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 Valve
Image Courtesy: Mechatronics Control
Let 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.
Principle
An 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.
Purpose
The 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 Valves
The unloading valves are classified into two types.
They are as follows.
- Direct operated pressure Unloading Valve
- Pilot operated pressure Unloading Valve
Direct Operated Unloading Valve
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 Valve
Unloading 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.
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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
Double Pumps with Unloading Valve
The 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.
Advantages
- The pressure of unloading valve is typically less of the relief valve.
- The oil getting hot can be avoided by re-circulation with second pump in circuit.
- Life span of O-rings can be extended by avoiding oil becoming hot.
- Less energy consumption.
Reference: Fluid Power Control Systems by MD Faiyaz Ahmed.
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Figure 11. A shuttle valve is used to select the higher load pressure when two functions are simultaneously shifted, giving the system load pressure priority.
Figure 11 shows the circuitry that can change the load-sense logic. A shuttle valve connects the two sense ports of the two 4-way valves. The unloader spool is now motivated, not by a 4-way&#;s position in the stack but by the higher load pressure. If more than two 4-way spools are in the stack, the highest pressure priority can be maintained by connecting shuttles in binary fashion. That is, each pair of 4-way spools is connected to its own shuttle. Their outputs are paired with one other shuttle, and that output is then paired with one other pair, until they are &#;Christmas treed&#; together until a single output feeds back to the unloader spool. In this manner, two spools require one shuttle, four spools require three shuttle valves, eight spools require seven shuttle valves, and so on.
Simultaneous operations
As is the case with more conventional open-center valves, simultaneous operation of several functions can become complex. System designers are faced with complex mathematical configurations, many of which can be solved only with simulation methods and computers. Operators can have difficulty in training because the behavior of the controls can depend on the cylinder loading and the relative loading between cylinders.
None of these problems is solved when going from conventional open-center designs to the unloader/compensator design. In fact, simultaneous operation of multiple functions can be even more difficult with unloader designs, analytically speaking, and operators can be surprised by sudden changes in load speeds. This is especially true when there is disparity between loads, and the 4-way valves are shifted one after the other.
All of the unloader/compensator designs shown until now provide for predictable output flow or actuator speed when only one function is shifted. To help make the point, here is a brief recap of single-function operation: First, the unloader compensator adjusts in an attempt to keep the load flow or speed more dependent on the amount of shift in the 4-way spool than on the load. Second, pump pressure rises only enough to support that operation, assuming the pump is not undersized. Third, the action of the compensator function eliminates the apparent, load-dependent dead zone of the conventional open-center valves. Fourth, all 4-way stacks are configured for parallel operation, not series operation. To my knowledge, this the only configuration that has been commercially viable.
However, when more than one 4-way spool is shifted, predictability can be problematic. For example, what happens when a heavily loaded spool is shifted, and after its load is set into motion, a second spool is shifted, but it has a very low load? In fact, consider that the second load is so small that it steals the flow from the high-pressure load. Flow stealing calls for a reduction in pressure at the pump. Will it drop until the high pressure load stalls, which would argue in favor of a rising pump pressure? Or will the system reach some new condition of equilibrium where both loads are moving at some &#;compromised&#; speed? Or maybe the system will break into oscillation and chatter as pressure rises and falls. Will the high pressure load drop?
These questions can be answered for specific cases but are difficult to answer in a general sense. The answers are best predicted by having good, detailed mathematical models of the valves that can be analyzed in a simulation program.
Unfortunately, no standard test methods exist to evaluate all the parameters needed for detailed mathematical modeling of dynamic responses. The critical item to evaluate is the dynamic impedance of the sense lines, which have earned total silence on all valve testing standards. Sense lines are always small and often long, meaning that they have modes of operation with laminar flow. But they are also subject to large differential pressures, which means they can at times have turbulent flow.
In the next issue we&#;ll discuss a logical extension of unloader/compensator valves, namely, valves that have a compensator dedicated to each 4-way spool as well as an unloader. Such systems ease the challenges in trying to predict the consequences of simultaneously shifting more than one 4-way spool without the need for detailed math models and computer simulations. But they, too, have limitations.
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