PSV sizing using standard cubic meter per hour but confused by the working fluids 3

[LuNL]

Hi dear all

Recently I got a interesting question from engineers.
While doing the sizing calculation for pressure safety valves, I assumed if I use standard cubic meter per hour, it should be okay no matter the fluid is air, or other industrial gases, like GNG. Since the molar volume at standard condition is the same , more or less.
However, when I compared the valve sizing calculation with API 520 standard, to AD 2000 Merkblatt A2, I find actually if you input , let say 5000 Sm3/h GNG , and 5000 Sm3/h air. They give different sizing results, and the orifice area can be twice different from each other.

I know there may be some properties related issues in the sizing procedure. Could someone help me cleaning things up here?

Big thanks !

 

[TD2K]

When you say that "it should be okay no matter the fluid is air, or other industrial gases, like GNG. Since the molar volume at standard condition is the same , more or less", you are forgetting the effect of molar mass on the required area.

It's easier to keep straight if you remember that standard volumetric flow rate is analagous to a mass flow rate. While 1 Sm3/hr = the same number of moles/hr regardless of the gas, the mass flow rate is very different. If you look at PSV sizing equations, it will become obvious very quickly why the area varies so much with different gases at a constant standard volumetric flow rate.

CNG likely has a MW of about 16. Air has a molar mass of about 29. For a constant standard volumetric flow rate, the PSV handling air has to be about 2x as large.

 

[LuNL]

HI
TD2K
Thanks for pointing out.
So is there any easy way to have a very general view about how big the PSV needs to be, before the preliminary sizing calculation?
Curious is, I saw most of supplier they give the tested Sm3/h of air when the valve is fully open, can I use that number as a reference , for gases whose molar weight is , at least smaller than 29 ?

Thanks !

 

[TD2K]

Your best method is to review and understand the PSV sizing equations. Most vendors will have them in their catalogues as will API 520.

All other things being equal, set pressure, allowable overpressure, temperature, the three variables that are gas specific are the MW, compressibility and the C term. The C term is a function of the ratio of the specific heats.

If you look at the equations, you will realize that the area increases proportionally to the square root of the MW masses. So, if you take a PSV that is rated in scfm of air, with CNG, it's going to pass (29 / 16 )^0.5 = 1.35 as much scfm of CNG as it does scfm of air (or whatever ratio you get for CNG depending on the actual MW).

C is a function of the ratio of the specific heat ratios. The specific heat ratio depending on the vapor in question can vary from close to 1 to over 1.5 so it's a significant factor. Tabulated values of C vary in API 520 vary from 315 up to 400 though 400 is not typical. Air has a C factor of 356 which is a reasonable value for a lot of gases but for any actual gas it could be high or low.

The compressibility factor is an indication how close the gas compares to what we call an 'ideal gas'. Z = 1 indicates ideal gas behavior, the old PV=nRT equation. Z can be greater than 1 though usually not much and can be less than 0.4 near what is called the critical point. The effect on the capacity is like molar mass, the square root of the ratio of the Z. So a gas with a Z = 0.5 compared to air with Z = 1 would have a capacity (1.0 / 0.5 )^0.5 = 1.41. The gas with Z = 0.5 would have a capacity 1.41x scfm of the gas than that of the PSV with a capacity expressed in scfm of air.

 

[zdas04]

LuNL,
In case you didn't capture all of the subtlety in TD2K's excellent post, the short answer is "NO, there is not a simple method to do this and attempts to force it to be easy will lead you to bad decisions".

I have a spreadsheet which has a "properties" tab that has a column for the gases I see most often (i.e., air, CBM, Mancos Shale, 1300 BTU/SCF conventional gas, etc). For each of those gases I've calculated MW, k (or γ if your prefer), etc. Then I have an index variable on the properties sheet that puts the proper data in column "B" so the API 530 calc doesn't have to deal with which column to look in for properties. I use the GPSA Field Data Book compressibility calcs (which are really close to the EOS values I get from NIST's REFPROP.exe, and a lot easier). Then I can put in pressure and temperature and column of properties and get a useful number very quickly. Took a week to build the spreadsheet, but that was 15 years ago and I use the sheet several times a week to give me much better results than any rule of thumb ever would.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LuNL]

Thanks for splendid explanation guys !

So what is the purpose or how to use the table normally provided by manufacturers ? I mean the table that experimented with air, so the rated capacity is Sm3/h air. Is it due to money-wise method, for the manufacturer to only test something ?

 

[zdas04]

You'll have to wait until someone who uses those worthless tables (from an end-user perspective) can answer. I've always thought that the tables were there because manufactures thought that end users wanted them.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LuNL]

I thought so , ha ha !
It will be nice if someone can actually calibrate with various gases and make a reference for this.