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.
If you are looking for more details, kindly visit Huade Hydraulic.
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
Double Pumps with Unloading ValveIf you are looking for more details, kindly visit unloading valve in hydraulic system.
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.
AdvantagesReference: Fluid Power Control Systems by MD Faiyaz Ahmed.
If you liked this article, then please subscribe to our YouTube Channel for Instrumentation, Electrical, PLC, and SCADA video tutorials.
You can also follow us on Facebook and Twitter to receive daily updates.
Read Next:
Don't Miss Our Updates
Be the first to get exclusive content straight to your .
We promise not to spam you. You can unsubscribe at any time.
Invalid address
You've successfully subscribed !
Several types of pressure-control valves are found in fluid power circuits. Some keep the whole system from excess pressure while others only protect a portion of the system. Others allow flow to an isolated circuit after reaching a preset pressure. Some bypass fluid at low or no pressure when activated.
This chapter only covers relief valves and unloading valves because they are closely associated with hydraulic pumps. The other pressure-control valves are part of the control circuit and will be dealt with after directional control valves.
All fixed-volume pump circuits require a relief valve to protect the system from excess pressure. Fixed-volume pumps must move fluid when they turn. When a pump is unloading through an open-center circuit or actuators are in motion, fluid movement is not a problem. It is when the actuators stall with the directional valve still shifted that a relief valve is essential.
Pressure compensated pump circuits could run successfully without relief valves because they only move fluid when pressure drops below their compensator setting. (Most designers still use a relief valve in these circuits for reasons explained later.)
In either case, a relief valve is similar to a fuse in an electrical system. When circuit amperage stays below the fuse amperage, all is well. When circuit amperage tries to exceed fuse amperage, the fuse blows and disables the circuit. Both devices protect the system from excess pressure by keeping it below a preset level.
The difference is that when an electrical fuse blows it must be reset or replaced by maintenance personnel before the machine can cycle again. This requirement alerts the electricians to a possible problem and usually causes them to look for the reason before restarting the machine. Without the protection of a fuse, the electrical circuit would finally overheat and start a fire.
In a hydraulic circuit, a relief valve opens and bypasses fluid when pressure exceeds its setting. The valve then closes again when pressure falls. This means a relief valve can bypass fluid anytime . . . or all the time . . . without intervention by maintenance. (It also means the system can run hot even with a heat exchanger installed.)
Many fixed-volume pump circuits depend on this bypassing capability during the cycle, and some even bypass fluid during idle time. A well-designed circuit never bypasses fluid unless there is a malfunction, such as a control switch not closing or an operator overriding the controls. This eliminates most overheating problems and saves energy.
There are two different designs of relief valves in use: direct acting and pilot operated. Both types have advantages and work better in certain applications.
Some terms relating to relief valves and their function are:
' Overshoot: The actual pressure reading when a relief valve first opens to bypass fluid. (It can be up to twice the actual pressure setting.)
' Hysteresis: The difference in pressure between when a relief valve starts letting some flow pass (cracking pressure) and when full flow is passing.
' Stability: The fluctuation of pressure as a relief valve is bypassing at set pressure.
' Reseat pressure: The pressure at which a relief valve closes after it has been bypassing.
' Pressure override: The difference in the pressure reading from the time a relief valve first opens (cracking pressure) until it is passing all pump flow to tank.
Figure 9-1 shows a cutaway view and the symbol for a direct-acting relief valve. The valve has a poppet that is pressed against its seat by an adjustable spring. An adjusting knob can be change the force on the spring to raise or lower maximum pressure. The poppet remains seated while pump flow goes to the circuit and pressure is lower than the relief valve setting. If pressure tries to go above spring setting, the poppet is forced off the seat just enough to pass excess pump flow to tank.
The company is the world’s best Right Angle Check Valve supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.