|While conducting a hydraulic safety workshop for one of the largest equipment rental companies in the US recently, I was told, by a student, that a representative of an aerial lift manufacturer advised him to remove a cartridge valve from the base of a boom lift cylinder to relieve pressure from a boom cylinder after the engine failed.
Incidentally, this problem also applies to mobile and stationary hydraulic cranes.
In this instance, an aerial lift platform apparently tipped over. As it lay on its side, engine oil found its way into the engine cylinders. When righted, an attempt to start the engine proved fruitless because the engine had hydro-locked leaving the maintenance personnel wondering how to lower the boom.
Aerial platforms are equipped with cylinder safety locking valves, which are designed to operate only when the pump is operating.
The maintenance technician discussed the dilemma with his supervisor who prudently advised him to consult the manufacturer.
Upon discussing the matter with the manufacturers technical experts, he was told to carefully screw the safety valve out until it was free of the o-ring seal, which would allow the oil to leak out past the threads thus allowing the boom to lower very slowly.
Here is an illustration of the situation (Figure 1) -
Imagine that the mast/boom on the machine in the illustration is in the raised position and the engine/hydraulic pump is disabled the pump provides the pressure and flow needed to lower the boom/mast safely.
The inset shows a photograph of an actual safety locking valve (Figure 2), which is screwed into the closed-end of the cylinder.
The only way to lower the boom/mast is to remove the oil, which is trapped in the cylinder between the piston seals the safety locking valve. This leaves loosening and/or removing the valve as the only means of dissipating the unpredictable amount of stored energy from the cylinder - or to use the correct description - weight loaded accumulator!
This situation creates a profound safety hazard for a number of reasons:
|The pressure generated by the total weight of the boom/mast and the surface area of the piston(s), can be extremely high enough to easily penetrate the skin or blow out an eye.
|The flow and resultant velocity can be extraordinarily high.
|The cartridge, if inadvertently removed, becomes an aimless projectile, which could easily penetrate safety glasses, or a persons chest.
|The oil, if hot, could strike at any distance and cause life-threatening burns.
|The atomized oil could cause an inferno if it contacts a hot exhaust manifold, an exposed electrical panel, an electric motor, a shop heater (floor-standing or ceiling-mounted), etc.
A similar situation occurred at a mine in Globe, Arizona when a mechanic, following the advice of a factory expert, was screwing a safety valve out of its housing to lower the boom on an engine-disabled basket lift. Due to the fact that the threads restricted the leakage to a few drops per minute, the mechanic decided to, as he put it, give it another turn.
With only one or two threads left in the valve body, the pressure, and resultant force agianst the cartridge valve, sheared the threads, propelling it out unexpectedly. The ensuing jet of oil blew both the mechanic's safety glasses and hard hat off and sent him reeling backwards to the ground. His colleagues scattered in all directions.
Needless to say, the mast came crashing down causing severe mechanical damage. Fortunately, no one was injured, or worse, killed!
Cartridge Valve Manufacturers Offer the Same Advice -
While chatting with an applications engineer for one of the largest cartridge valve manufacturers in the U.S., I asked him what advice he offers personnel who are faced with the same dilemma - back the cartridge out and let the oil drip past the threads, was his rhetorical response he did admit that it was a dangerous procedure!
Pressures and Flow Rates are Usually Unknown -
A hydraulic cylinder, which is supporting a boom or a mast, is for all intents and purposes, a weight-loaded accumulator. The magnitude of the stored energy can be calculated by the weight (load), the area of the cylinder (s), and the cylinder rod stroke.
Here is a hypothetical example:
Lets say that the boom on a given crane weighs 4,000 lbs., and the area of the lift cylinder is 5 square inches. If the formula to calculate the pressure in the cylinder is force (weight) divided by area (piston), then the pressure is:
P = F/A
P = 4000 lbs divided by 5 square inches
P = 800-PSI
If the cylinder piston seal has a slight leak, which allows the pressure to equalize on both sides of the piston, the hazard increases exponentially.
Lets say that the piston rod area is 2 square inches. The weight 4000 lbs divided by the area 2 square inches makes the pressure rise to an astounding 2000-PSI!
Money is the Root of All Evil! -
Ironically, the reason why maintenance personnel are subjected to these, and other, life-threatening hazards, is due to the fact that making a hydraulic system safe costs money, and while OSHA et al, look the other way when it comes to hydraulic safety, there is no reason to make the investment.
Another reason why aerial lift and scissor lift manufacturers get away with the safety issues is because less than 1% of the personnel who work on the hydraulic systems are properly trained untrained people will heed bad advice when it comes from the so-called experts after all, they must know what they are talking about because they designed the machine!
Heed My Advice - DONT DO IT!!! -
Let me make it abundantly clear removing a cartridge valve while it is under pressure could lead to severe injury, death, and/or substantial property damage. You are usually called upon to remove a cartridge valve because an engineer failed to design-in safety for a number of reasons:
|Ignorance simply dont know what solutions are available.
|Assume that the problem will never occur.
|Frequency of problem doesnt justify the expense.
|Never personally subjected to risk.
|Costs too much corporate profits take precedence over safety.
Its your decision. Understand the reasons why the situation you are in has occurred in the first place, and then decide if you are willing to risk your life, and that of your colleagues, to compensate for poor engineering in concert with a blatant disregard for the safety of the people who work on and around this type of machinery.
If you are ever faced with this problem heres the best advice I can offer: call the engineer who designed the machine and ask for his/her advice on lowering the boom/mast.
If you are told to remove a cartridge valve plead ignorance ask for a demonstration! The only way to stop this problem in its tracks is to have the engineer, or the president of the respective company, come face-to-face with the wrath of the hot, pressurized oil.
You will quickly find that they are not prepared to risk their lives that's precisely why you, and not them, are doing it.
I recently received a letter from the wife of a maintenance technician who cared so much about hurting people as a result of his actions that he told the rental company that he would prefer not to work on personnel lifting machines until he was suitably trained (rental companies are notorious for allowing untrained personnel to work on aerial lift equipment).
They hired him knowing full well that they had no intention of training him. When he was finally ordered, by his supervisor to work on a personnel lifting machine, he politely refused citing his discussion during his interview. They apparently fired him on the basis of refusing to work.
I asked OSHA to intervene they were not interested. Remember, hydraulics is not a recognized occupational hazard. While OSHA et al, "talk the talk" about stored hydraulic energy, they dont "walk the walk" you are on your own choose wisely!
Ironically, there is a hydraulic safety product on the market that is designed specifically for the purpose of safely removing stored energy from hydraulic systems it only costs a few dollars! (www.safe-t-bleed.com)
Upon my discussing it with an engineer from one of the largest oil-field service companies in the country he told me to, go and play with my toys somewhere else I rest my case!
In fairness to aerial-lift, crane, basket-lift, and telescopic handler manufacturers who might have developed a safe method of lowering a boom/mast in an engine/pump disabled situation, feel free to submit your comments. Please state if you would like us to show your comments on this advisory. I wrote to a number of them on September 19, 2006 and asked them what they recommend with respect to the subject problem. Here are their comments (I will update the comments as, and when, they become available):
Fluid power safety doesnt just happen, it has to be pursued."
Rory S. McLaren
Fluid Power Safety Institute
The Fluid Power Safety Institute welcomes constructive dialogue regarding our safety bulletins. Your comments are welcome.