Table of Contents
The pump is a device which used to transfer fluid usually from a lower to an upper point. They are used to increase the mechanical energy level of water by which it can be raised to a higher level. Pumps are operated by the rotary or reciprocating mechanisms, and it consumes energy while performing mechanical work. Pumps are used for day to day life and for industrial application. Pumps are driven by manual operation, engine and electricity.
There exist a wide variety of pumps that are designed for various specific applications. However, most of them can be broadly classified into two categories, as mentioned below.
2.1 Positive Displacement Pumps
The term positive displacement pump itself defines that it is used to deliver or displace a fixed amount of liquid during each cycle of operation. These pumps provide very high pressures, but flow rates are generally low. The volumetric flow rate is determined by the displacement per cycle of the moving member times the cycle rate (e.g. rpm). So the design, size, and operating speed of the pump fix the flow capacity. The pressure (or head) that the pump develops depends upon the flow resistance of the system in which the pump is installed, and is limited only by the size of the driving motor and the strength of the parts. These pumps are used in fluid power applications to pump oil in heavy-duty machines (cranes, excavators, etc.).
2.1.1 Reciprocating Pump:
A reciprocating pump is a hydraulic machine which is used to converts the mechanical energy into hydraulic energy.
The piston moves to and fro within the cylinder and entrapped a specific amount of fluid and deliver it to the higher level. It is often used to handle a relatively small quantity of liquid and where delivery pressure is quite large. These pumps are used for low volumes of flow at high pressures.
- Suction Pipe
- Suction Valve
- Delivery Pipe
- Delivery Valve
- Piston and Piston Rod
- Crank and Connecting Rod
- Air Vessel
Water is sucked from the source through this suction pipeline, and it is said to be pump inlet.
The valve is placed between the suction pipe inlet and the cylinder. To prevent the liquid’s backflow, a check valve is provided on the suction line, which means the only one-directional flow is possible in this type of valve. During suction of liquid, it is opened, and during discharge, it is closed.
It is a pipe that is used to deliver the water from the cylinder to the desired location. It is an outlet pipe of the pump attached to the tank where the water is delivered.
Delivery valve or non-return valve is placed between the cylinder and delivery pipe outlet. During suction, this valve is kept closed, and it gets open while discharging the liquid.
The cylinder consists of an arrangement of piston and piston rod. This is a hollow cylinder which is made of steel alloy or cast iron.
A piston is a solid type cylindrical rod that moves forward and backward inside the hollow cylinder, to perform suction and liquid delivery. The piston is responsible for creating linear motion.1+
Crank is a solid circular disc which is connected to the power source like motor, engine, etc., for its rotation. Connecting rod connects the crank to the piston. As a result, the rotational motion of the crank gets converted into linear motion of the piston.
The strainer is used to remove or prevent solid particles from a water source at the suction pipe’s end. The pump may get damaged if solid enters in the suction line, so the strainer plays a vital role in the pump’s arrangement.
Air vessels are connected to both suction and delivery pipes to eliminate the frictional head and give a uniform discharge rate.
The reciprocating pump consists of a piston that moves forward and backwards in a close-fitting cylinder. The piston is connected to the crank through a connecting rod which helps to start its movement. When crank rotates, the piston starts moving in the cylinder. The crank is rotated using an electric motor.
Pipe of suction and delivery lines and a suction valve and a delivery valve are connected to the cylinder. These valves are generally non-return valves which prevent backflow of liquid.
As the crank rotates from θ=0º to 180º, the piston moves towards the right in the cylinder; creating a partial vacuum in the cylinder. Also, atmospheric pressure is acting on the liquid’s surface, which is more than the cylinder’s pressure. Due to the vacuum inside the cylinder, the liquid tends to move towards the cylinder, opens the suction valve, and enters the cylinder.
When a crank is rotating from θ=180º to 360º, the piston starts moving toward left in the cylinder, which increases the pressure of the liquid inside the cylinder more than the atmospheric pressure. Hence the Suction valve closes, and the delivery valve opens. The liquid is moved from cylinder to the delivery pipe, and then it is sent to a Required Height.
Types of Reciprocating pumps:
1 Piston Pump
The piston pump is one of the simplest types of pumps. This pump is mostly used for the requirement of high pressures. Piston pump can deliver high viscosity fluids like slurries, paints, heavy liquids.
It consists of a piston that reciprocates inside a cylinder. As the piston, goes away after the delivery stoke, low pressure is created in the cylinder which opens the suction valve, a new amount of liquid is sucked through the inlet check valve. On forward stroke, the fluid-filled inside the cylinder is compressed, which opens the delivery valve to deliver liquid.
2 Diaphragm Pump
The diaphragm pump is also known as a membrane pump. It contains two flexible diaphragms that reciprocate back and forth, creating a temporary chamber. The diaphragm is generally made up of a rubber, thermoplastic or Teflon and this type of pump has suitable non-return check valves to pump a fluid. The diaphragm moves when the motion is given by mechanical linkage, compressed air, fluid from a pulsating, external source, etc.
There are two types of diaphragm pumps mostly used in industries, i.e. air operated (AODD) and mechanically operated. AODD pumps are generally used where feed containing sludges, viscous fluid and slurries need to be pumped. This pump is designed to eliminate leakage issues, so there is no contact between pumping fluid and energy source. Limited head and capacity range and the necessity of check valves in the suction and discharge nozzles are some disadvantages of diaphragm pumps.
2.1.2 Rotary Pump
The name suggests that the pump is driven by rotating motion. The working principle of this pump is quite similar to the positive displacement pump. The only difference is instead of the piston it uses rotating elements like a vane, screw, gear that entraps the liquid in the suction side of the pump casing and forces it to the discharge side of the casing.
There is relative motion between rotating and stationary elements which causes the pumping action. It is essential that all clearances between rotating parts and rotating and stationary parts, be kept to a minimum to reduce slippage. Slippage is leakage of fluid from the discharge of the pump back to its suction. Rotary pumps do not require valve arrangements similar to reciprocating pumps.
Types of Rotary pumps:
A) Gear Pump
There are two basic types of gear pumps, internal and external gear pumps. The main differences between the two types of gear pumps are the gears’ placement and where the fluid is trapped.
- External Gear Pump:
It consists of two meshing gears that rotate inside a fit casing. As the teeth separate, the space between them increases and sucks a certain amount of liquid. The trapped liquid moves with the teeth till they get in contact again, and the space between them becomes zero. The only way the liquid has is the delivery port. With the large number of teeth usually employed on the gears, the discharge is relatively smooth and continuous. Small quantities of liquid are delivered to the discharge line in rapid succession. Gear pumps have higher working pressures and speeds, but these types of pumps tend to be noisy and special precautions may have to be made.
- Internal Gear Pump:
An internal gear pump works on a similar principle except for the two linking gears sizes are different with one revolving within the other. The rotor is a larger gear and an inner gear, and it has the teeth projecting inside.
Fluid supplies into the cavities and trapped with the teeth of gear because the gears continue to rotate next to the pump’s casing. The trapped liquid can be moved from the inlet side to the discharge side in the casing region. When the gears’ teeth become linked on the pump’s discharge surface, the amount can be decreased & the liquid is forced out beneath force.
B) Lobe Pump
Working principle of Lobe pumps is similar to external gear pumps where fluid flows around the casing’s interior. The lobes do not make contact, and the contact is prevented by external timing gears located in the gearbox. As the lobes come out of mesh, they create expanding volume on the pump’s inlet side. Liquid flows into the cavity and is trapped by the lobes as they rotate. Liquid travels around the interior of the casing in the pockets between the lobes and the casing. Finally, the meshing of the lobes forces liquid through the outlet port under pressure. Lobe pumps provide superb sanitary qualities, high efficiency, reliability, corrosion resistance, and good clean-in-place and sterilise-in-place (CIP/SIP) characteristics. Thus they are essential in food, beverages and pharmaceutical industries.
C) Screw Pump
It is equipped with screws that mesh together and rotate within a cylindrical cavity or liner. Screw pumps have two or more intermeshing screws rotating axially clockwise or counterclockwise. Each screw thread is matched to carry a specific volume of fluid. The fluid enters from the pump’s suction side and moves linearly along with these intermeshing screws to the pump’s discharge side. Due to the less clearance between screw and liner, the fluid gains more pressure while moving through the pump. These pumps are generally used to transfer high or low viscosity fluids.
D) Vane Pump
The pump consists of a cylindrically bored housing with a suction inlet on one side and a discharge outlet. A cylindrically shaped rotor with a diameter smaller than the cylinder is driven about an axis placed above the centerline of the cylinder. The clearance between the rotor and cylinder is small at the top but increases at the bottom. The rotor carries vanes that move in and out as it rotates to maintain sealed spaces between the rotor and the cylinder wall. The vanes trap liquid or gas on the suction side and carry it to the discharge side, where contraction of the space expels it through the discharge line. The vanes may swing on pivots, or they may slide in slots in the rotor.
In some cases, these vanes can be variable length and/or tensioned to maintain contact with the walls as the pump rotates.
Positive displacement pumps are used for various applications.
These pumps are sometimes called constant-volume pumps because they discharge constant fluid volume at once and maintain a constant speed and flow. Even if the system pressure varies, the flow remains constant.
- Variety of fluid types can be handled by these pumps, e.g. high, low and variable viscosity; shear-sensitive fluids; solids; and liquids with a high percentage of air or gas entrainment.
- Capacity is not affected by the operating pressure.
- They are more efficient than centrifugal pumps when handling viscous fluids.
- These pumps are self-priming.
- They can be designed without a seal.
2.2 Dynamic Pressure Pumps
Dynamic Head Pumps (DHP):
In this type, mostly centrifugal pumps are used. The kinetic energy of the fluid is increased, which is then converted to pressure. These pumps give huge quantities at low pressures. These pumps generate high rotational velocities then convert the resulting kinetic energy of the liquid to pressure energy. It is also called Kinetic pumps. The centrifugal pump design is often associated with the transfer of water but is also a popular solution for handling thin fuels and chemicals. They are used in liquid transportation to pump water from treatment stations to homes.
2.2.1 Centrifugal Pump:
- Suction pipe with a foot valve
- Delivery pipe
An impeller is an integral part of a pump. It is a rotating component in a centrifugal pump equipped with vanes or blades that rotate and move the pump’s fluid. These vanes or blades are connected with the shaft. When the impeller rotates, it utilises the energy getting from the motor to transfer fluid. Impeller design is the main factor to decide the efficiency of a pump.
A casing is a cover or housing in which most of the components are placed and protected. The casing receives fluid that needs to be pumped. It is generally used as a seal to prevent leakage. It is also used to support some of the critical parts such as shafts, bearings, etc.
A pipe whose one end is connected to the pump’s inlet and another end connected to the liquid sump is known as a suction pipe.
A foot valve is found at the end of a pipeline in a suction lift application. They function as a check valve; they also have a strainer attached to their open end.
A pipe whose one end is connected to the pump’s outlet and other ends delivers the water at a required height is known as a delivery pipe.
Centrifugal pumps consist of a stationary pump casing, and an impeller mounted on a rotating shaft. The pump casing provides a pressure boundary for the pump and contains channels to properly direct the suction and discharge flow. The pump casing has suction and discharge penetrations for the pump’s main flow path and typically has a small drain and vent fittings to remove gases trapped in the pump casing or drain the pump casing for maintenance.
The pump casing guides the liquid from the suction connection to the centre, or eye, of the impeller. The rotating impeller’s vanes impart a radial and rotary motion to the liquid, forcing it to the pump casing’s outer periphery, where it is collected in the outer part of the pump casing called the volute. The volute is a region that expands in cross-sectional area as it wraps around the pump casing. The volute’s purpose is to collect the liquid discharged from the impeller’s periphery at high velocity and gradually cause a reduction in fluid velocity by increasing the flow area. This converts the velocity head to static pressure. The fluid is then discharged from the pump through the discharge connection.
Applications of Centrifugal Pumps
- Centrifugal pumps are used in buildings for pumping the general water supply, as a booster and for domestic water supplies.
- The design of a centrifugal pump makes them useful for pumping sewage and slurries.
- They are also used in fire protection systems and for heating and cooling applications.
- Beverage industry: Used to transfer juice, bottled water, etc.
- Dairy industry: Used to transfer dairy products such as milk, buttermilk, flavoured milk, etc.
- Various industries (Manufacturing, Industrial, Chemicals, Pharmaceutical, Food Production, Aerospace, etc.) for cryogenics and refrigerants.
- Oil Energy: pumping crude oil, slurry, mud; used by refineries, power generation plants.
3 BASIC PUMP PARAMETERS
The primary pump parameters can be subdivided into those that deal with purely hydraulic/liquid aspects and those classified as more or less rotational.
The hydraulic variables consist of a head, capacity (or flow), and efficiency. We will examine each one in detail:
Head is simply a pressure unit commonly used in hydraulic engineering expressed in feet of the pumped fluid. So, the pressure is exerted from the weight of a given liquid’s height; So, the unit of feet (meters in the metric system of units). There are numerous forms and references to hydraulic head.
- Fluid friction head;
- Static suction head
- Pump discharge head.
In hydraulics calculations, the most relevant term used, i.e. differential head. It is the difference between the pressure on the pump’s discharge side and the suction side pressure.
Consider an infinitely long vertical pipe, connected to the outlet of a centrifugal pump. When operated, this pump developed discharge pressure which would lift the pumped liquid to an equilibrium height in the vertical pipe, equal to the pressure produced by the weight of that same column of liquid. This particular height is known as shut-off head because it would simulate the head produced when the flow is zero.
The pump flow rate or capacity Q is the useful volume of fluid delivered to the pump discharge nozzle in a unit time in m3 /s (l/s and m3 /h are also used in practice, as are GPM in the US). The flow rate changes proportionally to the pump speed of rotation.
The pump efficiency η is given with the characteristic curves. There are four efficiencies involved in centrifugal pump systems.
(1) Hydraulic efficiency:
A complete discussion of hydraulic efficiency will be provided when the subject of power is undertaken. In this course, efficiency means hydraulic efficiency, and it will be denoted with the symbol E.
(2) Mechanical efficiency:
Mechanical efficiency measures the losses between the drive output shaft and the impeller’s shaft input side. For instance, frictional losses in couplings would be a contributing factor to lower mechanical efficiency
(3) Drive efficiency:
Drive efficiency refers to the pump driver’s effectiveness, either an electric motor, magnetic drive, or a steam turbine. As efficiencies go, electric motor efficiencies are excellent, and there is little change with load or speed.
(4) Overall pump operational efficiency:
It is the product of the three preceding efficiencies.
The rotational (maybe they should be referred to as mechanical) variables are power, speed, and impeller diameter.
The pump input power P is linearly proportional to the fluid density r. For high-density fluids, the power limits of the motor and the torque limits must be considered.
In physics, power is defined as work per unit time. In the field of engineering, power is defined as the ability to do work. Units for power are the horsepower (hp) and the kilowatt (kW). The unit of horsepower is commonly used interchangeably with, and taken to mean the variable of power. There are three different horsepowers involved in centrifugal pump systems. These are
(1) Hydraulic horsepower
Hydraulic horsepower sometimes referred to as water horsepower (WHP), is the power imparted to the liquid by the pump. The following formula defines it
(2) brake horsepower:
Brake horsepower is the quantity that is generally provided by pump manufacturers on performance curves. F
(3) drive or motor horsepower.
Prime movers, also known as drives, are machines that convert natural energy into work. Drive horsepower is the nominal or nameplate power rating of the prime mover. The two primary types of drives for centrifugal pumps are electric motors and steam turbines.
- Rotation Speed
The rotational speed is the scalar quantity of the dynamics term known as angular velocity. The rotational speed is generally referred to simply as speed. The unit of revolutions per minute (rpm) is used for speed. AC induction motors generally drive centrifugal pumps. Variable frequency AC drives have gained popularity and allow for variable pump speeds and improved efficiencies. Speed is denoted by the symbol N.
- Impeller Diameter
Impellers are an integral part of pumps, and deciding its diameter is the main factor while designing pumps. Impellers are manufactured in various shapes and design, depending on their functions. The impeller is mounted on the pump shaft and within the pump’s casing. Centrifugal pump casings are fabricated to accept a given maximum impeller diameter allowing for a hydraulic clearance to facilitate fluid motion to the pump discharge point. Thumb rule for Impeller diameter is considered as two times the distance of a line passing through the pump shaft centre to the impeller periphery
PUMPS PERFORMANCE CURVE AFFINITY LAW PDF\
4 PUMP CALCULATIONS:
The pump flow rate or capacity can be calculated with the formula
Q = V *A
V= velocity of fluid flowing through pump, m/s
A= Cross-section are of pipe, m
- Static Suction Head (hS): Head resulting from the liquid’s elevation relative to the pump centre line. If the liquid level is above pump centerline, hS is positive. If the liquid level is below pump centerline, hS is negative. Negative hS condition is commonly denoted as a “suction lift” condition
- Static Discharge Head (HD): The vertical distance in feet between the pump centerline and the point of free discharge or the surface of the liquid in the discharge tank.
- Friction Head (hf): The head required to overcome the resistance to flow in the pipe and fittings. It depends on the size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid.
- Vapour Pressure Head (hvp): Vapour pressure is the pressure at which a liquid and its vapour co-exist in equilibrium at a given temperature. The vapour pressure of a liquid can be obtained from vapour pressure tables. When the vapour pressure is converted to head, it is referred to as vapour pressure head, hvp. The value of the hvp of liquid increases with the rising temperature and, in effect, opposes the pressure on the liquid surface. The positive force tends to cause liquid flow into the pump suction, i.e. it reduces the suction pressure head.
- Pressure Head (hp): Pressure Head must be considered when a pumping system either begins or terminates in a tank under some pressure other than atmospheric. The pressure in such a tank must first be converted to feet of liquid. Denoted as hp, pressure head refers to absolute pressure on the liquid reservoir’s surface, supplying the pump suction, converted to feet of head. If the system is open, hp equals atmospheric pressure head.
- Velocity Head (HV): Refers to a liquid’s energy as a result of its motion at some velocity ‘v’. It is the equivalent head in feet through which the water would have to fall to acquire the same velocity, or in other words, the head necessary to accelerate the water. The velocity head is usually insignificant and can be ignored in most high head systems. However, it can be a large factor and must be considered in low head systems.
- Total Suction Head (HS): The suction reservoir pressure head (hpS) plus the static suction head (hS) plus the velocity head at the pump suction flange (hVS) minus the friction head in the suction line (hfS).
HS = hpS + hS + hvS – hfS
- Total Discharge Head (Hd): The discharge reservoir pressure head (hpd) plus static discharge head (HD) plus the velocity head at the pump discharge flange (HVD) plus the total friction head in the discharge line (hfd).
Hd = hpd + hd + hvd + hfd
- Total Differential Head (HT): It is the total discharge head minus the total suction head
HT = Hd + HS (with a suction lift)
HT = Hd – HS (with a suction head).
- The pressure to Head Conversion formula
The static head corresponding to any specific pressure is dependent upon the density of the liquid according to the following formula:
NPSH: Net Positive Suction Head
NPSH is one of the most widely used and least understood terms associated with pumps. Understanding the significance of NPSH is very much essential during installation as well as operation of the pumps.
To avoid cavitation in centrifugal pumps, the fluid’s pressure at all points within the pump must remain above saturation pressure. The quantity used to determine if the pressure of the liquid being pumped is adequate to avoid cavitation is the net positive suction head (NPSH). The net positive suction head available (NPSHA) is the difference between the pressure at the pump’s suction and the saturation pressure for the liquid being pumped. The net positive suction head required (NPSHR) is the minimum net positive suction head necessary to avoid cavitation. The condition that must exist to avoid cavitation is that the net positive suction head available must be greater than or equal to the net positive suction head required. This requirement can be stated mathematically, as shown below. NPSHA ≥ NPSHR
NPSH= Suction Pressure- Vapour Pressure + Static Head – Friction Head
When Suction pressure is less than the vapour pressure cavitation takes place. Cavitation in a centrifugal pump has a significant effect on pump performance. Cavitation degrades the performance of a pump, resulting in a fluctuating flow rate and discharge pressure. Cavitation can also be destructive to pumps internal components. When a pump cavitates, vapour bubbles form in the low-pressure region directly behind the rotating impeller vanes. These vapour bubbles then move toward the oncoming impeller vane, where they collapse and cause a physical shock to the leading edge of the impeller vane. This physical shock creates small pits on the leading edge of the impeller vane. Each pit is microscopic, but the cumulative effect of millions of these pits formed over hours or days can destroy a pump impeller. A cavitating pump can sound like a can of marbles being shaken. Other indications observed from a remote operating station are fluctuating discharge pressure, flow rate, and pump motor current.
It is the ratio of pump input and output power.
It is calculated as
5 CHARACTERISTICS CURVES:
Characteristic curves of centrifugal pumps are used to understand the pump’s performance and behaviour when the pump is operated under different conditions (different elevations, capacity and speed). Characteristic curves can be defined as curves plotted from the results of several tests on the centrifugal pump.
The following are the essential characteristic curves for pumps :
1) Main characteristic curves :
The main characteristic curves of a centrifugal pump consisting of a variation of head, power and discharge concerning speed. For plotting curves of discharge versus speed, manometric head (Hm) is kept constant. And for plotting curves of power versus speed, the manometric head and discharge are kept constant.
2) Operating characteristic curves :
operating characteristic curves of a pump are plotted when the speed is kept constant, and there is a variation of the manometric head, power and efficiency concerning discharge.
The input power curve for the pump shall not pass through the origin. It will be slightly away from the origin on the y-axis, as even at zero discharge some power is needed to overcome mechanical losses. The head curve will have a maximum value of head when discharge is zero. This point is called a shut-off head.
3) Constant efficiency curves:
For obtaining constant efficiency curves for a pump, the head versus discharge curves and efficiency versus discharge curves for different speed are used. Combining curves (H vs Q curves and N vs Q curves) constant efficiency curves are obtained. For plotting the constant efficiency curves (also known as iso-efficiency curves), horizontal lines representing constant efficiencies are drawn on the N vs Q curves. The points at which these lines cut the efficiency curves at various speeds are transferred to the corresponding H vs Q curves. The points having the same efficiency to are then joined by smooth curves. These smooth curves represent the iso-efficiency curves.
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