It should also be remembered that hydraulic systems, and the fluids within them, often operate at relatively high temperatures. Therefore, carrying out maintenance work on a machine immediately after it has been shut down can result in burns from hot components or fluids. Even a temperature as low as 44°C, which is less than the operating temperature of many hydraulic systems, can cause burns to the skin. Mineral oil fluids are flammable, so care must be taken to ensure that any fluid spillages cannot come into contact with anything hot, such as welding or cutting equipment.
A much more serious danger with hydraulic fluids is the effect they have when they enter the bloodstream. This can happen if a high-velocity jet of fluid, caused by a pin-hole leak, comes into contact with the skin. Such injuries can result in blood poisoning and bacterial infection, potentially requiring amputation of fingers or limbs.
Therefore, hands must not be used to check for leaks, even if gloved. If such an injury does occur, even though it may appear to be quite minor at the time, medical attention must be obtained immediately.
Component failure
Fortunately, it is very rare for hydraulic components to fail in a dangerous manner.
As fluids are only very slightly compressible, an external failure of a component usually results in a rapid loss of pressure. Although this may result in a significant fluid spillage (and the hazards that this may result in), the sudden loss of pressure normally means that further mechanical damage is limited.
By contrast, compressed gases will have to expand significantly before their pressure reduces, and so are much more likely to create a damaging, explosive-type failure.
Normally, compressed gases are only present in hydraulic systems in accumulators, and these therefore require special safety precautions, as will be described. However, trapped air in a system, caused by ineffective bleeding, can also create a potential hazard.
One type of component failure that may require special attention is the separation of a flexible hose from its end fitting. In this situation the hose tends to flail around, which in itself can cause injury, and hot, flammable hydraulic fluid may be sprayed out in the process. Where such a potential danger exists, hose restraints or covers must be fitted to reduce the potential risk.
customer’s premises, additional safety requirements may have to be determined and complied with.
All maintenance personnel should have a good understanding of both the particular machine and its hydraulic system operation, and be provided with the necessary circuit diagrams, manuals, procedures, etc. All recommendations and instructions provided by the machine or system manufacturer must be followed. Also, the safety data sheet for the fluid being used should be consulted if necessary (e.g. if the fluid is an unfamiliar one).
Routine maintenance tasks (such as filter element changes, accumulator pre-charge checking and hose replacement) should have detailed written work procedures.
However, no matter how exhaustive the documentation is on a piece of machinery, there will always be times when a maintenance technician has to work on their own initiative. Even in undocumented or unforeseen situations the rigorous adoption of safe working practices means that accidents can be avoided. Such working practices include the following:
• Procedures and equipment. Have a clear understanding of what work or diagnostic procedures need to be carried out and what instrumentation, tools, etc., will be required. Having the right tools for the job is something common to all maintenance activities (Fig. 7.4). If more than one maintenance technician is involved, one individual should manage the process and be designated to coordinate or lead all activities.
• Personal protective equipment. Relevant personal protection equipment must be worn at all times. This may include safety shoes, hard hats, safety glasses, gloves, protective skin creams, ear defenders and high-visibility clothing (Fig. 7.5). If working in unfamiliar surroundings, the location of first-aid facilities should also be established.
• Put the machine in a safe condition. The machine must be put into a safe condition for maintenance work to be carried out. Wherever possible, loads that are supported by the hydraulic system should be placed in a position
fig. 7.4 Hydraulic test equipment
where they cannot move under the action of outside influences such as gravity, wind or waves. If this is not possible, loads should be mechanically secured or supported to ensure that they cannot move when maintenance work is being carried out.
• Switch off all pumps. The drive to the pump or pumps should be switched off and appropriate methods used to ensure that pumps cannot be restarted inadvertently. This could involve removing ignition keys from vehicles, locking motor starter switches in the off position, removing fuses and using appropriate signs (Fig. 7.6). The lead maintenance technician should be the only person designated to restart pumps after a shutdown.
TOP TIP
Remember that some systems incorporate more than one pump (such as a standby pump), so all pumps need to be isolated before maintenance work is carried out.
REMOVE IGNITION KEY OR DISCONNECT BATTERY
LOCK STARTER OPEN OR REMOVE FUSES
fig. 7.6 Lock out prime movers
fig. 7.5 Ensure personal protective equipment is worn at all times
• Drain all accumulators. If accumulators are incorporated in the system, they should be drained of fluid by opening the drain-down valve and checking with a pressure gauge that the fluid pressure has been released (Fig. 7.7). In some systems, automatic valves are fitted to accumulators to drain the fluid each time the system is shut down. Even if this is the case, the manual drain valve should
also be opened and a check made (by means of a pressure gauge) that the drain-down has occurred. (Note that some pressure gauges are fitted with a push-to-read valve, which requires a button to be pushed in before the gauge is connected to the system.) As an extra precaution the isolation valve can be closed to isolate the accumulator from the system. However, this isolation valve must not be used on its own as a safety precaution (i.e. the accumulator should always be drained of pressurised fluid before carrying out maintenance work).
Unless it is required to remove or work on the accumulator itself, it is not normally necessary to release the gas pressure from the accumulator, as this will be retained within the accumulator vessel itself by means of a rubber bag or piston. It may, however, be necessary to check the gas pressure in the accumulator (pre-charge pressure), and this must be done with the accumulator drained of fluid.
• Release any pressure in the system. Any pressure that may be locked into certain parts of the system can often be released by operating directional valves either electrically or via manual overrides. Checking machine functions using solenoid valve manual overrides must be carried out with extreme caution, however, as this may bypass programmed machine safety sequences.
Again, if pressure gauges or test points are included in the system, check that the pressure has decayed. Pressure bleed points should be incorporated in parts of the system where pressure is likely to be locked in (e.g. between a cylinder and a counterbalance valve). As fluids are only slightly compressible, only a relatively small release of fluid is normally necessary to relieve the pressure. If the pressure does not drop rapidly, this implies that something is creating pressure other than just the compressibility of the fluid. This could be an unsupported load, spring-loaded actuators or compressed gas in the system (from an accumulator or trapped air) (Fig. 7.8).
AUTOMATIC DRAIN VALVE
MANUAL DRAIN VALVE
THERMAL RELIEF VALVE
ISOLATION VALVE
ACCUMULATOR
PRESSURE GAUGE
fig. 7.7 Manually drain all accumulators
TOP TIP
Accumulators are potentially dangerous components and should be treated with extreme care.
• Shut off the electrical control supply. Isolate the electrical control voltage from the machine (i.e. the supply to electrically or electronically controlled valves, etc.) (Fig. 7.9). This may require the services of a qualified electrician.
TOP TIP
Remember – hydraulic systems often operate at temperatures that could cause injury to unprotected skin.
fig. 7.9 Lock-out electrical control supply
fig. 7.10 Temperature sensing gun
TRAPPED AIR ACTS LIKE AN ACCUMULATOR
fig. 7.8 Trapped air may be pressurised
• Use a temperature-sensing gun. When checking for heat generation or hotspots in a hydraulic system, people are tempted to use their hands as a temperature sensor. Needless to say this is not a good idea, for obvious reasons, and neither does it provide particularly quantifiable data. A far better and safer solution is to use an infrared temperature-sensing gun (Fig. 7.10), which can be pointed at various components or locations to discover where the heat is being generated.
• Use the correct components and adjust the settings appropriately. If a
component is being replaced, ensure that the new component is the correct one in all respects (pressure rating, solenoid voltage, mounting arrangement, thread form, etc.). Wherever possible use genuine replacement parts recommended by the machine manufacturer. Manufacturer’s recommendations for installation must be followed with regard to alignment, torque levels, air bleeding, etc. If an adjustable component is being replaced, if possible set the adjustment to approximately the same setting as the original component (Fig. 7.12). If this is
• Avoid fluid contamination and collect spillages. Carry out the diagnostic and/
or rectification work as required, ensuring that there is minimal opportunity for contamination to enter the system. Any fluid spillage that is likely to occur should be captured in a drip tray or other suitable container (Fig. 7.11).
fig. 7.11 Empty filter bowls into a container
fig. 7.12 Adjust new components to the approximate previous setting
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Changing a filter element is always likely to create some fluid spillage, so provision should be made for this in work instructions.
TOP TIP Be aware that not all adjustments operate in the same manner (i.e. screwing an adjuster in one direction may have opposite effects on different components). If in doubt the component data sheet should be checked.
Avoid mixing hoses and fittings from different manufacturers (as they are often matched together) and ensure that the correct thread forms are used. Some BSP fittings, for example, will screw into a metric ISO 6149 threaded port (and vice versa), but the required connection strength will not be obtained.
Non-conductive hoses must be used for equipment (such as man lifts) that may operate close to high-voltage power lines. Care must be taken that any replacement hoses fitted are to the original specification in this respect.
• Do not assume new fluid is clean. When replacing or topping up the system fluid, always filter the fluid to an appropriate level before it is put into the system reservoir. If the system is not equipped with a means of filling the reservoir via a filter, a portable filter cart should be used (Fig. 7.14). If changing the type of fluid (e.g. from mineral oil to fire resistant), seek advice from fluid and component suppliers.
not possible, set the adjustment to a safe condition for a restart. For example, when replacing a relief valve adjust the setting to a low pressure, whereas when replacing a counterbalance valve it is safer to adjust the valve setting higher.
• Replacing hose assemblies. Pay particular attention to the replacement of flexible hose assemblies. Hoses are one of the commonest components of a hydraulic system that require routine replacement, while at the same time offering one of the most likely opportunities for mistakes to be made. Hose assemblies must be chosen, manufactured and installed correctly, without twisting and undue bending, and be clear of anything likely to cause mechanical damage. Figure 7.13 illustrates some of the common errors made in the
installation of flexible hoses.
TOO SHORT TOO LONG
BEND TOO TIGHT
TWISTED RUBBING
TOO TIGHT WHEN FLEXED
fig. 7.13 Correct and incorrect hose installations
• Maintenance of electrical or electronic components. If maintenance is required on electrical or electronic components within the hydraulic system, only people certified to carry out such work must be allowed to do so.
• Do not work on a hydraulic system while it is running, unless absolutely necessary. In general, no work should be carried out on the hydraulic system while it is running, unless this has been allowed for in the design of the system.
For example, it could be acceptable to change a filter element in a duplex-type filter (Fig. 7.15) fitted with a changeover valve designed for this purpose, but
fig. 7.15 Duplex filter (Image courtesy of MP Filtri Ltd) fig. 7.14 Fluid-transfer cart (Image courtesy of MP Filtri Ltd)
a leaking fitting should not be tightened with the system running and under pressure.
However, there will inevitably be occasions when maintenance work has to be carried out with the machine in operation, or at least with the hydraulic system running. This could involve setting adjustable components, taking fluid samples, checking and recording pressure levels, etc. In such situations the risk levels are greater, and additional safeguards have to be taken to protect personnel from moving parts of the machine, etc.
As mentioned, checking machine functions using solenoid valve manual overrides must only be carried out with extreme caution when the hydraulic system is operational.
• Restarting the machine. Once the maintenance work has been completed, valves that were opened should be closed and those that were closed should be opened, interlocks should be removed, and everyone in the vicinity should be warned by appropriate means that the machine is about to restart.
• Leave the work site in a clean and tidy state. Once final adjustments have been made, if necessary, and the machine performance confirmed, all tools and instruments should be cleared away, any fluid spillages should be cleaned up, and the machine left in a clean and tidy state. Contaminated materials such as used filter elements, cleaning cloths, etc., should be disposed of appropriately and contaminated clothing removed.
• Record the work done in the machine log and reassess risk and safety. Any work carried out on the machine should be recorded in a log for the benefit of other personnel at a later date. Wherever possible, the root cause of
unexpected failures should be established and the risk analysis for the machine modified if necessary. Note that any significant modifications that are made to a machine or its hydraulic system may require it to be reassessed from the point of view of conformity to relevant safety legislation.
To a large extent, sudden catastrophic failures of hydraulic systems that may create potentially dangerous situations can be significantly reduced by establishing a routine preventive maintenance procedure, as described in Chapter 8. Anticipating failures before they occur avoids the stress of breakdown situations, when safety precautions are easily overlooked.
Finally, it is worth repeating that the number one requirement to ensure the safety of hydraulic systems is to ensure that all personnel involved are well trained, well informed and competent to carry out the tasks required of them.
TOP TIP
Wherever possible think about the causes of the failure and any consequences of it before restarting the machine.
FURTHER READING ISO 4413. Hydraulic fluid power. General rules and safety requirements for systems and their components.
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