This invention relates to fluid flow rate measurements. More specifically, this invention relates to determining fluid flow rates using measured motor speed and torque parameters.
Measurement of fluid flow through pipes and pumps is well known in the art. One method of determining fluid flow rates is to install gears, vanes, paddle wheel, turbines, etc., in the flow channel and determine fluid flow rate by the speed at which these devices turn. A second method is by measuring the differential pressure across a dedicated flow obstruction, such as a venturi, orifice plate, annubar, pitot tube, etc., and applying the well-known Bernoulli""s principle to obtain a velocity and, consequentially, a fluid flow rate. As an example of this principle, U.S. Pat. No. 5,129,264, entitled xe2x80x9cCentrifugal Pump with Flow Measurement,xe2x80x9d issued Jul. 14, 1992, to Lorenc, and assigned to the same assignee herein discloses using differential pressure and pump speed to measure fluid flow rates. Still other methods of fluid flow rate measurement employ electrical/magnetic or sonic measurement means. For example, Mag Meters determine fluid flow rates by measuring the change in a magnetic field caused by the velocity of the fluid flowing therethrough. Sonic devices use acoustical pulses, i.e., Sonar, and Doppler principles to measure fluid flow rates. Other non-intrusive methods measure the torque a variable speed electrical motor delivers to a pump by installing a torque shaft between the motor and pump. The motor or pump is then calibrated and a motor kilowatt input/Motor Brake Horsepower Output (BHP) calibration table is developed. Accordingly, knowledge of the kilowatt input value can be used to determine the output horsepower. However, this calibration is needed at several speeds and requires several different sized torque shafts.
Thus, current methods for determining fluid flow rates necessitate intrusion into, or require access to, the enclosures transporting the fluid. In some systems, such as caustic systems having lined pumps, intrusion is prohibited.
Hence, there is a need for a simple, accurate and reliable method for determining fluid flow rates without intruding into the fluid flow or having access to the enclosures transporting the fluid.
The present invention determines a fluid flow rate through a pump by first determining two flow rate values from a plurality of characterizing flow rate values corresponding to two known speed values selected from a plurality of known characterizing speed values at a known motor torque. The first one of the two known speed values is selected greater than a measured pump speed and a second one of the two known speed values is selected less than the measured pump speed. The fluid flow rate is then determined as being proportional to the two determined flow values at the known speed values and the pump speed.