Why do large-diameter orifice flow meters need online verification or calibration?

Most large-diameter orifice flow meters are installed in water supply mains, and it is difficult to get an opportunity to stop the water supply for inspection. Orifice flow meters are inconvenient to disassemble, and most of them are installed in underground instrument wells and cannot even be disassembled. However, they are important parameters for calculating water plant production, so verification or calibration issues are particularly important.

As we all know, the effectiveness of the flow meter is not only related to the orifice flow meter itself, but also to its installation environment and usage conditions. Sending it for inspection can only solve the problem of the orifice flow meter itself. Only online verification or calibration can solve this problem. Therefore, online verification or calibration of large-diameter orifice flow meters is of great significance.

1. Solve the problem that most orifice flow meters in use cannot be sent for inspection

The difficulty in sending large-diameter orifice flowmeters for inspection is a problem that needs to be solved urgently. This problem is particularly prominent in orifice flowmeters. However, at present, the proportion of large-diameter pipelines using orifice flowmeters is as high as over 70%. As water charges increase year by year and accounting becomes increasingly detailed, orifice flowmeters have undoubtedly become the focus of people's attention and have also put great pressure on metering staff. Everyone earnestly hopes that the problem of online calibration of large-diameter orifice flowmeters can be solved as soon as possible.

2. Eliminate additional errors caused by unsatisfactory installation and use conditions

There are two necessary conditions for a large-diameter orifice flowmeter to truly achieve accurate flow measurement: first, the accuracy and reliability of the orifice flowmeter itself must be good; second, the installation environment of the orifice flowmeter must meet the requirements of the specifications.

The current reality is that people have very high requirements for the accuracy of orifice flowmeters, but not enough consideration for reliability, and even less attention to installation and use conditions. People often spend a lot of money to buy highly accurate flowmeters, but because they ignore the adverse effects that the installation and use conditions of the flowmeter may have on its accuracy, the actual use effect is very poor, and many people even doubt the orifice flowmeter itself. This problem cannot be solved by simply removing the orifice flowmeter for inspection. On-site calibration is required to eliminate the additional errors caused by unsatisfactory installation and use conditions.

According to the information we have, more than 60% of the large-diameter orifice flowmeters in use have unsatisfactory installation and use conditions, and most of the additional errors caused by unsatisfactory installation and use conditions may be greater than the accuracy of the orifice flowmeter itself.

3. Complexity of the impact of flow meter installation environment and use conditions on flow meters

At present, there are two common problems with the installation environment and use conditions of large-diameter orifice flowmeters. One is that the minimum requirements for the front and rear straight pipe sections given in the instrument manual are rigidly applied, and the orifice flowmeter is installed in the unstable flow field area that is easily affected by the front and rear resistance parts; the other is that the design capacity does not match the actual flow, and a large pipe with a small flow often occurs.

The installation conditions of large-diameter orifice flowmeters are difficult to explain with one or two data. For example, if an orifice flowmeter is installed after a valve, the possible changes in its influence include: different types of valves, different valve openings, different flow points, and other combinations of conditions. Therefore, it should be ensured that the orifice flowmeter has as long a straight pipe section as possible, and do not take the simple reference data given in the manual as the so-called golden rule.

Large-diameter orifice flow meters used in the water supply industry are almost all differential pressure flow meters. Their principles are based on the functional relationship between point or line flow velocity and surface flow velocity under ideal flow field conditions. Unstable flow field or too low flow velocity will greatly reduce the accuracy of measurement.

4. Measurement uncertainty analysis

How to analyze the uncertainty of flow measurement in the online calibration of large-diameter orifice flowmeters is a question that we all care about. Both international standards and national specifications require that the calibration report should give an explanation of the uncertainty of the measurement results. Recently, experts from the National Institute of Metrology who are engaged in flow measurement have compiled the book "Analysis of Uncertainty in Flow Measurement". As a general teaching material for the national metrology technical regulations, it is currently a monograph guiding the orifice flow measurement industry in China to conduct uncertainty assessment. The book is based on the actual characteristics of the flow profession and is highly practical. Whether it is various flow standard devices or various flow meters, it covers them all, and is a rare good reference book in the flow industry.