An Emerson whitepaper, Fundamentals of Orifice Meter Measurement, shares the history of tried-and-true differential pressure (DP) measurement across an orifice plate (orifice plate flowmeters) to measure flow rate, which dates back to the end of the 1700s.
The first recorded use of an orifice device for fluid measurement was in 1797 by Giovanni B. Venturi, an Italian physicist whose work led to the development of the modern Venturi meter in 1886 by Clemons Herschel.
Orifice plate meters are still widely used today. In a Flow Control magazine article, Using orifice plate flowmeters to their full potential in flow applications, Emerson’s Steve Ifft highlights their popularity:
The orifice plate meter — where an orifice plate provides a restriction to the flow to create the necessary differential pressure (DP) — is one of the most popular and versatile flowmeters on the market today, accounting for 77 percent of all DP installations.
These DP measurement systems consist of the orifice plate, a DP transmitter to measure the high and low side pressures, and calculations to convert this DP in a flow rate to send back to the flow computer or control system.
When specifying orifice plate meter systems, Steve shares four considerations: process characteristics & flow conditions, flow turndown (ratio of maximum flow to minimum flow at or better than the stated accuracy), allowable pressure drop, and the gas fraction in liquid flow or liquid fraction in gas flow. Accuracy is maintained:
…in liquid streams that contain up to 5.5 percent by volume of gas in liquids and up to 2.2 percent by weight of liquids in gas.
He describes the options and their advantages and disadvantages for the orifice holding device which holds the orifice plate: orifice flange union (OFU), single-chamber orifice fitting, dual-chamber orifice fitting. The OFU is the simplest and lowest cost option but is difficult to swap out orifice plates since it requires line depressurization and separation of the pipe flanges. Single-chamber orifice fittings also require line depressurization and provide the ability to:
…precisely locate the orifice plate in the pipe and where there is no need for flange separation.
Dual chamber orifice fittings allow:
…for the removal of a plate under flowing conditions. The dual chambers are separated by a slide valve operated with a gear shaft.
Not all orifice plates come with a single bore. Multi-hole orifice plates, such as the Rosemount 1595 Conditioning Orifice Plate:
…distribute the flow through smaller bores rather than a single large bore. Tests have shown these plates measure accurately without the need for upstream flow conditioners or long, straight pipe sections.
The size of the bore/bores must be based on the range of flows over time. Read the article for more considerations in properly sizing orifice plate meters such as beta ratios, turndowns, maintenance considerations, stacking multiple DP transmitters handling different flow ranges, and impulse piping challenges and solutions.
Visit the DP Flow measurement section on Emerson.com for more on orifice plate primary elements and other technologies and services to meet the needs of your application. You can also connect and interact with other flow measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.