Electromagnetic flow meters reading erratic flow when control valve is open >40%

I am working with ONICON ft-3000 electromagnetic flow meters. The flow meters are positioned on drain lines which drain or "blowdown" water from water return lines. When the control valve on this drain line begins from closed, the flow meter reads very stable flow until the valve is opened greater than 40%. After 40%, the flow becomes very erratic, with spikes as much as +/- 25%. If the valve is above 40% and thus reading erratically, it must be closed to 30% before it begins reading stable flow again. The flow coming out of the pipe into the drain appears to be very stable as well, even with the valve open 100%.

There are two identical (or at least nearly identical) lines that have identical behavior. The thinking is that since we are able to get very stable readings from both lines, there is not an issue with grounding or any type of electromagnetic interference. There is a number of bends in this drain line, but the mechanical team seems confident this won't cause irregular flow. I'm not mechanically savvy, but I am not sure if they are correct.
You have piping design problem. Good luck!
Dave, could you point me in the right direction of how I can prove this? I am commissioning this system and need to provide evidence before I make it the mechanical teams problem. I would really appreciate it as I have very basic knowledge of fluid mechanics.
You need to know the fluid, it's pressure, and temperature, vapor pressure, pipe velocity and valve characteristics (globe, etc. and pressure drop). Then you grab a mechanical engineering hand book and get busy. Flow meters less that 20-50 pipe diameters down stream of a valve are not recommended, but you already know that. The issue appears to be multiphase flow, that's what you have to sort out. It may be as simple as placing the meter upstream of the valve, but now you have to re-evaluate the process design issues.
Hi srichardson_ProA,

What is the fluid - you mentioned water and "blowdown" - is this boiler condensate?
If so - please check conductivity limits for your ONICON ft-3000 electromagnetic flow meter - boiler blowdown will have very low conductivity.
This flowmeter may be slightly less sensitive than the others.
Adding salt to the condensate will fix this problem ... may cause other problems.

Check the manual and confirm instrument earthing meets specification, metallic upstream piping to body to metallic downstream piping. The manual will have a sketch of how to provide earth wiring.

You could also swap the flowmeters around and see if the issue follows the flow meter.