Why do hydraulic systems get hot?

Applying reliability principles to system design may offer an answer.


The hydraulic cylinder. Courtesy: Motion Industries and FLIR SystemsIs it really possible to design reliability into a hydraulic system? Let's consider one of oldest problems that plague many hydraulic system designers, reliability engineers and maintenance technicians. They ask, "My hydraulic system is running hot—what's causing that?"

First, we must understand that hydraulic horsepower either goes to work as energy or is wasted energy in the form of heat. If a hydraulic system is designed to be efficient and is operated and maintained properly, it won't get hot.

There must be a pressure drop for oil to flow in a hydraulic system. However, there are certain pressure drops that are unnecessary and create a given amount of heat. If you look at the pressure drop for a half-inch standard 90-degree fitting with a 22.10-psi drop per fitting and then compare that to the pressure drop of a long-radius 90-degree fitting, it is significantly less at 2.98 psi drop per fitting.

For illustrative purposes, let's say we have heat or wasted horsepower in a circuit with a pump discharge flow of 25 gallons per minute.

Standard 90-degree fitting

With a 22.10-psi drop multiplied by 25 gallons/minute (gpm), divided by a 1,714-psi constant, you get 0.322 wasted horsepower. Multiply that by 2,545 Btus/hour per 1 hp, or by 819.5 Btus per hour of heat that will be generated as a result of using this type of fitting.

If you think that is insignificant, go out and count the 90-degree fittings in one of your circuits. I think you will be surprised at the amount of heat being generated for no apparent reason. If your circuit had 20 of the 90-degree fittings, that would generate 16,390 Btus of heat that your system was not designed to eliminate.

At this point, many clients ask for a heat exchanger to "mask" the real problem of a system that wasn't designed properly. If you really think about it, you are paying extra money to produce this additional heat, and then paying more money to eliminate it with a cooling device. What you are paying for is the expenses of extra horsepower for an air-type cooler, the cost of treating the water with a water-type cooler, plus installation and maintenance.

the temperature of the hose leading to the standard 90-degree fitting at 134° F The actual temperature of the fitting itself at 137° F, which indicates a 3° F temperature rise per fitting. Courtesy: Motion Industries and FLIR Camera

Long-radius 90-degree fitting

With a 2.98-psi drop multiplied by 25 gpm divided by the 1,714-psi constant, you get 0.043 wasted horsepower. Multiply that by 2,545 Btus/hour per 1 hp, and you get 109.4 Btus per hour of heat generated as a result of using the long-radius fitting.

Using the same 20 fittings in the first example, you wind up with just 0.86 of wasted horsepower, generating 2,188 Btus per hour of heat.

The cost in dollars

How much is that costing you per year in dollars? As a general rule, at 440 V, a three-phase motor draws 1.25 amp per horsepower. For this example, let's assume our power factor (pf) is 1.0 and our plant is in Florida, where the average commercial electricity rate is $9.66/kWh.

With the standard 90-degree fitting, you're wasting 6.4 hp multiplied by 1.25 amps per horsepower for a 440 V electric motor, or 8 amps. At more than 8,760 hours of operation per year, you'd wind up with 53,345 kWh per year. And at $0.0966/kWh, that comes to $5,153.11/year per fitting.

With the long-radius 90-degree fitting, you're wasting 1.07 amps. Over the same 8,760 hours of operation, that's 7095.95 kWh/year, or just $685.43/year per fitting.

When trying to identify the heat source in a total hydraulic system, your new best friend will be an infrared thermal imaging camera.

Obviously, there are many other things that cause heat in a hydraulic system, such as:

  • Improperly set relief valves when used in conjunction with a variable volume pressure-compensated pump
  • Internal leaks around spools and piston seals
  • Mechanical binds causing elevated operating pressure to force the relief valve to crack open.

However, if you understand that heat is a byproduct that is unnecessary and can be eliminated in the design phase, you will be miles ahead.

Paul Craven, CFPHS, manages one of Motion Industries' repair shops. He is a fluid power specialist and is certified by the International Fluid Power Society as a fluid power hydraulic specialist. For more information, go to www.motionindustries.com.

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