Orifice Sizing

It doesn't matter. You're only getting close-to-a-guess values by using an orifice plate where sediment will accumulate on both sides of the plate and distort the flow profile. That means you're dealing with high uncertainties, and result is relative readings, lower/higher, not absolute.

So pick a number and live with it.

Good evaluation of the situation, so GA. The thing that worries me is that there is a lower viscosity with increased solids. Doesn't make sense.

Perhaps another method is needed.

A different type of flowmeter (e.g. magnetic) might be in order for this application.

This is a job for a coriolis mass flowmeter, not differential pressure across an orifice!

Line Size 20.0"

Now it's a job for a clamp-on ultrasonic flowmeter, not a coriolis mass or an orifice type!

Using and orifice plate for this kind of measurement is obviously not the best solution or option, but it can be used. Without the exact data the suggestions below are that suggestions, not true solutions.

First you would be using a segmental orifice plate not normal orifice plate, since the sediments will give an error measurement.

Second calculating a segmental, you will have to run all the variations and choose the mathematical average size so from clean to dirty to sediment the measurement will vary the least.

You should think about using other measurement methods that may be better for your application.

) This sounds like a classic case for Ultrasonic clamp-on doppler. Lots of suspended solids to trigger doppler, and clamp-on is non-intrusive.

I've personally never seen doppler get uncertainties better than ±20%, but my experience is limited.

2) The V-cone is a DP primary flow element with an internal bluff body that doesn't have the flow restrictions that conventional orifice plates have. I'm not sure how erosion (from suspended solids) to the bluff body would affect operation or uncertainty, but I'm sure the vendor could.

3) Others have suggested the magmeter, which is likely to be about as accurate as you can get with no flow restriction. I actually have to ask why a magmeter isn't the first consideration?