Layout pump NPSH

The pressure drop (top LHS of Fig. 2.1) depends on the diameter, length and type of pipe and fittings, the elements for process control, the configuration, the velocity and the layout for NPSH. The latter, NPSH, depends on the vapor pressure, the temperature, the design of the vortex breaker, the NPSH required by the pump, the vertical component of the configuration. The resultant pressure drop expressed as a function of the volumetric flowrate summarizes the system needs. 

A centrifugal pump is to be used to circulate liquid (density 800 kg/m3 and viscosity 0.5 mN s/nr) from the reboiler of a distillation column through a vaporiser at the rate of 400 em /s, and (o introduce the superheated liquid above the vapour space in the reboiler which contains liquid to a depth of 0.7 in. Suggest a suitable layout if a smooth bore 2.5 mm pipe is to be used. The pressure of the vapour in the reboiler is i tcN/nr and the NPSH required by the pump is 2 in of liquid. 

When the diagram is laid out, it is only necessary to show the relative elevation of the process connections to the equipment where they affect the process operation for example, the net positive suction head (NPSH) of pumps, barometric legs, siphons, and the operation of thermosyphon reboilers. Full details of pipe layout are usually shown in a different drawing, known as a piping isometric drawing. See Figure 5.19 for an example. 

Exhibit 4-27 illustrates how primary process considerations govern the elevation of the surface condenser and some of its auxiliary equipment. Because a pump is needed to remove the condensate from the hot well, the pump net positive suction head (NPSH) must be satisfied to operate satisfactorily. Vertical pumps are generally used in this application because the NPSH requirement is calculated from the bottom of the lower impeller for vertical pumps, compared with the centerline of the inlet nozzle for horizontal pumps. Setting the elevation of the inter-after condenser, usually located to the side of the suifice condenser, can be done with fewer restriaions. Ilie bottom of the inter-after condenser shell must not be more than 3 ft (900 mm) below the condensate return nozzle on the side of the hot well. The plant layout designer is then free to set the elevation as high as necessary. 

Net positive suction head NPSH is one of the most important terms a plant layout designer needs to understand when developing an equipment layout that includes pumps and ve.s.sel.s. The required net positive suction head is a mea.sure of the pressure drop of tlie liquid as it moves from the inlet of the pump to the eye of the impeller. It is a characterLstic of the pump that i.s generally determined by te.sting and is ex-pre.ssed in feet of water by the pump manufacturer. 

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