Centrifugal
Pump Fundamentals - Topics
- Centrifugal Pump
- Pump construction
- Considerations for Pump Selection
- Effect of Speed Variation on pump performance
- Effect of Impeller Trimming on pump performance
- NPSH
- System Resistance Curve
- Variation in system resistance curve
What is a Pump ?
Ø Pump is a machine used to convert
Mechanical energy into
Hydraulic energy
Ø Pump is a device to transfer liquid –
Over a distance
From a low level to a higher level
From an area of low pressure to high pressure
Combination of the above
Source: Google. Images
Terminology
used in pump selection
Capacity :
Capacity or pump flow rate Q is external volume flow per unit of time. Balance
water, leakage water etc. do not count as part of the capacity.
Head :
The head H of a pump is the useful mechanical energy transmitted by the pump to
the product. A centrifugal pump will generate a same head H for all fluids
irrespective of the density. The density determines the pressure within the
pump and forms part of the power Input
Speed :
With AC motor drives ( squirrel cage ) approx. pump speed are as follows
At 50Hz No of poles 2 4 6 8 Speed rpm 2900 1450 960 725
In practice, actual motor speed to be
considered.
Power :
The pump power input P of a centrifugal
pump is mechanical energy at pump shaft.
P(kW)= ( r X Q X H) / (367 X h) where is r in kg/dm3,
Q in m3/hr., H in m
The definition of NPSHA is simple: Static head + surface pressure head - the vapor pressure of your product - the friction losses in the piping, valves and fittings.
But to really understand it, you first have to understand a couple of other concepts:
- Cavitation is what net positive suction head (NPSH) is all about, so you need to know a little about cavitation.
- Vapor Pressure is another term we will be using. The product's vapor pressure varies with the fluid's temperature.
- Specific gravity play an important part in all calculations involving liquid. You have to be familiar with the term.
- You have to be able to read a pump curve to learn the N.P.S.H. required for your pump.
- You need to understand how the liquid's velocity affects its pressure or head.
- It is important to understand why we use the term Head instead of Pressure when we make our calculations.
- Head loss is an awkward term, but you will need to understand it.
- You will have to be able to calculate the head loss through piping, valves and fittings.
- You must know the difference between gage pressure and absolute pressure.
- Vacuum is often a part of the calculations, so you are going to have to be familiar with the terms we use to describe vacuum.
- Cavitation means cavities or holes in liquid. Another name for a hole in a liquid is a bubble, so cavitation is all about bubbles forming and collapsing.
- Bubbles take up space so the capacity of our pump drops.
- Collapsing bubbles can damage the impeller and volute. This makes cavitation a problem for both the pump and the mechanical seal.
- Vapor pressure is about liquids boiling. If I asked you, "at what temperature does water boil ?" You could say 212° F. or 100° C., but that is only true at atmospheric pressure. Every product will boil (make bubbles) at some combination of pressure and temperature. If you know the temperature of your product you need to know its vapor pressure to prevent boiling and the formation of bubbles. In the charts section of this web site you will find a vapor pressure chart for several common liquids.
- Specific gravity is about the weight of the fluid. Using 4°C (39° F) as our temperature standard we assign fresh water a value of one. If the fluid floats on this fresh water it has a specific gravity is less than one. If the fluid sinks in this water the specific gravity of the fluid is greater than one.
- Look at any pump curve and make sure you can locate the values for head, capacity, best efficiency point (B.E.P.), efficiency, net positive suction head (NPSH), and horse power required. If you cannot do this, have someone show you where they are located.
- Liquid velocity is another important concept. As a liquid's velocity increases, its pressure (90° to the flow) decreases. If the velocity decreases the pressure increases. The rule is : velocity times pressure must remain a constant.
- "Head" is the term we use instead of pressure. The pump will pump any liquid to a given height or head depending upon the diameter and speed of the impeller. The amount of pressure you get depends upon the weight (specific gravity) of the liquid. The pump manufacturer does not know what liquid the pump will be pumping so he gives you only the head that the pump will generate. You have to figure out the pressure using a formula described later on in this paper.
- Head (feet) is a convenient term because when combined with capacity (gallons or pounds per minute) you come up with the conversion for horsepower (foot pounds per minute).
- "Head loss through the piping, valves and fittings" is another term we will be using. Pressure drop is a more comfortable term for most people, but the term "pressure" is not used in most pump calculations so you could substitute the term "head drop" or "loss of head" in the system. To calculate this loss you will need to be able to read charts like those you will find in the "charts you can use" section in the home page of this web site. They are labeled Friction loss for water and Resistance coefficients for valves and fittings.
- Gage and absolute pressure. Add atmospheric pressure to the gage pressure and you get absolute pressure.
- Vacuum is a pressure less than atmospheric. At sea level atmospheric pressure is 14.7 psi. (760 mm of Mercury). Vacuum gages are normally calibrated in inches or millimeters of mercury.
- The curve for your pump. This pump curve is supplied by the pump manufacturer. Someone in your plant should have a copy. The curve is going to show you the Net Positive Suction Head (NPSH) required for your pump at a given capacity. Each pump is different so make sure you have the correct pump curve and use the numbers for the impeller diameter on your pump. Keep in mind that this NPSH required was for cold, fresh water.
- A chart or some type of publication that will give you the vapor pressure of the fluid you are pumping. You can find a typical vapor pressure chart in the "charts you can use" section in the home page of this web site
- If you would like to be a little more exact, you can use a chart to show the possible reduction in NPSH required if you are pumping hot water or light hydrocarbons. I will cover this subject in great detail in another paper.
- You need to know the specific gravity of your fluid. Keep in mind that the number is temperature sensitive. You can get this number from a published chart, ask some knowledgeable person at your plant, or or take a reading on the fluid using a hydrometer.
- Charts showing the head loss through the size of piping you are using between the source and the suction eye of your pump. You will also need charts to calculate the loss in any fittings, valves, or other hardware that might have been installed in the suction piping. You can find these charts in the "charts you can use" section in the home page of this web site
- Is the tank you are pumping from at atmospheric pressure or is it pressurized in some manner? Maybe it is under a vacuum ?
- You need to know the atmospheric pressure at the time you are making your calculation. We all know atmospheric pressure changes through out the day, but you have to start somewhere.
- The formulas for converting pressure to head and head back to pressure in the imperial system are as follows:
- sg. = specific gravity
- pressure = pounds per square inch
- head = feet
- You also need to know the formulas that show you how to convert vacuum readings to feet of head. Here are a few of them:
- Inches of mercury x 1.133 / specific gravity = feet of liquid
- Pounds per square inch x 2.31 / specific gravity = feet of liquid
- Millimeters of mercury / (22.4 x specific gravity) = feet of liquid
- NPSHA (net positive suction head available)
- NPSHR (net positive suction head required)
Cavitation begins as small harmless bubbles before you get any indication of loss of head or capacity. This is called the point of incipient cavitation. Testing has shown that it takes from two to twenty times the NPSHR (net positive suction head required) to fully suppress incipient cavitation, depending on the impeller shape (specific speed number) and operating conditions.
To stop a product from vaporizing or boiling at the low pressure side of the pump the NPSHA (net positive suction head available) must be equal to or greater than the NPSHR (net positive suction head required).
As I mentioned at the beginning, NPSHA is defined as static head + surface pressure head - the vapor pressure of your product - loss in the piping, valves and fittings .
In the following paragraphs you will be using the above formulas to determine if you have a problem with NPSHA. Here is where you locate the numbers to put into the formula:
- Static head. Measure it from the centerline of the pump suction to the top of the liquid level. If the level is below the centerline of the pump it will be a negative or minus number.
- Surface pressure head. Convert the gage absolute pressure to feet of liquid using the formula:
- Pressure = head x specific gravity / 2.31
- Vapor pressure of your product . Look at the vapor pressure chart in the "charts you can use" section in the home page of this web site. You will have to convert the pressure to head. If you use the absolute pressure shown on the left side of the chart, you can use the above formula
- Specific gravity of your product. You can measure it with a hydrometer if no one in your facility has the correct chart or knows the number.
- Loss of pressure in the piping, fittings and valves. Use the three charts in the "charts you can use" section in the home page of this web site
- Find the chart for the proper pipe size, go down to the gpm and read across to the loss through one hundred feet of pipe directly from the last column in the chart. As an example: two inch pipe, 65 gpm = 7.69 feet of loss for each 100 feet of pipe.
- For valves and fittings look up the resistance coefficient numbers (K numbers) for all the valves and fittings, add them together and multiply the total by the V2/2g number shown in the fourth column of the friction loss piping chart. Example: A 2 inch long radius screwed elbow has a K number of 0.4 and a 2 inch globe valve has a K number of 8. Adding them together (8 + 0.4) = 8.4 x 0.6 (for 65 gpm) = 5 feet of loss.
Let's go through the first example and see if our pump is going to cavitate:
Given:
- Atmospheric pressure = 14.7 psi
- Gage pressure =The tank is at sea level and open to atmospheric pressure.
- Liquid level above pump centerline = 5 feet
- Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.
- Pumping =100 gpm. 68°F. fresh water with a specific gravity of one (1).
- Vapor pressure of 68°F. Water = 0.27 psia from the vapor chart.
- Specific gravity = 1
- NPSHR (net positive suction head required, from the pump curve) = 9 feet
Now for the calculations:
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings
- Static head = 5 feet
- Atmospheric pressure = pressure x 2.31/sg. = 14.7 x 2.31/1 = 34 feet absolute
- Gage pressure = 0
- Vapor pressure of 68°F. water converted to head = pressure x 2.31/sg = 0.27 x 2.31/1 = 0.62 feet
- Looking at the friction charts:
- 100 gpm flowing through 2 inch pipe shows a loss of 17.4 feet for each 100 feet of pipe or 17.4/10 = 1.74 feet of head loss in the piping
- The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet
- Adding these numbers together, 1.74 + 0.6 = a total of 2.34 feet friction loss in the pipe and fitting.
The pump required 9 feet of head at 100 gpm. And we have 36.04 feet so we have plenty to spare.
Example number 2 . This time we are going to be pumping from a tank under vacuum.
Given:
- Gage pressure = - 20 inches of vacuum
- Atmospheic pressure = 14.7 psi
- Liquid level above pump centerline = 5 feet
- Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.
- Pumping = 100 gpm. 68°F fresh water with a specific gravity of one (1).
- Vapor pressure of 68°F water = 0.27 psia from the vapor chart.
- NPSHR (net positive suction head required) = 9 feet
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings
- Atmospheric pressure = 14.7 psi x 2.31/sg. =34 feet
- Static head = 5 feet
- Gage pessure pressure = 20 inches of vacuum converted to head
- inches of mercury x 1.133 / specific gravity = feet of liquid
- -20 x 1.133 /1 = -22.7 feet of pressure head absolute
- Vapor pressure of 68°F water = pressure x 2.31/sg. = 0.27 x 2.31/1 = 0.62 feet
- Looking at the friction charts:
- 100 gpm flowing through 2.5 inch pipe shows a loss of 17.4 feet or each 100 feet of pipe or 17.4/10 = 1.74 feet loss in the piping
- The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet
- Adding these two numbers together: (1.74 + 0.6) = a total of 2.34 feet friction loss in the pipe and fitting.
For the third example we will keep everything the same except that we will be pumping 180° F. hot condensate from the vacuum tank.
The vapor pressure of 180°F condensate is 7 psi according to the chart. We get the specific gravity from another chart and find that it is 0.97 sg. for 180° F. Fresh water.
Putting this into the pressure conversion formula we get:
- pressure x 2.31/sg. = 7 x 2.31 / 0.97 = 16.7 feet absolute
NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 16.7 - 2.34 = -2.74 feet.
We need 9 feet, so the pump is going to cavitate for sure.
A few notes about this last example:
- A negative NPSHA is physically impossible because it implies that the friction losses exceed the available head and that cannot happen. The rule when pumping a boiling fluid is: The NPSHA equals the Static Suction Head minus the Suction friction head because the suction surface pressure and the vapor pressure equalize one another. The absolute pressure in the tank is 34 -22.7 = 11.3 ft. The vapor pressure of the condensate in the tank converts to 16.7 ft of head (see above) so the condensate is boiling /flashing and reaching a state of equilibrium.
- When pumping a boiling liquid, the Static Head must exceed the Suction Friction Head (2.34 feet) by the amount of NPSH Required (9 feet) or: (9 ft. + 2.34 feet = 11.34 feet.) We can do this by raising the level in the suction tank an additional 6.34 feet to get the 11.34 feet required (6.34 feet + 5 feet existing = 11.34 feet)
- In some instances you could reduce the Suction Friction Head to get the same result, but in this example there is not enough friction head available to reduce.
- This example also allows you to shortcut NPSHA calculations any time you are pumping from a tank where the liquid is at its vapor pressure. Oil refineries are full of these applications.
- Pp = Absolute pressure expressed in psia. In an open system, Pp equals atmospheric pressure, Pa, expressed in psia.
- Pvpa = Vapor pressure expressed in psia.
- W = Specific weight of liquid at the pumping temperature in pounds per cubic foot.
Thanks: Mcnally Institute Source Link. http://www.mcnallyinstitute.com/11-html/11-12.html
System
characteristics curve
Every pump manufacturer would like to recommend the perfect pump for your application. To do this he would like you to provide him with an accurate system curve that would describe the capacity and head needed for your various operating conditions. Once he has your system curve he can plot his pump curves on top of the system curve and hopefully select something that will come close to your needs. Without this system curve neither one of you has much of a chance of coming up with the right size pump.
To create a system curve we plot the desired capacities against the required head over the total anticipated operating range or window 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. Some of the confusion begins when we realize that there are three different kinds of head:
STATIC HEAD.
This is the vertical distance measured from the centerline of the pump to the height of the piping discharge inside the tank. Look at figure "A" and note that the piping discharge is below the maximum elevation of the piping system. We do not use the maximum elevation in our calculations because the siphoning action will carry the fluid over this point once the piping is full of liquid. This is the same action that lets you siphon gasoline out of an automobile to a storage can.
The pump will have to develop enough head to fill the pipe and then the siphoning action will take over. The pump operating point should move back towards the best efficiency point (BEP) if the pump was selected correctly.
FIGURE "A"
PRESSURE HEAD.
If the vessel we are pumping to is pressurized, this pressure converted to head units, will have to be added to the static head. To convert pressure to head units use one of the following formulas:
DYNAMIC OR SYSTEM HEAD
As the liquid flows through the piping and fittings it is subject to the friction caused by the piping inside finish, restricted passages in the fittings and any type of hardware that has been installed in the system.
The resulting pressure drop is described as a "loss of head" in the system and can be calculated from charts you will find in the charts section of this CD This head loss is related to the condition of the system and makes the calculations difficult when you realize that older systems may have "product build up" on the piping walls, filters, strainers, valves, elbows, heat exchangers, etc., making the published numbers some what inaccurate.
A general "rule of thumb" states that the friction loss in clean piping will vary approximately with 90% of the square of the change in flow in the piping, and 100% of the square with the change of flow in the fittings and accessories. You calculate the change in flow by dividing the new flow by the old flow and then square the number. As an example:
At 200 gpm the piping resistance calculated from published charts (you can find these in the charts section) is seventy-five feet (75 Ft.). What will it be at 300 gpm?300 / 200 = (1.5)2 = 2.25 x 75 feet = 168.75 x 90% of the change = 151.88 feet of resistance head
In other words, when we went from 200 to 300 gallons per minute the piping resistance increased from 75 feet to 151.88 feet.
The loss through the fittings and hardware was calculated at 25 feet. What will the new loss be?
300 / 200 = (1.5)2 = 2.25 x 25 feet = 56.25 x 100% of the change = 56.25 new feet of head
In the original application, system loss was a combination of the loss through the piping and the loss through the fittings for a total of 100 feet at 200 gallons per minute.
When we increased the flow to 300 gallons per minute our system head changed to a total of 208.13 feet. This change would have to be added to the static and pressure heads to calculate the total head required for the new pump.
Please note that the pump is pumping the difference between the suction head and the discharge head so if you fail to consider that the suction head will be either added to or subtracted from the discharge head you will make an error in your calculations.
The suction head will be negative if you are lifting liquid from below ground or if you are pumping from a vacuum. It will be positive if you are pumping from a tank located above ground. If the suction head is pressurized, this pressure must be converted to head and subtracted from the total head required by the pump.
A centrifugal pump will create a head-capacity curve that will generally resemble one of the curves described in figure "B" The shape of the curve is determined by the specific speed number of the impeller.
Centrifugal pumps always pump somewhere on their curve, but should be selected to pump as close to the best efficiency point (BEP) as possible. The best efficiency point (BEP) will fall some where between 80% and 85% of the shut off head (maximum head).
The manufacturer generated these curves at a specific rpm. Unless you are using synchronous motors (you probably are using induction motors on your pumps) you will have to adjust the curves to match your actual pump speed. Put a tachometer on the running motor and record the rpm difference between your pump and the speed shown on the pump manufacturer's published curve.
You can use the pump affinity laws to approximate the change.
POSITIVE DISPLACEMENT PUMPS have a different shaped curve. They look something like figure "C":
The capacity, of a positive displacement pump will remain almost a constant as long as you do not alter the pump speed. Run it faster and it will pump more. The maximum head is determined by the strength of the pump casing and the horsepower (KW) available.
Surprisingly there are only a few system curve shapes that you will encounter.
Figure "D" describes the first one.
In this system the head remains a constant as the capacity varies. This is a typical application for a boiler feed pump that is supplying a constant pressure boiler with a varying steam demand.
This is also a very common application in many process systems, or aboard a ship that is frequently changing speeds (answering bells).
Filling a tank from the top and varying the amount of liquid being pumped is the normal routine in most process plants. The curve will look like this first one if the majority of the head is either static or pressure head.
The second system curve is the ideal one. Figure "E" describes it:
In this system the head and capacity remain a constant as long as the pump is running. This is the perfect pump application! We find this condition in a boiler circulating pump where the suction and discharge are at the same pressure.
Most tank circulating pumps have a single point rather than a system curve. A steady state, power-generating boiler is another example.
A steady state process pump operates at a single point also.
Figure "F" describes the next curve. We call this an exponential curve. In this system the entire head is system head so it will vary with the capacity. Look for this type of curve in a circulating hot or cold water heating/ cooling system or if you are pumping to a non pressurized tank a long distance from the source, with little to no elevation involved.
Filling tank cars is a typical application.
System curve "G" is a another curve. It is a combination of static, pressure and system heads.
This curve is generated if we are pumping to an elevated tank a long distance from the source and the amount we are pumping varies due to the system demands.
System figure "H" is the type you get if you are filling a tank from the bottom or attempting to use the centrifugal pump as an accumulator.
If the capacity is below 20 gallons per minute (4,5 m3/hr) you really should be using a positive displacement pump in this application or a really robust centrifugal pump.
Once the pump manufacturer has a clear idea as to the shape of your system curve and the head and capacity numbers needed, he can then select the proper centrifugal pump. The shape of his curve will be pretty much determined by the specific speed number of the impeller.
In addition to specific speed he can select impeller diameter, impeller width, pump rpm.; and he also has the option of series or parallel operation along with the possibility of using a multi-stage pump to satisfy your needs.
The sad fact is that most pumps are selected poorly because of the desire to offer the customer the lowest possible price. A robust pump with a low L3/D4 number is still your best protection against seal and bearing premature failure when the pump is operating off of its best efficiency point. Keep the following in mind as you select your pump:
- A centrifugal pump will pump where the pump curve intersects the system curve. This may bear no relationship to the best efficiency point (BEP), or your desire for the pump to perform a specific task.
- The further off the best efficiency point (BEP) you go the more robust the pump you will need. This is especially true if you have replaced the packing with a mechanical seal and no longer have the packing to act as a support bearing when the shaft deflects. Shaft deflection is always a major problem at start up.
- When you connect pumps in parallel you add the capacities together. The capacity of a pump is determined by the impeller width and rpm. The head of a centrifugal pump is determined by the impeller diameter and rpm. If the heads are different the stronger pump will throttle the weaker one so the impeller diameters and rpms must be the same if you connect pumps in parallel. Check the rpms on these pumps if you are experiencing any difficulties.
- If you connect the pumps in series the heads will add together so the capacities must be the same or one of the pumps will cavitate. You could also have a problem operating too far to the right of the best efficiency point with a possible motor "burn out".
- When you vary the speed of a centrifugal pump the affect is almost the same as changing the diameter of the impeller. This means that the variable speed motor will work best on a system curve that is exponential (Figure "F"). Unfortunately most process and boiler feed pump system curves are not exponential.
- Pump curves are based on a speed of 1750, 3500, 1450, or 2900-rpm. Electric induction motors seldom run at these speeds because of slip. You can estimate that a 2% to a 5% slip is normal in these pumps with the amount of slip directly related to the price of the motor.
- You should also keep in mind that if the motor is running at its best efficiency point that does not mean that the pump is running at its best efficiency point (BEP).
Do not trust piping diagrams to make your calculations. The actual system always differs from that shown on the diagram because people tap into the lines using the pumped fluid for a variety of purposes, and after having done so, forget to change or "mark up" the original diagram.
You are going to have to "walk down" the system and note the pipe length, the number of fittings, etc. to make an accurate system head calculation. Do not be surprised to find that the discharge of your pump is hooked up to the discharge of another pump further down the line. In other words the pumps are connected in parallel and nobody knows it.
Pressure recorders (not gauges) installed at the pump suction and discharge is another technique you can use to get a better picture of the system or dynamic head. These gages will show you how the head is varying with changes in flow. The trouble with these recording devices is they tell you what the present pump is doing. They do not tell you what pump should be in the system.
Pump selection is simple but not easy. Do not depend upon the knowledge of the local pump salesman to select the correct pump for you. In many cases he is prepared to sell his pump at a large discount to get the spare parts business. If you are purchasing pumps at too big a discount something is wrong, there is no free lunch.
Keep in mind that if several people are involved in the selection process each of them will add a safety factor to the calculated pump size. These factors added together can cause you to purchase a pump that is very much over-sized.
If you find that your pump curve dos not match your system curve, please keep in mind that you will have to change valve positioning, or modify the piping system. A VFD (variable frequency drive) changes only the pump cure
Thanks: Mcnally Institute Source Link.http://www.mcnallyinstitute.com/18-html/18-5.htm
Effect
of valve closing on the operating point
Pump Fundametals Book in PDF: http://www.pumpfundamentals.com/images/tutorial/tutorial.pdf
