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# Net Positive Suction Head: A Key Concept for Hydraulic Systems

## What is NPSH and why is it important

NPSH is an acronym for net positive suction head. It is a measure of how close the fluid at a given point in a hydraulic circuit is to boiling or flashing. Technically, it is the difference between the total head (static plus dynamic) at the suction side of a pump or turbine, and the vapor pressure of the fluid at that temperature.

Why does this matter? Because if the fluid pressure drops below its vapor pressure, it will start to form vapor bubbles. This phenomenon is called cavitation, and it can have serious consequences for the performance and integrity of pumps and turbines. Cavitation can reduce the flow rate, increase the noise and vibration, decrease the efficiency, and cause erosion and damage to the impeller blades.

To prevent cavitation, we need to ensure that there is enough NPSH at the critical points of the hydraulic system. There are two types of NPSH that we need to consider: available (NPSHA) and required (NPSHR).

NPSHA is the actual value of NPSH at a given point in the system. It depends on the system design, operating conditions, fluid properties, and atmospheric pressure. We can estimate or measure NPSHA using formulas or instruments.

NPSHR is the minimum value of NPSH needed to avoid cavitation at a given point in the system. It depends on the device design, such as pump or turbine geometry, flow rate, speed, etc. We can obtain NPSHR from the manufacturer's data or empirical methods.

The basic rule for avoiding cavitation is that NPSHA must be greater than or equal to NPSHR at all times. If this condition is not met, cavitation will occur and cause problems.

## How to calculate NPSH for different scenarios

Now that we know what NPSH is and why it is important, let's see how we can calculate it for different scenarios. We will use the following formulas and symbols:

NPSHNet positive suction head (m or ft)

pPressure (Pa or psi)

vVelocity (m/s or ft/s)

γSpecific weight of the fluid (N/m3 or lb/ft3)

gAcceleration of gravity (9.81 m/s2 or 32.2 ft/s2)

AAvailable

RRequired

sSuction side of the pump or turbine

eEntrance of the draft tube of the turbine

iInlet of the impeller of the pump or turbine

0Free surface of the fluid in an open tank

The formula for NPSHA at the suction side of a pump or turbine is:

NPSHA = hs - hv (1)

where hs is the suction head and hv is the vapor head of the fluid. The suction head can be calculated using the energy equation:

hs = ps / γ + vs2 / 2 g (2)

where ps is the static pressure and vs is the velocity of the fluid at the suction side. The vapor head can be calculated using the vapor pressure of the fluid at the given temperature:

hv = pv / γ (3)

where pv is the vapor pressure of the fluid. The vapor pressure depends on the temperature and can be obtained from tables or charts.

The formula for NPSHR at the inlet of the impeller of a pump or turbine is:

NPSHR = hi - hv (4)

where hi is the head at the inlet of the impeller and hv is the vapor head of the fluid. The head at the inlet of the impeller can be obtained from the manufacturer's data or empirical methods.

To illustrate how to use these formulas, let's consider two examples: a pump lifting water from an open tank and a turbine discharging water to a draft tube.

### Example 1: Pump lifting water from an open tank

In this example, we have a centrifugal pump that lifts water from an open tank at sea level to a higher elevation. The pump has a flow rate of 0.1 m3/s, a speed of 1800 rpm, and a NPSHR of 3 m. The water temperature is 20 C and its vapor pressure is 2.34 kPa. The suction pipe has a diameter of 0.15 m and a length of 10 m. The friction factor in the pipe is 0.02. The elevation difference between the free surface and the pump inlet is 2 m. We want to calculate the NPSHA and check if it is enough to avoid cavitation.

To calculate NPSHA, we need to find hs and hv using equations (1), (2), and (3). To find hs, we need to find ps and vs at the suction side.

To find ps, we apply Bernoulli's equation from point 0 to point s, assuming that there is no kinetic energy at point 0 and no losses in the pipe:

p0 + γ h0 + v02 / 2 g = ps + γ hs + vs2 / 2 g + hf (5)

where hf is the head loss due to friction in the pipe, which can be calculated using Darcy-Weisbach equation:

hf = f L v2 / D 2 g (6)

where f is the friction factor, L is the pipe length, and D is the pipe diameter.

Simplifying equation (5) and solving for ps, we get:

ps = p0 + γ h0 - γ hs - hf (7)

Substituting the given values, we get:

0.1 m3/s / (3.14 * 0.15 m2 / 4))2 / 0.15 m / 2 / 9.81 m/s2

ps = 101325 Pa + 19620 Pa + 19620 Pa - 1278 Pa

ps = 128287 Pa

To find vs, we use the continuity equation:

Q = A vs (8)

where Q is the flow rate and A is the cross-sectional area of the pipe.

Solving for vs, we get:

vs = Q / A

vs = 0.1 m3/s / (3.14 * 0.15 m2 / 4)

vs = 2.83 m/s

Now we can find hs using equation (2):

hs = ps / γ + vs2 / 2 g

hs = 128287 Pa / 9810 N/m3 + (2.83 m/s)2 / 2 / 9.81 m/s2

hs = 13.07 m + 0.41 m

hs = 13.48 m

To find hv, we use equation (3):

hv = pv / γ

hv = 2340 Pa / 9810 N/m3

hv = 0.24 m

Finally, we can find NPSHA using equation (1):

NPSHA = hs - hv

NPSHA = 13.48 m - 0.24 m

NPSHA = 13.24 m

We compare NPSHA with NPSHR and see that NPSHA is greater than NPSHR, which means that there is no risk of cavitation in this scenario.

### Example 2: Turbine discharging water to a draft tube

In this example, we have a reaction turbine that discharges water to a draft tube at sea level. The turbine has a flow rate of 0.5 m3/s, a speed of 1200 rpm, and a NPSHR of 5 m. The water temperature is 15 C and its vapor pressure is 1.7 kPa. The draft tube has a diameter of 0.5 m and a length of 15 m. The friction factor in the draft tube is 0.015. The elevation difference between the free surface and the draft tube entrance is -5 m. We want to calculate the NPSHA and check if it is enough to avoid cavitation.

To calculate NPSHA, we need to find he and hv using equations (1), (2), and (3). To find he, we need to find pe and ve at the draft tube entrance.

To find pe, we apply Bernoulli's equation from point e to point 0, assuming that there is no kinetic energy at point 0 and no losses in the draft tube:

pe + γ he + ve2 / 2 g = p0 + γ h0 + v02 / 2 g - hf (9)

where hf is the head loss due to friction in the draft tube, which can be calculated using Darcy-Weisbach equation:

hf = f L v2 / D 2 g (10)

where f is the friction factor, L is the draft tube length, and D is the draft tube diameter.

Simplifying equation (9) and solving for pe, we get:

pe = p0 + γ h0 - γ he - hf (11)

Substituting the given values, we get:

0.015 * 15 m * (0.5 m3/s / (3.14 * 0.5 m2 / 4))2 / 0.5 m / 2 / 9.81 m/s2

pe = 101325 Pa + 49050 Pa - 49050 Pa - 1917 Pa

pe = 100458 Pa

To find ve, we use the continuity equation:

Q = A ve (12)

where Q is the flow rate and A is the cross-sectional area of the draft tube.

Solving for ve, we get:

ve = Q / A

ve = 0.5 m3/s / (3.14 * 0.5 m2 / 4)

ve = 1.27 m/s

Now we can find he using equation (2):

he = pe / γ + ve2 / 2 g

he = 100458 Pa / 9810 N/m3 + (1.27 m/s)2 / 2 / 9.81 m/s2

he = 10.24 m + 0.08 m

he = 10.32 m

To find hv, we use equation (3):

hv = pv / γ

hv = 1700 Pa / 9810 N/m3

hv = 0.17 m

Finally, we can find NPSHA using equation (1):

NPSHA = he - hv

NPSHA = 10.32 m - 0.17 m

NPSHA = 10.15 m

We compare NPSHA with NPSHR and see that NPSHA is greater than NPSHR, which means that there is no risk of cavitation in this scenario.

If you want to learn more about NPSH, you may be interested in finding and downloading a pdf file with more information on this topic. A pdf file is a portable document format that can be viewed and printed on any device. It can also contain images, graphs, tables, and links to other sources.

There are many online sources that offer pdf files with detailed information on NPSH theory and applications. Some of them are:

• The Engineering Toolbox: Pumps - NPSH (Net Positive Suction Head)

• Pumps & Systems: NPSH Calculation: A Step-by-Step Guide

• Pump Fundamentals: Net Positive Suction Head (NPSH)

• Pump Magazine: Net Positive Suction Head - Cavitation & Pump Performance

• Flowserve: Net Positive Suction Head Considerations for Centrifugal Pumps

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• Click on the download icon or button and choose a location on your device where you want to save the file.

• You can open the pdf file with any pdf reader application and view or print it as you wish.

To use the pdf file for reference and learning purposes, you can do the following:

• Read the pdf file carefully and take notes of the main points and concepts.

• Compare the pdf file with other sources and check for consistency and accuracy.

• Use the pdf file to solve problems and examples related to NPSH.

• Share the pdf file with others who may be interested in NPSH.

## Conclusion

In this article, you have learned what NPSH is, why it is important for pumps and turbines, how to calculate it for different scenarios, and where to find and download a pdf file with more information on NPSH. You have also seen some examples of NPSH calculation for a pump lifting water from an open tank and a turbine discharging water to a draft tube.

NPSH is a key concept in hydraulic systems that helps to prevent cavitation and its negative effects on performance and damage of pumps and turbines. By understanding and applying NPSH concepts, you can design and operate hydraulic systems more efficiently and safely.

If you want to learn more about NPSH, we encourage you to download the pdf file that we have provided in this article. It contains more details and information on NPSH theory and applications. You can use it as a reference and a learning tool for your projects.

### FAQs

• What is the difference between NPSHA and NPSHR?

NPSHA is the net positive suction head available from the system at a given point. It depends on the system design, operating conditions, fluid properties, and atmospheric pressure. NPSHR is the net positive suction head required by the device at a given point to avoid cavitation. It depends on the device design, such as pump or turbine geometry, flow rate, speed, etc.

• How can I increase NPSHA?

You can increase NPSHA by increasing the suction pressure, decreasing the suction velocity, decreasing the fluid temperature, decreasing the elevation difference between the free surface and the suction side, or decreasing the friction losses in the suction pipe.

• How can I decrease NPSHR?

You can decrease NPSHR by choosing a device with a lower NPSHR value, reducing the flow rate, reducing the speed, or improving the impeller design.

• What are the units of NPSH?

NPSH is expressed in units of head, such as meters or feet. It is not expressed in units of pressure, such as Pa or psi.

• What are some signs of cavitation?

Some signs of cavitation are reduced flow rate, increased noise and vibration, decreased efficiency, and erosion and damage to the impeller blades.

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