Hydraulics 2 - Pressure control

Pressure controls perform the following functions:
1.Limiting the pressure in the system by controls
2.Compensating lost pressure in system by a balance mechanism of infinite positioning.
3.Maintenance of required pressure differential in the system.

This control is achieved by valves , which are as follows:
1.Relief valves
Direct acting relief valves : open at a set pressure (cracking pressure ) to relieve pressure in excess of set pressure - The volume is limited to the set pressure- once the set pressure is reached , this valve will close and wait for the system to build up pressure. this is a passive element in the system , which allows partial flow  or full flow as waste energy  . This wasted energy is called pressure override . In case where a complete over ride is required , pilot operated valves can be used to divert / relieve flow.

Pilot operated relief valve:This consists of two parts - pilot valve on the top - which is a pressure limiting valve - operating the main valve , to allow a full flow - by means of a hydro dynamic piston (balanced piston)

 

These valves can be remote operated by introducing an additional four way/ two way valve connected to venting port.

 These valves can be direct acting or controlled by multiple valves with different presets as shown actuated either by hydraulic or by other means such as pneumatic / electrical / electronic.

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Pilot operated sequence valves:

These differ from pilot operated relief valves - drain being external , rather than through balanced piston.
In operation , primary is connected to piston bottom and secondary is connected to bottom port and sequencing occurs at a slightly higher pressure than required, thus maintaining infinite positioning.

Unloading relief valve :
- Maximum pressure limiter
- Unload the system - pressure circuit , when maximum pressure is reached.
Relief valve iston is held in balance with a light spring by pilot set valve and check valve prevents reverse flow.

Operation:When preset pressure is reached , the valve is lifted up releasing / limiting pressure above the piston. Any rise in pressure will enable the valve to lift completely and unload the pressure side, with check valve allowing accumulator to maintain system pressure.When the pressure falls to approximately 85 %  of the set pressure , the valve seats back.

Pressure reducing valves:
1.Direct acting pressure reducing valves

A bleed passage is maintained to ensure no pressure surges in the system and smooth seating

2.Pilot operating pressure reducing valves:

Orifice passage through the valve operates the spool of pilot when pressure is high and also acts as a damping mechanism for the valve. When pressure drops , the pilot closes and valve seats but continues bleeding through the orifice towards low pressure side. The pressure transition is thus smooth.

 

 3.Back pressure valve

To operate  back pressure reducing valve as an unloading valve (this can be used to control pressure in system ), The bottom assembly is assembled in such a way that the remote pilot pressure  operates the balance piston, as shown below.

Sequence valve  is used to maintain constant primary pressure with secondary sequence action to occur as shown below

 

Sequence valve with internal check allows reverse flow as shown below

Counter balance valve:
As shown below when cylinder is raised , the flow is free through check valve. when lowering the flow is restricted by the spool , thus prevents free running. In case of Hoisting/ lowering systems , this prevents free running of wire during lowering due to weight added by the wire.

Brake valve: 
The brake valve prevents over speeding  when a heavy load is applied  during lowering as well as excess pressure build up.