Bends and fittings

Viscous Transport. Low velocity viscous laminar dow ia gas pipes is commonplace. Practical gas dow can be based on pressure drops of <50% for low velocity laminar dow ia pipes whose length-to-diameter ratio may be as high as several thousand. Under laminar dow, bends and fittings add to the frictional loss, as do abmpt transitions. 

If relief is via a bursting disc, the flow capacity of the relief system will normally depend on friction and choke points in the relief system. The only exception is where friction is not important (LE/D less than about 40), where equation (A6.4) can be used.) Where friction is significant, an isometric sketch of the route of the relief system will be required to determine the capacity. If the system is to be of constant diameter, then using the sketch, the total equivalent length, LE, of the route, including the frictional resistance of bends and fittings can be determined111. This can also be expressed in terms of total frictional velocity head loss, K ... 

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

Equation (4.23) applies to a theoretical pipe, but for a real pipeline additional allowances need to be made for pipe entry and exit losses, bends and fittings. Consider a fluid flowing in a typical process plant pipeline as shown in Figure 4.2. 

The frictional effects of bends and fittings are often expressed in terms of a quantity called the velocity head drop . To introduce this concept, we begin by observing that the specific energy lost to friction, F(J/kg), for incompressible flow follows from an integration of the Fanning equation (4.20) with friction factor and velocity constant over the length of pipe ... 

The total pressure drop due to all bends and fittings is found by summation of equation (4.50) ... 

Here Kj represents the total number of velocity heads lost to friction, from pipe friction, entrance loss and losses due to bends and fittings ... 

Bends and fittings. Piping systems seldom consist of straight pipes of constant diameter. It is therefore necessary to account for pressure drops and mechanical energy losses associated with bends, changes in diameter, valves and other fittings. 

Table 7.1 Loss coefficients for bends and fittings in turbulent How...

The first full scale experiment in a heating circuit have been carried out in the Northern district of Prague at a local thermal station. A maximum drag reduction of 12.8 % was achieved. The low drag reduction for the polymer concentration 225 ppm Separan AP 273 was caused by mechanical degradation of the polymer in a great number of bends and fittings in the system. 

Moller, K. and G. Elmqvist, Head Losses in Pipe Bends and Fittings , Tappi 63 101-104 (1980). 

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