Page 43 - Improving Machinery Reliability

Requirements Specification 15

INarrative Specifications Lead to Better Machinery
By far the most effective method of specifying equipment is the single narrative
document. Instead of using a series of disjointed individual specifications and sta-
pling them into a stack of reference leaflet, the narrative document serves to blend
all applicable references into a unified whole. In developing this single narrative, the
responsible project engineer will use thought processes that tend to uncover over-
sights, weaknesses, and deficiencies in the procurement and design efforts for
machinery installed in process plants.
Again, the single narrative pulls together only the truly relevant information,
whereas the cross-referenced individual plant specification approach tends to be
extremely bulky, leaving it to the vendor to find relevant specification clauses, and
depriving the purchaser’s project engineer from detecting oversights or other defi-
ciencies. Experience confirms that additional cost incurred in developing single nar-
rative specifications is often recovered before the plant starts to manufacture on-
specification product at full capacity.
A single narrative supplement should spell out all requirements that add to, delete
from, or provide explanatory details to the API focal point specification. A typical
narrative specification for centrifugal compressors would state that “the following
requirements are additions, deletions, modifications, and clarifications to API Stan-
dard 617, Sixth Edition, 1995.” The narrative specification would not rewrite applic-
able, unchanged portions of API 617. Instead, it would identify supplementary or
overriding requirements using the paragraph identification numbers used in the API
standard. Here is a typical example:

2.12 Nameplates and Rotation Arrows
2.12.1 A nameplate should be securely attached at an easily accessible point on
the equipment and on any other major piece of auxiliary equipment.
2.12.2 Rotation arrows shall be cast in or attached to each major item of rotating
equipment. Nameplates and rotation arrows (if attached) shall be of AIS1 Standard
Type 300 stainless steel or of nickel-copper alloy (Monel or its equivalent).
Attachment pins shall be of the same material.
2.1 2.3 The purchaser’s item number, the vendor’s name, the machine serial num-
ber, and the machine size and type, as well as its minimum and maximum allow-
able design limits and rating data (including pressures, temperatures, speeds, and
power), maximum allowable working pressures and temperatures, hydrostatic test
pressures, and critical speeds, shall appear on the machine nameplate.

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