Special Section: Flow
Sizing orifice plates
Meeting modern expectations
As common as flow measurements are using orifice plates, there are various thoughts regarding design, application, rules of thumb, and field practice. Factors that can be considered include measurement errors as % full scale, % rate, bias error, ambient temperature induced errors (largely corrected by smart transmitters), and signal-to-noise ratio deteriorating at low flow rates. InTech invites other thoughtful insights on the subject.
By Allan G. Kern, P.E.
Orifice plates with differential pressure (DP) transmitters remain the workhorses of fluid flow measurement in the process industries, due to their proven robustness, ease of use, adaptability to a broad spectrum of applications, familiarity, and economy. The weak side of orifice plates, where otherwise properly applied and installed, is limited turndown, with a nonlinear loss of accuracy at lower flow rates due to the square-root nature of the flow/DP relationship.
With modern instrumentation and today’s more stringent demands regarding material balances, yield and loss accounting, energy management, environmental reporting, and safety systems, users have developed greater expectations and requirements regarding accuracy of their flow measurement systems.
When sizing orifice plates, some new rules of thumb can be applied to significantly improve orifice plate turndown and accuracy, while gaining extended measurement range, in most applications. This can be accomplished for the cost and effort of revising the calculation, buying a new orifice plate, and re-configuring the transmitter, activities that are routinely carried out in any case.
Sources of error
There are many potential sources of error in orifice plate flow measurement. Many of them have been minimized in today’s world or are outside of our control, such as variations in pipe diameter, orifice plate machining tolerances, and standardized flange taps. Modern DP transmitters have high accuracy (ca. 0.1%). The greatest sources of error today will come from temperature deviation from design (for liquids) and temperature, pressure, or specific gravity deviations (for gases). The best practice where these parameters vary from design values is online compensation, utilizing built-in control system functions.
The remaining most common source of error is DP measurement error, whether due to transmitter inaccuracy, static pressure effects at high pressures, or imperfect field installation. The effect of measurement error can be greatly reduced by employing appropriate rules of thumb when “sizing” the orifice plate, i.e., when calculating the orifice size, differential pressure, and maximum flow.
Selecting full-scale DP
The orifice flow measurement error figure shows the effect of a 1-inch DP measurement error on accuracy for three different full-scale DPs (50, 100, and 200 inches of water). The square root nature of the relationship amplifies the effect at lower flow rates, making it essential to avoid this operating region. One way to do this is to size the orifice for a greater full-scale DP, which moves the curves downward into the higher-accuracy region in the figure.
Based on an assumption of a potential 1-inch DP measurement error and a goal of less than 2% resultant error in flow (orifice plates are commonly considered “2% devices”), an orifice plate sized for 50-inches full-scale DP (a common design practice) meets this criteria only above 50% of flow, for a turndown of only 2:1. A full-scale DP of 100 inches (the most common design practice today) meets this criteria above 25% of flow, for a turndown of 4:1. And a full-scale DP of 200 inches (an uncommon practice today) meets this criteria above 10% of flow, for a turndown of 10:1.
Faced with today’s more stringent performance expectations, what does the figure say about reducing this error from 2% to 1%? An orifice plate sized for 50-inches full-scale DP only meets this requirement at near full-scale flow (>90%). A full-scale DP of 100 inches only meets this above 50% of flow, or a 2:1 turndown. And a full-scale DP of 200 inches meets these criteria down to 25% of flow, for a turndown of 4:1.
For any given flow, any of these choices is most likely completely acceptable and would likely go unscrutinized, i.e., in most cases, an orifice can be sized for anywhere from 50 to 200 inches full-scale DP, while staying well within the beta ratio and other guidelines. Consequently, based on an often arbitrary choice, turndown can vary from 2:1 to 10:1, and accuracy from 4% or more to 1% or less.
Selecting maximum flow rate
Another often somewhat arbitrary choice in orifice sizing is the maximum flow rate. As this discussion shows, selecting an unnecessarily high maximum flow rate will compromise accuracy at lower (normal) flow rates, so selecting a maximum flow rate based on infrequent conditions carries an accuracy penalty under normal conditions and should be avoided to the extent possible.
Many users do not realize that with modern smart transmitters, which are configured by the end user, not calibrated, the maximum flow measurement limitation is the upper range limit (URL) of the transmitter (often 200 to 500 inches, depending on the make and model), not the configured upper range value (URV), which is the orifice sizing full-scale DP. This removes the incentive to increase the full-scale DP in order to capture infrequent high flow conditions, since the limitation is the transmitter URL, not the configured URV.
Taking advantage of this can have subtle and initially confusing implications on traditional 4-20 milliamp analog input systems, but on digital systems, such as Fieldbus, the practice is simply to configure the control system “high” scale, or URV, equal to the orifice sizing full-scale DP, and set the control system “extended” scale based on the URL of the transmitter. This practice allows the orifice sizing full-scale DP to be chosen appropriately for normal conditions, thereby maximizing accuracy, while taking advantage of the full measurement range of the transmitter to capture infrequent high flow rates.
Modern safety systems also create increased incentive for orifice plate accuracy. Safety system transmitters are traditionally given a reduced range in order to improve accuracy around the trip setting. But in modern safety systems, design calls for the safety transmitters to have the same range as the control system transmitter in order to provide diagnostic discrepancy alarms.
There are a few caveats to shrinking the orifice and increasing the DP in order to improve accuracy, but they are not usually significant. As mentioned, the beta ratio, which is the ratio of orifice diameter to inside pipe diameter, should remain within the established design range of ca. 0.3 to 0.7. The table shows increasing the full-scale DP from 50 to 200 inches will decrease the beta ratio from ca. 0.60 to 0.45, still well within range on both ends.
Second, there can be an energy penalty for increased permanent pressure loss, which is typically 50–90% of DP. This amounts to ca. 3 PSIG additional loss when switching from 100 to 200 inches full-scale DP, when at full-scale flow. In most cases, this is not significant, and the pressure is lost elsewhere in the process, for example, across a control valve.
Third, with a higher DP, there is the possibility of cavitation or flashing in liquid service. This is not usually an issue and is typically flagged by the orifice sizing software.
New rules of thumb
For greater orifice plate accuracy and turndown, use a larger full-scale DP. Consider using 200 inches as a default, rather than 50 or 100 inches.
Avoid selecting an unnecessarily high maximum flow for sizing. Utilize the capabilities of modern smart transmitters to capture infrequent high flow rates.
Use the figure to gauge if expected accuracy and turndown are satisfactory, or if improvements could be easily captured by selecting a higher full-scale DP.
ABOUT THE AUTHOR
Allan Kern (email@example.com) has 30+ years of experience in process control and has authored numerous papers on multi-variable control, inferentials, decision support systems, safety instrumented systems, distillation control, and other topics, with an emphasis on practical process control effectiveness. Kern is a professional control systems engineer in California, a senior member of ISA, and a member of the InTech editorial advisory board.