Page 103 - Improving Machinery Reliability

Vendor Selection arid Bid Conditioning 75

user that there are perhaps elements of pump design which will influence the success
of even the best available seal system. In some cases, the seal manufacturer will have
to deal directly with the pump manufacturer in efforts to define such pump modifica-
tions and might be necessary to accommodate optimum seal configurations.
The results of these efforts are shown in Reference 13, which documents signifi-
cant increases in seal mean time between failures at three British oil refineries. This
is plotted later in Figure 5-4.

Gas-Lubricated Mechanical Seals for Pumps. The gas sealing technology used
in gas compressors has been successfully applied to the emission-free sealing of liq-
uid pumps since the early 1990s. Gas-lubricated seals are thus an option to be
weighed against both conventional seals and such higher-cost alternatives as canned
motor and/or magnetically driven sealless pumps. While sealless pumps have their
place, the potential purchaser should carefully study anticipated installation and
operating and maintenance costs before making an informed choice.
Gas-lubricated seals employ a pressurized gas supply system and are available as
single or double seals. Figure 2-14 shows a double mechanical gas seal in the unvented
seal chamber of a conventional centrifugal pump. The seal or barrier gas, typically
nitrogen, is introduced at a pressure of 0.2 Mpa (3psi) higher than the product pressure.
Since many single-type gas seals find application as safety seals, the reader is referred
to Chapter 13, pages 550-558, which describe these seals in greater detail.

Good Proposal Data Lead to Comprehensive Bid Tabulation
As mentioned earlier, it is essential that completed API data sheets be submitted
with each proposed pump. The proposal package must also include performance
curves and typical pump cross-sectional drawings. In addition to these, the vendor
must state minimum allowable flow and NPSH required (NPSHR) for the entire
capacity range. Because minimum allowable flow could be governed either by ther-
mal or mechanical considerations, the vendor should be asked to specify his basis.
With these data and any notable exceptions given by the various bidders, a com-
prehensive bid tabulation can now be constructed. Careful review will narrow this
bid tabulation to two or three principal manufacturers, as illustrated in Figure 2- 15.
These would be the manufacturers whose equipment more consistently offered high
performance by demonstrating such features as low risk suction specific speeds and
operation near the best efficiency point (BEP). (For a more detailed treatment of this
topic, refer to Chapter 3, “‘Reliability Review for Centrifugal Pumps.”)
Engineers involved in pump-failure analysis have long suspected that cavitation
erosion can plague even pumps in pumping circuits where the available net positive
suction head (NPSHA) exceeds the manufacturer’s certified required value, NPSHR.
NPSHR tests by vendors are principally concerned with determining the performance
drop-off point. A drop of 3% in total dynamic head is usually considered indicative of
cavitation, and whatever NPSH is available at that point is though to be the NPSH
required by the pump. However, it should be realized that significant mechanical
damage may be encountered by long-term operation of some pumps with inadequate