Pipe Fittings and Bends

In general, pardally due to the size and cost of maintaining vacuum in a piping system, the lines are not long (certainly not transmissions lines), and there is a minimum of valves, fittings, and bends to keep tire resistance to flow low. 

Pressure drop in the transmission pipes is a combination of pressure losses in the pipes and pipe fittings7. Pipe fittings include bends, isolation valves, control valves, orifice plates, expansions, reductions, and so on. If the fluid is assumed to be incompressible and the change in kinetic energy from inlet to outlet is neglected, then ... 

It should be noted that Equations 13.9 and 13.10 apply to smooth pipes, whereas the pipes used for transmission of fluids usually have some surface roughness, which increases the friction factor. However, for short fluid transmission pipes, the overall pressure drop is usually dominated by the pressure drop in the pipe fittings (valves, bends, etc). Thus, for short transmission pipes, there is little point in calculating the straight pipe pressure drop accurately. If the transmission pipe is long (>100 m) and straight, then the Fanning friction factor can be correlated as7 ... 

J = frictional loss due to fittings and bends, expressed as equivalent fractional loss in a straight pipe... 

The preceding analysis clearly neglects a number of factors that may have an influence on the optimum economic pipe diameter, such as cost of capital or return on investment, cost of pumping equipment, taxes, and the time value of money. If the preceding development of Eq. (39) for turbulent flow is refined to include the effects of taxes and the cost of capital (or return on investment) plus a more accurate expression for the frictional loss due to fittings and bends, the result is t... 

J = frictional loss due to fittings and bends, expressed as equivalent fractional loss in a straight pipe, or column k = designation for column in simplex algorithm matrix with maximum value of Cj - Zj... 

Detailed test section is shown in Figure 6. The test section was provided with pressure taps (piezometric ring) at various points in the upstream section and downstream section, sometimes on the pipe fitting like bend, elbow, etc. The static pressure at the different points was measured by means of simple manometer or piezoresistive pressure transducers. The main idea of putting long upstream and... 

Plain-end pipe used for socket-weld joints (Fig. 10-126) is available in all sizes, but fittings and valves with socket-weld ends are limited to sizes 3 in and smaller, for which the extra cost of the socket is outweighed by much easier alignment and less skill needed in welding. The joint is not so resistant to bending stress as the butt-welded joint but is otherwise equal, except that for some fluids the crevice between the pipe and the socket may promote corrosion. ANSI B16.il—1973,... 

Effects of Water Hammer. Water hammer has a tremendous and dangerous force that can collapse floats and thermostatic elements, overstress gauges, bend mechanisms, crack trap bodies, rupture fittings and heat exchange equipment, and even expand piping. Over a period of time, this repeated stress on the pipe will weaken it to the point of rupture. 

A tabulated form as in Table 19.3. This shows flows versus pipe length and diameter for both copper and steel. Such tables are included in British Gas lM/16 and British Standard BS 6891. Note that Table 19.3 includes allowances for elbows, tees and bends. Allowances, equivalent to numbers of pipe diameters, must be made for all pipe fittings, which cause an additional pressure loss. Further details for allowances to be made for pipefittings, including valves and nonreturn valves, are given in Table 19.4. 

Pipe fittings such as bends, tees and crosses, unless large enough to enable spray application of the lining, are usually lined by casting using internal mould formers. The pipe fitting is assembled with the internal mould and preheated, the polyurethane mixture injected and the whole is oven cured. 

Any obstruction to flow will generate turbulence and cause a pressure drop. So, pipe fittings, such as bends, elbows, reducing or enlargement sections, and tee junctions, will increase the pressure drop in a pipeline. 

The loss in terms of velocity heads can be estimated by counting the number of flow contractions, expansions and reversals, and using the factors for pipe fittings to estimate the number of velocity heads lost. For two tube passes, there will be two contractions, two expansions and one flow reversal. The head loss for each of these effects (see Volume 1, Chapter 3) is contraction 0.5, expansion 1.0, 180° bend 1.5 so for two passes the maximum loss will be... 

The method of calculating frictional losses is described in Chapter 2. It may be noted here that losses occur as the fluid flows through the plain pipe, pipe fittings (bends, valves), and at expansions and contractions such as into and out of vessels. 

ISO 3503 1976 Assembled joints between fittings and polyethylene (PE) pressure pipes -Test of leakproofness under internal pressure when subjected to bending ISO 3663 1976 Polyethylene (PE) pressure pipes and fittings, metric series - Dimensions... 

Viscous systems are the subject of continuing research by the US DIERS Users Group and in Europe. Research projects include the flow of high viscosity two-phase mixtures in safety valves, and the effect of bends and pipe fittings on high viscosity two-phase flow in pipes. 

The length of straight pipe of given roughness which is equivalent in frictional resistance to the actual piping which contains bends and fittings. 

Correlations are available for pressure drops in flowthrough pipe fittings, such as elbows, bends, and valves, and for sudden contractions and enlargements of the pipe diameter as the ratio of equivalent length of straight pipe to its diameter. 

The flow in pipe fittings, e.g. bends and valves, is generally too complex to determine theoretically. For turbulent flow, these minor losses are approximately equal to the square of the flow velocity. Thus, we define a loss coefficient, K,... 

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