Page 65 - Improving Machinery Reliability

Requireinents Specification 37

ments are often achieved at the expense of mechanical reliability. Specifically, tight
labyrinth clearances may indicate high test-stand efficiency, but they are also prone
to cause mechanical contact during operational upsets. Should this happen, the user
is not only losing a fair measure of compressor efficiency, but is also running the
risk of mechanical damage.
Extra capacity looks good in light of future uprate potential, but it may cost in
terms of requiring large recycle streams to avoid compressor surge during initial
operation of the compressor. Similar concerns exist for centrifugal pumps operating
at flows substantially below their best efficiency point (BEP). This element is further
explained in Chapter 2, “Selecting Major Machinery Vendors.”
In today’s energy-conscious production and procurement environment, strong
contractual safeguards are recommended. In virtually all cases, the user must be pre-
pared to pay the cost of certified and witnessed performance tests. If these tests are
to be conducted in the field, it is important for vendor and purchaser to agree before-
hand on test-equipment types, test-equipment location, and applicable test proce-
dures to be followed.

Assessing First Cost Versus Maintenance Cost

Typical maintenance costs for major equipment installed and operated in the con-
tinental United States are given in Chapter 4, “Benchmarking Maintenance and Reli-
ability.” We have arranged the text in this sequence because maintenance costs gen-
erally tend to emerge into real focus after the plant has started up. However, from the
point of view of the engineer who specifies machinery or selects machinery from
among several competing offers, the future cost of maintenance should be of concern
now. An example will illustrate why this is significant.
Let us assume a plant can satisfy a given compression service by using either a
single electric-motor-driven centrifugal compressor sized for 100% capacity, or three
gas-engine-driven reciprocating compressors sized for 50% capacity. The process
engineers, cost estimators, and project engineers have developed data showing the
foundation requirements, plot-plan arrangement, piping complexity, and utility costs
for the two different approaches. Their balance sheet reflects the principal elements
and is given in Table 1-2.
The initial judgment might favor integral gas-engine reciprocating compressors
for this service. However, the picture may change when we apply current mainte-
nance statistics for motor-driven centrifugal compressors and integral gas-engine-
driven compressors, Table 1-3, and project these costs over the next few years. The
importance of paying attention to future maintenance cost is further illustrated in
Chapter 5, dealing with life-cycle costs studies.

Specifying Machinery Documentation Requirements

Safe operation, proper surveillance, and cost-optimized maintenance of machinery
requires that a good deal of machinery-related data be available and made accessible