Flowmeters, differential pressure

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

Linearizing the output of the transmitter. Functions such as square root extraction of the differential pressure for a head-type flowmeter can be done within the instrument instead of within the control system. 

Differential-pressure flowmeters, 11 656-663 advantage of, 11 657 Differential pressure flow rate sensors, 20 680... 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

Differential pressure meters are widely used. Temperature, pressure, and density affect gas density and readings of differential pressure meters. For that reason, many commercial flowmeters that are based on measurement of differential pressure often have integral temperature and absolute pressure measurements in addition to differential pressure. They also frequently have automatic temperature and pressure compensation. 

Head-type flowmeters include orifice plates, venturi tubes, weirs, flumes, and many others. They change the velocity or direction of the flow, creating a measurable differential pressure, or "pressure head," in the fluid. Head metering is one of the most ancient of flow detection techniques. There is evidence that the Egyptians used weirs for measurement of irrigation water flows in the days of the Pharaohs and that the Romans used orifices to meter water to households in Caesar s time. In the 18th century, Bernoulli established the basic relationship between the pressure head and velocity head, and Venturi published on the flow tube bearing his name. 

The detection of pressure drop across a restriction is undoubtedly the most widely used method of industrial flow measurement. If the density is constant, the pressure drop can be interpreted as a reading of the flow. In larger pipes or ducts, the yearly energy operating cost of differential-pressure (d/p)-type flowmeters can exceed the purchase price of the meter. The permanent pressure loss through a flowmeter is usually expressed in units of velocity heads, v2/2 g, where v is the flowing velocity, and g is the gravitational acceleration (9.819 m/s2, or 32.215 ft/s2, at 60° latitude). 

In laminar flow elements, the pressure drop and flow are in a linear relationship. The laminar flow element can be used in combination with either a differential-pressure- or a thermal-type flow detector. These flowmeters provide better rangeability at about the same cost as sonic nozzles. 

In this approach a gas flowmeter is used to determine the amount adsorbed. It can be of a differential type, as in Figure 3.7 (e.g. with a differential catharometer or a differential pressure drop flowmeter) or a simple form with either a sonic nozzle (Figure 3.8) or a thermal detector (Figure 3.9). The last provides a signal which depends on the heat capacity, thermal conductivity and mass flow of the gas it is usually referred to as a mass flowmeter although there is no direct measurement of mass. 

Now, we shall present an inexpensive means to meter small flows of gas. Accurately controlling and measuring such flowrates is often difficult. Control valves with small trims coupled to differential-pressure transmitters having small orifices (or long capillaries) are prone to plugging and calibration troubles. On the other hand, most mass flowmeters are expensive for small-scale uses. 

The experimental apparatus consists of a closed-loop circuit which includes a pump, a filter, two flowmeters, two pressure transducers, a differential inductive pressure transducer and two K type thermocouples for the determination of the inlet and outlet temperatures (Figure 10). The test section comprises the channel between two plane bronze blocks separated by a foil whose thickness fixes the distance between the brass walls. A series of foils with several thickness enables the width to be varied (Figure 10). Details are given in [25]. 

Figure 3 shows the test section and instrumentation. Ten wall temperatures on the tube external surface were measured with 0.5 mm diameter calibrated type E thermocouples electrically insulated from the aluminium. Fluid inlet and outlet temperatures were measured with 1 mm diameter calibrated type K thermocouples. Cah-bration was carried out with a Rosemount 162-CE platinum thermometer. Due to the high thermal conduchvity of the aluminium and the low thickness of the tube walls the measured temperature is very close to the wall temperature in contact with the fluid (the difference less than 0.01 K). The inlet fluid pressure was measured with a calibrated Rosemount type 11 absolute pressure sensor. Two calibrated differential pressure sensors measured the pressure loss through the test section. A Rosemount Micro-motion coriolis flowmeter was used to... 

ASTM C 522 covers the measurement of airflow resistance and the related measurements of specific airflow resistance and airflow resistivity of porous materials that can be used for the absorption and attenuation of sound. The method describes how to measure a steady flow of air through a test specimen, how to measure the air-pressure difference across the specimen, and how to measure the volume velocity of airflow through the specimen. The airflow resistance, R, the specific airflow resistance, r, and the airflow resistivity, rQ, may be calculated from the measurements. The apparatus includes a suction generator or positive air supply arranged to draw or force air at a uniform rate through the specimen. A flowmeter is used to measure the volume velocity of airflow through the specimen, and a differential-pressure-measuring device measures the static-pressure difference between the two faces of the specimen with respect to atmosphere. 

Meters that measure differential pressures over the flowmeter and such pressure changes that can be interpreted as flowrates. Such flowmeters with a large number of designs include orifices, venturi tubes, pitot tubes, elbow taps, etc. Fluids that result in changes of the cross-sectional area due to erosion, corrosion, or deposition of solids obviously change the calibrations. These meters tend to be relatively cheap but are often not very accurate. 

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. 

During an experiment the mass flow rate is controlled by means of the Coriolis flowmeter, and the pressure drop in the loop is measured by a differential pressure cell. Make-up gas is added to the loop in order to keep the static pressure constant when the hydrates form. Measurement of the make-up gas flow rate is a way to observe the hydrate growth kinetics and amount formed. Knowing the amount of gas necessary to saturate the oil phase at 75 bar, when the temperature is decreased from 20°C to 4°C, and assuming that crystals of hydrates form in stoichiometric conditions, it is possible to evaluate the rate of conversion and the mass of water consumed. 

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