|Topic: Worker gets blamed for accident caused by poor hydraulic system design! -
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 it was supporting, to fall on him.
While we agree that the vehicle should have been blocked, we vehemently oppose the fact that the vehicle manufacturer/designer escaped unscathed.
Herein lies the reason why "hydraulics" will continue to remain outside the boundaries of defined occupational hazards. Legitimate "hydraulic" accidents are often times "masked."
What if the circumstances were different? For example, outriggers are invariably used on drilling rigs and cranes. There is no requirement, that we are aware of, that calls for blocking the vehicle once it is in-situ on the outriggers, ready to do its job.
In many cases the rear wheels are off the ground, to stabilize the machine. Outriggers are also used to level a machine that is operating on uneven terrain.
|This worker, or a colleague, could quite easily have been killed "on-the-job! -
Let's imagine that a machine is set-up for a drilling operation early in the morning while the ambient temperature is extremely low. The mast is raised, the outriggers fully extended, and the drill is readied for work.
The lengthy drill-rod is hoisted into place, and the drilling operation commences.
The sun rises, and the ambient temperature steadily climbs. The sun "bakes" on the exposed outrigger cylinders, and the inevitable thermal expansion process begins. Oil is trapped within the confines of the cylinder and the load-holding valves, and has no way to escape.
After drilling for a few minutes at full rotating speed, half of the drill-rod disappears beneath the ground. At that moment, the cylinder unexpectedly fails (bursts) due to the excessive pressure caused by thermal expansion.
The sudden loss of the cylinder on one side of the drill rig causes it to tilt violently to one side. This reaction causes two potentially disastrous situations:
1. The inertia created by the dynamics of the tall mast swinging in reaction to the sudden tilting of the drill-rig, can cause the vehicle to roll over on top of the operator, and/or bystanders.
2. The drill-rod twists and snaps as it rotates, leaving the piece that is still attached to the drill-head whipping around violently, in close proximity of the drill-operator and his/her assistants. Being struck by the heavy drill-rod will almost certainly result in death.
This is one of the numerous "design oversights" that are caused by people who "dabble" with hydraulic system design.
This is not the first time this type of failure has occurred. A similar incident occured when a group of mechanics were given the task of building a make-shift drill-rig. They made the same mistake. Fortunately, the mast collapsed when the mechanics were out of harms way.
Mechanics are often times asked to "design" and build their own hydraulic systems with little or no fromal training, They are often times advised by untrained sales personnel who are quick to recommend dual, pilot-operated check valves for "safety."
|Root-cause analysis -
People who design hydraulic systems should be well aware of the problems associated with thermal expansion, and should thus design accordingly. Thermal expansion MUST be taken into account in the selection of a load-holding valve.
A pilot-operated check valve without thermal expansion capability should never be used if heat is a factor, and the oil is confined. On the other hand, a counterbalance valve, while capable of holding a load, will open at a predetermined pressure and prevent this problem.
In addition, if the designer "specs" a cylinder that has a pressure rating equal to the system's main pressure relief valve setting, pressure intensification can occur if the lines are blocked, and the seals leak, in a double-acting cylinder, rod-up, vertical load, application. Pressure intensification can lead to similar problems.
Needless to say, while this accident could have been prevented if the machine was properly blocked - as it should have been - the accident would, in all probability, have been avoided if the machine was properly designed in the first place. Therefore, in our opinion, the designer(s) of the hydraulic system must accept part ownership for this accident.
Incidentally, the use of quick-disconnects can lead to similar failures because they do not cater to thermal expansion.
They offer one other problem. If the pressure rating of a quick disconnect is equal to the system's main pressure relief valve setting, in addition to the thermal expansion problems, there is the possibility that the live-end connection is inadvertently incomplete.
When pressure is applied to the closed-end of the cylinder, the pressure in the live-end will intensify in proportion to the pressure relief valve setting, and the rod-to-bore ratio.
(A worker was killed when he was struck in the chest by a poppet from a quick disconnect that failed for this very reason!)
Many of these pitfalls can be avoided if our design engineers get hydraulic safety training while at university.
Communication and caring are the keys to eliminating accidents.
|The accidents we report are real, and the victims are real. The safety guidelines we provide are to help companies and individuals work safely with hydraulics. All guidelines we provide are general, and are not intended for one specific hydraulic system or machine.
WHEN YOU PURCHASE HYDRAULIC-POWERED MACHINERY AND/OR EQUIPMENT, WE HIGHLY RECOMMEND THAT ONCE YOU DETERMINE THAT IT CAN DO THE WORK YOU WANT IT TO DO, YOU THEN DETERMINE HOW MUCH SAFETY HAS BEEN BUILT INTO IT BY THE MANUFACTURER. IN ADDITION, WE RECOMMEND THAT YOU, WITH THE INPUT FROM YOUR OPERATORS, MECHANICS, ENGINEERS, PRODUCTION PERSONNEL, AND SAFETY DEPARTMENT, FORMULATE A LIST OF MINIMUM SAFETY STANDARDS FOR THE EQUIPMENT YOU PURCHASE. ALTHOUGH WE LIKE TO BELIEVE THAT MOST MANUFACTURERS ARE CONCEREND ABOUT SAFETY, SOME BELIEVE THAT CERTAIN SAFETY MEASURES ARE AN UNNECESSARY EXPENSE. WE FURTHER RECOMMEND THAT ALL THE SAFETY STANDARDS AND RECOMMENDATIONS THAT YOU DEVELOP SHOULD BE REVIEWED BY YOUR SAFETY DEPARTMENT, ENGINEERING DEPARTMENT, AND BY THE RESPECTIVE MACHINE OR EQUIPMENT MANUFACTURER PRIOR TO MAKING THEM POLICY - TOTAL SAFETY CAN ONLY BE ACHIEVED WITH A TEAM EFFORT.
|The Fluid Power Safety Institute does everything possible to insure that the information and drawings contained in these documents is accurate, and the procedures are deemed safe and reliable. However, these are general recommendations only and might not be applicable to all situations.
You MUST have your engineering department and service department read these recommendations and make the necessary changes for your specific application.
The Fluid Power Safety Institute is not responsible for actions taken by untrained and/or unauthorized persons. ALL hydraulic system service, repair, and troubleshooting should be done by trained, authorized persons ONLY.