Page 41 - Improving Machinery Reliability

Requirements Specification 13

purchase a well-proven “oldtimer,” which will require preventive maintenance?
Should we allow a short spacer length to be supplied with this coupling, or would
not a longer spacer be far more tolerant of the anticipated misalignment between the
driver and driven machine? A typical modern centrifugal compressor should be fur-
nished with a generous spacer, preferably 20-30 in. (500mm-750mm) long. This
spacer length ensures that driver-to-driven-machine angular misalignment stays
within acceptable limits during temperature transients. However, compressor and
driver vendors must be aware of the potential impact of this selection criterion and
must design the machinery to remain insensitive to dynamic disturbances in spite of
the increase in overhung weight. Couplings should preferably be sized for future
uprate or maximum allowable shaft torque carrying capacity. This selection guide-
line will benefit the coupling hub engagement which, incidentally, should be a
hydraulic dilation fit instead of the thermal heat-shrink method of yesteryear. Liber-
ally sized shaft ends and coupling hubs will allow the safe use of interference fits on
the order of 1.5-2 mils per inch (mm per m) of diameter without having to resort to
key and keyway combinations.
Data sheet page 6 (Figure 1-7) is the last of the many API data sheets to contain
material related to the centrifugal compressor proper.
In the “Shop Inspection and Tests” column, we have checked off some rather
indispensable requirements. Also see the arrow that points to testing of compressor
and driver combined, so-called string testing. Contrary to general belief, string test-
ing in the vendor’s shop is very rarely justified. Early field installation and testing of
the entire train makes more sense, both technically and economically.
Although not specifically listed on the API data sheets, disassembling and
reassembling the compressor to check bearings, seals, and internal condition is advo-
cated for two reasons: first, it affords an excellent opportunity to develop a photo-
graphic record of these procedures for future reference by personnel engaged in turn-
around maintenance and emergency repairs. Second, disassembling and subsequent
reassembly is required for mechanical run testing of the spare rotor.
And that just about sums it up, Many of the rhetorical questions raised here were
meant to alert the specifying user engineer to the need to know what to purchase and
why to purchase it. The point is: the user must specify based on knowledge. The data
sheets represent a summary checklist or tabulation of extent of supply rather than a
specification. Therefore, a specification supplement or similar complementary
instructions must accompany the API standard and the API data sheets.
Resourceful and forward-looking equipment purchasers or owner companies
should require the manufacturer or vendor to develop and provide machinery instal-
lation instructions (Figure l-8), equipment commissioning instructions (Figure l -9),
proposed instrument checkout guidelines or sequences (Figure 1 -IO), illustrated
spare parts cross-reference tables (Figure 1-1 l), equipment startup instructions (see
Figure 1-26), and other helpful documents. If these requirements are included in the
bid request or invitation to submit cost proposals, they may be provided at very rea-
sonable cost. On the other hand, attempting to acquire this important documentation
at a later date may prove frustrating, expensive, or futile.