Pipes size selection

If the motive power to drive the fluid through the pipe is available free, for instance when pressure is let down from one vessel to another or if there is sufficient head for gravity flow, the smallest pipe diameter that gives the required flow-rate would normally be used. 

If the fluid has to be pumped through the pipe, the size should be selected to give the least annual operating cost. 

Typical pipe velocities and allowable pressure drops, which can be used to estimate pipe sizes, are given below  

Rase (1953) gives expressions for design velocities in terms of the pipe diameter. His expressions, converted to SI units, are  

Simpson (1968) gives values for the optimum velocity in terms of the fluid density. His values, converted to SI units and rounded, are  

Piping systems must be designed so as not to impose unacceptable stresses on the equipment to which they are connected. 

The weight of the pipes, their contents, insulation, and any ancillary equipment  

Loads imposed by the operation of ancillary equipment, such as relief valves  

Thermal expansion is a major factor to be considered in the design of piping systems. The reaction load due to pressure drop will normally be negligible. The dead-weight loads can be carried by properly designed supports. 

Flexibility is incorporated into piping systems to absorb the thermal expansion. A piping system will have a certain amount of flexibility due to the bends and loops required by the layout. If necessary, expansion loops, bellows, and other special expansion devices can be used to take up expansion. 

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Nolte, C. B. (1978) Optimum Pipe Size Selection (Trans. Tech. Publications). 

Optimum exit pipe size selection pump liquids at >1.5 m/s hydrocarbons with low conductivity <0.3 m/s Suction pipe of larger diameter to prevent cavitation NPSH requirement [m] = rpm/(5400 volumetric flow rate [L/s]) ... 

Table 7-1. Alloy Pipe Size Selection for Various Payout Times...

PULIDO-CALVO I, GUTifiRREZ-ESTRADA t c et al. (2008) Pipes size selection of water distribution systems of fishfarms. Aquacultural Engineering 39(1) 43-52. 

The design procedure requires trial and error calculations. Pipes are available in fixed sizes and so the procedure adopted here is to select a pipe size and determine the saltation velocity from Equation (8.1). The system pressure loss is then calculated at a superficial gas velocity equal to 1.5 times the saltation velocity [this gives a reasonable safety margin bearing in mind the accuracy of the correlation in Equation (8.1)]. The calculated system pressure loss is then compared with the allowable pressure loss. The pipe size selected may then be altered and the above procedure repeated until the calculated pressure loss matches that allowed. 

In order to select the pipe size, the pressure loss is calculated and velocity limitations are estabHshed. The most important equations for calculation of pressure drop for single-phase (Hquid or vapor) Newtonian fluids (viscosity independent of the rate of shear) are those for the deterrnination of the Reynolds number, and the head loss, (16—18). 

Pipe-Wall Thickness. Once the design pressure and temperature have been established and the pipe material and size selected, the wall thickness is calculated using the appropriate section of the code. In rare cases, a thin pipe must be made thicker to withstand handling. Occasionally the thickness is affected by external loads or vibrations. All codes prescribe essentially the same design formula for metallic hoUow circular cylinders under internal pressure ... 

To be used as guide, pressure drop and system environment govern final selection of pipe size. 

The pitch of the coils and the area covered can be selected to provide the heat transfer area required. Standard pipe sizes from 60 imn to 120 mm outside diameter area are often used. Half-pipe construction can produce a jacket capable of withstanding a higher pressure than conventional jacket design. 

Calculations are best carried out in a tabular form. The value of < 4 is 29.7 mm. In practice the next larger standard pipe size would be selected. 

If more than one standard pipe size is indicated, calculate the wall thickness for each standard pipe size based on required maximum allowable working pressure and select a standard wall thickness for each -Size. 

As piping drawings are developed, re-evaluate those lines where estimated pressure drop was a criterion in size selection, taking into account the actual piping configuration and effects of control and piping components. 

Pressure drop through line systems containing more than one pipe size can be determined by (a) calculating the pressure drop separately for each section at assumed flows, or (b) determining the R totals iox each pipe size sep>-arately, and then converting to one selected. size and using that for pressure drop calculations. For example, using... 

From Table 2-4, select 6 ft/sec as design velocity for estimating pipe size. 

The system of Figure 2-27 consists of 125 feet of unknown size schedule 40 steel pipe on the discharge side of a centrifugal pump. The flow rate is 500 gallons per minute at 7o°F. Although the tank is located above the pump, note that this elevation difference does not enter into the pipe size-friction drop calculations. How ever it will become a part of selection of the pump for the serrice (see Chapter 3). For quick estimate follow these steps ... 

Determine Cj and C2 from Figure 2-31 and Table 2-11 for the steam flow rate and assumed pipe size respectively. Use Table 2-4 or Table 2-8 to select steam t eloc-ily for line size estimate. 

Calculation of condensate piping by two-phase flow techniques is recommended however, the tedious work per line can often be reduced by using empirical methods and charts. Some of the best are proprietary and not available for publication however, the Sarco method [42] has been used and found to be acceptable, provided no line less than VA" is used regardless of the chart reading. Under some circumstances, w hich are too random to properly describe, the Sarco method may give results too small by possibly a half pipe size. Therefore, latitude is recommended in selecting either the flow rates or the pipe size. 

Refer to the large pipe multipliers shown in the table on the chart, and select the pipe size whose factor is equal to or smaller than the result of step (11) above. This is the pipe size to use, provided a sufficient factor of safety has been incorporated in the data used for the selection of pipe size. 

Figure 7-20. Recommended API-520 piping for safety relief valve installations. Reprinted by permission, American Petroleum Institute, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part ll-lnstallation, API RP-520, 3rd Ed., Nov. 1988.

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