what is meant by NPSH for pumps and how it was calculated?

Question

explain about NPSH in simple way. and wats the diff b/w NPSH available and required.

Answer 1

Net Positive Suction Head (NPSH) and Cavitation :
The Hydraulic Institute defines NPSH as the total suction head in feet absolute, determined at the suction nozzle and corrected to datum, less the vapor pressure of the liquid in feet absolute. Simply stated, it is an analysis of energy conditions on the suction side of a pump to determine if the liquid will vaporize at the lowest pressure point in the pump.

The pressure which a liquid exerts on its surroundings is dependent upon its temperature. This pressure, called vapor pressure, is a unique characteristic of every fluid and increased with increasing temperature. When the vapor pressure within the fluid reaches the pressure of the surrounding medium, the fluid begins to vaporize or boil. The temperature at which this vaporization occurs will decrease as the pressure of the surrounding medium decreases.

A liquid increases greatly in volume when it vaporizes. One cubic foot of water at room temperature becomes 1700 cu. ft. of vapor at the same temperature.

It is obvious from the above that if we are to pump a fluid effectively, we must keep it in liquid form. NPSH is simply a measure of the amount of suction head present to prevent this vaporization at the lowest pressure point in the pump.

NPSH Required is a function of the pump design. As the liquid passes from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH Required is the positive head in feet absolute required at the pump suction to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure. The NPSH Required varies with speed and capacity within any particular pump. Pump manufacturer's curves normally provide this information.

Net Positive Suction Head (NPSH) NPSH Available is a function of the systetm in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction. Fig. 4 shows four typical suction systems with the NPSH Available formulas applicable to each. It is important to correct for the specific gravity of the liquid and to convert all terms to units of "feet absolute" in using the formulas.
PB = Barometric pressure in feet absolute.
VP = Vapor pressure of the liquid at maximum pumping temperature, in feet absolute.
P = Pressure on surface of liquid in closed suction tank, in feet absolute.
Ls = Maximum static suction lift in feet.
LH = Minimum static suction head in feet.
hf = Friction loss in feet in suction pipe at required capacity

In an existing system, the NPSH Available can be determined by a gauge on the pump suction. The following formula applies:

NPSH available
=PB - VP+/-Gr+hv

Where
Gr = Gauge reading at the pump suction expressed in feet (plus if above atmospheric, minus if below atmospheric) corrected to the pump centerline.
hv = Velocity head in the suction pipe at the gauge connection, expressed in feet.

Cavitation is a term used to describe the phenomenon, which occurs in a pump when there is insufficient NPSH Available. When the pressure of the liquid is reduced to a value equal to or below its vapor pressure the liquid begins to boil and small vapor bubbles or pockets begin to form. As these vapor bubbles move along the impeller vanes to a higher pressure area above the vapor pressure, they rapidly collapse.

The collapse, or "implosion" is so rapid that it may be heard as a rumbling noise, as if you were pumping gravel. In high suction energy pumps, the collapses are generally high enough to cause minute pockets of fatigue failure on the impeller vane surfaces. This action may be progressive, and under severe (very high suction energy) conditions can cause serious pitting damage to the impeller.

The accompanying noise is the easiest way to recognize cavitation. Besides possible impeller damage, excessive cavitation results in reduced capacity due to the vapor present in the pump. Also, the head may be reduced and/or be unstable and the power consumption may be erratic. Vibration and mechanical damage such as bearing failure can also occur as a result of operating in excessive cavitation, with high and very high suction energy pumps.

The way to prevent the undesirable effects of cavitation in standard low suction energy pumps is to insure that the NPSH Available in the system is greater than the NPSH Required by the pump. High suction energy pumps require an additional NPSH margin, above the NPSH Required. Hydraulic Institute Standard (ANSI/HI 9.6.1) suggests NPSH margin ratios of from 1.2 to 2.5 times the NPSH Required, for high and very high suction energy pumps, when operating in the allowable operating range.

In designing a pumping system, it is essential to provide adequate NPSH available for proper pump operation. Insufficient NPSH available may seriously restrict pump selection, or even force an expensive system redesign. On the other hand, providing excessive NPSH available may needlessly increase system cost.

Suction specific speed may provide help in this situation.

Answer 2

NPSH is an acronym for Net Positive Suction Head. It shows the difference, in any cross section of a generic hydraulic circuit, between the pressure and the liquid vapor pressure in that section.

In pump operation, two aspects of this parameter are called respectively NPSH (a) Net Positive Suction Head (available) and NPSH (r) Net Positive Suction Head (required), where NPSH(a) is computed at pump inlet port, and NPSH(r) is the NPSH limit the pump can withstand without cavitating.

Answer 3

NPSH= net positive suction head. This is usually obtained from the pump curve. There are two kinds of NPSH's one is the available and called NPSHA, and one is the required and called NPSHR. The manufacturer of pumps does know the density or the vapour pressure of the fluid the user is going to pump, this is why he gives the available net positive suction head for his pump.
To calculate the NPSH one uses the following equation:
NPSH = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your fluid- loss in the piping, valves and fittings

Answer 4

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)..