Line Sizing And Fluid Velocity In Hydraulic Systems

TECH TIP

LINE SIZING AND FLUID VELOCITY IN HYDRAULIC SYSTEMS

LINE SIZE SELECTION

Selecting the proper line size for a hydraulic system is critical to get maximum performance and life from your hydraulic components. The four basic line types in a hydraulic system are pump suction, return (Low pressure <100 PSI), medium pressure (500 to 2000 PSI), and high pressure (2100 to 5000 PSI). Under-sizing fluid lines will result in high pressure loss and system overheating. Over sizing lines increases the cost of the system. Lines should be sized as follows:

Suction: 2-4 feet/sec.

Return: 10-15 feet/sec.

Medium Pressure: 15-20 feet/sec.

High Pressure: 20-25 feet/sec.

On extremely long lines the pressure drop through the pressure and return lines must be accounted for to insure sufficient pressure is available at the actuator.

(See pressure drop charts included with this document).

FLUID VELOCITY:

Velocity through a line can be calculated with the formula:

Velocity = GPM x 0.3208
Area

Velocity = Feet Per Second (fps)

GPM = Gallon per minute fluid flow

A = Square inch inside area of conductor

The Following velocity charts are a quick way to check velocity through a line. Included in the charts is the I.D. of the line. If the particular tube, pipe or hose you are using is not listed, reference a line with a similar I.D. for approximate velocity.

Approximate Velocity in GPM – Schedule 40 Pipe

Size	I.D.	2fps	4fps	10fps	15fps	20fps
3/8”	0.493”	1.2	2.3	5.9	8.9	11.9
1/2″	0.622”	1.9	3.8	9.5	14.2	18.9
3/4″	0.824”	3.3	6.6	16.6	24.9	33.2
1”	1.049”	5.4	10.7	26.9	40.4	53.9
1-1/4”	1.38”	9.3	18.6	46.6	69.9	93.2
1-1/2”	1.610”	12.7	25.3	63.5	95.2	126.9
2”	2.067”	20.9	41.8	104.6	156.9	209.2
2-1/2”	2.469”	29.8	59.7	149.2	223.8	298.5
3”	3.068”	46.1	92.1	230.4	345.6	460.8
4”	4.026”	79.4	158.7	396.8	595.2	793.6
6”	6.065”	180.1	360.2	900.5	1350.7	1801

Approximate Velocity in GPM – Schedule 80 Pipe

Size	I.D.	2fps	4fps	10fps	15fps	20fps	25fps
3/8”	0.423”	0.9	1.75	4.4	6.6	8.8	11
1/2″	0.546”	1.5	2.9	7.3	10.9	14.6	18.2
3/4″	0.742”	2.7	5.4	13.5	20.2	27	33.7
1”	0.957”	4.5	8.9	22.4	33.6	44.8	56
1-1/4”	1.278”	8	16	40	60	80	100
1-1/2”	1.5”	11	22	55.1	82.6	110.2	137.7
2”	1.939”	18.4	36.8	92	138.1	184.1	230.1
2-1/2”	2.323”	26.4	52.8	132.1	198.2	264.2	330.3
3”	2.9”	41.2	82.3	205.9	308.8	411.8	514.7
4”	3.826”	71.7	143.4	358.3	537.5	716.7	895.9
6”	5.761”	162.5	325.1	812.5	1218.7	1624.9	2031.2

Approximate Velocity in Tubing

Size	I.D.	2fps	4fps	10fps	15fps	20fps	25fps
3/8”x.049wall	0.277”	0.38	0.75	1.88	2.81	3.75	4.69
1/2″x.065wall	0.37”	0.67	1.34	3.35	5.03	6.7	8.37
3/4″x.109wall	0.532”	1.39	2.77	6.93	10.4	13.9	17.31
1”x.120wall	0.76”	2.83	5.65	14.1	21.2	28.3	35.33
1-1/4”x.120wall	1.01”	5	10	25	37.4	50	62.4
1-1/2”x.120wall	1.26”	7.77	15.5	38.8	58.3	77.7	97.12

PRESSURE DROP:

Pressure drop through the conductor must be accounted for in long runs of pipe, hose or tubing. This will insure you have sufficient pressure available at your actuator to do work and sufficient horsepower available at the pump. You can use the following charts to estimate total pressure drop through a conductor based on fluid velocity. Included in the charts is the I.D. of the conductor, so if the particular tube or pipe you are using is not listed you can reference a conductor with a similar I.D. for approximate pressure drop.

Fluid viscosity plays a very large part in calculating pressure drop through the system. The charts below are based on ISO grade 46 hydraulic oil with a viscosity of approximately 225 S.S.U. at 100 deg. F and .88 specific gravity. Actual pressure drop will increase as viscosity increases.

Approximate Pressure Drop (PSI) per 1 Foot of line length – Schedule 40 Pipe Size

I.D.	2fps	4fps	10fps	15fps	20fps	25fps
3/8”	.493”	0.18	0.36	0.91	1.36	2.98	4.4
1/2”	.622”	0.11	0.23	0.57	1.35	2.23	3.29
3/4″	.824”	0.06	0.13	0.32	0.95	1.57	2.31
1”	1.049”	0.04	0.08	0.34	0.7	1.16	1.71
1-1/4”	1.38”	0.02	0.05	0.24	0.5	0.82	1.21
1-1/2”	1.610”	0.02	0.04	0.2	0.41	0.68	1
2”	2.067”	0.02	0.04	0.15	0.3	0.5	0.73
2-1/2”	2.469”	0.01	0.03	0.12	0.24	0.4	0.59
3”	3.068”	0.01	0.02	0.09	0.18	0.3	0.45
4”	4.026”	0.01	0.02	0.06	0.13	0.22	0.32
6”	6.065”	0.004	0.01	0.04	0.07	0.12	0.18

Approximate Pressure Drop (PSI) per 1 Foot of line length – Schedule 80 Pipe

Size	I.D.	2fps	4fps	10fps	15fps	20fps	25fps
3/8”	0.423”	0.25	0.49	1.23	1.85	2.46	5.32
1/2”	0.546”	0.15	0.30	0.74	1.11	2.62	3.87
3/4″	0.742”	0.08	0.16	0.40	1.08	1.79	2.64
1”	.957”	0.05	0.10	0.39	0.79	1.30	1.92
1-1/4”	1.278”	0.03	0.05	0.27	0.55	0.91	1.34
1-1/2”	1.5”	0.02	0.04	0.22	0.45	0.74	1.09
2”	1.939”	0.02	0.04	0.16	0.33	0.54	0.80
2-1/2”	2.323”	0.02	0.03	0.13	0.26	0.43	0.63
3”	2.9”	0.01	0.02	0.10	0.20	0.33	0.48
4”	3.826”	0.01	0.02	0.07	0.14	0.23	0.34
6”	5.761”	0.005	0.01	0.04	0.08	0.14	0.20

Approximate Pressure Drop (PSI) per 1 Foot of line length – Tubing

Size	I.D.	2fps	4fps	10fps	15fps	20fps	25fps
3/8” x 0.49” Wall	0.277”	0.61	1.22	3.04	4.56	6.08	7.60
1/2” x 0.065” Wall	0.37”	0.31	0.63	1.57	2.35	3.13	6.19
3/4″ x 0.109” Wall	0.532”	0.18	0.35	0.88	1.32	2.92	4.32
1” x 0.120” Wall	0.76”	0.08	0.16	0.39	1.06	1.76	2.60
1-1/4” x 0.120” Wall	1.01”	0.04	0.09	0.37	0.74	1.23	1.82
1-1/2” x 0.120” Wall	1.26”	0.03	0.06	0.28	0.56	0.93	1.37

CONCLUSION

By using the information supplied in this tech tip and selecting the proper line sizes for your hydraulic equipment you will enjoy better performance and longer life from your hydraulic components. Flow will be more laminar, shock potential reduced, leak potential greatly lowered, fitting and connector life increased, wear and tear exposure on system components reduced and concern for excessive heat build-up minimized.

Note: “Tech Tips” offered by Flodraulic Group or its companies are presented as a convenience to those who may wish to use them and are not presented as an alternative to formal fluid power education or professional system design assistance.

Experts in fluid power, electrical and mechanical technologies.

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