Basic principles and optimization plans of ultra-high pressure pneumatic pressure reducing valves

Abstract: This article introduces the working principle, main structural design calculation and test conditions of the ultra-high pressure pneumatic pressure reducing valve, and analyzes the structural characteristics of the valve.
Keywords: ultra-high pressure pneumatic pressure reducing valve valve seat design
1 Introduction
​ The ultra-high pressure pneumatic pressure reducing valve is an important component of the pneumatic brake system. Because the gas viscosity is small, it is easy to leak, and the system working pressure is high. The input pressure of the valve is 11~13MPa, and the maximum output pressure is 7MPa. Therefore, the sealing and durability of the valve have become prominent issues. The ultra-high-pressure pneumatic pressure reducing valve introduced here breaks through the traditional structure [1], and the important parts and components are optimized and designed, so that the valve has no leakage under high pressure conditions, and other performances also meet the use requirements. ​
2. Working principle
The working principle of ultra-high pressure pneumatic pressure reducing valve is shown in Figure 1. When there is no external force acting on the pressure head, the gas from the air source enters the air chamber at the lower part of the valve body through the input port. The air inlet valve is pressed against the air inlet valve seat under the action of air pressure and return extension spring with swivel shackle, and there is no gas output from the valve output port. When the pressure head is acted upon by an external force F, the pressure head moves downward, and the return spring 1 is compressed by the balance spring. The exhaust valve is pressed down and contacted with the exhaust valve seat to isolate the output port from the atmosphere. The pressure head continues to move downward and is pushed open. Intake valve, the passage of compressed air controlled by the intake valve enters the actuator cylinder behind the valve. As the cylinder pressure increases, the opening of the intake valve gradually decreases until the intake valve closes when the output pressure p2 is balanced with the force on the pressure head. When the external force is eliminated, the intake valve moves upward and closes under the action of air pressure and the force of return spring 2. At the same time, the pressure head and the exhaust valve are reset under the force of the return spring 1 and the exhaust pressure, the exhaust port is opened, and the original output gas is discharged into the atmosphere from the exhaust valve through the muffler.

Figure 1 Structural working principle diagram
Now let’s study the state of the exhaust valve when it is in a certain equilibrium position. Ignoring the gravity and friction of the pressure head, exhaust valve, etc., the force balance equation of the exhaust valve is:
F＝p1A1＋p2（A2－A1）＋Fs＋Ff（1）
Where: Fs – the sum of the elastic forces of the two return springs;
Ff – the friction of the sealing ring;
A1, A2 – are the effective pressure areas of the intake and exhaust valves respectively,
A1=π(d12-d012)/4,
A2=π(d22-d022)/4;
d——Exhaust valve seat diameter;
​ d01——The diameter of the lower section of the ejector pin;
​ d02——The diameter of the upper section of the ejector pin. ​
From formula (1), the output pressure p2 of the valve is proportional to the force F on the pressure head (see Figure 4). ​
3. Design and calculation
The design of ultra-high-pressure pneumatic pressure reducing valve generally involves selecting the structural type of the valve based on the given design parameters and working conditions, and then selecting and calculating the structural parameters. ​
Usually the given parameters include: air source pressure, maximum output pressure of the valve, ventilation capacity, maximum operating force and stroke, etc. The contents of design and calculation include: selecting the structural type, determining the structural size of the valve according to the ventilation capacity and working pressure, designing the balance variable pitch compression spring according to the stroke and operating force, etc. ​
The structural design of the valve focuses on the sealing structure of the intake valve, exhaust valve and valve seat. Because the gas viscosity is low and the working pressure is high, it is easy to leak. The structure of the valve is shown in Figure 1. ​
(1) Calculation of ventilation capacity
The ventilation capacity of the valve refers to the inflation and exhaust time of the valve under the given air source pressure, valve output pressure, actuator cylinder and the volume of the pipeline behind the valve. ​
Ventilation capacity depends on the area of the intake and exhaust channels. The valve is in the process of inflation and exhaust for a very short time, and we ignore the influence of heat exchange, that is, adiabatic inflation and adiabatic exhaust. In addition, depending on the working pressure of the valve, the valve is inflated at the speed of sound and exhausted at the speed of sound. Therefore, the effective area Aa of the air inlet passage of the valve is calculated according to the following formula [2]:
​
In the formula: V——the total volume of inflation;
K——Specific heat ratio, when adiabatic inflation, K=1.4;
T——The temperature of the air, the temperature of standard air T=293.15K;
t1——Inflation time;
R——Gas constant, R=287.1N*m/kg/K;
p1——valve input pressure;
p2——valve output pressure;
p20——The pressure in the cylinder before the start of inflation. ​
∵A1=Aa
∴According to the structure (see Figure 1 and Figure 2), the diameter of the air inlet
​
According to the principle of equal area, the axial distance (opening) between the intake valve and the valve seat
hc≥(d12－d012)/(4d1)(4)
The effective area of the air release channel is calculated according to the following formula
​
In the formula: t2——exhaust time;
p20——Initial exhaust pressure in the cylinder;
Pa – external pressure. ​
The meaning of other symbols is the same as that of formula (3). ​
Vent hole diameter (see Figure 1 and Figure 2)
​
Axial distance (opening) between the air release valve and the valve seat
h2≥(d22-d022)/(4d)(7)
(2) Calculation of exhaust valve seat diameter
It is known from the working principle of the valve that the diameter d of the exhaust valve seat directly affects the pressure regulation accuracy of the valve. If the diameter is large, the pressure regulating accuracy of the valve is high; otherwise, the pressure regulating accuracy of the valve is low. However, the diameter of the exhaust valve seat is limited by the operating force. The diameter of the exhaust valve seat (see Figure 3(b)) can be obtained from equation (1)
​
Where: Fmax – the given maximum control force. ​
Under the premise of satisfying the control force value, the diameter of the exhaust valve seat should be as large as possible. ​
(3) Design of intake and exhaust valves
The design of intake and exhaust valves mainly includes structural type, material selection and geometric size determination. The valve structure adopts metal rubber-coated valves (the so-called metal rubber-coated valves are rubber directly vulcanized on the metal skeleton). It takes advantage of the advantages of high elasticity and low sealing specific pressure of the rubber material, so that the valve has a good compensation function during the working process; in addition, it takes advantage of the strength and stiffness of the metal material. The valve processing and manufacturing technology is good and the manufacturing cost is low. ​
The selection of rubber material is mainly based on its mechanical properties and the operating temperature of the valve. ​
The thickness of the vulcanized rubber is selected according to the valve seat profile height h, and the rubber compression amount is preferably (20~25)%. ​
The metal skeleton of the intake and exhaust valves should be made of brass because of its good bonding performance with rubber. ​
(4) Design of intake and exhaust valve seat profiles
The valve seat profile is in direct contact with the rubber surface of the valve. During the working process, the rubber surface is deformed and acts as a seal, which also has a great impact on the life of the valve. The valve seat profile structure is shown in Figure 2 (where: Figure 2(a) is the intake valve seat, Figure 2(b) is the exhaust valve seat). The height h range in the figure is the valve seat profile, and R is the sealing surface. A small R value means the valve has high sensitivity; a large R value means the valve has a long life. After optimized design, R has a better value in the range of 0.3 to 0.5. The roughness of the valve seat surface also affects the sealing performance and life of the valve. The roughness Ra should not be greater than 0.4μm.
In Figure 2, b is the supporting surface. It is used to limit excessive deformation of the rubber surface and protect the rubber surface. ​
(5) Design of balance spring
​According to the valve performance analysis, the balance spring has the same diameter as the exhaust valve seat, which directly affects the valve’s pressure regulation accuracy. The smaller the stiffness of the pressure reducing spring, the better the pressure regulation accuracy of the valve. But the stiffness is too small and the spring travel is too long. It is limited by a given stroke, and the spring stiffness should be designed according to the given parameters:
k=Fmax/(h1+h2)(9)
With the spring stiffness, elasticity and stroke, the spring can be designed. The stiffness of the two return springs can be designed to be the same and smaller than the stiffness of the balance spring. ​
4. Test
In order to test the performance of the valve, the schematic diagram of the designed test system is shown in Figure 3. ​
The relationship between the output pressure of the valve and the operating force is shown in Figure 4. Figure 5 shows the gas filling (exhausting) characteristics of the valve when the gas tank volume is 2L, the input pressure is 11MPa, and the operating force is rapidly applied (removed) on the pressure head. After testing and application, all technical performances of the valve meet the requirements, and some indicators exceed similar products. It has the characteristics of simple structure, compactness, small size, light weight, long life and good maintainability.