Liquid Pumps Technical Equations and Information Tables

Efficiency Formulas for Rotary Positive Displacement Pumps

Pipe Velocity

  • It is best to allow 10 pipe diameter upstream and 5 pipe diameter downstream leading to the pump suction.
  • Try to keep pipe velocities around 10 ft/sec for good practical results.
  • Doubling the pipe diameter increases the capacity of the pipe four times.

Rules of Thumb for Motors

At 3600 rpm: motor develops 1.5 ft/lb per hp

At 1800 rpm: motor develops 3.0 ft/lb per hp

At 1200 rpm: motor develops 4.5 ft/lb per hp

At 575 volts, a 3-phase motor draws 1.00 amps per hp

At 460 volts, a 3-phase motor draws 1.25 amps per hp

At 230 volts, a 3-phase motor draws 2.50 amps per hp

Approximate rpm at Full Load for Medium Sized Motors

Poles rpm 60 Hz Synch speed rpm 50 Hz Synch speed
2 3500 3600 2850 3000
4 1750 1800 1450 1500
6 1150 1200 950 1000
8 850 900 700 750

 

Loss of Air Pressure Due to Friction

(Per 100 Feet of Pipe, 100 psi Initial Pressure)

Free-air
capacity in cfm
(at 14.7 psi)		 Equivalent
capacity in cfm
(at 100 + 14.7 psi)		 1/2" 3/4" 1" 1-1/4" 1-1/2" 2"
10 1.28 1.38 0.09 0.03 0.007    
20 2.56 1.42 0.34 0.10 0.026 0.012  
30 3.84 3.13 0.74 0.23 0.056 0.026  
40 5.13 5.55 1.28 0.38 0.096 0.044 0.013
50 6.41 8.65 2.00 0.60 0.146 0.067 0.020
60     2.84 0.84 0.210 0.095 0.027
70     3.85 1.12 0.280 0.130 0.036
80     5.01 1.44 0.360 0.160 0.046
90     6.40 1.85 0.450 0.200 0.058
100     7.80 2.21 0.550 0.250 0.069
125     12.40 3.41 0.850 0.380 0.107
150     18.10 4.91 1.200 0.540 0.150
175       6.80 1.640 0.730 0.200
200       8.79 2.120 0.950 0.260
250         3.300 1.480 0.400
300         4.710 2.100 0.570
350         6.450 2.860 0.770
400         8.300 3.700 0.990
450           4.650 1.270
500           5.790  1.560 
600           8.45 2.230
700             3.000
800             4.000
900             5.050
1000             6.200

Loss of Air Pressure Due to Pipe Bends

(Per 100 Feet of Straight Pipe)

Angle of pipe bend  Loss of pressure in psi per pipe ID 
1/2"  3/4" 1" 1-1/4" 1-1/2" 2"
45° 0.73  0.92  1.18  1.55  1.85  2.35 
90°  1.60  2.00  2.50  3.40  4.00  5.10 

Flow of Air through Orifice

(with Discharge of Orifice at 14.7 psia and 70°F)

Supply 
pressure 
(psig) 		 Flow of air in cfm per orifice size 
1/32"  1/16"  3/32"  1/8"  5/32"  3/16"  7/32"  1/4"  9/32"  5/16" 
65 1.15 4.49 10.10 17.90 27.90 40.30 55.20 71.80 89.90 111.70
70 1.21 4.77 10.80 19.10 29.70 42.80 58.80 76.40 95.70 118.80
75 1.30 5.06 11.40 20.20 31.50 45.40 62.30 81.00 105.50 126.00
80 1.37 5.35 12.10 21.10 33.30 48.00 65.80 85.60 107.40 133.10
85 1.44 5.64 12.70 22.50 35.10 50.60 69.40 90.30 113.20 140.30
90 1.52 5.92 13.40 23.70 36.90 53.20 72.90 94.80 119.00 147.50
95 1.59 6.21 14.00 24.80 38.70 55.70 76.50 99.40 124.90 154.60
100 1.66 6.50 14.70 26.00 40.50 58.30 80.00 104.60 130.70 161.80
125 2.03 7.94 17.90 31.70 49.50 71.40 97.78 127.10 159.80 197.50
150 2.40 9.28 21.20 37.50 58.40 84.40 115.40 150.40 189.00 233.30