Solving Hydraulic System Overheating Problems
I was asked recently to investigate and solve an overheating problem in a mobile hydraulics application. The hydraulic system that was overheating 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:
PLtotal = PLpump + PLvalves + PLplumbing + PLactuators
If the total input power lost to heat is greater than the heat dissipated, the hydraulic 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 hydraulic system’s components. This can occur well below 180°F, depending on the fluid’s viscosity grade.
Maintaining stable hydraulic 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 hydraulic 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 ¾” pressure hose at 24 gpm is 800 psi. The pressure drop across the same length of 1″ return hose is 200 psi. The formula for these calculations are 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 hydraulic system’s heat exchanger. This, when combined with the system’s normal heat load (inefficiencies) was causing the hydraulic 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 hydraulic 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.
Need help troubleshooting your hydraulic system?
Related Articles
Part one of a four-part series that will cover alignment fundamentals and thermal growth, and highlight the importance of field measurements through two case studies. Despite the best efforts to precisely align rotating machinery shafts, dynamic movement (commonly believed to be due to the thermal growth of the machine casings) has resulted in machines operating at less than optimum alignment conditions. This vexing problem has plagued machine reliability professionals for decades.
Part one of a four-part series that will cover alignment fundamentals and thermal growth, and highlight the importance of field measurements through two case studies. Despite the best efforts to precisely align rotating machinery shafts, dynamic movement (commonly believed to be due to the thermal growth of the machine casings) has resulted in machines operating at less than optimum alignment conditions. This vexing problem has plagued machine reliability professionals for decades.
See More
Contrary to popular opinion, a centrifugal pump is not designed to develop a specific head at a certain capacity as requested by the pump purchaser. In fact a pump is designed and built to produce a whole range of head-capacity conditions as identified by it’s performance curve. The pump will operate on that curve if it is driven at the designated speed.
Contrary to popular opinion, a centrifugal pump is not designed to develop a specific head at a certain capacity as requested by the pump purchaser. In fact a pump is designed and built to produce a whole range of head-capacity conditions as identified by it’s performance curve. The pump will operate on that curve if it is driven at the designated speed.
See More
The majority of the failure mechanisms that significantly reduce the reliability and life of a centrifugal pump are caused by operation away from the pump best efficiency point (BEP). To achieve best-in-class life, it is essential to either operate close to the pump BEP (which is always the preferred method) or to provide provisions that anticipate the various effects of off-BEP operation and attempt to mitigate these consequences.
The majority of the failure mechanisms that significantly reduce the reliability and life of a centrifugal pump are caused by operation away from the pump best efficiency point (BEP). To achieve best-in-class life, it is essential to either operate close to the pump BEP (which is always the preferred method) or to provide provisions that anticipate the various effects of off-BEP operation and attempt to mitigate these consequences.
See More
Last time, we pretty much ignored ladder diagrams. This time I'm giving you lots of them. We'll look at the ins and outs of turning those pretty pictures into powerful text. You'll gain insight into the block commands and the various structures that can be built using List. You'll leave with full knowledge of the perils and pitfalls.
Last time, we pretty much ignored ladder diagrams. This time I'm giving you lots of them. We'll look at the ins and outs of turning those pretty pictures into powerful text. You'll gain insight into the block commands and the various structures that can be built using List. You'll leave with full knowledge of the perils and pitfalls.
See More
The affinity laws accurately predict the affect of changing the speed of a centrifugal or rotary pump, and they also do a fairly good job of predicting the affect of changing the diameter of a centrifugal pump. In another paper (02-01) we discussed the affinity laws as they apply to centrifugal pumps, but in this paper we will look at their use with rotary pumps.
The affinity laws accurately predict the affect of changing the speed of a centrifugal or rotary pump, and they also do a fairly good job of predicting the affect of changing the diameter of a centrifugal pump. In another paper (02-01) we discussed the affinity laws as they apply to centrifugal pumps, but in this paper we will look at their use with rotary pumps.
See More
What they finally realized is that the majority of their day-to-day work orders were being generated to repair things that were happening at the failure mode level, which was determined to be reactive maintenance. How times have changed! These days, the majority of those senior level managers have been promoted into positions within the reliability group and their technicians are now using improvement-based maintenance techniques. They have never looked back since!
What they finally realized is that the majority of their day-to-day work orders were being generated to repair things that were happening at the failure mode level, which was determined to be reactive maintenance. How times have changed! These days, the majority of those senior level managers have been promoted into positions within the reliability group and their technicians are now using improvement-based maintenance techniques. They have never looked back since!
See More
Mass unbalance in a rotating system often produces excessive synchronous forces that reduce the life span of various mechanical elements. Unbalance is basically very simple case caused by an asymmetry in the rotating element that results in an offset between the shaft centerline and center of mass. Asymmetry can be an off-center weight distribution, or it can be a thermal mechanism that produces uneven heating and bowing of the rotor, or it can be an electrical effect that produces uneven magnetic field.
Mass unbalance in a rotating system often produces excessive synchronous forces that reduce the life span of various mechanical elements. Unbalance is basically very simple case caused by an asymmetry in the rotating element that results in an offset between the shaft centerline and center of mass. Asymmetry can be an off-center weight distribution, or it can be a thermal mechanism that produces uneven heating and bowing of the rotor, or it can be an electrical effect that produces uneven magnetic field.
See More
