On split-range control of level and pressure
Thread Content
This post was last edited by chenzx on 2025-7-15 08:54. Hello everyone; the attached diagram shows the proportional control of the liquid level and pressure in a methanol flash tank. I’m not quite understanding either of these proportional controls. For example, the valve opens to send the fluid to the tank area when the level is 50–100%; does that mean it’s necessary to send it to the tank area when the level is above 50%? If the liquid level control value is 50%, are different control valves used to adjust the level around 50%? Pressure is also divided into ranges; the control pressure for the flash tank is 0.4 MPag. So what pressure corresponds to the 0–50% range? How does the valve operate? Thank you so much!
1. In proportional control, 50% refers to the signal on the control valve being at 50% of its value; for 4–20mA, 50% corresponds to 12mA； 2. In the level range control system, both valves are FC, that is, air-actuated valves. When the current is between 4 and 12 mA, the opening degree of valve A ranges from 0 to 100%; above 12 mA, valve A remains fully open. At 12 mA, valve B begins to open, and as the current ranges from 12 to 20 mA, its opening degree also increases from 0 to 100%. Once valve B is open, valve A stays fully open ； 3. In pressure range control, valve A operates in FO mode while valve B operates in FC mode; the air-operated/normally closed behavior of these two valves is different. The acting mode of the controller remains the same in both cases, and an increase in either flow rate results in a decrease in pressure. Therefore, it is assumed that the acting mode settings for the electrical valve positioners of these two valves should be different. The adjustment process of this system is similar to that of liquid level zoning.
This post was last edited by chenzx on 2025-7-15 08:40. Thanks for the replies. I still have some questions. For the level control valve, does 4–12 mA correspond to a liquid level of 0–50%? If the controlled liquid level is 60%, does this 12mA also correspond to a 50% liquid level? It is easy to understand that both liquid level control valves operate in a reverse manner: as the liquid level rises, the valves open wider. For pressure control valves, what pressure corresponds to 4~12mA? Also, I don’t understand that these two pressure control valves – A is of direct-acting type while B is of reverse-acting type; does that mean the valve closes further when the pressure increases? I hope to get your guidance again; thank you!
This post was last edited by jlshnlhj on 2025-7-18 08:56. It was originally posted by chenzx on 2025-7-15 08:24. Thanks for the replies. I still have some questions. For the level control valve, does 4–12 mA correspond to a liquid level of 0–50%? What if the controlled liquid level is 60%? ... Pressure and liquid level are generally controlled using P-I control; the output value depends on the magnitude and duration of the deviation between SV and PV, as well as the P and I parameters, and is unrelated to whether the PV value is x%. The value to which the control valve should be adjusted is determined through P-I calculations or manually. Valve a operates to achieve full closure/opening within the range of (0-50%), and if PV still does not equal SV, the output continues to increase; then valve b starts to act to also achieve full closure/opening within the range of (50-100%).
In proportional control, 50% corresponds to the signal point on the control valve, and it is independent of the controlled variable； The different proportional gains, integral times, initial operating points, and error magnitudes of a controller collectively determine its output ； In pressure range control, the air-open/air-close characteristics of the two valves differ, as does their direction of operation; however, the controller has a single direction of operation. Therefore, one of the two control valves should have its electric valve positioner set to reverse operation.
Thank you all for your responses. As I work in the field of processing, I am not familiar with the principles of instrument control; I pay more attention to the operational parameters, and in particular to the behavior of the valves when these parameters change. Pressure is another aspect that interests me – I’m not quite clear on the difference between direct-acting and reverse-acting valves.
It probably refers to FC and FO; this is determined by the safety aspects of the process flow. For example, if there is a gas supply failure (no gas supply) or an interruption in the control output signal (broken connection) for PV-101A/B, should this valve be in the open or closed position? That is, whether the flash vapor is sent to the flare or to the fuel gas network determines whether the valve is FC or FO.