PSV Sizing and ACFM vs SCFM

[BanditKeith]

Hey y'all. Been working on a project and something has been bugging me for quite a while. Not sure if it's the best place to put it. Sorry in advance.

We're looking to install a temporary air compressor while our instrument air system is down for several maintenance activities. The unit is quite larger than what we need (1500 cfm @ 100 psig). We've also determined we definitely need a PSV for our temporary installation. One thing that caught our attention was that their PSVs are rated and certified at 2884 scfm. The rental company says the PSV is certified for the output capacity, but based on the operating conditions, we expect an actual capacity of around 12300 scfm using the combined gas law. This capacity seems quite large to size a PSV for, but I've confirmed my conversion with some of the engineers out here, and they're fine with it. More importantly though, this capacity is much larger than what the PSVs are rated and certified for. Should we be concerned about this? Is it possible they're equating SCFM and ACFM to be the same, or is the PSV being grossly under sized? I know ACFM and SCFM are not the same, especially at large operating pressures. In any event, we're likely going to get our own PSV for our required capacity and set point.

I'm new at PSV sizing (this is my first time), so any input will be appreciated.

 

[zdas04]

Sizing a PSV is not something that should be done without study. Purchase a copy of API 521 and read it. It has the equations (which are in mass flow rate, and SCF, not ACF which would be very different from one pressure to another).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[TD2K]

I suspect your 12,300 scfm is a little high. I think you've taken 1500 cfm * (100 + 14.7) / 14.7 = 11,704, do you have a temperature correction also in there?

What size engine/motor does this temporary compressor have? I installed an air compressor that was rated for about 2700 scfm at 125 psig discharge pressure, that compressor required an 800 Hp motor. I think you've assumed the discharge flow rate is 1500 cfm at 100 psig, I suspect it's the suction flow to the compressor at atmospheric pressure. Most air compressors I've seen are rated for the suction flow (it may also be something like 14.4 psia rather than 14.7 psia but that's not going to be hugely different here). If it is 1500 cfm at 100 psig air pressure, you are going to have one big driver attached to that compressor.

As a check, I would ask the company renting you this compressor what capacity the discharge PSV needs to be rated for and what the 1500 cfm flow rate applies to.

 

[zdas04]

As TD2K says, air compressors are usually rated at suction ACFM. It turns out that all that is doing is defining a local "standard". I don't get a different PSV in Denver if I do my calcs with the pressure and temperature locked in at an imaginary 14.7 psia and 60F or if I lock it in at 13.1 psia and 72F, as long as I lock it in. I had a really hard time with the air compressor industry's approach of rating their equipment based on ACF of the suction until I realized that they were just setting a better standard for that process than the usual 14.7-ish psia.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[chicopee]

The compressor relief valve discharge capacity should be based on the free air delivery of the compressor.

 

[BanditKeith]

Had a chat with the rental company and the manufacturer of the compressor. Sounds like the 1500 cfm is the free air delivery at the discharge and is rated at ambient conditions, so this definitely sounds like a standard cfm (scfm) capacity after all.

So is this what "free air delivery" typically means? A google search reveals quite a number of terms for compressor ratings: acfm, scfm, icfm, fad, etc. Sounds like different companies have slightly different ratings. Looks like the best way to avoid such confusion in the future is to check with the vendor and understand what the rated capacity specifically refers to right away.

This is still quite a large unit we're renting (600 bhp I believe), but knowing that the capacity is in scfm clarifies things much more now.

Thanks for your guys' replies. This was definitely a good learning experience.

 

[Latexman]

Basically, a gas volume or volumetric flow rate doesn't mean crap, unless the conditions (T and P) are stated, or implied (scfm). Even when implied, you have to be sure all parties have the same understanding. The most thorough solution is to always state the flow and conditions together, i.e. 1500 cfm at 60 F and 14.7 psia or 1500 scfm (60 F and 14.7 psia).

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529

 

[TD2K]

I like to think of scfm is just another version of a mass flow rate. 1500 scfm is 3.95 lbmole/min (379.6 scf/lbmole). For dry air, that's 114.4 lb/min or 6866 lb/hr.

I think if people were asked how big of a PSV was needed for 6866 lb/hr of air you'd be hearing "What pressure? What temperature? What physical properties?". There are tabulated tables for PSV capacities for air, water and steam but you still need to do some corrections to use those tables.