Liquid Pumps
Technical Equations and Information Tables

Efficiency Formulas for Rotary Positive Displacement Pumps

Liquid HP =

GPM x psi 1747

Volumetric efficiency

=

Actual GPM Theoretical GPM

Overall pump efficiency

=

HP of fluid discharge HP of driving motor

Mechanical efficiency

=

Overall pump efficiency Volumetric efficiency

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Pipe Velocity

Velocity (ft/sec)

=

GPM x 0.321      (pipe ID in inches)2

• 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.

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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

 

hp =	Torque ft/lb x rpm 5252

Torque (ft/lb) =

hp x 5252 rpm

Torque (in/lb) =

hp x 63,025 rpm

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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

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Atmospheric Pressure Conversion

Altitude in feet	Pressure in psia	Altitude in feet	Pressure in psia
0	14.7	5000	12.23
100	14.64	7000	11.34
300	14.54	10,000	10.11
500	14.43	15,000	8.29
700	14.33	20,000	6.76
1000	14.17	25,000	5.45
1500	13.92	30,000	4.36
2000	13.66	40,000	2.72
3000	13.17	50,000	1.68
4000	12.69	60,000	1.04

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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)	Loss of pressure in psi per pipe ID
		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	7.69		2.84	0.84	0.210	0.095	0.027
70	8.97		3.85	1.12	0.280	0.130	0.036
80	10.25		5.01	1.44	0.360	0.160	0.046
90	11.53		6.40	1.85	0.450	0.200	0.058
100	12.82		7.80	2.21	0.550	0.250	0.069
125	16.02		12.40	3.41	0.850	0.380	0.107
150	19.22		18.10	4.91	1.200	0.540	0.150
175	22.43			6.80	1.640	0.730	0.200
200	25.63			8.79	2.120	0.950	0.260
250	32.04				3.300	1.480	0.400
300	38.45				4.710	2.100	0.570
350	44.86				6.450	2.860	0.770
400	51.26				8.300	3.700	0.990
450	57.67					4.650	1.270
500	64.08					5.790	1.560
600	76.90					8.45	2.230
700	89.71						3.000
800	102.50						4.000
900	115.30						5.050
1000	128.20						6.200

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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

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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

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