• Isolated Footing Design Example and Excel Sheet
• Pump Sizing and Selection Made Easy

Liu, Virginia Tech University January 1, Viscosity, power consumption, commercial availability and lifecycle cost analysis are all important considerations in pump sizing. An automated spreadsheet method helps engineers take those factors into account in centrifugal pump selection Many aspiring chemical engineers enter industry after university study without sufficient practical knowledge about how to properly size pumps. A number of recent articles provide useful guidelines for sizing and selecting pumps, but these articles focus on certain specific aspects of proper pump sizing, while leaving out others [1—4].

Chemical engineering literature does not fully cover other essential aspects of pump sizing and selection — including the viscosity correction, power consumption, commercial availability and lifecycle cost analysis. The initial purchase price of a pump is only a small fraction of the total lifecycle cost. There are situations in which purchasing a less expensive pump actually leads to greater energy-usage costs. This results in a higher lifecycle cost see Example 1, below.

Without a proper understanding of the pump selection process, engineers cannot effectively make both economic and practical decisions. This article aims to fill in some of the gaps in understanding and provide a straightforward method for pump sizing and selection.

Along with this article, we have created a useful Microsoft Excel spreadsheet to assist with centrifugal pump sizing. The automated Excel spreadsheet assists in calculating the key parameters for pump sizing and selection. Since the majority of the pumps used in the chemical process industries CPI are centrifugal pumps, this article focuses on that equipment category, rather than the other general classes of pumps, such as rotary and positive displacement pumps.

Example 1. Pump Sizing and Selection The following is a pump sizing problem to illustrate the calculations in this article. You are told to purchase a pump for your manufacturing facility that will carry water to the top of a tower at your facility. Assume BHP is 32 and 16 horsepower for the 3,rpm and 2,rpm pumps, respectively, for all pump choices in the composite curve. Assume all of the pumps are viable for your required flowrate.

The suction-side pipe and discharge-side pipe diameters are 4 and 3 in. The suction tank elevation S is 12 ft, and the discharge tank elevation D is ft. Assume that both hd,f and hs,f are roughly 10 ft. The pump curves in Figure 3 illustrate the following pump options to choose.

Notice that most of the TDH comes from the significant elevation difference between the suction and discharge side. Now that two pumps are feasible from the perspective of TDH requirements, you can compare the economics. At first glance, it is tempting to choose Option 1, since the initial investment is significantly lower. Although Option 2 has a higher initial cost, the lifetime cost over five years is dramatically lower.

The problem shows that, in selecting a pump, the costs associated with power consumption and maintenance are critical pieces of information for making an informed decision. Pump sizing overview The concept of a pumping system is rather simple. The suction side refers to everything before the pump, while the discharge side refers to everything after the pump. Figure 1 illustrates a simplified pumping system. A key parameter in characterizing a pump is the total dynamic head TDH , which is the difference between the dynamic pressure of the discharge side and the suction side.

The dynamic pressure represents the energy required to do the following: 1 to raise the liquid level from the suction tank to the discharge tank; 2 to provide liquid velocity inside both suction and discharge piping; 3 to overcome frictional losses in both suction and discharge piping; and 4 to pump the liquid against the pressure difference between the suction and discharge tanks.

Figure 1. The following components are needed to calculate total dynamic head: suction and discharge elevation; fluid velocity; friction loss and dynamic head; and tank pressure Six steps to pump sizing. In order to size a pump, engineers need to estimate the temperature, density, viscosity and vapor pressure of the fluid being pumped.

Pump sizing can be accomplished in six steps, as follows: Find the total dynamic head, which is a function of the four key components of a pumping system, such as the one shown in Figure 1 Correct for the viscosity of the fluid being pumped, since pump charts and data are given for water with a viscosity of 1 cP. The viscosity of other process fluids can differ dramatically Calculate the net positive suction head NPSH to select a pump that will not undergo cavitation Check the value of suction-specific speed to see if a commercial pump is readily available see section on suction-specific speed later in this article Check for potentially suitable pumps using a composite performance curve and an individual pump performance curve Compare the energy consumption and lifecycle cost of operating the selected pumps Pump Selection, Example 2 An additional pump selection problem is shown Example 2.

For this example, consider a discharge line that is 50 ft schedule 40, 4-in. The velocity is The elevation difference on the discharge side is 17 ft, the total dynamic suction head is 50 ft, and the pressure on the discharge side is Solution: For choosing the appropriate pump, see Figures 3 and 4. Both pumps should be analyzed by performing a lifecycle cost analysis using the pump efficiencies from the individual pump performance curves.

Calculating friction losses Pumps must overcome the frictional losses of the fluid in order for the fluid to flow in the suction and discharge lines.

These frictional losses depend on pipe roughness, valves, fittings, pipe contractions, enlargements, pipe length, flowrate and liquid viscosity. To calculate the frictional head losses, in feet of liquid being pumped, on the suction hs,f and discharge hd,f side of the pump, Equation 1 can be used. The same equation can be applied to calculate the frictional losses of the discharge side, but with the appropriate values correlating to the discharge side of the pump.

The first term in Equation 1 represents the frictional losses from the fluid flowing through a straight piece of pipe. The second term represents the frictional losses due to valves, fittings, pipe contractions and enlargements. We have provided the values for the typical resistance coefficients and pipe surface roughness from the chemical engineering literature in the Excel spreadsheet discussed in this article.

An illustration of this solution can be observed in Example 2 on page We also implement the same heuristic within the Excel spreadsheet. The Darcy friction factor fD can be calculated using the Churchill equation, Equation 2 , which is applicable for all values of Reynolds number Re.

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### Isolated Footing Design Example and Excel Sheet

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## Pump Sizing and Selection Made Easy

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