Problems with self-acting control valves

I’m not very familiar with it; let’s discuss it. It’s a self-acting type operated by a controller, and the pressure can range from 0.2–0.8 Mpa before the valve and 0.5–100 Kpa after the valve. If the pressure before the valve exceeds 0.8 Mpa, then a pressure regulator is necessary. This is what I wrote down before

This depends on the pressure adjustment range indicated on your valve or in its manual. For values outside the adjustment range, secondary depressurization is required for reference.

I’m not quite sure what “valve closing pressure of 0.5 KPA, valve opening pressure of 0.2 KPA” means. If the pressure is 0.5 kPa when the valve is fully closed and 0.2 kPa when it is fully open, the stable pressure downstream of this valve will be approximately 0.5 kPa. When selecting a self-acting valve, if the flow coefficient Cv is too low or too high, it may result in instability.

The set pressure behind the valve is a value, not a pressure range.

Self-acting control valves typically have a pressure reduction range of 10 to 20 times; 30 times is already considered quite high, and values exceeding 30 times require multi-stage pressure reduction

You need to determine the pressure value that needs to be adjusted; it should be controlled after the valve. According to what you said, this value should be somewhere between 0.2 and 0.5. The pressure before your valve is 0.8, so the pressure difference between the two sides is quite large – it might not be possible to adjust it

This should be the nitrogen seal valve for a 2KPA atmospheric pressure tank. 1. In a choked flow, the flow rate is related to the cross-sectional area and independent of the downstream pressure. 2. The flow rate at 0.2 KPA must meet the process flow requirements. As long as the flow is replenished, the pressure in the tank will not drop. 3. 0.5 is the shut-off pressure; it must be closed tightly to prevent the breathing valve from activating, which is why a soft seal is preferred. 4. Continuous control is not required between 0.2 and 0.5. 5. The self-acting valve is a proportional control P-type with a mechanical structure. This series belongs to micro-pressure nitrogen-sealed valves, and typically comes equipped with a controller to improve pressure control accuracy. Imported products are common, but there are now also many domestic products.

For self-acting control valves (pressure-controlled type after the valve), “valve closing pressure of 0.5 KPA” refers to the value of pressure increase when the valve behind it is closed； “\"Valve opening pressure of 0.2 KPA\" refers to the pressure value set for the valve to open behind it during normal operation. These are the parameters inherent in the characteristics of a self-acting control valve: when the valve downstream closes, the pressure downstream rises. The pressure-sensing mechanism of the self-acting control valve detects this increase in pressure, and it takes some time for the valve to close completely; that is why there is a parameter known as the \"valve closing pressure\".

The flow capacity of the valve also matters; if the operating flow rate is greater than the valve’s flow capacity, the pressure will be low. If the flow rate downstream of the valve fluctuates greatly, the pressure downstream of the valve will also fluctuate significantly. The extent of these fluctuations depends on the valve itself, with considerable differences existing between different brands.