Oil and Gas

Five reasons orifice meters lead the way

Modern flowmeters are a network node too.

By Steve Ifft July 25, 2019

While the last few years saw significant innovations across the oil & gas and flow industries, some older technologies continue to stand the test of time.

One such is differential pressure (DP) flowmeters, which remain the most widely used flow technology in industrial process measurement and control, and natural gas custody transfer measurement applications.

The most common form of DP measurement is done with orifice plates that consist of an orifice with a concentric round hole machined to a specific size. Like all DP meters, the orifice plate meter cross-references the physical laws of conservation of mass and energy. The orifice provides the restriction to flow, while pressure taps on the upstream and downstream side of the restriction provide the necessary differential pressure. Figure 1 shows a schematic of a traditional orifice plate installation.

Here are five reasons why orifice meters lead the way in DP and flow measurement and why they are as relevant today as they were years ago, particularly through the integration of modern networking innovations.

1. Highly accurate measurements

First, orifice plate meters provide accurate measurements.

With mass flow rate ─ e.g. million standard cubic feet per day (mmscfd) ─ being the primary output of a flowmeter, flow rate uncertainty is one of the most important performance specifications for DP meters. Here, orifice meters have a mass flow rate uncertainty in the range of ±0.7% of rate as shown in the American Petroleum Institute (API) orifice meter standard (API 14.3 [1]), which compares favorably to heavily calibrated turbine, ultrasonic and vortex meters.

One area not fully understood around orifice meters concerns turndown ratio – the ratio of the maximum to minimum flow rate within a stated flow measurement uncertainty. Some years ago, when a manometer tube indicated the DP and, prior to modern instrumentation, orifice meters had a flow rate turndown limited to 3:1. Today’s advanced digital DP transmitters used with orifice meters make accurate 14:1 flow turndown possible with a single orifice plate and single transmitter.

2. Ease of use & maintenance

Orifice meters are easy to use and maintain and therefore cost-effective. They incorporate readily available instruments to read differential pressures and meet design requirements. Orifices can be sized using standards developed by the American Gas Association (AGA), the American Petroleum Institute (API) or the International Organization for Standardization (ISO). These standards provide requirements for every part of an orifice plate measurement. Orifice meter calculations are embedded in most flow computers. The details of orifice meter equations more widely known and understood.

No need for calibration exists. Meter performance is reproducible so standards can predict the discharge coefficient to low uncertainty without calibration. Hence, the uncalibrated orifice meter has a gas mass flow rate uncertainty of ±0.7% of rate. Other competing meters require calibration to reach such uncertainty levels.

3. All-conditions operation

Orifice plate meters operate in varying, and often harsh, conditions and over large flow ranges. The ability to switch the orifice plate to one of a different beta gives the meter the rare ability to be used long-term over large flow ranges — either when there is a significant reduction in flow or whenever operations can cause flows to surge and exceed the maximum meter flow rate specification.

The dual chamber orifice meter design allows a plate to be replaced in minutes, without taking the flow offline (see Figure 2), enabling the meter to be reconfigured in service to match long term changing flow conditions.

Another recent advance is that of multi-hole orifice plates combining orifice plates and flow conditioners to create an orifice measurement device with short upstream piping installation requirements. Multi-hole orifice plates have traditional single hole orifice plates but distribute the flow through several smaller bores rather than a single large bore.

Orifice plate meter technology also overcome long or blocked impulse lines, using integrated designs that mount the transmitter to the primary element body. Impaired signals due to condensation in gas system impulse lines or gas bubbles in liquid lines are addressed through heat-traced impulse tubing, sediment traps and blowdown valves.

4. Compliance standards met

Orifice plate meter use is approved by the main regulatory, standards and industry organizations for the custody transfer of gases.

Orifice plates must be in strict compliance with standards for lower uncertainty developed by organizations, such as ISO and API. The international standards that apply are AGA Report No. 3, API 14.3 and ISO 5167 Part 2 with natural gas applications. Other standards also cover common applications such as steam, liquid, gas, clean fluids, and other standards industrial applications.

5. Networking innovations

Orifice meters evolve to meet measurement challenges through the embracing of digital, networking and industrial internet of things (IIoT) innovations.

Orifice meters today are integrated within the plant, the generated flow information being actionable and shareable across different domains.

While the orifice meter has at its core a simple technology premise, it may have one of the most modern, comprehensive and easy-to-understand diagnostic suites of all flowmeters.

Emerson Rosemount orifice plate flowmeters include a number of technology innovations. This includes advanced electronics that integrate the different components (flow computer, primary element, DP transmitter, manifold, etc.) required for traditional mass flow measurement and negate the need for separate flow computers or distributed control systems (DCS) calculations for measuring mass flow.

Orifice meters today remain one of the most advanced and versatile flowmeters on the market, embracing the latest digital and networking innovations and providing actionable business intelligence across plants for closed loop control, general purpose monitoring and custody transfer applications (among other applications). And the best is yet to come.

Steve Ifft
Author Bio: Steve Ifft is product line director, Daniel Differential Products, Emerson.