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