Orifice Plate and Flow Transmitter

If the orifice plate calculation sheet disagrees with the installed range (datasheet), either the calculations are incorrect, the installed range (datasheet) is incorrect, or perhaps there is some other parameter not being taken into account. I don't have orifice plate software handy right now and I'm not in the mood to crunch through Bernoulli's equation to see which of the two figures seems more reasonable, but I'd say it's time to locate some orifice plate software of your own and do a complete re-calculation to see where the error lies.

Your estimation of 1.78 bar DP for 400 m3/hr is correct assuming the orifice plate really does generate 250 mbar DP at 150 m3/hr. If this is truly the case, then what is needed is a new orifice plate (with a larger d/D "beta" ratio) to produce more realistic differential pressures. A differential pressure of 1.78 bar is tremendous, and might actually *damage* the orifice plate!

Also, bear in mind the limited turn-down ratio of most orifice flowmeter installations. Increasing the top-end range of the flowmeter also increases the lowest flow rate you can measure with reasonable accuracy. If operations personnel want the range increased, they must also realize they will compromise low-flow accuracy as a direct consequence of this. Perhaps they are already aware of this fact, but it's worth repeating.

Omar,

Using this data:

the pipe size is 8'', d/D is 0.35858938, so, the orifice size is 7.98" x 0.35858938 = 2,8615432524", If you can please tell us the pipe ID, the 7.98" is only a reference from pipe SCH 40 table.

So,

temp. 25 C, viscosity 0.72 cP, flowing pressure 9.3 Bar, dp 250 mbar, densty 994.3 kg/m3 and orifice size 2,8615432524"

According an orifice software you will be having the following flow 64.3 m3/hr

Can you check directly on your orifice plate What is the orifice size?

What is the liquid?

I agree with Tony there is a error in someplace of your data

Best regards

JP

If you want to have same pressure drop at the plate then you must increase the oriffice diameter as follows:

d2/D=d1/D*(q^2/(1-(d1/D)^4*(1-q^2))^0.25; q= Q2/Q1

This assumes that the flow discharge coefficient is constant which is valis if the ratios d/D are not too far from each other.

If you accept a pressure drop change then you should correct the obtaine value with sqrt of pressure drops ratio.

The 3 values you give Q-Δp are coherent although a difference does appear.

You can find all equations in wikipedia under "oriffice plate calculations" and very well explained.

The flow sizing programs will flag a warning at about 85m3/hr flow rate that plate deflection needs to be checked.

Standards like AGA 3-2 have tables for plate thicknesses at various betas for various line sizes.

The vendor table below summarizes typical recommendations on plate thickness.

I've always considered the relative low cost and replaceability of orifice plates an advantage, so replacing with a re-sized plate can be an option.Living with plate deflection might be an option, too.

I was surprised when I read how little (relatively) little error a deflected plate showed, after reading a paper:

"Yeyssandier and Chisman (1984) tested 8- and 10-inch bent plates with original [beta] ratios of 0.5 to 0.6 and deflections of up to 0.54 inch in the direction of flow. Their study also concluded that measurement errors become more negative with increasing deflection angle, but the largest under-measurement error observed in the study was 3.3% for a deflection of about 1/2 inch. Ting (1993) flow calibrated 4- and 6-inch orifice plates with beta ratios between 0.25 and 0.75, then recalibrated the same plates after they had been bent over a mandrel to deflection angles between 1[degrees] and 10[degrees]. Except for [beta] = 0.25, the measurement error increased with increasing [beta] and deflection angle. From his test results, Ting concluded that bent plates might cause under-measurement errors up to 4.5%, less than the errors reported by Burgin for similar deflection angles."

http://www.thefreelibrary.com/Guidelines+for+measurement+errors+caused+by+buckled+orifice+plates.-a0182406220