John Flood, Krohne America, Inc.
Flow Velocity Measurement. Flow velocity is calculated based on Faraday's law of induction, which states that if a conductive liquid flows through a magnetic field a voltage will be induced. This voltage is directly proportional to the average flow velocity of the medium. With this, and the tube diameter of the primary head, the signal converter calculates the volumetric flow rate:
U = K1 · B · V · D (1)
U = voltage
K1 = instrument constant
B = magnetic field strength
V = mean flow velocity
D = electrode spacing/tube diameter
In the Tidalflux, two coils generate a magnetic field that is perpendicular to the nonmagnetic measuring tube (see Figure 1). The electrodes are not placed opposite each other in the middle of the pipe as is the case in conventional electromagnetic flowmeters, but rather at a point near the bottom representing ~10% of the tube diameter. The flow velocity can therefore still be determined with certainty in a measuring tube filled to only 10% of its diameter:
Q(t) = V · A (2)
Q(t) = actual flow
V = flow velocity
A = wetted cross section of the pipe, computed by means of capacitive level measurement
Capacitive Level Measurement. The integrated capacitive level measurement system consists of a detection plate on one side of the measuring tube and four transmission plates on the other side, all embedded in the Irathane (polyurethane) flowtube liner (see Figure 2). After the liner is cast in place, the level sensor is completely covered and fully insulated from the liquid to be measured. A high-frequency voltage induced into these transmission plates influences the current in the receiver plate, which is then measured. The bottom transmission plate is always completely covered by the liquid of interest and is used as a reference by which the offsets caused by varying fluid conductivity can be determined and compensated for. With these parameters, the filling level in the tube can accurately be determined:
C = K2 · A (3)
C = capacitance
A = wetted cross section.
The compact electronics, which are mounted on the measuring tube, calculate the filling factor, defined as the relation between the filled area of the measuring tube at partial filling and at complete filling. This filling factor b is between 0 and 1 and is a function of the flow. Combining the actual flow velocity at partial filling with the filling factor b yields:
qpart = Vpart · K3 ·b (4)
Vpart = velocity of partially filled flowmeter
K3 = flowmeter constant
The level data are linked via serial digital communication to a remote flow converter that provides the necessary flow outputs. In full pipe conditions, the Tidalflux functions as a conventional electromagnetic flowmeter.
Calibrating the level measurement feature addresses both static (no-flow) and dynamic calibration. For the former, the correction factors for the level meter are determined by a number of filling levels and fluids compared with a reference level measuring system. By using a 3rd degree polynomial, the so-called a-function is determined. This function is stored in the EPROM of the level electronics and used by the microprocessor to calculate an output signal, which is in proportion to the filling fraction and independent of the properties of the liquid.
For dynamic calibration, a specially designed calibration facility has been designed for partially filled flowmeters. The partially filled flowmeter is calibrated against a reference magmeter, a process that allows corrections to be found at a number of filling levels and flow velocities. This is the b function, and is also stored in the EPROM of the level measurement electronics.
The magmeter has no moving parts to wear out and no obstructions in the flow to collect dirt or debris, and requires no sluice underpass. Its single set of electrodes minimizes potential downtime. The Irathane flowtube liner has been found to be resistant to abrasion and corrosion in this application, based on thousands of successful installations of conventional magmeters.
The use of a continuous pipe brings its own set of benefits: because the pipe is closed, rather than opening out into a flume, hazardous materials in the flow stream do not endanger personnel working nearby. There is only a minimal chance of plugging from surface contamination, and no restriction in the flow stream. The continuous pipe diameter eliminates the risk of flooding that can occur when a fast-moving stream coming from a large inlet is forced into a narrower inlet.
In partially filled measuring tubes there is usually a formation of waves on the liquid surface. These ripples, especially when they are on the level of the electrodes, could cause a fluctuation in the sensor's output signal (see Figure 4). With the help of fuzzy logic and special filtering circuits included in the level measurement system, this problem is eliminated.
John Flood is Magmeter Product Specialist, Krohne America, Inc., 7 Dearborn Rd., Peabody, MA 01960; 508-535-6060, fax 508-535-1720.