The City of Charleston considered a number of flowmeter technologies to help monitor flow through a 32 inch ductile iron discharge line at the Patrick Street Lift Station before selecting a transit-time ultrasonic flowmeter for the application.
The Patrick Street station went into service in 1956. Originally there were four 100 horsepower pumps installed with constant speed motors. Variable frequency drives were installed in 1986. In 1995, the city's Sanitary Board began an upgrade program at the station.
As part of the program, the city purchased and installed a doppler flowmeter for the 32 inch line. However, it became apparent very quickly that the meter was not able to operate throughout the entire range of service. It behaved erratically and had difficulty maintaining accurate and repeatable performance. Repeated attempts to rectify the situation failed to solve the problems.
As a result, the Sanitary Board began an investigation to find a replacement. A number of different flowmeter technologies were considered including propeller, magnetic and transit-time ultrasonic.
The city decided to go with a transit-time flowmeter, in part due to its clamp-on, non-intrusive nature. The non-intrusive design eliminated the concern of mechanical parts wearing and fouling. Since the meter's transducers remain external to the pipe, there is no associated pressure drop. In addition, the cost of the clamp-on transit-time flowmeter was relatively low in comparison to a 32 inch magmeter and could be installed easily with little expense.
A transit-time flowmeter uses the effect of the liquid's flow velocity on an ultrasonic signal to calculate flowrate. The meter uses a minimum of one pair of transducers that act as transceivers. The transducers are placed on the pipe with one transducer located upstream of the other.
The upstream transducer sends an ultrasonic signal to the downstream transducer and the signal travel time is measured by the flowmeter's internal computer. Next, the downstream transducer sends a signal to the upstream transducer and this signal travel time is then measured. When there is flow in the pipe, the liquid's flow velocity speeds up the signal moving downstream, reducing its travel time, and slows down the upstream signal, increasing its travel, creating a time difference between to the two signals. This time difference is proportional to the liquid's flow velocity. Based on the known pipe inner diameter, the instantaneous flowrate is calculated.
Contrary to what still is commonly believed, state-of-the-art transit-time flowmeters are suitable for service on liquids that contain suspended solids and/or entrained gases. Historically it was thought that transit-time flowmeters were limited to liquid applications where there was little to no solids or bubbles. Advancements in the design of these meters over many years now permit the application of this flowmeter technology to liquids that, in some cases, may have a relatively high level of solids and bubbles.
Some of these improvements include wide beam transducer technology, variable frequency transmission and phased based signal discrimination. As illustration of this improvement, it is interesting to note that transit-time flowmeters now routinely are applied to virtually all full pipe applications found in a wastewater treatment plant.
Charleston decided to evaluate transit-time flowmeters from three different manufacturers. Each manufacturer was required to install a meter for a period of five days of continuous satisfactory operation. During this period, each meter was subjected to the complete range of operational conditions including periods of high aeration as a result of cavitating pumps and low wet well levels.
After an analysis of the evaluation data, it was determined that the Controlotron System 1010N provided the required performance throughout the five day period. The other meters tested were not able to operate at all times due to the high aeration level.
The Controlotron meter was able to handle the high level of aeration due to a signal detection method designed specifically to tolerate high levels of entrained gases or suspended solids.
As a result of the flowmeter-testing program, the city purchased and installed the Controlotron 1010N transit-time flowmeter. The meter was installed in 1996 and has been in continuous operation since that time. The meter has provided accurate and repeatable performance and allowed the city to solve its flowmeter problem at the Patrick Street Lift Station.
