Pumps and Sytems, March 2009
Editor's Note: This is the second in a series of five articles based on the Hydraulic Institute's new Positive Displacement (PD) Pumps: Fundamentals, Design and Applications e-Learning course. To read the previous article, click here . To read the next article in the series, click here.
Positive displacement (PD) pumps are used in a myriad of applications across multiple industries. Users have found them to be the solution to many specific pumping challenges; however, due to their size, simplicity and ruggedness, they often are not as well understood as other pump types.
Technologies within the extensive positive displacement family cover a broad range of horsepower, fluid and pressure applications. These products merit increased consideration in a user's pump selection process. To assist pump users with a proper understanding of definitions, applications, installation, operation, maintenance and testing procedures, the Hydraulic Institute publishes ten ANSI/HI Standards covering PD pumps including: Air Operated, Controlled Volume Metering, Reciprocating and Rotary.
ANSI/HI standards perform a vital function in pump industry commerce and serve important roles in minimizing misunderstandings in the marketplace. The Hydraulic Institute has extended its mission to include the development of a pump knowledge and education portfolio in response to member and pump user needs. Among the first key elements are a re-launch of the Centrifugal Pump e-Learning course and the development of a new Positive Displacement Pump course covering fundamentals, design and applications.
Last month, we provided an overview of the curriculum and an overview of positive displacement pumps, as well as the 12 benefits of PD pumps.
This installment will focus on "Positive Displacement Pump Hydraulics," which introduces the fundamental physical concepts and fluid properties that affect positive displacement pump selection and operation.
Since many of these properties affect positive displacement pumps differently than centrifugal pumps, it is critical to understand the interaction between the pump and the fluid, and how the operation of positive displacement pumps differs from centrifugal pumps. Without this foundation of fundamental principles, it would be difficult to effectively learn about the myriad of positive displacement pumps (to be presented in the next three articles).
On the most basic level, pumps are used to provide pressure and/or flow so that the pump user can accomplish a specified task. With this premise in mind, note that positive displacement pumps create flow, not pressure. The pressure on the pump is a function of the system's reaction to the delivered flow due to pipe losses, restrictions and elevation changes. See Figure 1 for an explanation of gauge versus absolute pressure; these two different reference points have caused confusion through the years.
Figure 1. Gauge versus Absolute Pressure
Since positive displacement pumps theoretically generate flow independent of discharge pressure, Figures 2 and 3 show how the delivered flow rate is affected by the differential pressure and speed. This is a fundamental difference between positive displacement pumps and centrifugal pumps. The delivered flow rate is the theoretical flow rate minus the internal slip of the pump. The slip is the internal leakage that occurs in the pump due to clearances, viscosity and differential pressure, and will vary between pump types and applications.
Figures 2 and 3.
Of course, nothing comes for free, and every pump requires a certain amount of power to perform work. The pump input power is comprised of the theoretical liquid horsepower and the internal power losses at the operating point. The theoretical liquid horsepower is the work done to move the theoretical volume of fluid from inlet to outlet pressure and is solely based on the physical dimensions of the pumping elements, the operating speed and the differential pressure. On the other hand, the internal power losses account for the mechanical and viscous losses that occur as the pump operates. It is typical that the mechanical loss is the major component when operating at low viscosities, while the viscous loss is
