The speed of hydraulic oil passing through a line can reach up to 15 to 30 feet per second. This incredible speed is achieved due to the pressure inside the system. Thus, when you close the valve rapidly, a pressure spike occurs. Hydraulic oil is a non-compressible material. Unlike air, it can only compress by 0.5% when pressurized to 1000 psi. Upon closing the valve rapidly, the pressure spike that occurs can raise the system’s pressure 4 or 5 times above the regular operating pressure.
As the average shock spike duration is 25 milliseconds, the pressure gauge fails to respond fast enough and to provide an accurate indication. You need pressure transducers to record such pressure spikes. Leakage and damage in the lines and components can occur due to these spikes if they are not adequately absorbed or dampened. If oil starts dripping out of the line at the speed of one drop per second, it will result in a loss of 405-gallon oil in a year's time. If the oil costs 600 Rs per gallon, this single leak will cost you 243000 Rs in a year.
A shock suppressor consists of nitrogen that is separated from the oil using a rubber bladder and the nitrogen is pre-charged at half of the maximum system’s pressure. It acts as a hydraulic accumulator but unlike a hydraulic accumulator, it can directly be mounted on the line. The suppressor has to be installed as close as possible to the area where the shock occurs. For instance, if shock arises from the rapid closing of a directional valve then install the suppressor close to the pressure port of the valve. After installing the suppressor when the shock spike occurs, the nitrogen will compress, absorbing the pressure spike. They are also useful in systems that have 90 degrees bends in piping or tubes. The shock suppressor also helps in reducing noise. This is useful in systems that have high-volume pumps or accumulators.
Avoid using 90-degree fittings in pipes and tubes when you are plumbing a system. When oil inside your system takes high velocity 90 degree turns, it gives rise to turbulence and generates shocks. This results in leakages at the fitting elbow. To reduce turbulent flow of the oil through the line, use 45 or 90 degrees sweep ell.
Properly clamping the pipes is essential for reducing the shock generation in the system because pipes can move due to hydraulic shocks. This eventually leads to wearing and leakage of the pipe. Note that conduit clamps, U-bolts and beam clamps are not suitable to withstand the shocks of a hydraulic system.
It is recommended to have a distance of 5-8 feet between clamps, the spacing depends on the size of the line. Clamps should be tightened regularly for eliminating movement which results in pipe leakage. Make sure to install a clamp within 6 inches of termination point.
Properly placed hoses also help in absorbing shock in the system. Install a hose at the pump’s outlet before it enters the main header or manifold. If the cylinder is not mounted vertically, you have to install hoses prior to the cylinder or valve as well. Make sure to have hoses that are long enough to affect the timing of the pressure spike but not so long that they may rub against another hose, beam, catwalk or other structure.
The pressure of a hydraulic motor or cylinder is often arbitrarily adjusted for achieving the maximum speed of the machine by the plant’s knob-turner and the pressure only builds up high enough to move the load. Hence, the maximum pressure limiter in your system needs to be set 200 psi higher than the pressure needed to move the load. Pressure settings higher than that will generate excessive shocks when you start and stop the actuator.
Communicate with all your plant personnel about the importance of proper pressure adjustment. Apart from lowering the risk of leakage and shock, lowering pressure reduces energy electricity consumption and heat generation of the system.
These were a few ideas that will help you in reducing or even eliminating shock and leakages in your Hydraulic systems. In summary – you have to properly adjust the system's pressure by installing shock suppressers and accumulators where needed. Ensure that the system’s piping and hosing is correctly done, and add or adjust pilot chokes as needed.