How to create a bypass control system

Hello,

I would like to facilitate my model with a bypass control system by which a certain amount of the inflow (above a given value) should bypass the WWTP directly to the recipient while the rest continues through the treatment process. I wonder what kind of controller I shall use in order to set up the system?  How I shall modify the parameters? In other words how can I bypass the excess flow?

I am modelling an activated sludge WWTP with phosphorus removal (ASM2d).

Regards,
Moses

You could use the following combination of blocks:
- a (multi)[b]probe [/b][after the influent block] that measures the influent flow rate
- a [b]2-way fraction splitter[/b] [after the probe] that either directs the flow to the plant or by-passes it
- a [b]"threshold" control model[/b] that receives the signal from the probe and sets the split fraction of the splitter: 0 (= no by-pass) when flow is lower than a given threshold; 1 (= full by-pass) when flow is greater than the threshold

The latter control model needs to be implemented in MSL - and associated to one of the existing control icons, e.g. the "onoff" icon.

I have set a bypass and I would like to get  a clarification to improve my confidence. The incoming flow is in m3/d, I'm assuming that y_Max and y_Min should be in m3/d as well. Is that right? I have set y_Max =113,000 and y_Min=10000, u_Off=0 and u_On=1. How will i know that bypass is working as I describe.
Regards
Moses

Yes: if the incoming interface link is between the signal of the probe (m3/d) and "Y_M" of the controller, then "Y_M" as well as "Y_Max" and "Y_Min" are in m3/d.

How will it work? Assuming that you used the standard OnOff model (I guess), it will work as follows (also have a look at the [b]Models Guide[/b]; or [b]F1[/b] when the block is selected on the canvas):
[list]
[li]Y_M < Y_Min: u = u_On = 1[/li]
[li]Y_M > Y_Max: u = u_Off = 0[/li]
[li]Y_Min < Y < Y_Max: u does not change[/li]
[/list]

I don't believe this is the kind of behaviour you would like from a by-pass ...

Dear Enrico
I'm using OnOff controller, I want the flow above 113,000 m3/d must be bypassed.According to what you directed me, I should change it.
Thanks
Moses

Dear Enrico
I would like to know more from your previous explanation, Y_Min >measured value (Y_M) then you allow bypass (u_on=1) and when Y_Max <measured value (Y_M) then you don't allow bypass (u_off=0). In my opinion, I'm thinking that the aim of bypass is to bypass the exceeded flow. I was expecting to bypass when Y_M>Y_Max. Did I understand it right or not? If i'm wrong I would like to know more about another constant ratio controller in terms of how it work. I have already read Model guide for it.
Regards
Moses

[quote author=Moses link=topic=2465.msg5763#msg5763 date=1433322597]
.. Y_Min >measured value (Y_M) then you allow bypass (u_on=1) ..
[/quote]
That's not exactly what I said/meant :)
In the [b]standard[/b] "OnOffBand" controller, the condition Y_M < Y_Min is 'associated with' the signal u_On.
The actual value that u_On is to take is entirely up to you, i.e. the type of control you've implemented.
Assuming you have:
[list]
[li]used a 2-way fraction splitter (as the by-pass valve)[/li]
[li]connected the "out_2" terminal of the splitter to the by-pass flow (and therefore "out_1" goes to your plant)[/li]
[/list]
then the signal of the controller ("u") will set the percentage of by-pass flow:
[list]
[li]u = 0 (no by-pass, all to the plant) for Y_M<Y_Min --> u_On = 0[/li]
[li]u = 1 (by-pass, no flow to the plant) for Y_M>Y_Max --> u_Off = 1[/li]
[/list]

However, as I mentioned earlier, [b]this may not be the best solution[/b]: because a) you probably want to cut-off flow rate in excess of a certain threshold (rather than divert the whole incoming flow); and b) you may not want a "band" (Y_Min - Y_Max) but rather that particular threshold.

So the "OnOffIdeal" model may be a better option (solves point b).
You still need to vary "u" in function of Y_M, e.g.:
u = Y_M - Y_Threshold