With changes in the pressure before the pressure relief valve and the self-acting control valve, can the pressure after the valve remain stable?
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As I understand it, a pressure relief valve calculates the pre-tension of the spring based on the pressure difference before and after the valve as well as the flow rate at the time of design. When the pressure before the valve remains constant, if the pressure behind the valve changes, the spring will cause the valve disc to move, thereby restoring the pressure behind the valve to its set value. However, if the pressure before the valve changes, for example if it increases, then the pressure behind the valve will also increase (although not by the same amount). Without manually adjusting the pre-tension of the spring, the pressure behind the valve cannot be automatically restored to its set value. Although the structure of a self-acting control valve differs from that of a pressure reducing valve, their basic principles are the same; it is still the balance among the pressure before the valve, the pressure after the valve, and the spring preload that determines its operating state. If the pressure before the valve remains constant while the pressure after the valve changes (in the case of a control valve with pressure sensing at the outlet side), it can still return automatically to the set value. However, if the pressure before the valve changes, the pressure after the valve will inevitably change as well. Without adjusting the spring preload, it is not possible to return automatically to the set value. In fact, pressure reducing valves and self-acting control valves have similar principles; the only difference lies in their structural design and the precision of pressure regulation. I’m not sure if my understanding is correct I would appreciate some guidance from experts.
The pressure relief valve will not. The pressure regulator connected to our gas cylinders is set to maintain a very low outlet pressure; even as the pressure inside the cylinders drops from 15 Mpa to 1 Mpa, the outlet pressure remains stable. Provided that the pressure relief valve is not damaged. If it breaks, it may not provide any pressure relief
In your case, the pressure difference is quite large; it seems that the gas velocity at the valve opening has reached the critical value. As a result, the gas flow rate is no longer related to the pressure before the valve. As long as you use gas steadily, changes in the pressure before the valve have no effect on what happens after the valve. If the gas velocity is far from reaching this critical level, then this situation does not apply
Principle of the stand-alone pressure relief valve: P2 balances with the force of the pre-tension spring. As a result of this balance, the flow rate passing through the throttling area of the valve spool’s orifice matches the flow load downstream. The upstream pressure is throttled; if P1 increases after installation, the flow rate passing through increases, which leads to an accumulation of flow at the downstream side and an increase in P2. As a result, the throttle orifice closes automatically to reduce P2 and match it to the load at the downstream side ； If the load downstream increases, P2 will decrease; in that case, the valve opening is increased to match the load and maintain P2 at a stable level. Therefore, the PCV is a variable throttle driven by flow rate, but its balancing principle is pressure. Pressure is a superficial aspect; the core issue is the flow load. PCV needs to provide a minimum (P1-P2) value in order to maintain the \"stiffness\" of the P2 pressure; that is, when the flow rate increases, the drop in P2 should not be too large, which is what constitutes \"precision\". That minimum value is achieved through the guaranteed flow rate; the valve core maintains the pressure at P2 by allowing that flow rate to pass through. Naturally, if P1 is very high, to the point of blocking the flow, that’s fine as well; the flow orifice will simply close down automatically.
This is indeed related to the downstream flow rate; the flow rate directly affects the downstream pressure. If the downstream flow rate remains stable, then changes in upstream pressure within certain limits will not affect the downstream pressure；