Hello All:

I have a rather unique application that I wanted to bounce off all "the experienced" out there. I am controlling a gas flow application that uses canisters to process the gas. Each canister is limited to a maximum flow and since there is a varing load, we have banks of modules that are selectable in combinations. The control system
can activate any combination of modules from 1 to 18. The typical load requires approximately 9 to 10 modules to be online. This is where the dilemma arises. Occasionally the variation in load demand (reading of inline flow meter) causes an operating point that teeter-totters between the selection of modules (i.e. between 9 and
10) and every time a transition occurs, an initial surge is generated in the system that takes a few minutes to settle out and therefore sets up a sort of unstable process. Once the demand increases and generates an operating point well inside the bracket of selection, the problem subsides. To remedy this, we are thinking
of building in some sort of hysteresis into the calculation (of modules via flow) to try to avoid this. Does anyone have any better ideas?

Thank you, in advance, for you input.

Brian Romano
PACsys, LLC

 

I would go with the hysteresis. You may want to
have hysteresis settings for each canister in case
future loads are a problem. The size of the
undesirable fluctuations should indicate the settings.

Timers could also be used, but timer values are
not as easy to guess at, and they're not as
intuitive.

Anthony Kerstens P.Eng.

 

Dear Brian,

Insert a pressure control loop in the combined header to the load and place a gap type VPC on the control valve in that loop. The valve position controller (VPC) should be provided with a neutral band of say 70 to 90% opening, so that at 90 a new canister is opened and at 70 one is closed.

Best regards, Bela

 

Brian... if you were to borrow a common principal from the food processing industries, I think you may find an answer. It is quite common in food
container filling applications to have "bulk" and "fine" feeders. What we do is... fill using the bulk equipment to a given point then finish the fill using the higher resolution of the fine feeder. Could you not use this same concept. As in... set the set point of your calculation a little on the low side but high enough so that the desired flow could be reached by modulating one more canister. Then allow your main calculation to provide the bulk of the flow then have a modulating control regulate the flow on the
fine canister... just a thought.

Best Regards... Rick Kelly

Chief Technician
Natural Cuts
Cheese Operations
Kraft Canada
(613) 537-8069 V
(613) 537-8057 F
[email protected]
http://trondata.on.ca