Pneumatic control in the PID diagram represents

The red area definitely represents the liquid level; when the liquid level is between 0% and 50%, the purified water valve moves from fully open to fully closed, while when the liquid level is between 50% and 100%, the demineralized water valve moves from fully closed to fully open.

What you’re referring to is likely the adjustment values for central control operations; they are related to the opening degree of the valves on-site. It’s not the liquid level.

The liquid level control diagram for the water injection tank shows that, in terms of design, desalinated water and purified water are used in equal amounts, with a slight preference for desalinated water. 1. The level controller operates on a feedback principle: the valve closes when the liquid level is high ； 2. Then it should be 100~50% (output of the LIC controller) corresponding to 100~0% for desalinated water valve B ； 3. 50~0% (output of the LIC controller) corresponds to 100~0% for valve A for purified water. The red digit represents the output of the level controller; 100~0% indicates the degree of opening of the valve. Since all these valves are FC valves, which are air-actuated and operate in a direct manner, it can be considered that the valve position setting for valve A is reversed. ) With this design, purified water is added first; if that is not sufficient, desalinated water is then added. It meets the energy-saving requirements better.

Uniform reply: The drawings prepared by the design institute were not done properly; incorrect data was entered. In practice, it should be as described to 4L

In the liquid level range of 0-50%, the purified water valve is controlled, and its position moves from fully open to fully closed; the speed of this opening and closing is determined by the rate of change in liquid level； In the liquid level range of 50–100%, the desalinated water valve is under control, and its opening degree increases gradually from 0 to 100%. There is no issue with the valve position design for the purified water valve. Please take a close look at the request made by the original poster; it leans towards replenishing desalinated water. That means: 1. During normal operation, the set level for the liquid level in the water injection tank should be above 50%, in other words, the liquid level in the tank must remain above 50% to meet the requirements related to the addition of demineralized water. As long as the liquid level stays above 50%, demineralized water can be continuously added; purified water is only needed when the output volume from the tank is high and the liquid level cannot stay above 50%. Until the liquid level rises above 50% again and desalinated water supply is resumed, the purified water make-up valve remains closed. 2. Both the demineralized water valves and purified water valves shown in the diagram are of the FC type; therefore, it cannot be concluded directly that there is a problem with the design of the valve failure locations. Perhaps, due to the special requirements of the process fault interlock, it is not allowed to supply desalinated water and pure water in the event of a failure in the gas shutdown valve; this is also a possibility. If supplementation is required, it should also meet the requirements of demineralized water; that is, the demineralized water valves should be designed to open in case of a failure, while the purification water valves should close in such cases. 3. When the water injection tank is empty, purified water is first added to it; once the liquid level reaches over 50%, the valve for adding purified water is closed completely, and the valve for adding deionized water is opened. This is inconsistent with what you said, namely that desalinated water should be added if the purified water is insufficient; instead, it is preferable to use desalinated water, and purified water should only be added in special cases in order to meet energy-saving requirements.