Page 184 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 155

We have seen exceptions made when deviations from the rule-of-thumb were
judged minor, or in situations where the pump manufacturer was able to demonstrate
considerable experience with ANSI pumps under the same, or even more adverse
conditions.

Cost Justification. If your company is interested in seeing the cost justification for
purchasing the generally stronger API pumps instead of normally satisfactory ANSI
pumps, consider statistical approximations,
Suppose that under average conditions of maintenance effectiveness and installa-
tion care (foundation, baseplate stiffness, grouting, piping configuration, etc.), ASNI
pumps in hydrocarbon services required service every 18 months vs. three years for
API-610 8th Edition pumps. Assume further that each maintenance event requires
the rather conservative expenditure of $6,000, including burden and overhead costs.
Knowing that you are likely to have a $600,000 fire for every 1,000 pmp failures
means that each failure event will incur a $600 cost adder. All of this translates to:

ANSI Pump repair cost, per year $4,000
Fire event adder 400
Probable cost per year: $4,000
API Pump repair cost, per year $2,000
Fire event adder 200
Probable cost per year: $2,200

This would yield a yearly maintenance cost advantage of $2,200 in favor of API
pumps under the stated average maintenance effectiveness conditions. Add to it pro-
duction loss credits, and the number increases. Also, how many service interventions
per year equal one maintenance worker, or one reliability/technical support person,
or one planner/supervisor/purchasing specialist? Or just how much would it be worth
to your plant if your reliability engineers could spend more of their time on predic-
tive maintenance and failure avoidance rather than having to struggle with equip-
ment failures? Certainly a subject worth pondering.
Between-Bearing Pumps. There is some evidence that overhung impeller con-
struction is occasionally more prone to maintenance or downtime than impeller-
between-bearing construction. In view of this, some contractors and pump pur-
chasers have, in the past, applied rather arbitrary selection guidelines.
One such guideline limits impeller diameters for overhung pumps to 15 in.
(38 1 mm). Another guideline calls for in-between-bearing geometries whenever the
product of horsepower and rpm (rotational speed) exceeds 900,000. For example, at
3,600 rpm, motor ratings in excess of 250 hp would favor selecting between-bear-
ing pumps.
How rigidly should these rules be applied, if at all? We would consider them
appropriate far screening purposes. Allow deviations if the pump is of double-volute
design and if calculated shaft deflections do not exceed 0.002 in. over the entire
operating flow range, from zero to full BEP flow. Alternatively, allow deviations if
the pump manufacturer can demonstrate long-term, satisfactory experience under
identical (or more severe) operating conditions.