Safety flow switch

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?

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.

This is what I use for the same type of application.

http://www.ifm.com/products/us/ds/SI5000.htm

nonuke said:

This is what I use for the same type of application.

http://www.ifm.com/products/us/ds/SI5000.htm

Click to expand...

I always check Ifm first but this time the problem is that the max medium temp is 80 °C.

@RET
I also think that a switch would be better solution.

You can use the old style paddle switch that goes up to 150 ºC

This one goes up to 140c

https://www.ifm.com/ifmus/web/si5.htm

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

DeKemp said:

This one goes up to 140c

https://www.ifm.com/ifmus/web/si5.htm

Click to expand...

140 ºC only for a CIP cycle. It means that can only support high temperature for about 30 minutes and not for a long production time.

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/

kallileo said:

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/

Click to expand...

That is the old style paddle switch that can go up to 150 ºC I mentioned before

kallileo said:

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/

Click to expand...

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.

Something like this?
http://koboldusa.com/sites/default/files/product_files/Kobold-classic-glass-rotameter-KDV.pdf.
You can order switches as an option.