Pumps & Systems, February 2009
Editor's Note: This is the first 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 next article in the series, click here .
Positive displacement 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 enable coverage of a broad range of horsepower, fluid and pressure applications. These products, therefore, 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.
Figure 1. Positive Displacement Pump Family Tree.
ANSI/HI standards perform a vital function in pump industry commerce and serve important roles in minimizing misunderstandings in the marketplace. The Hydraulic Institute, however, 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. These, and future courses, will be hosted within HI's new educational portal, http://www.pumplearning.org/.
Curriculum Overview
The PD pump e-Learning course is a five module internet-delivered learning program designed to provide users with broad and comprehensive knowledge of positive displacement pumping technologies. Material is highly visual and interactive, designed to allow students to take full advantage of the latest internet technology.
Content is arranged in independent modules with each one focusing on markets and applications, as well as providing basic recommended technical terms and fundamentals for an understanding of positive displacement pump hydraulics. The first two modules in the series are:
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Why Positive Displacement Pumps
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Positive Displacement Pump Hydraulics
Three other modules are each devoted to a specific positive displacement pump technology. To enhance the users learning experience, these modules rely heavily on color photographs of pumps and pumping installations.
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Rotary Pumps , including: Vane, Rotary Piston, Flexible Member, Lobe, Gear, Circumferential, Piston, Progressing Cavity, Timed Screw, Untimed Screw
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Reciprocating Pumps , including: Power, Direct Acting, Power Diaphragm, Air Operated Double Diaphragm, Air Operated Piston
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Metering Pumps , including: Torque Sources, Drive Mechanisms, Capacity Control, Liquid End Reviews
Multiple 1500-hp Rotary Pump Heavy Crude Loading Station.
Modules are designed for either self-instruction, instructor lead courses by the twelve PD pump company sponsors or as HI sponsored webinars. Each module, designed to stand alone or combined with others, includes an examination and completion certificates suitable for submittal for PDH or CE credit. [af1]
Pump education courses typically highlight rotodynamic (centrifugal and vertical) pumps, and a good knowledge of that technology is helpful in understanding positive displacement pumps. Many subjects are common, but certain terms and concepts are unique because PD pumps involve an entirely different technology.
Centrifugal vs. PD Pumps
In simple terms, a centrifugal pump impeller moves a stream of liquid from the pump suction to a discharge cone where velocity is gradually decreased and converted to pressure energy. A positive displacement pump, however, moves a set volume of liquid. Pressure is obtained as liquid is forced through the pump discharge into the system, thereby converting energy to pressure.
One example of this principle is demonstrated by reciprocating motion where the movement of a piston forces liquid out of a closed cylinder, which has (inlet) suction and discharge valves to control flow. This forms one of the major PD technologies, reciprocating pumps. In portions of their operating range, reciprocating pumps are the single technology that can successfully provide the necessary pumping solution.
Rotary pumps constitute the second major positive displacement category in which a pumping chamber and a pumping element are actuated by the relative rotation of the drive shaft to the casing. This family is distinguished by having no valves on the inlet or discharge. These types of pumps are available in a number of different pumping principles, each with its own features and benefits that provide specific pumping solutions.
The third major category is controlled volume metering pump (CVMP). These types of pumps are often known as chemical injection feed pumps or dosing pumps. Essentially, these are reciprocating positive displacement pumps configured to accurately dispense a set volume of liquid in a specified time period. They may include one of several types of mechanisms for varying the effective displacement. These types of pumps are used in applications requiring highly accurate, repeatable and adjustable rates of flow.
Pumping Solution Products
Technologies within PD pumps are often called "pumping solution products," as they perform that function for applications across a broad range of process conditions. For example, rotary PD pumps can handle highly viscous product (3,000,000 SSU) while reciprocating pumps handle water thin liquids. PD pumps handle flow rates from less than 1-gpm to 15,000-gpm, and pressures from a few psi to 70,000-psi and higher. PD pumps, at constant speed, are constant flow rate devices, but centrifugal pumps are variable flow rate devices. Generally, PD pumps require some type of pressure protection, and certain designs will require pulsation control. System design requirements are different from centrifugal pumps.
PD pumps may be found almost anywhere, but a generally accepted view is that 90+ percent go into applications within these top six industrial market segments:
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Oil and Gas
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Water and Wastewater Treatment
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Chemical
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Food, Beverage and Pharmaceutical
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Power
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General Industrial (Marine/Medical/OEM)
Many of these industries represent multiple markets. Oil and gas, for example, has distinctly different applications for PD pumps across its segments: exploration, production, pipeline, processing and distribution marketing.
The food and beverage market is another key positive displacement market with multiple segments such as beverage, bakery, confectionary, dairy and
