Section 1: Working Principle of Pressure Relief Valves
Compressed air at pressure P1 enters from the left end, passes through the valve orifice 10, and its pressure drops to P2 for output. The adjustment of P2 can be achieved by the pressure regulating springs 2 and 3. By rotating the knob 1 clockwise, the compression springs 2 and 3, as well as the diaphragm 5, cause the valve core 8 to move downward, increasing the opening of the safety valve orifice 10 to raise P2. If the knob 1 is rotated counterclockwise, the opening of the orifice 10 decreases, resulting in a decrease in P2.
If P1 spikes instantaneously, P2 will rise accordingly, causing the pressure inside diaphragm gas chamber 6 to increase. This results in a greater thrust on diaphragm 5, disrupting the original force balance and causing diaphragm 5 to move upwards. A small portion of air flows out through overflow hole 12 and exhaust hole 11. As the diaphragm moves up, due to the action of the reset spring 9, the valve core 8 also moves upwards, closing the intake valve orifice 10, enhancing the saving effect and reducing the output pressure until a new balance is achieved, and the output pressure returns to its original value. If the input pressure drops instantaneously, the output pressure will also decrease, diaphragm 5 will move down, the valve core 8 will follow, opening the intake valve orifice 10 wider, reducing the saving effect, and the output pressure will also return to its original value. Rotate the knob 1 counterclockwise to relax the adjustment springs 2 and 3. The gas force on diaphragm 5 exceeds the force of the pressure-regulating spring, causing the diaphragm to curve upwards and close the intake valve orifice 10 due to the reset spring's effect. Rotate the knob 1 again to disengage the top of the intake valve core 8 from the overflow valve seat 4. The compressed air in the diaphragm gas chamber 6 is then released through overflow hole 12 and exhaust hole 11, putting the valve in a non-output state.
In summary, the overflow relief valve reduces pressure through the energy-saving effect at the intake port, maintains pressure stability through the force balance on the diaphragm and the overflow effect of the overflow holes; adjusting the spring allows the output pressure to change within a certain range. To avoid the pollution of a small amount of gas discharged from the overflow relief valve affecting the surrounding environment, a pressure reducer without an overflow valve (i.e., a general pressure reducer) can be used.
When the output pressure of a pressure-reducing valve for chlorine gas is high or the pipe diameter is large, direct pressure adjustment with a pressure spring will necessarily result in an excessively high spring stiffness. This causes significant fluctuations in output pressure when the flow rate changes, and the valve structure size will also increase. To overcome these deficiencies, a pilot-operated pressure-reducing valve can be used. The operating principle of the pilot-operated pressure-reducing valve is fundamentally the same as that of the direct-acting type. The pressure-reducing gas used in the pilot-operated pressure-reducing valve is supplied by a small direct-acting pressure-reducing valve. If the small direct-acting pressure-reducing valve is mounted inside the valve body, it is called an internal pilot-operated pressure-reducing valve; if it is mounted outside the main valve body, it is called an external pilot-operated pressure-reducing valve. Figure 14-2 shows the structural diagram of the internal pilot-operated pressure-reducing valve. Compared to the direct-acting pressure-reducing valve, this valve incorporates an orifice plate expansion section consisting of nozzle 4, baffle 3, fixed orifice 9, and gas chamber B. Even a slight change in the gap between the nozzle and the baffle will cause a significant change in the pressure within chamber B, which in turn will result in a substantial displacement of diaphragm 10. This displacement controls the up and down movement of valve core 6, allowing the inlet valve orifice 8 to open wider or narrower, thereby enhancing the sensitivity of valve core control, i.e., improving the precision of pressure stabilization.
Section II: The Fundamental Function of Pressure Relief Valves
(1) Pressure Regulation Range: This refers to the adjustable range of the pressure relief valve's output pressure P2, within which a consistent precision is required. The pressure regulation range is mainly related to the stiffness of the pressure-regulating spring.
(2) Pressure characteristic: It refers to the property that causes fluctuations in the output pressure due to changes in the input pressure when the flow rate (g) remains constant. The better the pressure characteristic of the pressure reducing valve, the smaller the fluctuations in the output pressure. The output pressure must be lower than the input pressure by a certain value for it to not fundamentally change with the input pressure.
(3) Flow Characteristics: This refers to the property where the input pressure is consistent, while the output pressure varies with the change in output flow rate, g. Ideally, when the flow rate, g, changes, the change in output pressure should be minimal. Generally, the lower the output pressure, the less it fluctuates with changes in the output flow rate.
Section 3: Selection of Safety Valves for Dechlorinated Chlorine Gas
Select the type and pressure reduction accuracy of the pressure reducing valve based on the application requirements, and then choose its diameter according to the required maximum output flow rate. When determining the gas source pressure of the valve, it should be greater than 0.1 MPa higher than the maximum output pressure. The pressure reducing valve is generally installed after the water separator and filter, and before the oil mist eliminator or setpoint regulator, and attention must be paid not to reverse the inlet and outlet connections; when the valve is not in use, the knob should be loosened to prevent the diaphragm from being deformed frequently under pressure, which may affect its performance.
