Regarding the stages of nitrogen sealing
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The pressure at the top of the tank is controlled using segmented control; the set pressure for PICA-103 is 0.65 MPag, and a valve operation dead zone has been designed. When the pressure is below 0.64 MPag, the PV-103A valve on the top of the tank opens to supply N2 into the tank ； When the pressure is above 0.66 MPag, the PV-103B valve opens to vent to the flare header. The process control description outlines how this function should be implemented in a DCS system. It’s different from the usual split routes
Are they two valves with solenoids? If it is a single-control solenoid valve, choose either high or low pressure, and then use an assignment module to send the value to the DO solenoid valve. If controlling the opening degree is required, an assignment module can also be used; by setting the desired opening degree value and sending it out, it will be possible to control the MV of that PID
After thinking about it, your description of the process flow isn’t clear. First, is this tank used for generating gas or for storing gas? And is nitrogen directly supplied to the top of the tank? Should it be added to the inlet air pipeline instead? Or add it to the outlet pipeline? Where is the branch leading to the flare located: on the tank inlet pipeline, on the tank itself, or on the outlet pipeline? This location must be clearly specified.
It’s recommended to include a simple diagram for clarity, to facilitate discussion.
For this provided by the design institute, how can the split points and split curves be determined? Strictly speaking, shouldn’t it be called segmented travel? Another approach is to use two PID loops.
This post was last edited by hxch150804 on 2021-12-6 at 08:11. As you can see, once this diagram is shown, it’s clear that it deals with step-controlled operation. However, there are mistakes in the diagram: if the nitrogen valve above is valve A, which is used to supply nitrogen, then the valve below is valve B, which is used to control the amount of exhaust gas released. In detail diagram C, the labels for A and B are reversed. What is meant here is that initially, the pressure of the equipment is zero; the nitrogen valve is in the closed position, while the exhaust valve is in the fully open position. When automatic mode is activated, when the pressure is low, the exhaust valve gradually closes, trying to increase the pressure by retaining pressure. If the pressure still does not reach the set value, the nitrogen valve is opened to supply more nitrogen in an attempt to raise the pressure. This is a process of dealing with low pressure； Since process gas (gas) is added as part of the process, the pressure rises; in that case, the nitrogen valve is closed. If the pressure remains high, then the exhaust valve is opened to allow air to be released outside. This is how the entire process operates. So I think A and B are marked backwards. One final question: this storage tank should also have pipelines for supplying process gas, so why aren’t they shown?
The valves should not be incorrectly labeled; the lower the pressure, the more the valve A opens, resulting in a greater amount of nitrogen being supplied – there is no logical issue with this. The current issue is: in such a case, how can control be implemented in a DCS system? In my opinion, there need to be set points for different ranges; isn’t it inaccurate for the design team to provide only pressure values? If we want to meet the design requirements within the system, what should be done? For now, the only idea I have is to use two PID controllers with the same measurement value, along with interlocks. Is it because some domestic design institutes have not provided accurate split curve data, which makes things more difficult for us later on in the process?
This post was last edited by hxch150804 on 2021-12-6 at 08:42. A PID function block along with two proportional control blocks, plus two operators, can also be used; MV1 controls valve A with a range of 0-50, while MV2 controls valve B with a range of 50-100. However, it’s necessary to set the forward and reverse operation mode of the valves correctly. I usually use two proportional control blocks to control each valve, allowing me to specify their forward or reverse operation mode through the proportional control settings. Try it slowly.
What Detail C means: When the pressure is low, Valve A is fully open; it almost closes at 0.64. When the pressure rises to 0.66, Valve B opens, and the higher the pressure, the greater its opening degree. The diagram provided shows only the nitrogen sealing control system.