Principle feature of pressure control valve is that , hydraulic forces are restricted by spring forces. These valve maintains the desired pressure level in various parts of circuit.Pressure control valves senses the pressure at certain point in circuit and takes the action.lt maintains the required pressure by -
* Diverting high pressure fluid flow to low pressure area , thereby limiting the pressure based on this principle, it is classified as -
1) Pressure reducing valve (PRV) 2) Pressure relief valve (PRV) 3) pressure Sequence valve (PSV) 4) Pressure unloading valve (PUV) 5) Counter balance valve (CBV)
* Reducing the flow to the particular region of the circuit , based on this principle pressure reducing valve works. * The pressure valves are classified into two types according to the operating feature – ‘I) Normally open type (NO) 2) Normally closed type(NC) * The pressure reducing valve is normaly open type (NO) which partially closes outlet port to maintain constant pressure in regulated line.They are used to limit the pressure in certain portion of circuit to valve lower than rest of the circuit .
These are used for clamping light metal objects or limiting the force on cutting tools and works on the principle of throttling through orifice having variable area. These are having further types – 1) Direct operated pressure reducing valve 2) Pilot operated pressure reducing valve
Terms of pressure reducing valve
1) Cracking pressure – The pressure at which valve opens , called as Cracking pressure
2) full-flow Pressure – As the flow through valve increases , poppet is forced further off its seat , causing compression of spring.When valve is bypassing its full rated flow , the pressure is called full flow pressure.lt is always higher than cracking pressure.
3)Sealing/Closing pressure – The pressure at which valve seases to pass the fluid after being open is called as closing or sealing pressure.
Construction of pressure reducing valve -
The high pressure fluid from positive displacement pump is continuously fed to the first actuator as well as it is diverted to second actuator via port ‘D’. The spool of the valve is held open by spring. On the top side of main spool pressure setting knob is employed to set the required pressure or force inside the spring. a separate drain connection is given at the top to accomplish upward movement of the spool.Port ‘E’ is diverted from the output to the bottom of the spool.
Working od pressure reducing valve
As the pressure at the outlet side of the valve increases , it acts as passage E against lower end of spool. An imbalanced exists because opposite end of spool is open to drain.This cause the spool to move upward by compressing the positioning spring and restricts the flow of fluid at the outlet until the pressure drops at specified level.Therefore the pressure at outlet remains constant regardless of inlet pressure.
To overcome the limitations of direct operated PRV piot operated PRV’s are used-1 (Direct operated pressure reducing valve are suitable for constant flow rate and low pressure application. 2)Relationship between spring force and system pressure is proportional to each other as the system pressure increases the force required to overcome the pressure increases , which will change the spring design.