Question: This question relates to PRV pressure. “Is our tank’s MAWP defined in units of PSIG or PSIA? Can we adjust the PRV set pressure to match MAWP (or lower), instead of 10% higher?”
Tank Guru: It’s definitely PSIG (gauge), not absolute. And in the case of tank containers with a working pressure higher than 3 bar, the set-to-discharge pressure of the PRV (also called SRV) will generally be set at 10% above MAWP.
Relief valve pressure settings are specified in 49CFR Sec. 178.275 (g) 6. For a link directly to the regulations, click here: http://www.gpo.gov/fdsys/pkg/CFR-2010-title49-vol2/pdf/CFR-2010-title49-vol2-sec178-275.pdf. In summary, tanks with an MAWP higher than 3 bar have the “set-to-discharge” pressure of the PRV set at 10% above the MAWP (110% of two-thirds of the Test Pressure). This applies to tanks conforming to Tank Instruction T11, which make up the majority of the world’s fleet of chemical tank containers.
However, once you get up to an MAWP of 6.89 bar or higher (such as with T20 or T22 tanks) it’s not uncommon to have the safety relief valve set right at the MAWP (instead of 10% above MAWP) on certain tanks, like older DOT-51 tanks. With ASME vessels, sometimes the tank manufacturer followed ASME rules versus DOT — and they don’t align perfectly, as relief valves on ASME vessels are generally set at MAWP instead of higher.
With that being said, it’s hard to argue with wanting a high-pressure tank to start venting excess pressure as soon as it reaches MAWP, rather than allowing the internal pressure to exceed MAWP by 10% first. Therefore when you have tanks with a working pressure of 6.89 bar (100 psig), for instance, sometimes you will see the SRV’s set at 100 psig and sometimes you will see them set at 110 psig.
It’s important to note that these valves are purely emergency venting devices; spring-loaded to discharge excess pressure in order to protect the tank from exploding. Leave them set exactly as the manufacturer intended (the tank’s manufacturer will have performed venting calculations and submitted them to the Class Society as part of the tank’s initial approvals), and ensure that PRV’s are serviced regularly.
Also, some valves are dual-acting and provide vacuum relief in addition to relief from over-pressurization. Again, it’s important that the relief valves are checked regularly to ensure proper function. https://www.youtube.com/watch?v=fIWDzp03_qo