Pressure drop bends

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. 

Compared with elbow fittings, bends with a centerhne radius of three or five nominal pipe diameters save the cost of joints and reduce pressure drop. Such bends are not suited for instaUation in a bank of pipes of unequal size when the bends are in the same plane as the bank. 

Cascade coolers are a series of standard pipes, usually manifolded in parallel, and connected in series by vertically or horizontally oriented U-bends. Process fluid flows inside the pipe entering at the bottom and water trickles from the top downward over the external pipe surface. The water is collected from a trough under the pipe sections, cooled, and recirculated over the pipe sections. The pipe material can be any of the metallic and also glass, impeiMous graphite, and ceramics. The tubeside coefficient and pressure drop is as in any circular duct. The water coefficient (with Re number less than 2100) is calculated from the following equation by W.H. McAdams, TB. Drew, and G.S. Bays Jr., from the ASME trans. 62, 627-631 (1940). 

Thermal Expansion in Flare Header - Sliding-type expansion joints may be used in flare headers as an alternative to piping expansion loops, if required to achieve a reduction in pressure drop or where expansion bends may result in liquid surging, subject to the following conditions ... 

The equivalent length of the main flare header is then calculated from the flare stack to the last safety valve, taking into consideration the straight length of the pipe and approximate equivalent lengths for bends, etc. If the achial location of the flare stack is not known by that time, it maybe assumed to be 500 ft from the last piece of equipment. Later on, even if it varies from 500 ft, it will not affect the pressure drop calculation at all compared with the entire length of the pipe. 

If large compressor plant requires clear headroom for cranes, air intakes may have to mn through under floor piping or ducting. Intake ducts must be of a cross-sectional area sufficiently large to avoid excessive pressure drop, and the number of bends should be kept to a minimum. The ducts should be of non-corrosive material and care should be taken that extraneous material cannot enter the duct. The duct should be cleaned thoroughly before connection to the compressor. 

Sometimes insufficient differential across the regenerated catalyst slide valve is not due to inadequate pressure buildup upstream of the valve, but rather due to an increase in pressure downstream of the slide valve. Possible causes of this increased backpressure are an excessive pressure drop in the Y or J-bend section, riser, reactor cyclones, reactor overhead vapor line, main fractionator, and/or the main fractionator overhead condensing/cooling system. 

The pressure drop in the Y or J-bend section could be from improper fluidization or a flaw in the mechanical design. There are often fluffing gas distributors in the bottom of the Y or along the J-bend that are designed to promote uniform delivery of the cataly.st into the feed nozzles. Mechanical damage to these distributors or too little or too much fluffing gas affect the catalyst density, causing pressure head downstream of the slide valve. 

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 pressure drop over the inlet and outlet pipes could also be estimated, taking into account the bends, and expansions and contractions. 

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 ... 

Water flows through a 30° pipe bend at a rate of 200 gpm. The diameter of the entrance to the bend is 2.5 in., and that of the exit is 3 in. The pressure in the pipe is 30 psig, and the pressure drop in the bend is negligible. What is the total force (magnitude and direction) exerted by the fluid on the pipe bend ... 

Pipeline Routing and Bend Design. Direction changes should be kept to an absolute minimum in order to keep the number of bends that are considered to be the major attrition sources as low as possible. This helps also to reduce the conveying line pressure drop and with it the gas expansion effect. 

New test-design procedures for the accurate prediction of pipeline pressure drop, including the effects due to horizontal/ vertical flow and bends. 

Good flow splitting design is dependent on the accurate prediction of the pressure drop caused by the various bends, branches and straight sections of pipe. This can be achieved by employing the above branch model(s), proven for the particular material and application, coupled with the accurate pipeline test-design procedure described in Sec. 2.4 of this chapter. 

Pan, R., and Wypych, P. W., Bend Pressure Drop in Pneumatic Conveying of Fly Ash, Powder Bulk Solids Conf., Reed Exhibition Companies, USA, Proc, pp. 349-360, Rosemont, Illinois, USA (1992b)... 

Wypych, P. W., and Pan, R., Pressure Drop due to Solids-Air Flow in Straight Pipes and Bends, Freight Pipelines, (G. F. Round, ed.), pp. 49-67, Elsevier Science Publishers BV, Amsterdam, Netherlands (1993)... 

As the wave front moves forward, the reflected overpressure on the face of the structure drops rapidly to the side-on overpressure, plus an added drag force due to the wind (dynamic) pressure. At the same time, the air pressure wave bends or "diffracts" around the structure, so that the structure is eventually engulfed by the blast, and approximately the same pressure is exerted on the sides and the roof. The front face, however, is still subjected to wind pressure, although the back face is shielded from it. 

For example, due to the inevitable friction and impact of the channel surface to fluid flows, particularly at the bending part of the flow channels, how to reduce fluid flow rate changes or fluid pressure drop from inlet to outlet has to be considered in the fluid field design. One key part of the flow field... 

Fig. 41. (a) 3D simulation of a DPF placed downstream of a 45° bend in the exhaust, (b) Soot mass and velocity distribution at the inlet face at different soot loads of the filter, (c) Evolution of radial soot mass profile (along the dotted line) at different soot loads of the filter and resulting pressure drop (see Plate 15 in Color Plate Section at the end of this book). 

Only the irreversible frictional pressure drop should be included in the calculation of upstream pressure drop, not the momentum pressure drop required to accelerate the fluid to the velocity at inlet to the valve. The irreversible frictional pressure drop includes both friction in the inlet contraction from the reactor.(K = 0.5 for a sudden contraction1151) and friction in the piping, bends and any 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. 

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