we have a mixing vessel having load cell of Siemens (capacity 1000 kg). During our batch manufacturing we gives the command to PLC to dispense 717 kg of Propellant. It start to dispense and after dispensing of 717 kg it stopped, but on PLC the value was not stopped. it rises and stopped at 804 kg. it is due to load cell or how? we don't find the answer after our investigation. kindly suggest the probable causes to this.

[email protected]

 

could be all kinds of things. might be that dispensing mechanism was told to shut off but stayed open longer than desired, possibly due to a mechanical issue.

might have been a blockage in the line somewhere that cleared after the dispensing mechanism shut off and dropped a load into the batch reactor.

there are a half dozen other equally possible things that might have gone wrong as well. considering the limited information you gave us to work with, it is doubtful that you will get much more of an answer than wild guesses like I have suggested.

--
Bob

 

I agree with Bob, just not enough information. I'm confused about whether the correct amount or the higher amount was actually dispensed to the load cell when all was said and done or if the PLC just continued to count for some reason even though it stopped dispensing at the correct amount.

 

Is over value is same every time? Adjust the free fall of system. may be it count weight in air.
Regards,

Zahid Ali Khan,
Rox Tech Engineering,
[email protected].

 

1) The totalized weight value could be damped by averaging, the effect being that instantaneous load changes are not instantly reflected in the measurement, but gradually over time. This is not at all uncommon on agitated vessels, where the load cell signal is heavily damped to avoid 'seeing' the effects of agitation.

If the decision to shut off comes too late, because the value for making the control decision is 'delayed' due to damping, then the result is too much stuff, too much weight.

The averaging action could be in the load cell electronics or it could be in the PLC.

2) Just because the valve turns off does not necessarily mean that at that moment that all the media downstream of the valve has transferred to the vessel so that its weight is measured. The final transfer can depend on the physical layout.

 

1. where is the shut-off valve located relative to the mix tank, line size, nominal flow rate or fill time? If the valve is locate some distance away from the tank you may have to deal with line drainage.

2. do you correct for tare weight prior to filling?

 

Your no. 2 is exactly the point I was going to make. In the last application I did that involved automatically dispensing to a weighed vessel, I had to close the valve about 2 kilos before the setpoint. That was the weight of product in "limbo" -- already past the valve but not yet weighed by the load cells.