Structural design and performance testing of the electromagnetic proportional pressure relief valve

Feng-xia HAN

Mechanical and Electrical Practice Center，Beijing Information Science and Technology University，Beijing 100192，China

1.Introduction

Pressure relief valve is an important safety accessory in middle-high pressure liquid analytical instruments.Currently，during the course of using liquid chromatography or high pressure solvent extraction instrument，different working pressures are needed depending on the requirements of different objects.In order to satisfy the safety and operation reliability for the system，the pressure of the system must be adjustable.However，the figure of the existing pressure relief valve in the market is set by the factory in accordance with the mostly nominal standard series，which means that users can not adjust it according to their own needs.Some pressure valves are adjustable but the price is too high.Therefore，this paper describes the development of a self-designed electromagnetic relief valve，which could be adjusted according to the actual set pressure with a compact structure，long service life，and relative low cost.

2.The working principle of the proportion electromagnetic relief valve

The proportional solenoid valve is a fluid(gas)flow or pressure control solenoid valve.The output liquid(gas)flow or pressure could be adjusted conveniently by controlling the input current.Therefore，the proportional solenoid valves are widely used in the liquid(gas)pressure in open-loop or closed-loop control system.The proportional solenoid has a special magnetic circuit structure，when a current signal is input the coil，the coil generates an electromagnetic field to force the movable iron core to move against the spring force.At the point of the balance of the electromagnetic force and the spring force，the movement of core will stop，therefore，at the combined action of the electromagnetic force and the spring force，the movable iron core will move continuously in proportion to an electrical signal which is proportional to the controlled output fluid(gas)flow or pressure.Therefore，the proportional control could be reached［1］.

Electromagnetic proportional pressure relief valve is a normally open pressure control valve，which can control the released pressure automatically and accurately with a pressure sensor.The principle is shown in Figure 1.

Figure 1.The principle of electromagnetic proportional pressure relief valve

The proportional solenoid valve with a standard analog input signal and feedback signal has a PID regulation.Pressure sensor mounted on the output point compares the piping pressure with the setting value，the results are then transmitted to the PID controller.PID controller adjusts itself according to the results of the comparison values，through controlling the input current，the proportional solenoid could control the high-lowering supplement to keep the system pressure constant［2］.

3.The design of the electromagnetic proportional pressure relief valve

3.1.The structure design of the electromagnetic proportional pressure relief valve

According to the practical requirements，the electromagnetic proportional valve is designed with a normal working pressure range of 5 ～ 20 MPa，the highest working temperature(+50℃)and the lowest temperature(-20℃).The structure of the solenoid proportional relief valve is shown in Figure 2.

Figure 2.The structure of the electromagnetic proportional pressure relief valve

The electromagnetic proportional pressure relief valve includes a proportional solenoid valve holder，valve holder，plunger rod，compression spring，valve body，guide sleeve，plunger seal untie，valve port seal unit，and a valve head.

3.1.1.Selection of proportional solenoid

The working principle of the proportional solenoid is as follows:for a given input current，the proportional solenoid will engender a certain displacement，the proportional solenoid will push rod to produce the same displacement and push the spool valve to move towards the head block of the valve with the decrease of opening degree and the system pressure will get increased.When the input current gets decreased，the displacement of the proportional solenoid will be decreased，under the action of spring force，the piston rod will produce a reverse displacement with the increase of opening degree and the system pressure will get decreased.

The design adopts the force-controlled proportional solenoid.The change of current in coil could adjust the output of electromagnetic force within the effective working stroke region of the solenoid armature.The rated current of the proportional solenoid is 0.8 A，the rated suction is 80 N and the rated travel is 3.6 mm.

3.1.2.The spring design calculations

The material of the valve spring is 60Si2Mn and the compression spring is designed to meet GB/T 12241—2005［3］and the both ends contact closely.The diameter of spring wire could be selected according to the following formula:

Where，C is spring index;K is curvature coefficient;P2is the maximum working load of the spring;［τ］is the allowable stress of spring material.

The number of spring active coils could be determined as follows:

Where，G is the modulus of elasticity of spring material，λ is the spring stiffness.The number of spring turns could be obtained:n1=n+2.

Based on the calculations，the parameters of the relief valve spring are as follows:the spring wire diameter d=1.5 mm，spring diameter D1=14 mm，height of the spring H=15 mm，effective number of turns of the spring n=4，the number of spring turns n1=6.

3.1.3.Sealing design

Sealing structure of the electromagnetic proportional pressure relief valve includes both a valve port sealing unit and piston rod unit.

The contact sealing methods is adopted for the designing of the valve port and the valve port sealing structure is shown in Figure 3.

Figure 3.Valve port seal structure

In order to obtain a good sealing performance，the sealing surfaces must have sufficient contact pressure.The desired contact pressure on the sealing material is related to material，width，surface roughness and medium pressure.With the limitations of the solenoid thrust，the sealing contact pressure can not be too high.In this design，comparative tests of the polyether ether ketone(PEEK)and the poly tetra fluoroethylene(PTFE)have been conducted to determine which one is more suitable material for the valve port with the same working width and roughness.The comparative results show that the hardness of PEEK material is too high while PTFE shows a good sealing performance.

The sealing between axial piston rod and the valve body adopts the canted coil Spring-energized Reciprocating.When the seal is assembled，the exposed edge of the canted coil spring contacts with the high pressure liquid.Such a sealing structure is shown in Figure 4.

Figure 4.Spring energized seals reciprocating

The canted coil spring is embedded in the sealing region.The patented canted-coil spring provides a near-constant spring contact force over a wide range of working deflection.The structure of the dynamic sealing flange ring improves the sealing performance，strengthens the wipe function，and reduces the friction;therefore，a longer service life is expected.

In this design，the plunger rod and the valve body are made of stainless steel，surface finish is down to 1.6 μm.Tolerance of the plunger rod is g6，valve body is H7.Under the action of medium pressure，the canted spring in the seal tension to ensure the liquid not to leak to the outside of the high pressure valve through the interference-fit between the inner ring of the seal to the plunger rod and the outer ring to the valve body.

3.2.The characteristics of the electromagnetic proportional pressure valve

The characteristics of the electromagnetic proportional pressure relief valve are as follows:①the proportional solenoid could make the output pressure controllable.It is also convenient to regulate the pressure;②the range of output pressure could be continuously adjusted from 5 to 20 MPa;③by using the PID controlling proportional solenoid，the parameters of the PID could be adjusted according to the output pressure and the range of change is in a relative wide range;④the use of the canted coil Springenergized Reciprocating enhances the sealing capability and wipe function，meanwhile it could reduce the friction and possess a longer service life.

4.The testing of the electromagnetic proportional pressure relief valve

4.1.Testing purpose

The testing purposes of the electromagnetic proportional pressure relief valve are as follows:①to test whether the accuracy of the pressure regulator or the pressure fluctuation range could meet the requirements of the instrument analysis;②to test whether the service life of the valve is long enough to support the instrument working stably and reliably within the expected working life［4-5］.

4.2.Testing device

Testing device consists of solvent reservoirs，high pressure bump，control board，touch screen，and high pressure pipelines［6-7］.The principle of the testing device is shown in Figure 5.

Figure 5.Principle of the testing device

4.3.Testing method and test results analysis

1)The testing method of pressure fluctuations:the desired pressure could be set through the“pressure setting”button of the testing device，and the fluctuation of actual pressure can be displayed dynamically on the screen.The measure points of 5 MPa，10 MPa，15 MPa，and 18 MPa are selected to test pressure fluctuations［8-10］.

2)The testing method of the working life of the valve:for a given working pressure，the action time of electromagnet is set to test whether the pressure range of the relief current is within the maximum range of current for the solenoid.The maximum current of the electromagnet of the valve is 0.8 A，maximum output torque is 80 N and the working pressure is chosen as 15 MPa.

Based on the testing results as shown in Table 1，Table 2 and Figure 6，the following conclusions could be drawn:①the design can fully meet the requirements of the pressure range of 0～20 MPa，with the pressure fluctuation of±1 MPa;②electromagnetic proportional pressure relief valve acts 82，000 times and the controlled pressure accuracy of ±0.5 MPa meets the requirements.Since the action is evaluated 40 times per day，the valve life span will be more than five years.

Table 1.Pressure fluctuation test results

Table 2.Life test results

Figure 6.Service life testing curve

5.Conclusion

This paper describes the design and the testing method for electromagnetic proportional pressure relief valve.It adopts proportional solenoid as a driving element and the pressure can be dynamically adjusted to meet the laboratory analytical instrumentation requirements at different pressures.The sealing ability could be enhanced by using the canted coil springenergized Reciprocating seals.Such electromagnetic proportional valve has been successfully applied in a certain type of a prior sample processing equipment in the laboratory.The valve has been verified that it is structurally safe and reliable with a stable performance.This successful development could break the dependence of importing similar valves，and it could provide a reference method to develop other similar products.

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