Pressure Control Valves
Need of pressure control valves :
1. To relief the excessive pressure if oil pressure is greater than working pressure of the system :
When working pressure is exceeded by the pump oil pressure, it may cause hazards and damage of pipe lines and other components hence as a safety device, pressure relief valves are provided.
2. Regulating and reducing pressure in certain portions of the circuit :
When actuator 1 requires pressure of 30 bar while actuator is operating at pressure of 15 bar. In this case,the pressure of the oil is to be reduced from 30 bar to 15 bar. This function is performed by pressure reducing valve.
3. Unloading system pressure :
When it is required to unload the pressure to save the system from undesired heat energy, this function is performed by unloading valve.
4. To achieve sequential operation of actuators in circuit with pressure control:
When the numbers of actuators are to be operated one after another in a sequence, with the help of pressure control, sequence valve of pressure control is used.
In this example, the object is moved from X to Y by actuator 1 and after complete movement of actuator 1, the oil will pass through sequence valve to the actuator 2 and moves the object from Y to Z.
Types of Pressure Control Valves :
● Pressure control valves are classified according to their function, type of connection, size and pressure operating range.
● Commonly used pressure control valves are :
1. Pressure relief valve - To relieve excessive pressure in the system
2. Pressure reducing valve - To reduce pressure of oil in certain portion of circuit
3. Unloading valve - Unload the pressure in the system
4. Sequence valve - To actuate actuators in a sequence
Types of Pressure Control Valves
1. Pressure Relief Valve
i) Direct operated pressure relief valve
ii) Pilot operated pressure relief valve
2. Pressure Reducing Valve
3. Unloading Valve
4. Sequence Valve
5. Counterbalance Valve
(a) Pressure Relief Valve:
Definition : When pressure control valve is used to limit the pressure in a system to a prescribed maximum value by draining some or all the pump output to the reservoir tank when designed set pressure is reached, it is known as pressure relief valve.
Principle :
It works on the principle of lifting the valve element with oil pressure against the spring (designed set) pressure. When set pressure is greater than oil pressure, the valve is closed and opens when oil pressure exceeds the set pressure.
Function of pressure relief valve:
● When working pressure is exceeded by the pump oil pressure, it may cause hazards and damage pipe line and other components hence as a safety device pressure relief valves are provided.
● It will relief pressure which is more than working pressure to the reservoir by lifting valve element.
● It will divert flow to the reservoir till the pressure is equal to the working pressure.
Types of pressure relief valve :
i) Direct operated pressure relief valve
ii) Pilot operated pressure relief valve
i) Direct operated pressure relief valve :
Construction:
It consists of following main parts:
(a) Poppet: It is the main valve component of conical shape seated in the valve body. Its upper end is attached to the spring which exerts compressive force on it.
(b) Spring : It is provided to set the designed pressure of the hydraulic system with the help of setting knob.
(c) Pressure setting knob : It is a screw which can be adjusted to set the spring force as per requirement.
(d) Valve body : It is provided with the inlet and drain ports and accommodate all above components.
Working :
● It is normally closed valve connected between pressure line and the oil reservoir.
● When inlet oil pressure is less than the spring force, it means that it is insufficient to overcome the spring force, the valves remains closed. The pressure of oil is safe for the system.
● When the oil pressure is greater than spring force, it pushes the poppet against the spring force and unseated the poppet. Now the valve opens and oil flow from inlet port to the reservoir. The valve will remain open until the excessive pressure is diverted to the tank.
● Cracking pressure : The pressure at which the valve first opens is called the cracking pressure.
● It is essential for every hydraulic system to provide pressure relief valve as a safeguard against the pressure override.
Limitations of direct operated pressure relief valve
1. They are suitable where the flow rate and the system are reasonably smaller.
2. There is no much variation in system pressure of flow rate.
3. It is not feasible to use bigger spring due to more space requirement for higher system pressure.
To overcome above problems, pilot operated pressure relief valve are designed and used.
II) Pilot operated pressure relief valve (Compound relief valve) :
Definition: When the valve element is actuated indirectly through the internal pilot line to relief the excessive pressure it is known as pilot operated pressure relief valve.
Principle :
It works on the principle of throttling by orifice produced in the main spool which operates the valve element using spring and throttle oil pressure.
Construction :
It consists of following main parts:
(a) Main spool : The main spool is seated with the help of spring force and it is provided with orifice.
(b) Poppet and spring : It is provided as a second stage pressure controlling element which is connected with the help of pilot line. The spring is used to seat the poppet.
(c) Adjusting screw : It is provided to vary the pressure setting.
(d) Port P and R: They are provided for inlet and diverting flow to the tank.
Working :
● Oil will enter through main port P and passed through orifice in the main valve spool.
● When it flows through the orifice, there is a pressure drop due to throttling effect. If the pressure of the oil is less than the set pressure the valve will remain in closed position
● However, if there is increase in pressure due to some reasons, the throttle value of a pressure in the chamber above main spool will also change to a higher value and thus act against the poppet through pilot line.
● As oil pressure is higher it will lift the poppet and creates pressure difference between throttled oil pressure and supply oil pressure. The main spool automatically moves up to open port P to port R, so that oil will be drain out to the tank.
(b) Pressure Reducing Valve:
Definition : When a pressure control valve is provided to maintain a constant pressure to a part of system that is lower than the pressure in the rest of the system, it is known as pressure reducing valve.
Principle:
It is a normally open type valve, which utilizes the spool movement to reduce the pressure by partly blocking outlet port.
Construction :
It consists of:
(a) Sliding spool : It is a main valve element which mounted with the help of spring.
(b) Pilot line : Pilot line is provided to control the movement of sliding spool.
(c) Inlet and outlet ports are provided in a valve body with a drain for leakage of oil.
(d) Setting knob : It is provided for setting of valve as per requirement.
Working :
1. Normally open position :
● When the main supply pressure at the inlet is below the valve setting, the fluid will flow freely from the inlet to the outlet. The valve will remain open.
2. Actuating position :
● When inlet pressure is greater, the oil will flow from the outlet passage through internal pilot line and acts on the sliding spool to oppose the spring force.
● When outlet pressure rises to the valve setting, the spool moves to partly block the outlet port. Now only enough flow is passed to the outlet to maintain preset pressure and reduced pressurized oil is supplied to the system.
(c) Unloading Valve:
Definition : As name indicates, when pressure control he is a lead the oil energy at a low pressure to called unloading valve.
Principle:
It is spool operated valve which is actuated through external pilot line and unloads the pressurised oil from pump
Construction and working:
● It consists of sliding spool mounted with the help of spring. The spring force is set with the help of setting screw.
● The external pilot line is provided through which spool is pushed up, against the spring force.
● The pump port P and the unloading port R is provided.
● When it is required to unload the pressure from the pump the oil is allowed to enter through pilot line and lifts the spool in the upward direction.
● Now the port P and Port R get connected and oil will go to the reservoir to unload the pump.
● This unloading valve is generally located before the pressure relief valve.
● The advantages of unloading valve are it helps to reduce heat energy to a great extent and used in systems where two pumps provide a large volume of oil at low pressure and one of them must be unloaded during a specific period.
(d) Sequence Valve:
Definition : When a pressure control valve is placed in a such manner that it will actuate the operations in a sequence one after another, it is called as sequence valve.
Principle :
● It is spool operated valve in which internal pilot line is used to build up pressure to actuate the sub circuit in a sequence.
● Examples where sequence valve can be used:
1. Movement of material from one location to another.
2. Machining and clamping of workpiece.
In this example, there are two operations (1) Clamping of job (2) Drilling. The sequence valve is used to actuate sequence of clamping and drilling. First clamping is done by hydraulic cylinder and it holds w/p firmly, then build up pressure will open the sequence valve and rotates the hydraulic rotor for drilling.
Construction and working:
● The sequence valve consists of spring loaded spool, setting screw knob, valve body with inlet port, port for sub circuit and drain port.
● The internal pilot line is provided to operate the spool. Drain is provided for any internal leakage of oil.
● This valve is mainly placed in the oil path of two actuators where sequence is desired. When the first actuator completes it operation, the oil will flow towards the inlet of the sequence valve.
● It will enter in the pilot line and exerts a pressure from bottom side of the spool. The spool moves upw when sufficient pressure is build up and connects inte port to the sub circuit port to supply oil to the manor actuator.
● Sequence valve are commonly used in pressure dependent sequencing circuits.
(e) Counterbalance Valve:
Definition: When control valves are used in hydraulic systems working with overriding (running-away) or suspended load to create back pressure at the return line o the actuator to prevent losing control over the load, it is known as counterbalance valve.
● It is providing counter (opposite) force of oil to prevent rapid falling (balance) of heavy load due to gravity or overrunning.
● Counterbalance valve is also a type of normally closed valve which permits the flow of fluid in one direction but will offer resistance in fluid flow in other direction until pressure reaches up to a level at which valve opens.
● In the case of lowering, the vertically loaded hydraulic cylinder with higher speed due to gravity, we could not use flow control valve to reduce flow of oil and we will have to go for counterbalance valve here.
● If hydraulic cylinder falls and stops suddenly at the end of motion then it may damage the load, cylinder and also it may harm the surrounding people.
Principle :
● Counter Balance Valve is basically a relief valve but it is used to set up a back pressure in a circuit to prevent load from falling
● In such applications, the counterbalance valve creates a back pressure to prevent the movement of piston rod of cylinder.
Construction and Working:
● It mainly consists of sliding spool operated with the help of spring, internal pilot line and check valve.
● Counterbalance valves are used with cylinders to safely hold suspended loads and deal with over-running loads.
● The pressure and the weight of the load will cause the valve to open, allowing the load to lower. The load dropping too fast would cause a pressure drop in the external pilot line. The counterbalance valve spool would then partially close, preventing an uncontrolled drop of the load.
● At the present pressure (due to load) acting at port A, the valve remains closed under the spring force. The fluid n the port A is trapped thereby prevents the movement of the load.
● When the pressure in the port A increases beyond certain value, it acts on the spool from downward direction.
● The spool moves against the spring force and provides the passage for the fluid to tank. This allows descending of the load. As this valve gets actuated line pressure, it is known as direct operated counterbalance valve.
Applications :
● They are frequently employed in vertical presses, loaders, lift trucks and other machines that must maintain a particular position or hold a suspended load.
