Modelling Steam generator with P controller and 4-20mA signal to valve

In summary, the conversation discusses the implementation of controls for a sodium steam generator model in Octave. The individual is confused about the relationship between the P controller output signal and the valve that controls the feed water pump discharge. They also mention using a signal conditioner to convert voltage input to a 4-20mA signal and discuss different methods for controlling the water level in the boiler. Finally, they mention some helpful resources for learning about controllers.
  • #1
jakub.gongol
4
0
Hi .

I am modeling a sodium steam generator in Octave. I build a set of differential equations and solve them with Octave DAE solver. My model works just fine. Now i would like to start implementing controls but I am quite confused with P conroler output signal and with what it actually controls

For example: I am trying to control the level of water in the boiler . Mesuared value is 4-20mA signal that represents water level. What I want to control is valve that throttles the Feed water dpump discharge.

I implemented a control equation:

t= time
l= water level [m]
l_set =required water level [m]


f=Kp*(e)=Kp*(l_set-l(t)).

I know that the The characteristic of the valve is linear (4mA(0% open) - 20mA(100% open)).

What is the relation between value f and the signal that i send to the valve. Do I send the f to the valve?? That does not seem right since when e=0 then f=0 I would be telling the valve to shut to 0% open when the error is 0.(neglect the 4mA signal...I can add bias) That is not right or is it??

How can I model this problem with a differential or algebraic equation??

Thank you
Jacob
 
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  • #2
What you need is a signal conditioner to take a voltage input, say 0 to 1 volt, and outputs, 4-20mA. Conditioners like this

DRF-VDC_VAC_m.jpg


... are designed to accept a broad range of input signals, such as ac and dc voltage and current, frequency, temperature (thermocouple and RTD), and process transducers, and provide standard process outputs of either 4 to 20 mA, or 0 to 10 Vdc.

http://www.omega.com/pptst/DRF-VDC_VAC.html
 
  • #3
Hmmm

Start at the simplest mental model of the boiler.
Usually they're made from a huge round steel cylinder that may be vertical(as in PWR) or horizontal(as in fossil plant and some non-US nukes ).
So there is some relation between the amount of mass in the boiler and the level of the water.
If your boiler is vertical that's roughly linear, if horizontal it's not.

What you are controlling with flow is rate of water addition to the boiler.
So you'd want error to drive valve in the direction to correct that error - low level opens valve just as in a toilettank..


In a real plant it is typical to measure both the feedwater flow into and the steamflow out of your boiler. Next subtract them to make a "flow error" signal. This might get multiplied by some modest gain.
Flow error is then added to your level error signal and the result sent to the valve.
That's called "Three element" boiler level control.
It has the advantage that when steamflow changes, the valve immediately receives a signal to make feedwater flow match it , so you won't get much of a level perturbation.

Usually some integral is applied to the level error signal so that the system will eventually drive measured level to what is desired.

http://marineengineeringonline.com/marine-boiler-water-level-control/
marine_boiler_water_level_control.jpg


You'll want to learn about controllers, too

http://www.ni.com/white-paper/3782/en/
 
  • #5


I would like to commend you on your work so far in modeling the sodium steam generator in Octave. It is important to have a solid understanding of the differential equations and their solutions before moving on to implementing controls.

In terms of the P controller and the 4-20mA signal to the valve, it is important to understand that the P controller is used to adjust the valve's position based on the measured water level. The output of the P controller, f, represents the desired position of the valve, which is then converted into a 4-20mA signal that is sent to the valve.

In your control equation, f=Kp*(l_set-l(t)), the value of f is directly proportional to the difference between the desired water level (l_set) and the actual water level (l(t)). This means that as the difference between the two values decreases, the value of f will also decrease, indicating to the valve to close and reduce the flow of feed water.

In terms of modeling this problem with a differential or algebraic equation, you can use the same control equation, but you will need to incorporate it into your existing set of differential equations. This will allow you to simulate the behavior of the system with the P controller in place.

I hope this helps clarify the relationship between the P controller, the 4-20mA signal, and the valve in your model. Keep up the good work and don't hesitate to reach out for further clarification or assistance. Good luck with your research!

Best,
 

1. What is a P controller and how does it work in a steam generator?

A P controller, or proportional controller, is a type of feedback control system commonly used in industrial processes such as steam generators. It works by adjusting the output of a control signal in proportion to the difference between the desired setpoint and the actual process variable. In a steam generator, the P controller would adjust the valve opening based on the 4-20mA signal to maintain a specific steam pressure.

2. How does the 4-20mA signal control the valve in the steam generator?

The 4-20mA signal is a common current loop signal used in industrial control systems. In this case, the signal is sent from the P controller to the valve, which is equipped with a 4-20mA input. The varying current in the loop is converted into a corresponding valve position, allowing for precise control of the steam flow.

3. What are the benefits of using a P controller and 4-20mA signal in a steam generator?

The use of a P controller and 4-20mA signal allows for more accurate and precise control of the steam generator. This results in better efficiency and reliability, as well as reduced energy consumption and maintenance costs. Additionally, the use of standardized signals such as 4-20mA allows for easy integration with other control systems.

4. Can a different type of controller or signal be used in a steam generator?

Yes, there are various types of controllers and signals that can be used in a steam generator, depending on the specific needs and requirements of the system. Some examples include PID controllers, which incorporate integral and derivative components, and different types of signals such as 0-10V or digital signals.

5. What factors should be considered when choosing a P controller and 4-20mA signal for a steam generator?

When choosing a P controller and 4-20mA signal for a steam generator, factors such as the required accuracy and response time, the size and complexity of the system, and the availability and compatibility of components should be taken into account. It is important to select components that are suitable for the specific application and can work together effectively to achieve the desired control outcomes.

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