Water flow rate measurements with a calibrated Pitot tube are best made in pipes at a point that there is a fairly well developed flow profile. As such, flow rates are typically measured in pipes that have at least six pipe diameters of straight length from a flow obstruction or bend. Flow rates can be measured in pipes as small as 4 inches to over 20 ft in diameter. Typically, two mutually perpendicular 20-point traverses are made to obtain the average velocity in the pipe. The flow rate is calculated from the product of the average velocity and the cross-sectional area in which the traverses are conducted. Access to the flow is made through full port ball or gate valves. Please contact CleanAir for guidance regarding pitot tap size and placement to ensure the accuracy of pitot tube flow rate measurements.
In applications where pitot taps cannot be installed in a straight run of pipe, dye-dilution flow rate techniques may be a viable alternative. Dye-dilution flow measurements are especially well suited for "once-through" applications to determine pump, condenser or helper cooling tower flow rates. The flow rate accuracy does not depend on pipe cleanliness, pipe geometry or turbulence at the measurement point and flow measurements can typically be performed with existing taps.
In this technique, fluorescing Rhodamine WT dye is introduced at a precisely known injection rate into the flow stream under investigation. At a downstream location after the dye is thoroughly mixed, the concentration of the dye in the system water is measured with a calibrated fluorometer. The downstream concentration is proportional to the mass flow rate of the water. The straightforward mass balance is mathematically expressed as:
Ci = concentration of dye at the injection point
mi = mass injection rate of the concentrated dye
Co = concentration of dye at the downstream sampling point
Mo = mass flow rate of the flow stream under investigation
Calibrations of the fluorometer are made via very accurate dilutions of the Rhodamine WT dye using water from the system to be evaluated. Because the calibrations are made with the water from the tested system, the calibrations compensate for any background fluorescence or interferences due to sedimentation in the tested water. Temperature corrections are also made to address dye fluorescence response with temperature.
Because chlorine in the system water will react with the dye, chlorination or other oxidizing chemical treatment systems must be taken out of service prior to the generation of calibration solutions and throughout the dye dilution test efforts.
Rhodamine WT was developed for water flow studies and has been chosen by the Army Corp of Engineers for inflow studies and is approved by the EPA for water flow measurements. Typical test concentrations at the sampling station range from 10-20 ppb. The test concentration becomes further diluted upon discharge into a receiving body of water.
In closed loop applications where pipe taps cannot be installed, a strap-on ultrasonic flow rate measurement may be an option. Ultrasonic flow rate measurement accuracy is a function of flow profile, pipe material of construction, pipe thickness and homogeneity. Ultrasonic flow rate measurements are best made in new or clean steel pipe with long straight runs to ensure well developed velocity profiles. Ultrasonic flow rate measurements are sometimes used to complement other flow rate measurements for test programs when an increased uncertainty in the smaller flow rates does not detract from the test objective. For example, ultrasonic flow rates may be used to quantify auxiliary flows around a steam condenser when the main condenser flow is measured via dye-dilution or the make up into a cooling tower cold water basin where the tower water supply is measured via pitot tube traverse of the hot water risers.