to the reservoir, adjust the relief valve to its lowest possible pressure setting. This will avoid an unexpected condition when the hydraulic power is applied to the circuit with this newly-installed relief valve.

Once the relief is properly installed and set at its minimum setting, turn on the fluid power. There may be noise coming from the relief valve, as flow may be racing through the relief when it is at its lowest pressure setting. Next, begin to increase the relief valve setting and monitor the pressure gauge associated with the circuit of this new relief valve. This method for setting a system relief valve is common to a circuit with a fixed displacement pump.

When the system includes a relief valve used in a pressure compensated pump circuit, this setting procedure becomes a little more complicated!

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As described above, install the relief valve in the circuit with the pressure port connected to the pressure line and the tank port connected directly to the reservoir. Ensure that the relief valve is adjusted to its lowest possible pressure setting. Again, this avoids an unexpected condition when the hydraulic power is applied to the circuit with this newly-installed relief. Once properly installed, set both the pump compensator and the newly-installed relief valve to their minimum settings. Start the hydraulic pump and begin to turn the system relief valve to nearly the maximum adjustment position. Now begin to adjust the pump pressure compensator to a setting about 200 PSI above the final desired system pressure setting. Next, begin to reduce the setting on the system relief valve until the pressure on the pressure gauge begins to drop below the pressure setting of the pump compensator. Now, lock the system relief valve at the position that remains the 200 PSI above required pressure. Last, set the pump pressure compensator setting to the actual system pressure that is required.

Remember, the system pressure relief valve must be set about 200 PSI above the pump pressure compensator setting, or the two pressure settings will fight each other. This would result in the system acting as if it were a fixed displacement system. Loud noises can be generated by the pump and this condition will also begin to develop high heat generation in the hydraulic fluid!

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