5.3 VALUATION OF CENTRIFUGAL COMPRESSOR PERFORMANCE IN CONDITIONS OTHER THAN IN DESIGN

 Let us consider the performance curves of a centrifugal compressor when operating conditions are other than in design.

We know the machine type (MCL, PCL, BCL, 2BCL etc.) its geometry (perfect definition of stages composing the machine, chosen when designing the same) as well as the operating speed range.

We wish to define the performance when the operating parameters are different from those in design.

The previous paragraphs have described the main factors influencing the machine performance. Now we wish to estimate the influence of such parameters, in quantity values and their effect on the machine operation.

It is clear that during operation, different and combined parameters (contemporaneous changes in suction pressure, type of gas etc.) can occur; it is therefore impossible to foresee the entire series of cases.

To have some data about the several, possible per formances, let us express two different "off-design families" schematically: changes in suction conditions (type of gas, suction pressure and temperature) and chages due to possible field deteriorations.

5.3.1 Variations in inlet parameters

Three different, possible variations in the inlet parameters were supposed for natural gas compression service:

1. molecular weight (gas composition)
2. suction temperature
3. suction pressure

The characteristic curves, herewith enclosed, are obtained by computer for a BCL 404/A centrifugal compressor and show its full range of operation.

From these curves we can note the movement of the surging line, that of the max. flow line etc. due to the changing inlet conditions.

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5.3.2 Variations due to possible field deteriorations

As regards the influence of the possible field deteriorations, the compressor may undergo during its operation, on performance, we applied simplifying assumptions in order to simulate actualities.

Particularly we considered the following cases:

1. fouling on impellers and diaphragm bundles
2. increase in labyrinth seals‘ clearance

The characteristic curves herewith enclosed are obtained by computer, for the same service in the same way as in the diagram for changes in inlet parameters.

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