INTRODUCTION
Legislation exists in most industrialised countries to ensure that machinery is designed, manufactured and operated according to recognised safety standards in order to protect machine operators. Even stricter legislation may apply to hydraulically powered machinery designed to operate in the vicinity of the general public, such as road sweepers, refuse vehicles and movable bridges.
However, when troubleshooting or maintenance work has to be carried out on machinery, some of the operational safety features may need to be overridden or bypassed in order to carry out such work. For example, a machine guard may have to be temporarily removed in order to gain access to an actuator for test purposes.
It therefore goes without saying that, for anyone working within hydraulic system maintenance, the number one consideration must be safety. No matter how diligently safety standards have been applied, ultimately the safety of personnel maintaining or troubleshooting hydraulically powered machinery depends strongly on both the competence of the individuals involved and the adoption of safe working practices and procedures.
By their nature, hydraulic systems are used to move heavy loads and generate large forces, and they sometimes use potentially dangerous fluids, so the potential for hazardous situations to arise is significant.
In some applications, hydraulic cylinders and motors can be fitted with mechanical braking devices, which are normally spring applied and hydraulically released.
Typically these are used as ‘parking’ brakes. That is, they prevent movement (due to fluid leakage) when the actuator is stationary. Hydraulic motors operating a winch, for example, are often fitted with a spring-applied brake, and cylinders holding vertical loads can use a rod lock device to prevent creep when the cylinder is parked.
While these devices may provide adequate safety during normal machine operation, a higher level of safety will be required during maintenance work on a machine, particularly where work on the actuator itself may be required. Any failure of the mechanical connection between the actuator and load, for example, could still cause a dangerous situation, as could inadvertent release of the brake due to an incomplete release of pressure in the system. The same also applies to hydraulic components fitted to control loaded actuators. For example, pilot-operated check valves or counterbalance valves must not be relied on to provide adequate safety during maintenance operations on the machine or hydraulic system.
Knowledge of the machine function is therefore vital to determine which parts of the machinery could move in potentially dangerous ways. Practices must then be adopted in order to prevent such movements occurring, and ideally these should be documented in machine maintenance manuals.
Fluid pressure
Pressures up to 350 bar (5000 psi) are not uncommon in modern hydraulic systems, and many operate at higher pressures still. A pressure of 350 bar is approximately possible, the loads must be mechanically secured in such a way that they are unable to move under their own weight.
Similarly for vehicles, before carrying out any work it must be ensured that the vehicle is parked on level ground or chocked to ensure that it cannot move (Fig. 7.2).
fig. 7.2 Ensure vehicles and their components are secured
TOP TIP
When carrying out maintenance work on a hydraulic system do not rely on the system itself to prevent dangerous movement of machine components. Always mechanically secure moving parts of the machine that could create hazards to maintenance or other personnel.
100 times the pressure of a domestic water supply (which as plumbers know can cause a lot of problems if not adequately contained). High pressures acting on even small areas can create very large forces. For example, a pressure of 350 bar acting on a piston as small as 100 mm (4 in) diameter will create a force of almost 28 tonnes (31 US tons) (Fig. 7.3).
100 mm
350 bar
~28 tonnes
fig. 7.3 Hydraulic power creates large forces
This means that maintenance work on a hydraulically powered machine should ideally be carried out with no pressure in the system. When this is not possible (e.g.
when setting valve adjustments), it must be ensured that all personnel are clear of any moving parts of the machine.
Shutting down the drive to system pumps will not necessarily reduce the pressure throughout the system to zero, as pressure can sometimes become ‘locked in’ to parts of the system. Any unsupported load will create pressure in the system, even when the drive is shut down. Also, if accumulators are used in the system, the system pressure may be maintained unless the fluid is drained from the accumulators on shutdown.
A further danger of high fluid pressure is caused by small ‘pin-hole’ leaks (typically in flexible hoses), where the fluid escapes at very high velocity, as described in the next section.
Hydraulic fluids
As mentioned in Chapter 3, certain fluids used in hydraulic systems are potentially dangerous to humans because of their chemical composition. Fire-resistant phosphate esters, for example, can cause skin irritation, and much more serious problems if they are ingested or come into contact with the eyes. Mineral oil fluids obviously carry the risk of fire if they come into contact with hot surfaces, especially when sprayed as a mist from a high-pressure leak. Water-based fluids have the potential to form bacterial growth, which again can be harmful to health, and just about any hydraulic fluid when spilled on the floor can create a slip hazard.
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.