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. First, let’s take a minute to make sure we understand the difference between design and design basis. It’s not just about the relief valve!
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 2008b: Section 11
- (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……. Etc…
We like to cover both 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.
The design bases can change per RAGGAEP or code a plant falls under. If a company falls under the Uniform Mechanical Code (UMC) or the California Mechanical Code (CMC) and some city specific jurisdictions, they may not be allowed to have relief valves and headers discharge to the atmosphere.
1994 UMC §1119 Ammonia Discharge
Ammonia systems shall be provided with an emergency discharge into a tank of water provided exclusively for ammonia absorption. At least 1 gallon of fresh water shall be provided for each pound of ammonia in the system. The water used shall be prevented from freezing without the use of salt or chemicals. The tank shall be substantially constructed of not less than 1/8-inch or No. 10 M.S.G. (2.51 mm) steel. The horizontal dimensions of the tank shall be equal to or less than one half of the height. The tank shall have a hinged cover or, if of the enclosed type. shall have a vent hole at the top. Pipe connections shall be through the top of the tank. The discharge pipe from the pressure-relief valves shall discharge ammonia in the center of the tank near the bottom but not more than 30 feet (9104 mm) below the surface of the water.
In 2009, the requirements were modified reducing the required tank size: