How to Select the Right Centrifugal Pump: A Brief Survey of Centrifugal Pump Selection Best Practices
144
How to Select the Right Centrifugal Pump: A Brief Survey of Centrifugal Pump Selection Best Practices
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Product Details
| ISBN-13: | 9781504922661 |
|---|---|
| Publisher: | AuthorHouse |
| Publication date: | 07/30/2015 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 144 |
| File size: | 6 MB |
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How to Select the Right Centrifugal Pump
A Brief Survey of Centrifugal Pump Selection Best Practices
By Robert X. Perez
AuthorHouse
Copyright © 2015 Robert X. PerezAll rights reserved.
ISBN: 978-1-5049-2267-8
CHAPTER 1
Introduction to Centrifugal Pumps
Pumps are required throughout the process industry to move liquids from point A to point B, as shown in Figure 1.1 below where a pump takes suction flow from an overhead process vessel and pumps it into the process. Pump duties may involve moving liquid from a lower pressure to a higher pressure, lifting the liquid from a lower elevation to a higher level, or a combination of both of these requirements. Liquid normally wants to move from a higher pressure to a lower pressure and from a higher level to a lower level. Getting liquid to move against its normal tendencies requires the addition of energy. That's where pumps come in. Pumps are devices that transform mechanical energy into useful liquid energy.
Centrifugal pumps are one of the simplest of all the pump designs. They have one moving part, called the rotor. The rotor has an impeller that accelerates liquid from its suction eye or inlet (see Figure 1.2) to a maximum speed at its outer diameter.
The liquid is then gradually decelerated to a much lower velocity in the stationary casing, called a volute casing. As it slows down, due to the increasing cross sectional area of the casing, pressure is developed, until full pressure is developed at the pump's discharge. This simplicity of design and operation is what makes centrifugal pumps one of the most reliable of pump designs, assuming they are applied properly.
This process of converting velocity to pressure is similar to holding your hand outside of a moving automobile. As the high velocity air hits you, it slows down and pushes your hand back due to the pressure developed. Similarly, if you could insert your hand into the pump casing at the impeller exit and "catch" the liquid, you would feel the pressure produced by dynamic action of the impeller. When any high velocity stream slows down, pressure is created. (This effect is called Bernoulli's Principle, which simply states that energy is always conserved in a fluid stream.) The greater that the impeller diameter or rpm is, the greater the exit velocity is and, therefore, the higher pressure developed at the pump's discharge.
Another benefit of centrifugal pumps is that they can cover a wide range of hydraulic requirements. Thus, they can be used in a wide range of flow and pressure applications. They can easily provide flows from less than 10 gallons per minute (37.9 liters per minute) to well over 10,000 gallons per minute (gpm) (379 000 liters/minute). Centrifugal pump impellers can easily be staged, that is, arranged so that one impeller's output is directed to a subsequent impeller; over 4500 pounds per square inch (psi) (31026 kPa) of pressure can be generated.
One key disadvantage of centrifugal pumps is that their efficiencies are usually less than positive displacement pumps. Whereas positive displacement pumps can deliver efficiencies greater than 90 percent range, centrifugal pump efficiencies can range from less than 30% to over 80%, depending on the type and size. Here are few samples of centrifugal pump efficiencies:
1. 10,000 gpm centrifugal pump — 75 to 89%
2. 500 gpm centrifugal pump — 60 to 75%
3. 100 gpm magnetic drive pump — 40%
4. 100 gpm canned motor pump — 35%
This is the price you pay for simplicity.
Head versus Pressure
Before we can talk about centrifugal pump performance, we must understand the difference between pressure, which is usually given in pounds per square inch (psi) and liquid head, which is usually given in feet, inches, or mm of liquid. In Figure 1.3, we have three pressures gauges connected to the bottom of three different 115.5-feet (35.2 m) liquid columns. Notice that pressure gauges read, from left to right, 60 psi (414 kPa) on the liquid with a specific gravity (S.G.) of 1.2, 50psi (345 kPa) on the liquid with a specific gravity of 1.0, and 35 psi (241 kPa) on the liquid with a specific gravity of 0.70. From these observations, we can infer that, given a fixed column height, the denser, or heavier, the liquid in a column, the greater the expected reading at the pressure gauge. Conversely, we can state with confidence that a lighter liquid will result in a proportionately lower reading at the pressure gauge. The formulas used to determine exactly how pressure relates to head and vice-versa is given below.
Where SG is the specific gravity of the fluid.
To use these formulas, you need to understand the liquid property specific gravity, which is simply the ratio of the liquid's density to the density of water (62.4 lbs/ft3 or 1000 kg/m3) at a given temperature. The denser the liquid is, the higher its specific gravity will be.
Let's go through a few simple examples.
1. First, we will assume we have a 100-foot column of water. Water has a specific gravity of 1.00, so the pressure you would expect at the bottom of the column to be (100 x 1)/2.31 or about 43.3 psi.
2. Next, we will assume we have a 100-foot column of gasoline, which has a specific gravity of 0.75. (This means one cubic foot of gasoline weighs 75 % the weight of one cubic foot of water.) We can expect the pressure gauge to read (100 x 0.75)/2.31 or 32.5 psi.
3. Finally, we will assume we have a 100-foot column of brine, which has a specific gravity of 1.2. (This means one cubic foot of brine weighs 120 % the weight of one cubic foot of water.) We can expect the pressure gauge to read (100 x 1.2)/2.31 or 51.94 psi.
If the specific gravity, or density, of your liquid is known, we can readily convert pressure head into pressure. Although pressure and head are not identical quantities, they are closely related and are often used interchangeably.
Head is a convenient form of stating pressure in centrifugal pump applications. Most manufacturers test their pumps with water, so you need to be able to determine how your pump will perform on the liquid you are pumping. If a pump is capable of delivering 200 feet (61 m) of head, you can conclude your pump will add 200 feet (61 m) of liquid head to whatever pressure you have on the suction of the pump, regardless of the specific gravity of the liquid being pumped. Let's also say you are pumping gasoline and that you have 50 feet of suction pressure. You can now calculate the discharge pressure of the pump. (Here we will assume there are no pressure losses in the pump's piping.)
Pd = (Hs + HPump) × S.G./2.31
In the equation above, Pd is the Discharge Pressure, Hs is the suction head in feet, Hpump is the pump's head rise in feet, and S.G. is the specific gravity. Plugging the values provided we get:
Pd = (50 + 200) × 0.75/2.31 = 81.2 psig
It is important to note that, in our example, the pump's discharge pressure (Pd) is in gauge pressures (psig) as opposed to an absolute because we chose to state SH in feet of liquid above atmospheric pressure.
Another way of looking at pump head is to imagine what would happen if you opened a bleeder near the pump's discharge nozzle to see how high the pumping liquid would shoot up due to the generated head. Using the example above, where the pump is adding 200 feet of head to 50 feet of suction pressure above atmospheric pressure, you should expect to see a liquid stream soar 250 feet into air, excluding valve losses and air resistance.
Centrifugal Pump Performance
Table 1.1 shows a listing of typical performance data for a centrifugal pump. You will notice the table lists "head" (a form of pressure that the pump produces), "NPSHr" (the amount of suction head over the vapor pressure required for normal operation), the required horsepower, and the pump efficiency for five different flow conditions. This is typical of pump test data. The pump to be tested is placed on a well-instrumented test stand so that these performance values can be determined. From the table below, you can readily see that at 400 gpm you can expect 195 feet of pressure head; in addition, 32.8 horsepower will be required to operate the pump at this flow condition. Similarly, at 800 gpm, you can expect only 120 feet of pressure, and 48.5 horsepower will be required to operate the pump at this flow. Similar data is obtained and is used in the metric system but flow is in liters per minute, head is in meters, NPSH is in m, and power is in kilowatts.
Pump manufacturers usually convert this tabular performance data into graphical formats called pump performance curves. These curves provide pump users a means of visualizing the performance data and allowing them to quickly determine how changes in flow can affect pressure and horsepower. If you look at Figure 1.4, you will see a pump performance curve generated from our pump performance data in Table 1.1.
You can see that:
• Pump head is fairly constant until the flow increases above 400 gpm (approximately) and then drops rapidly. The flatness of the head curve at lower flows is why we call centrifugal pumps constant head devices.
• NPSHr, or suction requirement, increases steadily as flow increases.
• Horsepower also increases as flow increases. (It is important to note that not all pump curves have a continuously rising horsepower curve like this one. In general we can say that radial flow pumps have continuously rising horsepower versus flow curves, mixed flow pumps have relatively flat horsepower versus flow curves, and axial pumps have continuously falling horsepower versus flow curve.)
This visual display of pump performance data makes them useful as a troubleshooting and analysis tool.
Basic Centrifugal Pump Construction
Most centrifugal pumps have the following common elements (see Figure 1.5):
1. Impeller, which adds energy to the liquid by accelerating it
2. Pump casing and volute, which contains the liquid being pumped and decelerates the liquid expelled by the impeller
3. Shaft seal, which allows rotation of the rotor while preventing product leakage around the shaft. Mechanical seals are commonly used in process applications
4. Shaft and bearings, which maintain the position of the rotor with respect to the pump casing
Mechanical seal and bearing failures tend to be the most common component failures within centrifugal pumps. High vibration, shaft deflection, high operating temperatures, chemical attack, and the build-up of solids can cause mechanical seals to fail prematurely, while bearing life can be curtailed by excessive shaft vibration and lubricant contamination. Extra attention should always be given to mechanical seals, bearings, and their corresponding support systems to ensure they are properly designed and applied. Once the designs of these key elements have been optimized for the given application, detailed operating and maintenance procedures should be developed and then faithfully followed to ensure reliable and economical pump operation.
Types of Centrifugal Pumps
Pumps come in all shapes and sizes to fit the hydraulic requirement of the industry. Here are just a few common designs:
[ILLUSTRATION OMITTED]
Fixed Versus Variable Speed Operation
There are two design choices when specifying centrifugal pumps. You can either opt for a fixed or constant speed application or a variable speed application. Variable speed operation allows the user to vary the developed pump head by varying the pump speed to meet the process requirements. As the pump speed increases its developed head increases, therefore increasing the flow and the horsepower demand (see Fig. 1.10).
The simplest installation and therefore the lowest cost design option tends to be a fixed speed pump driven by a constant speed electric motor. In fixed speed applications, control valves are often used to modulate the pump flow. By opening or closing a control valve, the pump flow is affected by increasing or decreasing the back-pressure on the pump. Make sure to size the control so that it can handle 110% to 125% of the rated pump flow, for upset conditions.
In contrast, variable speed applications are more expensive options due to their inherent complexity. Variable speed pumps can be driven by electric motors with variable frequency drives, steam turbines, or gas turbines. Typically the potential cost savings realized by variable speed operation justifies the additional cost of variable frequency drives. Steam turbine drives are usually selected whenever there is an excess supply of steam and gas turbines tend to be used if readily available fuel sources are more convenient than electricity.
In this book, we will concentrate on fixed speed application; however all the concepts presented here related to variable speed applications. A good practice is to select your pump based on maximum or rated speed condition. Since the highest energy conditions are seen at the maximum speed, it makes sense to ensure the pump you select fits best at your highest process demands.
Pumping Systems
When multiple pumps are piped together, as shown in figure 1.11, we create pumping systems that offer process facilities and pipelines multiple capabilities. Pumps can either be piped in series (in a line) or in parallel (side by side). When pumps are piped in series their pressures add together. When pumps piped in a parallel arrangement, their flows add together. In parallel, as each additional pump is turned on, more and more flow can be realized by the system. Keep in mind that pump and piping systems must be carefully designed to adequately handle the overall system requirements. Every aspect of a piping system should be considered to ensure that the production goals can be realized.
The Importance of System Head Curve
Every pump manufacturer would like to supply the perfect pump for every application they quote. However to do this, the manufacturer requires the future pump user to provide them with an accurate system head curve that describes the capacity and head needed for your operating conditions. In this section, we will define what a system head curve is, why they are important, and how they are generated.
A system-head curve is an analytical or graphical representation of the relationship between the flow and the hydraulic head requirements of a given piping system. Hydraulic head requirements are related to the suction head, the discharge head, and the hydraulic head losses of the piping system. Since hydraulic head requirements and losses are functions of the flowrate, size and length of pipe, and size, number and type of fittings, each system head curve tends to have a unique shape.
Normally, at least one point on the system curve is given to the pump manufacturer in order to help him select the pump properly. However, it is highly desirable to graphically superimpose the entire system curve over the head-capacity curve of the pump in order to better understand the interactions between the pump performance curve and the system head curve. By definition, the intersection of the pump performance curve with the system-head curve defines the operating point of the pump and piping system. Once the system curve is defined, we can plot various pump curves on top of the system curve and hopefully select one that matches the process needs. Without this system curve, there is not much of a chance of coming up with the right pump.
To create a system curve we must first plot the desired capacities against the required head over the total anticipated operating range of the pump. The head will be measured in feet or meters and the capacity will be measured in gallons per minute or cubic meters per hour. The general equation for a system curve is given by the following equation:
Hsystem = (Pdownstream -Pupstream) + Elevation + All line losses
Putting this equation in words, we would say that the system head is the sum of 1) the difference between the downstream pressure and the upstream pressure, 2) the elevation difference between the downstream liquid level and the upstream liquid level, and 3) all the line losses. For the remainder of the chapter we will use the following shorthand version of the equation:
Hsystem = (Pds -Pus) + Hstatic + L
Hstatic is defined as the vertical distance from the surface of the liquid in the suction tank or pit to the level of the liquid level in the discharge tank. Hstatic can be positive or negative.
(Continues...)
Excerpted from How to Select the Right Centrifugal Pump by Robert X. Perez. Copyright © 2015 Robert X. Perez. Excerpted by permission of AuthorHouse.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
Contents
Dedication, vii,Acknowledgement, ix,
Preface, xi,
Chapter 1 — Introduction to Centrifugal Pumps, 1,
Chapter 2 — Can I Use a Centrifugal Pump?, 23,
Chapter 3 — How Good Pumps Become Bad Actors, 45,
Chapter 4 — 10 Pump Selection Rules of Thumb, 65,
Chapter 5 — Bid Tabulations, 75,
Chapter 6 — The most important day in a pump's lifetime, 82,
Chapter 7 — The Pump Selection Process, 85,
Appendix A — How to Calculate Pipe Friction Head Losses for Newtonian Fluids, 89,
Appendix B — Mechanical Seal Selection Primer, 104,
Appendix C — Centrifugal Pump Selection Checklist, 118,
Appendix D — Useful Conversions, 122,
Index, 127,