What is Net Positive Suction Head in Pump? Formula, Calculation


In this article, we will learn what is the net positive suction head or NPSH along with formula, calculation, NPSH available, NPSH required, etc. Let’s explore!

Net Positive Suction Head or NPSH Basic Concept

Net Positive Suction Head or NPSH Concept

Let’s try to understand the basic concept of Net Positive Suction Head or NPSH! How long pump will last? This is the question we come across lots of times. But it is not just about the design of the pump which has a stake in its long life but its proper selection for a specific task is also important.

  • Among the different factors during pump design, net positive suction head or NPSH is one of the main factor which needs to be considered during the deisgn.
  • If mistakes are done while selecting a pump then it will lead to early pump failures, increasing maintenance cost with time, wastage of time in maintenance which will result in high production cost as well wastage of man-hours of labors. 
  • So, we will lean here this aspect of deciding the pump design & application and the main challenges with respect to Net Positive Suction Head.

Net Positive Suction Head Important Aspects

  • Net Positive Suction Head plays a very important role in the proper selection of both centrifugal and positive displacement type pumps. This pump has its own specifications for the same.
  • The absence of available or sufficient NPSH results in cavitation. Liquid cavitation has bad effects in the pump.
  • It reduces the pumps efficiency as well as its performance.
  • It reduces pumps reliability as well as internal parts constantly seek a replacement.
  • This NPSH has its own types, their difference, and their calculation is also must to fully understand this concept.

What is Net Positive Suction Head or NPSH? Definition

NPSH Explanation

Let’s try to understand the NPSH of a centrifugal pump. Before starting a centrifugal pump, priming is done, which means filling the casing of the pump with water. This is achieved by closing the delivery valve and allowing water to accumulate in the casing.

  • When water is collected inside the pump then the only the pump is started and subsequently the delivery valve is opened. As soon as the shaft rotates, the impeller rotates at high speed.
  • It pushes water away from center as grooves guides this fluid. This is because of centrifugal force as it always acts away from center. This centrifugal force is making water thrown away at the periphery results in the build of high pressure.
  • The momentum of the impeller is getting transferred to water and water is getting dragged with it. In absence of air inside the casing, when the impeller starts rotating, the pressure developed at the eye is lower when compared with outside pressure which results in the suction of water from the sump.
  • This low pressure at the impeller as well as at the suction of the pump is associated with net positive suction head.
  • This low pressure has a limitation with respect to the fluid vapor pressure.

Take a scenario, if the fluid pressure will reduce below its vapor pressure, what will happen? It’s simple! The fluid will start to have vapor and cavitation will start. To avert this problem, NPSH comes into the picture! Let’s define NPSH!

NPSH Definition

NPSH can be defined in many ways. Some are explained down below.

  • It is defined as the total head which is available at the pump inlet above the vapor pressure head of the liquid.
  • It is also defined as the difference between absolute pressure head available at the inlet to pump and vapor pressure head and addition of velocity head in this difference.
  • It can also be defined as a total required head which we need to make liquid flow through the suction pipe to the impeller.

At end of this article, you’ll know all about NPSH and other possible solutions to increase it and you will be able to identify some of the root causes and their possible solutions for pump cavitation.

Net Positive Suction Head Basic Terms

Before we learn NPSH, we need to understand the following terms. NPSH is related to vapor pressure, absolute pressure, head, etc. So, let get the basic idea of these terms.

Vapor Pressure

Vapor pressure is the pressure at which a liquid begins to vaporize, i.e. boil. Vapor pressure is usually measured in units in terms of mercury height (mm Hg).

Vapor pressure head can be written as follows:

hv = pv / ρg                                 

Where

  • hv = vapor pressure head (m)
  • pv = vapor pressure (Pa, N/m2)
  • ρ = density of the vapor (kg/m3)
  • g = gravitational acceleration (m/s2)

Absolute pressure

It is the sum of atmospheric pressure and gauge pressure.

Absolute Pressure can be written as the summation of Gauge Pressure & Atmospheric Pressure. It is denoted as ha

ha = hatm + hg

  • ha = absolute pressure at the pump suction
  • hg = gauge pressure of the fluid

Head

It is the difference in elevation/height between two points in a body of fluid expressed in meters of fluid.   

It has the following types,

Suction Head

Suction head means the head at suction. Basically, it is the head at the pump impeller eye. However, as the actual head at the pump impeller cannot be measured, normally, the suction head is considered the head at the suction flange.

The suction refers to the inward flow of liquid via a conduit, such as a length of pipe, into the pump and eventually to the impeller’s eye. Suction is always negative pressure since fluid must be drawn in. This head is transferred to the fluid via a revolving impeller and its vanes.

The head difference between a suction flange and impeller is very minimal and can be ignored.

So, at the suction flange, the fluid head will have two components.

  • Heat due to static
  • Head due to fluid velocity

Hence, we can write the suction head as follows

Suction head = static head + velocity head

hs = ps / ρg + vs2 / 2 g                                

Where

  • hs = suction head at suction flange (m)
  • ps = static pressure in the fluid at suction flange (Pa (N/m2))
  • vs = velocity of fluid at suction flange (m/s)
  • ρ = density of the vapor (kg/m3)
  • g = gravitational acceleration (m/s2)

Pump discharge head

The pressure head is at the outlet of the pump.

Fluid friction head

The fluid losses its pressure whilst passing through the pipe, this is called fluid friction head. It is denoted as hfs

Why pump needs NPSH?

Pressure drop is seen between two places – suction flange and eye of the impeller.

  • Velocity increases between two points. They are suction flange and impeller vanes.
  • Losses amounting to turbulence and friction between suction flange and impeller.

So it is impossible to construct a pump where these two losses don’t take place. That is why ensuring NPSH is a must. We all know NPSH is just a pressure gap maintained over vapor pressure.

Net Positive Suction Head or NPSH Definition, Formula

This term is used whenever a pump is used in industry. Minimum suction conditions for pumps are given in specific terms of NPSH. Pump manuals specify the NPSH required for a specific pump model.

NPSH Formula or Equation

From the definition of suction head,

  • Suction head = hs = ps / ρg + vs2 / 2 g  

From the definition of vapor pressure head,

  • Vapor pressure head = hv = pv / ρg

In the simplest language, we can say,

NPSH = Pressure head at the inlet or suction – vapor pressure head

Let’s see the formula or equation of NPSH,

  • NPSH = hshv
  • NPSH = ps / ρg + vs2 / 2 g – pv / ρg
  • NPSH = ps / ρg – pv / ρg + vs2 / 2 g

NPSH = Ps/ρg – Pv/ρg + Vs2/2g

Here NET means, the difference between two pressures is there. It is not a singular quantity itself. Small components of pressure have been deducted from the same. Basically good elements of pressure are added and negative elements of pressure are subtracted.

Here POSITIVE means needed pressure may be minuscule in value but it is still non-zero quantity. It is a must that is greater than zero.

Here SUCTION means pressure calculated from the suction side of the pump because the suction side is the reason for pump failure in 60-70% cases.

How to Derive Net Positive Suction Head? Equation

We already got, NPSH = Ps/ρg – Pv/ρg + Vs2/2g ———-(1)

Let’s try to understand the detailed formula or equation of net positive suction head at different conditions.

Pump is Placed above Tank Elevation

Take a simple example. A pump is installed to lift fluid from an open tank (atmospheric) to other places.  Here, the pump is placed above the tank elevation.

From Bernoulli’s equation, we can say that

The energy at open tank surface = Energy at the pump suction + loss

ha = hs + hfs————————-(2)

Where,

  • ha = Absolute head at tank open surface (m)
  • hs = head at suction (m)
  • hfs = head loss in the pipe from the tank surface to pump suction (m)

Now,

Pressure head (ho) at open tank surface can be written as,

  • ho = po/ρg
  • ho = patm/ρg

where Patm = atmospheric pressure

As vapor pressure is written in absolute pressure, we need to write the atmospheric pressure in the absolute head.

Absolute head (ha) can be written as,

  • ha = ho + hg [hg = gauge pressure]
  • ha = patm/ρg + hg ———————-(3)

Pressure heat (hs) at the suction of pump can be written as,

  • hs = ps / ρg + vs2 / 2g – he ——————-(4)

Where

  • he = elevation or height difference between tank surface and pump center. It is written as (-) as it is placed above the tank elevation so it is negative.

From the main equation,

  • ha = hs + hfs
  • patm /ρg + hg = (ps / ρg + vs2 / 2g – he) + hfs [Putting the vales from equation (2), (3) & (4)
  • patm /ρg + hg = (ps / ρg + vs2 / 2g – he) + hfs
  • ps / ρg + vs2 / 2g = patm /ρg + hg – he – hfs

Now, NPSH shall be as follows,

  • NPSH = Ps/ρg – Pv/ρg + Vs2/2g
  • NPSH = (Ps/ρg + Vs2/2g) – Pv/ρg
  • NPSH = (patm /ρg + hg – he – hfs)- Pv/ρg
  • NPSH = ha – he – hfs– hv [as, ha = patm/ρg + hg]

NPSH     = (ha he – hfs ) – hv

Where,

  • ha = Absolute head
  • hs = static suction head
  • hfs = head lost due to friction
  • hv = vapour head

Pump is Placed below Tank Elevation

If the pump is placed below the tank elevation, then he will be positive.

Hence, the equation can be written as,

Pressure heat (hs) at the suction of pump can be written as,

  • hs = ps / ρg + vs2 / 2g + he

From NPSH equation, we will get,

  • NPSH = ha + he – hfs– hv

NPSH     = (ha + he – hfs ) – hv

NPSH General Equation or Formula

Hence, based on the NPSH derivation, we can write,

  • NPSH     = (ha he – hfs ) – hv
  • NPSH     = (ha + he – hfs ) – hv

If we summarize, we can write,

NPSH     = (ha ± he – hfs ) – hv

Understanding & Explanation of NPSH Equation

NPSH Part#1 Suction Pressure

Suction pressure is measured near the suction nozzle just near the impeller. It is negative in nature. It has to be added with atmospheric pressure so as to obtain absolute pressure. Gauge elevation must be considered and action should be taken. It must be added (if the gauge is above datum) or subtracted (if the gauge is below datum). Even though it is very negligible, the velocity head in the pipe at the gauge connection should be added to obtain total (stagnation) pressure.

NPSH Part#2 Vapour Pressure 

Vapour pressure is far more difficult to calculate than suction pressure. This is because it is similar to determining when a liquid will turn into a gas. So it will differ from liquid to liquid. Some liquids, such as butane and ammonia, have high vapor pressures and hence they must be kept under pressure, or they will start to boil (flash). A container of ammonia gas will boil and turn into noxious gas.

Cool water has a low vapor pressure when compared to water at high temperatures. If this water container is kept open in some jar, then it will not boil but it will evaporate with time.

As a result, the likelihood of chilly water boiling here is minimal. But if this same water container was placed on the moon it would boil away, similar to the ammonia. The reason for this is that air pressure on the moon is zero, similar to a vacuum. To put it in simple terms, the vapor pressure of water at 80 degrees Fahrenheit is around 0.5 psia.

Simply put if the pressure of the water, if it is reduced below 0.5 psia, the water will start boiling.

Types of Net Positive Suction Head or NPSH – Availble & Required

NPSH can be of two types required NPSH and available NPSH. For the pump to operate correctly, the required NPSH should be equal to the available NPSH.

Available NPSH

When the pump is installed at the site this actual value has to calculate and should be matched with the required NPSH. It is calculated by the following equation –

     NPSH     = (ha ± hs – hfs ) – hv

It is very important that available NPSH should be greater than the required NPSH. This reduces the cavitation inside the pump.

Required NPSH

It is the value that is given by manufacturers to buyers by catalogue or brochure. This value keeps varying with pump design, operating speed of the pump, and capacity of the pump. This value is different for different models of the same company.

These parameters are meticulously computed by the firm itself through real-world experimentation. In order to get these values pump is tested with different suction lifts. By this minimum value is obtained. This value gives the highest efficiency.

The noise of the pump is also minimum in this case. This makes it cavitation-free.  This will help in the improvement of the life term of this pump.

Is NPSH dependent on Temperature?

Vapour pressure is indeed a function of temperature. this is because as the temperature of the liquid increases, its vapor pressure will keep on increasing until the critical temperature is achieved.

At this point of critical temperature, vapor pressure always vanishes. And when temperature increases than that of critical temperature, there will be no difference between a liquid and a gas. It is all fluid.

Units of Net Positive Suction Head or NPSH

For centrifugal pumps, NPSH values are the value of height in feet or meters. For rotary and reciprocating pumps, NPSH values are mostly expressed in pressure units such as pounds per square inch (psi), kilopascals, or bars.

One fact is important, NPSH values are neither gauge pressures nor absolute pressures. Psia means absolute pressure denoted by ‘a’ and ‘g’ and Psig shows the gauge pressure. As NPSH is a pressure measurement performed above vapor pressure, thus its unit is feet or meters.

Liquid Cavitation

When a liquid flows from the pump’s inlet to the point where it receives energy from the impeller, it causes a pressure drop. We know that liquids can vaporize (boil) at very low temperatures when they are subjected to low pressures.

Liquid cavitation is the phenomenon of sudden formation and collapse of low-pressure bubbles (cavities) in the pumped liquid, which is caused by the mechanical rotation of the pump impeller. A synonymous term used for cavitation is partial vaporization and liquid flashing.

We all know that this process creates noise, vibration, and not only this but also damages many pump’s internal components. Acoustically it is characterized as the sound produced by a centrifugal pump as if there are small rocks that are guided in fluid suspension.

The result is noise and vibration which ranges from barely audible to quite loud and violent. To check if cavitation will occur or not, the reduced pressure at the pump inlet should be compared to the vapor pressure of the pumped liquid.

It is widely observed that incoming liquid is most likely to vaporize in the vicinity of the vane tips of the eye of the impeller in a centrifugal pump. Another damaging property of cavitation is that it drastically reduces not just the pump’s health but also its performance as it attempts to drive a binary combination of liquid and vapor.

Causes of Inadequate NPSHA

Let’s try to understand the role of NPSHA in potential cavitation problems and how to rectify them. Let’s dive deep into making a list of the causes behind inadequate NPSHA. For that, we only have to look at the right side of the classical NPSHA equation in order to develop an understanding of contributing factors to inadequate NPSHA.

  • NPSHA gets reduced if suction line loss (hL), or liquid vapor pressure (hV) are greater when compared to absolute pressure (Ha). 

It is easy to replace the pump but considering various angles it will be great if NPSHA is improved by the understanding system.

How to Adjust System Variables to Achieve Desired NPSH?

Try to change some physical system variables before changing the pump, follow the below steps:

  • Either increase level of liquid in suction side or reduce the height of the pump’s elevation.
  • If it’s possible try to reduce fluid’s operating temperature that way vapour pressure of the liquid will reduce.
  • If possible, try to increase the superimposed pressure which is in the suction vessel vapour space.
  • One of easiest solution will be enlarging suction line size or reduction in its length, which will help in lowering the frictional head losses.

Frictional Head Loss on Suction Line

Suction line frictional head losses are attributable to the following parameters.

They are as follow-

  • Contractions and expansions;
  • Strainers and foot valves;
  • Valves and pipe fittings and;
  • Straight pipeline length fluid friction.

In all cases, the straight pipeline friction losses act as a function of the square of the mean fluid velocity, hL = f (V2). These frictional head loss functions are explained by, the Darcy-Weisbach formula:

hL = 4fLV2 /2gD

Where,

  • f = friction factor
  • L = pipe line length in m
  • V = mean fluid velocity in m/s
  • D = pipe diameter in m
  • g = gravitational acceleration in m/s2

If we observe carefully, this formula itself gives us all possible solutions.

  • By reducing the length of pipe
  • By achieving reduction in mean fluid velocity
  • By ensuring that the suction pipe diameter is bigger  

Any of the above conditions will reduce the value for hL. So by reducing the value of the hL component in the NPSHA formula we can increase the numerical quantity of NPSHA.

Design of Pump Based on NPSH

NPSHR can be drastically reduced by achieving slower rotational speeds. This is the result of a concept that was developed in 1937-8. It is known as Suction Specific Speed.  It is defined as,

Nss = N√Q/NPSH3/2

Where,

  • N = pump rotational speed, rpm
  • Q = pump capacity, gallons per minute

It has been observed and from which a knowledge base has been developed which tells that cavitation mostly occurs when values of NSS increases more 10,000. Also, preference should be to pump having lower suction specific speed rather than to that of higher speed for the same condition and same work.

  • Also it has been observed if suction designs are doubled in size they can offer reductions in NPSHR of over 20%.
  • While Use of an impeller eye having large diameter reduces NPSHR by achieving reduction in the entrance fluid velocities.

Other Parameters

As NPSH is losses calculated in pump pressure in suction side either due to friction, vapour pressure and velocity of pump. Not only this but also pump’s own characteristics too have bearing on same. These are –

  • Operating speed of pump
  • amount of discharge
  • designs on suction side
  • vane angle/ impeller type
  • thickness of impeller

NPSH Curves

Just to refresh concepts again, consider you are driving a car. It runs on fuel. So NPSHr will be that specific amount of fuel needed to run a car. While NPSHa is the amount of fuel available in the fuel tank. So obviously car will not run if NPSHr is greater than NPSHa.

what net positive suction head pump formula calculation curves
What net positive suction head pump formula calculation curves

In the same way, this NPSHr also varies in real conditions. Which is plotted on this graph. The X-axis shows the head and Y-axis shows the amount of flow. This NPSHr is dependent on pumps design.

Determinants of NPSHr-

  • Suction side diameter
  • Impeller eye diameter
  • Blades designs, angles of impeller
  • Tolerances to various parts
  • Gaps provided to wear rings

As we can see these determinants are fixed as per design so theoretical NPSHr plotted on a graph looks linear – an inclined line varying with flow rate. So at the minimum flow rate, practical NPSHr looks flat.

It is at one point when flow rate increases NPSHr curve rises exponentially till it meets theoretical NPSHr. This is so because the pump is trying to compensate for the loss of efficiency it incurred in the early phase when the flow rate was low.

Conclusion

Whilst selecting and buying a centrifugal pump, the most neglected conditions are the ones in the pump’s suction system. Many times in the centrifugal pump selection process, less attention is given to satisfy the parameters of the total dynamic head (TDH) and capacity.

The importance of suction conditions is frequently neglected which gives rise to pump operational problems. The finding of a pump’s Net Positive Suction Head is the analytical tool that is solely for making sure of suitability to suction conditions.

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