Is poor design the real reason why this worker was killed? -
By Rory S. McLaren - Director
According to a Safety Alert issued by the Canadian Petroleum Council, a Saskatchewan worker was killed when the hydraulic outrigger jack-cylinder failed, causing the digger truck unit that it was supporting, to fall on him.
While we agree that the vehicle should have been blocked while it was undergoing service and/or repair, we vehemently oppose the fact that the vehicle manufacturer and designer apparently escaped conviction.
Safety must be the nucleus of hydraulic system design -
It is a basic design rule, albeit unwritten, that all structures, i.e. masts, booms, derricks, etc., that are elevated and supported with hydraulic actuators, should have some type of safety device to prevent unexpected and uncontrolled movement of the structure, in the event of a transmission line failure.
Counterbalance valves and pilot-operated check valves are the most common types of valves used for these types of applications.
This unwritten design rule must surely apply to outriggers, since the failure of a transmission line connected to an outrigger cylinder could result in a major catastrophe.
Outriggers are used to provide critical support for cranes. They are also used to stabilize machines. In many cases, they lift heavy equipment off the ground to level them when they are working on uneven terrain.
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Inconsistent rules of engagement! -
However, the rules of engagement seem to differ between operation, and maintenance and/or repair.
For example, there are a handful of manufacturers that equip their machines with mechanical locks on the outriggers. Accordingly, one can only assume that outrigger safety locks are installed at the discretion of the manufacturers.
Consequently, it is safe to assume that operators are not required to block a machine once it is set on its outriggers - even if it is several feet off the ground!
However, if the machine is undergoing service or repair, the mechanic is required to block a machine for safety!
Ironically, an operator, in the course of his/her work, might have to climb on top of, or crawl beneath a machine.
In another scenario, the outriggers might be supporting a drill-rig. During the drilling operation, the steel drill pipe is rotating at very high speed as it grinds its way through the earth's crust. This while the operator stands just a few feet away.
If any one of the outrigger transmission lines should unexpectedly fail, it could cause the drill steel to bend and snap, hurling steel in all directions. Alternatively, the inertia of the mast, as it swings in unison with the machine, could cause it to fall over. Who knows how many people could be in striking distance!
Let's not forget to address the issue of design oversight! -
Many engineers find themselves "in the thick" of hydraulic system design the minute they step into their first jobs - this without even a day's worth of hydraulic training!
Sure they covered "fluid dynamics" in college, but "pumps" and "pressure relief valves?" These are horses of a different color!
There is no doubt that mistakes will be made - some more serious than others!
Hopefully, these unfortunate individuals will learn before there is an accident that leads to severe injury or death.
It is most unfortunate that engineers are "thrown to the lions," so to speak, when they are given hydraulic design projects. However, that is more a cultural problem than it is an engineering problem.
Thermal Expansion - it could come back to haunt you! -
For the benefit of our "apprentice" design engineers, be very careful when using pilot-operated check valves to check cylinders.
A pilot-operated check valve allows oil to flow in one direction, and seals in the reverse direction. The seal it provides is extremely effective. The only way that it can be opened is with pilot pressure.
If a cylinder is locked in the fully-raised (rod-extended) position, and the oil is trapped between the piston seal and the pilot-operated check valve located in the closed-end of the cylinder, it creates an undesirable situation - if the cylinder is exposed to a high ambient temperature, it will cause the oil within the cylinder to expand.
If one were to apply the "rule-of-thumb" for expansion, the pressure within the cylinder will increase by approximately 55-60 PSI (approximately 4 bar) for each 1ºF increase in ambient temperature.
The pressure inside the cylinder could quite easily exceed the cylinder's terminal pressure value.
Conclusion -
The FPSI™ wants to advise all parties concerned, about the hazards associated with elevated loads. |
Here is a list of things to consider:
Engineers -
| 1. |
Make safety the nucleus of all hydraulic design. |
| 2. |
Design excellence will only be achieved when maintenance and operations personnel are respected and appreciated members of the design team - their input is vital to safe and efficient design! |
| 3. |
If you do not have design experience, attend in-service schools. Also, serve an "apprenticeship" along-side a qualified colleague. |
| 4. |
Ask hydraulic component manufacturers for assistance. They generally have excellent applications engineers who can help you. |
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Maintenance and Operations -
| 1. |
Never work on top of, or underneath, any mechanism that is supported by an unblocked hydraulic actuator - assume that it will fail when you are most vulnerable! |
| 2. |
Take time to block loads. Make sure the blocks are adequate. Make sure they are in the correct place and won't move if what they are supporting comes into contact with them. |
| 3. |
If you are using cables or chains to support loads, make sure they are capable of withstanding the weight of the load. If necessary, get a second opinion from a qualified individual who understands cable and chain strengths. |
| 4. |
If a machine is equipped with a locking device(s) use it/them. They are put there for a reason - to preserve your life and limb! |
| 5. |
Don't haphazardly adjust valves in a hydraulic system. What might resemble a pressure relief valve might be the safety override for a load-holding valve. |
| 6. |
Do not attempt to set valves if you are not properly trained. A valve that is set incorrectly can result in an accident that can cause severe injury or death. |
| 7. |
If a load-holding valve is defective (usually indicated by a drifting cylinder) red-tag the machine immediately. Continued use might result in an accident that could cause severe injury or death. |
| 8. |
Never ride on a machine that is not specifically designed to carry or transport personnel. A small problem can lead to a catastrophic accident! |
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Supervisors -
| 1. |
Provide employees with a written job-safety-breakdown for safely blocking a load. Blocking methods should not be left to the discretion of unskilled personnel. |
| 2. |
Provide suitable training for people so they know how to safely block loads. |
| 3. |
When purchasing equipment, discuss the equipment's safety features with the manufacturer. Get the facts about lockout, safe de-energization and verification, blocking, servicing, troubleshooting, etc. |
| 4. |
Make sure employees have the proper training to work on hydraulic systems. Remember their lives are in your hands! |
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