Case studies

1. High flow readings

The output from a rectangular flume open channel flowmeter could be viewed on the computer screen of the site SCADA control system. A trend display of the flow measurement indicated the flow rate regularly went high and often it would go full scale. The functionality and calibration of the ultrasonic flow measurement device had been checked several times and no fault found. It was therefore assumed that the flowmeter was reading correctly and it needed re-ranging to prevent it going full scale. Flowcheck was commissioned to survey the installation and determine if the flowmeter could be recalibrated to bring the high flow rates within range. On surveying the installation it was discovered that at high flow rates the level downstream would rise and the flume would become drowned, consequently the flowmeter read artificially high. The cause of the problem was traced to a partial blockage in an underground section of pipe 30m downstream from the flume. When the pipe was cleared of rags and some heavier debris the flowmeter read correctly without the need to alter its range or calibration.

    2. Error between two flowmeters

    The inlet flow to a wastewater treatment works was monitored at two locations:

    • Downstream from the inlet screens, using a rectangular flume open channel flowmeter
    • At the inlet to the primary settlement tanks, using a closed pipe electro-magnetic flowmeter

    Both meters should have recorded the same flow rates as there were no inlets or outlets between the two locations. Readings between the two flowmeters however, constantly deviated by an average of more than 20%. A survey was conducted on both meters and the following errors found.

    • Although the flume was clean there were heavy deposits of solids in the approach channel
    • The calibration of the ultrasonic device that measured the level and converted it into units of flow was incorrect
    • The calibration factor programmed into the electro-magnetic flowmeter's converter did not match that of its sensor

    The above problems were resolved during the course of the survey and upon completion the deviation between the two meters was negligible.

    3. Pumps deliver insufficient flow

    A wastewater pumping station had recently been refurbished and three new centrifugal pumps had been installed in a duty, assist, standby configuration. There was no flowmeter on the rising main from the pumphouse but it was believed that the pumping station was not delivering the required flow rate. To resolve the problem our Client decided to replace one of the new pump units with a larger pump and starter panel. The estimated cost of the works was £30,000. To obtain the duty requirements for the new pump, to ensure it could be specified correctly, Flowcheck were commissioned to perform a hydaulic survey on the rising main.

    The survey revealed

    • Short sections of new pipe between each pump and the common rising main were generating large friction heads
    • Air entrapment within the assist and standby pumps was restricting the delivered flow rates
    • The new pumps were designed to deliver greater heads than required by the system

    As a result of the hydraulic survey the scope of work on the pumping station was changed

    • Minor modifications were performed on the sections of pipe between each pump and the common rising main
    • Air relief valves were installed on the discharge side of each pump
    • The pump impellers were replaced with a type designed to deliver greater flow rates at lower heads

    The consequence of the revised works was:

    • The existing starter panels and pumps were retained
    • The flow rate delivered by each pump was increased by approximately 50%
    • The pumps operated reliably
    • Considerable savings were made. The remedial works including our survey fee was only 25% of the original budgeted

    4. Fault finding

    A pumping station refurbishment had been completed and was at the commissioning stage. The refurbishment had comprised of installing two new pumps and a new electro-magnetic flowmeter in the common rising main. Upon completion of the installation the new flowmeter indicated that the pumps were not delivering the required flowrate. Prior to delivery the pumps had been fully tested at the factory and were certified to meet the required flow/head duty point. The new flowmeter was also supplied with a manufacturer's calibration certificate. The commissioning engineer for the project had to determine:

    • If the pumps had been correctly specified
    • Was there a blockage in the rising main that was restricting the flow?
    • Was there a fault with the pumps?
    • Was there a fault with the flowmeter?

    Flowcheck were commissioned to perform a survey to identify the source of the problem. Non-intrusive ultrasonic flow measurement equipment was installed on the rising main from each pump and pressure measurement equipment was installed on the pumps' suction and discharge sides. A performance test was carried out on each pump and the flow rate measured on both the ultrasonic flow measurement equipment and on the site electro-magnetic flowmeter.

    From the results it was clear that the pumps were meeting their required duty points but there was a constant deviation between the electro-magnetic flowmeter and the ultrasonic reference meter. A close inspection of the electro-magnetic flowmeter revealed a one digit difference between the serial number on the sensor head and the one on the transmitter. This identified that the sensor head and transmitter installed were not a matched pair and it proved that, following factory calibration, the correct transmitter had been misplaced. Despite the client having a calibration certificate an error had occurred and the wrong transmitter had been installed. Once the source of the problem had been identified the correct transmitter was traced and the problem rectified. The flowmeter then indicated the pumping station was delivering the required flow rate.

    5. Locating a pipe blockage

    After a few years of service a modern effluent pumping station which pumped down a new rising main was failing to deliver the original design flow rates. The pumping station had been installed with two variable speed pumps and even at maximum speed they were unable to deliver sufficient flow. An initial survey revealed the reduced flow rate was due to abnormally high pressure in the main, this clearly indicated a partial blockage. The rising main was several miles long and Flowcheck were employed to identify the section of pipe where the blockage was located. The main was buried throughout its length and could only be accessed in a chamber at the pumping station and at a series of air valve locations.

    To perform the survey we installed a pressure transducer and data logger to a port located in the side of each air valve, we also installed a non-intrusive flowmeter and data logger in the chamber at the pumping station. The data loggers were synchronised to the same time and configured to record at two second intervals. When all the equipment was installed the pumps were run for a period of several hours. Data from the loggers was collected and compiled onto a single spreadsheet. Graphs were then plotted which clearly displayed trend data and any time lags in the system.

    The results revealed there was a gradual blockage over the length of the pipe and the restriction was greatest in the sections of pipe closest to the pumps. This disproved the original theory of a single blockage at a low point in the main. Armed with this information our Client exposed a section of the main and cut out a length of the pipe. On inspection it was found that the effluent had formed a large build up of jelly type sludge which completely covered the bore of the pipe. Once the nature and location of the blockage had been identified the problem was rectified by passing a series of pigs through the main. This cleared the blockage and the pumping station was returned to its original design performance.

    6. Air entrapment

    As part of a large scheme a wastewater pumping station was to be modified to deliver greater volumes of liquid. Flowcheck were commissioned to survey the rising main from the pumping station to ensure it was capable of handling the increased flow rates. From the pressure and flow rates measured, the Ks value (effective roughness of the pipe bore) was calculated; the result was then compared to the recommended Ks value for the type of pipe used. As the figure calculated from the on-site measurements was significantly higher than the recommended value for the type of pipe, it was evident a partial blockage was present within the pipe.

    The survey results also revealed some inconsistencies in the pressure and flow relationship which indicated that the severity of the blockage changed during the test period. From this it was concluded that the most likely cause of the blockage was air entrapment at one or more locations over the length of the rising main. As a consequence of the survey further investigations were carried out and it was discovered that the wrong type of air valves had been installed. The air valves were replaced with the correct type and Flowcheck were commissioned to perform a repeat survey. The second survey revealed the problem had been resolved as the effective Ks value, calculated from the actual measured pressure and flow rates, was much closer to the recommended figure for the type of pipe installed.

    7. WwTW unable to achieve design flow rates

    Following completion of a major capital project that was designed to increase the treatment capacity of a WwTW, it was disputed whether the works was achieving the desired maximum FtFT flow rate. Within the project three new velocity area flow meters had been installed, at locations within the inlet works, to provide flow measurement signals for the flow to works and flow to treatment penstock controllers. Flowcheck were commissioned to perform a flow survey of the inlet works to determine the accuracy of the new flow meters and to establish if the design FtFT flow rates could be achieved.

    To prove the accuracy of the site flow meters we installed a Nivus OCM Pro flow meter in the same channel as each of the three site velocity area flowmeters. Our Engineers also installed level transmitters at key locations around the inlet works and monitored signals from some existing site instrumentation. All the signals were fed to a series of data loggers and the measurements were recorded at one minute intervals over a period of several weeks.

    The first sets of logged data identified that the three site flow meters were very slow to respond to a change in flow rate and that the control penstocks were not responding to the changing loads. Our Engineers modified the set up configuration of the three site flow meters to rectify the response speed problems. Further logging of the site data was then able to confirm the accuracy of the site flow meters by comparison to the Nivus devices.

    Once the performance of the flow meters had been established the survey results revealed problems with the automatic control of the penstocks that governed the flow to works and FtFT. The logged data clearly showed that the two penstocks were operating out of phase with each other and this was resulting in unstable oscillating flow patterns. Consequently sewage was regularly being passed to the storm tanks even during dry weather flow conditions. The source of the problem was traced to the penstock controllers being unsuitable for the type of application they were being used for. Replacement penstock controllers were then installed by the main contractor.

    As a consequence of our survey works the source of the problems were identified and rectified, it was also confirmed that the inlet works could pass forward the design FtFT flow rates.

    8. Storm pumping station survey

    As part of a scheme to prevent raw sewage being pumped to river during storm conditions, a survey was commissioned to determine if a storm pumping station was meeting its design feed forward flow rate. An initial survey of the site, using clamp-on flow meters and data loggers to capture a storm event, had proved unsuccessful due to the pump run duration period averaging only 40 seconds; this was too short to enable the flow meters to stabilise correctly.
    During dry weather conditions there was no liquid in the storm wet well and operating the pumps would not be permitted.

    To facilitate the pump tests it was necessary to install temporary pipework to enable the discharge from each pump to be recirculated back into the inlet wet well. This required major civil works to be undertaken as the recirculation pipe section comprised of approximately 20m of DN500 steel pipe suspended 1m above ground. An electromagnetic flowmeter was installed in the recirculation pipe to measure the flow rate during the pump tests. Each pump had to be tested individually thus enabling two pumps to be left in service in the event of storm conditions occurring during the survey period.

    Pumping station work

    The pumping station dry well was classified as a confined space and entry into it had to be conducted in accordance with confined space entry procedure NC4, non standard entries. This required a minimum team of four trained personnel each time access was needed. To minimise the number of entries, pressure transmitters were installed on the suction and discharge sides of each pump and the signal cables were extended above ground. All the pump test procedures could then be performed from the safe area outside the dry well chamber.

    The outcome of the survey confirmed that each storm pump was capable of meeting its designed feed forward flow rate. Prior to the survey our client had planned to build a new pumping station adjacent to the existing site. By performing the pump tests we were able to prove that the existing pumping station was capable of being utilised within the new sewage pumping scheme and that a new pumping station was not required. Although the cost of the survey works and the installation of temporary recirculation pipework ran into tens of thousands of pounds our client saved in excess of £2 million that had been budgeted for to build a new storm pumping station.

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