Reciprocating Compressor Inspections

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-Oil System
– Pulsation Dampers (pressure vessels)
– 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; Pre-Inspection meetings 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. Non-Destructive Examination of Cylinders (liquid penetrant, magnetic particle, ultrasonic, radiographic). Cylinders are normally accepted on the basis of visual examination and successful hydrostatic tests. Supplementary NDE 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. When NDE is specified, it should always be witnessed (radiographs interpreted when radiography is specified) and should be identified as a witness point on the compressor data sheet. (See Section 620 for definitions.)

3. Hydrostatic Tests. Cylinder hydrostatic tests are always performed and should always be witnessed. The purposes of the hydrostatic test are to prove the structural integrity of the cylinder, to reveal leaks caused by material flaws that extend through the cylinder wall, and to reveal leaks caused by machining errors or damage to machined surfaces. The gas side of the cylinder and cylinder heads are tested separately from the cooling water side to verify there is no leakage from one side to the other.

4. Helium or Air Pressure Test. API 618 or the job specification may require helium or air pressure tests in addition to the hydrostatic tests. Gas tests are more likely to find small leaks than hydrostatic tests; gas tests are made with the cylinder submerged in water (a helium probe is sometimes used for helium tests instead of submergence). Since compressed gas has a great deal of stored energy, high-pressure gas tests should always be preceded by a successful hydrostatic test for safety. When a helium or air test is required, it should always be witnessed.

5. Compressor Valve Leak Test. This test measures the amount of time for a fixed volume of gas behind a valve to drop from one defined pressure to another. Special fixtures are required for this test which is sometimes specified for all of the compressor valves. If the test is required, it should be witnessed.

6. Alignment of Cylinders to Frame. Concentric and axial alignment of frame, crosshead guide, distance piece, and cylinder are carefully measured. Witnessing the alignment check should be considered for large machines; consult a machinery engineer to determine if it is warranted. Note that if these alignments are not done correctly in the shop, then field alignment will be much more difficult. Although the vendor would still be liable, getting them to take care of it could be very troublesome. Manufacturers will usually resist fitting the distance pieces and cylinders on large units.

7. Piston Rod Runout and Piston/Head Clearance. These measurements on the assembled compressor provide assurance that cylinder alignment is satisfactory, machining/assembly of crosshead/piston rod/piston are correct, and that the manufacturer’s specified head clearance is in fact present. These measurements should always be witnessed. Dimensional check of the compressor against the outline drawing, and visual inspection for defects and damage are done at this time. These steps would also be difficult in the field, as would getting the vendor to correct any problems. The vendor might claim some external factors.

8. Mechanical Run Test. This test is usually run without pressure-loading the cylinders to verify that the compressor is mechanically sound. Cylinder heads are removed after the test for inspection of the cylinder liners. This test (and post-test inspection) should always be witnessed. For very large compressors, manufacturers do not have the facilities to make mechanical run tests.

9. Final Inspection. This may not be required if the compressor is shipped disassembled as many are. For compressors that ship assembled, final inspection is similar to item 16, Section 630. The following are inspection points for auxiliary equipment and drivers.

10. Drivers. (See the Driver Manual)

11. Gears. (See item 18, Section 630)

12. Pulsation Dampers. (See the Pressure Vessel Manual). Checks are made for:
a. A high degree of internal cleanliness and adequate preservation of cleanliness.
b. Flange faces being in the same plane for nozzles in a single bottle that connect to two separate cylinders.

13. Lube-Oil System. (See item 20, Section 630)

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