Calculation Net Positive Suction Head
NPSH = Net Positive Suction Head.
NPSH is the pressure available at the pump suction after vapor pressure is subtracted
It is calculated as:
Static head + surface pressure head - the vapor pressure of your product - the friction losses in the piping, valves and fittings
It thus reflects the amount of head loss that the pump can sustain internally before vapor pressure is reached.
It is therefore some measure of the head loss that a pump is able to endure internally before vapor pressure is achieved.
NPSH is obtained by the subtraction of vapor pressure of fluid at suction of pump from the total pressure of fluid. The formula for NPSH is as follows:The components of NPSH are defined through the following formula:
NPSH = System pressure at suction flange – Vapor pressure of the fluid at the suction flange
Total bearing at the suction side is usually measured in either in ft of head or m head and it is got by adding up the static head, velocity head and pressure head required.
Vapor pressure of the fluid is the pressure at which the evaporation of the fluid commences with reference to a certain temperature. This value is obtained from the thermodynamic chart or as may be gotten from the data sheet of the fluid to be used and which would be supplied by the manufacturer.
The NPSH with reference to a pump is the fixed NPSH that has to be provided in order to prevent caviation. This value is provided to it by its manufacturer and it depends on the type of the pump as well as on the nature of the fluid that is intended to be pumped.From this it can therefore be deduced that NPSH to be available at the suction side of a pump should always be greater than the NPSH required so as to be sure that a pump will not cavitates. If the NPSH available is lower than the NPSH then the pump cavitates and hence yields low efficiency, wears down the casing of the pump and may even lead to the complete destruction of the pump.
As a consequence it is necessary to check the NPSH available on the suction side of the pump and shall compared with the NPSH the pump required before going for the suction. This can be done by calculation of the fluid pressure and temperature on the suction side of the pump whereby the NPSH is calculated from the formula NPSHA = NPSHR = The above formula tells the required NPSH that the pump must be able to achieve.
Pumps - NPSH (Net Positive Suction Head)
Net Positive Suction Head or NPSH is one of the most important design characteristics of pump, specifically centrifugal one.Cavitation is the formation of vapor bubbles in the liquid being pumped This the pressure head or energy available at the suction of a pump or impeller. Cavitation is generation of vapor bubbles in the liquid being pumped This is the head or energy available at the suction of a pump or impeller. Hydraulically on cavitation, it is undesirable to the pump in a respect to erode the lining of the pump, reduce the efficiency of the pump and is likely to develop some symptoms. Here's a brief overview of NPSH:A brief introduction to NPSH is presented as follows:
1. NPSH Available (NPSHA):
- NPSHA is used to refer to the total suction head energy that is available at the intake of the suction pump. It considers the power pressure; velocity; elevation of liquid in suction line; and vapor pressure of the liquid.
- NPSHA is normally determined or estimated to be more than the NPSHa that is required by the pump or in other words NPSHA> NPSHR.
2. NPSH Required (NPSHR):
- NPSHR is the net positive suction head required to give the minimum energy at suction end of the pump without causing cavitation.It is an attribute of the pump; in most cases the supplier of the pump provides this.
- If the NPSHA is greater than the NPSHR then the pump will be non-cavitating in its operation. If NPSHA is lesser that NPSHR then cavitation occur and affect the performance of the pump and also destroy the pump.
3. Factors Affecting NPSHA:
- The NPSHA depends with suction systems and design, the temperature of the liquid to be pumped, the level difference of the pump in relation to the liquid source and pressure in the suction line.
- This pressure can be maintained by the right design of the suction piping with straight runs of pipes, filters, and the correct pipe size.
4. Effects of Cavitation:
- They are as follows; lowering of the pumping head, vibrations and Acoustic noise, deterioration of various parts of the pump and shortening of the life of the pump.
- It is thus incumbent upon the operators of systems in which cavitation is likely to occur to always see that NPSHA is greater than NPSHR.
5. NPSH Margin:
- That is why, it is a tradition to have some difference between the required NPSHA and the net positive suction head available at the required operating conditions to avoid unintended cavitation.
In conclusion, it can be stated that Net Positive Suction Head, or NPSH, is an important variable that provides for the right working of pumps and especially, the centrifugal ones, by avoiding cavitation. It is therefore crucial to have a correct NPSHA greater than NPSHR to achieve high availability and performance of the pumps. This can be realised through proper design, convection with other systems in the operation of the suction system and the right maintenance practices.
NPSH Calculation || pump head calculation || NPSH pump calculation
NPSH Calculation
Net positive suction head (NPSH) is the minimum pressure required at suction which avoids creation of vapour bubbles at the entrance to impellers. It is a condition in which the liquid available in the pump is allowed to form vapor bubbles and then compress them until they implode, doing likewise to the pump and also reducing its efficiency.
There are two types of NPSH:
- NPSH required (NPSHr): These represent the minimum NPSH that should be available at inlet of the pump in order to avoid cavitation.
- NPSH available (NPSHa): This is the actual NPSH that is available at the pump inlet It is the NPSH that must be provided to the pump to avoid overheating while it is being operated or tested at the specific rpm at which it was designed to run.
NPSHa is calculated using the following equation:
NPSHa = Pa + (z * ρ * g) - (v^2 / 2g)
where:
- Pa is the atmospheric pressure
- z is the vertical distance from the liquid surface to the pump inlet
- ρ is the density of the liquid
- g is the acceleration due to gravity
- v is the velocity of the liquid at the pump inlet
NPSHr stands for Net Positive Suction Head required, and this can be specified by the pump manufacturer or calculated and this is normally available in the datasheet of a particular pump.
NPSHa refers to the net positive suction head which is available; NPSHr refers to the required net positive suction head; if NPSHa is below NPSHr, then cavitation occurs.
- Lowering the pump inlet
- Increasing the liquid level in the reservoir
- Using a larger pump inlet
- Using a pump with a lower NPSHr
Pump Head Calculation
Pump head is the total energy a pump imparts to the pumped fluid. It is employed to deal with the static head, friction loss and minor head losses in the pipining system.
Pump head is typically measured in feet or meters.
To calculate pump head, the following equation is used:
Pump Head = (P2 - P1) / ρ * g + (z2 - z1)
where:
- P1 is the pressure at the pump inlet
- P2 is the pressure at the pump outlet
- ρ is the density of the liquid
- g is the acceleration due to gravity
- z1 is the elevation of the pump inlet
- z2 is the elevation of the pump outlet
NPSH Pump Calculation
NPSH pump calculation is defined as calculation of the amount of NPSH necessary in a pump in order to prevent caviation. The above calculation is essential to be certain that the specific pump will work as expected and to the optimized performance.
To perform a NPSH pump calculation, the following information is needed:
- Flow rate
- Liquid type
- Pump type
- Pump inlet elevation
- Pump outlet elevation
Once this information is gathered, the following equation can be used to calculate the NPSH required:
NPSHr = H - (P2 - P1) / ρ * g - (z2 - z1)
where:
- NPSHr is the net positive suction head required
- H is the pump head
- P1 is the pressure at the pump inlet
- P2 is the pressure at the pump outlet
- ρ is the density of the liquid
- g is the acceleration due to gravity
- z1 is the elevation of the pump inlet
- z2 is the elevation of the pump outlet
If the NPSHa is lower than the NPSHr, cavitation will occur. To prevent cavitation, the NPSHa must be increased. This can be done by:
- Lowering the pump inlet
- Increasing the liquid level in the reservoir
- Using a larger pump inlet
- Using a pump with a lower NPSHr
A further important thing to which regard must be paid is the fact that the NPSH pump calculation is not exact and only helps to evaluate the chances of cavitation, although there are other factors as well. To obtain a better result, it is advisable to use a computational tool or to address a pump engineer.
Useful information on NPSH, NPSHA and NPSHR
NPSH, NPSHA, and NPSHr
Net positive suction head (NPSH) is a measure of the pressure that is available at the suction side of a pump with the idea purpose of avoiding cavitation. The main impediment to the function of the pump is cavitation; which is the formation and subsequent collapse of bubbles of vaporized liquid.
There are two types of NPSH:
- NPSH required (NPSHr): This is the minimum NPSH that is required at the suction of the pump in order to avoid cavitation.
- NPSH available (NPSHa): This is the actual NPSH that is available at the pump suction This is the actual NPSH that is available at the pump suction In this situation then we have:
NPSHa is calculated using the following equation:
NPSHa = Pa + (z * ρ * g) - (v^2 / 2g)
where:
- Pa is the atmospheric pressure
- z is the vertical distance from the liquid surface to the pump inlet
- ρ is the density of the liquid
- g is the acceleration due to gravity
- v is the velocity of the liquid at the pump inlet
NPSHr is normally determined by the pump manufacturer and it is provided by the manufacturer on the datasheet of the pump.
If the NPSHa is less than the NPSHr cavitation will occur. To prevent cavitation, the NPSHa must be increased. This can be done by:
- Lowering the pump inlet
- Increasing the liquid level in the reservoir
- Using a larger pump inlet
- Using a pump with a lower NPSHr
NPSH is therefore a key factor of consideration and importance in any selection process of a pump as well as in its operation. But it is advocated with a lot of pressure that in as much as possible; the NPSH of the pump must be kept to the minimum to avoid the occurrence of cavitation.
Here is some useful information on NPSH, NPSHA, and NPSHr:
- NPSH is that particular factor that should not be spared while in any conditions one is in doubt of using a centrifugal pump.
- NPSH is the extra head that is available at the suction that caters for head loss occurring in suction line of the pump as a result of internal degradation and friction.
- NPSHa is dependent of the head of the liquid sources, atmospheric pressure on the inlet velocity of the liquid to the pump.
- As observed NPSHr is a function of the pump design and operating conditions.
- When choosing a pump the NPSHr should be lower than the NPSHa available.
- Where the NPSHa is less than the NPSHr, cavitation happens and this leads to the damaging of the pump, besides the fact that its efficiency will also be affected.
Here are some tips for increasing NPSHa:
- Lower the pump inlet closer to the liquid source.
- Increase the liquid level in the reservoir.
- Use a larger pump inlet pipe.
- Use a pump with a lower NPSHr.
If there is any ambiguity regarding the level of NPSH required for any of the process application in question, it is far better to seek the help of a pump engineer.
NPSH Calculation: A Step-by-Step Guide
NPSH Calculation: On the other hand we have prepared the following general step by step guide:
Is required to determine the net positive suction head (NPSH) necessary to prevent cavitation in a pump. Here's a step-by-step guide to calculate NPSH:Below is the procedure for calculating NPSH:
step1:Collect Information Here are the information that you need for the calculation:
Hs (Suction Head): To singulate: quantify or define the suction head (Hs) at pump intake. These are the pressure head of the liquid in suction line, the velocity head of the liquid in suction line and the elevation head of suction line.
Pv (Vapor Pressure): Establish the vapor pressure, (Pv), of the liquid at the pumping temperature. Vapor pressure is always dependent on the temperatures of the system under consideration and is stated in pressure units which include psi and Pa.
Hf (Friction Head Loss): If possible try to determine or estimate the friction head loss (Hf) for the suction pipe. This is because it considers the heat losses by condensation inside tubes, and all external tubes of connection of the fittings.
Step 2: Calculate NPSHA Use the following formula to calculate NPSHA:NPSHA calculation – Use the following formula to arrive at the value of NPSHA:
NPSHA = Hs - (Pv / ρg) - Hf
Where:
NPSHA (Net Positive Suction Head Available): This figure above is gotten from the above calculation and represents the suction head energy at the inlet of the pump.
Hs (Suction Head): A pressure head (measured suction head – velocity head – elevation head).
Pv (Vapor Pressure): Vapor pressure of the liquid as determined to pumping temperature.
ρ (Density of Liquid): The type of liquid to be pumped and particularly, the viscosity of this liquid determines the best pump to purchase. It has certain dependence on the type of the liquid used and the temperature at which it is applied.
g (Acceleration Due to Gravity): Gravitation, the acceleration towards the centre of the earth or about 9. 81 m/s² or 32. 2 ft/s².
Hf (Friction Head Loss): The losses which occur due to friction in the suction pipe.
Step 3: Compare NPSHA to NPSHR Consult the pump manufacturer's documentation to determine the required Net Positive Suction Head (NPSHR) for the specific pump model. NPSHR is typically provided in meters or feet.
Step 4: Evaluate the Result Compare the calculated NPSHA to the NPSHR:
If NPSHA > NPSHR: The pump is operating with an adequate margin of positive suction head, and cavitation is unlikely.
If NPSHA < NPSHR: It is possible there may be cavitation problems with the pump as well. In such circumstances, it might be necessary to modify suction arrangements, premises suction head, pipe loss, or pump type.
Step 5: Control of Operating Margin Again a safety margin should be kept such that the NPSHA is well above the NPSHR in light of fluctuations that may occur in operations and changes in the suction system.
We should always recall that rotodynamic pump NPSH is designed during the design phase while NPSH should be regularly checked during actual pump operation to help reduce cavitation tendencies and maximize the pump effectiveness.
application of NPSH in a centrifugal pump
Application of NPSH
NPSH is one of those aspects which is crucial when choosing and using the centrifugal pump equipment. It should be important to make certain that the pump possess adequate NPSH to minimize the chances of cavitation.
But if the NPSHa < NPSHr then the flow will be cavitated. This causes wear and tear on the pump and results in decrease efficiency of the pump. It also produces noise and vibration to some extent.
To prevent cavitation, the NPSHa must be increased. This can be done by:
- Lowering the pump inlet closer to the liquid source.
- Increasing the liquid level in the reservoir.
- Using a larger pump inlet pipe.
- Using a pump with a lower NPSHr.
However, for new applications, you want to know, for example, the required NPSH for the specific application, and in this case, it is always advisable to seek a pump engineer’s opinion.
Here are some examples of how NPSH is applied in centrifugal pumps:
- In a water supply system for instance, the NPSHa would be obtained from the water tank elevation and the atmospheric pressure.
- Where it is used, NPSHa can be obtained from the elevation of the chemical storage tank and the vapor pressure of the chemical in a chemical processing plant.
- In a power plant, NPSHa would be derived from the specific elevation of the condenser and the temperature of the water which is pumped.
One thing that should be noted, is that NPSH is not the only index that affects the phenomenon of cavitation. There are other conditions influencing the pump, for instance pump design, operating conditions of the pump.
Just as you have specific guidelines to follow whenever you decided to give your car a brand new paint, so are the guidelines that will help you determine the NPSH of your centrifugal pump through calculation.
What is NPSH (NET POSITIVE SUCTION HEAD) and why dose it mater?
Net Positive Suction Head (NPSH) is one of the important flow characteristics and is used in the design consideration of pumps especially centrifugal pumps. It is the energy level or pressure head available at the suction side of a pump to avoid the tendency of the liquid to vaporize within the pump. NPSH is a basic constituent of fluid dynamics and is used in the Initial Design and Maintenance of pumps.. Here's why NPSH matters:
Cavitation Prevention:
- Cavitation is a situation where in a pump there is formation of vapor bubbles at the suction side because of a reduction in pressure; this is of great concern to NPSH.
- Itae also lead undesirable effects such as decrease in pump efficiency, vibration, noise and also lead to wearing out of some of the pump components.It can also lead to decreased pump life.
Maintaining Pump Performance:
- Thus, it is possible to maintain a sufficient NPSH that will prevent the emergence of cavitation problems and ensure the stability of the pump’s design characteristics. The consequences of cavitation are always negative because it results in the reduction of pump performance and efficiency as far as the required flow rate or head.
Longevity of Pump:
- Cavitation results in the erosion, and pitting of the impellers and other parts of the pump mainly leading to reduced component life. Abide by the National Pump Spare Parts House and you will be sure that the life of the pump has been enhanced.
Efficiency and Energy Savings:
- An irreversibly pumped with sufficient NPSH means that the used power is less in comparison with when it is pumped with low NPSH. Cavitation is deleterious as can be observed from the fact that there could be significant energy loss in a cavitated pump.
Reliability:
- The increase of NPSH will secure the reliability of the pump. Incidents of cavitation lead to substantial losses in terms of outage for repair and constant checkups.
Safety:
- Cavitation can cause vibration and noise, they can be a potential hazards to people involved on the vicinity of the pump. The collapse of vapor bubbles can also produce shock, which may cause severe impact on the structural parts of the pump and other equipments, also.
Environmental Impact:
- Damage resulting from cavitation also causes fluctuation of pressures causing water hammer effects that may help in the destabilization of the entire system. This can have environmental and operational impacts, the most notable in the case of industrial uses.
Design Considerations:
- It is necessary for pump designers and engineers to pay attention on NPSH when choosing and ordering pumps for some specific service. Deficiency of NPSH can cause problems in operation.
In conclusion, net positive suction head (NPSH) is one of the important specification of the pump and it speaks about the reliability of the pump. This ensures that the suction head under which it is working is well above the NPSHA eliminating all forms of operational problem as well as safeguarding the pump. As the present research has demonstrated, the effective management of NPSH is crucial in the course of design, installation & operation of the pump handling systems in the application areas such as water & wastewater industries, manufacturing, or oil & gas sector, etc.
10 Things You Need to Know about NPSH
Here are 10 important things to know about Net Positive Suction Head (NPSH):
Definition: Net Positive Suction Head means NPSH for short. It is the measure of the pressure head available at suction side of a pump in order to avoid cavitation.
Cavitation: Cavitation includes the formation and collapse of vapor bubbles in a pump because of low pressure that is present at the suction side. It can cavitate fluid and hence damage some of the parts of the pump besides causing a drop in efficiency.
NPSHA: NPSHA is the overall energy at the suction of the pump read in terms of Net Positive Suction Head Available energy. It consists of the suction pressure, velocity head and elevation head of the liquid other than the vapor pressure and the friction losses.
NPSHR: NPSH (Net Positive Suction Head Required) is the head which is needed for safe running of the given pump. They are an attribute of the pump and are to be provided by the Specific Gravity Pumps manufacturer.
Cavitation Consequences: Cavitation make the pump to perform inefficient, vibrate a lot and make a lot of noise, moreover it has a short life cycle of the parts of the pump.
NPSH Margin: Hence it is suggested that there should be a safe margin between NPSHA and NPSHR so that cavitation is not realized in actual operating conditions.
Calculation: It can be computed using the same formula that has been described in a previous work by the author: NPSHA = Hs – (Pv / ρg) – Hf.
Proper Suction Piping: It is thus pertinent that suction piping should not use any restrictions and that the right sizes of line fittings only should be used and the pipes laid straight.
Temperature Effects: These show that; the greater the temperature, the greater the vapor pressure of a liquid and the lesser the temperature the lesser the vapor pressure of a liquid.
System Considerations: NPSH is not only a characteristic of the pump but also depends on the suction pipe, pipe layout, pipe geometry and setting, fluid type and its level.
Here are 10 things you need to know about NPSH:
- NPSH is short for the net positive suction head. It is concerned with the capacity of energy attainable at the pump suction to avoid cavitation.
- In cavitation a vapour cavity forms and then collapses to form a void in some liquid. This can be detrimental to the pump and as such decreases the efficiency.
- There are two types of NPSH: This latter shall be easily differentiated from the necessary net positive suction head (NPSHr) as well as the attainable net positive suction head (NPSHa). NPSHr: is the minimum NPSH which is required to be available at the suction of the pump with the help of which cavitation is not possible.
NPSHa = Pa + (z * ρ * g) - (v^2 / 2g)
where:
- Pa is the atmospheric pressure
- z is the vertical distance from the liquid surface to the pump inlet
- ρ is the density of the liquid
- g is the acceleration due to gravity
- v is the velocity of the liquid at the pump inlet
NPSHr is usual to be designated by the pump maker to as found and is often incorporated in the datasheet.
When NPSHa is less than NPSHr, then cavitation arises in the system.
- Lowering the pump inlet closer to the liquid source.
- Increasing the liquid level in the reservoir.
- Using a larger pump inlet pipe.
- Using a pump with a lower NPSHr.
Other requirements that must be considered when selecting a centrifugal pump as well as when operating it are the NPSH. Cavitation should be prevented at all manifestation and therefore it is useful to ensure that the pump has the optimum NPSH.
NPSH is also applied in the other categories of pumps such as reciprocating pumps, and rotary pumps.
NPSH depends on the liquid being pumped, the type of pump and conditions under which the latter operates.
If in doubt between short and long term NPSH, it is always recommended that you get the pump engineer to advice on the specific NPSH for the particular application.
- NPSH is the crucial one among the factors that decide the problem-free long life of the centrifugal pumps.
- NPSH is the energy that is available at the suction flange of the pump to offset energy that is dumped by the pump internally and energy that is dumped in the suction piping system.
- It was also found that NPSHa depends on the height of the liquid supply, the pressure in the atmospheric surrounding, and the flow rate of the liquid at the suction of the pump.
- As it has been mentioned, NPSHr depends on the pump design and the operation conditions.
- Before choosing a pump, it is necessary to fix on one that possesses an NPSHr that is less than the available NPSHa.
- If the NPSHa is less than the NPSHr, then cavitation takes place and this effect leads to the decrease of the pump efficiency and damage of its material.
The present article explains what NPSH means and how it impacts centrifugal pumps – allowing you to contribute towards the best performance of the installed units.
Familiarity and control over NPSH is important for proper and efficient flow of pumps in utility, municipal & industrial water and wastewater, chemical and petrochemical, food and beverage, pharmaceutical, power generation industries to name a few. Effective NPSH calculations and system design minimise cavitation and maintains pumps efficiency and durability.