This sounds crazy I know but I have an Engineer wanting me to devote manpower and materials to install an additional dp cell across an existing orifice plate to monitor a backflow condition. I was wondering if the existing dp cell could be set up to monitor bi-directional flow.

When I asked if the orifice was beveled or square I was told accuracy on reverse flow wasn't an issue... he only wants to detect a reverse flow. If I can re-range the point on the DCS to go from a negative flow to the original positive flow and change the calibration range of the dp cell to measure bi-directional flow, I can save on manpower and on materials to give this guy what he needs...Am I crazy or not?

 

It's not crazy, really, just a cheap way of measuring reverse flow. I've encountered reverse flow measurement situations in chilled water plant "decoupling" headers where it was important to know when flow reversed. The use of a paddle type flow switches was tried but it proved to be unreliable. We ended up with a magnetic flow meter, which was a pretty pricey fix.

Since reverse flow accuracy isn't important per your engineer and you don't want the expense of installing another DP transmitter and orifice plate, I suggest reconfiguring your existing DP transmitter (I assume that you have a decent one that will allow you to do this, and that you're doing your square root extraction in your DCS). You'll also have to do a little programming in your DCS to make this work. (Or let your engineer figure that part out.)

You can re-range your DP transmitter so that the transmitter puts out 4mA at the max desired reverse flow (negative) DP, with the max desired positive flow DP at 20mA. Zero DP for zero flow determination would end up at some point in between.

For example, if you want a max negative DP of negative 50 inwc with a max positive DP of 200 inwc, you're DCS scaling would be -50 to 150 inwc for 4-20mA. Your DCS code would need to look at the first 25% of the signal for reverse flow determination (or 4-8mA), and the remaining 75% (8-20mA) for calculation of the forward flow value.

If you have a way to actually measure the reverse flow with a portable ultrasonic while measuring the reverse flow DPs, you could even create a curve for your DCS to approximate the actual reverse flow. It wouldn't be very accurate but it's a lot less expensive than the alternative.

 

The least expensive way to do it is with a paddlewheel flow sensor. They are inherently bidirectional. If you use a hall-effect sensor, you can read quadrature and determine direction of flow. There are many companies who can do this. It will have the advantage of leaving your dP sensor alone, and still getting the measurement.

Walt Boyes
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If your engineer only wants to tell if the flow is in reverse and doesn't care about flow rate, your existing DP setup will do that. At Zero flow you get 4 mA, with reverse flow you will get <4mA (even if the transmitter has square root turned on). If your input module ignores less than 4mA, consider using a second input in series with a higher input impedance, e.g. 300 Ohms to give 1.2 Volts, or use a 0-5V input in parallel for reverse flow.

Hope this helps,
Roy