From a recent sequence of correspondence, we discern that
certain rulings were being made years ago, although the
rationale used to make these rulings has not been explained.
This example involves a reliability engineer in Australia. He
works for a polyethylene manufacturer that formerly belonged to
MNXC, a leading multinational corporation. The plant inherited
operating and maintenance manuals from MNXC.
The reliability engineer was aware that MNXC in its
maintenance practices manual had documented proven-best
practices for spare-pump operation. MNXC was said to have
claimed that the more frequently a pump is started, the lower
its mean-time-between-repairs (MTBR). This, MNXC explained, was
attributable to pump startup being the most severe or stressful
transient operation to which pump seal(s) and bearings are
The Australian reader informed us that MNXCs
maintenance practices manual advocated a four-category
criticality ranking of all pumps and to then swap the pumps as
Emergency: Every 4 weeks
Vital: Every 12 weeks
Normal: Every 52 weeks
The reader also notedquite correctlythat the
MNXC maintenance practices manual
recommendation does not seem to follow logic. It was apparent
to this reliability professional that, for
its most critical equipment, MNXC applied a frequent start
strategy which, as they correctly inferred, would yield the
shortest MTBR. The Australian engineer summarized his
correspondence by recalling that we at HP were rather
familiar with MNXCs practices and asked for our opinion
on the matter.
Well, the manual didnt exist at MNXC when I opted for
early retirement in 1986. In the intervening years, I have had
ample opportunity to research the issue and to observe
prevailing practices in many countries. I subsequently alluded
to pump swap-over topics in books and articles on
As to the issue raised by the reader, there are several
possibilities. Some, I will admit, are alluded to with a twist
of irony, but they deserve to be mentioned here:
a) There is confusion with the terminology
swap vs. test run. The old manual might
advocate a swap once a year, but (elsewhere) asks
the operators to test run the second pump for at least four
hours every month.
b) The old owner company, MNXC and the person(s) compiling
the manual have overlooked the fact that letting a pump sit
idle for 52 weeks and to then expect it to run flawlessly is
stunningly naive and willultimatelycost the owner
dearly. Experienced reliability professionals know well
that, after a year, the bearings of the stand-still
spare pump will have degraded due to two
One action is micro-vibration transmitted from
adjacent running equipment; such vibratory motion causes the
oil film to be wiped off. The result is metal-to-metal contact
and false brinelling (Fig. 1), usually at the race near the
lowermost bearing balls.
The second typical action results from corrosive
damage, unless, of course, dry-sump oil mist is used for both
lubrication and stand-still protection.
c) Whoever made the rules is inexperienced and/or bases
this advice on the premise that one has to do something
in order to justify being on the companys
d) The owner company is planning to save money and will
later sell the asset to some unsuspecting buyer. The enterprise
will look goodlow operating cost, high profits. Two years
later, the facility will be well on its way to becoming maintenance-intensive and
1. False brinelling caused by vibration of
Personally, I believe what a leading producer of petrochemicals (LPC) advocated and
practiced in 1986 is still correct: Swapping every four weeks
will protect the bearings and will keep the stagnant liquid
product in the piping and seal regions from partially
vaporizing. Periodically swapping pumps will also serve as a
training exercise to keep operators knowledgeable and
An LPC engineer told us, at a conference in 2008, that LPC
is now swapping pumps every six weeks. Thats sufficiently
close to our old four-week rule, and I can live with that. The
new rule that the reader mentioned in his note
makes no sense, but Im past arguing.
The wheel is being reinvented even as we speak.
Interestingly, pump MTBFs at some plants have declined since
2002. The new rule undoubtedly contributes to the
MTBF decline. In any event, the important answer and technical
explanation is found in b), above. You can disregard the rest.
Heinz P. Bloch is Hydrocarbon
Processings Reliability/Equipment Editor.
As the author of 18 textbooks and over 490 papers or
articles, he advises process plants worldwide on reliability improvement
and maintenance cost-reduction
opportunities. His latest text, Pump Wisdom:
Problem Solving for Operators and Specialists,
John Wiley & Sons, Hoboken, New Jersey, 2011,
sheds additional light on the matter.