Page 49 - Improving Machinery Reliability

Requirements SpeciJication 21

general “catch-all” specifications are appropriate, the user should apply these with
discernment. By referring only to those paragraphs or clauses that really pertain to
machinery and auxiliaries furnished by the vendor, the specifying engineer will
reduce the probability of unexpected problems later in the job. Issuance of a perti-
nent specification package leads to more accurate cost proposals, generally lower
prices, and higher quality machinery.

Considering Uprateability and Low Failure Risk
An early decision to provide for future capacity increases or power output uprates
may prove highly advantageous in plant debottlenecking or future expansion situa-
tions. More often than not, the resulting pre-investment costs are surprisingly low,
especially when unexpected mechanical reliability improvements result from the
decision to pre-invest.
A process gas compressor for a specialty chemical plant will serve as an example.
This compressor required a throughput of 9500 cfm (16,140 m3/hr) to compress a
medium molecular weight gas from about atmospheric pressure to approximately
120 pig (8.3 bar). The vendor’s initial offer was for a compressor with a maximum
throughput capability of 11000 cfm (18,660 m3/hr). When encouraged to propose an
alternative selection, the vendor submitted a marginally more expensive machine in
the next larger casing size. Not only did this machine exhibit an uprate potential to
16000 cfm (27,180 m3/hr) but it proved mechanically superior, a true workhorse of a
compressor which, a good 20 years later, had weathered more abuse than the plant
manager cares to remember.
The procurement of uprateable centrifugal compressors usually involves investigat-
ing the feasibility of removing the last impeller and moving all preceding impellers
into the location previously occupied by the next higher stage. Only a new first-stage
impeller would have to be bought later. Figure 1-12 illustrates this principle.
Another way of reducing the pre-investment cost difference would be to purchase
the spare rotor (and one spare diaphragm and probably the coupling) to represent the
most probable uprate case. An investigation of relevant process parameters would be
required to determine whether the present plant requirements could be safely accom-

Original Upraled
1 2 3 4 New 1 2 3

Figure 1-12. Compressor uprate through downward movement of stages.