WTP now achieving desired disinfection results with proper chlorine dosing
Figure 1. Chlorine Tanks
The staff of a municipal water treatment plant had been experiencing problems with the facility’s disinfection treatment system, which relies on chlorine for water purification. Chlorine remains a popular drinking water disinfectant around the world because of its effectiveness and relatively low cost. Wherever chlorine is in use, accurate measurement of the gas is essential for successful disinfection, water quality and safety purposes.
The gas readings provided by the existing flowmeters at the city’s plant were inconsistent and causing problems. It was determined that the chlorinator system lacked suitable flow measurement turndown capability (measuring range) and was not repeatable at lower flow rates, which meant that the monthly totalized chlorine usage was not consistent with the actual treatment cycles (Figure 1).
Poor control over the amount of chlorine being dispensed resulted in either excessive, wasteful chlorine use or potentially hazardous and expensive re-treatment. The addition of too much chlorine affects water taste, wastes expensive chlorine gas and adds the cost of extra residual chlorine removal. With too little chlorine added, the disinfection treatment process is incomplete, and the water requires costly additional alternative treatment or re-treatment.
The city’s water treatment plant had been in operation for decades with many types of equipment in use. The disinfection system originally relied on site-gauge rotameters for gas measurement. Differential pressure (dP)-type orifice plate flowmeters were added later on for automated control purposes to increase operational efficiency.
At the minimum gas flow rates the system required as demand fluctuated, the orifice plate dP meters were unable to measure accurately under the low-flow conditions when little pressure differential was available. The limited flow range of the dP meters also could not support the changing dose rates required with fluctuating water demand.
The plant needed a different gas flowmeter solution that would be appropriate for service in a 1-in.-diameter pipe at a flow rate of 150 to 2,000 lb per day. The plant’s geographic location required that the new flowmeters for the disinfection system would need to function over an operating temperature range of 60°F to 100°F at a pressure of 0 to 10 psig.
Figure 2. ST100L Flowmeter With Vortab Flow Conditioner
The chosen flowmeter would measure chlorine and no other gases. Adding to the accuracy issue, however, the meter would be installed in a cramped equipment location where inadequate straight-pipe run was present, which is a challenge for any velocity based instrument.
Effective measurement would require the flowmeter to function well in the transitional zone, where the gas flow profile was transitioning from laminar to turbulent. Instead of relying on dP technology, a mass flowmeter would allow a simple, direct means of reconciling monthly throughput compared against the change in weight of the chlorine gas containers that were installed on load cell technology scales.
A New Meter
After contacting the application engineering team at Fluid Components Intl. (FCI), the engineers at the water department selected the Model ST100L mass flowmeter with built-in Vortab flow conditioner (Figure 2). This meter is an in-line, spool piece instrument that combines intelligent transmitter electronics and a precision thermal sensor for accurate measurement in line sizes up to 2 in. over a wide turndown range.
The new meter, with its constant power technology thermal sensor (Figure 3), was installed in the system’s chlorine gas inlet line to the chlorinator panel. To ensure maximum corrosion resistance and longest service life in the highly corrosive chlorine gas environment, the meter’s entire sensor assembly, including flow elements, flow body and Vortab flow conditioner elements, are fabricated entirely of Hastelloy C-276.
FCI’s gas flowmeters are typically calibrated in an National Institute of Standards and Technology traceable flow laboratory using the actual gas to be measured and at the installation’s temperature and pressure conditions. However, chlorine gas presents safety concerns during the calibration process, which renders that process infeasible. It also has been thoroughly established that air equivalency calibrations for chlorine gas are inaccurate and unrepeatable. FCI solves this problem by combining a lab-based equivalency basic calibration with an onsite, in-situ calibration adjustment against the site’s rotameters, all performed by a field service technician. This achieved the highly accurate and repeatable measurement needed by the client. The onsite calibration matching proved to be the best solution because the totalized flow readings from the Model ST100L and the weigh scale comparison were now consistently aligned.
The new meter chosen by the plant engineers measures air/gas flow from 0.25 to 1,000 SFPS (0.07 to 305 NMPS), with turndowns of 100:1 and with accuracy of ±0.75% of reading, ±0.5% of full scale. To match present and future distributed control system, programmable logic controller or SCADA needs, the plant’s engineers would be able to choose from multiple output options including triple 4-20 mA analog, frequency/pulse, or certified digital bus communications of HART, Foundation Fieldbus, Profibus PA and Modbus RS485.
The plant team appreciated the new meter’s graphical, multivariable, backlit LCD readout, which provides a continuous display of all process measurements, alarm status and service diagnostics. Its four-button user keyboard is activated through the glass, which means the user never needs to remove lids or open up the unit at the installation site. The instrument also includes a USB port for PC interface and an Ethernet port for service needs.
Low maintenance and long life were important features that the team considered when selecting its new meter. The meter it chose features no moving parts and requires virtually no maintenance over its long service life, even in the most rugged industrial applications and installations.
Figure 3. Thermal Dispersion Sensing Principle of Operation
The meter’s enclosure is NEMA4X/IP67 rated and features four separate conduit ports to isolate all wiring. Additional pedigrees include global agency approval for hazardous environments (ATEX, IECEx, FM, FMc, Inmetro, NEPSI and EAC/TR CU) and SIL compliance. The electronics/transmitter is available for installation as either integral with the flow element or remotable (up to 1,000 ft).
To ensure a smooth installation and successful operation, the meter’s design includes an integral Vortab flow conditioner. The flow conditioning component ensures optimal installed performance by overcoming the plant’s limited piping straight run and the flow range occurring in the transitional flow region.
The in-line flow conditioner eliminates both swirl and velocity profile distortions produced by process equipment obstructions and/or inadequate straight run of pipe and ducting. It also mitigates temperature and media stratification, which can be present at low flow rates.
The flowmeters were installed and commissioned without any issues. They are in service on the chlorine gas inlet lines and achieving consistent accurate and repeatable flow measurement results. The treatment plant is now achieving the desired disinfection results with proper chlorine dosing at significant cost savings due to reduced chlorine use, avoiding re-treatment and lessened residual chlorine removal processes.