Concept of operating cases- compression

[rotw]

Hello,

As you know, generally equipment datasheet are specified in terms of operating cases. For example a centrifugal compressor datasheet may have few cases (summer, winter, normal, rated, etc.) specified.
The API 617 recognizes theses operating conditions specified and includes requirement that apply to these operating conditions (example Maximum Allowable Working Pressure based on maximum discharge pressure between all operating cases, motor rated power calculations, etc).

From my experience, the machinery designer normally verifies the equipment based on the operation (example: looking maximum operating speed) ; But there are situations based on API 617 where only the operating cases will be considered to verify the design unless more stringent rules are applied for instance due to manufacturer's policies.

My question is how true is that on the field to have discrete operating conditions ? If you specify a machine with say 4 operating cases. Is that true that the machine on the field and the process will make the machine to operate exactly or in the vicinity of the operating cases as specified ? Its seems hard to believe so, I would expect the operation to be varying between the specified cases either to transition from one case to the other or simply operate continuously with significant offset to the case as specified. In short: having the design and operation verified against a set of specified cases seems to be theoretical and an idealization.

What is your experience on site or in the field ?

 

[BigInch]

Is that true that the machine on the field and the process will make the machine to operate exactly or in the vicinity of the operating cases as specified

It is true, if your control system (or operating staff) is good enough. The system should always be operated between design cases. Design cases are often set by practical limits such as well flow, city demand on the year's average coldest day, max and minimum environmental temperatures, max/min downstream pressures, etc. If anything changes, you'll need some control to keep the compressor within your limits, otherwise it will find its own running position, which may not be appropriate for your needs.