Pump Wisdom

Essential Centrifugal Pump Knowledge for Operators and Specialists

Robert X. Perez, Heinz P. Bloch

• Publisher: Wiley-AIChE
• Year: 2021
• ISBN:
  9781119748236
• Edition: 2
• Topic: Technik & Maschinenbau
• Subtopic: Chemie- & Biochemietechnik

Pump Wisdom

Explore key facets of centrifugal pump ownership, installation, operation, and troubleshooting

The Second Edition of Pump Wisdom: Essential Centrifugal Pump Knowledge for Operators and Specialists delivers a concise explanation of how pumps function, the design specifications that must be considered before purchasing a pump, and current best practices in lubrication and mechanical seals.

Readers will encounter new startup and surveillance tips for pump operators, as well as repair versus replacement or upgrade considerations for maintenance decision makers, new condition monitoring guidance for centrifugal pumps, and expanded coverage of operator best practices.

This latest edition of Pump Wisdom: Essential Centrifugal Pump Knowledge for Operators and Specialists includes expanded coverage of areas critical to achieving best-in-class pump reliability, including commonly encountered issues and easy-to-follow instructions for getting centrifugal pumps to operate safely and reliably.

This book also provides:

• Comprehensible and accessible explanations of pump hydraulics
• Simple explorations of the mechanical aspects of pumps with coverage of bearings, seals, impeller trimming, lubricant application, and more
• Safety tips and instructions for centrifugal pumps

Perfect for chemical, petroleum, and mechanical engineers, Pump Wisdom: Essential Centrifugal Pump Knowledge for Operators and Specialists is also an ideal resource for operators, managers, purchasing agents, machinists, reliability technicians, and maintenance workers in water and wastewater plants.

1
Principles of Centrifugal Process Pumps

Pumps, of course, are simple machines that lift, transfer, or otherwise move fluid from one place to another. They are usually configured to use the rotational (kinetic) energy from an impeller to impart motion to a fluid. The impeller is located on a shaft; together, shaft and impeller(s) make up the rotor. This rotor is surrounded by a casing; located in this casing (or pump case) are one or more stationary passageways that direct the fluid to a discharge nozzle. Impeller and casing are the main components of the hydraulic assembly; the region or envelope containing bearings and seals is called the mechanical assembly or power end (Figure 1.1).

Many process pumps are designed and constructed to facilitate field repair. On these so‐called “back pull‐out” pumps, shop maintenance can be performed, while the casing and its associated suction and discharge piping (Figure 1.2) are left undisturbed. Although operating in the hydraulic end, the impeller remains with the power end during removal from the field.

The rotating impeller (Figure 1.3) is usually constructed with swept‐back vanes, and the fluid is accelerated from the rotating impeller to the stationary passages into the surrounding casing.

In this manner, kinetic energy is added to the fluid stream (also called pumpage) as it enters the impeller's suction eye (A on Figure 1.3), travels through the impeller, and is then flung outward toward the impeller's periphery. After the fluid exits the impeller, it gradually decelerates to a much lower velocity in the stationary casing, called a volute casing, where the fluid stream's kinetic energy is converted into pressure energy (also called pressure head). The combination of the pump suction (inlet) pressure and the additional pressure head generated by the impeller creates a final pump discharge pressure that is higher than the suction pressure [3].

Figure 1.1 Principal components of an elementary process pump.

Source: SKF USA, Inc. [1].

Pump Performance: Head and Flow

Pump performance is always described in terms of head H produced at a given flow capability Q, and hydraulic efficiency η attained at any particular intersection of H and Q. Head is customarily plotted on the vertical scale or vertical axis (the left of the two y‐axes) of Figure 1.4; it is expressed in feet (or meters). Hydraulic efficiency is often plotted on another vertical scale, the right of the two vertical scales, i.e. the y‐axis in this generalized plot.

Figure 1.2 Typical process pump with suction flow entering horizontally and vertically oriented discharge pipe leaving the casing tangentially.

Source: Emile Egger & Cie. [2].

Head is related to the difference between discharge pressure and suction pressure at the respective pump nozzles. Head is a simple concept, but this is where consideration of the impeller tip speed is important. The higher the shaft rpm and the larger the impeller diameter, the higher will be the impeller tip speed – actually its peripheral velocity.

Figure 1.3 A semiopen impeller with five vanes. As shown, the impeller is configured for counterclockwise rotation about a centerline “A.”

Figure 1.4 Typical “H–Q” performance curves are sloped as shown here. The best efficiency point (BEP) is marked with a small triangle; power and other parameters are often displayed on the same plot.

The concept of head can be visualized by thinking of a vertical pipe bolted to the outlet (the discharge nozzle) of a pump. In this imaginary pipe, a column of fluid would rise to a height “H”. If the vertical pipe would be attached to the discharge nozzle of a pump with higher impeller tip speed, the fluid would rise to a greater height “H+”. It is important to note that the height of a column of liquid, H or H+, is a function only of the impeller tip speed. The specific gravity of the liquid affects power demand but does not influence either H or H+. However, the resulting discharge pressure does depend on the liquid density (specific gravity or Sp.G.). For water (with an Sp.G. of 1.0), an H of 2.31 ft equals 1 psi (pound‐per‐square‐inch), while for alcohol, which might have a Sp.G. of 0.5, a column height or head H of 4.62 ft equals 1 psi. So if a certain fluid had an Sp.G. of 1.28, a column height (head H) of 2.31/1.28 = 1.8 ft would equal a pressure of 1 psi.

For reasons of material strength and reasonably priced metallurgy, one usually limits the head per stage to about 700 ft. This is a fairly important rule‐of‐thumb limit to remember. When too many similar rule‐of‐thumb limits combine, one...