Most people are familiar with friction loss and velocity requirements when it comes to pump discharge lines and the impact these factors have on overall system performance. In the world of hydraulic power packs, hydraulic tools and hydraulically driven submersible pumps, the same factors must be considered to insure that enough energy is being efficiently transferred through the power unit and on down to the hydraulic motor powering the tool or submersible pump.
Each size of hydraulic hose has limitations when it comes to how much flow can be pushed through the Inside Diameter (ID) without adversely affecting both working pressure on the input side and back pressure on the return side of the system. So, choosing the right diameter hose is critical for eliminating inefficiencies which result in heat generation and excess back pressure in the motor.
The general rule for velocity in a pressure line is 10-20 feet per second and on the return side it should be between five and ten feet per second. The use of different ID hoses will alter this velocity, and the right size hose will insure that your system runs efficiently. This can be the case whether you are running 50 feet or 500 feet of hydraulic line out to the tool or submersed pump. The link below provides an industry standard reference to hose selection as it relates to velocity.
As mentioned above, the size and length of the hose run could also create an adverse impact on both the high pressure fluid delivery and the system back pressure. In fact, the hose size can have a greater effect on the tool performance even if the velocity of the fluid is within range.
For example, if a power unit is outputting 8gpm at 2500psi and you are using ½” (-8) hydraulic hose for delivery, the pressure loss across a 50 foot run is 33psi and the velocity of the fluid is 13f/s (ideal). If you add hose to a length of 150 feet the psi loss jumps to 99psi while the velocity remains the same. And, if the one way hose length increases to 300 feet, the pressure loss jumps to 300psi with the same 13f/s velocity. So at 300 feet out, the system pressure drops by over ten percent and the added friction also builds harmful heat which will eventually damage the critical hydraulic components as the oil quality degrades. And of course the performance of the tool or pump will drop off by at least the same percentage.
Not using the right size hose on the return side does not result in a pressure loss, rather it creates a pressure build. This increase in back pressure, when not controlled, can lead to failure of the lip seal in the hydraulic motor which can lead to mechanical seal failure and possibly a loss of system hydraulic oil in to the material being pumped. Before providing some sample numbers, consider that any lip seal in a hydraulic motor is typically rated for 100psi. With that in mind, using the same inputs as above (8gpm at 2500psi), and the same starting length of 50 feet of ½”ID hose, the system back pressure would be 33 psi at 13 f/s (the system pressure of 2500psi has already been expended doing the work). Moving to 150 feet of return hose jumps the back pressure to 99psi or the limit of most lip seals, and a 300 foot hose run results in back pressure of nearly 200psi.
In both cases above, up-sizing the hydraulic hose one size to 5/8” (-10) has a dramatically positive effect on system pressures. On the input side, the pressure loss of 300psi drops down to 67psi and on the return side, the back pressure drops to the same 67psi. These new pressures realized are dramatically better and will help insure that your system runs more efficiently.
The other thing to consider as you set up the hydraulic lines is the number of quick disconnects or other connections across the length of the run. It is always better to have fewer connections as each junction creates friction which impacts both delivery and back pressure.
Taking the time to think about the hoses and making sure to choose the right hose will go a long way towards insuring that your hydraulic system runs efficient and trouble free.