I was asked recently to investigate and solve an overheating problem in a mobile application.
The system comprised a diesel-hydraulic power unit, which was being used to power a pipe-cutting
saw. The saw was designed for sub-sea use and was connected to the hydraulic power unit on the
surface via a 710-foot umbilical. The operating requirements for the saw were 24 gpm at 3000 psi.
Why do hydraulic systems overheat?
Heating of hydraulic fluid in operation is caused by inefficiencies. Inefficiencies result in
losses of input power, which are converted to heat. A hydraulic system's heat load is equal to
the total power lost (PL) through inefficiencies and can be expressed as:
PL total = PL pump + PL valves + PL plumbing + PL actuators
If the total input power lost to heat is greater than the heat dissipated, the system will
eventually overheat.
Hydraulic fluid temperature - how hot is 'too hot'?
Hydraulic fluid temperatures above 180°F (82°C) damage most seal compounds and accelerate
degradation of the oil. While the operation of any hydraulic system at temperatures above 180°F
should be avoided, fluid temperature is too high when viscosity falls below the optimum value
for the system's components. This can occur well below 180°F, depending on the fluid's viscosity
grade.
Maintaining stable fluid temperature
To achieve stable fluid temperature, a hydraulic system's capacity to dissipate heat must
exceed its inherent heat load. For example, a system with continuous input power of 100 kW and
an efficiency of 80% needs to be capable of dissipating a heat load of at least 20 kW. It's
important to note that an increase in heat load or a reduction in a system's capacity to
dissipate heat will alter the balance between heat load and dissipation.
Returning to the above example, the hydraulic power unit had a continuous power rating of 37 kW
and was fitted with an air-blast heat exchanger. The exchanger was capable of dissipating 10 kW
of heat under ambient conditions or 27% of available input power (10/37 x 100 = 27). This is
adequate from a design perspective. The performance of all cooling circuit components were
operating within design limits.
Pressure drop means heat
At this point it was clear that the overheating problem was being caused by excessive heat load.
Concerned about the length of the umbilical, I calculated its pressure drop. The theoretical
pressure drop across 710 feet of 3/4" pressure line at 24 gpm is 800 psi. The pressure drop across
the same length of 1" return line is 200 psi. The formula for these calculations is
available here. The theoretical heat load
produced by the pressure drop across the umbilical of 1,000 psi (800 + 200 = 1000) was
10.35 kW. The formula for this calculation is available here.
This meant that the heat load of the umbilical was 0.35 kW more than the heat dissipation capacity
of the system's heat exchanger. This, when combined with the system's normal heat load
(inefficiencies) was causing the system to overheat.
Beat the heat
There are two ways to solve overheating problems in hydraulic systems:
- decrease heat load; or
- increase heat dissipation.
Decreasing heat load is always the preferred option because it increases the efficiency of the
system. In the above example, the heat load of the umbilical alone was nearly 30% of available
input power, a figure that would normally be considered unacceptable. Decreasing this heat load
to an acceptable level would have involved reducing the pressure drop, by replacing the pressure
and return lines in the umbilical with larger diameter hoses. The cost of doing this for what was
a temporary installation meant that, in this case, the most economical solution was
to install additional cooling capacity in the circuit.
Continuing to operate a hydraulic system when the fluid is over-temperature is similar to
operating an internal combustion engine with high coolant temperature. Damage is guaranteed.
Therefore, whenever a hydraulic system starts to overheat, shut it down, identify the cause
and fix it.
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"This book has the potential to save many
organizations lots of m0ney. It should be on the bookshelf of every engineer, supervisor, planner and
technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more
Alexander (Sandy) Dunn
Plant Maintenance Resource Center
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