Sqrt (Hs / Ps ) is a measurement of Actual Flow rate ?

[rQuestionEngineering]

Question 1.
Sqrt (Hs / Ps ) is a measurement of Actual Flow rate ?
See X -axis .

Question 2.
Why discharge temperature is not a critical input ?

 

[georgeverghese]

Q1. Yes, it is, provided suction temp and mol. wt ( and compressibility factor) are the same as that used to design the flow element on the suction line. Else mol. wt and/or suction temp correction should be applied-talk to your field instrumentation engineer for details for the correction formula.
Q2. Discharge temp need not be a factor influencing the location of the surge line on a Q-h plot, provided h is polytropic head in kJ/kg (or kNm/kg). Or you can use the corresponding units in the fps system of measurement (ft.lbf/lbm ?). But the y axis on this plot is not in polytropic head units.

Obviously, the surge line from compressor vendor should be developed using the same gas composition ( or as near as you can get it to be so) as that seen in actual operations

 

[compguy22]

h is differential head produced by flow element. See section 7 of this article (the author is the inventor of this flow coefficient):

https://asmedigitalcollection.asme....V003T07A017/2402148/v003t07a017-92-gt-428.pdf

The article indicates that the coefficient with Pd/Ps produces a universal and invariant map for purpose of anti-surge control; this also tend to differ from what is stated just above, in that the article shows that the use of the said coefficient yield to no discernable changes of operating point location on compressor envelope over a relatively wide range of MW's. Namely the author has tested mixtures with MW covering 14 to 24 g/mol without noticeable changes and certainly it go much wider.

In multistage machines however the locus of the surging/choking impeller may change vs. MW's so it is something to be kept in mind in addition to the effet due to k-value mentioned in the article.

 

[georgeverghese]

@compguy,
It does look like the DCS dump screenshot details match that described in this article from DR. Dont know how widespread the use of this "self compensatory" scheme for surge line location is in industry. Am familiar with the more conventional approach described in Fig.5 - system 3 and the description in section 9 of the article. In this conventional scheme, even the actual k value is computed by the control algorithm, since temp and pressure are measured both on suction and discharge. This query from the OP appears to be related to the "missing TT on the discharge line " on the DR control scheme for the compressor he has.

Ultimately, the compressor vendor and the capacity/surge control designer needs to justify the control scheme proposed. I have not examined this article in detail.

 

[compguy22]

please, what does DR stand for?
edit: ok I think its Dresser-Rand. The machine the OP is GE but the scheme has become widely I suppose and the patent I guess is public domain.