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Evidence gained through my many years working in the hydraulic repair industry suggests that the failure of maintenance personnel to follow proper procedures when re-commissioning hydraulic systems after carrying out maintenance work, is a significant cause of infant mortality in hydraulic equipment.
Incorrect commissioning during startup can result in damage to hydraulic components through cavitation, aeration and inadequate lubrication. In many cases, this damage will not show itself until the component fails hundreds or even thousands of service hours after component failure.
A common misconception among maintenance personnel with limited training in hydraulics is that because oil circulates through hydraulic components in operation, no special attention is required during installation beyond bolting the component into position and connecting its hoses. This is not the case.
Damage to hydraulic components during initial startup can be prevented by:
Depending on the type of equipment in operation, detailed commissioning procedures may be available from the machine manufacturer on 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.
If the system is down as a result of a major component failure:
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.
Change out the filters.
Change or flush the hydraulic fluid.
When installing pumps and motors, check the drive coupling for fit on the pump or motor shaft. Loose fitting couplings cause accelerated wear of the drive shaft 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 cylinders with clean hydraulic fluid through the service ports. This reduces the risk of air compression within the cylinder (dieseling) on startup, which will result in damage to the cylinder and its 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. Failure to do so will result in damage to the motor through inadequate lubrication on startup. 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.
Pumps - After installing the pump(s) and connecting service lines:
Open the intake line isolation valve at the reservoir.
On pumps with a flooded inlet, that is, when pump inlet is below reservoir fluid level, 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 a flooded housing (see below).
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 damage to the pump through inadequate lubrication on startup. 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.
On piston-type pumps with a flooded housing, that is, when pump case and inlet are connected internally and are below reservoir fluid level, carefully loosen the uppermost plug in the pump case to allow trapped air to escape. This ensures that the case is full of fluid. Failure to do so will result in damage to the pump through inadequate lubrication on startup.
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.
Check that all pipe and hose connections are tight.
Confirm reservoir fluid level is above the minimum.
CAUTION! Confirm all controls are in neutral to ensure the system will start unloaded. Take safety precautions to prevent machine movement in the event that the system is activated during initial startup.
If the prime mover is electric, momentarily start and then stop the electric motor to visually confirm the direction of motor rotation is correct for the pump. Rotating the pump in the wrong direction can damage the pump.
Start the prime mover and run at the lowest possible rpm.
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.
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!
Allow the system to run at idle and unloaded for ten minutes. Monitor pump(s) for unusual noise or vibration, inspect system for leaks and observe reservoir fluid level.
Operate the system without load. Stroke cylinders slowly, taking care not to develop pressure at the end of stroke to avoid compression of trapped air, 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.
With the system at operating temperature, check and adjust settings of circuit protection devices according to manufacturers’ specifications.
Cycle the system under load to verify correct operation.
Inspect the system for leaks.
Shut down the prime mover, remove all gauges fitted during commissioning, check reservoir fluid level and, if necessary, fill to the correct level.
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 training of new maintenance employees.