Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/875060
24 | September - October 2017 | www.machinerylubrication.com HY DR AULICS By Al Smile y, GPm HydrAulic conSultinG Hydraulic oil in a pressure line can travel at a speed of 15-30 feet per second, depending on the pres- sure. When a valve is rapidly closed to block flow or a cylinder fully strokes, a pressure spike will occur. Unlike air, hydraulic oil is generally consid- ered to be non-compressible. Oil will only compress 0.5 percent when pressurized to 1,000 psi. When a pressure spike occurs in the system, the pressure can increase four or five times above the normal operating pressure. Since the average duration of a shock spike is 25 milliseconds, the pressure gauge cannot respond fast enough to give an accurate indication. Pressure transducers are normally used to record pressure spikes. Shock spikes that are not properly dampened or absorbed can result in leakage and damage to the lines and components in the system. One drop of oil that drips once per second will result in a loss of 405 gallons in a year's time. At the cost of $9 per gallon, that one leak costs $3,645 in one year. Shock Suppressors A shock suppressor acts very much like a hydraulic accumulator except that it can be mounted directly in the line. The suppressor is pre-charged with dry nitrogen. The rubber bladder separates the nitrogen from the oil. The recommended pre-charge of nitrogen is half the maximum system pressure. The suppressor should be installed as close as possible to where the shock is occurring. For example, if shock is generated by the rapid closing of a directional valve, install the suppressor near the pressure port of the valve. When the valve rapidly closes and the shock spike occurs, the nitrogen will compress and absorb the pressure spike. The suppressor is also useful in systems that contain 90-degree bends in the piping or tubing. An additional benefit of the shock suppressor is that it also reduces noise. This can be useful in systems that utilize high- volume pumps and/or accumulators. System Piping and Hoses When plumbing a system, eliminate the use of 90-degree fittings in the pipe and tubing. When oil enters the fitting at a high velocity and rapidly takes a 90-degree turn, turbulence will exist and shock will be gener ated. This Best Ways to Hydraulic Oil Eliminate Leakage Nitrogen Charging Valve Hydraulic Oil (Red) 3-Baffle Chamber Diffuser Tube A cross-section view of a shock suppressor The use of 90-degree fittings in piping and tubing should be avoided when plumbing a system. Nitrogen Charge (Blue) Bladder (Black Line)