Water Well Journal

July 2015

Water Well Journal

Issue link: http://read.dmtmag.com/i/529509

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Page 43 of 83

W hether it's repairing an irrigation pump to supply badly needed water to rapidly wilting crops, deepen- ing a well for a family of five, or installing a brand new well and water system for a new subdivision—we are usually embroiled in tasks we routinely execute each day, but are generally deemed a higher priority during the so-called peak summer season. As you venture out day to day to tackle your tasks, I am sure learning more pump theory is not on your agenda. That's the reason this month's column will kick off a three-part series on the practical elements of the various components found in a typical water system you should always be able to find. Even though there will be some repetition from past columns and some theory included in each article, I will make it as painless as possible and only use it when needed to help explain the practical aspects. This first part will delve into the practical components of pumps and what you really should know about them to help you troubleshoot problems we all seem to run up against at one time or another. The second installment will discuss the many practical and often overlooked diagnostic characteristics of the drivers, most notably the electric motors used to actu- ally operate the pumps. The final installment will provide an overview on the controls and ancillary devices used as an interface between the operating environment, namely water, and the pump and motors used to deliver the water. The Pump When we refer to the definition of a pump, we are defining a machine that imparts energy to a fluid in order to move that fluid. Remember that term "energy"—since we are adding this energy to translate the fluid from a resting state, or what is called potential energy, to a moving state to become kinetic energy. There are two basic classes of pumps seen in common use today. Energy is imparted to the fluid to cause this transfer or a pressure increase in both cases. The first class is displacement pumps. Often used where precise volume output is required, such as chemical feed, these pumps are also referred to as positive displacement, gear, and rotary pumps. The second category is dynamic pumps. The main differ- ence between the two is the way energy is added to the fluid to be converted to a pressure increase. In dynamic pumps energy is added to the fluid continuously through the circular motion of impeller blades. These rotating blades raise the momentum of fluid and the momentum is then converted to pressure energy through a diffuser or case in the pump outlet. In positive displacement pumps the energy is periodically added to the fluid and the pump has reciprocating motion through pistons. When the fluid enters the pump through valves, the reciprocating piston begins to press the fluid— resulting in the fluid being pushed out of the pump with a resultant pressure rise. For our purposes, the rest of this discussion will be limited to dynamic pumps, specifically centrifugal pumps. Even though positive displacement pumps are also important to many of us who work with them regularly (for example, in chemical metering or injection pumps), the relative volume of centrifugal pumps used over positive displacement pumps is far greater. Determining Horsepower The dynamic pump, whether it is an end-suction centrifu- gal, a vertical turbine, or a submersible—they all work on the same basic principle. They all use the force, generally in horsepower, that is transferred from a driver, usually an elec- tric motor, into a pump. The horsepower is then applied to move a given volume of water against a given amount of re- sistance, or what is called total head. The total head, techni- cally referred to as total dynamic head (TDH), includes all of the vertical component (water surface to water surface, or lift), pressure (in psi, converted to head by multiplying by 2.31), friction losses, and minor losses. Calculating the TDH is not always an easy thing to do. Enough practice and an understanding of the components of head are needed for an accurate computation. However, if you lack the exact friction losses of the system, use a 10% add-on of the vertical lift and pressure for friction loss and you will usually be "in the ball park" for most water systems. This is where the initial factoid of what you should really know about pumps comes in. The relationship between the amount of mechanical horsepower it takes to move a given ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS Knowing the practical components of a pump helps you troubleshoot them. 42 July 2015 WWJ waterwelljournal.com A curve with reasonable accuracy is created for all operating conditions between the two most extreme plotted points.

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