Page 51 - Improving Machinery Reliability

Requirements Specificatiort 23

180

160

140

120

100
80

60

blw 20 30 50 70 100
Figure 1-13. Permissible inlet velocity as function of mole weight and gas temperature.

The uprate speed thus calculated must not, however, exceed the mechanical limi-
tations of impeller materials. This limit is typically reached at a peripheral speed of
950 fps (289 mhec). If N uprate would exceed this limitation, an additional impeller
would have to be added. Centrifugal compressors can usually be built with a blank
stage, Le., de-staged and ready to be modified at a later date.
Specifying shaft dimensions, couplings, and thrust bearings for the uprate case
will inevitably reduce the risk of machine distress and premature failure. In fairness,
it must be said that these measures will result in marginally higher friction losses at
thrust bearings and couplings, thus causing a slight increase in horsepower require-
ments over base-case, normally sized machinery.
The most effective way of reducing failure risk at the specification stage is to
examine vendor experience. Detailed experience lists should be submitted for the
purchaser's review. These lists must clearly identify the locations where identical
compressor impellers, shaft stresses, couplings, impeller or turbine blade stresses,
machinery bearings, bearing spans, speeds, etc. have been successfully used in the
past. A detailed review of the vendor's claimed component experience should be
made before issuing purchase orders, and this review should include telephone con-
tact with other users. In some cases, a plant visit a continent away will be well worth
it. We know of instances where, in preparation for the procurement of diesel genera-
tors for South East Asia, the US-based review engineer visited an installation in
West Africa, and where a U.S. manufacturer's steam-turbine experience was
reviewed by a site visit to a location halfway around the globe. It was well worth it,
in both cases.