Flow measurement manometer differential pressure

Water at 60°F is flowing through a pipeline, and the line contains an orifice plate for flow measurement. The pressure difference between the upstream and the downstream sides of the orifice plate is to be measured using an available U-tube manometer which is 48 in. high. If the maximum expected pressure drop across the orifice plate is 3.55 psi, what is the minimum specific gravity required of a manometer liquid that can be used to measure this differential pressure Density of water at 60 F = 0.999 g/cm. ... 

Example Pitot-static tube is placed within duct flowing air with a temperature of 200°F. The static pressure within the duct is 1 psig and the atmospheric pressure is 14.3 psia. A U-tube manometer measures a differential pressure of 4 inches of WC between the total pressure... 

The pitot tube is the standard method for measuring air velocity in ductwork and can accommodate the presence of solvent vapours and high temperatures. However, dust laden air may cause false readings. In use the head of the pitot tube must face into tfie gas flow. The standard pitot tube consists of a pair of concentric tubes. The inner tube has an open end that faces into the gas stream and measures the total gas pressure. The outer tube, which is sealed to the inner tube at its leading end has a series of holes at right angles to the gas flow that measure the static pressure of the gas. The tubes are connected to the opposite ends of a manometer that measures the differential pressure which is proportional to the gas velocity. 

Nozzle arrangements for various applications vary considerably. For subcritical flow measurement at the outlet end, where nozzle differential pressure p is less than the barometric pressure, flow should be measured with impact tubes and manometers as shown in Figure 20-3. 

Differential Pressure Measurement Differential manometers are instruments that measure the difference in pressure between two adjacent rooms. Cleaner environments must have a higher pressure than adjacent, less clean environments to prevent flow of relatively dirty air into the cleaner environment. This differential pressure must be monitored and controlled. 

The pitot tube is a device for measuring v(r), the local velocity at a given position in the conduit, as illustrated in Fig. 10-1. The measured velocity is then used in Eq. (10-2) to determine the flow rate. It consists of a differential pressure measuring device (e.g., a manometer, transducer, or DP cell) that measures the pressure difference between two tubes. One tube is attached to a hollow probe that can be positioned at any radial location in the conduit, and the other is attached to the wall of the conduit in the same axial plane as the end of the probe. The local velocity of the streamline that impinges on the end of the probe is v(r). The fluid element that impacts the open end of the probe must come to rest at that point, because there is no flow through the probe or the DP cell this is known as the stagnation point. The Bernoulli equation can be applied to the fluid streamline that impacts the probe tip ... 

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. 

C. Flow Measurement by Pressure Drop across an Orifice. Another common scheme for the measurement of flow is based on the determination of the pressure drop on either side of a constriction, such as an orifice or venturi. Either a liquid-filled differential manometer or a pressure transducer with associated digital readout may be used for this pressure measurement. The flow rates determined by these meters are in units such as cm3/s, and it is necessary to make a correction for total pressure to convert these to standard cm3/s or mol/s. 

The flowpath restriction, such as an orifice, causes a differential pressure across the orifice. This pressure differential is measured by a mercury manometer or a differential pressure detector. From this measurement, flow rate is determined from known physical laws. 

Example 8.4. A venturi meter is to be installed in a 100-mm line to measure the flow of water. The maximum flow rate is expected to be 75 m /h at 15°C. The manometer used to measure the diflerentia pressure is to be filled with mercury, and water is to fill the leads above the surfaces of the mercury. The water temperature will be 15°C throughout, (a) If the maximum manometer reading is to be 1.25 m and the venturi coefficient is 0.98, what throat diameter, to the nearest millimeter, should be specified for the venturi ( ) What will be the power to operate the meter at full load if the pressure recovery is 90 percent of the differential pressure ... 

The model is three-dimensional. In order to comprehensively analyze the distribution of flow field, 260 sampling points for 4 levels in goaf are sampled in a short time, and then the gas concentration of samples was analyzed and tested by GC-4000 A gas chromatograph. The sampling points are also used as pressure measurement points. The static pressure of points are absorbed by the YJB-1500-type compensator micro-manometer, and the wind speed and static pressure of intake and return airway were measured by electronic wind instrument and U-type differential pressure meter. 

The inverted differential U tube, in which the manometric fluid may be a gas or a hght liquid, can be used to measure hquid pressure differentials, especially for the flow of slurries where solids tend to settle out. Additional details on the use of this manometer can be obtained from Doolittle (op. cit., p. 18). 

A liquid hydrocarbon is fed at 295 K to a heat exchanger consisting of a 25 mm diameter tube heated on the outside by condensing steam at atmospheric pressure. The flow rate of the hydrocarbon is measured by means of a 19 mm orifice fitted to the 25 nnn feed pipe. The reading on a differential manometer containing the hydrocarbon-over-water is 450 mm and the coefficient of discharge of the meter is 0.6. 

Water is flowing through a 150 mm diameter pipe and its flowrate is measured by means of a 50 mm diameter orifice, across which the pressure differential is 2.27 x 104 N/m2. The coefficient of discharge of the orifice meter is independently checked by means of a pitot tube which, when situated at the axis of the pipe, gave a reading of 100 mm on a mercury-under-water manometer. On the assumption that the flow in the pipe is turbulent and that the velocity distribution over the cross-section is given by the Prandtl one-seventh power law, calculate the coefficient of discharge of the orifice meter. 

The volumetric flow rate of the teed gas is determined with an orifice meter, with a differential mercury manometer being used to measure the pressure drop across the orifice. Calibration data for this meter are tabulated here ... 

A venturi meter is a device to measure fluid flow rates, which in its operation resembles the orifice meter (Section 3.2b). It consists of a tapered constriction in a line, with pressure taps leading to a differential manometer at points upstream of the constriction and at the point of maximum constriction (the throat). The manometer reading is directly related to the flow rate in the line. 

The remedy is to select a meter no larger than necessary to handle the minimum flow rates expected, even if the pipe and meter sizes do not match. The velocity through the meter at minimum flow should have range between 2 and 5 fps and at maximum flow have a range of 8-12 Ips (1 fps = 1 ft/s = 0.3048 m/s). In some cases, this may involve the use of pipe reducers to adapt the water line to the meter size a practice which is acceptable provided that the resulting pressure loss is not excessive. Manometers (measuring pressure differential) can be used to calibrate Venturi type meters, whereas only controlled water tests can be used to accurately calibrate propeller, turbine, and magnetic type meters. 

In measuring the flow of fluids in a pipeline, a differential manometer, as shown in Fig. El.26, can be used to determine the pressure difference across an orifice plate. The flow rate can be calibrated with the observed pressure drop. Calculate the pressure drop p — pi in pascal. 

A horizontal venturi meter having a throat diameter of 20 mm is set in a 75-mm-ID pipeline. Water at 15°C is flowing through the line. A manometer containing mercury under water measures the pressure differential over the instrument. When the manometer reading is 500 mm, what is the flow rate in gallons per minute If 12 percent of the differential is permanently lost, what is the power consumption of the meter ... 

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