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작성일 : 11-01-21 12:12
Calculating net positive suction head (NPSH)
 글쓴이 : 날클
조회 : 5,920   추천 : 0  

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.

Lets look at each of these concepts in a little more detail :

  • 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.

To calculate the net positive suction head (NPSH) of your pump and determine if you are going to have a cavitation problem, you will need access to several additional pieces of information:

  • 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:

To convert surface pressure to feet of liquid; use one of the following formulas:

  • 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

There are different ways to think about net positive suction head (NPSH) but they all have two terms in common.

  • NPSHA (net positive suction head available)
  • NPSHR (net positive suction head required)

NPSHR (net positive suction head required) is defined as the NPSH at which the pump total head (first stage head in multi stage pumps) has decreased by three percent (3%) due to low suction head and resultant cavitation within the pump. This number is shown on your pump curve, but it is going to be too low if you are pumping hydrocarbon liquids or hot water.

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.

In the following examples we will be looking only at the suction side of the pump. If we were calculating the pump's total head we would look at both the suction and discharge sides.

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.

NPSHA (net positive suction head available) = 34 + 5 + 0 - 0.62 - 2.34 = 36.04 feet

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

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

 

  • 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.

NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 0.62 - 2.34 = 13.34 feet. This is enough to stop cavitation also.

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 = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings

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.

If you are given the absolute and vapor pressures in psia, and you forgot how to convet to feet of head; you can use the following formula, providing you know the specific weight of the liquid you are pumping :

  • 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.

 
 

 
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