As a process engineer in the oil and gas industry, sizing a pressure safety valve (PSV) for a positive displacement pump is an essential task to ensure the safe operation of the pump and protect the system from overpressure. Here's a step-by-step guide on how to size a PSV for a positive displacement pump using SI units:
Step 1: Gather necessary data:
Pump capacity (Q), usually in cubic meters per hour (m³/h)
Maximum allowable working pressure (MAWP) of the protected equipment, in bar
Set pressure (SP) of the PSV, typically a percentage of the MAWP, e.g., 110% of MAWP
Backpressure (BP), if any, in bar
Relief pressure (RP), which is the sum of the SP and BP
Specific gravity (SG) of the fluid
Pump's rated speed (N), in revolutions per minute (RPM)
Step 2: Determine the required relief flow rate (Qr): For a positive displacement pump, the required relief flow rate is the same as the pump capacity (Q).
Qr = Q
Step 3: Calculate the required orifice area (A): Use the following formula to calculate the required orifice area, where K is the specific gravity of the fluid (water = 1), and C is the discharge coefficient (usually around 0.975 for most PSV designs).
A = (Qr * K) / (C * 0.61 * sqrt(RP))
Make sure the units are consistent, e.g., m³/h for flow rate and bar for pressure.
Step 4: Select the appropriate PSV size: Compare the calculated orifice area (A) with the standard orifice areas provided by the PSV manufacturer. Select the PSV size that has an orifice area equal to or slightly larger than the calculated area.
Step 5: Confirm the PSV selection: Verify the selected PSV size with the manufacturer's datasheets or technical support, ensuring that the valve's set pressure, backpressure, and material compatibility meet the requirements of your application.
Remember to follow relevant industry standards and guidelines, such as API 520 and API 521, for the selection, installation, and maintenance of PSVs in the oil and gas industry. Additionally, always consult with experienced professionals and cross-check your calculations for accuracy and safety.
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