What is It?

Unloading valves are very similar to balanced, pilot-operated relief valves. The only difference is where the pilot pressure comes from.

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In unloading valves when external pressure is applied to Port X that is greater than the pilot setting, the pilot poppet will crack open, allowing the main poppet to open. System flow will then unload from Port P to Port T.

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Schematic Symbols

Unloading valve symbols are also drawn in a very similar way as some other types of valves.

The function of many pressure valves on a schematic can be clarified by their location within the hydraulic system.

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new to hydraulics - I have a bank of 4 hydraulic pumps @ nominal 90 gpm each powering various hydraulic equipment (reactor agitators, etc.). The pumps each deliver to an unloading valve and then to system or reservoir. As the fluid is returned (from plant) it goes through a cooling heat exchanger and then back to reservior. Pumps are turned off/on depending on required load. When we only have one pump on and a low flow condition the unloading valve sends flow back to reservior and the cooling heat exchanger is by-passed - this results in a very high temperature increase if not discovered. Question, can I re-route discharge from the unloading valve from the reservior to system return line where it will cath the exchanger? Thanks. Still learning terminology - It is a Vickers CG10F30 remote pressure compensator (pump is Vickers PVB90) - and as you said is really just a really a relief valve to reservior - the temperature rise would then be from the recirculating fluid continuing to move across the hydraulic pump. So I can either 1) install a recirc pump with cooler on reservior, 2) install a valve from supply to return inside the plant (to maintain a min flow to return (to heat exchanger)), or 3) install discharge of relief valve to return. Do these options sound correct? Each pump has its own relief valve - for option no. 3 I would need to tie all 4 relief discharges to an inlet to return line (so flow goes through heat exchanger). Thanks again! Depending on how low the flow, especially when too low, you can generate considerable heat.

You MIGHT be able to reroute, BUT it sounds like the heat exchanger you are thinking about going to is after the process equipment and pretty much at the end of the lines. If this is true, you might have lost considerable pressure before you reach the exchanger. If that is the case, it is possible that the heat exchanger has a design pressure much lower than the pump discharge and of course it would not be acceptable to reroute high pump discharge pressure into a lo pressure exchanger. Please find some engineer and have him CHECK THE RATINGS of the exchanger, its relief valve settings if any, piping and everything inbetween.

Perhaps better suggestion for present is, first, check the minimum flow requirements of the pumps. (API actually suggests min op flows not lower than 60% BEP to avoid long term damage.) It sounds like you are operating at very low flow rates. Probably much lower than intended. If not, maybe you are operating at low flows for much longer time than intended in the original design. Review your current operating practices and current flow rates versus the intent of the original design. It just may be a change in pumps or modification of the pump could be the best solution. Doesn't hurt to check that first.

I'm not up on Vickers as we use Rexroth for almost all valve applications. The old Vickers book I have verifies that you are using a standard relief valve( no unloading function to it ) and when no work is being done all the fluid is going back to tank across this relief at whatever it is set for( system pressure). Rexroth and Vickers both make solenoid operated unloading/relief valves that are commonly used for this function. In additon to unloading the pump when no work is being done it helps on motor startup as the pump is unloaded for a few seconds to allow the motor to come up to fully RPM under no hydraulic load. I think you will need to change the subplate mount you have. The Rexroth valve would be a DBW 30 I think. Maytag There is a lot of info missing that might determine the best route for you. The PVB will have 3 lines on it-a 3 1/2 -4 inch suction, a pressure line and a case drain that should be run unrestricted back to tank-this case drain fluid should be hotter than any other fluid in the system-if everything is working correctly. Regardless of the control option there should be little/no flow over the relief valve in a static system pressure. The PVB is a swash plate piston pump- when system demand calls for flow the swash plate swivels (the pump comes on stroke). When demand is satisfied the pump destrokes-without a relief valve there would be pressure spikes considerably higher than system pressure-this should be the only time fluid should pass over the relief in this orientation.
There is no cardinal rule that PVB style pumps couldn't be unloaded but it is not necessary-if there is little fluid moved back to tank I would be concerned about the filtration and might put in a recirc unit(kidney loop) with both a filter and heat exchanger. Maytag I guess it depends on your definiton of high and what pumping medium you are using. All the pumps and valves I'm familiar with are flow rated @ either 40 C or 100 F. I am in a steel mill where we use water glycol and it needs to be kept between 110-120 F to be ideal-this probably would be hotter than some people would want unless they had researched their pump specs.
Pressure compensated controled pumps are common on large central systems but they are not the most energy efficient. But it only uses very little energy to provide makeup fluid if no work is being done(makeup for case drain and any fluid required thru out the system) but not to include fluid bypassing over a relief valve that is either faulty or set lower than the pump is trying to produce. An amp probe would shed lite on how much work is being done with the system at rest. Maytag