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.
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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 meat packaging.
Selection Consideration: Twelve Benefits of PD Pumps
In many markets, there are applications that clearly should be positive displacement and applications that are clearly centrifugal. It is important for the user or specifying engineer to recognize, however, that there are also a broad range of applications where both types should be considered and selection should be based on the results that the user desires.
In such consideration, there are reasons why positive displacement pumps make an ideal solution to specific pumping requirements. Twelve suggested reasons to use PD pumps are summarized below, grouped by fluid characteristics, process conditions, environmental system requirements and flow control. Additionally, Figure 3 provides a matrix of these 12 reasons compared to the primary markets of PD pumps. Some may surprise you.
Figure 3. Matrix Market Application vs. Desired Pump Characteristics
Fluid Characteristics
High Viscosity
Selected rotary technologies and air operated piston pumps easily handle highly viscous fluids. Due to high friction losses in centrifugal pumps, their flow rate and efficiency start to drop above 500 SSU. Flow and efficiency in a rotary pump, however, typically increase with viscosity. PD pumps can handle fluids with viscosities of several million SSU.
Low and Variable Viscosity
PD pumps, such as vane or air operated double diaphragm (AODD), are often applied on very thin fluids. Other liquids, such as oil, have viscosities that vary with temperature. With variable viscosity liquids, a moderately small change in viscosity may have a large effect on centrifugal efficiency but little effect on PD pump efficiency.
Low Shear Pumping Required
In many fluid applications, liquid shear is not a problem; however, it is critical in some applications. PD pumps excel in the handling of shear sensitive fluids.
Solids Handling Capability
Progressing cavity pumps handling high solids content sludge in a waste treatment plant and reciprocating pumps are applied on coal slurry pipeline with solids contents as high as 40 percent by weight. This is sometimes a surprising PD pump characteristic, but widely varied applications serve as examples.
Multi-Phase Flow
A constant source of liquid is a centrifugal pump requirement, but unfortunately all processes do not provide such constant sources. If there is insufficient liquid, a gas bubble forms in the suction and causes loss of prime (the pump stops pumping). PD pumps, on the other hand, are capable of handling a high percentage of air or gas entrainment.
Process Condition
High Pressure
Beyond the range of centrifugal pumps are many chemical, sandblasting and high-pressure water-cutting applications where PD pump technology dominates. Figure 4 provides an overview of the pressure and capabilities among pump technologies.
Figure 4. Technology Flow Pressure Range Chart.
Low Flow
Flow below 100-gpm and above 200-psi provides excellent application opportunities for PD pump technology.
Efficiency
For viscous fluids where both PD and centrifugal pumps can operate, PD pumps can often be 10 to 40 points more efficient than centrifugal pumps.
Combination of High Pressure/Low Flow-Efficiency Demand
Any of the previous three characteristics individually are a reason to use PD pumps; however, in applications where all of these conditions occur simultaneously, a PD pump solution is ideal.
Figure 4 provides an overview of the pressure and capabilities among pump technologies.
Environmental System Requirements
Sealless Pumping (No Shaft Seal)
Magnetic drives and canned motor pumps are available in PD pump designs. The requirement is also met by designs where the pumping environment does not have a shaft penetration, such as peristaltic or diaphragm pumps.
Self-Priming and Inlet Conditions
The ability to self-prime is a useful feature for PD pumps as it allows substantial flexibility in system layout and eliminates the need for suction priming systems. PD pumps are self-priming, have excellent suction lift capabilities (raising liquids on the suction side) and are capable of drawing down to near vacuum.
Flow Control
Constant Flow Against Variable System Pressure
At a constant speed, PD pumps deliver practically constant flow. Flow is constant even if the system pressure varies, which is a desirable condition in certain systems.
Accurate Repeatable Measurement
Since a PD pump is a constant flow device, certain designs that limit slip are ideal for metering fluids in or out of systems. This application, of course, requires accuracy and repeatability. It also may need flow variation, which is typically obtained mechanically or electronically by speed variation.
There is a universe of standard PD solutions in addition to the bakers dozen described here. As these pumps also must meet many other requirements, manufacturers provide products with special options such as jacketing, non-corrosive materials and built-in pressure relief valves.
Some PD units have duty cycle limits that users are advised to investigate. It is important to note PD pumps are constant torque devices. In variable speed applications, VFD drives must be rated with that understanding.
Fundamentals of PD Pumps: Online Learning
Extensive material is provided in the HI PD pump e-Learning modules to allow for expanded and detailed understanding of these items and applications. The course contains more than 500 screens of information in five separate modules for 10 hours of credit work for those seeking PDH or CE credit.
Pumps & Systems, February 2009