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Error Analysis and Correction of Water Supply Meters

Date: 2019-03-19
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In industrial production and life, in order to ensure the reasonable and safe distribution of water supply load and to provide basic basis for economic accounting, most water supply pipelines are equipped with flow meters for detection.  In the design process, the metering flowmeter is often selected based on the nominal diameter of the pipeline and the average flow rate, which can meet the metering accuracy requirements under normal water supply conditions. However, in the actual operation process, there are various operating conditions, and very large metering errors often occur when the installation is completely standardized, resulting in large metering loss of the water supply unit, while the calibrated accuracy of the metering meter is still within the normal range.  The following is an analysis of several situations encountered by the author in his work on water supply management.


1 Error and Correction of Flow Measurement in Variable Frequency Constant Pressure Water Supply System


With the development of frequency conversion technology, more and more water supply enterprises are applying frequency conversion and constant pressure water supply equipment in water supply projects. Constant pressure water supply has brought remarkable energy-saving benefits to production and life, which is an inevitable development direction.  However, in some water supply areas where the water consumption varies greatly between day and night, the frequency conversion and constant pressure water supply has brought great influence on metering.


A unit has a water supply area, and in 1999, a frequency conversion constant pressure water supply renovation was carried out.  After the renovation is completed, the monthly water consumption of current users in Rizhao is reduced by 15000m3 (before the renovation, the water supply unit uses intermittent water supply, and users set up a constant pressure water tower and reservoir to balance the water consumption).  At first, the water supply unit believed that the water consumption was also reduced due to the decline in the user's production. However, it was still the same in the following months. Most of them used 15000m3/ month less water than before, and the few used 10000m3/ month less water than before. Even if the user's production increased, the water consumption remained unchanged.  Only then did the water supply units gradually realize the seriousness of the problem, and after careful observation and monitoring, they finally found the proof of the problem.


The Dg200 water supply pipeline is used in this water supply area. Vortex flowmeter is used for measurement. Under normal water supply condition, meter error is very small. However, from 23: 00 a.m. every day to 7: 00 a.m. next day, the water consumption for production and living of users in the area suddenly decreases. After observation, when the water consumption of users is less than 80m3/h, the measurement error increases to about -20%, and the water consumption is less than 50m3/h, vortex flowmeter hardly measures, with the maximum error reaching -100%. This is the real reason why the water consumption of users decreases greatly after constant pressure water supply.


It turns out that the range of vortex flowmeter used by water supply unit is too narrow, and the problems were not exposed when centralized intermittent water supply was used before.  After constant pressure water supply, the metering requirements under different flow regimes cannot be met. When the flow rate is small, the metering error is too large, that is to say, the critical flow rate value of the meter is too large, resulting in a large accumulated flow error.


After the reason was found out, the water supply unit made a careful comparison and analysis, and finally chose the ultrasonic flowmeter with no head loss and a speed measuring range of 0.01-12m3/h to replace the original vortex flowmeter, thus fundamentally correcting the measurement error caused by excessive flow change.  After more than two years of operation, the metering has been normal, and the cumulative loss of metering has been reduced by more than 350,000 m3, with remarkable benefits.


2 Measurement Error and Correction for Users with Small Water Quantity


Another example is that a unit has many public buildings with large construction area, but the water consumption is small.  However, in the design process, taking into account the requirements of building fire protection water standards, most water supply pipelines are above Dg100, and metering water meters of the same caliber are also used.  Under the actual normal water supply situation, the user's water consumption deviates greatly from the water supply capacity of the water supply pipe, mostly below the critical flow rate of the meter, and even lower than the minimum flow rate of the meter in some places, resulting in larger metering errors.  To solve this problem, we adopt the following two methods.

(1) shunt water supply

According to the user's daily water load, all the daily water pipelines are reconfigured as required, which not only ensures the normal water demand, solves the problem of large measurement error, but also does not affect the safety of fire fighting water.  There is a big problem in this transformation: the investment is relatively large, and the transformation is also difficult.  It is suitable for construction and renovation with simple water use arrangement, and it is better to design the building in place at the initial stage of infrastructure construction.

(2) reconstruction of large pipes and small watches

When it is very difficult to rebuild by shunt, we sometimes replace the meter with a small caliber meter.  Replacement of gauge diameter shall be calculated to ensure that the flow rate and pressure behind the gauge have little influence and do not violate the fire protection requirements.  In this way, although the measurement error cannot be eliminated fundamentally, the error is also corrected to a large extent.  This kind of transformation must pay attention to the straight pipe sections before and after the transformation of the meter should conform to the installation specifications, otherwise, additional installation errors will occur.


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