Modelling Steam generator with P controller controling 4-20mA valve

[lordgogi]

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

 

[ione]

Not my field of expertise, but you might find something at the link below

http://www.hindawi.com/journals/ame/2013/125936/

 

[davefitz]

It would seem to be necessary that the value f should be set equal to the demand error for more ( or less) valve opening. If f=0 then the change in valve demand =0 and the valve actuator would hold the valve in a set position.

Most large boilers use a 3 element PID feedwater controller. The primary element for trim is the error in drum water level setpoint. A rate of change of steam output from the boiler to the consumer is the second element which may also send a parallel signal to the heat input ( boiler master ) controller. The 3rd element may be the error is drum pressure setpoint, which also may send a parallel signal to the heat input ( boilermaster) controller.

"Whom the gods would destroy, they first make mad "

 

[IRstuff]

Is this for school? Student posting is not allowed.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[lordgogi]

No, it is not school.

 

[MatthewL]

Lordgogi,

f is the output to the valve. Kp is the gain, in this case measured in mA/m (milliampere per meter). There should be an additional value p(s) which is equal to 4 mA. The full equation is f = Kp * (l_set - l(t)) + p(s). The value for f is bounded between 4 and 20 to limit the valve to closed and open. The Kp needs to be determined by you, depending on the needs of the system. For example, if l_set is 10 meters, and you need to have the input valve open 100% once the level reaches 9 meters, the required Kp would be:

Kp = milliamp range/maximum span = (20mA - 4mA)/(10m - 9m) = 16mA/m

So if the level is 9.5m, the output to the valve is = 16mA/m * (10m - 9.5m) + 4ma = 12mA. This is straight proportional control, and will induce an offset in the actual level in the tank due to its nature. Check out a controls handbook for more details. You may want to add integral and derivative control to the controller to remove the offset, but it will induce some sinusoidal behavior to the level. Since most controls are handled by computer nowadays, there isn't any savings by not using integral and/or derivative controls.

Check the documentation for Octave, if it is looking for a 0 - 100 response for f, then Kp is 100/maximum span and there is no p(s) value (rather, it is = 0), giving a Kp value for the example of 1/m and an output of 50 for f. In any case, Kp is equal to the range of response divided by the maximum span.

Regards,

Matt

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