Pipes pressure differences

Shirato, Gotoh, Osasa, and Usami [J. Chem. Eng. Japan, 1, 164— 167 (January 1968)] present a method for determining the mass flow rate of suspended sohds in a liqiiid stream wherein the liquid velocity is measured By an electromagnetic flowmeter and the flow of sohds is calculated from the pressure drops across each of two vertical sections of pipe of different diameter through which the suspension flows in series. 

Centrifugal blowers or turbines usually cannot generate enough pressure difference to overcome the added resistance of the recycle pipes. In addition, some components may condense out in the cooler, especially with high-boiling materials or at high pressures. These must be recycled by a liquid pump through an evaporator. This in turn makes them approach a steady-state slowly. ... 

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

Any obstruction inserted into a duct or pipe that creates a measurable pressure difference can be used as a flow meter. The three basic standardized flow measurement devices presented above are perhaps more suitable for laboratory work than installation as permanent ductwork instruments in ventilation applications. They are sensitive to flow disturbances, relatively expensive, require considerable space, and have a narrow measurement range and a high permanent pressure loss. For these reasons, numerous attempts have been made to develop instruments without these drawbacks. Some of them, like the... 

In order to cover all the lines in the facility, pipe and valve tables are normally included. Each pipe pressure class is assigned a designation. Sometimes it is necessary to assign two classes for a single designation. For ample, in Table 15-1 A, L, and AA are all ANSI 150 cl , but they contain different fluids. 

It is found, as a result of experiment, that the pressure difference (AP) between two ends of a pipe in which a fluid is flowing is a function of the pipe diameter d, the pipe length /, the fluid velocity u, the fluid density p, and the fluid viscosity p. 

If the pressure difference over the pipe is unaltered, what will be the ratio of the volumetric flowrates of the two liquids ... 

The rate of flow of gas under adiabatic conditions is never more than 20 per cent greater than that obtained for the same pressure difference with isothermal conditions. For pipes of length at least 1000 diameters, the difference does not exceed about 5 per cent. In practice the rate of flow may be limited, not by the conditions in the pipe itself, but by the development of sonic velocity at some valve or other constriction in the pipe. Care should, therefore, be taken in the selection of fittings for pipes conveying gases at high velocities. 

Oil with a viscosity of 10 mN s/m2 and density 900 kg/nr is flowing through a 500 mm diameter pipe 10 km long. The pressure difference between the two ends of the pipe is lO N/m2. What will the pressure drop he at the same flowrate if it is necessary to replace the pipe hy one only 300 mm diameter Assume the pipe surface to be smooth. 

Water flows at between 3000 and 4000 cm3/s through a 50 mm pipe and is metered by means of an orifice. Suggest a suitable size of orifice if the pressure difference is to be measured with a simple water manometer. What is the approximate pressure difference recorded at the maximum flowrate ... 

What size of orifice would give a pressure difference of 0.3 m water gauge for the flow of a petroleum product of specific gravity 0.9 at 0.05 m3/s in a 150 mm diameter pipe ... 

All toilets on an aircraft are connected with one or more central tanks through vacuum pipes. This is illustrated in Figure 4. Solids are transported to the tanks by high vacuum, creating an air displacement of over 60 m per second. Up to flight altitudes of 4-5000 m, an electrical pump provides the necessary low pressure. In altitudes exceeding this level, the low pressure outside the aircraft provides the necessary pressure difference amounting to 0.5-0.6 bar. 

Example 4-1 Manometer. The pressure difference between two points in a fluid (flowing or static) can be measured by using a manometer. The manometer contains an incompressible liquid (density pm) that is immiscible with the fluid flowing in the pipe (density pf). The legs of the manometer are connected to taps on the pipe where the pressure difference is desired (see Fig. 4-2). By applying Eq. (4-7) to any two points within either one of the fluids within the manometer, we see that... 

Water is flowing in a horizontal pipe bend at a velocity of 10 ft/s. The radius of curvature of the inside of the bend is 4 in., and the pipe ID is 2 in. A mercury manometer is connected to taps located radically opposite each other on the inside and outside of the bend. Assuming that the water velocity is uniform over the pipe cross section, what would be the manometer reading in centimeters What would it be if the water velocity were 5 ft/s Convert the manometer reading to equivalent pressure difference in psi and Pa. 

You must select an orifice meter for measuring the flow rate of an organic liquid (SG = 0.8, /x = 15cP) in a 4 in. sch 40 pipe. The maximum flow rate anticipated is 200 gpm, and the orifice pressure difference is to be measured with a mercury manometer having a maximum reading range of 10 in. What size should the orifice be ... 

In general, the velocity profile will be curved but as equation 1.33 contains only the local velocity gradient it can be applied in these cases also. An example is shown in Figure 1.13. Clearly, as the velocity profile is curved, the velocity gradient is different at different values of y and by equation 1.32 the shear stress r must vary withy. Flows generated by the application of a pressure difference, for example over the length of a pipe, have curved velocity profiles. In the case of flow in a pipe or tube it is natural to use a cylindrical coordinate system as shown in Figure 1.14. 

A simple interpretation of this choking condition is as follows. The gas flows as a result of the pressure difference P — P2. When the gas speed reaches the speed at which a pressure wave propagates relative to the gas, any pressure wave generated will be unable to travel upstream but will remain stationary relative to the pipe. Thus, if the pressure in the reservoir into which the gas discharges is reduced below Pw, the fact cannot be transmitted upstream and so the flow rate will not change. 

Both equations 8.11 and 8.12 refer to horizontal pipes. When the pipe is not horizontal, the total pressure difference (P —P2) must be corrected for the pressure difference due to the static head between the two pressure taps. Thus, equation 8.11 should be replaced by... 

It is essential to appreciate that the pressure difference measured by. a manometer automatically eliminates the static head difference. This is shown in Figure 8.1(b). The static head pg(zi — z2) in the pipe is exactly balanced by the extra static head above the right hand limb of the manometer. Consequently, if Ah is calculated from Azm using equation 8.4, no further correction for the static head should be made. 

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