Pressure reducing valve and pressure relief valve

Most of us are very much familiar with pressure relief valve and pressure reducing valve. Both control pressure in the hydraulic system. Since both are pressure controlling device still both are different from each other. Therefore, we will learn in this article how they differ with each other, where they are using and their construction. Also, we will see some special purpose valve.

Pressure relief valve

Consider a simple hydraulic system without a relief valve for instance.

Fig 1. Hydraulic system without pressure relief valve
Fig 1. Hydraulic system without pressure relief valve

When the pump starts and does the work by lifting the load up to its maximum limit. But when it reaches its limit what will happen? The pump is still running and try harder to push more oil in the system and ultimately pressure will increase tremendously and damage the weaker component. Therefore we need something which will limit the pressure in the system. This component is a pressure relief valve.

Fig. 2 Hydraulic system with pressure relief valve
Fig. 2 Hydraulic system with pressure relief valve

Pressure relief valve: A pressure relief valve is a hydraulic component which provides safety to the hydraulic system from excessive pressure. This is normally closed but when the pressure in the hydraulic system increase beyond the set limit of a pressure relief valve, it opens the path and allow hydraulic oil to flow into the tank. In this way, it releases the excessive pressure from the system and provides protection. As soon as pressure reduces in the system, pressure relief valve comes in the initial position and close the open path again.

Fig. 3 Pressure relief valve symbol
Fig. 3 Pressure relief valve symbol

As shown above, an arrow shows the closed path for hydraulic fluid and there is also spring tension which forces the relief valve to remain closed. But when the pressure in the hydraulic system is enough or more than relief valve spring tension, a pilot line will push the arrow against the spring tension and open the path. In this way, the pressure is relieved from the system and save the components.

Construction of pressure relief valve

Fig. 4 Pressure relief valve
Fig. 4 Pressure relief valve

A pressure relief valve can be designed in much more ways but here, here it is a most common type pressure relief valve. It consists of two passages one is inlet port and the other is outlet port. Inlet port is connected with the pressurise hydraulic system while on the other hand outlet is connected with the tank.

As in above figure 4, you can see there is a valve seat which is normally closing the inlet and seat holder is forced against a spring. At the top, there is a screw which will set the spring tension or in other word set the pressure of the relief valve. Usually, the screw is protected with a cap so, the unauthorised person can not change the pressure setting and also it protects the screw from rusting.

Pressure Reducing Valve

Pressure reduction valves are used as pressure relief valves to limit the hydraulic pressure in a system. However, instead of reducing inlet pressure, they reduce the outlet pressure. It divides a hydraulic system into different sub-systems. For Example, If a hydraulic system there are numbers of operations and each has its own capacity. As per every hydraulic components, there may be a different pressure requirement but with only one pump this cannot be achieved but with the use of pressure reducing valve it can easily achieve.

Pressure reducing valve is normally open valve and it allows the pressurised fluid to flow but when the pressure at the output of the valve is more than the valve pressure setting, a pilot pressure push the arrow down and the path will close. The valve will not allow pressurised fluid to flow in the system until the pressure drops lower than valve pressure setting.

Fig. 5 Pressure reducing valve symbol
Fig. 5 Pressure reducing valve symbol
Fig. 6 Example of Pressure reducing valve
Fig. 6 Example of Pressure reducing valve

In the above example, there are two hydraulic actuators for clamp A and B which holds a workpiece. A pressure relief valve. A pressure reducing valve. And a direction control valve and positive displacement pump. The pressure relief valve setting is 300 bar. So, maximum system pressure is 300 bar. When the direction control valve is in the first position both the clamp cylinders extend but in clamp A the pressure will be maximum as system pressure but in clamp B due to the pressure reducing valve setting, the pressure will be 200 bar. As soon as the pressure increases in clamp B line the pressure reducing valve will close and not allow the pressurised fluid to flow. Hence, the pressure will limit at 200 bar.

For better understanding you may read click here this article.

Construction of pressure reducing valve

Fig. 7 Construction of pressure reducing valve
Fig. 7 Construction of pressure reducing valve

A common pressure reducing valve consists of an inlet port (C), an outlet port (D) and a pilot line (E). There is a spool against the spring tension which opens and close the path for pressurised fluid. As the pressure increase at the outlet port (D) beyond the spring tension (i.e. Pressure setting), pilot line (E) shift the spool against the spring and make the opening narrow. Due to the pressure difference between inlet and outlet this spool continuously maintain an almost constant pressure.

Hope, you like this article and learned the basic of pressure relief valve and pressure reducing valve.

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