The aerodynamic performance of a stage, of a section, or of a complete casing can be evaluated with an optional test using the procedures covered in the ASME Power Test Code (PTC-10, 1965), Compressors and Exhausters.
Aerodynamic performance refers to the shape of the head-versus-capacity characteristic curve. The ASME test is conducted to determine whether the compressor meets the quoted power and head at one operating point on the curve for one speed. This point is usually the normal operating point, but can be any other point as specified. The surge point is determined during the test, and at least four other points are taken along the speed line including one at a capacity beyond that of the point in question. Specification CMP-MS-1876 calls for additional points to be taken for variable speed machines. These additional points result in a higher cost for the performance test, but experience has shown that the added cost is justified.
Acceptance criteria are given in API 617 and CMP-MS-1876. Note that some vendors use the term “head rise-to-surge (head RTS),” and that this term is not numerically equal to “pressure RTS.” RTS is an important factor as it can have a major influence on the operating stability of the machine, and on the design of the anti-surge system. RTS tends to become a problem with gases that are heavier than air and have relatively low acoustic velocities. Propane and propylene are examples of gases that can have a very flat characteristic from the normal operating point to the surge point. Accordingly, a small change in the system resistance would effect a major change in capacity causing a potentially unstable situation. CMP-MS-1876 requires a guarantee on the RTS quoted by the vendor.