How to achieve nitrogen sealing of storage tanks and DCS control
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Our storage tank is designed with a nitrogen supply valve A and a nitrogen exhaust valve B.； Now it is planned to use split-range control to keep the tank pressure between 1-5Kpa, without nitrogen supplementation or nitrogen discharge. I would like to ask, generally the PID setting value has only one value. How to set the two values of 1KPa and 5KPa? How to program and configure such hierarchical control of intervals? Looking for advice.
The split-range control of nitrogen sealing of storage tanks can be achieved through the DCS system. For your problem, you can take the following steps to achieve it: 1. Set up a pressure sensor in the DCS system to obtain real-time pressure data of the storage tank. 2. Use the PID control algorithm to compare the real-time pressure data of the storage tank with the set value to obtain the deviation value. 3. Calculate the opening of the control valve based on the deviation value, and adjust the openings of nitrogen supply valve A and nitrogen exhaust valve B to keep the tank pressure between 1-5Kpa. 4. In order to meet your needs, two set values need to be set in the DCS system, namely 1KPa and 5KPa. Different setpoints can be used in different control phases through programming. For example, when the tank pressure is less than 1KPa, nitrogen supply valve A is fully closed and nitrogen discharge valve B is fully open. ； When the tank pressure is between 1KPa and 5KPa, the openings of nitrogen supply valve A and nitrogen discharge valve B are adjusted according to the PID algorithm. ； When the tank pressure is greater than 5KPa, nitrogen supply valve A is fully open and nitrogen discharge valve B is fully closed. In short, the split-range control of nitrogen sealing of storage tanks can be achieved through the PID control algorithm, set values and programming configuration in the DCS system. .
Isn't it true that one PID can only have one SP setting value? for example: Set 2kpa, the actual pressure reaches 3kpa, then the PID output should be 0% ； When the pressure continues to rise, the pid output will continue to remain at 0%, so that means the exhaust valve will never open?
Split-range control is a very mature and complex control. For specific DCS configuration, you can refer to the configuration examples of DCS manufacturers. I have Zhejiang Zhongkong ECS700 here. If you need it, I will send it to you.
There is only one PID controller in the split-range control system. The PID controller is given according to the process requirements. In this example, it can be set to 2kPa ((5-1)/2). When driving, the controller is manually operated so that the controller output is at the working point (i.e., the middle position of the dead zone). At this time, both valves are in a closed state. When the pressure in the tank deviates beyond the dead zone, the corresponding valve is opened/closed to supply air or exhaust air.
It feels like you don't need to adjust the valve or PID at all. Just use simple switching logic. If the value is lower than 1, the intake valve will open, if the value is higher than 5, the exhaust valve will open. 1-5 will not operate.
This post was last edited by jlshnlhj on 2023-6-13 21:27. If the pressure only needs to be maintained between 1-5Kpa, the 6th floor solution can be used. 2. If the pressure needs to be maintained at a certain set value between 1-5Kpa, use 1.2.3 on the second floor. Controlled by a regulating valve, the diagram on the first floor shows the split-range scheme: 1. Simply set the split range on the valve positioner. But if you want to know which valve is currently used, the actual valve position needs to be converted. 2. Add polyline module (%): 0-x, 100-0, y-100, 0-100. The actual valve position can be known through the output value of the polyline module. Yokogawa can use the Control Signal Splitter Block (SPLIT) module. It is best to use ST16 to manage it so that it is in CAS mode and the Signal Distribution Switch Switching is in position 3.
If you check the introduction of split-range adjustment, you will understand. There is only one setting value. The output of PID controls two valves. They work according to their respective signal segments to stabilize the pressure. It is not as complicated as you think. There is a dedicated module for split-range adjustment DCS, which can be used directly. In addition, select the regulating valve failure mode, FC for air supply, and FO for exhaust.