The working principle and Optimal Design of ultra-high pneumatic
Ultra-high pressure pneumatic reducing valve is an important component of pneumatic braking system,. Because the gas viscosity is small, easy to leak, and the system working pressure is high, the valve input pressure is 11 ~ 13MPa, the maximum output pressure is 7MPa, therefore, valve sealing and durability become the prominent problem. Here presented ultrahigh-pressure pneumatic reducing valve breaks through the traditional structure [1], and optimized design the key components, makes the valve without leakage under high pressure, and other properties also meet the using requirements.
2、working principles
The working principle of ultra-high pressure pneumatic reducing valve is shown in Figure 1. When the pressure head with no external force, air source gas enters into body down air chamber from the input port, intake valve presses tightly with the seat under the pressure and reset spring effects, valve output port without gas output. When the pressure head is affected by the external force F, pressure head moves down, through balancing spring compression to reset spring 1, taking exhaust valve to contact with seat, makes output port isolated from the atmosphere, the pressure head continue moves downward, pushes out the gas input valve, compressed air enters into actuator cylinder behind valve through the passage controled by gas input valve. As the cylinder pressure increases, the intake valve opening gradually decreases until the output pressure p2 is balance with the pressure head force, the intake valve closes. After the external force removes, the intake valve moves up and closes under the effect of air pressure and reset spring 2 force. At the same time, the pressure head and the exhaust valve reset under the reset spring 1 force, and exhaust pressure actions, exhaust port opens, the original output gas is exhausted to atmosphere by exhausted valve through the muffler.
Now reaserch exhaust valve state when at a certain balanced position. Ignore the pressure head, exhaust valve gravity and friction, exhaust valve balanced bearing force equation is
F＝p1A1＋p2（A2－A1）＋Fs＋Ff（1）
in formula：Fs――the elasticity sum of two reset springs；
Ff――sealing ring friction；
A1、A2――respectively are the input, outpur gas exhaust valve effective pressure area
A1＝π（d12－d012）/4，
A2＝π（d22－d022）/4；
d――exhaust valve seat diameter
d01――ejector lower segment diameter
d02――ejector upper segment diameter
From equation (1), we know that valve output pressure p2 is proportional to the pressure head force F (see Figure 4)
3、Design and calculation
Design ultra-high pressure pneumatic reducing valve is generally according to the given design parameters and working conditions, choosing the valve structural type, and then select and calculate the structural parameters. We recommend broad chemical pumps, pharmaceutical factory selecting Shanghai Sanxing pupm factory, Shanghai Yihai corrision resistant pump manufacturing Co., Ltd. Shanghai new Sanxing Water Supply and Drainage Equipment Co., Ltd.
Usually given parameters are: gas supply pressure, valve maximum output pressure, ventilation capacity, maximum control force and stroke, etc. Design and calculate contents are: the selected structural type, according to the ventilation capacity and working pressure to determine the valve structural size, according to stroke and manipulate force design to balance the spring and so on.
The valve structural design is focuses on inlet valves, exhaust valves and valve seat sealing structures, because the gas viscosity is small, and the work pressure is high, so it is easy to leak. Valve structure shown in Figure 1.
(1)Ventilation capacity calculation
Valve ventilation capability refers to under the conditions of given air supply pressure, valve output pressure, actuator cylinder and after valve pipe volume, valves, inflatable and exhaust time.
Ventilation capacity depends on the intake channel and exhaust channel area. The time of valve in inflation and exhaust process is very short, we have neglected the impact of heat exchange, namely, inflatable insulation and thermal insulation exhaust. In addition, according to the valve working pressure, the valve is inflated and exhaust with the speed of sound. Therefore, the intake valve passage effective area Aa is determined by the formula [2]
K――Specific heat ratio, when thermal insulation inflated，K＝1.4；
T――air temperature，standard air temperature T＝293.15K；
t1――inflating time；
R――gas constant，R＝287.1N*m/kg/K；
p1――valve inlet pressure
p2―― valve outlet pressure
p20―― Pneumatic cylinder internal pressure before the start of inflation
∵A1=Aa
(2) Calculation of the exhaust valve seat diameter
From the valve working principle known, the size of exhaust valve seat diameter d directly affects the valve regulating pressure precision. If its diameter is large, then the valve regulating pressure precision is high; on the contrary, the valve regulating pressure precision is low. However, the exhaust valve seat diameter is also limited by the manipulate power. Exhaust valve seat diameter (see Figure 3 (b)) can be determined by equation (1)
(3) input, exhaust valve design
Inlet and outlet valve designs are primarily including structural type, materials selection and geometry size determination. Valve structure use metal rubber-covered valves (it means directly vulcanizing rubber on metal frame). It uses advantages of rubber material high elasticity and low sealing ratio pressure, makes the valve with good compensation function in the working process; it also takes advantage of the strength and stiffness of metal materials. Valve processing and manufacturing technology is good, manufacturing cost is low.
Rubber material selection primarily on the basis of its mechanical properties and valve operating temperature.
The thickness of vulcanized rubber is according to valve seat height h for selection, rubber compression volume is suitable at (20 ~ 25)% .
input, exhaust valve metal frame is preferred to use brass, because its good combination performance with rubber
(4) input, exhaust valve seat surface design
Valve seat surface connects with valve rubber surface directly, in the work process, makes the rubber surface deformation, affected as a sealing role, but also a great influence on the service life. the valve seat surface structure as shown in Figure 2 (where: Figure 2 (a) is the intake valve seat, Figure 2 (b) is the exhaust valve seat). The figure height h is within the context seat surface, R for the sealing surface. R value is small, valve with high sensitivity; R value is large, valve wih long life. Through optimum design, R takes well in the range of 0.3 ~ 0.5. Valve-seat surface roughness also affects the valve sealing and life, the roughness Ra should not be larger than 0.4μm.
It is used to restrict the excessive rubber surface deformation, affects to protect rubber surface.
(5)Balance spring design
According to valve performance analysis, balance spring and exhaust valve seat diameter is the same, directly impact on regulating pressure precision. the smaller reducing spring stiffness, the better pressure regulating precision. But the stiffness is too small, spring travel is too long. It is restricted by given stroke, should be based on the given parameter design of spring stiffness.
k=Fmax/(h1+h2)(9)
With the spring stiffness, elasticity and stroke, we can design the spring. Two reset spring stiffness can be designed as the same, moreover, its stiffness is less than the stiffness of spring balance.
4、 test
In order to test valve performance, design test system schematic shown in Figure 3.
Valve output pressure and manipulate power relationships shown in Figure 4. Figure 5 is a cylinder volume of 2L, input pressure is 11MPa, under the pressure head rapidly exert (excluding) to manipulate power operating conditions, the valve inflate(exhaust) gas properties. After testing and application, the valve technical performance meets the requirements, some of the indicators exceed similar products. And also with simple and compact structure, small size, light weight, long life and good maintainability and other features.
