Establishing Relieving or Set Pressures

The pressure at which the valve is expected to open (set pressure) is usually selected as high as possible consistent with the effect of possible high pressure on the process as well as the containing vessel. Some reactions have a rapid increase in temperature when pressure increases, and this may fix the maximum allowable process pressure. In other situations the pressure rise above operating must be kept to some differential, and the safely valve must relieve at the peak value. A set pressure at the maximum value (w hether maximum allowable working pressure of vessel, or other, bill insuring protection to the weakest part of the system) requires the smallest valve. Consult manufacturers for set pressure compensation (valve related) for temperatures 200°F. 

Applied Process Design for Chemical and Petrochemical Plants 

Compensation Factors for Safety Relief Valves Between Atmospheric Test Temperature and Actual Operating Temperature [24] 

When the pressure rise in a system is gradual and not explosive in nature, a safety or safety relief valve is the proper device, but when it is critical to completely depressure a system or the rate of pressure increase might be expected to be rapid, then a rupture disk is the proper device. Properly designed a pilot operated valve may be selected after checking its performance with the manufacturer. 

Often a system (a group of vessels not capable of being isolated from each other by block valves, or containing restriction to flow and release of pressure) may need a relief valve set reasonably close, sat +15% to 20% when system is below 1000 psig above, typically use 7% to 15% above as set criteria related to normal operating pressure to catch any pressure upswing. Then this may have a backup valve set higher (but within code) to handle further pressure increase. Or, the second device may be a rupture disk. It is not unusual to have two relief devices on the same equipment set at different pressures. 

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