Pressure and Oil Injection in Hydraulic Systems

Hydraulic system safety: where does the danger come from?

A hydraulic system transmits large forces in small volumes using pressurised oil. This is the power that lowers a press ram, drives the blade of a press brake, closes the mould of an injection moulding machine and raises the bucket of a construction machine. In metal and machinery workshops, the automotive supplier industry and construction sites in Bursa and the surrounding provinces, hydraulic equipment is everywhere, and maintenance teams work with these systems every day.

In hydraulic system safety, the danger does not come only from the running pump. Energy can remain in the system even when the machine has stopped:

• Accumulators store pressurised oil and keep the line under pressure after the pump has stopped.
• Cylinders and loads: A raised ram, a lifted bucket or a suspended platen descends under gravity when a valve is opened.
• Trapped pressure: Oil trapped between closed valves is released suddenly when a connection is undone.
• Heat: After operation the oil and the housing are hot and carry a risk of burns.

So in hydraulic equipment, maintenance safety starts not with "I've switched the machine off" but with "I've released all the energy and verified it".

Oil injection injury: a small hole, major damage

Pressurised oil can escape from a pinhole leak as a fine, invisible jet. This jet pierces the skin and spreads oil into the tissue, between muscle sheaths and tendons. A safety alert from the Northern Ireland OHS authority states that serious cases have been reported at pressures above 100 bar, but that there are observations of injection injuries occurring even at pressures as low as 7 bar. Most industrial hydraulic systems operate at pressures far above this.

What makes an oil injection injury dangerous is that it looks trivial at first: a small point like a needle prick on a finger, a slight ache. Within hours swelling, pain and discolouration begin. Oil spreading into the tissue both causes direct damage and impairs circulation; delay in treatment can result in loss of the limb.

The most common scenario is searching for a leak by hand. When a wet fitting is seen, the employee runs their hand or finger along the line to try to find where the leak is. The correct method is to depressurise the system or, if it is under pressure, to run a piece of cardboard or wood along the line, away from the body. Gloves and workwear do not provide reliable protection against injection.

Hydraulic hose bursts and whipping

A hydraulic hose burst creates two separate hazards: pressurised oil spraying out, and a hose that has come off its fitting whipping around. A burst is usually not sudden; it gives warning signs first. Findings to look for when inspecting hoses are:

Measures against bursts and whipping:

• Protective sleeves or shields are fitted to hoses running close to the operator, preventing sprayed oil from reaching the face.
• Hose whip restraints or clamps are used on high-pressure hose connections.
• Hoses are clamped so that they do not rub against sharp edges, hot surfaces or each other.
• Sudden valve movements that increase pressure surges, and incorrectly set relief valves, are checked during maintenance.

Hydraulic pressure release: before starting maintenance

Annex I item 2.13 of the Regulation on Health and Safety Requirements for the Use of Work Equipment states that maintenance work may only be carried out when the work equipment is switched off, isolated from its energy supply, all its movement stopped and not running, and that the necessary measures are taken where this is not possible. Item 2.14 also requires devices for isolating energy sources to be clearly visible and identifiable. In a hydraulic system, "isolated from its energy supply" means more than switching off the electrical isolator. Hydraulic pressure release is carried out in this order:

1. Stop the machine in the normal way and bring moving parts to a safe position; if possible, lower the ram or arm to its lowest point.
2. Isolate, lock and tag the electrical supply. Make sure nobody can start the pump.
3. Mechanically support loads that could descend under gravity. Use a ram safety block, support stand or safety pin. Do not rely on a hydraulic valve and go under a raised load.
4. Discharge the accumulator. Use the discharge valve in the manufacturer's instructions; see on the accumulator's own gauge that it has discharged.
5. Release residual pressure. With the energy isolated, operate the control valves in both directions, in accordance with the manufacturer's instructions, to release pressure trapped in the lines.
6. Verify zero pressure. Do not undo any connection until you have seen on a gauge or test point that the pressure is zero.
7. Wait for the oil to cool. Hot oil and housings cause burns.

These steps are written machine by machine into the workplace's lockout and tagout procedure. For the general rules, see our article on LOTO lockout and tagout.

Rules for replacing hoses

A hydraulic hose is not a part to be replaced with "fit whichever hose fits". The following rules apply when replacing hoses:

• Correct specification: The new hose must be equal or superior to the old one in working pressure, inside diameter, temperature range and compatibility with the fluid used. The marking on the hose is compared with the machine documentation.
• Correct fitting and crimping: The hose and fitting are selected from a compatible system; crimping is done with a suitable press by someone who knows the job. Old fittings are not fitted to a new hose.
• Correct installation: The hose is not twisted when fitted, not bent below its bend radius, and enough clearance is left for moving parts.
• Cleanliness: The inside of the hose and the connection ports are protected from dirt; dirty oil damages valves and causes new leaks.
• Testing: After installation, the system is brought into service with nobody nearby, starting from low pressure; leak checks are done by eye and with cardboard, not by hand.
• Records: The hose replaced, the date and who did it are entered in the maintenance record. The regulation requires maintenance work on machines with a maintenance log to be entered in the log on the day (Annex I item 2.13.1).

Labelling hoses with dates and keeping a hose list for each machine makes it easier to replace aged hoses during planned maintenance.

Hydraulic maintenance safety: PPE and working arrangements

Hydraulic maintenance safety is not only about pressure. The following are also planned for the maintenance team:

• Eye and face protection: Goggles against oil splashes when undoing connections, and a face shield if work on a pressurised system cannot be avoided. For selection, see our article on safety goggles and face shields.
• Slips: Spilled hydraulic oil makes floors very slippery; absorbent material is kept ready on the maintenance trolley.
• Fire: Oil mist from a leak can ignite if it reaches hot surfaces or welding sparks. Hot work near a hydraulic unit is done under a permit.
• Crushing: Nobody enters the movement area of cylinders and arms without the load being mechanically supported.
• Periodic inspection: Periodic inspection of hydraulic equipment such as presses, press brakes and lifting equipment is carried out within the periods in Annex III. Under Art. 14/A-2 of the regulation, the person who maintains a piece of equipment may not carry out its periodic inspection.

For the general risks of maintenance technicians see our article on maintenance technician risks, and for rules specific to presses our article on press safety. We prepare the risk assessment of hydraulic equipment, machine-specific pressure release instructions and training for the maintenance team as part of our OHS specialist service. For other risks in the sector, see our metal and machinery page.

Frequently Asked Questions

What should be done if hydraulic oil enters the skin?

The employee is taken to A&E without delay and the physician is told that this is a hydraulic oil injection. Even if the wound looks small, surgical assessment is needed; delay can lead to serious tissue loss.

How can a hydraulic leak be searched for safely?

If possible, the system is depressurised. If it has to be searched for under pressure, a piece of cardboard or wood is run along the line away from the body. A leak is never searched for by hand or with a finger.

Does hydraulic pressure remain when the machine is switched off?

Yes. The accumulator, lines between closed valves and cylinders holding raised loads can hold pressure after the pump has stopped. Before maintenance, the accumulator is discharged, loads are mechanically supported and zero pressure is verified with a gauge.

When should a hydraulic hose be replaced?

The hose is replaced if the outer cover is worn and the reinforcement is visible, or if there is blistering, cracking, hardening or a leak at the base of a fitting. The replacement interval recommended by the manufacturer is also written into the maintenance plan.