Relief Systems – Understanding Design and Design Basis

In compliance audits all over the country, we’re finding that the requirements for documenting the relief systems are not being met in many PSM programs. How bad is the situation? About 1/3rd of the relief documentation we see shows that the relief system DOES NOT meet the chosen design basis. About 90% of the remaining relief documentation doesn’t address the chosen design basis enough to make a case either way.

First, let’s take a minute to make sure we understand the difference between design and design basis.

Design Basis: The design code, standard, or RAGAGEP chosen to serve as the requirements. You can think of it as the conditions for success.

Design: How the facility actually met the Design Basis. This should include the “work” behind the design – the measurements, calculations, etc.

Let’s say we chose IIAR2-2008b as our Design Basis. Your documentation is likely limited to some spreadsheets provided by an engineer that shows the measurements and calculations that the engineer did to show that the piping design met the RAGAGEP.

That’s great. We definitely need that and that’s why you hired an engineer in the first place. But there is more to meeting the design basis than doing advanced math! You need to include an explanation of how all the other requirements were met. To give you an idea, of the sorts of things that you would need in your documentation for just IIAR-2 2008:

  • (11.1.1) The relief system meets ANSI/ASHRAE 15-2007 Section 9.4 [ref.4.1.4] excepting section 9.4.3
  • (11.1.2) If stop valves exist downstream of a relief device, that they are locked open when in service. Reference to a car-seal program to control the valve position.
  • (11.1.2) If stop valves exist downstream of a relief device, the relief calculation includes the pressure drop across the stop valve. Reference to the engineering calculations to back this up.
  • (11.1.3) Relief valves are installed as near to equipment being protected as possible.
  • (11.1.4) Unless hydrostatic the relief is connected at the highest practical point.
  • (11.1.5) If installed in refrigerated spaces, precautions have been taken to prevent moisture buildup in the valve or relief line. Reference maintenance procedures or PHA section as necessary.
  • (11.1.6.1) The relief valves are set to function at a pressure equal or lower than the design pressure of the protected system.
  • (11.1.6.2) If installed, rupture discs are set to function at a pressure equal or lower than the design pressure of the protected system.
  • (11.1.6.2) If installed, rupture discs conform to Section VIII, Division 1, ASME Boiler and Pressure Vessel Code.
  • (11.1.6.2) If installed, rupture discs ahead of relief valves are at least as large as the relief valve inlet.
  • (11.1.6.2) If installed, rupture discs have provisions in place to detect pressure buildup between the disc and a relief valve. Reference to the maintenance procedure (such as daily walk-through) and any automated systems in place.
  • (11.1.7.1) Pressure relief valves settings are set by the manufacturer.
  • (11.1.7.1) Pressure relief valves are marked to conform with Section VIII, Division 1, ASME Boiler and Pressure Vessel Code.
  • (11.1.7.2) Rupture discs are marked to conform with Section VIII, Division 1, ASME Boiler and Pressure Vessel Code.
  • (11.1.7.3 & 11.1.8) The capacity of the relief devices are stamped on the valve or available.
  • (11.2.1) Pressure vessels relief protection complies with Section VIII, Division 1, ASME Boiler and Pressure Vessel Code
  • (11.2.2) If a pressure vessel is capable of being isolated by stop valves, it must have overpressure protection.
  • (11.2.3) Reliefs are sized in accordance with IIAR2-2008b 11.2.7. Reference to the engineering calculations to back this up.
  • (11.2.5) Vessels with internal volume of 10 cubic feet or greater are equipped with three-way valves and dual pressure relief valves or a single valve if the vessel is on the low-side, can be pumped out and the other vessels are protected in accordance with IIAR2-2008b 11.2.7.
  • (11.2.6) When pressure is relieved into other vessels, that the back pressure is taken into account and that the affected vessel has its reliefs sized for the combined capacity of both vessels. Reference to the engineering calculations to back this up.
  • (11.2.7) The discharge capacity of the relief device meets the requirements of this section. Reference to the engineering calculations to back this up.
  • (11.2.7) When one relief device serves multiple vessels, the discharge capacity of the relief device meets the requirements of this section. Reference to the engineering calculations to back this up.
  • (11.2.7) When combustible materials are used with 20ft of a pressure vessel, the modified formula is used. Reference to the engineering calculations to back this up.
  • (11.2.8) The rated discharge capacity of the relief valve was determined in accordance with Section VIII, Division 1, ASME Boiler and Pressure Vessel Code. Reference to the engineering calculations to back this up.
  • (11.2.8) The capacity marked on the nameplate of the relief device is expressed in lb/min air or in standard ft3/min (SCFM) of air at 60°F
  • (11.2.9) The rated discharge capacity of rupture discs is calculated with the equation in this section. Reference to the engineering calculations or manufacturers documentation to back this up.
  • (11.2.9) Provisions to prevent plugging the piping when the rupture disc relieves are present.
  • (11.3.1) No stop valves are present on the inlet or outlet piping of the reliefs with procedures in place conforming to ASME Boiler and Pressure Vessel Code Section VIII Appendix M. Reference to the procedures if applicable.
  • (11.3.1) The size of the inlet piping to a pressure relief device is not less than the inlet size of the pressure relief device.
  • (11.3.3) Relief piping conforms to IIAR2-2008b 11.3.3, 10.2.1.5, 10.2.1.6, 10.2.1.7, and 10.2.1.8.
  • (11.3.4) The size of the relief pipe is not less than the outlet size of the relief device.
  • (11.3.4) For headers, relief piping is sized to accommodate all the relief devices that are expected to discharge simultaneously at the lowest pressure setting with consideration given to pressure drop in all downstream piping. Reference to the engineering calculations to back this up.
  • (11.3.5) Where piping or components may contain liquid that could be isolated during operation or service, IIAR2-2008b 11.4 for hydrostatic protection is also applied.
  • (11.3.6) All atmospheric reliefs relieve to the outdoors.
  • (11.3.6.1) The maximum length of the discharge piping is determined by the method in IIAR2-2008b, Appendix A. Reference to the engineering calculations to back this up.
  • (11.3.6.2) Atmospheric relief piping has provisions for draining moisture. Reference to maintenance procedures.
  • (11.3.6.3) The relief point of a relief device to the atmosphere is 20’ or more from any window, ventilation intake, or personnel exit.
  • (11.3.6.3) The direction of discharge is vertically upwards.
  • (11.3.6.4) The discharge from pressure relief devices is 15’ or higher above the adjacent grade or roof level and arranged to avoid spraying of refrigerant on persons in the vicinity. Reference to PHA as necessary to deal with catwalks and platforms.
  • (11.4) This section is applied when equipment or piping sections are isolated manually or automatically.
  • (11.4.1) Only trained technicians taking all necessary precautions to prevent overpressure due to hydrostatic expansion will manually isolate equipment. Reference to training, LO/TO program, written SOPs, etc.
  • (11.4.3) Equipment or piping that can be isolated automatically is protected by a hydrostatic relief device.
  • (11.4.3) Hydrostatic relief devices are not used as shut-off valves.
  • (Appendix A) Relief design is calculated to conform to this section.
  • (Appendix E) Relief design for positive displacement compressors is calculated to conform to this section.

We like to cover all these things in a single narrative with appendices for engineering work. You can do this any way you like as long as you address the requirements of your chosen RAGAGEP. One method is to use GCAP’s IIAR2-2008b RAGAGEP Audit Checklist provided for all GCAP PSM students.

About Brian Chapin

PSM / RMP Compliance Consultant
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