Centrifugal Compressor Inspections

The principal inspection points are listed below in the normal manufacturing sequence. The cost of testing is usually added to the purchase cost in a compressor quotation.

1. Pre-Inspection Meeting. Held to review specifications and order requirements at the point of manufacture to verify there will be compliance. This visit should always be made at the compressor manufacturer’s plant and normally at the
manufacturing plants for:

– Lube- and seal-oil consoles
– Overhead seal oil tank (pressure vessel)
– Gear (speed changer)
– Driver (prime mover)

The Pre-inspection meetings will help resolve ambiguities that may delay final shipment or result in equipment that is not what the user specified. They also verify that manufacturers understand our inspection and testing requirements and are aware of the required witness points. These meetings should be held for all except small utility compressors.

2. Review of Compressor-Casing Fabrication Drawings (by Purchaser’s inspector prior to start of fabrication). This visit should always be made for compressors with fabricated casings. The casings are pressure containing parts, but compressor manufacturers may use joint designs and materials that do not meet normal minimum requirements for pressure vessels or pressure piping (required by API 617).

3. Visual Inspection of Fabricated or Cast Casings Before Machining. This visit should always be made for fabricated casings, even when sound joint designs are on the fabrication drawings. Actual weldments frequently have major flaws that can be found visually; weld repairs must be made before machining since some distortion from welding is inevitable. Cast casings should be visually examined prior to machining to verify that they do not have significant visible defects.

4. Non-Destructive Examination of Fabricated or Cast Casings (liquid penetrant, magnetic particle, ultrasonic, radiographic). Company compressor specifications will usually require some degree of NDE in addition to visual examination and a successful hydrostatic test. Supplementary NDE beyond that contained in the specification should not be added unless it is clearly justified by the service conditions, material characteristics, or established specification requirements. A materials engineer, QA engineer, or both should be consulted if supplementary NDE is being considered for other reasons. NDE in itself is frequently inexpensive; it is the resulting repairs to upgrade castings which have been subjected to NDE that can be very expensive. When some form of supplementary NDE is specified, an acceptance standard must always be specified as well. Whenever NDE is specified, it should always be witnessed (radio graphs interpreted when radiography is specified) and should be identified as a
witness point on the compressor data sheet. (See Section 620 for definitions.)

5. Visual Inspection of Welded Baseplates Before Machining. This visit should always be made for large, or critical compressor baseplates. Weld quality and inadequate weld size have been problems on large baseplates.

6. Hydrostatic Test. A casing hydrostatic test is always performed. Witnessing the test is always warranted. Helium testing, if specified, should also be witnessed. (See item 4, Section 650.)

7. Visual Inspection of Welded or Cast Impellers Before Heat Treatment and Machining. This is warranted in most cases. Welding the impellers is difficult. The customer does not specify a quality standard, and the compressor manufacturer will not be likely to initiate weld repairs on completed impellers since another round of heat treatment and machining would be required. Cast compressor impellers frequently have significant defects but the compressor manufacturer may be inclined to use them anyway.

8. Overspeed Test and Subsequent NDE of Impellers. Both should always be witnessed to check for cracks and distortion. Impellers are made from highstrength alloys and often have hub stresses close to the material yield point.

9. Stacking and Incremental Balancing of Rotor. This is usually not witnessed. It can require days of inspector time since the rotor is balanced several times during the course of assembly. If witnessing incremental balancing is being considered, consult a machinery specialist.

10. Final Balancing of Rotor. This should normally be witnessed along with dial indicator measurement of runouts at bearing journals, thrust bearing faces, and all points along the rotor with close clearances. Impeller wobble caused by warpage during weld repair is also checked at this time.

11. Runout Checks for Proximity Probes. This should be done before the compressor is assembled and should be witnessed. Its purpose is to verify that mechanical and electrical runouts of the rotor surfaces are low enough for the vibration monitoring system to operate satisfactorily without electronic compensation. (See the General Machinery Manual for additional information.)

12. Final Assembly of Compressor. This should usually be witnessed to verify internal clearances are correct, parts do not have significant visible flaws, and internal damage is not done during assembly. This will require a resident inspector for one week or more.

13. Mechanical Run and Performance Tests. One of these is usually specified to verify that the compressor is mechanically sound and to prove it will meet performance requirements. A mechanical test is often required by the manufacturer’s own internal specifications. These tests should always be witnessed by a machinery engineer or mechanical specialist.

14. Dismantling After Test. As a minimum the bearings and seals are removed and inspected when the mechanical run or performance test is completed.

Inspection is done by the machinery engineer or mechanical specialist who
witnessed the mechanical run or performance test.

15. String Test of All Job Equipment. A string test is a mechanical running (and sometimes performance) test of all the ordered equipment assembled together on the job baseplates to make sure everything operates satisfactorily. Consult a machinery specialist if a string test is being considered. A string test should always be witnessed.

16. Final Inspection. This is always done after compressor, driver, piping, and instrumentation are installed on the baseplate, but before painting is finished. Final inspection includes but is not limited to:

a. Review of equipment against specifications and data sheets line by line.
b. Dimensional check against reviewed outline drawings.
c. Verification that all required piping and appurtenances are present.
d. Visual inspection for defects or damage.

The following are inspection points for auxiliary equipment and drivers.

17. Drivers. (See the Driver Manual.)

18. Gears. Degree of testing and inspection will vary with speed and load carrying requirements. The following are usually witnessed.

a. Gear contact and backlash check in contact-checking stand.
b. Gear contact and backlash check in casing.
c. Mechanical run test (unloaded or loaded) if run test is specified.

19. Overhead Seal Oil Tanks. Checked for a high degree of internal cleanliness.
(Also see the Pressure Vessel Manual for information on inspection of vessels.)

20. Lube and Seal Oil Consoles. The following are usually witness points:
a. Visual inspection of components prior to assembly for weld quality and cleanliness.
b. Pressure tests of completed systems.
c. Operational and cleanliness tests per API 614.
d. Final inspection before finish painting.

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