Hydraulic Hose Sizing

Horsepower to Flow

First of all, we must determine fluid flow rate from the horsepower and pressure.

Horsepower = Pressure × Flow / 1714, or:

Flow Rate: Q = (HP * 1714) / P

Pump Displacement: CIPR = Q * 231 / (RPM * 0.97)

(CIPR = Cubic Inches Per Revolution)

HP	RPM	PSI	Flow Formula	Flow Rate (GPM)	Displacement Formula	CIPR
28	3600	3000	(28 * 1714) / 3000	15.997 GPM	15.997 * 231 / (3600 * 0.97)	1.058
50	2700	3000	(50 * 1714) / 3000	28.5 GPM	28.5 * 231 / (2700 * 0.97)	2.514
50	2700	2500	(50 * 1714) / 2500	34.28 GPM	34.28 * 231 / (2700 * 0.97)	3.02

Hose Sizing

These are the formulas and guidelines for determining sizes for the three hose types: Pressure, Return and Suction. They are determined from the fluid velocity in each type of hose. There are two sets of recognized values used are:

SAE Values

Pressure: 15 ft/sec

Return: 10 ft/sec

Suction: 4 ft/sec

NFP Association Values

Pressure: 20 ft/sec

Return: 15 ft/sec

Suction: 5 ft/sec

The more conservative SAE values are gaining acceptance in hydraulic designers. The formula for calculating the hose size is as follows:

V = Q / (3.117 * pi/4 * D ^ 2)

or

D = (Q / (3.117 * pi/4 * V) ) ^ 1/2

Where:

V is fluid velocity in ft/sec

Q is fluid volume in gallons/min

D is hose inside diameter in inches

Note: The sizing guidelines say to always round up.

Now, let's calculate the hose sizes:

Supply (Pressure) Hose

Flow Rate	SAE Formula	SAE Diameter	NFP Formula	NFP Diameter
13 GPM	(13 GPM / (3.117 * pi/4 * 15 ft/sec) ) ^ 1/2	0.595 in	(13 GPM / (3.117 * pi/4 * 20 ft/sec) ) ^ 1/2	0.515 in
16 GPM	(16 GPM / (3.117 * pi/4 * 15 ft/sec) ) ^ 1/2	0.660 in	(16 GPM / (3.117 * pi/4 * 20 ft/sec) ) ^ 1/2	0.571 in
28.5 GPM	(28.5 GPM / (3.117 * pi/4 * 15 ft/sec) ) ^ 1/2	0.881 in	(28.5 GPM / (3.117 * pi/4 * 20 ft/sec) ) ^ 1/2	0.763 in
32.28 GPM	(32.28 GPM / (3.117 * pi/4 * 15 ft/sec) ) ^ 1/2	0.937 in	(32.28 GPM / (3.117 * pi/4 * 20 ft/sec) ) ^ 1/2	0.812 in

Return Hose

Flow Rate	SAE Formula	SAE Diameter	NFP Formula	NFP Diameter
13 GPM	(13 GPM / (3.117 * pi/4 * 10 ft/sec) ) ^ 1/2	0.729 in	(13 GPM / (3.117 * pi/4 * 15 ft/sec) ) ^ 1/2	0.595 in
16 GPM	(16 GPM / (3.117 * pi/4 * 10 ft/sec) ) ^ 1/2	0.808 in	(16 GPM / (3.117 * pi/4 * 15 ft/sec) ) ^ 1/2	0.660 in
28.5 GPM	(28.5 GPM / (3.117 * pi/4 * 10 ft/sec) ) ^ 1/2	1.08 in	(28.5 GPM / (3.117 * pi/4 * 15 ft/sec) ) ^ 1/2	0.881 in
32.28 GPM	(32.28 GPM / (3.117 * pi/4 * 10 ft/sec) ) ^ 1/2	1.14 in	(32.28 GPM / (3.117 * pi/4 * 15 ft/sec) ) ^ 1/2	0.938 in

Suction Hose

Flow Rate	SAE Formula	SAE Diameter	NFP Formula	NFP Diameter
13 GPM	(13 GPM / (3.117 * pi/4 * 4 ft/sec) ) ^ 1/2	1.15	(13 GPM / (3.117 * pi/4 * 5 ft/sec) ) ^ 1/2	1.03 in
16 GPM	(16 GPM / (3.117 * pi/4 * 4 ft/sec) ) ^ 1/2	1.28 in	(16 GPM / (3.117 * pi/4 * 5 ft/sec) ) ^ 1/2	1.14 in
28.5 GPM	(28.5 GPM / (3.117 * pi/4 * 4 ft/sec) ) ^ 1/2	1.70 in	(28.5 GPM / (3.117 * pi/4 * 5 ft/sec) ) ^ 1/2	1.53 in
32.28 GPM	(32.28 GPM / (3.117 * pi/4 * 4 ft/sec) ) ^ 1/2	1.81 in	(32.28 GPM / (3.117 * pi/4 * 5 ft/sec) ) ^ 1/2	1.62 in

Notes:

The 28 hp Briggs & Stratton engine HP is rated at 3600 RPM.
The 50 hp Volkswagen engine is believed to deliver the peak torque at 2700 RPM (needs confirmation)