Page 182 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 153

The following sample problem will illustrate this particular method:

Example: Pump-6 x 8 x 10 overhung process pump
Speed-3,560 rpm
Design conditions-I ,600 gpm
300 ft TDH
31 ft NPSH,
0.85 S.G. hydrocarbon
Pump design data-22 ft NPSHR at BEP
14,000 Nsss at 1,600 gpm (BEP)
From MCSF chart: MCSF = 56% of BEP
OS6 x 1,600 = 896 gpm
From NPSH margin chart:

NPSHA 31
=-- - 1.41
NPSH, * 22
(* at BEP)
K = 0.735
MCSF Corrected = 0.735 x 896 = 6.59 gpm
Rounding off, we would specify 660 gpm minimum continuous stable flow.
There are five points to remember:

1. Nsss is always defined at the pump best efficiency point at maximum design
impeller diameter.
2. Nsss = (rpm)(gpm)o~5/(NPSHR)o~75
3. gpm is per eye; for double suction pumps use one-half the total pump flow at
BEP.
4. When determining NPSH margin, use NPSHR at the BEP FLOW.
5. The determination of minimum continuous stable flow by using Figures 3-40
through 3-42 is approximate and occasionally tends to be conservative, i.e.,
lower flows are sometimes acceptable.

However, while it may be possible for some pumps to operate at flows away from
BEP without experiencing hydraulic problems, it is important to recognize that unac-
ceptably high radial forces could act on an impeller. Figure 3-45 compares the effect
of three casing designs on radial force.37 Unless a given pump is designed for sus-
tained low flow operation, excessive shaft deflection, high vibration, and problems
with bearings and seals may result.
These observations strongly suggest that the life expectancy of centrifugal-pump
components is influenced by tradeoffs or interaction of pump suction-specific speed
(Nsss), throughput percentage referring to flow at BEP (Q actual/Q @ BEP), NPSH
margin (NPSHA/NPSHR or NPSHA minus NPSHR), head rise per stage, and casing
design. A closer examination of References 33 through 37 will support this writer's