Sizing centrifugal pumps

Impervious graphite centrifugal pumps, pipe fittings, and valves were developed because most chemical processes require the movement of Hquids. Graphite pipe and fittings in sizes ranging from 25 to 635 mm ID are used to convey corrosive fluids. 

The centrifugal pump is the type most widely used in the chemical industiy for transferring liquids of aU types—raw materials, materials in manufacture, and finished produc ts—as well as for general services of water supply, boiler feed, condenser circulation, condensate return, etc. These pumps are available through avast range of sizes, in capacities from 0.5 mVh to 2 X 10 mVh (2 gal/min to 10 gaJ/min), and for discharge heads (pressures) from a few meters to approximately 48 MPa (7000 Ibf/iu"). The size and type best suited to a particular apphcation can be determined only by an engineering study of the problem. 

The precise flow-decay pattern will depend on the type, size, and dimensions of the pump. Flow for a typical centrifug pump will begin to decay at the NPSH point, but some additional fluid transfer will usually occur before a steady backflow of air through the pump begins. At that point, the pump s priming is completely lost. 

As the flow rate and quantity of liquor are the most important controllable variables in developing design data, a feed pump suitable for accurate and continuous flow is required. Depending on the size of the pilot column system, the use of peristaltic, diaphragm, piston-type or centrifugal pumps are recommended. The feed pump should be used in combination with a volumetric or gravimetric flow control... 

Positive pumps are normally fitted with relief valves. These are not usually fitted to centrifugal pumps unless the process material is likely to explode if it gets too hot. As an alternative to a relief valve, such pumps may be fitted with a high-temperature trip. This isolates the power supply. Or a kick-back, a small-diameter line (or a line with a restrietion orifice plate) leading from the delivery line baek to the suction vessel, may be used. The line or orifice plate is sized so that it will pass just enough liquid to prevent the pump from overheating. Small-diameter tines are better than restriction orifice plates as they are less easily removed. 

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 ... 

This chapter presents information on rating, sizing, and specifying process pumps. The emphasis will be on centrifugal pumps, which are by far the most widely used in the process industries however, applications of other types of pumps will also be discussed (see Table 3-1). 

To properly accomplish a good and thorough rat-ing/sizing of a centrifugal pump, the plant system designer should at a minimum ... 

Dusts, particle sizes, 225 Dusts, hazard class, 521-523 Explosion characteristics, 524 Efficiency, centrifugal pumps, 200 Ejector control, 380 Ejector systems, 343, 344, 351 Air inleakage, table, 366, 367 Applications, 345 Calculations, 359-366 Chilled water refrigeration, 350 Comparison guide, 357, 375 Evacuation lime, 380, 381 Charts, 382 Example, 381 Features, 345... 

Rupture disk, 455 Safety relief valves, 454, 467, 481 Specifications, centrifugal pumps, 209 Spray nozzle particle size, 225 Standards and Codes, 31, 32, 33 Static electricity, 536 Static mixing, 332 Applications, 336 Calculations, 337, 338 Materials of construction, 337 Principles of operation, 335 Type of equipment 334-338... 

For the designer, the use of a standardised component size allows for the easy integration of a piece of equipment into the rest of the plant. For example, if a standard range of centrifugal pumps is specified the pump dimensions will be known, and this facilitates the design of the foundations plates, pipe connections and the selection of the drive motors standard electric motors would be used. 

The efficiency of centrifugal pumps depends on their size. The values given in Figure 10.62 can be used to estimate the power and energy requirements for preliminary design purpose. The efficiency of reciprocating pumps is usually around 90 per cent. 

When the pressure increase across the centrifugal pump is less than 125 psi (9 kg/cm2) and the flow rate is less than 300 GPM (70 m3/hr) the pump specified should be large enough to meet expansion needs.39 This possibility should also be considered for other situations. A smaller impeller can be used to meet the lower initial requirements. Then when expansion occurs this can easily be replaced with the normal-sized one. The engineer must check to see that under both situations enough power is produced and there is an adequate NPSH. 

The power required to drive a centrifugal pump and the pressure that the pump will develop depend upon the size (diameter) and speed (angular velocity) of the impeller, the volumetric flow rate through the pump, and the fluid properties. However, if the fluid is not too viscous (e.g. less than about 100 cP), the pump performance is essentially independent of the fluid viscosity. Under these conditions ... 

This shows that the pump head is determined primarily by the size and speed of the impeller and the pump efficiency, independent of the flow rate of the fluid. This is approximately correct for most centrifugal pumps over a wide range of flow rates. However, there is a limitation to the flow that a given pump can handle, and as the flow rate approaches this limit the developed head will start to drop off. The maximum efficiency for most pumps occurs near the flow rate where the head starts to drop significantly. 

You must chose a centrifugal pump to pump a coal slurry. You have determined that the pump must deliver 200 gpm at a pressure of at least 35 psi. Given the pump characteristic curves in Appendix H, tell which pump you would specify (give pump size, speed, and impeller diameter) and why What is the efficiency of this pump at its operating point, what horsepower motor would be required to drive the pump, and what is the required NPSH of the pump The specific gravity of the slurry is 1.35. 

Two different size pumps are said to be geometrically similar when the ratios of corresponding dimensions in one pump are equal to those of the other pump [Holland and Chapman (1966)]. Geometrically similar pumps are said to be homologous. A set of equations known as the affinity laws govern the performance of homologous centrifugal pumps at various impeller speeds. 

The process engineer s job is to size both the centrifugal pump and the control valve. The bigger the control valve, the less pressure drop it will take. This means a lower-head pump can be used and energy costs will be tower... 

The designer must specify the maximum flow rate that is required under the worst conditions and the minimum flow rate that is required. Then the valve flow equations for the maximum and minimum conditions give two equations and two unknowns the pressure head of the centrifugal pump APj> and the control valve size C . 

J4. Reactant liquid is pumped into a batch reactor at a variable rate. The reactor pressure also varies during the batch cycle. Specify the control valve size and the centrifugal pump head required. Assume a flat pump curve. 

In short process lines slurries are readily handled by centrifugal pumps with large clearances. When there is a distribution of sizes, the fine particles effectively form a homogeneous mixture of high density in which the settling velocities of larger particles are less than in clear liquid. Turbulence in the line also helps to keep particles in suspension. It is essential, however, to avoid dead spaces in which solids could accumulate and also to make provisions for periodic cleaning of the line. A coal-oil slurry used as fuel and add waste neutralization with lime slurry are two examples of process applications. 

Pressure drop. Good control requires a substantial pressure drop through the valve. For pumped systems, the drop through the valve should be at least 1/3 of the pressure drop in the system, with a minimum of IS psi. When the expected variation in flow is small, this rule can be relaxed. In long liquid transportation lines, for instance, a fully open control valve may absorb less than 1% of the system pressure drop. In systems with centrifugal pumps, the variation of head with capacity must be taken into account when sizing the valve. Example 7.2, for instance, illustrates how the valve drop may vary with flow in such a system. 

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