Selecting a Pump for the Total System

Author: Ray Hardee, P.E.

Introduction

I am often asked the best way to arrive at a design point for pump selection.  My initial response is to understand how the pump elements will interact with the processes and control elements. This is necessary to get a clear view of the system as a whole, and to meet the systems design requirements.  When all the elements of the piping system are working together, the system will meet its design requirements while minimizing the total lifetime costs.  

In this two part article we will look at a simple system consisting of a collection tank, a pump, distribution tank, and the interconnecting piping as shown in figure 1 below.  This simple system is often used for distribution of water, bulk materials transfer, industrial waste collection, and agricultural irrigation.

Figure 1.  System showing the three principal elements found in every piping system.  We will size the system elements to meet the system’s design requirements.

Figure 1.  System showing the three principal elements found in every piping system.  We will size the system elements to meet the system’s design requirements.

Designing the System

When creating a piping system the first step is to understand its design requirements that must be met.  Once this is done we can size the process equipment, determine the method of control, and select the pumping equipment.  After all the systems are completed the plant is ready to come online. 

Design Requirement of the Drain Collection System

The function of the example system is to collect floor drains around the plant and sent then to a waste treatment system where the water can be reused.  The floor drains flow into the collection tank by gravity, the transfer pump moves the fluid to the distribution tank.  The distribution tank provides a constant source of fluid to the plants waste treatment system.  The floor drains have the property of water at 80 °F and it must be processed prior to reuse within the plant or prior to release to the environment.  

System Requirements

Based upon the projected plant operation it has been determined that the flow collection system must be sized to handle a maximum flow rate of 400 gpm.  Since the flow collection system is open to atmosphere and the process fluid is water at 80°F we have sufficient system requirements to begin our system design.  

The Process Elements

We will look at the tanks first.  The collecting tank capacity was sized to accept the design flow rate for 30 minutes, thus the tank was sized for 12,000 gallons.  Based upon the capacity and the weight of the full tank the foundation for the tank can be determined.  The weight of the tank and the available space within the plant will be used to determine the location and elevation of the collection tank.  In this example the tank elevation is 100 ft above the plant datum. 

The size and elevation of the distribution tank is based upon the needs of the capacity of the plants waste treatment system.  The elevation and location of the distribution system has been determine by the needs of the waste treatment system.  As a result the elevation of the distribution tank is established at 120 ft.  

The elevation of the collection and distribution tank is used in determining the systems static head.  The change in tank elevation is 20 ft.  To account for the difference in liquid levels in the tanks the level in the collection tank will be set to the lowest possible level of 1 ft, and the level in the distribution tank will be set to the tanks highest level of 12 ft.  As a result the static head of the system is 20 ft plus 11 ft or 31 ft. 

Based upon the plants pipe specifications the pipe material used in the waste collection system will be steel schedule 40 pipe.  Using the design flow rate of 400 gpm and the sizing velocity guidelines found in the pipe specification a 4 inch diameter pipe was selected.  Based on the estimated distance between the collection and distribution tank the head loss in the pipeline can be determined. 

The number and type of check valves and isolation valves can be selected to allow for proper system operation and maintenance.  Since the pipe routing has not been established an estimate on the number of elbows used for head loss calculations for the pipelines can be estimated. Adding the system’s static head and dynamic head loss in the pipeline results in head requirement to pump selection can be calculated.     

The Control Elements

Looking at the control elements, since this is relatively simple system the controls will be based on the level in the collection tank.  The pump will automatically start on a high level in the collection tank to prevent overflowing the tank.  The pump will automatically stop on a low tank level to prevent the pump from running dry.   

The Pump Elements

Based on the design flow rate of 400 gpm and the sum of the process elements static head and dynamic head a design point can be used for pump selection.  Based on the customer’s requirement a submersible sump pump will be used in this application.  Using the design point the pump supplier can determine a specific pump and motor size to meet the needs of the waste collection system.  

Wrapping up the System Design

Now that the process, control, and pump elements have been design the necessary equipment can be ordered and installed within the plant.  Once the construction is finished on the waste collection system it can be placed in service. This is the process that is followed on most system based on previous designs and it has worked this way for many years.  

Each one of the elements in the system is controlled by specific engineering disciplines.  The process group determines the type of process and capacity of the entire plant.  The civil department determines the plants footprint, and structural support and locations for large equipment and tanks.  The mechanical departments determine pipe sizing and routing needed to connect the equipment in the system.  The mechanical department may also determine the design requirements for pump selection.  The controls department typically determines the required instrumentations and method of controls. 

This process has been in use for years because it has always worked based on previous designs.  The first time anyone can see how the entire system works is when the plant is placed in operation.  During the initial plant startup is not the best time to discover problems within the plant design.  As a result if possible the plant will be utilized to make the product so as to obtain a return on the plant investment.  

In next months article we will look at ways to simulate the operation of the entire system during the design process and what can be done to improve simulate how the system should operate after completion.