Flowmeter zeroing

The indicated flow of acetic acid is 9000 liters per day. The instrument technician checks the flowmeter to see if it has drifted, by opening valve B, with A and C closed (see Fig. 6.7). It should go back to zero— but a reading of 2000 liters per day is noted. The full range on the flowmeter is 10,000 liters per day. What is the real flow rate of the acetic acid The answer is not 2000 liters. Why Because flow varies with the square root of the orifice plate pressure drop. To calculate the correct acetic acid flow ... 

The lesson is that near the top end of its range, the indicated flow is likely to be accurate, even if the meter is not well zeroed, or the measured AP is not too accurate. On the other hand, flowmeters using orifice plates cannot be very accurate at the low end of their range, regardless of how carefully we have zeroed them. Digitally displayed flows also follow this rule. 

Coriolis flowmeters can be used in both gas and liquid services, but are not suited for the measurement of their mixtures. The flow detection capability can range from 0 to 30,000 kg/min (0 to 65,000 lb/min), and these meters are available in sizes from 1 to 250 mm (0.05-10 in.). Their measurement error is 0.1% AF +/- zero offset which is about 0.01% within a 10 1 range. This error rises to 1% FS within a 100 1 range. This flowmeter can operate at temperatures from cryogenic to 450°C (850°F), and at pressures from atmospheric to 345 bar (5,000 psig). 

The basic rangeability of this meter is the same as that of an orifice plate (3 1), but if two (a high span and a low span) transmitters are used, and the flow element is accurately calibrated over the complete flow range, it can be increased to 10 1. This performance can be obtained from all properly calibrated d/p flow elements, not just from the V-cone design. The V-cone flowmeter should be installed horizontally so that the two pressure taps are at the same elevation. This guarantees that the d/p cell will detect a zero pressure differential when there is no flow. 

The sample flow which is split off from the main stream is burned in the second component of the energy flowmeter. This second component is called a Flow-Titrator . The Flow-Titrator performs two functions. It controls the sample stream flow to maintain zero differential pressure at the flow separator. It also mixes the sample flow with a stoichiometric quantity of air and bnms the mixture. The Flow-Titrator design is patterned after the Therm-Titrator, which is a commercial instmment mannfactured by Precision Measurement Incorporated for measuring the calorific value of natural gas. It has been established in previous experiments that the ratio of the main flow to the sample flow is always the same and is independent of the flow magnitude or the gas composition. Thus the air flow in the Flow-Titrator is in direct proportion to the energy flow in the main hne. 

The bottom flowmeter was off-zero, and showed a substantial flow. 

Axial-Flow Transverse Momentum Mass Flowmeter. Figure 8.15 shows a schematic of an axial flow transverse momentum flowmeter. Substantially all of the fluid flows through both the impeller and the turbine. The impeller and the turbine are geometrically similar cylinders mounted in a cylindrical flow conduit on an axis coinciding with the conduit centerline. Each element is mounted on a separate shaft. Both the impeller and turbine are composed of several straight vanes located at the periphery of the elements and parallel to the centerline of the conduit. A means is provided for measuring the torque on the turbine shaft. If the impeller were locked (not rotating), the torque on the turbine shaft would be zero. 

Electromagnetic flowmeters offer the advantage of no moving parts in the flow stream, possible two-phase fluid capability, direct mass flow measurement, good time response, and linear calibration. For reproducible results, the flow velocity profile must be symmetric with respect to the axis of the pipe. Problems with this flowmeter include zero point drift with temperature, complex electronics, and difficulty in maintaining a hydrogen leak-proof flow tube. 

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