ASHRAE 15 Chiller PRV Capacity

[anubis512]

I'm working on a chiller installation project. Our client has detailed pressure relief valve records, so we've asked the chiller vendor for their relief valve calculations. They gave some very basic info and deferred to the ASHRAE 15, Section 9.7.5 Equation:
C = f*D*L
Where:
C = Capacity (#/min, air)
f = Refrigerant Factor
D = Vessel Diameter
L = Vessel Length

I'm confused because ASHRAE also says pressure relief has to be in accordance with ASME, since they're pressure vessels, but I can't find any sort of derivation/explanation of where that equation came from. Also, this capacity doesn't have any sort of orifice area or set pressure component...so how exactly is it in accordance with ASME?


Thanks a lot

 

[don1980]

ASME Sec VIII contains a set of rules for protecting pressure vessels that are built to this code. These rules place certain constrains on the relief design (e.g. type of relief devices allowed, max set pressure, max pressure accumulation, etc.). However, the critical steps of establishing a sizing basis and choosing a sizing calculation methodology are intentionally left to the user.

Since ASHRAE refrigeration vessels are pressure vessels, built according to ASME Sec VIII, ASHRAE says to apply the ASME Sec VIII overpressure protection rules (relief design constraints). Then, ASHRAE provides guidance for sizing the relief valves on refrigeration machines - this is guidance which is not found in ASME Sec VIII.

Once the required relief capacity is calculated, using the ASHRAE equation, you select a PSV that provides the necessary flow. The PSV manufacturers provide the necessary information regarding the capacity of their valves.

When you understand what I explained above, you'll recognize that the ASHRAE text is not contradictory.

 

[anubis512]

Thanks, that makes a lot of sense. It's always confused me, as a young engineer, that codes typically say "x is required"...and then leave the determination of x up to the user.

With regards to relief valves, I understand what you've said. At the same, the API 520/521 methodology for sizing a relief valve for a fire scenario, the basis of the ASHRAE equation, is much more detailed. I'm assuming the ASHRAE equation is a simplified version for refrigerants...but no one I can find is aware of where it came from, so I'm curious.

 

[don1980]

There are a number of different methodologies for calculating relief load due to fire exposure. As with all other scenarios, ASME doesn't tell the user how to do this, nor does ASME endorse a particular method. The API 521 heat input methodology has become the most widely used one for pressure vessels. It has proven to be a good method, and I recommend its use when you have a choice of methodologies (fire exposure to pressure vessels), but it's not the only method out there. For example, some people use the NFPA-30 method, which yields a different result than that from API 521. For refrigeration equiipment, ASHRAE is another method (another different method) for fire relief. I have no knowledge of the basis or historical origin of this ASHRAE method (perhaps others can explain that), but it yields different results than the API 521 and NFPA 30. This ASHRAE method is the one that's generally used for pressure vessels within a refrigeration unit.

Since ASME doesn't endorse any particular fire heat input methodology, organizations such as API, NFPA and ASHRAE are free to publish their own. There's no necessity for any of these methods to be derived from the others, or in any way related to the others.