Hydraulic safety, general practice reminders and troubleshooting

Excessive heat and noise are prime indicators of fluid system malfunction on fiber, paper, board and tissue machine lines. But before jumping in to fix a fluids problem - there are several safety factors to consider before and during maintenance. In this article we list these, as well as basic fluids troubleshooting tips.

Safety and general practice reminders

Only trained personnel should perform maintenance or add new devices. This includes familiarity with the machine being worked on (safety issues, design and operation, and control circuits). Refer to applicable sections in the operating and maintenance manuals that came with the original equipment and rebuilt equipment as well as mill procedures.

As a general rule, both hydraulic and pneumatic systems must be locked out and depressurized before starting maintenance or repair work on any fluid system.

Never by-pass safety features, such as a guards or interlocks.

Before shutting down the system, familiarize yourself with the mill’s lock-out procedures.

Before depressurizing circuits, make sure that all loads are in the lowered position or are mechanically secured.

When shutting down a hydraulic system:

1. Always begin a hydraulic system shutdown by stopping the pumps.
2. Next, close the applicable isolation valves in the system (before performing the maintenance or repair).
3. After stopping the pumps, and before starting any maintenance or repair work, lock the motor starters for the electric motors so that there is no chance of anyone accidentally starting them. If there are no lockable motor starters, remove the motor fuses.
4. Put your name on secured locks and removed fuses. Never remove someone else's lock-out tags.

When shutting down a pneumatic system:

1. Always begin pneumatic system shutdown by closing the hand valve which isolates the system from the mill air supply.
2. Next, bleed trapped air from the circuit.

Verify that there is no pressure in the system, such as in pressure accumulators, air receivers, pipelines, or actuators which need to be depressurized.

Use appropriate tools and methods for repair and maintenance work.

Avoid spilling oil on the floor. Clean up any oil spills immediately.

Before removing a valve:

• If a valve has manual override controls, release any remaining pressure between the valve and actuator.
• Bleed off pressure repeatedly in both directions.
• Close the shut-off valve to the valve panel.

Use appropriate gaskets or suitable sealing compound to seal threaded connections in the system, as required.

Do not use cotton rags or other lint cloth. When changing a cylinder, plug hose ends to keep impurities out. Always pay attention to cleanliness when servicing hydraulic or pneumatic systems.

Re-tighten disconnected pipe and hose couplings immediately after maintenance work. Prior to finishing the maintenance work, verify that all connections that have been serviced or repaired are tight. Re-tighten as necessary.

Before energizing the system, make sure that no one else is in the vicinity of the machine, and no other maintenance work is in progress. Never remove anyone else's lock-out tags. When starting the pumps, maintain a safe distance from the areas where maintenance or repair work took place.

Before opening isolation valves, make sure that no one is working between the machine components. System pressurization may cause machine components to move unexpectedly.

Once the system is pressurized, test the repaired component. Check for leaks. If any are found, the system may need to be depressurized again to repair them. Do not tighten leaky fittings under pressure.

WARNING: Hydraulic and pneumatic systems are under high pressure. Do not test for hydraulic or pneumatic leaks with your hands. Use another material (wood, cardboard, etc.) to avoid risk of injection of hydraulic fluid or air under the skin. Seek emergency help for any injuries.

Make sure that all signs and notices attached to the machine or instrument components are in place and clean.

Keep instrument boxes and cabinets closed.

General troubleshooting principles

Familiarity with circuits

When troubleshooting fluid circuits, it is important to have a full understanding of all circuit components and the relationships between them. Therefore, the first step in troubleshooting any circuit should be to study the circuit diagrams and the fluid section of the maintenance manual(s) that came with the original equipment as well as any manuals that accompanied rebuilds.

Once you have a full understanding of a malfunctioning circuit, you can begin to isolate the cause of the malfunction by checking the performance of each component.

Pressure settings

Always check the operating pressures first. Some pressure settings are specified on the circuit diagrams.

If a specification is unavailable, set the pressure to the lowest level that will allow adequate performance of the system, without exceeding the maximum rating of the components.

Electrical controls

Most fluid circuits have at least two operating modes (ie: extend/retract or raise/lower). Each operating mode requires that the solenoid valves in the circuit be in a specific state (energized or de-energized). The text associated with each circuit diagram should explain which valves should be energized and which should be de-energized. This information, combined with the applicable electrical drawings should help you determine whether all the conditions have been met for a particular operating mode. If the conditions have not been met, or other controls are malfunctioning, correct that problem first, then proceed to check the fluid components themselves.

Trouble indicators

Trouble is indicated whenever circuit performance deviates from normal. As a result, the best way to learn how to recognize trouble is to become familiar with the normal operation of each circuit. Some general guidelines for recognizing trouble indications are as follows:

• Excessive heat means trouble. A warmer than normal tank return line on a relief valve indicates operation at relief valve setting. Hydraulic fluids which have too low a viscosity will increase the internal leakage of components and increase the heat rise.
• Typically, if oil temperature exceeds 140°F (60°C), check the cooling system. If no cooling system fault is found, locate and replace any devices showing reduced volumetric efficiency. Monitor the temperatures of different devices to detect major leaks.
• Excessive noise could mean wear, misalignment, cavitation, or air in the fluid. Contaminated fluid can cause a relief valve to stick or chatter. These noises may be the result of contamination, excessive viscosity, low reservoir level, or leaking suction or pressure lines.

Component replacement notes

If the actuator has failed, start troubleshooting from the actuator and work toward the power unit components.

If the actuator does not receive sufficient oil flow or pressure, start troubleshooting at the power unit and work towards the actuator.

New valves must be adjusted and locked or tuned to the settings given for the position concerned.

On computerized systems, try using the help and maintenance screens to locate and identify possible hydraulic operating problems.

For more information on fluids safety and troubleshooting, there are several Valmet articles covering different aspects of fluids maintenance, or contact your Valmet representative for training options.