Safety flow switch

kallileo

Lifetime Supporting Member
Join Date
Jun 2008
Location
Thessaloniki, Macedonia, Hellas
Posts
353
We have a water heating system for a pasteurizer with 2 in-line heaters connected in series.
I need to insure that there is water flow in the system before switching on the the heating elements in order to protect them from burn out in case the pump is not working.
The pipe line diameter will probably be 1", water temperature at the outlet of the heater 100-110 degrees Celsius and the pressure 6-7 bar.
I was thinking of using a flow switch or a flow meter both providing input to a PLC. Which of them would be the best and more reliable solution?

Safety flow switch.png
 
I would go with a flow switch, which would be easier to wire, test and troubleshoot. You don't have to worry about signal scaling, signal failure monitoring, input impedance or any of that for a flow meter, especially if you're just wanting to know whether or not there's flow.
 
We use the si5 as a flow switch for our Homogenizers. They work well. I think I have seen one failure over 4 years and it was completely drenched with water and chemicals
 
It seems that I have found what I need. IFM si5 isn't not in temperature range for our application.

http://mcdonnellmiller.com/flow-swi...s-fs4-3-general-purpose-liquid-flow-switches/

I don't have the time to look up the specs on that particular unit, but since this is a heating application, you'll want to make sure your switch doesn't work off of a differential temperature principle. Many years ago, I installed a flow switch just after a heat exchanger in a CIP system to verify flow prior to opening the steam valve.

It turns out that the switch utilized a small heating element along with an internal temperature sensor. If the sensor measured temperature was less than the heating element temperature, it assumed flow was present and the switch closed. Unfortunately when the steam valve opened and the water heated up, the sensor proceeded to open the switch under the assumption that higher temperature = no flow. We ended up using a different unit.
 

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