Orion Engineering, PLLC


PRV Seminar


Does your plant have Pressure Relief Devices? Do you also have any of the Highly Hazardous Chemicals listed in OSHA Regulations (Standards – 29 CFR) Process Safety Management of Highly Hazardous Chemicals.-1910.119. This standard requires the employer to have a compilation of written process safety information if the quantity is above the threshold value listed in Appendix A of the standard. This process safety information shall include information pertaining to the hazards of the highly hazardous chemicals used or produced by the process, information pertaining to the technology of the process, and information pertaining to the equipment in the process. Information pertaining to the equipment in the process shall include Relief System design and design basis.


Typical Findings

Our experience indicates that 25% to 50% of existing relief valves will be inadequately sized or poorly installed. The majority of failures occur because the inlet piping to the relief valve is too long or too small. This results in a pressure drop above the ASME 3% limit and means that the pressure in the equipment will rise above the MAWP beyond the allowance provided by ASME. The second largest component of failures is pressure drop in the outlet piping exceeding the ASME 10% limit. This results in valve chatter which reduces the relieving capacity of the system and can easily damage the valve enough to prevent complete reseating.


Our spreadsheet minimizes the time required to produce the calculations and minimizes errors. Our software, with a five-year track record, provides an analysis of each pressure / vacuum relief device (valves, rupture disks, and vents) based on the appropriate code (ASME, NFPA, API, ASHRAE) and over/ under pressure situation. It can handle gases, liquids and two phase flow.

What ORION ENGINEERING’S Analysis considers:

bulletClosed Outlets
bulletInstrument Failure
bulletCooling Water Failure
bulletAutomatic Valve Failure
bulletEntrance of a Volatile Fluid
bulletHeat from External Fire
bulletPump(s) – Isolated
bulletChemical Reaction or Decomposition
bulletCompressor(s) – Isolated
bulletAbnormal Fluid Input
bulletThermal Expansion of Liquids
bulletAbnormal Heat Input
bulletPower Failure (electrical, steam, etc.)
bulletHeat Exchanger Tube Rupture

Our Calculations include:

bulletPhysical Properties
bulletInlet & Outlet pressure drops
bulletOrifice sizing for several cases
bulletStructural Load due to reaction force

Our Output Documentation

Complete documentation is provided for each device as required by PSM.

Documentation includes all calculations, a sketch of the proposed or redesigned piping/equipment protected by the device, the design basis, the worst case cause of relief, the required / selected relief device orifice area, a relief device spec sheet, and supporting catalog cuts. The output can easily be tailored to your relief record form.


Orion Engineering is pleased to offer this service to our industrial clients and would be happy to provide a written proposal for your specific situation.

What do we need to complete the calculations?

bulletAccurate P&ID
bulletIdentification of relieving chemical
bulletOperating conditions
bulletPiping / equipment arrangement or a short plant visit

What do you get back?

bulletCalculations stamped by registered Professional Engineer in your state.
bulletRecommendation for piping or device change.
bulletSketch indicating calculation basis.
bulletDocumentation indicating the method used for confirming the device.
bulletComfort that your device is adequate and that you have satisfied the legal requirements.

How much does it cost?


For WOG systems we estimate about 10 hours of our office time plus some plant time to gather data. Other systems will require more analysis time.

I have a number of similar systems and a lot of devices.

bulletThere is a quantity discount available.
bulletThere is a discount for multiple similar systems.

Can I get a sample of the documentation package?


We will be happy to mail you a completed sample of our documentation.

I need to convince some of our managers, can you help?


Orion Engineering offers a relief device seminar that should resolve any questions. Contact us to make arrangements.



API  - American Petroleum Institute


ASHRAE - American Society of Heating and Refrigeration Engineers


ASME - American Society of Mechanical Engineers


Inlet Piping - The length and size of the pipe leading from the protected vessel to the inlet of the pressure relief device.


MWAP - Maximum Allowable Working Pressure the legal limit of pressure in the protected vessel.


NFPA - National Fire Protection Association


Orifice - The critical opening in the relief device through which all fluid must vent to prevent vessel overpressure or perhaps failure.


Outlet Piping - The length and size of the pipe leading from the pressure relief device outlet to the to the atmosphere or blow off tank.


Two-Phase Flow - A situation in which both a liquid and a gas or vapor attempt to vent through the relief device orifice or a pipe at the same time.

Home Up PRV Seminar

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Last modified: 12/21/04