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# Specifying Specific Specific Gravity

## Specifying Specific Specific Gravity

(OP)
I am sizing some PSVs for a natural gas compressor station.
On the spec sheets the vendor gave me it asks for Oper. Sp. Gr.
Since HYSYS is not giving me a SG in the properties tab (other than the petroleum SG(60/60) which I am not sure what this is.
The MW of the gas is 20.78. So in other work I have been doing, I have reported the SG to be 0.72, (20.78/28.85).
However I am confused about the wording Oper. Sp. Gr.
Does this mean that I take the mass density of the gas at operating conditions (120F, 1200psig)
and divide it by the mass density of air at 120F, 1200psig.
If so the SG would be 0.91.
Any help is much appreciated.
Thank you.

### RE: Specifying Specific Specific Gravity

if you have doubts call the vendor asking to explain

### RE: Specifying Specific Specific Gravity

Jacob B,
If you are not sure what HYSYS is giving you - you need to ask youself the question "Am I competent to use HYSYS?" If you are not sure of the outputs from HYSYS then ask someone who does in your Company. Remember GIGO is the acronym!!!

### RE: Specifying Specific Specific Gravity

Upload the specification sheet so we could see what is all about. For gas PSV sizing you need to know Molecular weight of the gas at relieving conditions - for pure gas systems this will always be the same number but for liquid-vapor systems the composition (and hence the Mw)of vapor phase will vary depending on pressure and temperature. As you correctly pointed out, Mw is directly related to specific gravity. Instead of specifying Mw at relieving conditions, perhaps the vendor is asking you to provide them with Specific Gravity at reference (in this case relieving) conditions. If that is the case, you can easily read in HYSYS stream properties the Mw of the vapor phase at relieving conditions and dividing it with SG of air at reference conditions.

In any case I would ask the vendor for clarifying this input data as well as stating the reference conditions (0 degC, 15degC, 20 degC, 1bar, 1atm, etc. etc.).

Dejan IVANOVIC
Process Engineer, MSChE

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