Comparison of ultrasonic flow meters and orifice plate flow meters and application of ultrasonic flow meters in petrochemical metering

Abstract: This paper elaborates on the advantages and disadvantages of ultrasonic flow meters and orifice plate flow meters through extensive theoretical derivations and credible application examples, comparing them in terms of technical performance, on-site installation and use, long-term use, and economy. It proposes that ultrasonic flow meters should be given priority for high-pressure and high-flow-rate measurement, and describes the time-difference method for ultrasonic flow meter measurement of finished oil and the ultrasonic flow calibration technology for standard volume tube actual flow.
Foreword
In my country's long-distance and gathering pipeline engineering practice, orifice plate flowmeters, especially advanced orifice plate valves, have long held a dominant position. However, with the large-scale development of my country's oil and gas industry, orifice plate flowmeters are increasingly showing their limitations due to their own structural constraints in high-pressure, high-flow-rate measurement. In recent years, some new types of flowmeters, based on theoretical and practical successes abroad, have actively entered the domestic market and achieved a series of successful experiences. In particular, ultrasonic flowmeters have significant advantages in high-pressure, high-flow-rate applications and are poised to replace advanced orifice plate valves. Due to a misconception, many people believe that ultrasonic flowmeters have good performance but are expensive. Is this really the case? Through a series of comparisons, we can arrive at a more accurate conclusion.
1. Usage requirements of orifice plate flow meters
The operating conditions, application range, and pipeline requirements for orifice plate flow meters (flow rate is proportional to the square of the differential pressure):
(1) Fluid: It should be a single-phase, homogeneous Newtonian fluid that does not undergo phase change or precipitate impurities when passing through the throttling device, and there should be no form of material adhering or accumulating in the throttling device.
(2) Pipeline: Only applicable to round pipes, with certain restrictions on pipe diameter, long straight pipe sections upstream and downstream, and the inner surface roughness and roundness of the 10D upstream and 4D downstream straight pipe sections of the throttling device must strictly comply with specific regulations.
(3) Flow pattern: The flow should be continuous and stable, not pulsating; a typical and fully developed velocity distribution (turbulent velocity distribution) should be formed before being affected by the throttling element, and streamlines should be parallel to the pipe axis, and it should not be a rotating flow.
2. Comparison of technical performance (see Table 1)

Table 1. Comparison of technical performance between ultrasonic flow meters and orifice plate flow meters

2.1 Range Ratio
Due to its structural characteristics, the orifice plate flow meter completes the measurement by using a throttling element, so its range ratio is usually only 1:3, and can reach up to 1:10. In contrast, the ultrasonic flow meter has no flow obstruction element, and its range ratio can reach 1:200.
These two data points indicate that if a measurement scheme is to be implemented, assuming its flow range is from 1 m³/h to 40 m³/h, only one process metering loop is needed when using an ultrasonic flow meter, while multiple loops are required when using an orifice plate flow meter.
2.2 Pressure Loss
Because orifice plate flow meters have flow obstructions in their structure, while ultrasonic flow meters do not, it is obvious that the pressure loss of orifice plate flow meters is very large, while the pressure loss of ultrasonic flow meters is negligible. The energy consumption calculation for the throttling device is as follows:
The following energy consumption calculation is based on the gas consumption parameters of a typical user: gas consumption of 160 × 10⁴ m³/d and gas pressure of 0.6 MPa.
Formula for calculating pressure loss of throttling device: (maximum differential pressure 50 kPa, β = 0.68)
δP=（1-0.24β-0.52β2-0.16β3）ΔP
=0.5486×50
=27.43kPa
Energy consumption calculation formula for throttling device: (compressor efficiency η = 0.8)
W = δp × QV / η
= 27430 × 18.5185 / 0.8
=634953W
2.3 Accuracy
Theoretically, orifice plate flow meters can achieve a metering accuracy of 1%. However, extensive practical experience has shown that due to their poor anti-interference capabilities, the highest on-site accuracy is only 2%, typically around 3%. Ultrasonic flow meters, on the other hand, can achieve an accuracy of 0.5% or even higher.
2.4 Measurement of pulsating flow
Since orifice plate flow meters measure flow based on the differential pressure signal across the orifice, pulsating flow can cause inaccurate differential pressure across the orifice. Therefore, orifice plate flow meters are not suitable for measuring pulsating flow. However, ultrasonic flow meters can measure the intensity of pulsating flow and eliminate its interference, making them suitable for measuring pulsating flow.
2.5 Measurement of bidirectional flow
Orifice plate flow meters achieve their measurement purpose based on a throttling element, which has strict directionality; therefore, orifice plate flow meters cannot measure bidirectional flow. Ultrasonic flow meters, on the other hand, depend only on the propagation time of the ultrasonic signal in the fluid, and thus can measure bidirectional flow.
2.6 Measurement of Humidity Gas
Orifice plate flow meters are not suitable for measuring humid gases. If the gas being measured is humid, liquid can easily accumulate upstream of the orifice plate flow meter, causing changes in the differential pressure between the upstream and downstream sides. Since orifice plate flow meters measure flow rate based on this pressure difference, changes in the differential pressure will prevent the meter from accurately measuring the gas flow rate. Ultrasonic flow meters, on the other hand, have a self-detection function. If the gas being measured is humid and affects the ultrasonic flow meter, the instrument itself can correct for this. Therefore, ultrasonic flow meters are suitable for measuring humid gases (humid gas volumetric composition content less than 5%).
2.7 Cleaning the metering pipeline
Orifice plate flow meters have flow obstructions that prevent cleaning balls from passing through. Therefore, when an orifice plate flow meter is installed on a pipeline, it cannot clean the metering line online; the pipeline can only be cleaned by removing the orifice plate flow meter. Ultrasonic flow meters, however, do not have this problem.
2.8 Eddy Current Influence
Orifice plate flowmeters measure gas flow using the differential pressure method. Eddy currents directly affect the differential pressure across the orifice plate, making orifice plate flowmeters highly sensitive to eddy currents and requiring long straight pipe sections to meet measurement accuracy requirements. The new international standard ISO 5167 has raised the requirements for the length of the upstream straight pipe section of orifice plate flowmeters: the upstream straight pipe section must be at least 44D; if a manifold exists upstream of the orifice plate flowmeter, the length must be at least 145D. (See "International Flow Measurement Academic Dynamics and Development Trends" (China Metrology, 2002) or ISO 5167-2). During the renovation of the South Mocam gas metering station under the UK Hydrogen Company, the National Engineering Laboratory (NEL) conducted a detailed fluid dynamics analysis. NEL concluded through extensive testing that the original ISO 5167 requirement of an upstream 18D straight pipe section was indeed insufficient to meet the measurement accuracy requirements, and the revised ISO 5167-2 requirement for the length of the straight pipe section was essential.
Most multi-channel ultrasonic flow meters have the ability to analyze the intensity of eddies, which can eliminate the influence of eddies on flow measurement and make them insensitive to eddies.
2.9 Influence of Flow Velocity Distribution
Due to the limitations of its structural principle, orifice plate flow meters require a uniform flow velocity distribution during measurement. However, due to the complexity of the metering pipeline in the field, the flow velocity distribution of gas in the pipeline cannot be uniform and symmetrical. Therefore, orifice plate flow meters are very sensitive to asymmetrical flow velocity distribution.
Ultrasonic flow meters can correct for asymmetrical flow velocity distribution.
2.10 Repeatability
For orifice plate flow meters, the accuracy and repeatability decrease as the edge of the orifice plate wears during use. In contrast, ultrasonic flow meters have no pressure loss, no indication drift, and high repeatability.
2.11 Comparison of Process Piping Complexity
For orifice plate flow meters, the narrow range ratio, numerous metering pipelines, and long upstream and downstream straight pipe sections make the on-site process pipelines complex.
Ultrasonic flow meters have a wide range, short upstream and downstream straight pipe sections, and simple process piping.
2.12 Comparison of Repair and Maintenance Rates
Orifice plate flow meters have flow obstruction components, which can easily lead to liquid accumulation upstream. For natural gas with high sulfur content, the orifice plate wears quickly, resulting in a high maintenance rate.
Ultrasonic flow meters have no moving parts, and the ultrasonic probe made of special materials can resist H2S corrosion, making maintenance simple.
2.13 Comparison of One-Time Investments
Because of their narrow range, orifice plate flow meters require more metering pipelines for the same flow measurement requirements. Although the direct investment in metering instruments is low, the initial investment in related valves, temperature transmitters, pressure transmitters, straight pipe sections, manifolds, etc. is high.
While ultrasonic flow meters are more expensive than orifice plate flow meters, they have a wider range and require fewer metering loops, resulting in lower initial investment in actual field operations.
3. Comparison of on-site installation and use (see Table 2)

Table 2 Comparison of on-site installation and use

(1) Length of straight pipe section
The straight pipe section of the orifice plate flowmeter must be at least 44D. If there is a manifold upstream of the orifice plate flowmeter, the length of the upstream straight pipe section must be at least 145D. (See "International Academic Dynamics and Development Trends in Flow Measurement" (China Metrology, 2002) or ISO 5167-2).
The upstream and downstream straight pipe sections of the ultrasonic flow meter are required to be 10D and 5D respectively (GB/T 18604-2001, "Measuring the Flow Rate of Natural Gas with Ultrasonic Gas Flow Meters").
(2) Impact of installation
For orifice plate flow meters, the installation conditions directly affect their metering accuracy, and the concentricity requirements for on-site installation are very high.
(3) Conditions of use
Because of the principle of orifice plate flowmeters, their field use conditions must match the design conditions, resulting in poor adaptability to pressure and flow.
Ultrasonic flow meters are highly adaptable to different environments, are insensitive to fluctuations in pressure and flow, and have strong overload capacity.
4. Comparison of long-term use
(1) Precision variation
Over time, the orifice plate flow meter will lose accuracy due to wear at the inlet edge of the orifice plate and bending deformation.
Ultrasonic flow meters can maintain high accuracy for a long time because they are wear-free and do not exhibit reading drift.
(2) The impact of dirt
Because orifice plate flowmeters use throttling elements, dirt and grime will accumulate upstream of the orifice plate during long-term use, causing inaccurate differential pressure signals and directly affecting measurement accuracy. Dirt and orifice plate passivation can cause measurement deviations of 2% to 10% or more.
The ultrasonic flow meter has a hollow pipe section and the probe is located at the top of the instrument. Dirt and dirt are not likely to affect the operation of the probe and will not affect the measurement accuracy. Moreover, the flow meter can detect dirt and dirt, correct and alarm, and clean in a timely manner.
(3) Troubleshooting
Since the instrument characteristics of an orifice plate flowmeter depend on the geometry and size of the throttling element, the throttling element needs to be checked frequently. Once the throttling element changes, it must be replaced. The lifespan of the throttling element depends on the composition, flow rate, and pressure of the gas.
The ultrasonic flow meter itself has a strong self-diagnostic function. It will alarm if it is not in normal condition and automatically record the data during the alarm period. The ultrasonic probe has a service life of at least 8 years and can be replaced online.
(4) Spare parts
Orifice plate flow meters require multiple sets of throttling elements because the throttling elements are frequently worn and deformed; ultrasonic flow meters only require one set of probes, which can be used interchangeably.
(5) Routine maintenance
Orifice plate flow meters require frequent maintenance and inspection of parameters such as the geometry of the throttling element. Replacing the orifice plate online is difficult to guarantee against leakage, leading to inaccurate differential pressure and compromised metering accuracy. Ultrasonic flow meters, on the other hand, are maintenance-free and have robust self-diagnostic capabilities.
(6) Mandatory inspection cycle
Orifice plate flow meters are inspected annually, typically using a geometric calibration method. Ultrasonic flow meters are inspected every three years and can be calibrated online.
5. Economic Comparison
5.1 One-time investment
Because of its wide range, the ultrasonic gas flow meter simplifies process piping compared to an orifice plate flow meter, reducing the need for related pressure regulating equipment, valves, transmitters, etc., thus significantly saving investment.
In addition, because ultrasonic flow meters require short upstream and downstream straight pipe sections, they greatly reduce the floor space required.

Figure 1. Comparison of occupancy (space) and cost between orifice plate flow meters and ultrasonic flow meters.

As shown in Figure 1, if the straight pipe section of the orifice plate flowmeter is calculated as 44D, to complete the measurement task under the conditions of Q=48000m3/h, T=40°C, and Pmin=30Bar, the orifice plate flowmeter requires four 24-inch process lines, eight valves, and a floor area of ​​333 square meters. In contrast, using an ultrasonic flowmeter only requires two 30-inch process lines, four valves, and a floor area of ​​94 square meters. The South Morecambe gas station, a subsidiary of HRL (Hydrogen Hydrogen Resources Limited), has a daily gas transmission capacity of up to 50 million N m3/h. This station initially used six orifice plate flowmeters in 1985, but during the 1998 upgrade, three ultrasonic flowmeters replaced the orifice plates. Practice has proven that ultrasonic flowmeters have significant advantages over orifice plate flowmeters in terms of measurement accuracy, maintenance costs, and daily operation.
5.2 Long-term operating costs
Due to the long-term operation of flow meters, there are the following costs: equipment maintenance costs, calibration costs, replacement parts costs, and operator labor costs.
(1) Equipment maintenance costs
Because ultrasonic flow meters are maintenance-free instruments, while orifice plate flow meters require frequent maintenance, the maintenance costs of the latter are much higher than those of the former.
(2) Calibration Fee
Calibrating the differential pressure transmitter for an orifice plate flowmeter requires significant manpower and resources. Furthermore, the orifice plate flowmeter also needs a density meter, requiring annual calibration, which adds to the calibration cost. In addition, although orifice plate flowmeters can be calibrated using a geometric dry calibration method, it is difficult to guarantee concentricity when reinstalling them after calibration.
(3) Orifice plate flow meters require frequent replacement of throttling elements, which increases costs; ultrasonic flow meters do not require replacement of parts, the probe can be replaced and cleaned under pressure, and other parts basically do not need to be replaced.
(4) Because the pressure loss of the orifice plate flow meter is very large, it increases the load on the compressor. As proven by years of practice, the operating cost increases with the increase of pressure loss.
(5) The labor cost of operators is much higher for orifice plate flow meters, which require maintenance and switching operations. Ultrasonic flow meters are unattended and require almost no maintenance. Therefore, the cost of operating and maintaining ultrasonic flow meters is much lower than that of orifice plate flow meters.
6. Application of ultrasonic flow meters in petrochemical metering
6.1 Time-of-flight ultrasonic flow meter for measuring refined oil
An ultrasonic flow meter is a derivation-based flow meter, and its basic principle is shown in Figure 2. The time it takes for a high-frequency acoustic pulse to travel from upstream transducer A to downstream transducer B is...
T1 = L/(C + Vcosθ)
The time it takes for the signal to travel from downstream transducer B to upstream receiving transducer A is...
T2=L/(C-Vcosθ)
In the formula, L is the distance between the sound channels; C is the speed of sound in the liquid; θ is the angle between the sound channel and the pipe axis; and V is the average axial velocity in the pipe.
From the above formula, we can derive
V=(L/2cosθ)*[(T2-T1)/(T1*T2)]

Figure 2. Basic principle diagram of time-difference ultrasonic flow meter

Theoretically, this measurement method can only measure the fluid velocity in the sound channel. We want to obtain the average flow velocity at the measured cross-section of the pipe, but in actual pipeline operation, there are always manifolds, reducers, tees, and elbows, which inevitably generate asymmetric flow, transverse flow, and vortex flow at the measured cross-section. This significantly reduces the accuracy of the average flow velocity measurement. Intelligent ultrasonic flow meters employ multi-channel measurement and monitor the flow meter's operating status through the diagnostic and alarm functions of intelligent calibration software; adjust the automatic gain of each channel; measure the flow velocity distribution of each channel and calculate the sound velocity of each channel to correct for transverse flow, vortex flow, and asymmetric flow; when the density and viscosity of the measured liquid change, the density and viscosity of different oils can be deduced by measuring the sound velocity. Thus, ultrasonic flow meters can replace density meters, solving the problem of handling mixed oil sections.
6.2 Standard Volume Tube Actual Flow Calibration Ultrasonic Flow Rate Technology
A bidirectional standard volume tube is a standard volumetric mechanical device equipped with two (or four) detection switches and a drain ball or piston at the inlet and outlet of a U-shaped standard tube section. When the first detection switch is activated, the drain ball enters the standard section; when this ball activates the second detection switch at the other end of the U-shaped tube, the drain ball leaves the standard section.
The measurement principle of ultrasonic flow meters dictates a fixed time delay between the pulse and the actual flow velocity (flow rate). However, fluid disturbances in a pipeline are complex, including multiple eddy currents and non-axial velocity components. The ultrasonic flow meter detects and calculates the fluid velocity along one or more sampling channels by utilizing the forward and reverse time differences between the ultrasonic transmitter and receiver. For these reasons, the following characteristics should be considered by design engineers when verifying the stability of an ultrasonic flow meter:
(1) In order to control the inaccuracy of the instrument coefficient of the ultrasonic flow meter under test within ±0.027%, it is recommended that the design engineer consider the nominal diameter and calibration operation number of the ultrasonic flow meter in the design of the metering system to select the standard volume of the bidirectional standard volume tube. The relationship is shown in Table 3.

Table 3 Recommended Standard Volume of Volumetric Tubes

(2) During each calibration process, before the discharge ball triggers the first detection switch, it begins to record the pulse signal emitted by the ultrasonic flow meter under test. The medium flow rate should be kept stable at this flow point of the ultrasonic flow meter under test. Otherwise, it may cause poor repeatability of the ultrasonic flow meter or systematic error of the flow meter gain.
(3) The four-way reversing valve should be in a stable flow and pressure state when the reversing medium enters the U-shaped pipe section. If the drain ball is driven into the standard volume tube detection switch, a disturbance flow will be generated directly before the drain ball triggers the detection switch. This will greatly reduce the instrument coefficient and repeatability of the ultrasonic flow meter.
7. Conclusion
In summary, ultrasonic gas flow meters offer significant advantages over orifice plate flow meters in terms of safety, technical performance, initial investment, and long-term operating costs. For illustrative purposes, this article uses relatively large gas consumption volumes for comparison in its calculations and examples. Practical calculations generally show that ultrasonic gas flow meters are more advantageous for flow meters with a diameter of DN200 or larger. For flow meters with a diameter of DN150 or smaller, orifice plate flow meters are more economical due to the inherent price of ultrasonic gas flow meters. However, to ensure measurement accuracy, the use of more precise metering instruments is recommended.