Hi,

Can anybody advice me on the below situation.

In one of a project we are using multivariable flow transmitter with dual chamber orifice plate.

The measuring flow dp range is 0-100 inch of WC and flow is 0 to 5 MMSCFD. The upstream pressure of the orifice is 350 psig.
We sized the orifice with beta ratio less than 6.

With the above situation, could any body advice the low cut off DP value?

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Is there a standard for your industry or application?

The United States' Bureau of Land Management has a standard (and API has a standard):

<quote>Unless otherwise approved by BLM, the low flow cutoff shall not be set higher than 1.5 times the manufacturer's basic uncertainty specification for the differential pressure transducer/transmitter, expressed in inches of water column, or 0.5", whichever is less* (Note: This is an additional requirement to API 21.1.4.2.3).

Example 2: What is the maximum allowable low-flow cutoff for an EFC with a 0-300" multivariable transmitter, spanned down to 0-200"? The performance specifications for this EFC state that the differential pressure accuracy is ±0.1% FS (full span).

Answer: The span in this example is 0-200", which gives an accuracy of ±0.2" (±0.1% of 200"). Therefore, the maximum allowable value for the low-flow cutoff is 1.5 times 0.2", or 0.3".

from:
UNITED STATES DEPARTMENT OF THE INTERIOR BUREAU OF LAND MANAGEMENT
Notice To Lessee/Operators of Onshore Federal and Indian Oil and Gas Leases Within the Jurisdiction of the Wyoming State Office
(NTL 2004-1)
Standards for the Use of Electronic Flow Computers Used On Differential Type Flow Meters For Gas Measurement

http://www.blm.gov/pgdata/etc/media...y/og/ogdocs.Par.7786.File.dat/04wy-efcntl.pdf

Example 2: What is the maximum allowable low-flow cutoff for an EFC with a 0-300” multivariable transmitter, spanned down to 0-200"?

The performance specifications for this EFC state that the differential pressure accuracy is ±0.1% FS (full span).

Answer: The span in this example is 0-200", which gives an accuracy of ±0.2" (±0.1% of 200").

Therefore, the maximum allowable value for the low-flow cutoff is 1.5 times 0.2", or 0.3".

The API standard:

Manual of Petroleum Measurement Standards
Chapter 21-Flow Measurement
Using Electronic Metering Systems
Section 1-Electronic Gas Measurement
FIRST EDITION, SEPTEMBER 1993

1.4.2.3 Low Flow Detection
A low flow cutoff point for differential meters should be determined by the contractually concerned parties based upon a realistic assessment of site conditions.

I've seen low flow cutoffs as high as 10% of max flow.

 

> Is there a standard for your industry or application?

> The United States' Bureau of Land Management has a standard (and API has a standard):
---- snip ----

Thanks for valuable advice. I got some how guideline to resolve the issue.