What is a Pressure-Reducing Valve and How is it Used in ...

When it comes to hydraulics – for certain large machines and equipment – there are branches of a fluid power circuit that need to operate at a lower pressure. A pressure-reducing valve assists with that important job, which can prevent machine damage and a hefty bill.

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A pressure-reducing valve maintains lower pressures in branches of hydraulic systems. The system relief valve or other pressure-setting device determines upstream main system pressure. A pressure-reducing valve is normally an open, 2-way valve that allows system pressure fluid to flow through it until a set pressure is reached downstream. It then shifts to throttle flow into the branch.

The forces exerted by pressure downstream actuates a pressure-reducing valve. A pressure drop across the valve’s spring-based main spool is created by the forces establishing the working pressure needed. The desired pressure setting is maintained by the position of it’s main spool adjusting continuously.

Basically, the pressure-reducing valve controls the downstream pressure or outlet. Here is a more in-depth description of 3 pressure-reducing valve’s functions in hydraulic settings:

1. Self-Relieving Type Air Line Regulator

For a pneumatic circuit, the air line regulators reduce pressure. Energy can be saved by reducing pressure whenever possible, because air in the supply line to a machine is at maximum pressure. For example, about half of the input energy would be lost without a properly adjusted regulator with a machine requirement of 60 psi and a compressor setting of 120 psi. The air-driven machine consumes more compressor horsepower than what is necessary when it works at the higher pressure.

For the retraction strokes of air cylinders, air line regulators are also very useful. A cylinder’s retraction stroke consumes less compressor horsepower when it saves air from reduced pressure.

2. Pressure-Reducing Valve with Bypass Check Valve

A standard reducing valve is normally open. When downstream pressure goes higher than its setting, the valve closes, which blocks the flow. Pressure downstream may try to increase. When there’s resistance from an opposing cylinder, for instance, a reducing valve also blocks the reverse flow. Something could burst or get mechanically damaged if the accelerated pressure in the downstream line would continue.

In multiple actuator circuits, it is often impossible to size all actuators to operate at maximum system pressure. Here’s a good example of this: a cylinder needs a certain number of pounds of force, but, even at maximum pressure, a standard bore does not produce enough. In this case, the designer must go to the next largest standard bore. However, the next largest bore produces too much force. Doing this can cause machine or part damage. Instead, install a pressure-reducing valve in the branch circuit with the over-sized cylinder to lower that branch’s pressure to generate the required cylinder force.

3. Pressure-Reducing-Relieving Valve with Bypass Check Valve

Maximum outlet pressure is set by a reducing-relieving valve. When the outlet pressure tries to go higher (due to high temperatures or outside sources), it relieves fluid to the tank. A reducing-relieving valve has a full-flow line to tank with an integral relief valve. Trapped fluid relieves to the tank when pressure in the downstream circuit rises 3-5 percent above reduced pressure. Adjusting the reduced pressure automatically sets the maximum relief pressure.

Note: There is always a drain line open to the tank to control oil flow with hydraulic pressure-reducing valves. When the pressure-reducing valve’s outlet is lower than its inlet, drain oil flows. This creates a small amount of heat in the system. Blocking the drain line lets the outlet pressure rise to system pressure by forcing the valve wide open.

How Does a Pressure Reducing Valve Work?

A Pressure Reducing Valve holds a consistent set point downstream of the valve.

For reducing pressure in high pressure application, you can use a High Pressure Control Valve package set up for pressure reducing application.

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In this video, we’re going to look at the production flow through this Pressure Reducing High Pressure Control Valve Package configured with a Diaphragm-Controlled High Pressure Pilot.

How the Diaphragm-Controlled 30 HPG High Pressure Pilot Works

• Beginning with the control valve in a closed position and the adjusting screw not yet calibrated to a desired set point, the upstream pressure begins to flow—in this example, up to a pressure of 400 PSI.
• No supply pressure is available yet to move the valve open.
• Upstream pressure flows into the Drip Pot, which knocks out liquid from the supply gas which could be detrimental to other instruments.
   
• The supply gas regulator reduces the upstream pressure to 30 PSI;to provide supply pressure to the pilot after passing through a filter.
  
  

   

• This supply pressure moves into the pilot, where the flow is met and blocked by the pilot plug.
• When the adjustment screw is unthreaded, there is no downward pressure on the pilot diaphragm assembly. Once the adjusting screw is threaded farther into the bonnet, the spring puts pressure on the assembly, which positions the pilot plug to open the flow of supply pressure into the control valve actuator.
  
  

   

• This supply pressure pushes up on the control valve diaphragm working against the tension of the spring. This opens the valve and allows upstream pressure to move downstream.
  
  

   

• Next, downstream gas moves through the sense line protector. This device blocks pressure to the pilot when it exceeds the working pressure of that pilot.
  
  

   

• The downstream pressure in the pilot tells the valve what to do next. With the set point at 100 PSI, this pilot will keep the valve open until 100 PSI is reached.
• As the pressure approaches the set point, there is enough force pushing up the pilot diaphragm assembly to move it up and reposition the pilot plug.
• Now, supply pressure is blocked from going into the control valve actuator, and instead the diaphragm pressure is rerouted out through the pilot and vented.
  
  

   

• This allows the control valve to travel to its fail position, which in this case is closed.
• Once pressure has vented, and there is no longer enough force to overcome the pilot spring, the pilot diaphragm assembly will move back down.
• This will seat the top of the pilot plug and unseat the bottom, allowing supply pressure back into the control valve actuator.
• If flow conditions are consistent, the valve will throttle open and closed by only a small percentage to maintain the set point.

In this example, the downstream pressure will hover around 99 PSI, where the valve is barely open, to 100 PSI, when the valve is closed.

This can also be seen on the travel indicator as it only moves between the lower two marks.

As this cycle repeats, liquids continue to fall out of the supply gas into the drip pot where it will need to be drained regularly, depending on the conditions.

How the Bellows-Controlled 150 PG High Pressure Pilot Works

In this video, we’re going to look at the production flow through this pressure reducing control valve package, configured with a Bellows Controlled High Pressure Pilot.

• Beginning with the control valve in a closed position and the adjusting screw not yet calibrated to a desired set point, the upstream pressure begins to flow – in this example, up to a pressure of 400 PSI. 

   

• No supply pressure is available yet to move the valve open at this time.
• Upstream pressure flows into the Drip Pot which knocks out liquid from the supply gas which could be detrimental to other instruments.

   

• The supply gas regulator reduces the upstream pressure to 30 PSI to provide supply pressure to the pilot.

   

• This supply pressure moves into the pilot where the flow is blocked by the pilot plug.
• When the adjustment screw is unthreaded, there is no downward pressure on the pilot diaphragm assembly. Once the adjusting screw is threaded, the spring puts pressure on the assembly which positions the pilot plug to open the flow of supply pressure into the control valve actuator.
  
  

   

• This supply pressure pushes up on the control valve diaphragm working against the tension of the spring.
  
  

   

• This opens the valve and allows upstream pressure to move downstream.
• Next, downstream gas moves through the sense line protector. This device protects the pilot from over pressurization. It blocks the downstream pressure when it exceeds the adjustable limit and reopens when inlet pressure drops below the limit.
  
   

   

• The downstream pressure in the pilot tells the valve what to do next. With the set point at 150 PSI, this pilot will keep the valve open until 150 PSI is reached.
• As the sense pressure approaches the set point, it causes the bellows to contract, actuating the bellows stem to move it upward against the diaphragm assembly.
  
  

   

• This compresses the pilot spring and closes off the flow of supply pressure to the control valve actuator.
• This also unseats the top of the pilot plug which allows the pressure to vent.
• This allows the control valve to travel to its fail position, which in this case is closed.
• Once pressure has vented, and there is no longer enough pressure to overcome the pilot spring, the pilot diaphragm assembly will move back down.
  
   

   

• This will seat the top of the pilot plug and unseat the bottom, allowing supply pressure back into the control valve actuator.

The rapid but stable repositioning of the pilot plug produces a throttling action, and the valve will open and closed by only a small percentage to maintain the set point.

As this cycle repeats, liquids continue to fall out of the supply gas into the drip pot where it will need to be drained regularly, depending on the conditions.

To speak with an expert about how a Pressure Reducing High Pressure Control Valve works, contact your local Kimray store or authorized distributor.

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