water pump

What is a water pump?

A water pump is a mechanical device for moving and transferring all kinds of liquids that, by increasing the pressure, causes the liquids to move to a higher height (with increasing head) or even lower (pond or tank). Pumps have many different applications in different industries. Pumps use a variety of energy sources, including manual, electric, internal combustion, and wind-powered pumps. Pumps come in a variety of sizes, from microscopic sizes for medical work to large industrial pumps. .

In other words, the pump takes mechanical energy from an external source, such as an engine, and transfers it to the fluid that is passing through it. As a result, the fluid energy increases after leaving the pump. Pumps are used to transfer or move fluid from one point to another in a piping or hydraulic system. There are different types of pumps, each with a specific application.

Units of measurement in the pump

In the following, we will examine a series of technical and keywords that are used in the pump manufacturing industry, along with their meanings. All sizes and units of measurement are technically expressed.

flow (discharge)

The rate at which a fluid passes through a section per unit time is defined as the flow rate and is usually denoted by Q. Flow measurement is done in bulk and based on two categories of units of measurement:

  • Mass units: The rate of passage of a mass mass of a fluid in a certain fraction of time is calculated, such as Kg / h
  • Volumetric units: The rate at which a volumetric mass passes through a fluid is calculated in units of time, such as lit / min (liters per minute) and m 3 / s (cubic meters per second).

In other words, the amount of fluid or liquid that passes and exits at a certain unit of time through a specific point such as the pump outlet or pipe cross section, is called flow, which can be in liters per minute, liters per second and Or cubic meters per hour. It should be noted that there is a similar correlation between the amount of fluid passing through the pipe and the amount of electricity passing through a wire, so that the amount of hydraulic head is equal to the amount of voltage and electrical potential energy and the amount of hydraulic current is equal to the amount of ampere and electric current. . As the thinner the conduction wire, the lower the current through the wire, the lower the diameter and cross section of the water pipe. Just as we need a voltage difference to transmit electricity to a power cable through a wire, a special head must be defined to transfer fluid through a pipe.

Head (height) or pump pressure

That is, how high can a pump (under standard conditions) raise the fluid under pressure? For centrifugal water pumps, the head or water pressure is expressed in meters (m) or feet (ft), and for rotary pumps and positive displacement, the pump pressure is expressed in bar, PSI and kPa (kpa). . In other words, Head means height and refers to the difference in level. For example, a pump with a flow rate equal to Q per second and a height of 30 meters is able to pump a quantity of Q liters of fluid per second to a height of 30 meters. The amount of pumping height of each pump is calculated based on the impeller diameter and the rotation speed of the pump motor, and the type of fluid being pumped is not important. In other words, in the above example, the pump is able to pump a quantity of Q liters of pumped fluid, including pure water, oil or mercury, etc. to a height of 30 liters per second, and the difference is only in the amount of pump power consumed to pump fluids. Be.


The amount of pressure is the amount of pressure applied to each unit of the ground (for example, Kg / cm 2 ) and care must be taken not to confuse it with the amount of height. In the case of liquid pumping, the amount of pressure that the liquid exerts on the ground surface is equal to the product of the pumping height multiplied by the specific weight of the pumped liquid. Therefore, the volume of several kilometers of air on the surface of the earth, on the surface of the sea produces only a pressure equal to one Kg / cm 2 , ie a pressure close to about one atmosphere. But the same amount of volume of compression fluid produces 700 to 800 times the air pressure. Because the specific gravity of the liquid is 700 to 800 times greater than the weight of the air. Keep in mind that the amount of water pressure at a height of 10 meters is something close to one Kg / cm 2 . By installing a manometer at the pump outlet, the following pressures can be measured.

  • Oil – specific gravity 2.1 Kg / cm 2 = 00.7 * 0.001 * 30 * 100 = 0.7 Kg / cm3
  • Water – specific gravity 0.3 Kg / cm 2 = 00.1 * 0.001 * 30 * 100 = 1.0 Kg / cm3
  • Mercury – specific gravity 40.8 Kg / cm 2 = 13.6 * 0.001 * 30 * 100 = 13.6 Kg / cm3

Pump height drop

As the fluid passes through the pipes, filters or valves and due to friction with their inner wall, the amount of fluid flow and consequently the pumping height is reduced to a certain extent, which is called height drop. Like electric current, which increases with decreasing current (ampere), the current drop inside the cable increases. As the fluid flow rate increases, the flow rate decreases and therefore the pumping height decreases. Therefore, as the fluid passes through more pipes, filters and valves, the height drop increases in the same proportion.

Fluid mass

The specific gravity of a fluid or liquid is the weight of that fluid / liquid in a particular unit of measurement, usually measured in units of 3 Kg / dm or Kg / l. It should be noted that each 1dm is equal to 1 liter.

Note: Now, according to the relationship between the amount of flow and the amount of pumping height, the pumps can be divided into the following groups:

  • Pumps with low flow rate and high pumping height (piston and rotary pumps and small centrifuges)
  • Pumps with normal flow rate and height (centrifuges)
  • Pumps with high flow rate and pumping (pumps with propeller-shaped impellers and Helio Centrifugal)
  • Centrifugal motors or Heliocentrifugal or pumps with propeller-shaped impellers rotate and their rotation speed is calculated in rpm. In this group of pumps, by changing the flow rate, the operation of the pump does not change and the pumping height remains constant. Therefore, in order to change the operating mode of the pump, the rotation speed of the motor must be changed.

The energy applied to the fluid passing through the pump, which is related to the height and concentration of the fluid itself, is called the output power.

Output power

The amount of power (energy) applied to the fluid by the pump is called the output power of the pump, which depends on the three factors of flow, height and weight of the fluid.

For example, the output power of the pump used to pump gasoline is much lower than the output power of the pump used to pump sulfuric acid because the weights of the two fluids are different. All pumps are able to pump fluids with the help of electric motors or motors that are installed inside the device. The power required by the pump to operate and pump fluids is called power consumption.


The power input to the pump by the motor and its transfer to the fluid is called the power consumption of the pump. Due to factors such as fluid friction with pipes or natural hydraulic drops in the device, the output power of the pump is always less than its consumption power, which is usually less than one and is calculated as a percentage, which is called the percentage of pump efficiency. .


By dividing the output power of the pump by the power consumption, the efficiency of the pump is obtained. For example, a pump that has an efficiency of 75%, ie only the pump returns a percentage of power consumption and 25% of it is wasted due to fluid friction with pipes and other devices or heat inside the pipe and device. As a result, the higher the efficiency of the pump, the lower the power dissipation percentage. Therefore, its energy consumption is lower.

If the output power of the two pumps is 1 HP while the efficiency of the first pump is 50 and the other is 60%, we conclude that the amount of power required to provide the output power of 1 HP for the first pump is 2 HP and for the second pump is only 1.67 HP is.

Therefore, the most important parameter to determine the quality of the device and the amount of energy savings is the efficiency parameter.

Classification of pumps based on function

There are different categories of pumps according to function, internal structure, method of fluid transfer, material, pumping liquid, installation status, etc. One of the common and comprehensive methods for classifying pumps is the method of energy transfer to fluid, which is divided into two methods: dynamic and displacement:

  1. Positive Displacement Pump
  2. Dynamic Pump or Non Positive Displacement Pumps

Positive Displacement Pump

Positive displacement pumps are classified into two types: rotary (reciprocating) and reciprocating (reciprocating):

Types of rotary pumps

  • Gear pump
  • Lob pump
  • Vane pump
  • Progressive vacuum pump
  • Pre-pump
  • Screw pump

Types of reciprocating or Reciprocating pumps

  • Floating piston pump
  • Diaphragm pump

What is a Rotary Pump?

In this type of pump, the displacement is done by rotating the gear, camshaft or blades. As mentioned earlier, this type of pump has a great variety, which we will describe in the following of each type of this product.

What is a Gear Pump?

One of the most common pumps to increase the hydraulic power of the fluid is the gear pump. Gear pumps use gears to move liquids and are produced in two types of internal and external gears. This type of pump is a positive type of displacement pumps because it moves a certain and constant amount of liquid in each cycle.

What is a Lobe Pump?

Lobe pump or ear pump is used in various industries such as paper making, food, biotechnology, pharmaceutical, chemical. The reason for various applications of lobe pump is high hygienic quality, reliability, corrosion resistant in steam environments.

What is a Vane Pumps?

Wayne pumps or vane pumps are a type of positive displacement pumps (Positive Displacement Pump) and rotary that are very similar to gear pumps (Gear Pumps). In a vane pump, the fins are mounted on a rotor that is located inside the cavity. In some cases, these vanes have different lengths to maintain the connection of the walls in which the pump rotates. They are commonly used in high pressure hydraulic pumps and power and supercharge systems.

What is a Mono Pump or Progressive Cavity Pumps?

Also known as Mardoni or single-screw pumps, these pumps advance fluid through discrete cavities on a helical rotor. The volumetric flow rate is proportional to the rotational speed of the rotor. Therefore, these pumps are suitable for measuring liquids and pumping viscous fluid.

What is a Pump or Peripheral Pump?

In pre-pumped pumps, the suction chamber is larger than the drain, which reduces the pressure in the suction chamber, making it easier for fluid to enter the pump. But the discharge part is smaller and this factor pushes the fluid more intensely. When the pump is turned on, the pump impeller directs the fluid from the suction port to the outlet.

What is a Screw Pump?

The screw pump is a positive displacement pump. For this reason, with each rotation, a certain volume of fluid is displaced. Therefore, with increasing rotational speed, fluid flow also increases. In all screw pumps, the rotation speed of the pump is determined according to the viscosity of the fluid. If the viscosity increases, a lower rotation speed is selected.

What is a Reciprocating Pump?

In this type of pumps, energy transfer should be done periodically. The driving force in these pumps is usually an electric motor. In these pumps, the piston moves up and down inside the cylinder. The function of this system is that if the piston moves to one side, a suction is created inside the cylinder and the fluid moves in through the valve. When the piston returns, this sucked fluid is expelled through another valve. These pumps are used in viscous and oily fluids. In the following, we will introduce the types of reciprocating pumps.

What is a Diaphragm Pump?

Diaphragm pump is another type of positive or discontinuous displacement pump. In a diaphragm pump, the current is cut off and connected by the movement of this diaphragm. Diaphragm pumps are used to move fluids with different viscosities (very concentrated fluids such as honey or resin and liquid glue to very dilute fluids such as water). In these pumps, unlike other pumps, there is no impeller and the action of moving the fluid by a flexible plate, which is usually made of elastometer, is used.

Types of diaphragm pumps

  • Manual diaphragm pump
  • Electric diaphragm pump
  • Pneumatic or pneumatic diaphragm pump

It is more used in the pneumatic system industry due to the lack of need for electricity.

 Ability to pump diaphragm

  • Create appropriate output pressure up to 1200bar
  • Ability to work with viscous fluids
  • Efficiency above 97%
  • Ability to work with minimal lubrication
  • Ability to immerse in fluid
  • Dry operation capability
  • No mechanical flood
  • Can be used in flammable or explosive environments
  • Suction capability of barrels and tanks with various dimensions

The barrel pneumatic diaphragm pump works with compressed air and has a high suction power, and as a result, it is used to pump all kinds of fluids.

What is a floating or planar piston pump?

These types of pumps have a rotating mechanism and are from the group of positive displacement pumps, whose principles of operation in pumping fluid are the same as reciprocating pumps, with the difference that instead of one cylinder and piston, they have a large number of cylinders and pistons. Pumps are used in cases where high outlet pressure (from 100 to 1000) times is required. In these pumps, by rotating the rotary mechanism, the pistons go back and forth inside the cylinder and draw the oil into the cylinder chamber and then pump it into the system. Piston pumps are generally made in two types, stainless steel and bronze.

Types of piston pumps

  • Carwash pump
  • Home car wash pump
  • Industrial car wash pump
  • Hot and cold water car wash pump
  • Electric car wash pump, gasoline car wash pump and diesel car wash pump
  • Fog pump
  • Sprayer pump

Dynamic Pump or Non Positive Displacement Pumps

In this type of pump, a rotating actuator blade converts kinetic energy into pressure or velocity. In the following, we will introduce the types of this product.

What is a Centrifugal Pump?

Centrifugal pumps are the most widely used and widely used type of pump in the industry. It accounts for about 75% of pump consumption in the industry. They are considered as ideal pumps because these pumps always provide a certain amount of fluid flow at a constant pressure in any situation. These pumps are based on centrifugal force. The working mechanism of such pumps directs the fluid out of the center when the pump blade rotates inside it, and when the fluid exits the center, the replacement fluid under atmospheric pressure or artificial pressure and higher towards the center. The fluid that comes out is drawn and discharged through the outlet path. This fluid will have a pressure, the amount of which can be calculated by the impeller step and its rotational speed.

The centrifugal pump can be classified according to the type of shell, the type of impeller and how the shaft is located and the number of floors.

In terms of shell

A) Screw pump

In these pumps, the screw or screw housing is designed so that the duct opens in the direction of rotation of the impeller. As a result, as the cross section increases, it decreases and the dynamic pressure (due to velocity) becomes static pressure.

B) Spray pump (diffuser)

In these pumps, the impeller is surrounded by fixed blades called guide vanes. These blades open apart as they move away from the center, and as the cross-sectional area increases, the speed decreases and the pressure increases.

In terms of shell cutting

A) Integrated shell

The body of single-stage centrifugal pumps is usually disposed of uniformly. To mount the impeller and internal parts of the pump, often two sides or at least one side of the shell is open.

B) Multi-piece shell

In this case, the shell can be cut in different directions. If the cutting plate is along the axis, the shell is cut horizontally. If the cutting plate is vertical, the shell is called a vertical cut.

Suction and thrust opening condition

The location of the suction and thrust openings in the pumps is also different and in some it is adjustable. This feature is not available in one-piece pumps. In multi-pump pumps, changing the position of the thrust opening allows the pump to better adapt to its position in the network, which leads to less use of connections and reduced costs.

In terms of butterflies

The impellers are divided according to the mechanical structure, how the liquid enters the impeller and its exit direction.

A) Mechanical building

Depending on the type and viscosity of the transfer fluid, the impeller can be open, semi-open and closed.

  • a) Closed license
  • b) Semi-open impeller
  • c) Open license

The open impeller is used as sludge and for dredging. Semi-open impeller is used to transfer viscous fluids such as sewage, pulp, sugar solution, etc. In the closed impeller model, the impeller blades are placed between two wrapping plates, which are used to transfer fluids with low viscosity. The closed impeller has a higher efficiency than the other two impellers.

B) Partition in terms of liquid entering the pump

Pumps are divided into single suction and double suction models in terms of fluid entering the impeller. A significant problem with single-suction pumps is their axial hydraulic imbalance. By adding a rim to specific parts of the impeller, it is possible to balance the axial forces on it. The balance of dual suction pumps will result in better performance in pumping high capacity water.

C) Classification based on the direction of liquid flow

In terms of liquid flow, impellers are divided into three categories: radial flow impellers, axial flow impellers and mixed flow impellers. Radial flow impeller is used in cases where high head and low flow are required. The axial flow impeller is used for high flow and low head and the mixed flow impeller is used for pressure and medium flow.

Classification of centrifugal pumps in terms of number of floors

Pumps can be single-stage or multi-stage, or in other words, single-stage or multi-stage.

Division based on the direction of the butterfly period

The direction of the butterfly period can be backward and forward.

Advantages of centrifugal pumps

  • simple design
  • The price is right
  • Occupy little space
  • Variety in the genus of butterflies
  • Availability
  • Generate uniform pressure
  • Possibility to change the performance by turning the impeller (of course, it should be noted that turning more than ten percent of the efficiency changes and the similarity rules of the pump will no longer apply)
  • Because they can be used in high speeds, they can be connected directly to the electric motor.
  • Uniform volumetric flow of fluid
  • Low running costs compared to other pumps

Disadvantages of centrifugal pumps

  • Normally does not provide high pressure and flow.
  • They are very expensive for high pressures (class pumps).
  • They need to be sealed.
  • They do not pump highly viscous fluids effectively.

What is an Axial Flow Pump?

These types of pumps are centrifugal, which are used in both linear and ground types according to different capacities. Cast iron body is made of two types of circulating pumps and steel body circulating pumps and are able to pump fluids up to 130 to 100 degrees Celsius. These pumps are often used for water circulation.

One of the important points in ground and linear pumps is that linear pumps usually only produce discharge. While ground pumps produce both flow and pressure.

Circulator pump features

  • They have little functional sound.
  • These types of pumps are made in two types of single-phase and three-phase and single-cycle, two-cycle, three-cycle and four-cycle.
  • The motor speed of these pumps is 1500 and 3000.
  • The body is mostly made of cast iron and steel.
  • The butterfly is made of cast iron, steel and brass.
  • These electric pumps are available in two types: intermediate (coupling) and Atarum or dry motor without intermediate.
  • These types of pumps do not have negative suction.

What is the difference between mechanical pumps and positive displacement?
  • Displacement pumps are used for small amounts of flow at high pressures and for viscous liquids, and dynamic pumps are usually used for medium pressures and high flows.
  • In displacement pumps, the desired energy is directly converted to pressure, and in mechanical pumps, the added energy is first converted rapidly and then converted to pressure in a diffuser or diffuser.
  • In displacement pumps, the maximum pressure is determined according to the system pressure, which means that as long as the pump drive has power, the pump reaches its required level of the system. The maximum energy (pressure or head) in dynamic pumps is limited by the complete closure of the outlet valve at zero flow.
  • In displacement pumps, energy is sent to the fluid at certain periods, and in dynamic pumps, energy is sent continuously to the fluid and fluid.

What is cavitation in a pump?

 Cavitation (also known as corrosion, bubbling, cavitation, vacuuming) is a phenomenon in which the reduction of fluid pressure relative to the partial pressure of liquid vapor (p_v) causes evaporation and bubbles to form. These bubbles enter the high pressure area with water from the low pressure area and burst (collapse). In this case, water moves towards the empty space created by the collapse and microjets are formed at high speed. This phenomenon is seen in centrifugal pumps, ship propellers, torpedoes and dam overflows.

Improper design of the suction basin causes the formation of vortices in the fluid flow pattern and therefore the suction of compressed air bubbles into the pump is often seen, which is one of the major problems in pumps.

The most common of these is pump impeller wear when there is a sudden change in the direction of fluid movement. Cavitation is usually divided into static and transient.

The most important effects of cavitation

  • Changes in fluid hydrodynamics (reduction of head and flow)
  • Damage to the boundaries between solid and fluid (impeller corrosion)
  • Create vibration and vibration
  • Create imbalance

What is the cavitation force?

When the bubbles enter the high pressure area from the low pressure area, they collapse and hit the walls of the pump with the speed of sound, which makes a lot of noise. The speed of sound in water is 4800 feet per second, the force of which is calculated as follows:

H= V2/2g = (4800FT/sec)2 ÷ 2 (32.16FT) = 358209FT

Pressure (psi) = (HEAD * Sp.gr) ÷ 2.31 = 358209 ÷ 2.31 = 155069 psi

The calculated number indicates that the cavitation phenomenon can cause the pump to fail in a short time. The following figure shows the pressure gradient when cavitation occurs in a centrifugal pump:

The fluid enters the pump from point a and at point b the pressure drops to a pressure lower than the vapor pressure and a bubble forms (due to evaporation) and when the fluid moves towards the outlet pressure zone (d), at point c Condensation occurs and bubbles burst.

Types of cavitation

  1. Evaporative cavitation
    • Classical cavitation (NPSH failure)
    • Internal re-circulation cavitation
  2. Air aspiration cavitation
  3. Vane passing syndrome cavitation
  4. Turbulence cavitation

Evaporative cavitation

The above definition of cavitation is called evaporative cavitation, which is the most common type of cavitation and covers 70% of cavities. Also, because (NPSH A ) is less than (NPSH R ) this phenomenon occurs, it is also called NPSH failure. Most damage to the pump impeller occurs in this type of cavitation.

Sometimes the sound of pebbles hitting the impeller inside the pump can be heard. If we close the discharge valve slowly and the sound is cut off, it indicates cavitation problem.

NPSH R What is the net positive positive height required for suction?

The NPSH R parameter  (the same as the net positive suction head) plays an important role in selecting pumps with high inlet fluid temperatures. In fact, a fluid evaporates when its pressure is too low or its temperature is too high, and NPSH R  refers to the minimum amount of pressure required to prevent cavitation. In order to prevent cavitation, the system pressure must always be higher than the liquid vapor pressure at operating temperature at all stages of suction, impeller entry and discharge.

The cavitation phenomenon occurs when the available net positive suction height of NPSH A  is lower than that recommended by the manufacturer. NPSH A  depends on various factors such as ambient pressure, physical properties of the fluid, diameter of the suction pipe and so on. This parameter is calculated according to the following formula:

NPSHA = {(P1 + Pb – Pv)/γ} – Z1 – Hr

1max  = {(P 1 + P b ) / γ} – {NPSH R + (P v / γ) + H r }

  • 1 : surface difference between the pump shaft and the surface of the pumped fluid
  • 1 : The amount of air pressure on the surface of the fluid. In case of suction of fluid from open tank or contact of fluid surface with air, the value of P1 is equal to 0
  • b : Atmospheric pressure
  • v : Vapor pressure of the fluid being pumped at source temperature
  • c: specific gravity of the fluid
  • r : Head drop in suction pipe

Most centrifugal pump manufacturers recommend that NPSH A  be at least 0.5 m longer than NPSH R to prevent cavitation  . Some sources suggest that NPSH  should be approximately 20% higher  than NPSH R. In any case, this difference should not be less than 0.5 meters.

Note: As the pump flow increases, the value of NPSH  increases, which is greater in the right range of the best operating point (BEP). That is why it is important to select the pump in the BEP range.

Solutions to prevent injuries due to cavitation performance

  • Speed ​​reduction that reduces the amount of head caused by the drop.
  • Increase the impeller diameter and decrease the speed (it should be noted that reducing the speed does not prevent the supply of the head).
  • Using two smaller pumps in parallel will reduce head drop.
  • Reduction of P v (decrease of fluid temperature)
  • Consider the pump suction port larger than the thrust port

Cavitation disposal methods

  • Increased pressure in the suction of the pump (for example, in boilers due to the high temperature of the fluid to prevent cavitation, a device called a deaerator is used to create artificial pressure before the pump)
  • Reduce pump flow
  • Do not use the valve at a distance close to the suction pump
  • Correction of piping in pump suction 
  • Washing and descaling in pipes and fittings, especially in pump suction
  • Use the inductor in the suction pump
  • Use a pump with less NPSH R than a NPSH A piping system
  • Increase NPSH A piping system

The following figure shows an example of installing a centrifugal pump to pump fluid with a flow rate of 235 liters per minute to pump water at four different temperatures.

Now we examine the performance of the pump in 4 different modes. If the NPSH required to supply the set head is 3.25 m, we can obtain the desired vapor pressure and fluid mass temperature from the relevant tables. Suction head drop is also calculated according to the following formula:

  • Case 1: Installation at sea level with a water temperature of 20 ° C

1max  = {(P 1 + P b ) / γ} – {NPSH R + (P v / γ) + H r )

Z1max = {(101325 ÷ (998.3 * 9.81)} – {3.25 + (2338 ÷ (998.3 * 9.81)) + 2.040} = 4.8

According to the above formula, it can be understood that the pump is able to pump water with a temperature of 20 ° C from a maximum depth of 4.82 meters. Note that as the flow rate increases to more than 2351 liters per minute, so does the NPSH and the suction head drop. As a result, the available suction head is reduced to less than 4.82 meters. By reducing the flow rate, the reverse is done.

  • Second case: installation at sea level with a water temperature of 60 degrees Celsius

Z1max = {(101325 ÷ (983.2 * 9.81)} – {3.25 + (1992 ÷ (983.2 * 9.81)) + 2.040} = 3.15

According to the above formula, it can be understood that the pump is able to pump water with a temperature of 60 ° C from a maximum depth of 3.15 meters.

  • Third case: installation at sea level with a water temperature of 90 degrees Celsius

Z1max = {(101325 ÷ (965.2 * 9.81)} – {3.25 + (70110 ÷ (965.2 * 9.81)) + 2.040} = -1.99

According to the formula, it can be understood that the fluid level should be at least 1.99 meters above the pump axis.

  • Fourth case: Install the device 1500 meters above sea level and with a water temperature of 50 degrees Celsius

Z1max = {(101325 ÷ (988 * 9.81)} – {3.25 + (12335 ÷ (988 * 9.81)) + 2.040} = 3.89

According to the above formula, it can be understood that the pump is able to pump water with a temperature of 50 ° C from a maximum depth of 3.89 meters. As can be seen, with increasing fluid temperature, the maximum suction height decreases.

It is best to always set the device in such a way that the operation of the device is not disturbed by a slight change in the set values, such as natural changes in the amount of atmospheric pressure. This is even more important for fluids close to the boiling point because the slightest change in fluid temperature causes large changes in the operating conditions of the device. 

Internal return cavitation

If the pump outlet is closed, the fluid returns from the high pressure area to the low pressure area and cavitation occurs in two ways. Areas of damage to the impeller are shown in the figure below.

  1. The fluid returns to the pump at high speed and the temperature rises and overheats.
  2. The fluid passes through the joints and high speed and temperature cause it to evaporate.

This phenomenon occurs when we close two dissimilar pumps in parallel. For example, the first pump has a head 10 times and the second pump has a head 8 times. Now, if the system pressure is 9 bar, the one-way valve of the second pump is closed, and if the command pressure switch does not turn off the second pump, internal return occurs in this pump.

Air suction cavitation

This cavitation occurs due to the entry of air with water in the pump and is very common in pumps with negative suction.

Cavitation insufficiency of passage through the blade

This phenomenon occurs if the distance between the pump impeller and the chamber is short (the empty space between the impeller and the chamber should be 4% of the impeller diameter). This cavitation mostly affects the blade tips. 

Turbulence cavitation

This cavitation occurs due to turbulence in the flow and occurs in the following conditions:

  1. Create a vortex in suction 
  2. Creating turbulence with sharp elbows and fittings and filters

To prevent this cavitation, it is recommended not to install pumps, fittings and equipment at a distance of 5D from the suction port of the pump. The NFPA also states that this distance is 10D for fire pumps. The suction pipe inside the tank must also be equipped with an anti-vortex plate (Anti Vortex).

How to calculate the pump head?

To transfer fluid from one point to another inside the system, pressure is required, this pressure must overcome the resistance of the system; Which is called the head. The overall head is equal to the sum of the static head, the compression head, and the dynamic head.

The static head of the pump is equal to the value of the pumping height, which is generally calculated in meters. To calculate the pumping height of the pump, you must measure the amount of suction head and the amount of suction head during pump operation.

Depending on how the device is installed, the height of the pump is calculated in the following two ways:

  1. The value of the suction head is negative, in which case the level of the pumped fluid is lower than the suction pipe.
  2. The value of the suction head is positive, in which case the level of the pumped fluid is higher than the suction pipe.

In the first case, the value of pumping height is equal to the sum of suction and discharge heights, and in the second case, it is equal to subtracting the amount of suction height from the value of heights. Note that the connections of the suction and discharge altimeters are connected to openings of the same diameter so that the difference in fluid concentration does not change their values. Dynamic head (friction head) is the pressure drop inside the pipe due to friction between the fluid with the pipe wall and fittings and is calculated using the formula of Haysen-Williams and Darcy-Wiesbach.

Haysen-Williams equation

This equation is an empirical relation and its advantage over other relations is that its coefficient of friction is not a function of the Reynolds number. But the disadvantages are that it is valid only for water and the effects of temperature and viscosity are not considered in this regard.

P= (4.25 * Q1.85) ÷ (C1.85 * d4.87)

  • m:  Friction resistance (pounds per square inch / foot)
  • Q: Pumping flow (gallons per minute)
  • C: Friction drop coefficient
  • d: True inner diameter of pipe (inches)

In the SI system it is equal to:

P= 6.05 * {(Q1.85) ÷ (C1.85 * d4.87)}

  • m:  Friction resistance (load per meter)
  • Q: Pumping flow (liters per minute)
  • C: Friction drop coefficient
  • d: True inner diameter of pipe (meters)

Darcy-Würbach equation

∆P = 0/000216f (LρQ2/d5)

  • P∆: Head wasted
  • Q: Pumping flow (gallons per minute)
  • L: length of its length
  • ρ: Fluid density (pounds per cubic foot)
  • C: Friction drop coefficient
  • d: True inner diameter of pipe (inches)

In fast calculations with appropriate accuracy to calculate the dynamic head in water supply and heating circulation systems, 50% is added to the length of the pipeline and it is multiplied by the allowable drop per meter of pipe depending on the type of system.

Also, if the pump destination has pressure, the pump must overcome this pressure (for example, in boilers, the pump must also overcome the boiler pressure), which is called the pressure head.

To calculate the head of submersible pumps, only the head of the pump is sufficient. Then, by adding the amount of dynamic head and the difference in height between the fluid level and the manometer, the pump head can be calculated.

How to calculate pump power?

The amount of pump power is calculated in kilowatts (kW) or horsepower (HP).

  • Q: Dubai
  • H: Height in meters
  • c: specific gravity of the fluid

Pump power is calculated by one of the following formulas:

How to calculate the output power of the pump motor?
  • P1: Engine power consumption
  • P2: Engine output power
  • P3: Engine output power

Change the pump head by changing the engine speed

Pump performance is related to engine speed. If cavitation does not occur, the law of similarity can be used according to the following formulas:

For example by doubling the engine speed ny

  • x : The flow rate doubles.
  • x : The height is quadrupled.
  • 2x : Power consumption is eight times.

Butterfly diameter similarity rules

Q1 / Q2 = D1 / D2

H/ H= (D1 / D2)2

P1 / P2 = (D1 / D2)3

Calculation of power, torque and current consumption of the motor

  • 1 : Pump consumption power
  • 2 : Pump consumption power
  • ~ V: AC power supply voltage
  • Hz: Frequency value
  • I: The amount of motor current consumption in amperes
  • Cosφ: power factor
  • 1 / min : engine speed
  • η: Output efficiency (which is obtained by dividing the initial power by the motor output power).
  • P: Number of motor poles
  • Cn: Nominal torque force of the motor

The speed of rotation of the motor without pressure is calculated according to the following formula:

By reducing the engine speed by 2 to 7% in no-load mode, the engine speed in full pressure mode can be calculated.

Pump motor speed
Frequency (Hz) 2 Pools 4 Bridge
50 3000 1500
60 3600 1800


Amount of power consumption

The amount of engine power output


Power factor

Nominal torque force of the motor

Relationship between power and frequency

Driver current value

The motor starting current is more than the amount of current at full pressure and generally 4 to 8 times that.



Details of capacitors

The amount of current consumed by a thorn is approximately equal to:

  • I: Amount of current in amperes
  • F: Voltage frequency
  • C: Capacitor capacity
  • V: Consumption voltage

Example: The amount of capacitive current consumption with a capacity of 14 microfarads that is connected to a 220 V power supply is equal to:

The approximate capacitance of the capacitor is also calculated according to the following formula:

Example: The capacitance of a capacitor connected to a 220V power supply is 1.4Amps

Electromotor start-up methods

Triangle start (Direct On Line) or DOL

The simplest method of starting induction motors is the triangle method, also known as DOL and single multiplication. The DOL starter includes a power circuit breaker (Circuit Breaker) or MCCB (automatic switch to protect against overload and short circuit), a contactor and a thermal relay for protection. The electromagnetic contactor can be disconnected by a thermal relay in the event of an error. The contactor is controlled by separate start and stop buttons and an additional contact (connection) is applied on it. In this method, direct electricity (three-phase or single-phase) enters the contactor and then connects to the electric motor.

The triangle method has a high starting current and also starts with maximum torque, which is not necessary in most industries and may cause tension in the rotating parts and couplings. When the motor accelerates, the current will start to decrease but will not decrease significantly until the motor reaches its highest speed. In many cases, the single multiplication method is the most efficient method and even the only setup method.

In pumps, this method is used up to a maximum power of 11 kW, because in larger motors, due to the high starting current of this method, the motor draws a lot of amps. 

Star-Triangle Start (Double Multiplication)

The motors that work by connecting to the triangle winding start by connecting to the star winding and the amount of amps consumed and their initial torque is reduced to one third of the starting value of triangle.. In this method, by reducing the current voltage, the torque is also reduced. The components used in star-delta start-up include 3 contactors, protective fuse, bi-metal, phase control and timer. The star-delta method can not provide the torque required by electric motors that start under initial load; to be used. In this method, it is recommended to start the alternator without load.

Soft start

As the name implies, it starts the engine smoothly and ends quickly, and also shuts off smoothly. In this method, the engine speed can not be changed. It works in a similar way to the star triangle method and reduces the voltage on each phase. When the alternator reaches its nominal speed, the soft starter should go out of circuit. Also not suitable for starting the engine under load.

AC drive

A drive is a device used to control the speed of an engine (by changing the speed). This equipment is also known by other names such as frequency converter, frequency regulator, drive, inverter and so on. In catalogs, the two terms are called variable speed drive (VSD) and variable frequency drive (VFD). Conventional AC drives operate in one of three ways to control the speed of an induction motor:

  • Inverter Voltage Converter (VVI)
  • Current Source Inverter (CSI)
  • Bandwidth Modulation (PWM)

All of these drives take AC power from the source and convert it to DC by a rectifier, then convert it back to AC power by an inverter whose voltage and frequency are controlled.

Protection system

It is best to connect the electric motors to a power supply with a suitable contactor with an overload protection system.


What is Inductor?

With the expansion of the industry, today the need for pumps with smaller sizes and of course high speed is important. Therefore, it is necessary to improve the quality of suction performance of pump impellers. Inductors are important parts that are installed in front of the main impeller of the pump with the ability to achieve more suction in the pump and rotate the impeller at the same speed. Inductors can increase the inlet pressure to the pump impellers and thus improve the suction performance of the pump.

Troubleshooting, repair and maintenance of the pump

If the pump is not working properly or the user is not skilled, problems such as vibration in the pump and overheating of the pump, corrosion of the pump impeller or rims and bearings that have been worn or the free distance between them have increased. In this article, we will briefly discuss the reasons for some defects and ways to fix them.

Pump vibration:

  • Reach critical speed
  • The pump needs lubrication
  • Fixation of parts with each other
  • Axis bending
  • The bearings are worn.
  • The foundation is not strong enough.

Reduce water pumping by pump:

  • Blockage of pipes
  • The pump must be vented.
  • Air has entered the inlet pipe.
  • Why does the pump get hot?
  • Or there is a mechanical defect.
  • High degree of pump speed
  • The viscosity of the fluid is higher than desired

Outlet pressure reduced:

  • A piece is stuck between the pump blades
  • Create cavitation inside the tubes
  • The pump rotation speed is low
  • Defective bowl
  • The pump rotor rotates in the opposite direction

Pumping cut off:

  • Low height
  • Air has entered the pipes
  • There is a leak in one of the pipes

Pump repairs

One of the most common problems that building occupants usually face is a problem with the building’s water pump . Although it is better to consult a service technician to repair a water pump, knowing the tips provided will give you a better understanding of the potential problem. When repairing the water pump, make sure that the ground is not wet and that the shoes are worn.

Vibration or loud noise of the pump:

This problem occurs due to unbalanced rotor, lack of foundation or the presence of foreign objects in the impeller or loose part on the shaft.

Ventilation pump:

If there is air inside the inlet pipe of the water pump and the inlet valve of the pump, the pump does not pump water and needs to be ventilated. Empty the pump completely, then close the vent tightly.

Complete operation of the water pump without shutdown:

Lack of adjustment of mechanical automatics, dehydration of water pump , drying and venting of water pump and sticking of mechanical automatic switch connections, cause this problem and should be adjusted by the pump service.

Pump operation without any valve open:

One of the reasons may be the perforation of the pipes. Another reason for this problem could be the failure of the toilet flotter (flash tank), which causes water to constantly flow into the well and is not detected because it does not show a leak. To make sure, you should open the water shut-off valve inside and close the water shut-off valve and the flash tank valve.

Automatic analog failure of water pump:

If the tank is under pressure, the main cause is automatic mechanical failure. In case of failure, the automatic adjustment source will crash. Another reason is the low quality of the automatic. The automatic switch has more problems than other pump devices.

Pump motor failure:

If the pump motor fails, the pump problem will usually be solved with the correct service of the electric motor. In case of damage to the pump capacitor or poor operation, the water pump will be defective and unusable.

Adjusting the water pressure in the pump:

To adjust the pressure, first close all the taps and adjust the pump by opening and closing a tap. Adjust this according to the number of building units so that the highest point of the building has a good water pressure.

Source of water pump pressure:

If the air inside the source is low, the pump will not work well and will have a problem. Pressure tanks require tank replacement and wind regulation, their tubes become defective in less than a year. If the water pump is turned on and off more than 5 times per minute, the tube may have a problem or the source may be under pressure.

Submersible pump

Immersing the pump and motor in the fluid to transfer it reduces the complexity and cost of the pumping station. Especially if the length of the sheath pipes is long, the reduction in the cost of these pumps can be significant. One of the available plans is to use engines filled with oil. Engine oil is separated from the pumped fluid by mechanical seals. Small sludge pumps are mostly made submerged.

These products are installed at the bottom of the hole and pulled up and down by a chain. No foundation operation required for installation. The oil is cooled by a finned heat exchanger located on top of the impeller. The oil is circulated by a impeller located under the rotor of the electric motor. An external tube directs the oil from the heat exchanger to the top of the engine. Engine oil is a special oil and if it leaks, it will not harm the environment.

Another common design for these pumps is a motor-cooled motor housing with fluid around it. There is no liquid inside these engines and the pumped fluid is separated from the engine environment by two mechanical seals. The upper mechanical seal is lubricated and cooled by an oil chamber (Figure below). The impeller of this pump is vortex type. In vortex pumps, the axial distance between the impeller edges and the front edge of the shell is large. This creates unrestricted flow paths. The impeller of these pumps performs the pumping operation by creating a vortex in the inner chamber of the pump housing and creating centrifugal forces. In fact, the inlet fluid is subjected to a lot of rotational forces. These pumps are suitable for pumping liquids with solid particles and fibers. Due to the lack of proper fluid guidance by the impeller, the efficiency of these pumps is about 30% lower than conventional centrifugal pumps with the same specific size and speed. The specific speed of these pumps may vary from 10 to 80.

Because the impellers of these pumps are less in contact with the fluid, these pumps are suitable for pumping contaminated fluids, sludge, liquids containing abrasive solids, gases, pulp and generally suitable for pumping any contaminants in the fluid. These pumps are also suitable for pumping applications of live fish and vegetables and the like. Small vortex pumps are commonly used in sludge pumping .

Tips on using different types of pumps

In the field of pump maintenance and maintenance, the following should be noted:

  • After opening and closing the pump, be sure to use special o-rings for sealing the pump and avoid gluing in the sealing parts.
  • When adding the cable, be sure to use the cable size of the pump (use ground wire).
  • Avoid loosening and tightening the oil screws.

Effects of cold on the pump

One of the common defects in the body of household water pumps is the bursting of the pump shell based on the cold air. In winter, the cold air reduces the temperature of the water in the pump body. This decrease in temperature causes the water volume to increase abnormally. It should be noted that water at zero degrees Celsius has the lowest density and maximum volume.

An abnormal increase in the volume of water in the pump chamber causes the pump chamber to not be able to withstand the extra pressure applied by the volume increase and to explode.

Existing bursts may appear as very small cracks in the body, but in some cases large cracks may appear in the body, which practically prevents the possibility of using the pump.

Therefore, it is recommended that the water in all electric pumps, especially household electric pumps, be removed during cold weather.

Pump troubleshooting methods

Troubleshooting pump bearings

Maintenance of rotary machines, including centrifugal pumps, is very important in the industry. Therefore, it is necessary to identify common defects in the components of these machines and take action to eliminate them.

Among the very important parts in pumps are the bearings used in them. Factors such as overloading, improper lubrication, inefficient sealing, dust entering the system and very tight compliance can cause many defects in rolling bearings.

Common defects in bearings include flaking, surface fatigue, and wear. The following is a brief introduction to these phenomena:

getting layered

Frequent application of load, excessive reduction of internal looseness, skewed bearing ring installation, rust or cracks on the roller groove during installation lead to scaling phenomenon. In this case, parts of the surface of the groove or bullet with a scale-like cross section are separated from it, and the occurrence of these defects will mean the end of the life of the bearing. This defect can be prevented by improving the lubrication conditions or improving the condition of the loads applied to the bearings. (Figure 1 – Spherical bullet has this problem)

Superficial fatigue

It is caused by lubrication problems such as low lubrication or low viscosity. In this case, after a while, the so-called groove surface comes out and cracks appear on the groove surface. This phenomenon causes the friction of the bullets in contact with the groove to increase and heat to be generated, eventually causing noise. By choosing the right viscosity for the bearing and using clean oil, this phenomenon can be prevented (Figure 2, the bearing groove has a problem).


In this case, the contaminant particles are removed from the bearing surface, metal destroys the smooth surface of the rollers, and depending on the type of abrasive particles, the surface of the roller is polished and mirrored, or it turns gray in color. Come. To prevent wear, you must improve the sealing and use clean oil to cool the bearings (Figure 3, the rollers have a problem).

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