Wrong Installation DP Plate

It is quite difficult to give a valid formula, it can be only said is that the pressure drop will be LOT less for same flow. I can give an example for orientation but which a priori not valid for your case.

If a thin plate with a small opening and sharp edge the coefficient is around 1.9, for a similar plate with a chamfer at 45° the same coefficient can be as low 0.46. Since you flow-meter is based on the pressure drop its indications will be less than the actual flow but I cannot say how much.

I think it would be a good solution to call the manufacturer which has much more precise informations about the devices, may be you are the first to have this problem and he can give a correction coefficient.

All the published studies I'm aware of indicate that a backwards installed orifice plate reads lower than actual flow.

1) Here you can read about the US nuclear industry's gaffs installing OP's backwards; citing a negative 15% error (indicated flow rate appears lower than true flow rate) in numerous installations:

http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1990/in90065.html

2) "Effects of Abnormal Conditions on Accuracy of Orifice Measurement", by Thomas B. Morrow, P.E.

Morrow cites a 8-25% range of error.

"Orifice Plate Installed Backwards"
Burgin did not discuss the effect of installing an orifice plate with the bevel facing in the upstream direction. It is known that installing a beveled orifice plate backward in an orifice fitting or orifice flange can cause the orifice meter to under-measure the flow rate by from 8% to 25%. Figure 1 shows data for backward facing orifice plates reported by Ting for 4-inch and 6-inch meters and by Witte for a 10-inch meter tube in the MRF. Some 2-inch and 4-inch data from the MRF is also shown. Note that the under-measurement error is reasonably well correlated by the orifice β ratio."

3) Rick Bell's paper, "Causes and solutions to your lost and unaccounted for gas"

cites the 8-25% error (page 6)

4) A 2003 article in Flowcontrolnetwork.com, "Operational Factors that Affect Orifice Meter Accuracy", by T. B. Morrow, D. L. George, and M. G. Nored is no longer posted in the magazine's archives, so here's the relevant part, including the equation used for calculating error:

Backwards-Facing Beveled Orifice Plates

A related topic is the backwards installation of a beveled orifice plate. When an orifice plate is accidentally installed backwards (with the beveled edge upstream and the square edge facing downstream), the pressure drop across the plate is reduced, and the flow rate can be under-registered by as much as 20 percent. Morrow and George, [5] among others, demonstrated that a backwards-facing orifice plate causes a measurement error that is a function of the following.

· Orifice beta ratio, b

· The ratio of orifice bore to plate thickness, e/E

· The ratio of orifice plate thickness to meter tube diameter, E/Dnom

· Reynolds number

Figure 2 compares backwards-facing orifice plate test data from the Gas Technology Institute

Metering Research Facility with results from Ting [6] and Witte. [7]

Morrow and George have proposed the following correlation for a backwards-facing orifice plate to estimate the percentage measurement error as a function of the beta ratio, b, plate thickness to meter tube diameter ratio, E/Dnom, and bore thickness to plate thickness ratio, e/E.

%DQ = -18.93 + 12.91b - 34.04 (E/Dnom) - 8.900 (e/E) + 13.64 (e/E)^2

7) This work appears to be a copyrighted piece, not available on the web.
"Effects of Orifice Plates Installed Backwards and Meter Tube Roughness on Discharge Coefficients"
Author(s): DARIN L. GEORGE, Ph.D., PAUL J. LaNASA, GERALD L. MORRISON, THOMAS B. MORROW
Source: American Gas Association 2001
Organization: American Gas Association www.aga.org
Year Published: 2001

Abstract/Introduction: This paper presents new results of experimental and Computational Fluid Dynamics (CFD) studies on operational effects that influence orifice meter accuracy.

The effects on the orifice discharge coefficient of (1) orifice meter bore thickness, (2) backwardfacing orifice plates, (3) pressure differentials less than 25 inches of water column, (4) roughened meter tube surfaces, and (5) the use of isolating flow conditioners to counteract the effects of meter tube roughness will be discussed.

8) CEESI has a huge bibliography of flow related technical documentation at

but only about 1 of 10 articles are available for download.

I had previously found an article that stated,

"Edge rounding or the plate in backwards has the same general effect with differing magnitudes. The backward plate under registers flow as high as 25%, the rounding of the edge is dependent on the magnitude of rounding" but I have lost its title and the current search engine can't find the text.

Unless you cannot live with the very large percentage of deviation of measured flow to actual flow and also if it is a safety related system that this flow measurement is fed into, it would be wise to shutdown and correct the problem.