Terminology used in Pumping System
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Casing: the body of the pump which encloses the impeller.
Cavitation: the sudden collapse of gas bubbles due to the sudden pressure increase
Centrifugal force: a force associated with a rotating body. In the case of a pump, the rotating impeller pushes fluid on the back of the impeller blade, imparting motion. Since the motion is circular there is a centrifugal force associated with it. The force pushes the fluid against a fixed pump casing thereby pressurizing the fluid
Control volume: limits imposed for the theoretical study of a system. The limits are usually set to intersect the system at locations where conditions are known
Datum plane: a reference plane. A conveniently accessible known surface from which all vertical measurements are taken or referred to.
Discharge Static Head: the difference in elevation between the liquid level of the discharge tank and the centerline of the pump. This head also includes any additional head that may be present at the discharge tank fluid surface.
Enthalpy: a thermodynamic property of a fluid. The enthalpy of a fluid consist of the energy associated with the fluid at a microscopic level (related to the temperature of the fluid) plus the energy present in the form of pressure at the inlet and outlet of a system.
Equipment: refers to any device in the system other than pipes, pipe fittings and isolation valves.
Equipment head difference: the difference in head between the outlet and inlet of an equipment.
Friction: the force produced as reaction to movement. All fluids produce friction when they are in motion. The higher the fluid viscosity, the higher the friction force for the same flow rate. Friction is produced internally as one layer of fluid moves with respect to another and also at the fluid/wall interface.
Friction head difference: the difference in head required to move a mass of fluid from one position to another at a certain flow rate
Head: refers to the pressure produced by a vertical column of fluid
Heat loss: the heat lost by a system (i.e. the heat lost due to friction).
Heat transfer: the heat lost or gained by a system. This book has not considered the application of equipment that produce a significant change in the fluid temperature.
Impeller: the rotating element of a pump which imparts movement and pressure to a fluid
Internal energy: a thermodynamic property. The energy associated with a substance at a molecular level .
Iteration: a method of solving an equation by trial and error. An iteration technique is used to solve equations where the unknown variable cannot be explicitly isolated. A frequently used technique is the Newton-Raphson method.
Kinetic energy: a thermodynamic property. The energy associated with the mass and velocity of a body.
Laminar: a distinct flow regime that occurs at low Reynolds number (Re < 2000). It is characterized by particles in successive layers moving past one another in a well behaved manner.
Mercury (Hg): a metal which remains liquid at room temperature. This property makes it useful when used in a thin vertical glass tube since small changes in pressure can be measured as changes in the mercury column height. The inch of mercury is often used as a unit for negative pressure
Negative pressure: pressure that is less than the pressure in the external environment..
Net Positive Suction Head (N.P.S.H.): the head in feet of water absolute as measured or calculated at the pump suction flange, less the vapor pressure (converted to feet of water absolute) of the fluid
Newtonian: a fluid whose viscosity does not change with the amount of strain it is subjected to
Operating point: the point on the system curve corresponding to the flow and head required to meet the process requirements
Performance curve: a curve of flow vs. Total Head for a specific pump model and impeller diameter
Pipe roughness: a measurement of the average height of peaks producing roughness on the internal surface of pipes. Roughness is measured in many locations, and is usually defined in micro-inches RMS (root mean square).
Potential energy: a thermodynamic property. The energy associated with the mass and height of a body above a reference plane.
Pressure: the application of external or internal forces to a body producing tension or compression within the body. This tension divided by a surface is called pressure.
Shut-off head: the Total Head corresponding to zero flow on the pump performance curve
Specific gravity: the ratio of the density of a fluid to that of water at standard conditions
Strain: the ration between the absolute displacement of a reference point within a body to a characteristic length of the body.
Stress: in this case refers to tangential stress or the force between the layers of fluid divided by the surface area between the layers.
Suction Static Head: the difference in elevation between the liquid level of the source of supply and the centerline of the pump. This head also includes any additional head that may be present at the suction tank fluid surface
Suction Static Lift: the same definition as the Suction Static head. This term is only used when the pump centerline is above the suction tank fluid surface.
Siphon: is a system of piping or tubing where the exit point is lower than the entry point.
System: the system as referred to in this book includes all the piping with or without a pump, starting at the inlet point (often the fluid surface of the suction tank) and ending at the outlet point (often the fluid surface of the discharge tank).
System curve: is a plot of flow vs. Total Head that satisfies the system requirements.
Turbulent: a type of flow regime characterized by the rapid movement of fluid particles in many directions as well as the general direction of the overall fluid flow
Vapor pressure: the pressure at which a liquid boils at a specified temperature
Velocity Head difference: the difference in velocity head between the outlet and inlet of the system
Viscosity: a property, which measures a fluid's resistance to movement. The resistance is caused by friction between the fluid and the boundary wall and internally by the fluid layers moving at different velocities
Frequently Asked Questions on Pumps
What is the difference between head and pressure?
To start, head is not equivalent to pressure. Head is a term, which has units of a length or feet. In the following equation (Bernoulli's equation) each of the terms is a head term: elevation head h, pressure head p/g and velocity head v2/2g. Head is equal to specific energy, of which the units are lbf-ft/lbf. Therefore the elevation head is actually the specific potential energy, the pressure head, the specific pressure energy and the velocity head is the specific kinetic energy (specific means per unit weight).
So what is the difference? Head is energy per unit mass whereas pressure is a force per unit area.
What is the total pressure drop for several pieces of equipment in the same line?
The pressure drop associated with each piece of equipment is additive.
What are fittings?
Fittings are all the miscellaneous pipe connections (tees, elbows, Ys, etc.).), sometimes known as hardware, required to run pipes and their branches in various directions to their destination. Manual valves are also considered fittings.
Why is the term pressure drop used when describing the effect of equipment on a system?
To drive fluid through a piece of equipment there must be a force at the inlet greater than the force at the outlet. These forces are converted to pressure, which is more convenient in a fluid system. The difference (or drop) in pressure between the inlet and outlet is proportional to the overall force pushing the liquid forwards. If we convert pressure drop to head then we obtain the pressure drop value in terms of head (i.e. fluid column height) or pressure head.
How can the same pump satisfy different flow requirements of a system?
If a pump is sized for a greater flow and head that is required for the present conditions, then a manual valve at the outlet of the pump can be used to throttle the flow down to the present requirements. Therefore, at a future date the flow can be increased by simply opening a valve. This however is wasteful of energy and a variable speed drive should be considered.
Is the head at the suction side of a pump equal to the N.P.S.H. available?
No, the N.P.S.H. available is the head in absolute fluid column height minus the vapor pressure (in terms of fluid column height) of the fluid.
Is the head at the discharge side of the pump equal to the Total Head?
No, the Total Head is the difference in head between the discharge and the suction.
What is the difference between the N.P.S.H. available and the N.P.S.H. required?
The N.P.S.H available can be calculated for a specific situation and depends on the barometric pressure, the friction loss between the system inlet and the pump suction flange, and other factors. The N.P.S.H. required is given by the pump manufacturer and depends on the head, flow and type of pump. The N.P.S.H. available must always be greater than the N.P.S.H. required for the pump to operate properly.
How is the pressure head at any location in a piping system determined and why bother?
First, calculate the Total Head of the system. Then, using a control volume, set one limit at the point where the pressure head is required and the other at the inlet or outlet of the system. Apply an energy balance and convert all energy terms to head. The resulting equation gives the pressure head at the point required.
What is the purpose of a variable speed drive?
All systems require a means of flow control. The plant's output requirements may change causing flow demand to vary and therefore the various systems throughout the process must be able to modify their output flow rate. To achieve this, pumps are sized for the maximum anticipated flow rate. The most frequent means of reducing the output flow rate is to have a line which re-circulates flow back to the suction tank. Another method is to have a valve in the discharge line which reduces the output flow rate when throttled. Either method works well, but there is a penalty to be paid in consumption of extra power for running a system, which is oversized for the normal demand flow rate. A solution to this power waste is to use an electronic variable speed drive. For a new installation this alternative should be considered. This provides the same flow control as a valved system without energy waste.
What is Total Head?
Total Head is the difference between the head at the discharge vs. the head at the inlet of the pump. Total head is a measure of a pumpís ability to push fluid through a system. This parameter (with the flow) is a more useful term than the pump discharge head since it is independent of a specific system. Also Total head, just as any head at any location in the system, is independent of the fluid density.
What is Friction Head?
Fluid layers move at different speeds depending on their position with respect to the pipe axis. The velocity is zero at the pipe wall and maximum at the pipe center. This difference in velocity between fluid layers is a source of friction. Another source of friction is the interaction between the fluid layers close to the pipe wall and the pipe roughness or the small peaks and valleys on the wall (for turbulent flow only). The sum of these two sources of friction is the total friction due to fluid movement. Friction head is the energy loss due to fluid movement and is proportional to the flow rate, pipe diameter and viscosity. Tables of values for friction head are available in many references. The Colebrook and Darcy equations provide a method of calculating friction head for Newtonian fluids. Another component of friction head is the pressure drop due to fittings. Many references supply the data for determining the friction loss due to fittings. The 2K method is recommended.
What is Velocity Head?
Velocity head is the kinetic energy of the fluid particles. Velocity head difference is the difference in kinetic energy between the inlet and outlet of the system.
What is Static Head or Total Static Head?
The static head or total static head is the potential energy of the system. It is the difference between the elevation of the outlet vs. the inlet point of the system.
What is N.P.S.H.?
The Net Positive Suction Head (N.P.S.H.) is the head at the suction flange of the pump less the vapour pressure converted to fluid column height of the fluid. The N.P.S.H. is always positive since it is expressed in terms of absolute fluid column height. The term "Net" refers to the actual head at the pump suction flange and not the static head. The N.P.S.H. is independent of the fluid density as are all head terms.
What information is required to determine the Total Head of a pump?
What information do I need to order a pump?
Total head, flow and fluid properties (i.e. temperature, PH, composition).
What is the best way to start a pump?
Start the pump with a closed discharge valve.
What does "centrifugal" refer to in centrifugal pump?
A centrifugal pump consists of an impeller rotating within a fixed casing or volute. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction. A force, which acts in a radial direction, is known as a centrifugal force. This force is the same one that keeps water inside a bucket, which is rotating at the end of a string.
What is the Best Efficiency Point (B.E.P.)?
The B.E.P. (best efficiency point) is the point of highest efficiency of the pump. All points to the right or left of B.E.P have a lower efficiency. The impeller is subject to non-symmetrical forces when operating to the right or left of the B.E.P.. These forces manifest themselves as vibration depending on the speed and construction of the pump. The most stable area is near or at the B.E.P.
What is the best way to measure Head and Flow?
Head: Total Head can be measured by installing pressure gauges at the outlet and inlet of the pump. The pump inlet pressure measurement can be eliminated if we can be sure what the pressure head is at that point. For example, if the pump suction is large and short and the inlet shut off valve is fully open and is the type of design that offers little restriction, then we can assume that the pressure head at the inlet of the pump is equal to the static head.
What is barometric pressure and why should I care?
Barometric pressure is the air pressure in absolute terms in the local environment. The air pressure is highest at sea level and gradually diminishes with elevation. Barometric pressure is often expressed in psia (pound per square inch absolute) or feet of water absolute. The barometric pressure at sea level is 14.7 psia or 34 feet of water absolute. Barometric pressure is used to calculate the N.P.S.H. available, which is required to determine if the pump will operate properly as designed.
What is my elevation above sea level and why should I care?
Your elevation above sea level varies with your location. Your local airport can give you their elevation and barometric pressure. The relationship between elevation and barometric pressure is well documented and available in many reference books as charts or tables. You can find your local elevation on a topographic map and determine the barometric pressure at your location. For example, the air pressure at sea level is 14.7 psia, at 10,000 feet it is 10.2 psia, and at 35,000 feet (the cruising altitude of most passenger jets) 3.5 psia. The local barometric pressure is required to calculate the N.P.S.H. available at the pump suction.
Ever see a movie where people and things are sucked out of an airplane after the bad guy shoots a hole through a window. Well at a 35,000 feet altitude, an object located over a 12" diameter hole (approximate size of a window) will be subject to a force of 1270 pounds, frightening isnít it?
The Colebrook equation gives the value of the friction parameter f with respect to the Reynolds number and the pipe roughness. When the Reynolds number is small, below 2,000 (laminar flow region), pipe roughness has no effect at all. When the Reynolds number is between 4,000 and 50,000, that is low velocity and/or high viscosity, then the influence of pipe roughness is as equally important as the effect of velocity. When the Reynolds number is large, above 50,000, that is high velocity and/or low viscosity, then the friction is entirely dependent on pipe roughness.
What is the effect of pipe fittings on the total pipe friction loss?
Any fitting inserted into a pipe run has an effect since it either obstructs the flow or re-directs it or both. Most common fittings have been studied and their effect quantified, the results are available in many reference books.
How can the Total Head of a system that has more then one outlet be determined and what is the effect compared to a system with one outlet?
One fluid path from inlet to a selected outlet is used for the calculation of Total Head. This path is assumed to require the highest Total Head, if there is a doubt about the head required for the other path then the calculation is done on the other path and a comparison is made. Also the velocity head input difference to the two separate branches needs to be added to the Total Head. This however is normally a small and negligible term.
How do you calculate pressure drop due to fluid friction?
The Colebrook equation is the most accepted formula for calculating the pressure or head drop due to friction in pipes for Newtonian fluids. This equation relates the friction factor to the Reynolds number and the pipe roughness. The friction factor is then used in the Darcy formula to calculate head drop. For non-Newtonian fluids, which is mostly slurries of one kind or another, the process is much more complicated and many factors are taken into account. Some of these factors are: particle size and distribution, settling velocity of the particles in the mixture, viscosity variation of the mixture, solids transportation mode, etc.
What is negative pressure ?
Pressure is said to be negative when it is less than the local barometric or atmospheric pressure.
What is relative and absolute pressure?
A control volume is a theoretical boundary which helps delimit the extent of a system, particularly all its inputs and outputs. The principles of conservation of mass and energy can then be applied within this region.
What is an energy balance?
Because of the principle of conservation of energy, any energy gain or loss in a system must be accounted for. Therefore, making an energy balance is the process of identifying all the sources of energy gain or loss and adding them up. The result must be equal to zero.
What is the system equation and how is it developed?
The system equation has on the left hand side the Total Head (difference between the pump discharge head and suction head), and on the right hand side, all the terms which impede fluid flow such as: friction, velocity, elevation difference, etc. An energy balance is used to derive the system equation.
Does a fluid system with no pump have a Total Head?
No, Total Head is a term that is used only for a pump.
What other devices can create pressure in such a way as to move fluid through a system?
An inductor can raise the pressure of a fluid by using another fluid at a higher pressure.
What happens if the damaged pump's performance curve has all points at a lower head than the good pump's performance curve?
The best that the damaged pump can do is to produce the head corresponding to its shut-off headD HC (point 2) at 0 flow. Since the head produced by the good pump is higher, there will be flow through the damaged pump in the reverse direction. The flow however will be impeded since the pump can produce some head. The system behaves as a branch system. The branch flow sees a head drop which is the sum of the shut-off head of the damaged pump, plus any friction loss, plus the static head of the suction tank on the inlet of the damaged pump.
What is laminar and turbulent flow?
Laminar flow is a very well behave flow usually occurring at low speeds for most fluids. In the laminar flow regime it is possible to determine theoretically the speed of any particle between the center of a pipe and the wall. Most fluids have to be carried at a much higher velocity which puts them in the turbulent flow regime. For turbulent flow, the fluid particles move in many directions, each particle reacts with its neighbor in an unpredictable fashion creating much higher internal friction than is present in the laminar flow situation. If you put dye in a laminar flow system, you will observe nice long streams of dye undisturbed by the surrounding liquid. The same dye inserted in a turbulent flow will immediately be dispersed through out the liquid.
Adjustable Speed Drives
Classifications of Drives
Variable Frequency Drives
A variable frequency drive controls the speed of an AC motor by varying the frequency supplied to the motor.
A variable frequency drive has two stages of power conversion
Types of Inverters
Comparison of Adjustable Speed Drives
Advantages of AC Variable drives
How to select an Adjustable Speed Drive
Advantages and Disadvantages of VSDs
Case study 1:
Pump HP : 75
Readings with Throttle control :
Readings with Variable Speed Drive :
Energy Conservation Techniques in Pumps
The following are different ways to conserve the Energy in Pumping System:
Preventive Maintenance Checks for Centrifugal Pumps and Drivers
The following are various preventive maintenance checks for centrifugal pumps that are to be carried daily, monthly, half yearly and Yearly :
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