Water Well Journal
Issue link: https://read.dmtmag.com/i/692787
Step 1: Determine column and lineshaft sizes: 6″ × 1″ water-
lubricated. Desirable flow range: 300-600 GPM. Use 150′ of
6″ × 1″ water-lubricated column and lineshaft.
Step 2: Determine total downthrust.
Total downthrust = Hydraulic thrust (+) Mechanical thrust
(2a) Hydraulic thrust = Bowl assy. "K" factor (obtained from
pump data) × TDH or SOH (in feet). "K" factor = 7 lb./ft ×
345′ (Use head at SOH) = 2415 lb. of hydraulic thrust
(2b) Mechanical thrust = Weight of impeller and bowl shaft
per stage × Number of stages (+) Weight of lineshaft per foot
× Total length
Weight of impeller and bowl shaft: Assume: 6″-6 lb./stg.–
8″-10 lb./stg.–10″-15 lb./stg.–12″-30 lb./stg.–14″-45 lb./stg.–
15″/16″-60 lb./stg.–17″/18″-75 lb./stg.–20″-110 lb./stg.
(Use the actual data from the manufacturer, when possible.)
Mechanical thrust = 15 lb./stage (for
10″ bowl) × 7 stages =
105 lb. (+) 2.67 lb./ft (for 1″) × 160′ = 532 lb.
Total downthrust = 2415 lb. (hydraulic) (+) 532 lb. (mechani-
cal) = 2947 lb. of thrust
Step 3a: Determine total horsepower.
(Figure 2 in May 2016 Water Works)
Bowl horsepower (from pump curve): 39.08 BHP (+) 1″ line-
shaft horsepower friction loss: .54 HP/100′ of shaft × 1.6
(160′) = .864 HP (+) thrust bearing HP loss: .0075 × pump
RPM/100 × total thrust/1000.
.0075 × 1760 RPM/100 × 2947 lb. thrust/1000 = .389 HP.
Total brake horsepower = 40.33 BHP
Select 40 HP, 1800 RPM vertical hollowshaft (VHS) electric
motor = ~40.33 BHP required
Step 3b: Check adequacy of 1″ C-1045 carbon steel lineshaft.
(Figure 2, Table 2a in May 2016
Water Works)
1″ lineshaft–OK for up to 57.5 HP @ 1770 RPM and 4000 lb.
thrust load.
Load = 40.33 HP < 57.5 HP allowed–OK. 4000 lb.> 2947 lb.–
OK.
Step 4: Verify shaft elongation will not result in impeller
dragging.
(Figure 2, Table 3 in May 2016 Water Works).
~2400 lb. of hydraulic downthrust with 1″ lineshaft = .127″
per 100′ ("C")
.127″/C × 1.6 (for 160′ total length of 1″ lineshaft + head
shaft) = .203″ total shaft stretch.
.203″ < .75″ allowed–OK (See pump curve or data sheet.)
In cases with more than 300′ of pump setting, the "relative
shaft stretch," which includes the total lineshaft stretch minus
the total column stretch, must be determined. This additional
design element will be explained in detail in a future
Water
Works column.
Suction Conditions
Even though this particular installation has no relevant is-
sues with the suction conditions, the topic still requires a brief
review.
As indicated on the performance curve, this specific pump
we have selected (Pump #1) has an NPSH
R
requirement that
varies with the flow from a low of 6′ up to 18′. The NPSH
R
at
the primary design condition of 500 GPM is approximately
10′. At sea level, this means the pump could actually lift water
about 15′ (see this discussion in the May 2016 issue of The
Water Works). The determination of this factor is as follows:
25′ (NPSH
A
) – 10′ (NPSH
R
) = 15′ Maximum suction lift
As the suction inlet of this pump will be well under the
projected lowest pumping water level (~60′), the NPSH
should not be a factor. Our next suction concern, if any, would
be to verify there is adequate submergence of water over the
pump inlet. Adequate submergence is a critical, but nonethe-
less often overlooked pumping condition. Generally, most
VTPs (including submersibles) require between 5′-10′ of
submergence over the inlet to avoid cavitation or a possible
"whirlpooling", cyclonic, or pre-rotation action developed
into the inlet.
WATER WORKS
continues on page 58
Table 1
Pump # Flow Rate Head SOH Efficiency Bowl Horsepower
1 500 GPM 260' 345' 84% 39.08 BHP
1 392 GPM 295' 81% 36.05 BHP
1 156 GPM 320' 50% 25.21 BHP
2 500 GPM 260' 395' 82.9% 39.60 BHP
2 392 GPM 300' 79% 37.59 BHP
2 156 GPM 355' 45% 31.08 BHP
3 500 GPM 260' 325' 81% 40.53 BHP
3 392 GPM 285' 75% 37.62 BHP
3 156 GPM (Q