Differential manometers

Repeat the first example in Chapter 5 where the cylinder body is charged with 100 ml of acetone and mercury is the operating fluid in the U-tube manometer (differential height is 14 in.). Discuss the potential sources of error in this measurement and how to minimize them. 

Pressure Manometer Differential pressure cell (mechanical or semi-conductor) Bourdon gauge... 

Pressure reducing valves should be of steel constmction, designed for minimum and maximum operation conditions. Pressure gauges should be of ak-kon constmction. Pressure rehef valves should be of the spring-loaded type. Rupture disks may be used only as auxkiary equipment. Differential pressure measurements using mercury manometers should be avoided in ammonia service. 

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). 

Change of manometric fluid. In open manometers, choose a fluid of lower density In differential manometers, choose a fluid such that the difference between its density and that of the fluid being measured is as small as possible. 

Differential pressures and subatmospherie pressures should be measured by manometers with a fluid that is ehemieally stable when in eontaet with the test gas. Mereury traps should be used where neeessary to prevent the manometer fluid from entering the proeess piping. Errors in these instruments should not exeeed 0.25%. 

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. 

Fluid manometers are devices where the readout of the pressure differential is the length of a liquid column. The most fundamental implementation of this principle is the U-tube manometer. This is simply a tube of U shape filled with manometer fluid, as shown in Fig. 12.16. The pressure differential is applied at both ends of the tube, making the manometer fluid move downward in one limb and upward in the other, until the forces acting on the fluid are in balance. 

For services where solids are present or evaporation of droplets on the mesh might leave a solid crust, it is usual practice to install sprays above or below the mesh to cover the unit with W ater (or suitable solvent) on scheduled (or necessary) operating times, as the plugging builds up. This is checked by a manometer or other differential pressure meter placed with taps on the top and bottom side of the mesh installation. 

Sulphuric acid of density 1300 kg/m3 is flowing through a pipe of 50 mm, internal diameter. A thin-lipped orifice, 10 mm in diameter is fitted in the pipe and the differential pressure shown on a mercury manometer is 0.1 m. Assuming that the leads to the manometer are filled with the acid, calculate (a) the mass flow rate of acid and (b) the approximate drop in pressure caused by the orifice in kN/m2. The coefficient of discharge of the orifice may be taken as 0.61, the density of mercury as 13.550 kg/m3 and the density of the water as OHIO kg/m ... 

The flow is also metered using a 15 cm orifice plate across which the pressure differential is 50 mm on a mercury-undcr-waler manometer. What is the coefficient of discharge of the orifice meter ... 

A gas of molecular weight 44 kg/kmol, temperature 373 K. and pressure 202.6 kN/m3 is flowing in a duct. A pitot tube is located at the centre of the duct and is connected to a differential manometer containing waler. if the differential reading is 38.1 mm waler, what is the velocity at the centre of the duct ... 

The flowrate of air in a 305 mm diameter duct is measured with a pitot tube which is used to traverse the cross-section. Readings of the differential pressure recorded on a water manometer are taken with the pitot... 

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. 

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. 

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