Warm greetings to all 🤩in the 24th post on #Day24 ! Today, we will begin to learn about Pumps, here we go with the 1st part 🚀
Introduction🖇
There are two broad categories of turbomachinery, pumps and turbines. The word pump is a general term for any fluid machine that adds energy to a fluid. Some people call pumps energy absorbing devices since energy is supplied to them, and they transfer most of that energy to the fluid, usually via a rotating shaft (Fig. 14–1a).
🚀The increase in fluid energy is usually felt as an increase in the pressure of the fluid. Turbines, on the other hand, are energy producing devices—they extract energy from the fluid and transfer most of that energy to some form of mechanical energy output, typically in the form of a rotating shaft (Fig. 14–1b).
The fluid at the outlet of a turbine suffers an energy loss, typically in the form of a loss of pressure. 🤔An ordinary person may think that the energy supplied to a pump increases the speed of fluid passing through the pump and that a turbine extracts energy from the fluid by slowing it down. This is not necessarily the case. Consider a control volume surrounding a pump (Fig. 14–2).
We assume steady conditions. By this, we mean that neither the mass flow rate nor the rotational speed of the rotating blades changes with time. (📌The detailed flow field near the rotating blades inside the pump is not steady of course, but control volume analysis is not concerned with details inside the control volume.) By conservation of mass, we know that the mass flow rate into the pump must equal the mass flow rate out of the pump. If the flow is incompressible, the volume flow rates at the inlet and outlet must be equal as well.
🌝Furthermore, the pump does not necessarily increase the speed of the fluid passing through it; rather, it increases the pressure of the fluid. Of course, if the pump were turned off, there might be no flow at all. So, the pump does increase fluid speed compared to the case of no pump in the system. However, in terms of changes from the inlet to the outlet across the pump, fluid speed is not necessarily increased. (The output speed may even be lower than the input speed if the outlet diameter is larger than that of the inlet.)
📌Note: The purpose of a pump is to add energy to a fluid, resulting in an increase in fluid pressure, not necessarily an increase of fluid speed across the pump.
An analogous statement is made about the purpose of a turbine:
📌The purpose of a turbine is to extract energy from a fluid, resulting in a decrease of fluid pressure, not necessarily a decrease of fluid speed across the turbine
🚀Fluid machines that move liquids are called pumps, but there are several other names for machines that move gases (Fig. 14–3).
A fan
is a gas pump with relatively low pressure rise and high flow rate. Examples include ceiling fans, house fans, and propellers.
A blower
is a gas pump with relatively moderate to high pressure rise and moderate to high flow rate. Examples include centrifugal blowers and squirrel cage blowers in automobile ventilation systems, furnaces, and leaf blowers.
A compressor
is a gas pump designed to deliver a very high pressure rise, typically at low to moderate flow rates. Examples include air compressors that run pneumatic tools and inflate tires at automobile service stations, and refrigerant compressors used in heat pumps, refrigerators, and air conditioners.
Pumps👯♀️
Some fundamental parameters are used to analyze the performance of a pump. The mass flow rate of fluid through the pump, m ., is an obvious primary pump performance parameter. For incompressible flow, it is more common to use volume flow rate rather than mass flow rate. In the turbomachinery industry, volume flow rate is called capacity and is simply mass flow rate divided by fluid density.
🚀The performance of a pump is characterized additionally by its net head H, defined as the change in Bernoulli head between the inlet and outlet of the pump
Pump Performance Curves and Matching a Pump to a Piping System 🌞
The maximum volume flow rate through a pump occurs when its net head is zero, H ! 0; this flow rate is called the pump’s free delivery. The free delivery condition is achieved when there is no flow restriction at the pump inlet or outlet—in other words when there is no load on the pump. At this operating point, V. is large, but H is zero; the pump’s efficiency is zero because the pump is doing no useful work.
📌At the other extreme, the shutoff head is the net head that occurs when the volume flow rate is zero, V . ! 0, and is achieved when the outlet port of the pump is blocked off. Under these conditions, H is large but V . is zero; the pump’s efficiency is again zero, because the pump is doing no useful work. Between these two extremes, from shutoff to free delivery, the pump’s net head may increase from its shutoff value somewhat as the flow rate increases, but H must eventually decrease to zero as the volume flow rate increases to its free delivery value. 🤯The pump’s efficiency reaches its maximum value somewhere between the shutoff condition and the free delivery condition; this operating point of maximum efficiency is appropriately called the best efficiency point (BEP), and is notated by an asterisk (H*, V . *, bhp*).
🚀Curves of H, hpump, and bhp as functions of V . are called pump performance curves typical curves at one rotational speed are plotted in Fig. 14–8.
🐢The pump performance curves change with rotational speed. It is important to realize that for steady conditions, a pump can operate only along its performance curve. Thus, the operating point of a piping system is determined by matching system requirements (required net head) to pump performance (available net head).
In a typical application, Hrequired and Havailable match at one unique value of flow rate—this is the operating point or duty point of the system. The steady operating point of a piping system is established at the volume flow rate where Hrequired and Havailable.
🤨On the other hand, the available net head of most pumps decreases with flow rate, as in Fig. 14–8, at least over the majority of its recommended operating range. Hence, the system curve and the pump performance curve intersect as sketched in Fig. 14–9, and this establishes the operating point. If we are lucky🤩, the operating point is at or near the best efficiency point of the pump. In most cases, however, as illustrated in Fig. 14–9, the pump does not run at its optimum efficiency. If efficiency is of major concern, the pump should be carefully selected (or a new pump should be designed) such that the operating point is as close to the best efficiency point as possible.🔎 In some cases it may be possible to change the shaft rotation speed so that an existing pump can operate much closer to its design point (best efficiency point).
🖇There are unfortunate situations where the system curve and the pump performance curve intersect at more than one operating point. This can occur when a pump that has a dip in its net head performance curve is mated to a system that has a fairly flat system curve, as illustrated in Fig. 14-10. Although rare, such situations are possible and should be avoided, because the system may “hunt” for an operating point, leading to an unsteadyflow situation. It is fairly straightforward to match a piping system to a pump, once we realize that the term for useful pump head (hpump, u) that we used in the head form of the energy equation is the same as the net head (H) used in the present chapter. Let’s consider, for example, a general piping system with elevation change, major and minor losses, and fluid acceleration. We begin by solving the energy equation for the required net head Hrequired:
Pump Cavitation and Net Positive Suction Head 🎙
When pumping liquids, it is possible for the local pressure inside the pump to fall below the vapor pressure of the liquid, Pv. (Pv is also called the saturation pressure Psat and is listed in thermodynamics tables as a function of saturation temperature.) When P & Pv, vapor-filled bubbles called cavitation bubbles appear. ☎️In other words, the liquid boils locally, typically on the suction side of the rotating impeller blades where the pressure is lowest (Fig. 14–17).
💡After the cavitation bubbles are formed, they are transported through the pump to regions where the pressure is higher, causing rapid collapse of the bubbles. It is this collapse of the bubbles that is undesirable, since it causes noise, vibration, reduced efficiency, and most importantly, damage to the impeller blades🥴. 📌Repeated bubble collapse near a blade surface leads to pitting or erosion of the blade and eventually catastrophic blade failure.
To avoid cavitation, we must ensure that the local pressure everywhere inside the pump stays above the vapor pressure. Since pressure is most easily measured (or estimated) at the inlet of the pump, cavitation criteria are typically specified at the pump inlet. It is useful to employ a flow parameter called net positive suction head (NPSH), defined as the difference between the pump’s inlet stagnation pressure head and the vapor pressure head,
💡Pump manufacturers test their pumps for cavitation in a pump test facility by varying the volume flow rate and inlet pressure in a controlled manner. Specifically, at a given flow rate and liquid temperature, the pressure at the pump inlet is slowly lowered until cavitation occurs somewhere inside the pump. The process is repeated at several other flow rates, and the pump manufacturer then publishes a performance parameter called the required net positive suction head (NPSHrequired), defined as the minimum NPSH necessary to avoid cavitation in the pump. The measured value of NPSHrequired varies with volume flow rate, and therefore NPSHrequired is often plotted on the same pump performance curve as net head (Fig. 14–18).
When expressed properly in units of head of the liquid being pumped, NPSHrequired is independent of the type of liquid. However, if the required net positive suction head is expressed for a particular liquid in pressure units such as pascals or psi, the engineer must be careful to convert this pressure to the equivalent column height of the actual liquid being pumped.
🔌Note that since NPSHrequired is usually much smaller than H over the majority of the performance curve, it is often plotted on a separate expanded vertical axis for clarity ( Fig. 14–15) or as contour lines when being shown for a family of pumps. NPSHrequired typically increases with volume flow rate, although for some pumps it decreases with V . at low flow rates where the pump is not operating very efficiently, as sketched in Fig. 14–18. In order to ensure that a pump does not cavitate, the actual or available NPSH must be greater than NPSHrequired.
📌Since irreversible head losses through the piping system upstream of the inlet increase with flow rate, the pump inlet stagnation pressure head decreases with flow rate. Therefore, the value of NPSH decreases with V, as sketched in Fig. 14–19.
By identifying the volume flow rate at which the curves of actual NPSH and NPSHrequired intersect, we estimate the maximum volume flow rate that can be delivered by the pump without cavitation (Fig. 14–19).
References📖
Fluid Mechanics by Yunus Chengel
Coulson&Richardson-Chemical Engineering Design Vol 6
Introduction to Fluid Mechanics by Edward J.
Additional Resources 🧪🔑
You can get deep insight about Process/Chemical Engineering from these sources😉:
https://www.instagram.com/p/CAXT-ZhlrRW/ Engineerium Mentoring Center Instagram page
https://www.facebook.com/engineeriummentoringcenter Engineerium Mentoring Center Facebook page
http://www2.eng.cam.ac.uk/~mpj1001/learnfluidmechanics.org/LFM_L6.html
https://www.youtube.com/watch?v=NCvYPclQNWM Heat Exchangers Explanation
https://t.me/ebookstorage/210-Introduction to Process Engineering and Design (2015)
https://t.me/ebookstorage/211-Elementary Principles of Chemical Processes
https://t.me/OilAndGas/18122Valve Sizing Sheet
https://t.me/ebookgate/1127Engineering Fluid Mechanics Book
https://coursemania.xyz/course.html?id=433291 Free course on Fundamentals of Fluid Mechanics
https://t.me/ebookstorage/178 Engineering Heat Transfer
https://t.me/ebookstorage/159 Fundamentals of Engineering Thermodynamics (9th Edition) (2018)
https://t.me/ebookstorage/18 Heat Transfer applications and principles
https://t.me/ebookstorage/171-Design and Operation of Heat Exchangers and their Networks (2020)
https://t.me/ebookstorage/215- Industrial Separation Processes (book)
https://t.me/ebookstorage/214- Advanced Process Engineering Control (book)
https://t.me/OilAndGas- Information about Oil&Gas (mainly arabic lang)
https://t.me/chemical_worlds- Chemical Engineering Books, Quizzes and GATE Study Group
https://t.me/chemical_environmental- Discussion group related to Chemical Engineering Problems
https://t.me/chemicalengineeringworld_cew- Everything related to Chemical Engineering
https://t.me/ebookgate- Chemical Engineering E-books (Telegram Channel)
https://www.youtube.com/channel/UCqioh32NOJc8P7cPo3jHrbg- Piping Analysis
https://www.youtube.com/channel/UCQfMyugsjrVUWU0v_ZxQs2Q -Mechanics of engineered devices
http://chemicalengineeringguy.com/- suggests a wide range of courses in Chemical engineering (you can find free courses on topic of Aspen HYSYS, Aspen Plus)
https://www.youtube.com/user/LearnEngineeringTeam- suggests working principles of every engineered devices, equipment and etch.
https://www.youtube.com/channel/UCR0EfsRZIwA5TVDaQbTqwEQ- suggests great information about pumps, compressors with animation.
Today we have begun learning about Pumps. Now, time to say goodbye👋🏻 until tomorrow and Stay tuned for more content 😉🌝✨!
✏️Note: If you need one of those books or links, you can contact me via my email or LinkedIn profile.
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