A significant cause of infant mortality in hydraulic equipment can be attributed to the failure of maintenance personnel to follow proper procedures, such as oil flushing, before startup or when recommissioning hydraulic systems after carrying out maintenance work.
Cavitation, aeration, and inadequate lubrication can all damage equipment that has not been properly commissioned or recommissioned. Often, this damage will not be revealed until a component fails. This can take hundreds, or even thousands, of hours, but the results can still be unexpected and catastrophic in some cases.
Common Misconceptions About Hydraulic Systems
There are common misconceptions among maintenance personnel who have limited training in hydraulics. One is that because oil circulates through the hydraulic components that no special attention is required during the installation beyond bolting the component into position and connecting the hoses. This couldn’t be further from the truth.
Damage to hydraulic components during initial startup can be prevented by:
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Developing equipment-specific commissioning procedures.
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Training your maintenance personnel to effectively carry out these procedures.
Depending on the type of equipment in operation, detailed commissioning procedures may be available from the machine manufacturer upon request. If this information is not available for a specific piece of equipment, appropriate procedures should be developed. If necessary, consult a fluid power engineer for assistance. The following generic procedure illustrates many of the issues that should be considered.
Pre-start Procedures
If the system is down as a result of a major component failure:
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Drain and clean the reservoir to ensure that it is free from metallic debris and other contamination. Failure to do so may result in damage to the pump(s) and/or other components on startup.
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Change out the filters.
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Change or flush the hydraulic fluid.
When installing pumps and motors, check the fit of the drive coupling on the pump or motor shaft. Loose-fitting couplings cause accelerated wear of the driveshaft and should be replaced.
On closed-circuit systems (hydrostatic transmissions), inspect the high-pressure hoses or pipes between the pump and motor, and replace any suspect lines. A burst hose or pipe in service can result in the destruction of the pump and/or motor through cavitation.
Cylinders – Before connecting service lines, fill the cylinders with clean hydraulic fluid through the service ports. This helps reduce the risk of air compression, or dieseling, which would result in damage to the cylinder and seals.
Motors – Fill the case of piston-type motors with clean hydraulic fluid through the highest case-drain port and connect the case-drain line. Failing to do so causes inadequate lubrication and will damage the motor upon startup. Vertically mounted units require special attention to ensure fluid levels are high enough to lubricate the front shaft bearings.
Pumps - After installing the pump(s) and connecting service lines:
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Open the intake line isolation valve at the reservoir.
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When the pump inlet is below reservoir fluid levels (pumps with flooded inlets), carefully loosen the intake line fitting at the pump to allow trapped air to escape. This ensures that the intake line is full of fluid. This step is not necessary with piston-type pumps that have flooded housing (see below).
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On piston-type pumps fitted with an external case drain line, fill the pump case with clean hydraulic fluid through the highest case drain port and connect the case drain line. Failure to do so will result in inadequate lubrication upon startup and will cause damage to the pump. Units that are mounted vertically with the shaft up require special attention to ensure that the fluid level in the case is high enough to lubricate the front shaft bearings.
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When the pump case and inlet are connected internally and are below the reservoir fluid level (piston-type pumps with a flooded housing), carefully loosen the uppermost plug in the pump case to allow trapped air to escape.
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This ensures that the case is full of fluid. Failure to do so will result in inadequate lubrication upon startup and damage to the pump.
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On closed-circuit pumps (hydrostatic transmissions), install a 0 to 900 PSI pressure gauge in the charge circuit. Refer to the machine manufacturer’s instructions for guidance.
Startup Procedures
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Check that all pipe and hose connections are tight.
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Confirm reservoir fluid level is above the minimum.
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CAUTION! Confirm all controls are in neutral to ensure the system will start unloaded. Take safety precautions to prevent machine movement if the system is activated during initial startup.
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If the prime mover is electric, momentarily start and then stop the electric motor to visually confirm the direction of the motor rotation is correct for the pump. Rotating the pump in the wrong direction can damage the pump.
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Start the prime mover and run at the lowest possible rpm.
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On closed-circuit systems (hydrostatic transmissions), monitor the pressure gauge previously installed in the charge circuit. If the manufacturer’s specified charge pressure, typically 110 to 360 PSI, is not established within 20 to 30 seconds, shut down the prime mover and investigate the problem. Do not operate the system without adequate charge pressure - damage to the transmission pump and/or motor will result.
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On variable-displacement pumps and motors with external, low-pressure pilot lines, carefully loosen the pilot line fitting at the pump or motor to allow trapped air to escape. This ensures that the pilot line is full of fluid. CAUTION! Do not bleed pilot lines carrying high-pressure fluid. Personal injury may result. If in doubt, do not bleed pilot lines!
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Allow the system to run at idle and unloaded for 10 minutes. Monitor pump(s) for unusual noise or vibration, inspect the system for leaks, and observe reservoir fluid level.
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Operate the system without a load. Stroke the cylinders slowing to avoid developing pressure at the end of the stroke. This helps prevent trapped air from becoming compressed, which damages the cylinder and seals through dieseling. The term “dieseling” refers to the combustion process in a diesel engine. Dieseling occurs in a hydraulic cylinder when free air mixes with the hydraulic fluid and combusts when pressurized. Dieseling can destroy the cylinder’s seals and in extreme cases, the cylinder itself. So, continue to operate the system until all air is expelled and actuators operate smoothly.
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With the system at operating temperature, check and adjust settings of circuit protection devices according to manufacturers’ specifications.
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Cycle the system under load to verify correct operation.
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Inspect the system for leaks.
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Shut down the prime mover, remove all gauges fitted during commissioning, check reservoir fluid level and, if necessary, fill to the correct level.
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Return machine to service.
Once commissioning procedures for a specific piece of hydraulic equipment have been developed and the necessary training has been provided, from a preventive maintenance perspective, ongoing costs are limited to the training of new maintenance employees. This is a much more predictable and manageable cost than unplanned downtime or more frequent component failures that come with poor commissioning and recommissioning procedures.