Page 183 - Centrifugal Pumps Design and Application

158 Centrifugal Pumps: Design and Application

Example of Pipeline Pump Selection

As described earlier, pipeline pumps must be capable of adapting to
change in pipeline throughput. The following exercise illustrates the
pump selection process for a crude oil pipeline, where condition changes
necessitate impeller changeouts, destaging, and volute chipping.
Pumps are required to transport crude oil from a developing oil field
through a trunk line to a tank farm 120 miles away. Routing, pipe size,
and pipe rating have been determined. A profile with hydraulic gradients
and typical system curves has been developed, and the field is expected
to produce 15,000 to 30,000 BPD (Figures 10-23 and 10-24). Note Mile-
post 085 (Figure 10-23) becomes a "control point." At least 1,200-ft sta-
tion head is needed to overcome the elevation and to insure 50 psi posi-
tive pressure at this high point. The projected future rate is 35,000 BPD,,
resulting in the following pipeline design conditions:

Initial J^iHH
Capacity (BPD) 15,000-30,000 35,000
Capacity (GPM) 437-875 1,021
Differential head (ft) 1,800-3,700 4,650
Differential head (psi) 694-1,246 1,792

A booster pump has been sized to provide adequate NPSH to the mainline
pumps. In this example, the head developed by the booster pump will be
disregarded.
First consideration is the "future" condition. With 0.89 specific grav-
ity and an estimated 76% pump efficiency, the total brake horsepower
required is approximately 1,400. The most economical energy source is a
local electric company. Starting current restrictions at the station site
limit motor horsepower size. To satisfy this restriction and to gain flexi-
bility of operation as well as partial capacity with one pump out of ser-
vice, two pumps driven by 700 HP motors operating in series are pre-
ferred. The ratings for each pump to meet future conditions, will then be
1,021 GPM at 2,325 ft.
3
A good selection would be a 4-in. pump with 5 stages and 10 /8-in.
diameter impellers (pattern 2008-H), for 465 ft per stage (Figure 10-25).
With 80% efficiency, the brake horsepower is 667. Performance curves
for single-pump operation and two pumps in series are plotted against the
system head curves (Figure 10-26). Operating points will be at intersec-
tions of pump curves and system curves. Lower capacities will require
throttling at the station discharge control valve, which, in effect,
produces a steeper system head curve. In this example, when flow is re-
duced to 885 GPM, the differential head developed by two pumps is