Going With Partial Flow

Fluid piping systems cannot always be engineered in configurations ideally suited for flow measurement devices. Oftentimes the limits of traditional metering devices are tested to minimize piping alterations. This is particularly true in the wastewater industry where the piping system must be designed to occasionally carry extremely large flows.

09/01/1998


KEY WORDS

 

  • Process control & instrumentation

  • Flow sensing/measurement

  • Mass flow sensing/measurement

Fluid piping systems cannot always be engineered in configurations ideally suited for flow measurement devices. Oftentimes the limits of traditional metering devices are tested to minimize piping alterations. This is particularly true in the wastewater industry where the piping system must be designed to occasionally carry extremely large flows. Traditionally, these lines have low flows at night, higher during the day, and very high in a rainstorm. The extreme flow range assures that the line will typically be partially full.

Today's increasing environmental concerns and accountability for waste processing costs are demanding metering devices with better accuracies, wider measuring ranges, and flexible installation parameters. Traditional electromagnetic flowmeter (EMF) technology has been favored due to its high accuracy, ability to easily handle fluid with high solid content, extremely wide rangeability, obstructionless flow path, minimal pressure loss, and low maintenance design. To avoid the inaccuracies caused by a partially filled pipe, traditional EMFs require specialized piping modifications, such as a U-shaped invert, to assure that they remain full of liquid at all times. Disadvantages include added pressure drop, collecting of sediment, and added cost. In large pipelines, including an invert will generally exceed the cost of a flowmeter.

Cost savings, other benefits

Parti-Mag II is a unique EMF system capable of accurately measuring flows in partially filled pipes. Designed as a replacement for flumes and weirs, Parti-Mag II contains advances which increase measurement accuracy and decrease external flow profile effects. 8 Parti-Mag II's full pipe accuracy is 1% of rate, while accuracy in the partially filled mode is 3-5% of rate, depending upon fill height, a two-fold advance when compared with a flume. By maintaining the +5% error limit throughout a 1,000 to 1 operating range, Parti-Mag II extends the measurement limitations to almost 10 times that of a flume.

Installation advantages

Installation parameters for free surface flow measurement devices are restrictive. Tight limitations are placed on pipeline slope, upstream/downstream channel configuration, and flow profile. Unlike other similar devices, Parti-Mag II will measure sub- or super-critical flows and therefore function with pipeline slopes up to 5%, eliminating costly design considerations such as stilling ponds and elevation changes. Additionally, the high limit on pipeline slope allows the metering run to adhere to the ATV 110 standard that details the minimum slope required to eliminate particulate settling, therefore eliminating pipe clean-out costs.

Operational principles

Parti-Mag II has incorporated advances in the measurement of both variables

necessary to calculate volumetric flow. The basic formula is:

Q = v x A

where:

Q= actual volumetric flow rate

v= average fluid velocity

A= cross sectional area of the flow

Under full pipe conditions, the meter functions as a traditional EMF. Faraday's Law of Induction determines the flow velocity. The law states that a conductive medium moving through a magnetic field will induce a voltage in the medium, which is proportional to its average flow velocity. The full pipe condition provides a constant cross-sectional area, which is easily calculated from the geometric configuration of the flow tube. In this case, the flow calculation is straightforward.

In a partially filled pipeline, PartiMag II's unique construction comes into play. As the fluid level drops, the Full Pipe Electrode senses the change and activates a correction algorithm. This mathematical correction alters the measurement method for velocity and the cross sectional area, and allows accurate flow measurement down to 10% fill height.

With traditional EMF technology, the voltage sensed at the electrodes is a "direct measurement" of average flow velocity. Under partially filled conditions, measured voltage must be corrected before flow rate is calculated. The correction factors are a function of fill height and are empirically determined during the meter's factory calibration. Data sets for each set of measurement electrodes are stored in the converter using electrically eraseable and programmable read-only memory (EEPROM) technology, therefore eliminating the necessity of on-site calibration.

Three pairs of measurement electrodes, located at different fill heights, are contained within the flowtube (see cross section). The optimally located pair measures the voltage. This electrode pair is selected by the converter as a function of fill height. Varying the height of the measurement electrodes provides an advantage when combating flow profile effects. Use of multiple electrode locations has reduced the adjacent straight run requirements to 5 pipe diameters upstream and 3 downstream, translating into substantial savings when considering total installation cost.

Fill height determination

Parti-Mag II uses an electrical solution to calculate the fill height. A high frequency signal with a sinusoidal waveform of fixed amplitude is applied through the bottom pair of electrodes. The resultant signal, as read by the three pairs of measurement electrodes, will accurately measure fill height, using the change in the detected signal's amplitude. Normalized ratios of the received signal to the injected signal are plotted to determine both the fill height and the correction factor for the velocity calculation.

Signal Processing

Updated data points for fill height and flow velocity are required for accurate measurement. Since both parameters use the same set of electrodes to derive the two signals, continually alternating the measurement function is required. The coil excitation for velocity measurement is made on a regular frequency, typically 7.5 Hz (or [1/8] of the line frequency). A signal generator for height measurement is capacitively coupled to the lowest electrode pair and operates between the dc pulses to the coils. This strategy allows the low-voltage signal induced by the flow velocity to be separated from the high-frequency voltage signal containing fill height information, and enables the microprocessor to capture the raw measurement data.

For more information about Parti-Mag II, visit www.controleng.com/info :





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