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Unless a brand new plant is being designed, users decide to replace a pump because of its age and wear or persistent reliability issues.

Plant engineers typically spend their time with the process to make sure machinery is working, water is flowing, power is produced, lights are up and no environmental problems are developing. They are not, as a rule, experts on any particular type of machinery.

They are basically generalists, having learned to rely on qualified suppliers, who are experts within their particular niche (pumps, centrifuges, boilers, generators, etc.). When a pump fails, it is usually replaced with a new one, without much analysis or discussion. If it continues to fail frequently, a new supplier is approached for a better, more reliable pump.

Occasionally, a relatively minor modification to the process, like an addition of a cooling (or heating) piping loop, for example, is needed. It may not be a particularly complex system, and hiring major design contractors may not be economical for such a small project. Yet, it might still be beyond the expertise of the plant engineers, maintenance and operating personnel.

So, how is a pumping system, simple or complex, actually designed? Details of pump performance curves, types, pressure, power or efficiency are usually not on the horizon at this initial stage.

All the plant knows is their requirements. Maybe they want to pump 1,000 gallons per minute (gpm) from a cold water tank 2 miles away to a heat exchanger and return the water to the tank. Thus, the details of the pump will start to emerge.

1. Before talking about a pump, consider the pipe.

Velocity of liquid in pipes ranges between 3 to 10 feet per second (ft/sec). If the velocity is too slow, the dirt, sludge or other contaminants can settle. If flow is too fast, abrasive wear will reduce the life of the pipe. Plant designers are familiar with the specific concerns for each application. A sludge stream will have a larger pipe than a clean water application. But for a &#;nonexpert,&#; a good starting point could be, say, 5 ft/sec. Solving for pipe diameter (1,000 gpm, 5 ft/sec), we get d = 9.1 inches, so we round it to 10 inches to fit available pipe sizes. For now, we will not consider pipe schedule, wall thickness, etc.

2. Now that we have the pipe, pressure is the next step.

Pressure comes from friction and elevation. We will assume no elevation changes along the pipe run. Friction losses are determined from a well-known Moody Diagram, from which a friction coefficient is found and then friction losses (h) are calculated (see Image 1).

Image 1. Moody Diagram to determine friction losses (Images courtesy of the author)

Image 1. Moody Diagram to determine friction losses (Images courtesy of the author)

This is the friction loss a pump pressure would need to work against.

The Moody Diagram has lots of helpful information on it: Reynolds number (Re), pipe type/age, roughness, and thus friction coefficient, as seen on Image 1, may range from 0.01 to 0.1, potentially an error. Fortunately, some of this can be simplified.

Re = 5 ft/sec x (10/12) (ft) / 10-6 = 4 x 106 - i.e. turbulent region and, from Image 1, we already cut down the friction factor to start from at least 0.2. If we reduce this region from the rough pipe and super smooth pipes, we find that an average will be around f = 0.03 for an iron pipe of 10 inches diameter.

Link to Huakai Anti-Corrosion Equipment

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3. We now can estimate the power.

See Equation 4. This will likely require a 40-horsepower motor. Note our &#;bold&#; assumption of the efficiency being 70 percent&#;a rough guess we need to refine now.

Image 2. Pump efficiency calculator results

Image 2. Pump efficiency calculator results

4. Go to the &#;Pump Efficiency Calculator&#; program.

Plug the numbers into pump-magazine.com/pump_magazine/pump_magazine.htm (See Image 2). The actual efficiency predicted by the program is 81.2 percent, better than our estimated 70 percent, and the motor could be smaller. However, also consider that the pump might &#;run out&#; on the curve some times when the flow is greater. So a slightly higher value on the motor horsepower would be prudent.

5. Refine the selection.

Now we need to refine our selection by the pump type, number of stages, speed of the motor (which may change overall size and efficiency) net positive suction head (NPSH) requirements, etc. But that is for the next time.

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What information is required to make a pump selection?

Our aim at MSE is to offer you the best solution to meet your application requirements and circumstances. The more information that you are able to provide us with; the more tailored and effective the solution that we can offer.

Basic information required for all pump selections

With the following information, we are able to make a basic pump selection;

Liquid name
Knowing the liquid that you are pumping is vital as it allows us to ensure that we offer a pump constructed from compatible materials; thus avoiding potential corrosion and abrasion issues. What is the chemical make up? Are there any solids present, if so what is the maximum particle size and concentration?

Flow rate
This will determine the size of the pump required. A higher flow rate requires a larger pump.

Pressure
The pressure at the inlet and outlet of the pump will determine the type and often the size of pump required. Knowing the pressure you are pumping against allows us to select the most suitable pump technology.  If you are unsure of your differential pressure; we can help to calculate it

Viscosity
There are many units of measurement for viscosity however we tend to work in centipoise cP or centistokes cSt. Viscosity is a measure of a liquid&#;s resistance to deformation caused by stress, or more plainly; the &#;thickness&#; of a liquid. Viscosity is typically higher for thicker liquids, for example; water has a viscosity of 1 cp at 20 degC whereas honey has a viscosity of approximately cp. Viscosity affects the type and size of the pump required, with higher viscosities usually requiring positive displacement units running at lower speeds rather than centrifugal pump solutions.

Density
The density or specific gravity of the pumping liquid at the operating temperature will affect how much power is required to achieve the required duty. This in turn will help us size a suitable drive or motor to operate the pump without a problem.

Temperature
This can affect the materials of construction for the pump, and the type of pump offered.

Additional information

To provide a more personalised selection, the following information is helpful to have; 

• Motor requirements
  Electric or air? If electric, what voltage and frequency do you require?

• Control
  Do you require any special controls for the pump? Will you be running at a fixed speed or is a variable speed drive required?

• Usage cycle
  Will the pump be run continuously or intermittently?

• Area of use
  Will the pump be operated in an ATEX area? If so, what classification is required?

• Suction set up
  How will you be feeding the pump?

• Vapour pressure
  Do you know the vapour pressure of your process liquid? This is more relevant when pumping at elevated temperatures.

• Certification
  Is any certification required with the pump? For example, are you operating in the food industry and need to comply with FDA guidelines?

• Budget
  Roughly, what budget do you have available for the project? Often there is more than one solution for a pumping application. 
  
  

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