Comparison between the hottest ultrasonic flowmete

Comparison between ultrasonic flowmeter and orifice flowmeter

in the engineering practice of long-distance transmission and gathering pipelines in China, orifice flowmeter, especially high-grade orifice valve, has occupied a dominant position for a long time. With the large-scale development of China's oil and gas industry, orifice flowmeter shows its limitations in the measurement of high pressure and large flow, which is increasingly restricted by its own structure. In recent years, based on the success of theory and practice abroad, some new flow meters have also actively participated in the domestic market and achieved a series of successful experience. In particular, ultrasonic flowmeter has obvious advantages in high-pressure and large flow occasions, and has the potential to replace advanced orifice valves. Due to misunderstanding, many people think that ultrasonic flowmeter has good performance but is expensive. Is this the case? We can get a more correct conclusion through a series of comparisons

1 requirements for the use of orifice flowmeter

service conditions, scope of use and requirements for pipelines of orifice flowmeter (flow is proportional to the square of differential pressure):

(1) fluid: it should be a single-phase, homogeneous Newtonian fluid, which will not undergo phase transformation and precipitate impurities when passing through the throttling device, and there should be no adhesion or accumulation of substances in any form in the throttling device

(2) pipeline: it is only applicable to round pipes. There are certain restrictions on the size of pipe diameter. There is a long straight pipe section at the upstream and downstream. Moreover, the roughness and roundness of the inner surface of the straight pipe section 10d at the upstream and 4D at the downstream of the throttling piece must strictly comply with the specific regulations

(3) flow pattern: the flow should be continuous and stable, not pulsating flow; A typical and fully developed velocity distribution (turbulent velocity distribution) has been formed before being affected by the throttling element. The streamline is parallel to the pipe axis and cannot be a rotating flow

2 comparison of technical performance table 1 Comparison of technical performance between ultrasonic flowmeter and orifice flowmeter

technical performance

orifice flowmeter

ultrasonic flowmeter

range ratio

1:

1:

pressure loss

great

none

accuracy

theoretically 1%; Generally 2% on site

generally 0.5%

measuring pulsating flow

inappropriate

suitable

measuring bidirectional flow

no

Yes

measuring wet gas

no

Yes

cleaning pipeline

no

Yes

eddy current effect

sensitive

insensitive

flow rate distribution influence

sensitive

insensitive

repeatability

low

high

process pipeline complexity

High

low

repair and maintenance rate

high

low

one-time investment

high

low

safety

low

high

2.1 range ratio

due to structural characteristics, orifice flowmeter completes measurement through throttling elements, so its range ratio is usually only 1:3, up to 1:10, while the ultrasonic flowmeter has no flow blocking parts, and its range ratio can reach 1:200

these two data show that if a measurement scheme is realized, assuming that its flow range is from 1m3/h to 40m3/h, only one process metering circuit is needed to use ultrasonic flowmeter, and if orifice flowmeter is used, multiple channels are needed to realize

2.2 pressure loss

because the structure of orifice flowmeter has flow blocking parts, and ultrasonic flowmeter does not have flow blocking parts, it is obvious that the pressure loss of orifice flowmeter is very large, and the pressure loss of ultrasonic flowmeter can actually be ignored. The energy consumption of throttling device is calculated as follows:

the following energy consumption is calculated based on a typical user's gas consumption parameter: gas consumption 160 × 104m3/d, with air pressure of 0.6MPa

calculation formula of pressure loss of throttling device: (maximum scale differential pressure 50kPa β = 0.68）

δ P=（.24 β - zero point five two β. sixteen β 3） Δ P

=0.5486 × 50

=27.43kpa

energy consumption calculation formula of throttling device: (compressor efficiency η = 0 8 ）

W= δ p × QV/ η

= 27430 × 18.5185/0.8

=634953w

2.3 accuracy

the metering accuracy of orifice flowmeter can reach 1% in theory, but a lot of practice has proved that due to the poor anti-interference ability of orifice flowmeter, the on-site accuracy can reach 2% at most, which is generally about 3%. The accuracy of ultrasonic flowmeter can reach 0.5% or even higher

2.4 measuring pulsating flow

because the plastic products of orifice flowmeter will help to reduce the weight of the car, the flow measurement is realized by the differential pressure signal before and after the orifice plate, and the differential pressure before and after the orifice plate will be inaccurate by the pulsating flow, so the orifice flowmeter is not suitable for measuring pulsating flow, while the ultrasonic flow meter can measure the intensity of pulsating flow and eliminate its interference, so it is suitable for measuring pulsating flow

2.5 measuring two-way flow

the orifice flowmeter realizes the measurement purpose according to a throttling element, which has strict directionality, so the orifice flowmeter cannot measure two-way flow. Ultrasonic flowmeter is only related to the propagation time of ultrasonic signal in the fluid, so it can measure bidirectional flow

2.6 measuring wet gas

orifice flowmeter is not suitable for measuring wet gas; If the measured gas is wet gas, it is easy to accumulate liquid at the front end of the orifice flowmeter, causing the upstream and downstream differential pressure to change, and the orifice flowmeter measures the flow according to the upstream and downstream differential pressure. If the differential pressure changes, the orifice flowmeter cannot accurately measure the gas flow. The ultrasonic flowmeter has a self-test function. If the measured gas is wet gas, the instrument itself can be modified when it affects the ultrasonic flowmeter. Therefore, the ultrasonic flowmeter is suitable for the measurement of wet gas (the volume component content of wet gas is less than 5%)

2.7 cleaning the metering pipeline

the orifice flowmeter itself has flow blocking parts, and the cleaning ball cannot pass through, so the metering pipeline cannot be cleaned when the orifice flowmeter is installed on the pipeline, and the pipeline can be cleaned only by removing the orifice flowmeter. For ultrasonic flowmeter, there is no such problem

2.8 vortex influence

orifice flowmeter uses the differential pressure method to measure the gas flow. Vortex directly affects the differential pressure at both ends of the orifice plate. Therefore, orifice flowmeter is very sensitive to vortex, and it requires a long straight pipe section to meet the requirements of measurement accuracy. The new international standard ISO 5167 has made higher provisions on the length of the upstream straight pipe section of the orifice flowmeter: the upstream straight pipe section of the orifice flowmeter must be at least 44d, and if there is a header upstream of the orifice flowmeter, the length of the upstream straight pipe section must be at least 145d. (see academic trends and development trends of international flow measurement (China Metrology, 2002) or ISO for details). During the reconstruction of the South mockham natural gas metering station subordinate to the British hydrocarbon source company, the British National Engineering Laboratory (nel) was asked to do a detailed hydrodynamic analysis. After a large number of tests, Nel concluded that the upstream 18D straight pipe section specified in the original iso5167 really could not meet the requirements for measurement accuracy, and the modified ISO regulation on the length of the straight pipe section was necessary

most multi-channel ultrasonic flow meters have the ability to analyze the eddy current strength, which can eliminate the influence of eddy current on flow measurement and are not sensitive to eddy current

2.9 influence of velocity distribution

orifice flowmeter requires uniform velocity distribution during measurement due to the limitation of structural principle. However, due to the complexity of on-site measurement pipeline, the velocity distribution of gas in the pipeline is impossible to be uniform and symmetrical, so orifice flowmeter is very sensitive to asymmetric convection velocity distribution

ultrasonic flowmeter can correct the asymmetry of velocity distribution

2.10 repeatability

for orifice flowmeter, with the wear of orifice edge in the process of use, the accuracy and repeatability of orifice flowmeter will decline, while ultrasonic flowmeter has no pressure loss, no indication drift, and high repeatability

2.11 comparison of process pipeline complexity

for orifice flowmeter, due to the narrow range, more metering pipelines, and the long upstream and downstream straight pipe sections, the on-site process pipeline is complex

the ultrasonic flow measurement range ratio is wide, the upstream and downstream straight pipe sections are short, and the process pipeline is simple

2.12 comparison of repair and maintenance rate

orifice flowmeter has flow blocking parts, which is easy to accumulate liquid in the upstream. For natural gas with high sulfur content, its orifice wear is fast, and the repair and maintenance rate is high

the ultrasonic flowmeter has no movable parts. The ultrasonic probe made of special materials can resist H2S corrosion and is easy to maintain

2.13 comparison of one-time investment

orifice flowmeter has more metering pipelines for the same flow measurement requirements due to its narrow range. Although the investment in direct metering instruments is less, the one-time investment in related valves, temperature transmitters, pressure transmitters, straight pipe sections, manifolds, etc. is more

the single meter price of ultrasonic flowmeter is higher than that of orifice flowmeter, but due to the wide range ratio, the whole metering circuit is less, and the actual on-site one-time investment is less

3 comparison of on-site installation and use (see Table 2) table 2 Comparison of on-site installation and use

orifice flowmeter

ultrasonic flowmeter

straight pipe section requirements

straight pipe section length is required to be more than 44d

first 10d, last 5D

installation impact

strict requirements on concentricity, complex installation

low requirements, flange connection, simple installation

service conditions

service conditions must meet the design.The condition

can be overloaded by 145%, Strong adaptability

(1) the length of the straight pipe section

the length of the straight pipe section on the orifice flowmeter must be at least 44d. If there is a header upstream of the orifice flowmeter, the length of the upstream straight pipe section must be at least 145d. (see academic trends and development trends of international flow measurement (China Metrology, 2002) or ISO for details)

the upstream and downstream straight pipe sections of ultrasonic flowmeter are required to be 10d and 5D (measuring the flow of natural gas with gas ultrasonic flowmeter - national standard gb/t)

(2) influence of installation

for orifice flowmeter, the installation conditions directly affect its measurement accuracy, and the concentricity of on-site installation is highly required

(3) service conditions

due to the principle of orifice flowmeter, its on-site service conditions must be consistent with the design conditions, and the adaptability of pressure and flow is poor

ultrasonic flowmeter has strong adaptability to the site, is insensitive to the fluctuation of pressure and flow, and has strong overload capacity

4 comparison of long-term use

(1) accuracy change

orifice flowmeter due to long-term use, the inlet edge of orifice plate is worn, and the orifice plate is bent and deformed, which will cause the loss of accuracy

ultrasonic flowmeter can maintain high accuracy for a long time due to no wear and no indication drift

(2) influence of dirt

because the orifice flowmeter is made of throttling parts, when used for a long time, dirt will accumulate in the upstream of the orifice, resulting in inaccurate differential pressure signal, which directly affects the measurement accuracy. Dirt and orifice passivation can cause measurement deviation of more than 2 ~ 10%

the ultrasonic flowmeter is a hollow pipe section, and the probe is on the upper part of the instrument. Dirt is not easy to affect the work of the probe, and will not affect the measurement accuracy. Moreover, the flowmeter can detect dirt, correct it, give an alarm, and clean it in time

(3) troubleshooting

since the instrument characteristics of orifice flowmeter depend on the geometry and size of throttling parts, it is necessary to check the throttling parts frequently. Once the throttling parts change, they must be replaced. The service life of throttling parts depends on the composition, flow and pressure of gas

the ultrasonic flowmeter itself has a strong self diagnosis function. Once it is not in normal condition, it will alarm and automatically record the data during the alarm. The service life of the ultrasonic probe is at least 8 years and can be replaced

(4) spare parts

orifice flowmeter