Myth Busters
Sizing a Pump Should Be Easier than Reading Greek
By Tom Aldridge
 When the ancient Greek mathematician Archimedes discovered the principles of density and buoyancy (known today as Archimedes’ Principle), he was in the bathtub. But that didn’t stop him from running down the street naked crying “Eureka!” (“I’ve found it!”) — or so the story goes. Contractors today have more composure, but they can also have similarly exciting reactions when applying the liquid principals of science to their pumping projects. Engineers, maintenance personnel, construction crews, utility contractors, municipal folks and even some homeowners should all be allowed to enjoy that satisfactory “I found it!” victory lap when discovering the perfect pump (clothed, of course).
Unfortunately, there are a number of obstacles to overcome in selecting the right pump for a project, especially when there may not always be a pump that is 100 percent applicable. Some of the obstacles and unknowns may include timing, technical knowledge, project details, availability and costs. However, by reviewing how a pump is selected and abolishing the typical myths surrounding pumps, contractors can find their Eureka moment when conquering the pump selection process.
General Pump Performance
Pumps are designed to operate at specific performance points that are defined along a curve to show the amount of fluid that can be pumped against a certain amount of resistance. This is the basic Flow-Head curve. In order to define a performance point, it is necessary to identify some basic information about the project:
What fluid is being moved?
Where from and to is fluid being moved?
How much fluid has to be moved and at what rate?
How deep does the fluid need to move from?
How high must the fluid be pushed?
How far does the fluid have to go?
What are the restrictions on space or access?
These items can give some general indication of the overall project and help define the desired performance. By simplifying the volume of fluid to be moved and how much time is desired to move the fluid, you can identify the flow rate. You can also determine the amount of fluid that is already moving in a system and has to be moved in order to keep the system operating. This gives the first part of the equation — the flow.
Looking at the characteristics of where you are pumping from and to, the amount of fluid and then how you want to carry the fluid (piping characteristics), you can determine the resistance that is being pumped against. Now you then have the second part — the head. There are a number of technical resources available to help determine these items, including handbooks on fluid dynamics, online calculators or references from pump and piping suppliers.
Myths Are for Kids
Now that we know the basics of how to identify the performance point and what it means in common sense terms, let us look at the reasons we identify this and dispel the common myths about pumps. In reality, pumps use energy and push fluid from Point A to Point B. The most common type of pump — the centrifugal — relies on the rotary motion to generate a low pressure area at the eye, or center, of the pump. Atmospheric pressure becomes the driver and pushes fluid into the eye, and the centrifugal motion moves the fluid along the blades or vanes of the impeller to the outlet. So, atmospheric pressure and pump design become limiting factors on a pump’s performance.
If That’s the Case, Will Any 6-in. Pump Do?
It is very easy to call and ask for a 6-in. pump in the construction industry. Nearly any supplier will have one to offer, and 6 in. is a common dimension for piping and hoses. But due to the differences in design, not all 6-in. pumps will operate the same. They may require different levels of horsepower depending on how much resistance they can operate against at the same flow. The impeller designs and clearances between moving parts will affect efficiency and basic performance. Dimensions on a pump are not necessarily performance indicators and primarily affect how the pump is connected to the system more than anything else.
 As noted earlier, by looking at the basic description of the system and answering two key items — how much and where — you can determine the desired performance. Sizing a pump may require some time and some research, but for the most part, the answers are right in front of you on a project. And the more you do it, the better you become at knowing what information can be estimated through common sense and experience.
Once you know what you have to pump, it is very easy to ask the next question: Why do I have to pump this? There will be many times that there will be other alternatives. But when it comes down to it, pumping is more of an insurance policy. If you consider the risks or results if you do or do not pump, the cost is sometimes very obvious. In a process system, if you can’t move the fluid, the process can’t continue. On a site, if you don’t move the fluid, you may not be able to work and there may be safety implications. Any of these delays or damages are typically more than offset by the cost of a temporary pumping system.
Planning Properly Is as Important as Pump Selection
Finding the best choice of pump to meet the performance required may not always be sitting in the yard waiting for someone to come rent it. With today’s economy, it is more and more difficult for pump suppliers to provide equipment on the fly since they and their suppliers may be minimizing inventory and increasing lead times. It is always better to plan on needing the pumping system and making sure it is available. Then, if it isn’t required, there won’t be last minute emergencies, delays and fees in order to get it.
Review and realize how critical pumping can be to the project so that you can identify the value. Eliminate the myths of pumping so that you understand why a certain pump is needed. Lastly, use the resources that are out there, including your pump suppliers, the Contractor’s Pump Bureau from the Association of Equipment Manufacturers (AEM) and the various publications to understand the basics of pumping. Pump selection does not need to be a mystery or chore. Based on the simple rules of thumb of how much and where, you can make a good decision on a pump system to add value to your project.
Thomas Aldridge is a Sales Engineer for Griffin Dewatering Corp. |
