ENERPAC has advised Australian owners of widely used double-acting hydraulic cylinders to ensure their equipment has relief valves. Enerpac says it has come across instances recently where the lack of a relief valve - or incorrect servicing of cylinders with relief valves - could have resulted in balls, couplers or other components being blown out with the velocity of a bullet, endangering operators.
While Enerpac double acting cylinders are all equipped with relief valves in accordance with ANSI B30.1, this safeguard is lacking in some other types of double-acting cylinders (ie, types which feature powered extension and retraction of the plunger and which require two hoses).
Because of the very high forces that can be built up in the retraction side of the cylinder if oil pressure is not released - about double those applying to the extension side - it is vital that oil must be able to flow freely in and out of both sides of the piston, says Enerpac Australia managing director Roy Coulson.
Mr Coulson says that to prevent problems arising, manufacturers such as Enerpac apply the clause contained in ANSI B30.1, which applies to general purpose portable jacks including hand or power-pumped types.
Section 1-184.108.40.206 of the standard says that each double-acting hydraulic jack shall be fitted with a relief valve on the retract circuit which bleeds the smaller effective area of the jack before any flow restricting components.
Siting of the relief valve can vary with the design of the cylinder. It may be located in the cylinder wall or in the stop ring or top gland. In these cases, the oil will vent to atmosphere and provide a visual indication that there is a problem.
Other cylinders may have the relief valve built into the piston so that the retract side vents to the pressure side, resulting in a balanced pressure. No visual indication is apparent, except that the cylinder pressurises but doesn't advance.
Under no circumstances should the relief valve be tampered with. The operator of any jacking system should thoroughly check all hose connections and valve and cycle the system under no load before applying any forces.
Any prospective purchaser of double acting jacks should check with the manufacturer or supplier that the jacks are designed and manufactured in accordance with ANSI B30.1, says Mr Coulson. Applied Power Australia 02 9743 8988.