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Mathematical and Simulation models for control valves

03/03/2007 7:17 AM

I´m developing an automation and control project for a water treatment process. By now, I´would like go for simulation before put into operation the designs.

For that I need a model for simulating control valves (proportional). It shouldn´t be so real, I only need to built a kind of block with two inputs and two outputs (a MIMO system). Those inputs are: the water flow , the signal from the controller. The outputs are: the water flow and the valve position. I sounds simple, but try it using simulink could be complex. At U I used to go for simulink in order to get simulations for controlling electronic systems but I never carry out sims for hydraulic models. (I also need the fuzzy blocks of simulink). I know software like Hysis or Fluidsim... but I don´t need such kind of reality in my simulations. In the other hand, I haven´t got the money for buy them. I only need to realise that the system won´t oscillate.

Could some one advise me? Thanks!

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#1

Re: Mathematical and Simulation models for control valves

03/05/2007 8:35 AM

Valve position shoule be a linear function of water flow through control valve, which is virtually the same thing. Water flow into system apparently provides input to proportion flow through valve -- is that correct? Need more info.

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Guru
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#2

Re: Mathematical and Simulation models for control valves

03/06/2007 6:31 AM

Flow through a control valve is proportional to the pressure drop across it. The missing data from the post is "what happens downstream?", which confounds issues, as increasing flow may cause reduced pressure drop at the valve depending on the process equipment configuration (which is why equal percentage characteristics, rather than linear, are frequently specified for a valve working on liquids).

Flow control can be achieved by proportional-only action at the controller, though the loop will always have an offset from the set point. To eliminate the offset, introduce a little integral action. Increasing integral action will result in a faster response to disturbances and more overshoot. Derivative action will tend to reduce the overshoot. Far too much integral action will result in oscillation that may be potentially damaging to the process or the components of the system.

If the selected controller has an "auto-tune" facility, like many off-the-shelf products these days, use that to determine the correct controller settings. If it hasn't then the only solution is to do it the time-consuming way, which is to disturb the loop in proportional-only, see what happens, and turn up the gain repeatedly until it just begins to oscillate. Once the period of oscillation is known, the controller parameters are then set to the values given in the Ziegler-Nicholls formulae either for fastest response or minimum overshoot, depending on circumstance. Currently Wikipaedia knows nothing about Zeigler-Nicholls, the textbook containing it is not to hand and it's a difficult thing to remember unless one is using it regularly. There may be a colleague nearby who can advise?

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