Pressure Relief Devices

A LEGAL REQUIREMENT FOR RELIEF DEVICE DOCUMENTATION

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.

EXPERIENCE DRIVEN DESIGN

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.

SOFTWARE DRIVEN CALCULATIONS

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:

	Closed Outlets
	Instrument Failure
	Cooling Water Failure
	Automatic Valve Failure
	Entrance of a Volatile Fluid
	Heat from External Fire
	Pump(s) – Isolated
	Chemical Reaction or Decomposition
	Compressor(s) – Isolated
	Abnormal Fluid Input
	Thermal Expansion of Liquids
	Abnormal Heat Input
	Power Failure (electrical, steam, etc.)
	Heat Exchanger Tube Rupture

Our Calculations include:

	Physical Properties
	Inlet & Outlet pressure drops
	Orifice sizing for several cases
	Structural 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.

FAQ’S

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?

	Accurate P&ID
	Identification of relieving chemical
	Operating conditions
	Piping / equipment arrangement or a short plant visit

What do you get back?

	Calculations stamped by registered Professional Engineer in your state.
	Recommendation for piping or device change.
	Sketch indicating calculation basis.
	Documentation indicating the method used for confirming the device.
	Comfort 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.

	There is a quantity discount available.
	There 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.

DEFINITIONS

	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.

Send mail to Webmaster with questions or comments about this web site. Last modified: 12/21/04

Send mail to Webmaster with questions or comments about this web site.
Last modified: 12/21/04