resources
sizing
There are several methods for sizing a pressure relief valve, depending on the specific system in which it is installed. In many cases, multiple operating scenarios may need to be considered, and all relevant conditions must be evaluated. Pressure relief valves are designed to release a specified flow rate under the particular conditions of the system to prevent overpressure. The flow rate used in sizing is determined by the amount of fluid or gas that must be relieved to avoid excessive pressure buildup.
The outcome of the sizing process defines the minimum required area for the valve to achieve the desired flow rate. This calculated area is then used to select the appropriate valve orifice size.
Various standards and organizations have established methods and calculations for valve sizing. Two of the most commonly used approaches for blocked flow sizing are from the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API). While both methods follow similar principles, the key difference lies in the discharge coefficient and orifice areas. API utilizes generic values for these parameters, while ASME relies on the specific orifice area of the valve series and the measured discharge coefficient for more precise results.
sizing tools
Use our Mercer Sizing Spreadsheet download to help you choose the right valve for the job, or use our capacity calculator for quick and easy flow rate calculations.
sizing forms
Use the following forms to provide us with the specifics of your application and we’ll take it from there. Just save the filled out form on your computer and then email it to insidesalesgroup@mercervalve.net.
valve shipping flyers
repair kit flyers
installation, operation, & maintenance manuals
literature & standards
Our Mercer Valve ISO 9001:2015 Certificate, General Information brochure, and the latest Product Line Card as PDF downloads.
general terms
Set Pressure
The point at which the pressure relief valve is set to open.
Pilot Operated Pressure Relief Valve
A valve where the opening and closing of the main valve is controlled by an auxiliary pressure relief valve called a pilot. The main valve is the primary relief device.
Superimposed Back Pressure
The pressure on the outlet of the valve at the time the valve opens. This back pressure is caused by other sources and will affect the set pressure of a conventional pressure relief valve. Superimposed back pressure can be variable or constant.
Over Pressure
The amount of pressure above the set pressure that is allowed for the valve to go to full lift and achieve full flow. For most applications, this is typically 10% or 3 psi, whichever is greater.
Back Pressure
Pressure on the outlet side of the valve. There are two types of back pressure: superimposed and built-up. Some types of back pressure can affect the flow rate and/or the set pressure.
Blowdown
The difference between when a valve opens (set pressure) and when the valve closes. Typically, blowdown is expressed as a percentage of the set pressure.
Conventional Pressure Relief Valve
A direct spring operated valve. The spring tension controls the opening and closing of the valve. The set pressure is affected by back pressure on this type of valve.
Built-Up Back Pressure
Pressure in the outlet of the valve caused by the flow of the valve after the valve opens. This type of back pressure does not affect the set pressure.
“Auto Seat Technology” ®
Mercer Valve’s soft seat relief valve design. ‘’Auto Seat Technology”® increases seat life, reduces repairs and gives the valve a consistent set pressure.