Page 180 - Improving Machinery Reliability

Muchirzevy Reliability Audits arid Reviews 151

ble-flow impeller is used, developing 280 ft of head at BEP capacity, and the manu-
facturer gives an NPSHR of 23 ft. For this pump the specific speed, Ns, would be

N(Q)0.5/(H)0.75 (3,56O)( 1 ,300)0.5/(280)0.75 = 1,875
=
This indicates that the upper portion of Figure 3-42 should be used. We calculate

Nsss = N(Q)0’5/(NPSH~)0.75 (3,560)( P,300)0.5/(23)0.75 = 12,225
=
Entering the horizontal scale at 12,225, we find that for double suction pumps in this
specific range, we should expect operation without damaging impeller-internal fluid
recirculation, as long as the flow exceeds 88% of the flow at BEP, or 0.88 x 2,600 =
2,288 gpm.
Recognizing that pumps in water service with a flow of 2,500 gpm or less and
heads of 150 ft or less operate at only moderate energy levels, Reference 36 suggests
that their minimum flow values can be set at 50% of the indicated recirculation val-
ues for continuous operation and 25% of the indicated recirculation values for inter-
mittent operation. For pumps of all sizes in hydrocarbon service, the minimum flow
values can be set at 60% of the indicated recirculation flow if continuous and 25% of
the indicated recirculation flow if intermittent operation is anticipated.
A similar graphic representation of minimum continuous stable flow was pro-
posed by C. C. Heald in 1983. His approach is to separate pumps into three size cate-
gories: A, B, and C:

A-Single stage, overhung, 2-in. discharge and smaller multistage, 2-in. discharge
and smaller
€3-Single stage, overhung, 34-in. discharge (3,600 rpm), 6-in. discharge and
larger (1,800 rpm and lower) single stage, double suction, 4-in. discharge and
smaller multistage, 3 discharge and larger
G-Single stage, overhung, 6-in. discharge and larger (3,600 rpm) single stage,
double suction, 6-in. discharge and larger

Next, the reviewer would calculate the pump suction specific speed, Nsss, and
enter Figure 3-43 at the calculated value. The Nsss-line intersection with the applica-
ble curve would show, on the horizontal scale, the percent-of-the-best-efficiency
point capacity for the maximum diameter impeller fitting in this particular pump.
Following this step, the reviewer would enter Figure 3-44 at the point indicating
ratio NPSHA/NPSHR, and read off a “minimum flow correction factor” K,, on the
horizontal scale. The two values thus found from Figure 3-43 and 3-44 would be
multiplied by the flow at best efficiency (BEP flow) of the maximum diameter
impeller and the result considered a reasonable approximation for minimum continu-
ous stable flow (MCSF).