One of the key advantages of direct-acting relief valves, in comparison to their pilot-operated cousins, is fast response. And their fast response makes direct-acting reliefs the valve of choice in situations where pressure spikes are possible and must be minimized. Secondary relief valves in an actuator circuit and the ‘spike relief’ for a variable-displacement pump are common examples.
But as a general rule, direct-acting relief valves suffer more from pressure override than their pilot-operated counterparts. Pressure override is the difference between the valve’s full flow pressure and its cracking pressure.
For example: if a relief valve is set so that it cracks open at a pressure of 1200 PSI and pressure rises to 1800 PSI when full flow from the system or sub-system is passing across it, the pressure override is 600 PSI (1800 – 1200 = 600). Which means in this example, the override is 50 percent of the cracking pressure (600/1200), which is not ideal.
If such a valve was employed to limit the output force from a cylinder — at setting of 1200 PSI and with 600 PSI of override, by the time the cylinder stalls out completely, its output force would be 50 percent higher than intended. For this type of application, the ideal situation is a pressure override of zero.
In a relief valve, pressure override is a function of spring compression, flow resistance and, depending on the design of the poppet, loss of area on which pressurized fluid can act as the poppet opens.
In the case of direct-acting relief valves, the influence of these issues can be minimized up to a point through clever design. For example, Rexroth’s DBD family of direct acting relief valves feature very low pressure override (almost zero) at flow rates up to 75 gallons or 280 liters per minute. But as already mentioned, this is the exception rather than the norm for a direct-acting relief valve.
If an application requires very low pressure override at flow rates above 75 gallons or 280 liters per minute, then a pilot-operated relief valve is the right choice. But as already mentioned, it comes with a slower response time.
Bottom line: failing to take pressure override into account can be a costly mistake in terms of system performance. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.