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A centrifugal pump is designed and
A centrifugal pump is designed and produced to supply whole range of head-capacity condition as identified by it’s performance curve. The pump will operate on thet curve if it is driven at the particular speed for which the curve is drawn. However, the actual condition on that curve at which the pump will run will be determined by the system in which it operates. So for all practical puepases, the system controls the pump and will determine the conditions at which the pump will operate, regardless of the head and capacity for which it was purchased.
This is a considerable advantage from the the safety aspect of the system, in that the centrifugal pump is not normally capable of over-pressurizing the system. However in order to understand how the centrifugal pump operates in a system, it is first necessary to understand some aspects of system design.
3.2 LIQUID FLOW IN PIPE
For those who may be unsure of manner in which liquid actually resond to flowing though pipes, the flowing basic guidelines are offered.
3.2.1 SPECIFIC GRAVITY
Speccific gravity is used frequently in the discussuin of fluids. It is the mane given ti the ratui of the density if a liquid to the density of cold water. Therefore, when dealing with cold water, its value of S.G is 1.0
3.2.2 PRESSURE IN A STATIC SYSTEM
When a body of liquid is a rest in a sytem, the relationship between the pressure showing on the gauge and the depth of the above it, will be as show.
P = gauge pressure in pounds per square inch
Hs = static head of liquid in feet
s.g. = specific gravity of the liquid being pumped
3.2.3 PRESSURE IN A FLOWING SYSTEM
When a body of liquid is moving in a system, the pressure will drop, as some of the energe by the static head is now being lost to fiction. Therefore, even when we maintain the level of water in the tank as shown in figure 3.2, to statis head, the pressure reading on the gaure will be less than when the liquid in the system was not flowing
P = gauge pressure in pounds per square inch
Hs = Static head of liquid in feet
Hf = friction head in feet
3.2.4 CHANGES IN AN EXISTING SYSTEM
Please note that the fllowing repationships are offered for those who wish to approximate the effect of changes in an existing system. For accurate data, it is recommended that the hydraulic institutes friction loss table are consulted, either in their engineering data book, or as show in chapter 14 this book
3.2.4.1 THE EFFECT OF CAPACITY CHANGE FRICTION
When the flow rate is changed without changing the pipe size, the qpproximate change in friction loss can be estimated as shown.
Q = Flow rate
Hf = Friction loss
In orther words, the friction loss will vary as square of the flow rate.
3.4.2.2 The effect of head change on flow rate
When the static head is changed again without changing the pipe size the appoximate change in flow rate can be estimated as shown
3.2.5 Pipe size changes in a system
The change in pipe diameter may be necessary to reduce friction losses or increase NPSH available to the pump. Therefore when capacity in unchanged, the friction loss is in INVERSE proportion to the 5th power of the change in the pipe diameter, as shown below.
To operate with the same head, such as from a lake or river, with the new pipe size, the fllowing approximation will apply.
D = pipe diameter
3.3 BASIC ELEMENT OF PUMP SYSTEM DESIGN
In designing any kind of pumping system, the first requirement is to determine the speed at which the task must be performed. In orther words, the flow needed through the system. In some systems, the flow rate will be determined by production requirement or by orther process considerations such as the flow rate needed to achieve the necessary temperature transfer in a liquid flowing though a heat exchanger. For the sake of this exercise, let us consider a batch process system where the average flow rate can be calculation by dividing the volume to be transferred, by the time allowed for that transfer.
The next requirement to e considered is how to overcome all the factor which hinder the movement of the liquid from one poit to another in the system. These are primarily gravity and friction and we will deal with them separately.
3.3.1 GRAVITY AND STATIC HEAD
If we considenr gravity as a force of nature that drives vertically downwards then, in a pumping system, we can oppase it by means of an energy factor we will refoer to as the total static head . this is simple the change in elevation through which the liquid must be lifted, and is measured vertically, regardless of the linear distance between the start and end poits in the system. As shown in figure 3.3, the static head.
Can be measured between the free surface of liquid in tank A, and the highest level of the discharge line.
In another system, where the pump dischar into tank B at a poit below the liquid level in that tank, the total static head in the system is the vertical distance from the free surface of the liquid in tank A to the free suface of the liquid in tank B . It should be noted that this also app;ies, even when the suction source is lower than the pump. This will be discussed in greater detail in chapter 4
Regardless of the layout of the system, the total static head is not a variable os the flow rate, therefore a gragh comraring the two would show up as a straight line.
3.3.2 FRICTION LOSSES AND FRICTION HEAD
Fiction is the resistance to flow in the piping system and must be caonsidered for three separate areas individually.
The Piping
The valves and fittings , and
Orther equipment , such as filters, heat exchangers, etc.
The friction losses in piping can most readily be obtained from the friction loss table available from a variety of sources such as the standards of the hydraulic in-stitute, for benefit of the reader, many of these table are repro-duced in chapter 14 of this boo. Table are also available to identify the losses thruogh the more common pipe fittings and valve types. However, any such losses in filters, heat exxhangers, etc…
Must be obtained from the original equipment manufacturer, or by measuring the equipment on site.
As the floe oncreases, so too does the friction loss but a far higher rate as shown in figure 3.6
3.3.3 Velocity head
Another factor that has to be overcome is the heat required to accelerate the flow of liquid through the pump. This is the difference in the values of velocity head at the suction and discharge nozzles of the pump.
As the linar velocity of the liquid in most system in maintained at lower than 10 ft/sec, the velocity head is usually an insignificant part the total, except low head application.
3.3.4 TOTAL HEAD
The combination of these values equals the total head of the system.
Total head = Static head + Friction loss + Velocity head
3.4 SYSTEM CURVE
When the total head is plotted against the flow rate, the resultant curve is known as the system curve
Therefore when a specific flow rate ia selected for a system, the system curve will identify the total head that must be overcome.
The flow rate through a system can only be supplied by a pump, and is therefore the capacity required from the pump.
The intersection of the pump performance curve and the system curve represents the poit at which the pump will operate as shown in figure 3.8
In system where the flow rate is maintained a constant level, the conditions identified in figure 3.8 will not change. In orther systems however, the operating condition of the centrifugal pump is constantly changing.
3.5 The effect of operating performance
.1 static g=head changes
.1.1 batch transfer system
In a batch transffer system for example, most obvious change is that which is created by emptying the supply tank, this results in a reduction in the liquid level in that tank and the equivalent increase in the static head that the pump must overcome.
When this happens, the system curve will move straight up on the graph, with the following three specific conditions occurring during a single batch.
At the startup poit,, the level of liquid in the supply tank will be at its highest, while the level in the discharge tank coula be zero. This will translate into a low value of static head
At the intermediate poit, the level of liquid in the supply tank will have dropped, and the level in the discharge tank will be greater. This will result in a higher value of static haed.
At the shutdown poit, the liquid in the supply tank will have been transferred entirely into the discharge tank, resulting in the maximum value of static haed. At this poit, the pump should be shutdown.
The system curve will assume the three positions shown in figure 3.9 as it moves steadily from startup (a) to shutdown (c) with the carresponding change in pump capacity. However , as the system approaches the shutdown poit, the pump performance will become unstable. This is due to gradual elimination of appropriate suction conditions which will be discussed in some detail in chapter 4.
.1.2 Pressurized system
In a pressurized type of system, such as a boiler feed system, the feed pump take it’s suction from a deaerator under vacuum and supplies a boiler under pressure. In this system, the differential pressure is not a function of the flow rate and will have similar consequences as the static head. Any change in pressure in either the deaerator or the boiler, will also cause the system curve to move up or down as indicated figure 3.9
.1.3 Closed loop system
A closed loop system in one in which the entire system is pressurized by the pump. To achieve this, the pumpage is fully con tained within a series of pipes and pressurized process equipment all the way from the pump discharge, thruogh the system, and back to the pump inlet. In such a layout, the static haed in the is effecively zero.
3.5.2 Friction head changes
.2.1 system control
A change in friction loss can be caused by a variety of conditions such as manual operation or automated controls operation or automate controls opening
This is a considerable advantage from the the safety aspect of the system, in that the centrifugal pump is not normally capable of over-pressurizing the system. However in order to understand how the centrifugal pump operates in a system, it is first necessary to understand some aspects of system design.
3.2 LIQUID FLOW IN PIPE
For those who may be unsure of manner in which liquid actually resond to flowing though pipes, the flowing basic guidelines are offered.
3.2.1 SPECIFIC GRAVITY
Speccific gravity is used frequently in the discussuin of fluids. It is the mane given ti the ratui of the density if a liquid to the density of cold water. Therefore, when dealing with cold water, its value of S.G is 1.0
3.2.2 PRESSURE IN A STATIC SYSTEM
When a body of liquid is a rest in a sytem, the relationship between the pressure showing on the gauge and the depth of the above it, will be as show.
P = gauge pressure in pounds per square inch
Hs = static head of liquid in feet
s.g. = specific gravity of the liquid being pumped
3.2.3 PRESSURE IN A FLOWING SYSTEM
When a body of liquid is moving in a system, the pressure will drop, as some of the energe by the static head is now being lost to fiction. Therefore, even when we maintain the level of water in the tank as shown in figure 3.2, to statis head, the pressure reading on the gaure will be less than when the liquid in the system was not flowing
P = gauge pressure in pounds per square inch
Hs = Static head of liquid in feet
Hf = friction head in feet
3.2.4 CHANGES IN AN EXISTING SYSTEM
Please note that the fllowing repationships are offered for those who wish to approximate the effect of changes in an existing system. For accurate data, it is recommended that the hydraulic institutes friction loss table are consulted, either in their engineering data book, or as show in chapter 14 this book
3.2.4.1 THE EFFECT OF CAPACITY CHANGE FRICTION
When the flow rate is changed without changing the pipe size, the qpproximate change in friction loss can be estimated as shown.
Q = Flow rate
Hf = Friction loss
In orther words, the friction loss will vary as square of the flow rate.
3.4.2.2 The effect of head change on flow rate
When the static head is changed again without changing the pipe size the appoximate change in flow rate can be estimated as shown
3.2.5 Pipe size changes in a system
The change in pipe diameter may be necessary to reduce friction losses or increase NPSH available to the pump. Therefore when capacity in unchanged, the friction loss is in INVERSE proportion to the 5th power of the change in the pipe diameter, as shown below.
To operate with the same head, such as from a lake or river, with the new pipe size, the fllowing approximation will apply.
D = pipe diameter
3.3 BASIC ELEMENT OF PUMP SYSTEM DESIGN
In designing any kind of pumping system, the first requirement is to determine the speed at which the task must be performed. In orther words, the flow needed through the system. In some systems, the flow rate will be determined by production requirement or by orther process considerations such as the flow rate needed to achieve the necessary temperature transfer in a liquid flowing though a heat exchanger. For the sake of this exercise, let us consider a batch process system where the average flow rate can be calculation by dividing the volume to be transferred, by the time allowed for that transfer.
The next requirement to e considered is how to overcome all the factor which hinder the movement of the liquid from one poit to another in the system. These are primarily gravity and friction and we will deal with them separately.
3.3.1 GRAVITY AND STATIC HEAD
If we considenr gravity as a force of nature that drives vertically downwards then, in a pumping system, we can oppase it by means of an energy factor we will refoer to as the total static head . this is simple the change in elevation through which the liquid must be lifted, and is measured vertically, regardless of the linear distance between the start and end poits in the system. As shown in figure 3.3, the static head.
Can be measured between the free surface of liquid in tank A, and the highest level of the discharge line.
In another system, where the pump dischar into tank B at a poit below the liquid level in that tank, the total static head in the system is the vertical distance from the free surface of the liquid in tank A to the free suface of the liquid in tank B . It should be noted that this also app;ies, even when the suction source is lower than the pump. This will be discussed in greater detail in chapter 4
Regardless of the layout of the system, the total static head is not a variable os the flow rate, therefore a gragh comraring the two would show up as a straight line.
3.3.2 FRICTION LOSSES AND FRICTION HEAD
Fiction is the resistance to flow in the piping system and must be caonsidered for three separate areas individually.
The Piping
The valves and fittings , and
Orther equipment , such as filters, heat exchangers, etc.
The friction losses in piping can most readily be obtained from the friction loss table available from a variety of sources such as the standards of the hydraulic in-stitute, for benefit of the reader, many of these table are repro-duced in chapter 14 of this boo. Table are also available to identify the losses thruogh the more common pipe fittings and valve types. However, any such losses in filters, heat exxhangers, etc…
Must be obtained from the original equipment manufacturer, or by measuring the equipment on site.
As the floe oncreases, so too does the friction loss but a far higher rate as shown in figure 3.6
3.3.3 Velocity head
Another factor that has to be overcome is the heat required to accelerate the flow of liquid through the pump. This is the difference in the values of velocity head at the suction and discharge nozzles of the pump.
As the linar velocity of the liquid in most system in maintained at lower than 10 ft/sec, the velocity head is usually an insignificant part the total, except low head application.
3.3.4 TOTAL HEAD
The combination of these values equals the total head of the system.
Total head = Static head + Friction loss + Velocity head
3.4 SYSTEM CURVE
When the total head is plotted against the flow rate, the resultant curve is known as the system curve
Therefore when a specific flow rate ia selected for a system, the system curve will identify the total head that must be overcome.
The flow rate through a system can only be supplied by a pump, and is therefore the capacity required from the pump.
The intersection of the pump performance curve and the system curve represents the poit at which the pump will operate as shown in figure 3.8
In system where the flow rate is maintained a constant level, the conditions identified in figure 3.8 will not change. In orther systems however, the operating condition of the centrifugal pump is constantly changing.
3.5 The effect of operating performance
.1 static g=head changes
.1.1 batch transfer system
In a batch transffer system for example, most obvious change is that which is created by emptying the supply tank, this results in a reduction in the liquid level in that tank and the equivalent increase in the static head that the pump must overcome.
When this happens, the system curve will move straight up on the graph, with the following three specific conditions occurring during a single batch.
At the startup poit,, the level of liquid in the supply tank will be at its highest, while the level in the discharge tank coula be zero. This will translate into a low value of static head
At the intermediate poit, the level of liquid in the supply tank will have dropped, and the level in the discharge tank will be greater. This will result in a higher value of static haed.
At the shutdown poit, the liquid in the supply tank will have been transferred entirely into the discharge tank, resulting in the maximum value of static haed. At this poit, the pump should be shutdown.
The system curve will assume the three positions shown in figure 3.9 as it moves steadily from startup (a) to shutdown (c) with the carresponding change in pump capacity. However , as the system approaches the shutdown poit, the pump performance will become unstable. This is due to gradual elimination of appropriate suction conditions which will be discussed in some detail in chapter 4.
.1.2 Pressurized system
In a pressurized type of system, such as a boiler feed system, the feed pump take it’s suction from a deaerator under vacuum and supplies a boiler under pressure. In this system, the differential pressure is not a function of the flow rate and will have similar consequences as the static head. Any change in pressure in either the deaerator or the boiler, will also cause the system curve to move up or down as indicated figure 3.9
.1.3 Closed loop system
A closed loop system in one in which the entire system is pressurized by the pump. To achieve this, the pumpage is fully con tained within a series of pipes and pressurized process equipment all the way from the pump discharge, thruogh the system, and back to the pump inlet. In such a layout, the static haed in the is effecively zero.
3.5.2 Friction head changes
.2.1 system control
A change in friction loss can be caused by a variety of conditions such as manual operation or automated controls operation or automate controls opening
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Một máy bơm ly tâm được thiết kế và sản xuất để cung cấp phạm vi toàn bộ điều kiện đầu-có khả năng như được xác định bởi đường cong hiệu suất của nó. Các máy bơm sẽ hoạt động trên đường cong tâm nếu nó là lái xe ở tốc độ cụ thể mà đường cong được rút ra. Tuy nhiên, điều kiện thực tế trên đường cong đó mà tại đó các máy bơm sẽ chạy sẽ được xác định bởi hệ thống trong đó nó hoạt động. Vì vậy cho tất cả thực tế puepases, Hệ thống điều khiển các máy bơm và sẽ xác định các điều kiện mà các máy bơm sẽ hoạt động, bất kể người đứng đầu và khả năng mà nó đã được mua.Đây là một lợi thế đáng kể từ các khía cạnh an toàn của hệ thống, trong đó các máy bơm ly tâm không phải là thường có khả năng hơn pressurizing hệ thống. Tuy nhiên, để hiểu làm thế nào các máy bơm ly tâm hoạt động trong một hệ thống, nó là cần thiết đầu tiên để hiểu một số khía cạnh của thiết kế hệ thống.3.2 CHẤT LỎNG CHẢY TRONG ĐƯỜNG ỐNGĐối với những người có thể không chắc chắn về cách mà trong đó chất lỏng thực sự resond để chảy mặc dù ống, các nguyên tắc cơ bản chảy được cung cấp.3.2.1 TRỌNG LƯỢNG RIÊNGLực hấp dẫn Speccific được sử dụng thường xuyên trong discussuin của chất lỏng. Nó là ti mane cho ratui mật độ nếu một chất lỏng để mật độ của nước lạnh. Vì vậy, khi giao dịch với nước lạnh, giá trị của nó của S.G là 1,03.2.2 ÁP LỰC TRONG MỘT HỆ THỐNG TĨNHKhi một cơ thể của chất lỏng là một phần còn lại trong một sytem, mối quan hệ giữa áp lực trên khổ và độ sâu của các bên trên nó, sẽ như hiển thị.P = đo áp lực trong pounds mỗi inch vuôngHS = tĩnh đầu của chất lỏng trong bàn châns.g. = trọng lượng riêng của chất lỏng được bơm3.2.3 ÁP LỰC TRONG MỘT HỆ THỐNG CHẢYKhi một cơ thể của chất lỏng đang chuyển động trong một hệ thống, áp lực sẽ giảm xuống, như một số energe bởi tĩnh đầu bây giờ bị mất với viễn tưởng. Vì vậy, ngay cả khi chúng tôi duy trì mức độ của nước trong hồ, như minh hoạ trong hình 3.2, đầu statis, đọc trên gaure áp lực sẽ ít hơn khi chất lỏng trong hệ thống đã không chảyP = đo áp lực trong pounds mỗi inch vuôngHS = tĩnh đầu của chất lỏng trong bàn chânHF = ma sát đầu trong bàn chân3.2.4 THAY ĐỔI TRONG MỘT HỆ THỐNGXin lưu ý rằng fllowing repationships được cung cấp cho những người muốn để ước tính tác động của những thay đổi trong một hệ thống hiện có. Cho dữ liệu chính xác, đó khuyến cáo rằng bảng mất ma sát thủy lực viện được tư vấn, hoặc trong cuốn sách dữ liệu kỹ thuật của họ, hoặc như hiển thị trong chương 14 cuốn sách này3.2.4.1 TÁC ĐỘNG CỦA BIẾN ĐỔI NĂNG LỰC MA SÁT Khi tốc độ dòng chảy được thay đổi mà không thay đổi kích thước đường ống, thay đổi qpproximate ma sát mất có thể được ước tính như được hiển thị.Q = tốc độ dòng chảyHF = tổn thất ma sátNói cách khác, tổn thất ma sát sẽ khác nhau như là hình vuông của tốc độ dòng chảy.3.4.2.2 các hiệu ứng của sự thay đổi đầu vào tốc độ dòng chảyKhi người đứng đầu tĩnh thay đổi một lần nữa mà không thay đổi kích thước ống thay đổi appoximate ở tốc độ dòng chảy có thể được ước tính như hiển thị3.2.5 thay đổi kích thước ống trong một hệ thốngSự thay đổi đường kính ống có thể cần thiết để làm giảm tổn thất ma sát hoặc tăng NPSH có sẵn cho các máy bơm. Do đó khi năng lực trong không thay đổi, tổn thất ma sát là in INVERSE proportion to 5 sức mạnh của sự thay đổi trong đường ống kính, như hình dưới đây.Để hoạt động với người đứng đầu tương tự, chẳng hạn như từ một hồ hoặc sông, với kích thước ống mới, xấp xỉ fllowing sẽ áp dụng.D = đường kính ống3.3 CÁC YẾU TỐ CƠ BẢN CỦA THIẾT KẾ HỆ THỐNG MÁY BƠMTrong việc thiết kế bất kỳ hình thức nào của hệ thống bơm, yêu cầu đầu tiên là để xác định tốc độ mà tại đó các nhiệm vụ phải được thực hiện. Nói cách khác, dòng chảy cần thiết thông qua hệ thống. Trong một số hệ thống, tốc độ dòng chảy sẽ được xác định bằng cách yêu cầu sản xuất hoặc xem xét quá trình khác chẳng hạn như tỷ lệ lưu lượng cần thiết để đạt được các cần thiết nhiệt độ chuyển trong một chất lỏng chảy mặc dù một bộ trao đổi nhiệt. Vì lợi ích của tập thể dục này, chúng ta hãy xem xét một hệ thống quá trình lô mà tốc độ dòng chảy trung bình có thể là tính toán bằng cách chia số lượng được chuyển giao, khi được cho phép để chuyển giao đó.Yêu cầu tiếp theo e xem xét là làm thế nào để vượt qua tất cả các yếu tố mà cản trở sự chuyển động của chất lỏng từ một poit khác trong hệ thống. Đây là chủ yếu là lực hấp dẫn và ma sát và chúng tôi sẽ đối phó với họ một cách riêng biệt.3.3.1 LỰC HẤP DẪN VÀ TĨNH ĐẦUNếu chúng tôi considenr trọng lực như là một lực lượng của thiên nhiên mà ổ đĩa theo chiều dọc xuống dưới sau đó, trong một hệ thống bơm, chúng tôi có thể oppase nó bằng phương tiện của một yếu tố năng lượng chúng tôi sẽ refoer để là người đứng đầu tất cả tĩnh. Điều này là đơn giản thay đổi độ cao mà qua đó các chất lỏng phải được nâng lên, và được đo theo chiều dọc, bất kể khoảng cách tuyến tính giữa poits bắt đầu và kết thúc trong hệ thống. Như minh hoạ trong hình 3.3, người đứng đầu tĩnh.Có thể được đo giữa bề mặt miễn phí của chất lỏng trong thùng A, và mức độ cao nhất của dòng xả.Trong hệ thống khác, nơi mà các máy bơm dischar vào thùng B tại một poit dưới mức chất lỏng trong xe tăng, người đứng đầu tất cả tĩnh trong hệ thống là khoảng cách thẳng đứng từ bề mặt miễn phí của chất lỏng trong bể A đến suface miễn phí của chất lỏng trong bể B. Cần lưu ý rằng điều này cũng app; IE, ngay cả khi hút nguồn là thấp hơn so với các máy bơm. Điều này sẽ được thảo luận chi tiết hơn trong chương 4Bất kể cách bố trí của hệ thống, người đứng đầu tĩnh tất cả không phải là một os biến tốc độ dòng chảy, do đó một comraring gragh hai sẽ hiện lên như là một đường thẳng.3.3.2 TỔN THẤT MA SÁT VÀ MA SÁT ĐẦUViễn tưởng là khả năng chống chảy trong hệ thống đường ống và phải là caonsidered cho ba khu vực riêng biệt cá nhân.É đường ống Van và phụ kiện, và thiết bị khác, chẳng hạn như bộ lọc, bộ trao đổi nhiệt, vv.Ma sát các tổn thất trong đường ống đặt dễ dàng có thể được lấy từ ma sát mất bảng có sẵn từ một số nguồn chẳng hạn như các tiêu chuẩn của thủy lực trong-stitute, vì lợi ích của người đọc, nhiều người trong số bảng được duced repro trong chương 14 của boo này. Bảng cũng có để xác định thiệt hại thruogh phổ biến hơn phụ kiện đường ống và Van các loại. Tuy nhiên, bất kỳ mất mát nào như vậy trong bộ lọc, nhiệt exxhangers, vv...Phải có được từ các nhà sản xuất thiết bị gốc, hoặc bằng cách đo lường các thiết bị trên trang web.Như tảng oncreases, do đó, quá hiện tổn thất ma sát nhưng một tỷ lệ cao hơn như minh hoạ trong hình 3.63.3.3 vận tốc đầuMột yếu tố khác đã được khắc phục là cần thiết để tăng tốc độ dòng chảy của chất lỏng thông qua các máy bơm nhiệt. Đây là sự khác biệt trong các giá trị của vận tốc đầu tại các họng hút và xả của máy bơm.Như linar vận tốc của chất lỏng trong các hệ thống hầu hết trong duy trì ở thấp hơn 10 ft/giây, vận tốc đầu là thường một phần không đáng kể tổng cộng, ngoại trừ các ứng dụng đầu thấp.3.3.4 TẤT CẢ ĐẦU Sự kết hợp của các giá trị này bằng đầu của hệ thống.Tổng số đầu = tĩnh đầu ++ tổn thất ma sát, vận tốc đầu3.4 HỆ THỐNG ĐƯỜNG CONGKhi người đứng đầu tất cả âm mưu chống lại tốc độ dòng chảy, đường cong kết quả được gọi là hệ thống đường cong Do đó khi một tốc độ dòng chảy cụ thể, ia lựa chọn cho một hệ thống, Hệ thống đường cong sẽ xác định người đứng đầu tất cả phải được khắc phục.Tốc độ dòng chảy qua một hệ thống chỉ có thể được cung cấp bởi một máy bơm, và do đó khả năng yêu cầu từ các máy bơm.Giao điểm của đường cong hiệu suất máy bơm và hệ thống đường cong đại diện cho poit mà tại đó các máy bơm sẽ hoạt động như minh hoạ trong hình 3.8Trong hệ thống nơi mà tốc độ dòng chảy được duy trì một mức độ không đổi, điều kiện xác định trong hình 3.8 sẽ không thay đổi. Trong các hệ thống khác Tuy nhiên, các điều kiện hoạt động của các máy bơm ly tâm đang không ngừng thay đổi.3,5 hiệu quả của hoạt động hiệu suất.1 tĩnh g = đầu thay đổi.1.1 lô chuyển hệ thốngTrong một hệ thống transffer lô ví dụ, thay đổi rõ ràng nhất là rằng đó được tạo ra bởi đổ xe tăng nguồn cung cấp, điều này kết quả trong việc giảm mức chất lỏng trong xe tăng và sự gia tăng tương đương đầu tĩnh các máy bơm phải vượt qua.Khi điều này xảy ra, Hệ thống đường cong sẽ chuyển thẳng lên trên đồ thị, với các sau ba điều kiện cụ thể xảy ra trong một lô duy nhất.Poit khởi động,, mức độ chất lỏng trong các bồn chứa cung cấp sẽ ở mức, trong khi mức độ trong xả xe tăng coula là số không. Điều này sẽ chuyển đổi thành một giá trị thấp tĩnh đầu cờ lêPoit trung gian, mức độ chất lỏng trong các bồn chứa cung cấp sẽ đã giảm xuống, và mức độ trong hồ xả sẽ lớn hơn. Điều này sẽ dẫn đến một giá trị cao hơn của tĩnh haed.Vào poit tắt máy, chất lỏng trong bể cung cấp sẽ có được chuyển giao hoàn toàn vào thùng xả, dẫn đến giá trị tối đa của tĩnh haed. Lúc này poit, các máy bơm phải tắt máy.Hệ thống đường cong sẽ giả sử các vị trí ba Hiển thị trong hình 3.9 khi nó di chuyển dần từ khởi động (a) để tắt máy (c) với sự thay đổi carresponding trong khả năng bơm. Tuy nhiên, như hệ thống phương pháp tiếp cận poit tắt máy, hiệu suất máy bơm sẽ trở thành không ổn định. Điều này là do các loại bỏ dần dần thích hợp điều kiện hút mà sẽ được thảo luận trong một số chi tiết trong chương 4..1.2 Pressurized hệ thốngTrong một loại áp lực của hệ thống, chẳng hạn như một nồi hơi nguồn cấp dữ liệu hệ thống, nguồn cấp dữ liệu bơm đi nó là hút từ một deaerator dưới máy hút và cung cấp một nồi hơi dưới áp lực. Trong hệ thống này, áp lực khác biệt không phải là một chức năng của tốc độ dòng chảy và sẽ có hậu quả tương tự như người đứng đầu tĩnh. Bất kỳ thay đổi nào trong áp lực trong deaerator hoặc lò hơi, cũng sẽ gây ra hệ thống đường cong để di chuyển lên hoặc xuống là chỉ định hình 3.9.1.3 vòng lặp đóng cửa hệ thống Một hệ thống vòng khép kín trong một trong đó toàn bộ hệ thống áp lực bởi các máy bơm. Để đạt điều này, pumpage là hoàn toàn con tained trong một loạt các đường ống và thiết bị áp lực công nghệ từ xả bơm, thruogh hệ thống, và quay trở lại các đầu vào máy bơm. Như vậy là bố trí, tĩnh haed trong các là effecively zero.3.5.2 thay đổi đầu ma sát .2.1 Hệ thống kiểm soátMột sự thay đổi trong ma sát mất mát có thể được gây ra bởi một loạt các điều kiện như hướng dẫn sử dụng hoạt động hoặc tự động điều khiển hoạt động hoặc tự động hóa điều khiển mở
đang được dịch, vui lòng đợi..
A centrifugal pump is designed and produced to supply whole range of head-capacity condition as identified by it’s performance curve. The pump will operate on thet curve if it is driven at the particular speed for which the curve is drawn. However, the actual condition on that curve at which the pump will run will be determined by the system in which it operates. So for all practical puepases, the system controls the pump and will determine the conditions at which the pump will operate, regardless of the head and capacity for which it was purchased.
This is a considerable advantage from the the safety aspect of the system, in that the centrifugal pump is not normally capable of over-pressurizing the system. However in order to understand how the centrifugal pump operates in a system, it is first necessary to understand some aspects of system design.
3.2 LIQUID FLOW IN PIPE
For those who may be unsure of manner in which liquid actually resond to flowing though pipes, the flowing basic guidelines are offered.
3.2.1 SPECIFIC GRAVITY
Speccific gravity is used frequently in the discussuin of fluids. It is the mane given ti the ratui of the density if a liquid to the density of cold water. Therefore, when dealing with cold water, its value of S.G is 1.0
3.2.2 PRESSURE IN A STATIC SYSTEM
When a body of liquid is a rest in a sytem, the relationship between the pressure showing on the gauge and the depth of the above it, will be as show.
P = gauge pressure in pounds per square inch
Hs = static head of liquid in feet
s.g. = specific gravity of the liquid being pumped
3.2.3 PRESSURE IN A FLOWING SYSTEM
When a body of liquid is moving in a system, the pressure will drop, as some of the energe by the static head is now being lost to fiction. Therefore, even when we maintain the level of water in the tank as shown in figure 3.2, to statis head, the pressure reading on the gaure will be less than when the liquid in the system was not flowing
P = gauge pressure in pounds per square inch
Hs = Static head of liquid in feet
Hf = friction head in feet
3.2.4 CHANGES IN AN EXISTING SYSTEM
Please note that the fllowing repationships are offered for those who wish to approximate the effect of changes in an existing system. For accurate data, it is recommended that the hydraulic institutes friction loss table are consulted, either in their engineering data book, or as show in chapter 14 this book
3.2.4.1 THE EFFECT OF CAPACITY CHANGE FRICTION
When the flow rate is changed without changing the pipe size, the qpproximate change in friction loss can be estimated as shown.
Q = Flow rate
Hf = Friction loss
In orther words, the friction loss will vary as square of the flow rate.
3.4.2.2 The effect of head change on flow rate
When the static head is changed again without changing the pipe size the appoximate change in flow rate can be estimated as shown
3.2.5 Pipe size changes in a system
The change in pipe diameter may be necessary to reduce friction losses or increase NPSH available to the pump. Therefore when capacity in unchanged, the friction loss is in INVERSE proportion to the 5th power of the change in the pipe diameter, as shown below.
To operate with the same head, such as from a lake or river, with the new pipe size, the fllowing approximation will apply.
D = pipe diameter
3.3 BASIC ELEMENT OF PUMP SYSTEM DESIGN
In designing any kind of pumping system, the first requirement is to determine the speed at which the task must be performed. In orther words, the flow needed through the system. In some systems, the flow rate will be determined by production requirement or by orther process considerations such as the flow rate needed to achieve the necessary temperature transfer in a liquid flowing though a heat exchanger. For the sake of this exercise, let us consider a batch process system where the average flow rate can be calculation by dividing the volume to be transferred, by the time allowed for that transfer.
The next requirement to e considered is how to overcome all the factor which hinder the movement of the liquid from one poit to another in the system. These are primarily gravity and friction and we will deal with them separately.
3.3.1 GRAVITY AND STATIC HEAD
If we considenr gravity as a force of nature that drives vertically downwards then, in a pumping system, we can oppase it by means of an energy factor we will refoer to as the total static head . this is simple the change in elevation through which the liquid must be lifted, and is measured vertically, regardless of the linear distance between the start and end poits in the system. As shown in figure 3.3, the static head.
Can be measured between the free surface of liquid in tank A, and the highest level of the discharge line.
In another system, where the pump dischar into tank B at a poit below the liquid level in that tank, the total static head in the system is the vertical distance from the free surface of the liquid in tank A to the free suface of the liquid in tank B . It should be noted that this also app;ies, even when the suction source is lower than the pump. This will be discussed in greater detail in chapter 4
Regardless of the layout of the system, the total static head is not a variable os the flow rate, therefore a gragh comraring the two would show up as a straight line.
3.3.2 FRICTION LOSSES AND FRICTION HEAD
Fiction is the resistance to flow in the piping system and must be caonsidered for three separate areas individually.
The Piping
The valves and fittings , and
Orther equipment , such as filters, heat exchangers, etc.
The friction losses in piping can most readily be obtained from the friction loss table available from a variety of sources such as the standards of the hydraulic in-stitute, for benefit of the reader, many of these table are repro-duced in chapter 14 of this boo. Table are also available to identify the losses thruogh the more common pipe fittings and valve types. However, any such losses in filters, heat exxhangers, etc…
Must be obtained from the original equipment manufacturer, or by measuring the equipment on site.
As the floe oncreases, so too does the friction loss but a far higher rate as shown in figure 3.6
3.3.3 Velocity head
Another factor that has to be overcome is the heat required to accelerate the flow of liquid through the pump. This is the difference in the values of velocity head at the suction and discharge nozzles of the pump.
As the linar velocity of the liquid in most system in maintained at lower than 10 ft/sec, the velocity head is usually an insignificant part the total, except low head application.
3.3.4 TOTAL HEAD
The combination of these values equals the total head of the system.
Total head = Static head + Friction loss + Velocity head
3.4 SYSTEM CURVE
When the total head is plotted against the flow rate, the resultant curve is known as the system curve
Therefore when a specific flow rate ia selected for a system, the system curve will identify the total head that must be overcome.
The flow rate through a system can only be supplied by a pump, and is therefore the capacity required from the pump.
The intersection of the pump performance curve and the system curve represents the poit at which the pump will operate as shown in figure 3.8
In system where the flow rate is maintained a constant level, the conditions identified in figure 3.8 will not change. In orther systems however, the operating condition of the centrifugal pump is constantly changing.
3.5 The effect of operating performance
.1 static g=head changes
.1.1 batch transfer system
In a batch transffer system for example, most obvious change is that which is created by emptying the supply tank, this results in a reduction in the liquid level in that tank and the equivalent increase in the static head that the pump must overcome.
When this happens, the system curve will move straight up on the graph, with the following three specific conditions occurring during a single batch.
At the startup poit,, the level of liquid in the supply tank will be at its highest, while the level in the discharge tank coula be zero. This will translate into a low value of static head
At the intermediate poit, the level of liquid in the supply tank will have dropped, and the level in the discharge tank will be greater. This will result in a higher value of static haed.
At the shutdown poit, the liquid in the supply tank will have been transferred entirely into the discharge tank, resulting in the maximum value of static haed. At this poit, the pump should be shutdown.
The system curve will assume the three positions shown in figure 3.9 as it moves steadily from startup (a) to shutdown (c) with the carresponding change in pump capacity. However , as the system approaches the shutdown poit, the pump performance will become unstable. This is due to gradual elimination of appropriate suction conditions which will be discussed in some detail in chapter 4.
.1.2 Pressurized system
In a pressurized type of system, such as a boiler feed system, the feed pump take it’s suction from a deaerator under vacuum and supplies a boiler under pressure. In this system, the differential pressure is not a function of the flow rate and will have similar consequences as the static head. Any change in pressure in either the deaerator or the boiler, will also cause the system curve to move up or down as indicated figure 3.9
.1.3 Closed loop system
A closed loop system in one in which the entire system is pressurized by the pump. To achieve this, the pumpage is fully con tained within a series of pipes and pressurized process equipment all the way from the pump discharge, thruogh the system, and back to the pump inlet. In such a layout, the static haed in the is effecively zero.
3.5.2 Friction head changes
.2.1 system control
A change in friction loss can be caused by a variety of conditions such as manual operation or automated controls operation or automate controls opening
This is a considerable advantage from the the safety aspect of the system, in that the centrifugal pump is not normally capable of over-pressurizing the system. However in order to understand how the centrifugal pump operates in a system, it is first necessary to understand some aspects of system design.
3.2 LIQUID FLOW IN PIPE
For those who may be unsure of manner in which liquid actually resond to flowing though pipes, the flowing basic guidelines are offered.
3.2.1 SPECIFIC GRAVITY
Speccific gravity is used frequently in the discussuin of fluids. It is the mane given ti the ratui of the density if a liquid to the density of cold water. Therefore, when dealing with cold water, its value of S.G is 1.0
3.2.2 PRESSURE IN A STATIC SYSTEM
When a body of liquid is a rest in a sytem, the relationship between the pressure showing on the gauge and the depth of the above it, will be as show.
P = gauge pressure in pounds per square inch
Hs = static head of liquid in feet
s.g. = specific gravity of the liquid being pumped
3.2.3 PRESSURE IN A FLOWING SYSTEM
When a body of liquid is moving in a system, the pressure will drop, as some of the energe by the static head is now being lost to fiction. Therefore, even when we maintain the level of water in the tank as shown in figure 3.2, to statis head, the pressure reading on the gaure will be less than when the liquid in the system was not flowing
P = gauge pressure in pounds per square inch
Hs = Static head of liquid in feet
Hf = friction head in feet
3.2.4 CHANGES IN AN EXISTING SYSTEM
Please note that the fllowing repationships are offered for those who wish to approximate the effect of changes in an existing system. For accurate data, it is recommended that the hydraulic institutes friction loss table are consulted, either in their engineering data book, or as show in chapter 14 this book
3.2.4.1 THE EFFECT OF CAPACITY CHANGE FRICTION
When the flow rate is changed without changing the pipe size, the qpproximate change in friction loss can be estimated as shown.
Q = Flow rate
Hf = Friction loss
In orther words, the friction loss will vary as square of the flow rate.
3.4.2.2 The effect of head change on flow rate
When the static head is changed again without changing the pipe size the appoximate change in flow rate can be estimated as shown
3.2.5 Pipe size changes in a system
The change in pipe diameter may be necessary to reduce friction losses or increase NPSH available to the pump. Therefore when capacity in unchanged, the friction loss is in INVERSE proportion to the 5th power of the change in the pipe diameter, as shown below.
To operate with the same head, such as from a lake or river, with the new pipe size, the fllowing approximation will apply.
D = pipe diameter
3.3 BASIC ELEMENT OF PUMP SYSTEM DESIGN
In designing any kind of pumping system, the first requirement is to determine the speed at which the task must be performed. In orther words, the flow needed through the system. In some systems, the flow rate will be determined by production requirement or by orther process considerations such as the flow rate needed to achieve the necessary temperature transfer in a liquid flowing though a heat exchanger. For the sake of this exercise, let us consider a batch process system where the average flow rate can be calculation by dividing the volume to be transferred, by the time allowed for that transfer.
The next requirement to e considered is how to overcome all the factor which hinder the movement of the liquid from one poit to another in the system. These are primarily gravity and friction and we will deal with them separately.
3.3.1 GRAVITY AND STATIC HEAD
If we considenr gravity as a force of nature that drives vertically downwards then, in a pumping system, we can oppase it by means of an energy factor we will refoer to as the total static head . this is simple the change in elevation through which the liquid must be lifted, and is measured vertically, regardless of the linear distance between the start and end poits in the system. As shown in figure 3.3, the static head.
Can be measured between the free surface of liquid in tank A, and the highest level of the discharge line.
In another system, where the pump dischar into tank B at a poit below the liquid level in that tank, the total static head in the system is the vertical distance from the free surface of the liquid in tank A to the free suface of the liquid in tank B . It should be noted that this also app;ies, even when the suction source is lower than the pump. This will be discussed in greater detail in chapter 4
Regardless of the layout of the system, the total static head is not a variable os the flow rate, therefore a gragh comraring the two would show up as a straight line.
3.3.2 FRICTION LOSSES AND FRICTION HEAD
Fiction is the resistance to flow in the piping system and must be caonsidered for three separate areas individually.
The Piping
The valves and fittings , and
Orther equipment , such as filters, heat exchangers, etc.
The friction losses in piping can most readily be obtained from the friction loss table available from a variety of sources such as the standards of the hydraulic in-stitute, for benefit of the reader, many of these table are repro-duced in chapter 14 of this boo. Table are also available to identify the losses thruogh the more common pipe fittings and valve types. However, any such losses in filters, heat exxhangers, etc…
Must be obtained from the original equipment manufacturer, or by measuring the equipment on site.
As the floe oncreases, so too does the friction loss but a far higher rate as shown in figure 3.6
3.3.3 Velocity head
Another factor that has to be overcome is the heat required to accelerate the flow of liquid through the pump. This is the difference in the values of velocity head at the suction and discharge nozzles of the pump.
As the linar velocity of the liquid in most system in maintained at lower than 10 ft/sec, the velocity head is usually an insignificant part the total, except low head application.
3.3.4 TOTAL HEAD
The combination of these values equals the total head of the system.
Total head = Static head + Friction loss + Velocity head
3.4 SYSTEM CURVE
When the total head is plotted against the flow rate, the resultant curve is known as the system curve
Therefore when a specific flow rate ia selected for a system, the system curve will identify the total head that must be overcome.
The flow rate through a system can only be supplied by a pump, and is therefore the capacity required from the pump.
The intersection of the pump performance curve and the system curve represents the poit at which the pump will operate as shown in figure 3.8
In system where the flow rate is maintained a constant level, the conditions identified in figure 3.8 will not change. In orther systems however, the operating condition of the centrifugal pump is constantly changing.
3.5 The effect of operating performance
.1 static g=head changes
.1.1 batch transfer system
In a batch transffer system for example, most obvious change is that which is created by emptying the supply tank, this results in a reduction in the liquid level in that tank and the equivalent increase in the static head that the pump must overcome.
When this happens, the system curve will move straight up on the graph, with the following three specific conditions occurring during a single batch.
At the startup poit,, the level of liquid in the supply tank will be at its highest, while the level in the discharge tank coula be zero. This will translate into a low value of static head
At the intermediate poit, the level of liquid in the supply tank will have dropped, and the level in the discharge tank will be greater. This will result in a higher value of static haed.
At the shutdown poit, the liquid in the supply tank will have been transferred entirely into the discharge tank, resulting in the maximum value of static haed. At this poit, the pump should be shutdown.
The system curve will assume the three positions shown in figure 3.9 as it moves steadily from startup (a) to shutdown (c) with the carresponding change in pump capacity. However , as the system approaches the shutdown poit, the pump performance will become unstable. This is due to gradual elimination of appropriate suction conditions which will be discussed in some detail in chapter 4.
.1.2 Pressurized system
In a pressurized type of system, such as a boiler feed system, the feed pump take it’s suction from a deaerator under vacuum and supplies a boiler under pressure. In this system, the differential pressure is not a function of the flow rate and will have similar consequences as the static head. Any change in pressure in either the deaerator or the boiler, will also cause the system curve to move up or down as indicated figure 3.9
.1.3 Closed loop system
A closed loop system in one in which the entire system is pressurized by the pump. To achieve this, the pumpage is fully con tained within a series of pipes and pressurized process equipment all the way from the pump discharge, thruogh the system, and back to the pump inlet. In such a layout, the static haed in the is effecively zero.
3.5.2 Friction head changes
.2.1 system control
A change in friction loss can be caused by a variety of conditions such as manual operation or automated controls operation or automate controls opening
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- information
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- my side of the story
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- tôi nhớ lại quá khứ của tôi
