What is a pump and how does it work?
A pump is a machine that imparts energy to a fluid in order to increase its energy and hence the pressure of the fluid (liquid or gas) and move it from one point to another.
Pumps can be classified into three categories:
- Fluid Transfer:
- Water supply and sewage
- Medical fluids
- Heating and refrigeration
- Fire fighting
- Power or Energy Transfer:
- Hydraulics
- Hot oil or water
- Chilled water
- Process pumps
- Boiler Feed pumps
- Processing:
- Water jets
- Cutting
- De-watering
- Injection pumps
Pump performance is described by its performance curve where capacity (gallons per minute - GPM) is plotted against developed head (FT). Efficiency (in %), required input power (brake horse power - BHP), and NPSHR (net positive suction head required in FT) are also shown on the pump curve. Pump characteristics such as speed (in RPM), pump size, pump type and impeller size are added for reference.
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What is a pump curve?
Also called a pump performance curve; a pump curve is a graph that represents a pump's water flow capacity at any given resistance. The curve on a bar graph that shows the performance characteristics of a pump. Variables include size, flow rate and resistance.
The term head refers to the differential head developed by a pump expressed in feet of liquid:
H = [Pd-Ps] x 2.31 / SG
where:
H = pump head, FT of liquid Pd = pump discharge pressure, PSIG Ps = pump suction pressure, PSIG SG = liquid specific gravity Since a pump is not a perfect machine, losses mean that pump efficiency is an important factor in many applications. The required input power, called brake horsepower (BHP), is calculated from: BHP = [Q x H x SG] / [3960 x E] where E = pump efficiency expressed as a decimal fraction.
The transmission of energy to the fluid can be accomplished in various ways. An impeller can be used as a rotating device which deflects liquid away from the center and converts kinetic energy into pressure energy. This group of "rotodynamic" pumps is the most common type and more commonly known as centrifugal pumps. With centrifugal pumps, the liquid is delivered in a continuous and uninterrupted flow.
By exerting force on the liquid with a reciprocating unit, rotating piston, set of gears, peristaltic hose or equivalent means , a fluid can be forced to move. This class, known as positive displacement pumps, includes piston pumps, certain types of vain pumps with oscillating pistons, gear pumps, screw pumps and other rotary pumps.
When a high velocity drive medium mixes with a lower velocity pumped medium, the drive medium imparts a proportion of its energy to the pumped liquid. This process is continuous and is employed by jet (also known as ejector) pumps.
Airlifts use pressurized air or gas, which is introduced into a container of liquid with a lower specific weight, to move the fluid in a continuous manner.
Hydraulic rams utilize shock energy to pump liquids by suddenly stopping a moving column of liquid which results in part of the column being pumped.
Reciprocating pumps are characterized by high efficiency, often times as high as 90%. Yet these pumps operate at low speeds and are relatively large in size, leading to higher operating and capital costs. Centrifugal pumps generally have slightly lower efficiencies but maintain high speeds, smaller sizes and lower operating costs.
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Where are pumps used?
Pumps can be used in various settings and to pump different types of liquids. Pumps can be used in the following applications:
- Residential - Small pumps serve many purposes in your home. Potable water may enter your house after being pumped from the well or through the municipal system. Your heating and/or cooling units use pumps to move fluids and air through those systems. High rise apartment buildings must maintain constant water pressure to ensure residents on the top floors receive the same pressure as those at street level.
- Industrial - Fire pumps protect buildings and people during the work day. These systems stand primed and ready in case of fire. Pumps generate high water pressures for de-scaling and boiler feed applications. Some food and drink preparation requires pumps to insert the finished product into packaging.
- Agricultural - Pumps support food production and agriculture by irrigating arid lands and areas with little rainfall. Wells and vertical pumps offer life supporting water to live stock as well as people.
- Scientific and medical - Pumps are used in scientific arenas in the manufacturing of pharmaceuticals, toothpaste and medicine. Chemicals require special pumps, called process pumps, that seal the processed liquids from the outside environment preventing contamination. Blood and bodily fluids are moved with pumps and pumping systems when in surgical/medical settings.
- Marine - Naval ships and other marine going vessels use pumps to de-water bilges, provide boiler feed, stabilize ballast systems, support weaponry and to turn sea water into potable water. Pumps are a very important component of a marine vessel’s safety and security system.
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What materials should be used with certain fluids?
While ANSI B73.1 identifies several materials that the pumps should be available in, carefully review of the material selection must be made to ensure that the material used by a customer is compatible with and will withstand chemical/erosion attacks. There are many sources available on the recommended material for a given fluid. The following list should be considered for demonstration purposes only and not construed as the final requirements or recommendation for material applications. * Please check with the manufacturer for proper material requirements.
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What is a centrifugal pump?
A centrifugal pump is a kinetic energy type pump, versus a displacement pump. It imparts energy to a liquid by means of centrifugal force produced by a rotating unit, usually an impeller. Whereas, a displacement pump imparts energy using pistons, plungers, screws, vanes, or gears.
Centrifugal pumps are popular because of its design simplicity, high efficiency, wide range of capacity and head, smooth flow rate, low operating costs, varied sizes, and ease of operation and maintenance.
Centrifugal pumps can be segmented into groups based on design, application, service, etc. These pumps can belong to several different groups depending on their construction and application. The following examples demonstrate various segments:
- Industry standards:
-
ANSI pump - ASME B73.1 specifications
-
API pump - API 610 specifications
-
DIN pump - DIN 24256 specifications (European standard)
-
ISO pump - ISO 2858, 5199 specifications (European standard)
-
Nuclear pump - ASME specifications
- UL/FM fire pump - NFPA specifications
- Number of impeller/s in the pump:
-
single stage - pump has one impeller only; for low head (pressure) service.
-
two-stage - pump has two impellers in series; for medium head service.
- multi-stage - pump has three or more impellers in series; for high head service.
- Impeller suction:
-
single suction - pump with single suction impeller (suction eye on one side of the impeller only). This design is subject to higher axial thrust imbalances due to flow coming in on one side of impeller only.
- double suction - pump with double suction impeller (suction eyes on both sides). This design has lower NPSHR than single suction impeller. Even though this pump is hydraulically balanced, it is susceptible to uneven flow on both sides of the impeller if the suction piping is not installed properly.
Pumps with more than one impeller are labeled according to the design of the first stage impeller.
- Type of volute:
-
single volute - single lipped volute which is easy to cast and is used with low capacity pumps. Pumps with single volute design maintain higher radial loads.
-
double volute - pump volute has dual lips located 180 degrees apart resulting in balanced radial loads; most centrifugal pumps are of double volute design.
- triple volute - has lips (cut-waters) 120 degress apart for exceptional radial thrus balancing and excellent performance across the range of the pump. The LaBour model of process pumps features a triple volute design.
- Shaft position:
-
horizontal - pump with shaft in horizontal plane; popular due to ease of servicing and maintenance.
- vertical - pump with shaft in vertical plane; used when space is limited, or when pumping from a pit or sump to increase the available NPSH.
- Orientation of case-split:
-
horizontal split - pump case is split horizontally into two pieces - the upper case and lower case. This type of pump is usually limited to temperatures up to 450 degrees F. Hotter applications can cause shaft misalignment due to uneven thermal expansion.
- vertical or radial split - pump case is split vertically and the split parts are known as a case and cover. These pumps are used for high temperature applications due to even thermal expansion of the shaft.
- Shaft connection to driver:
-
close-coupled or integral design - Used for light duty service, the impeller is mounted on the driver shaft with a special design. The pump-driver assembly is very compact, lightweight, and inexpensive.
- long-coupled - the pump and driver shafts are connected using a flexible coupling. A spacer coupling can be used to allow the removal of seals without disturbing the secured ends of the pump.
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What are the affinity laws?
The affinity laws of centrifugal pump performance express the effect, in mathematical equations, on pump performance due to changes in certain application variables. The affinity law variables which affect pump performance are: 1. RPM revolutions per minute. 2. Impeller diameter.
- Changing the Pump Speed (RPM):
-
When the impeller diameter of a centrifugal pump is held constant the effect of changing the speed (RPM) of the pump is in accordance with the following relationships:
- Capacity: Q1/Q2 = N1/N2 Head: H1/H2 = (N1/N2)2 BHP: BHP1/BHP2 = (N1/N2)3
Where subscript number 1 indicates the initial conditions and subscript 2 at the new conditions., and:
- BHP = Brake Horsepower H = Head, Feet N = Pump Speed (RPM ) Q = Capacity (GPM )
- Changing the speed (RPM) of a pump affects the flow, head and input brake horsepower of the pump in different proportions. Changing the speed affects the flow through the pump by a proportion equal to the increase or decrease in speed. The pump head is changed by the square of the proportion of speed change, while the brake horsepower is changed by the cube of the proportion of speed change.
- Changing the Impeller Diameter: When the speed (RPM) of a centrifugal pump is held constant the effect of changing the impeller diameter (D) is as follows:
-
Capacity: Q1/Q2 = D1/D2 Head: H1/H2 = (D1/D2)2 BHP: BHP1/BHP2 = (D1/D2)3 Where subset number 1 shows performance at the initial impeller diameter and subset number 2 shows performance at the new impeller diameter, and:
- Q = Capacity (GPM ) H = Head, Feet BHP = Brake Horsepower
In other words, capacity varies in proportion to speed, head to the square of the speed and power consumed in proportion to the cube of the speed.
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What is viscosity and why is it important?
Viscosity is defined as the property of resistance to flow in a fluid. Viscosity of a fluid governs its flow. Changing the diameter of a pump impeller affects the flow, head and input brake horsepower of the pump in different proportions. Changing the impeller diameter affects the flow through the pump by a proportion equal to the increase or decrease in diameter. The pump head is changed by the square of the proportion of diameter change, while the brake horsepower is changed by the cube of the proportion of diameter change.
The viscosity of a fluid will determine the type of pump and configuration required. Viscosity is sometimes influenced by temperature. In general, the viscosity of a liquid changes as follows:
-
as temperatures increase, viscosity decreases.
- as temperature decreases, viscosity increases.
The opposite is true for gases.
Compared with other liquids, water maintains unusual characteristics, in that it maintains both a low viscosity and a high viscosity index. Variations in viscosity are, therefore, seldom important in pump design, configuration and performance when pumping water. However, other, more viscous fluids can present challenges to pump designers and specifiers. You should contact your pump manufacturer for advice when you need to pump a viscous fluid.
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What is the most amazing pump of all?
The human heart works tirelessly from the moment it begins beating until the moment it stops. In an average lifetime, the heart beats more than 2.5 billion times and pumps 50 million gallons of blood, without ever pausing to rest. The body holds about 6 quarts (5.6 liters) of blood and the heart circulates blood throughout the body three times per minute, so that's 2,000 gallons of blood pumped over 12,000 miles (19,000 km) every day! Like a pumping machine, the heart provides the power needed for life.
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What is water hammer?
When a liquid is suddenly brought to rest within a piping system, by either a quick closing valve or a check valve, a shock load surge results, better known as waterhammer. The higher the liquid velocity and the longer the piping, the greater the effect of waterhammer. Due to its destructive nature, broken pipes, valves, fittings and pumps can result from waterhammer. Since the shock waves travel throughout the system, the results of water hammer are found in different parts of a system to that which caused the problem.
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Should I perform preventative maintenance on my pumps?
Operation and maintenance requirements vary with the type of pump, the type of installation and the type of fluid being pumped. Operating and maintenance recommendations specified by the manufacturer should take precedence and should be implemented upon installation.
A comprehensive and progressive record should be kept for each pump detailing components and procedures checked, preventative maintenance (PM) performed, maintenance intervals, operations and operational issues. A maintenance routine should be planned with the service conditions in mind rather than as an arbitrary routine or only when the pump malfunctions. A reduction of flow or performance can be determined fairly early by continuous monitoring of the pump and its records.
Preventative routine maintenance extends the life of your pump and associated equipment. PM prevents unscheduled pump downtime while often times indicating more serious problems that will occur without the scheduled PM. Even though PM appears to add costs to pump operation in the short term, PM actually saves money through planned repairs and part replacements rather than extensive, costly and unexpected shut downs.
Maintenance of the pump consists of periodically replacing the wearing parts such as sleeves, neck rings, packing material, lubricating mediums and others. Impellers and diffusers usually have a long life provided the pumped liquid is not corrosive or harmful. Damage may be caused by a combination of factors - corrosion, erosion, electrolytic action, graphitization and corrosion fatigue of the shaft.
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What is a process pump?
As defined by Webster's Ninth Edition, process is "a series of actions or operations conducing to an end; especially a continuous operation or treatment especially in manufacturing." So a process pump would be a pump used in the manufacturing of an end product. ANSI B73.1 Process Pumps are widely used in the chemical, pulp & paper, pharmaceutical, petroleum and general industries. These pumps are specifically designed to stand up to the heavy duty usage demands of process systems and operations. In addition, they can be fitted with a huge range of options to suit the varying demands of process operators.
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What does ANSI stand for?
American National Standards Institute. Per the ANSI website: "the American National Standards Institute (www.ansi.org) has served in its capacity as administrator and coordinator of the United States private sector voluntary standardization system for more than 80 years. Founded in 1918 by five engineering societies and three government agencies, the Institute remains a private, nonprofit membership organization supported by a diverse constituency of private and public sector organizations. Throughout its history, the ANSI Federation has maintained as its primary goal the enhancement of global competitiveness of U.S. business and the American quality of life by promoting and facilitating voluntary consensus standards and conformity assessment systems and promoting their integrity. The Institute represents the interests of its nearly 1,000 company, organization, government agency, institutional and international members through its office in New York City, and its headquarters in Washington, D.C.
ANSI does not itself develop American National Standards (ANS); rather it facilitates development by establishing consensus among qualified groups. The Institute ensures that its guiding principles -- consensus, due process and openness -- are followed by the more than 175 distinct entities currently accredited under one of the Federation's three methods of accreditation (organization, committee or canvass). In 1999 alone the number of American National Standards increased by nearly 5.5% to a new total of 14,650 approved ANS. ANSI-accredited developers are committed to supporting the development of national and, in many cases international standards, addressing the critical trends of technological innovation, marketplace globalization and regulatory reform."
Ref. ANSI B73.1 1991, ANSI B73.1 "covers centrifugal pump of horizontal, end suction single stage, centerline discharge design." ANSI B73.1 ensures that pumps supplied by ALL manufactures to this standard are interchangeable with respect to: size and location of suction and discharge flanges, mounting dimensions, shaft coupling size, base plate dimensions and foundation bolt holes. ANSI B73.1 not only covers dimensional interchangeability but also certain pump features to aid in installation and maintenance of the pump.
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Why should I use ANSI approved pumps?
In using an ANSI B73.1 pump, a user is assured that they have purchased a pump that was designed to meet the criteria set forth by both pump users and manufactures for the chemical industry. Not only does an ANSI B73.1 standard ensure dimensional interchangeability, but it also ensures that the customer is using a pump designed for easy installation and increased MTBF (mean time before failure).
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What is a self-priming pump and how does it work?
Priming is the addition of liquid into the pump casing which aides in the evacuation of captured air via a vent while creating a liquid seal inside the casing.
A self-priming pump develops a vacuum sufficient enough for atmospheric pressure to force the liquid to flow through the suction pipe into the pump casing without priming the pump. A self-priming centrifugal pump is especially designed with a large chamber at its discharge side that acts both as an air separator that removes the air from the liquid, and a reservoir that holds residual liquid used for priming or re-priming the pump. The pump has to be primed during the initial start-up but re-priming is done automatically without outside attention. Although self-priming appears to be a desirable feature for centrifugal pumps the trade-off is that pump efficiency is slightly compromised due to certain design constraints. This type of pump is popular with intermittent service such as drainage, sewage, and similar applications, but is less popular in continuous service applications where optimum efficiency is desirable and re-priming is seldom needed due to continuous operation.
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Why is specific speed important?
Specific speed is defined as the speed in revolutions per minute at which an impeller would operate if reduced proportionately in size so as to deliver a unit of capacity against a unit of total head. The performance of a pump can be determined by stating its speed, the head it generates and the rate of discharge. Yet, when comparing one pump to another, it is more useful to use one term which will provide a general description of the pump's performance characteristics.
Different types of impellers and flow patterns can best be compared by means of the specific speed. A pump can be classified as axial flow, mixed flow, or radial flow depending where the impeller discharges the liquid. An axial flow pump discharges the liquid in the axial direction compared to the pump shaft centerline, a radial flow pump discharges the liquid in the radial direction and a mixed flow pump is one that is a combination between a radial and an axial flow pump design.
Pumps have to be selected with the right specific speed impellers in order to operate satisfactorily in the desired application. High specific speed impellers (as fitted in axial flow and mixed flow pumps) are for high flow and lower head applications (such as ditch drainage, water supply operations, lift stations, flood control etc.) where huge amounts of water need to be moved quickly. Low specific speed impellers (such as those installed in radial flow pumps) produce higher heads (pressures) at moderate flow rates, typically this type of pump is used to move liquids to much higher elevation levels (e.g. water well pumps, process pumps, boiler feed pumps etc).
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What is Best Efficiency Point?
Best Efficiency Point (BEP) is the point at which the impeller diameter provides the highest efficiency. BEP is an important parameter in that many parametric calculations are considered when calculating BEP, such as size, , specific speed, viscosity correction, and headrise to shut-off. Professional users prefer that pumps operate within 80% to 110% of BEP for optimum performance.
As you move away from the BEP, the shaft will deflect and the pump will experience some vibration. Check with the pump manufacturer to determine safe deviation levels from the BEP. Operating your pump away from BEP not only reduces efficiency but it will increase maintenance costs and may damage the pump. You can change the operating point by installing control valves, changing the impeller diameter, installing recirculation lines or selecting a different size of pump.
Sometimes "safety" factors can mean that the system is designed with a pump which is too large. Such a pump will operate to the left of the BEP as shown on the manufacturer's curve. Thus, the radial loads generated by the impeller and supported by the bearings will be greater (at BEP, most pumps are designed to generate no hydraulically generated radial loads). So when you are designing a system try not to use any safety factors, get as much information about the required flows and the "system curve" (the profile of the resistance of the system to increasing flows) to allow the pump maker to select exactly the right pump for your operation.
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What is cavitation?
Cavitation occurs when the pressure of the liquid in the suction line of the pump falls towards the "vapor pressure" (the pressure at which the liquid will turn from a liquid into a gas, or vaporize/boil). This occurs when the liquid is drawn into the "eye" of the pump impeller. If there is not enough suction head (head of liquid at the suction side of the pump) to allow the liquid to enter the pump without it falling to its vapor pressure, then vapor bubbles will appear. As the pump adds pressure energy to the fluid these bubbles rapidly and violently collapse back into a liquid state. These collapsing bubble carry a lot of energy and they can cause significant damage as they implode. This process of vapor bubble formation and implosion is called "cavitation".
Different fluids cavitate under different conditions of pressure and temperature and different pumps require different suction heads (known as "Net Positive Suction Head Required [NPSHr]) to operate without damaging cavitation.
Cavitation can accelerate the corrosion, pitting and erosion of pumps and pump parts, shortening the life of the pump. Prior to start up, priming will remove the air, ensuring that the available NPSH in the suction line is greater than that required by the pump (NPSHa > NPSHr).
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Pump and system definitions:
Rate of flow (capacity)
The rate of flow of a pump is the total volume throughput per unit of time at suction conditions. It assumes no entrained gases at the stated operating conditions.
Speed (RPM)
The number of revolutions of the shaft. Speed is expressed as revolutions per minute.
Head
The height of liquid column generated by the pump, usually expressed in feet of water. This is the difference between the "suction" head (the head of liquid before the pump) and the discharge head (the head of liquid after the pump) plus the "velocity" head, representing the additional kinetic energy added to the fluid by the pump.
Velocity head (Hv)
Measure of the energy related to dynamic flow conditions or specifically flow velocity. Velocity head is specifically defined as the height through which the fluid would have to fall to attain that specific velocity.
Hv = V2/2g or Hv (feet) = 0,0155 V2 (where V is in feet/sec.) or Hv (meters) = 0.05 V2 (where V is in meters/sec.)
Elevation head (Z)
The potential energy of the liquid due to its elevation relative to a datum level measured to the center of the pressure gauge or liquid surface.
Best efficiency point (BEP)
The rate of flow and head at which the pump efficiency is a maximum.
Specific gravity (SG)
The density or specific weight of the fluid relative to the density or specific weight of water. The specific weight of clean water is 1.0 at normal ambient temperatures.
Single plane balancing (also called static balancing)
Correcting impeller imbalances by adjusting (removing or adding) the weight in one correction plane only. This is normally accomplished using balance rails or by spinning on balancing machines.
Dynamic Balancing (also known as two plane balancing)
Correcting impeller imbalances by adjusting (removing or adding) the weight in two correction planes. This is normally accomplished by spinning on balancing machines.
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