Bends in pipelines

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/veloc-ity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion. 

Water hammer may cause extreme stresses at bends in pipelines. Consequently, liquid pockets should be avoided in steam lines through the use of steam traps and sloping of the line in the direction of flow. Quick-opening or quick-closing valves may cause damaging water hammer, and valves of this type may require protection by use of expansion or surge chambers. 

Eastwood and Sarginson described the experimental investigation of the effect of transition curves on the head loss in flow through 90° bends in pipelines [41]. They observed that for purely circular bend the loss at the bend could be expressed in terms of the equivalent length (L ) of the straight pipe to cause the same loss. 

Equation (8.15) relates to pressure losses along lengths of straight pipe. Pressure losses are also associated with bends in pipelines and estimations of the value of these losses will be covered in the next section. 

In pneumatic conveying systems, bulk particulate materials are physically transported by air. Bends in pipelines, therefore, are particularly vulnerable to erosive wear, as are diverter valves and any other surface against which particles are likely to impact, including the pipeline itself to a limited extent. Where a pressure difference might exist on a plant, in the presence of abrasive particles, erosive wear will also occur, if there is a flow of air. A particular example here is with rotary air locks and screws used to feed materials into positive pressure pipelines. Even isolating valves will wear if they are not completely air-tight or fully shut. 

When dealing with water treatment applications you carmot avoid pipe flow calculations. We have a pipeline in which the throughput capacity of 500 Liter/sec. The flow is split into two pipelines and the inside diamter of the pipe is 350 mm. The length of the pipeline is 55 m. The entry loss is 0.70 and the exit loss is 1.00. There are two 45° bends and two 90° bends in the lines, (a) Determine the flow per pipe (b) determine the line velocity (c) determine the resulting hydraulic loss in meters. 

Because Eq. (22) is implicit in /, an iterative method of solution must be employed. However, the convergence of this iteration usually presents no difficulty (B5, D2). Bending and Hutchison (B5) noted that in pipeline network calculations it was not necessary to calculate exact friction factors for each overall network iteration. In their experience the problem could be solved satisfactorily with single updating of factors for each overall iteration. This truncation results in significant reduction of computing time. In all... 

A significant development in pipeline network computation in the last few years was the introduction of the so-called linearization method first by Wood and Charles (Wll) and later, independently, by Bending and Hutchi-... 

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)... 

In pipelines, especially those that are highly stressed from internal pressure, uniform and adequate support of the pipe in the trench is essential. Unequal settlements may produce added bending stresses in the pipe. Lateral thrusts at branch connections may greatly increase the stresses in the branch connection itself, unless the fill is thoroughly consolidated or other provisions are made to resist the thrust. Rock shield shall not be draped over the pipe unless suitable backfill and padding are placed in the ditch to provide a continuous and adequate support of the pipe in the trench. 

As vaporized methanol flows along the pipeline shown in Figure 8.1, it partitions into any produced water, along with water condensed from the gas. Hydrate inhibition occurs in the free water, usually at water accumulations with some change in geometry (e.g., a riser, bend, or pipeline dip along an ocean floor depression) or some nucleation site (e.g., sand, weld slag, etc.). 

Two typical sets of pressure measurement data for the vertically down and vertically up sections of pipeline are presented in Fig. 2. This shows the location of the pressure tappings and their proximity to the various bends in the pipeline. The data relate to the pneumatic conveying of a fine grade of pulverised fuel ash. Five different bulk particulate materials were investigated in the research programme, the other four being barytes, bentonite, cement and fluorspar. All five materials were capable of being conveyed in dense phase and hence at low velocity. 

Data obtained with cement and analysed in terms of an equivalent length of straight horizontal pipeline are presented in Fig. 13. This is for 90° bends having a bend diameter D, to pipe bore d, ratio of 24 1 in 53 mm bore pipeline in horizontal plane. Almost identical results were obtained when a similar analysis was carried out for the conveying of barytes. A simple correlation in terms of the conveying line inlet air velocity was not expected, but it was not possible to determine any effect of the position of the bends in the pipeline. Data obtained with barytes and analysed in terms of a pressure drop across a bend are presented in Fig. 14. Once again, almost identical results were obtained for the conveying of cement. These data are for the same bends reported in Fig. 13. 

A proper geometric design of the system in order to get a laminar flow with minimum turbulence, as in pipelines of large diameters and avoid abrupt changes or streamline bends. 

Sharp bends in the pipelines should be avoided as more erosion can occur. 

With a long pipeline, a 25 HP vacuum pump could be used. If the line could be shortened, a 10 HP unit would convey the same amount of plastic, resulting in power savings. One foot of vertical height in the line equals 2 ft of horizontal distance in its effect on conveying rates. Bends in the pipe add a considerable amount of equivalent footage. 

Eastwood, W. and E. J. Sarginson, The Effect of a Transition Curve on the Loss of Head at a Bend in a Pipeline , Proc. Instn. CivU. Engrs. 16 129-142 (1960). 

Besides frictional losses, other losses in pipelines are caused by resistance due to bends, internal features, changes in pipe cross-section, branches, junctions, etc. The pressure losses due to these various features can be calculated from the following general expression ... 

With vacuum systems, materials can be picked up from open storage or stockpiles, and they are ideal for clearing dust accumulations and spillages. Pipelines can run horizontally, as well as vertically up and down, and with bends in the pipeline any combination of orientations can be accommodated in a single pipeline run. Material flow rates can be controlled easily and monitored to continuously check input and output, and most systems can be arranged for completely automatic operation. 

Pneumatic conveying plants take up little floor space and the pipeline can be easily routed up walls, across roofs or even underground to avoid any existing equipment or structures. Pipe bends in the conveying line provide this flexibility, but they will add to the overall resistance of the pipeline. Bends can also add to problems of particle degradation if the conveyed material is friable and suffer from erosive wear if the material is abrasive. 

For materials that are abrasive, alternatives to conventional systems may have to be considered in order to reduce damage to the conveying system, particularly if the materials are not naturally capable of being conveyed in the dense phase mode, and hence at low velocity. A similar situation applies to materials that are friable, for considerable damage can occur to the conveyed particles. Particle degradation can occur in high velocity suspension flow, and erosion of bends in the pipeline and other plant surfaces subject to particle impact will occur if an abrasive material is conveyed in dilute phase. 

Oil-free air Oil-free air is generally recommended for most pneumatic conveying systems and not just those where the material must not be contaminated, such as food products, pharmaceuticals and chemicals. Lubricating oil, if used in an air compressor, can be carried over with the air and can be trapped at bends in the pipeline or obstructions. Most lubricating oils eventually break down into more carbonaceous matter which is prone to... 

Pressure drop Because of the change in direction, impact of particles against bend walls, and general turbulence, there will be a pressure drop across every bend in any pipeline. The major element of the pressure drop, however, is that due to the re-acceleration of the particles back to their terminal velocity after exiting the bend. The situation can best be explained by means of a pressure profile in the region of a bend, such as that in Figure 4.29. 

Figure 4.29 Pressure drop elements and evaluation for bends in a pipeline.

These values relate to steady flow conditions in pipelines remote from the point at which the material is fed into the pipeline, bends in the pipeline and other possible flow disturbances. At the point at which the material is fed into the pipeline, the material will essentially have zero velocity. The material will then be accelerated by the conveying air to its slip velocity value. This process will require a pipeline length of many metres and this distance is referred to as the acceleration length . The actual distance will depend once again on particle size, shape and density. The process was illustrated earlier in Figure 4.29 in relation to the pressure drop across a bend. 

Velocity The model for the erosive wear of pipeline bends in terms of velocity is ... 

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