Summary

Generally the pressure level in hydraulic systems will vary so as to provide the required torque or force from an actuator in order to drive an external load from the particular application.  During start and stopping situations and when the load is varying transiently the pressure may exceed the maximum safe value for the system.  In many systems several actuators will be driven by a single pump and, in addition to limiting the supply pressure it may necessary to reduce the pressure level supplied to individual services.  There are many different types of valves on the market and this chapter describes some of these and the operating principles involved.

1          Introduction

 

Major types of valves that are used to control the pressure level in hydraulic systems include:

• Relief valves for limiting the maximum system pressure
• Reducing valves for limiting the pressure in parts of a circuit at a lower level than in the supply system
• Load control valves to control the motion of an actuator or motor under the action of overrunning, or negative, forces.

 

The type that is employed in a given application depends on the particular requirements and system specification.

 

2          Relief valves

 

Relief valves are the most commonly used pressure control valve as they are required in all systems to prevent the generation of excessive pressures.  In many systems they are used in combination with a pump to provide a source of flow at constant pressure.

2.1       Single stage relief valve

Single stage relief valves, as with all valves, can use either a piston, or a spool, that is opened by a pressure force against a preloaded spring as shown in Figure 1 together with the ISO symbol that is used to represent it in a circuit diagram.  On opening, the valve allows some of the supply flow to be passed back to tank thus limiting the maximum pressure in the supply.  The valve needs to be sized such that all of the supply flow can be returned to tank at a supply pressure that does not exceed the maximum desired level.

Single stage relief valves are the simplest and lowest cost valve.  Considering the diagram of a poppet valve in Figure 2, the valve will start to open at its ‘cracking pressure’ when the force on its face due to the inlet pressure is equal to the spring preload.
 

                        Figure 1 Poppet and Piston Type Valves

As the pressure increases above this value the valve opens progressively thus allowing flow to pass through the valve.  This feature is referred to as the ‘pressure over-ride’.

The rate of the spring will determine the relationship between the valve displacement and the inlet pressure.  In order to minimise the free length of the spring requires the spring to have a high stiffness.  However, the higher the stiffness the greater will be the pressure over-ride.  There will be some hysteresis due to friction between the components that will result in the pressure being slightly higher when the valve is being opened than when it is being closed.

Valve force analysis

The valve design needs to include the effect of flow forces, which are discussed in the force analysis.  Considering the poppet valve shown in Figure 2 the force analysis needs to consider the pressure force on the valve face that will vary with the velocity of the fluid in the small valve opening.  Here the force due to friction is assumed to be negligible.

For the valve in the closed position:     

                                              

 

                        Figure 2 Single Stage Poppet Type Relief valve

 

 

For the valve open a distance y we have:

 

 

 

Because ysinq  < y, the pressure area has reduced and hence the force has also reduced.  There is a reduction in the pressure due to the increase in the fluid velocity which further reduces the force that can be analysed as follows:

The pressure force on the valve, which will depend on the pressure distribution on the valve face, will be :

However the pressure distribution is not known and the alternative approach is to obtain the force that results from the change in velocity which is often high and the associated change in momentum has to be created by pressure forces in the fluid.  The fluid generally enters the valve at a low velocity so that for a particle of fluid having a mass m, and an exit velocity u, the change in the momentum is given by mu.

The force required to create this change in momentum is given by:

Now the mass is related to the both the flow Q and the time period dt so that .  This gives for the rate of change of momentum:   where u is the change in velocity.

The change in axial momentum of the flow through the valve which is given by  neglecting the upstream velocity U1. 

Thus                

The flow angle q  depends on the conditions of the restriction it is the same as the face of the poppet.

Assuming that the downstream pressure P2 is zero, the pressure force on the face of the valve must equal the spring force.  Thus:

                                                                            (1)

where C is the spring compression force, k isthe stiffness and A1 = pd2 /4.
The term  is referred to as the flow force (or Bernouilli force) which acts in the direction to close the valve as it effectively reduces the upstream pressure force P1A1 that would act on the valve if it were closed.

Valve flow

The valve acts as a restrictor by reducing the pressure of the fluid.  Using the valve flow equation gives:

                       

                                                                                               (2)

For P2 = 0, CQ = CD for the valve orifice.

Also                                                                                                         (3)

Valve flow characteristics

Combining equations (1), (2) and (3) gives:

                                                                                    (4)

Where:

                       

and                 

                                                                                                   (5)

It is seen from equation 5 that the force coefficient K2 (due to the flow force) has an additive effect to the force coefficient K1 that arises from the spring stiffness.  Increasing the values of these coefficients reduces the level of flow that the valve will pass for a given upstream pressure P1.  The valve flow to pressure characteristics obtained from equation 5 are shown typically in Figure 3 where the zero flow intercept is referred to as the ‘cracking pressure’.

 

 

 

            Figure 3 Valve pressure flow characteristic from equation (5)

Numerical example

i)  Flow considerations

The flow though the valve at a given differential pressure can be found from the orifice equation:

where the velocity           

Now, taking DP = 100 bar,   r = 860 kg m-3

The area,

For d = l0 mm and a seat angle q = 45° at a valve opening of y =0.5 mm we get:

ii)  Force considerations

The flow force is given by:

Hence, the force is proportional to the opening and therefore behaves as a spring.

Typical values:

Spring stiffness of 200kNm-1
The spring preload required for the valve to crack open at 75 bar pressure
With the valve opening of 0.5 mm the spring force is 100 N
The flow force for DP = 100 bar is 97 N

Clearly, the flow force makes a significant contribution to the total force acting on the valve.

2.2 Two Stage Relief Valves

 

Figure 4 Two Stage Poppet Type Relief Valve

The two stage valve shown in Figure 4 uses a spring loaded pilot poppet (pilot relief valve) to sense the pressure level in the supply at A.  When this pressure causes the pilot relief valve to open the flow through the balancing orifice creates a pressure drop across the main valve poppet that has a spring preload in the region of 2 bar.  This causes flow from the supply to be returned to tank through the valve at a controlled level of the supply pressure. 

Two stage valves have a much reduced pressure over-ride as compared to single stage valves because the main spring is not required to be preloaded to the controlled pressure level and the valve poppet has a larger diameter than in the single stage valve.  This allows a reduced spring rate to be used that reduces the pressure over-ride which can be of advantage where it is required to control the supply pressure within close limits.

The pilot relief valve can be isolated from the main valve.  There can be more than one pilot valve which can be set at different pressures so that the connection of any one will operate the relief at the respective pilot set pressure.

                        
                                    Figure 5 Two Stage Cartridge Relief Valve

Figure 5 shows a typical cartridge type of two-stage relief valve.  Cartridge valves are available for installation in individual housings, sandwich mounting (stacking) and in special manifold blocks.

Relief valves are also available with electric control using proportional solenoids to provide the necessary force in place of a mechanical spring.  A major advantage of this type of valve is the ability to adjust the pressure setting from an appropriate voltage controlled amplifier.
 

 

   Figure 6 Dual Relief Valves in Actuator Circuit

Dual crossline relief valves, as shown in Figure 6 for limiting the pressure on both sides of a linear actuator, are available in a single casing.

 

 

 

3          Pressure Reducing Valves

A pressure-reducing valve is used to provide a sub-circuit with a supply of fluid at a pressure which is less than the pressure in the main circuit.  Figure 7 shows a schematic diagram.
 

 

                    Figure 7 Reducing Valve Operating Principles

The downstream pressure Pr acts on the first-stage, spring loaded, poppet valve and when this valve is open, the resultant flow through the orifice drilled through the spool valve creates a pressure differential between the two ends of the spool.  This moves the spool against a spring so that the spool throttles the flow between the supply and service ports.  If the downstream pressure rises above the required level, the spool moves to increase the throttling action (and vice versa).

 

                       Figure 8 Reducing Valve

The reducing valve in Figure 8 has a screw adjustment to change the set pressure.