Page 97 - Improving Machinery Reliability

Vendor Selection and Bid Conditioning 69

cal contents of API data sheets, the seal manufacturer can be requested to list the
“operating windows” within which the proposed seal will function reliably in the
pump tentatively selected. It is to be understood that the “operating window” refers
to the actual seal with the seal materials, balance ratio, flush plan, stuffing box lay-
out, etc., selected and disclosed by the vendor. Notice also that the pumps are desig-
nated “tentatively selected” because even a capable seal vendor may be unable to
offer his optimum seal for a given pump. Should this be the case, it might be appro-
priate to consider selecting a more suitable pump model for the intended service.
As mentioned earlier, it is recommended to mail the bid request information to
three or four capable mechanical seal manufacturers. Their response should be criti-
cally analyzed to ferret out significant deviations among the various proposals.
These deviations may range from materials of construction to different API flush
plans and from differences in basic configuration to differences in application phi-
losophy of stationary vs. rotating seal members. Reconciling the deviations or differ-
ences will assist the engineer responsible for final selection in determining whose
seal offer has the best chance of meeting the user criteria highlighted earlier.

Desirable Design Features identified

Evaluation of the various bids is made easier by recognizing desirable design fea-
tures incorporated in mechanical seals. Some of these merit closer consideration.

Cartridge Construction. Cartridge seals are designed for rapid installation on and
removal from pump shafts. The cartridge seal is an arrangement of seal components
on a shaft sleeve and in a seal gland constituting a single unit that is usually assem-
bled and pre-set at the factory. Both bellows and spring-type seals can be cartridge-
arranged if the pump stuffing box is large enough.
Cartridge seal units offer major maintenance advantages. Replacement is rapid
and there is far less risk of assembly error and assembly damage than with conven-
tional mechanical seal mounting. API 682 requires that cartridge seals be selected
for typical pumps in petrochemical plants.

Silicon Carbide Hard Face Material. Heat generated at the seal faces must be
rapidly conducted away if fluid vaporization and resulting problems are to be avoid-
ed. Depending on service conditions and pump design, either the rotating or station-
ary seal ring must be counted on to dissipate as much frictional heat as possible.
High thermal conductivity and hardness make silicon carbide the preferred seal face
material in many of the more severe applications.
Placement of 0-Rings. An advantageous seal design recognizes that O-ring life
can be reduced by close proximity to the heat source, by swelling due to chemical
attack, and by operation in a dynamic mode, especially in the presence of erosive
materials. Going to PTFE chevrons or wedges may allow operation at higher temper-
atures and reduced risk of chemical attack, but will lead to fretting of metal surfaces
in contact with it. Bellows seals eliminate many of these problems by using static