Test Correlation

in the discussion of gases and their correlation calculation, the point was made that the code can not be the final authority on gases. Do not expect the vendor to be all-knowing in the areas of all gas properties. The contract gas properties are the responsibility of the user. Much data are published on gases which, together with high-speed computers, make the job of defining gas properties somewhat easier today than it was when the code was written. This is not to say that all properties of all gases are as well defined as they might be. Gas mixtures, in particular, are always a problem.

The use of a gas mixture presents a two-part problem. If the state of the mixture is such that it may be considered a mixture of perfect gases, classical thermodynamic methods can be applied to determine the state of each gas constituent. If, however, the state of the mixture is such that the mixture and constituents deviate from the perfect gas laws, other methods must be used that recognize this deviation. In any case, it is important that accurate thermodynamic data for the gases are used.

Presently, sophisticated computer programs are available based on properties published by various scientific organizations. These programs are normally based upon the equations of state derived by Benedict Webb-Rubin [4] and Starling [5] for use with hydrocarbons. Modifica-

lions are often applied to cover other industrial gases. There are alternative equations of state that are more appropriate. Differences in values still may occur due to variations in thermodynamic properties published by different authorities and to differing mathematical techniques and assumptions used in programming. Therefore, an agreement must be reached prior to testing as to which authority and mathematical technique will be used. Three major parameters affecting performance are

• Reynolds number

Reynolds Number

PTC 10 has one correlation that has been found to be incorrect. Equation 5.27-1 will permit a Reynolds number correction for high Reynolds number gas (above 106), which is much too optimistic. ISO standards allowed no corrections, which is more nearly correct.

Work done by Wiesner [6] is a much more accurate approach. The subject has also been reported on more recently by Simon and Bulskamper [7 |. They generally agree with Wiesner that the variance of performance with Reynolds number was more true at low value that at high values. The additional influence above a Reynolds number of 106 is not much. It would appear that if a very close guarantee depended on the Reynolds number to get the compressor within the acceptance range (if the Reynolds number was high to begin with), the vendor would be rather desperate.

The other factors mentioned come into play with the inherent mismatch that occurs when an overall factor must be applied to correlate the flange-to-flange method. The correlation proposed in the code is based on work by Schultz [8] and was quite good for its day. When combined with modern calculation methods and equations of state, the philosophy is still valid.

A final tool available is the vendor's ability to generate a set of offdesign curves. By performing a stage-by-stage analysis of the proposed test gas, correlations are much easier to make. This is probably best illustrated by an example reported by Wong (9).

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