In certain specific applications, there is a need for completely unload pumps flow to the tank instead of relieving it over a relief valve. This can be done using unloading valves.
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Unloading ValveImage Courtesy: Mechatronics ControlLet us imagine a case of a system, where there are two pumps. Both pumps are required to perform specific work.
After completion of the job, there is a need to maintain the required pressure in the system by operating only one pump and the delivery of the other pump must be sent to the tank at low pressure. This job is done by an unloading valve.
PrincipleAn unloading valve is a pressure control valve that works on the principle of the hydraulic force as opposed to a spring force.
When pressure builds to the point, where hydraulic force is greater than that of the spring force, then the valve spool is shifted.
PurposeThe unloading valves are used for relieving the extra pressure in a system, that is at low pressure and connecting it to the tank when the delivery of the pump is not used.
The unloading valve may be controlled by a special cock or a pilot valve.
Types of Unloading ValvesThe unloading valves are classified into two types.
They are as follows.
A direct-acting unloading valve consists of a spool held in the closed state by a spring.
The spool blocks flow from the inlet to the tank port under normal operating conditions.
High-pressure fluid from the pump exerts a force against the pilot as it enters from the external pilot port.
When the system pressure increases to the force of the spring setting the fluid bypasses the tank.
When the pressure goes above the spring setting, the spool opens fully to dump the surplus fluid into the tank at little or no pressure.
Pilot Operated Unloading ValveUnloading spool is the addition in a pilot-operated unloading valve, it is not found in the pressure relief valve.
Without the unloading spool, this valve would function the same as any pilot-operated relief valve.
Pressure buildup within the pilot section would open a certain amount of fluid flow to the tank. It makes unbalances the poppet, allowing it to open and relieve excess pump flow to the tank.
Unloading spool receives a signal through the remote-pilot port when the pressure in the working circuit goes more than its setting.
Simultaneously, fluid pressure on the spring-loaded ball in the pilot section starts to open it.
Pressure drop on the front side of the unloading spool brings down back force and pilot pressure from the high-pressure circuit forces the spring-loaded ball completely off its seat
For more information, please visit unloading valve in hydraulic system.
Double Pumps with Unloading ValveThe primary use for an unloading valve is associated with a dual pump circuit. A high-pressure, low-flow pump along with a low-pressure, high-flow pump is used double pump operated circuit.
An un-loading valve is used with two pumps, create high discharge flow, and the other one able to create a high line pressure with low oil discharge.
Deliveries of both pump are discharged into the circuit until the pressure approaches the setting of the unloading valve.
At this stage, fluid from the high pressure-low flow pump is passed through CV1 to the cylinder through the directional control valve but not allowed by CV2.
In a particular application, sheet metal punch press in which the hydraulic cylinder must extend rapidly over a length with low-pressure but high-flow requirements. This occurs under no load.
However during the punching operation for short motion, the pressure requirements are very high, but the flow requirements are low as the cylinder travel is small.
It eliminates the requirement of having a very expensive high-pressure, high-flow pump.
As and when the punching operation initiates, the increased pressure opens the unloading valve to unload the low-pressure pump.
The purpose of relief valve is to protect the high-pressure pump from high pressure buildup at the end of cylinder stoke and when the directional control valve (DCV) is in its spring-centered mode.
The check valve protects the low-pressure pump from high pressure due to the high-pressure pump, which occurs during punching operation, at the ends of the cylinder stroke, and when the directional control valve (DCV) is in its spring-centered mode.
AdvantagesReference: Fluid Power Control Systems by MD Faiyaz Ahmed.
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Figure 3. Cutaway diagram of the unloader spool and 4-way valve as a variable orifice, showing how the unloader spool senses the differential pressure across the powered land.
Figure 3 is a simplified diagram and schematic consisting of the unloader spool in cutaway with the 4-way valve characterized only as a simple variable orifice, KVPL. A sense line connects the load to the spring cavity on one side of the unloader spool. Meanwhile, another sense line connects to the opposite side of the unloader spool. Other spools in the stack and other lands in the valve are omitted for simplicity in illustrating the operating principle behind unloading valve operation.
Note that the unloader spool&#;s bias spring, labeled KS, biases the unloader spool in the closed position. That is, the unloader function is normally closed. The unloader spool will not open until the pressure through the pump sense line exceeds the load sense pressure by the amount of bias spring plus flow forces acting on the unloader spool.
Valve designers select the spring so that the differential pressure required for cracking the unloader spool (just starting to open) requires from about 50 psi to as much as 250 psi. The higher pressure provides for better control, but it also results in higher unloading pressure and, thus, lower efficiency of the valve system.
Unloader valve issues
A few issues with unloader-type valves are worth discussing. The first is that the unloader spool processes two pressure signals in a way that attempts to maintain a constant pressure across the powered land, KVPL. In so doing, the unloader spool regulates the flow to the load, at least in this simplified scenario.
This raises the second point: As the load pressure changes, the powered land&#;s pressure drop is maintained, and flow tends to remain at a value determined by the degree of shift of the 4-way spool &#; not by the load or even pump pressure variation. This kind of control is easy for even the novice operator to use because of the nearly independent nature of the flow control. A human operator need not compensate for load pressure changes; the unloader does it automatically, somewhat.
However, this is a somewhat idealized description. Practical aspects bring us to a third point: Although many designers of these valves insist that the pressure drop across the powered land remains constant, indeed, it does not. When tested at a variety of loads and KVPL settings (differing amounts of 4-way spool shift), the differential pressure will vary between the cracking value up to as much as twice that amount. Nonetheless, the differential pressure drop will not, in this simple, single function scenario, vary by any large amount from the design cracking value.
This brings up a fourth point: Because the differential pressure drop is maintained at a relatively low level at all operating conditions, the flow forces acting on the 4-way spool are small, making it easier to shift and maintain than in other designs of previous discussions. This results in less operator fatigue, and when electrically activated with proportional solenoids, it also means that the solenoid current determines the flow, not the load and supply pressure. This contributes to operator friendliness of these valves.
A fifth point is that the flow regulation is imperfect. It can vary depending on supply pressure and load pressure, but not nearly as much as without the unloader function. Also, flow forces act to close the unloader valve and, in essence, make the bias spring seem stiffer than it is. The result is overcompensation; that is, as load pressure approaches the pressure rating of the valve, the load flow decreases.
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