Positive displacement flow measurement

Positive displacement flow meters, or PD meters, are used more than all other flow measurement devices [10] a typical PD flow meter is shown in Figure 4.22. Millions of PD... 

Flow, defined as volume per unit of time at specified temperature and pressure conditions, is generally measured By positive-displacement or rate meters. The term positive-displacement meter apphes to a device in which the flow is divided into isolated measured volumes when the number of fillings of these volumes is counted in some man-... 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

Flow Rate. The values for volumetric or mass flow rate measurement are often determined by measuring pressure difference across an orifice, nozzle, or venturi tube. Other flow measurement techniques include positive displacement meters, turbine flowmeters, and airflow-measuring hoods. 

Positive displacement vacuum pumps. Measurement of performance characteristics. Part 1 - Measurement of volume rate of flow (pumping speed) 12/93... 

Other sensors which are described in Volume 1 (Sections 6.3.7-6.3.9) are the variable area meter, the notch or weir, the hot wire anemometer, the electromagnetic flowmeter and the positive displacement meter. Some of these flowmeters are relatively less suitable for producing signals which can be transmitted to the control room for display (e.g. weir, rotameter) and others are used in more specialist or limited applications (e.g. magnetic flowmeter, hot wire anemometer). The major characteristics of different types of flow sensor are summarised in Table 6.1. Brief descriptions follow of the principles underlying the more important types of flowmeter not described in Volume 1. In many instances such flow sensors are taking the place of those more traditional meters which rely upon pressure drop measurement. This is for reasons of versatility, energy conservation and convenience. 

A. Flow Measurement by Displacement. A variety of flow meter designs are based on the positive displacement of a small amount of easily visualized material which does not alter the flow rate. A simple and easily constructed mass flow meter of this type, the soap-film meter, is based on timing the displacement of a soap film up a buret tube (Fig. 7.11). This type of flow meter is often placed at the exit of a gas chromatograph or small flow reactor. It is very... 

Flow, defined as volume per unit of time at specified temperature and pressure conditions, is generally measured by positive displacement or rate meters. The term positive displacement meter applies to a device in which the flow is divided into isolated measured volumes when the number of fillings of these volumes is counted in some manner. The term rate meter applies to all types of flowmeters through which the material passes without being divided into isolated quantities. Movement of the material is usually sensed by a primary measuring element that activates a secondary device. The flow rate is then inferred from the response of the secondary device by means of known physical laws or from empirical relationships. 

Positive-displacement (PD) flowmeters are used when the total quantity of the flowing process stream is of interest or when a recipe is being formulated in a batch process. These meters operate by trapping a fixed volume of fluid and transferring that volume from the inlet to the outlet side of the meter. The number of such calibrated "packages" of fluid is counted as a measure of total volumetric flow. These measuring devices are used in both gas and liquid services. 

A Flow Element (TTT410) is recommended to provide a measure of the fuel oil flow rate to the burner. Flow elements may include positive displacement meters, magnetic meters, orifice plates (stainless steel is recommended), vortex meters, mass flow meters, and others, provided that all of the materials are oil compatible and dimensional requirements are satisfied. 

Full-bore meters include variable-head meters such as venturi and orifice meters and variable-area meters such as rotameters. These will be described in some detail. Briefer descriptions are given of other full-bore measuring devices V-element, magnetic, vortex shedding, turbine and positive-displacement meters, ultrasonic meters, and mass flow devices such as Coriolis and thermal flowmeters. 

Flow. Nonintrusive sensors that can be maintained at the process temperature are ideally suited to measure the flow rate of feed and product streams. Magnetic flow meters are suitable and inexpensive choice for aqueous streams. Organic streams with low dielectric constants require a vibrating tube mass flow meter to satisfy these criteria. Although commonly installed, flow meters that operate by inducing a pressure drop proportional to the flow rate present restrictions for solids accumulation that may alter the calibration. An alternative approach is to monitor the rotational speed of a positive displacement pump. Accuracy of this method is subject to wear and tolerances in the pump. 

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