Page 95 - Improving Machinery Reliability

Vendor Selection and Bid Coriditionirig 67

Mechanical Seal Selection and Evaluation

Pump-application engineers will generally agree that seal and seal environmental
system selection on many pumps is becoming more complex and time consuming
than pump selection.’2 Still, an average of only 10%-35% of the allotted pump-engi-
neering time is generally spent on the mechanical seal system.13 Looking at the cost
of seal failures and such consequences as major fires, release of toxic materials, and
unit downtimes, there is reason to believe that more time should be spent on seal sys-
tems design. But before we decide who should spend this time, we should examine
the various selection practices prevalent in the petrochemical industry.
Seal selections made entirely by the pump vendor have generally proven to be
least reliable. The pump vendor is concerned that his competitor will underbid him,
and thinks that the engineer selecting the pump will only look at the initial, installed
cost without giving credit to the potential run-length extension and maintenance cost
avoidance of superior seal components or seal system designs. Consequently, the
least expensive seal is often selected, leaving plant operations or maintenance bur-
dened with an inherently weak seal. Furthermore, pump manufacturers seldom
receive experience feedback on seals furnished with their pumps. Seal selection by
the pump vendor alone should thus be discouraged.
Contractor’s or user’s standards have generally been applied with somewhat high-
er success. Unfortunately, many of these standards are full of generalities and give
little guidance on specific requirements. Very often the stated requirements do not
separate barely acceptable from truly successful seal systems. Lack of specific guid-
ance in an otherwise well-intended specification may significantly impair its useful-
ness and deprive the user of a low-risk sealing system.
Optimum seal selection practices should make extensive use of vendor experi-
ence. These practices must encourage the seal vendor to use his own gland design
and to recommend seal systems, not just seals. To properly advise the user, seal ven-
dors require full information on product composition, process conditions, crystalliza-
tion temperatures, solids entrainment, and the like. All of these data are highly rele-
vant if proper selection is to be ensured, and withholding data for “security” reasons
may cost the user dearly. Optimum seal selection consists of the following steps:

All relevant data must be disclosed to the seal vendor. If security is truly a valid
concern, disclosure should be preceded by signing confidentiality agreements.
At least three and preferably four major seal manufacturers with strong and capa-
ble representation in the user’s geographic location should be invited to submit
bids. The user should screen and verify vendor capability by such criteria as ability
to furnish engineered seal components, e.g., special pumping screws instead of
ineffective pumping rings, and by vendor’s willingness to stock appropriate spares
in the user’s geographic area.
The bid invitation should clearly state that the user is interested in buying a seal
sysrem, not just a seal.