A motion-less actuator does not confirm a hydraulic system is energy-less

I received a call from the owner of a fleet of refuse trucks.

He called me because he had concerns about the safety of his mechanics as it relates to the hydraulic systems on his fleet of refuse trucks. 

Here is an overview of our discussion:

The truck’s trash compactors are powered by hydraulic systems. His mechanics have to get inside the compactors to perform cleanup, maintenance and repair work. They also have to perform service and repair work on the vehicle's hydraulic systems. He felt that the safety instructions in the vehicle's service manual about making it safe for workers to work inside the compactor, or work on the hydraulic system, were vague. He apparently decided to call the manufacturer to get clarification on the issue. The manufacturers' representative, with whom he discussed the issue, advised him to have the company’s workers follow the steps provided in the vehicle's service manual, which are:

1. Shut off the engine.

2. Activate the compactor's control valve moving it back-and-forth while observing the compactor. If the compactor does not respond (move) while the control valve is being shifted back-and-forth, it is safe for a worker to enter the compactor.

3. If the compactor does respond (move) when the control valve is being shifted back-and-forth continue operating the control valve until the compactor stops moving. When it no longer moves when the control valve is shifted back and forth, it is safe for a worker to enter the compactor.

He asked the manufacturer's representative if there was a possibility there could be stored energy in the hydraulic system even though the compactor does not move when shifting the control valve back-and-forth. He said he had no reason to believe this could happen. He also mentioned the company had no record of any incidents related to his concerns. He asked if there was any way workers could confirm, beyond the fact the compactor did not move, if there is stored energy in the hydraulic system. He said no, and added that he "didn't think it was necessary."

The owner added that he was concerned that the hydraulic system could possibly have stored energy despite the fact that the engine was shut off, and workers had followed the safety recommendations in the vehicle's service manual. 

One of the statements made by the refuse truck manufacturer’s representative when asked by the truck owner if there is a possibility there could be stored energy in the hydraulic system even though the compactor no longer moved when shifting the control back and forth is particularly disturbing. He answered that he had “no reason to believe this could happen” without offering advice as to making certain it could not move.

     

I told the refuse truck owner that he had good reason to be concerned. I advised him that although a hydraulic actuator (cylinder or motor) does not respond when the control valve is activated, DOES NOT mean a hydraulic system does not contain stored energy, nor does it mean the actuator will not move unexpectedly, for the following reasons:

1. An actuator will not move below a certain pressure threshold because there is not enough force to overcome the load.  

2. There could be debris in slides, rollers, and/or concealed areas, which can prevent the compactor from moving. When a mechanic moves it may lurch forwards/backwards.

3. Compactor out of alignment, which causes it to bind.

4. Control valve is pilot-operated, which means it only functions when the pump is operating.

5. Wiring problem with a solenoid controlled control valve, which prevents it from shifting the valve.

6. Control valve seized: hydraulic system remains energized even though compactor remains stationary when valve is activated.

7. Control cable broken but break not visible.

8. Untrained person shifts the incorrect control valve.

9. Debris causes sliding mechanism to jam. Prevents compactor from moving when valve is activated, and does not release the stored hydraulic energy.

10. Pressure low due to internal wear. Sufficient pressure to cause injury, but not sufficient to move compactor.

11. Cylinder port relief valve malfunction: will not move cylinder in one direction, but cannot move in the opposite direction because rod is fully extended or retracted.

12. The actuator is equipped with load holding valves, i.e. counterbalance or pilot-operated check valves. Will not permit actuator to move if pump is shut off.

Regrettably, the same rules that apply to electrical safety do not apply to hydraulic safety even though, in this instance, the situation is, for all intents and purposes, identical. For example, if an electrician turns a light switch on and the light fails to illuminate, the electrician does not have to rely on guesswork to determine if the circuit is “dead.” A simple check with a voltmeter will confirm the situation. Regrettably, even though the hydraulic industry has the instruments needed to confirm whether or not a hydraulic system stores energy after shutdown, machinery and equipment designers typically don’t design them into hydraulic systems because it adds cost. Needless to say, OSHA ignores the problem.

When it comes to hydraulic systems don’t look to OSHA for help:

Regrettably, even though by law, workers must undergo energy control training (OSHA's lockout/tagout training), which includes information about the safe release of stored hydraulic energy, and protecting workers from the unexpected movement of equipment, OSHA ignores the fact that over 99% of the hydraulic systems operating in the USA do not have the means to either verify if a hydraulic system contains stored energy, or to safely remove/release it if a system does contain stored energy. Moreover, OSHA does not hold machinery and equipment manufacturers accountable for designing hydraulic systems, which are inherently unsafe by design from the point of view of stored hydraulic energy.

Hydraulic systems are particularly hazardous when it comes to stored energy, because, unlike electrical systems, where if a breaker is isolated the entire circuit is de-energized, a hydraulic system, depending on its magnitude and complexity, can store energy in numerous "zones," or "pockets." Therefore to make a hydraulic system safe to work on, a worker has to know where every potential energy "zone" is, and has to be able to safely remove the stored energy from each zone.

Here is my advice regarding this matter:

Get it in writing: If the refuse truck manufacturer, or any machine manufacturer for that matter, is confident that the safety procedures in the vehicle’s service manual are fail-safe, i.e., guarantees, without a shadow of a doubt, that if a worker follows the safety instructions provided in the service manual with respect to working inside the compactor, and performing service, repair, and maintenance work on the vehicle’s hydraulic system, they will have no problem with setting your mind at ease by giving it to you in writing.

The document must be formalized on the company’s letterhead, and signed by the company’s safety manager, and engineer. I further advise you to discuss with the manufacturer the need to install some type of mechanism that prevents the compactor from unexpectedly moving while a worker is performing work on same. Also, I highly recommend you refrain from allowing a worker to perform invasive work on the vehicle’s hydraulic system until such time as the manufacturer has assured you the hydraulic system is completely free of stored energy after following the recommended pre-work safety procedures.

Warning:

This general warning should be posted on ALL hydraulic systems:

WARNING: Performing a lockout/tagout in accordance with OSHA’s lockout and tagout protocols DOES NOT make a hydraulic system safe to work on. If, after isolating the power supply, a hydraulic actuator (cylinder or motor) does not respond when the directional control valve is activated, IT DOES NOT CONFIRM the hydraulic system cannot operate. You MUST verify if there is stored energy in the hydraulic system, and if there is, you must remove it safely. Also, a hydraulic system can store energy in numerous places. You MUST insure ALL “pockets” of stored energy are isolated.

Failure to follow this warning could cause an accident that can lead to severe injury, death, and/or severe property damage.

Have a question about a hydraulic safety matter, I will be happy to give you my opinion?

NOTE: I will never use your name or your company’s name in my response.