2. Impeller Diameter and RPM
3. Pump Characteristic Performance curve
4. Brake Horsepower
5. Cavitation
6. Time, Temperature and Pressure effects
There are two main characteristics to be considered when determining the correct pump:
1. The right type of pump for the application and liquid.
2. The pump performance or characteristic requirements.
First, in order to select the right type of pump you need to know the fundamentals. Pumps can be broken into two major categories. The first category is the positive displacement (PD) pump in which the liquid is moved by physically displacing that liquid from a confined space. Positive displacement pumps come in several configurations such as vane, gear, screw, progressive cavity, Archimedes' screw, piston, plunger, and others. Some of the very first pumps used by man for primitive irrigation were positive displacement pumps. PumpU will discuss PD pumps second.
The next major category is the centrifugal pump. The centrifugal pump adds energy to the liquid by first increasing the velocity of the liquid (adding kinetic energy) and then converting the kinetic energy to potential energy by directing the liquid into a progressively increasing volume, which in turn slows the velocity. Centrifugal pumps come in several configurations such as axial flow or propeller pumps, mixed flow or turbine pumps, and Francis vane and radial vane pumps. The last two categories represent the type of pumps dealt with most.
A centrifugal pump is a device, which converts driver energy to kinetic energy in a liquid by accelerating it to the outer rim of a revolving device known as an impeller. The impeller, driven by the pump shaft adds the velocity component to the liquid by centrifugally casting the liquid away from the impeller vane tips. The key idea here is that the energy created is kinetic energy. The amount of energy given to the liquid corresponds to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.
The kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The pump volute (casing) which catches the liquid and slows it down creates the first resistance. The case is the pressure vessel required to confine the liquid at the pressures generated by the system and the pump itself. The volute is the cavity cast into the case that progressively slows the liquid after the liquid has exited at high velocity from the impeller. When the liquid slows down in the pump casing some of the kinetic energy is converted to pressure energy. It is the resistance to the pump's flow that is read on a pressure gauge attached to the discharge line.
Note!!! A pump does not create pressure, it only creates flow! Pressure is a measurement of the resistance to flow. In Newtonian fluids (non-viscous liquids like water or gasoline) we use the term head to measure the kinetic energy which a pump creates. Head is a measurement of the height of a liquid column which the pump could create resulting from the kinetic energy the pump gives to the liquid (imagine a pipe shooting a jet of water straight up into the air, the height the water goes up would be the head). The main reason for using head instead of pressure to measure a centrifugal pump's energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change. So we can always describe a pump's performance on any Newtonian fluid, whether it's heavy (sulfuric acid) or light (gasoline) by using the term head.
Remember, head is related to the velocity, which the liquid gains when going through the pump.
To convert head to pressure (psi) the following formula applies:
PSI = Head (FT) x S. G.
2.31
Newtonian liquids have specific gravities typically ranging from 0.5 (light, like light hydrocarbons) to 1.8 (heavy, like concentrated sulfuric acid). Water is a benchmark, having a specific gravity of 1.0.
Impeller Diameter and RPM
The two main factors in determining how much head a pump creates are:
The Impeller Diameter
The RPM of the Impeller (revolutions per minute,)
Impeller Diameter
If the speed (revolutions per minute) of the impeller remains the same then the larger the impeller diameter the higher the generated head. Note that as you increase the diameter of the impeller the tip speed at the outer edge of the impeller increases commensurately. However, the total energy imparted to the liquid as the diameter increases goes up by the square of the diameter increase. This can be understood by the fact that the liquid's energy is a function of its velocity and the velocity accelerates as the liquid passes through the impeller. A wider diameter impeller accelerates the liquid to a final exit velocity greater than the proportional increase in the diameter.
RMPs (Revolutions Per Minute)
As the number of revolutions per minute of an impeller increases, the velocity (and head) imparted to the liquid passing through it increases as well. As the impeller revolves more rapidly the rate of increase in the liquid velocity is higher than the rate of rpm increase. In other words, an impeller spinning at 2000 RPMs generates more than twice the head of the same impeller spinning at 1000 RPMs.
A version of the single centrifugal is the multiple stage pumps that either have volutes cast into the case as in the horizontally split design or can have a diffuser as part of the stationary components at the discharge of each impeller. The diffuser provides the same function as the volute. The diffuser provides an expanding cavity for the high velocity liquid to slow and build pressure. In the multiple stage pump, the volutes or diffuser for each stage is the beginning of the passage that redirect the liquid back to the center of the pump to enter the eye of the next stage.
Specific Speed (Ns) is used to relate the hydraulic performance of a centrifugal pump to the shape and physical proportions of its impeller. Most pumps in the industry today range from a specific speed of 3000 down to 500. The 3000 index indicates a characteristic of higher flow rates at lower differential heads. The index of 500 indicates a characteristic of high differential head at relatively low flow rates.
Pump Characteristic Performance curve
Pump performance
A pump's performance is shown in its characteristics performance curve where its capacity (GPM) is plotted against its total developed head (FT), efficiency (%), required input power (BHP), and NPSHr (FT) The pump curve also shows its speed (in RPM) and other information such as pump size and type, impeller size, etc.
Pump Speed in RPMs
Also in the notice that most curves indicates performance at the speed of 3450
RPM (a common electric motor speed in 60 Hz countries). All the information given in the curve is valid only for 3450 RPM. Generally speaking, curves that indicate RPM to be between 3400 and 3600 RPM are used for all two pole (3600 RPM nominal speed) motors applications.
Flow
The pump's flow range is shown along the bottom of the performance curve. Note that the pump, when operating at one speed, 3450 RPM, can provide various flows. The amount of flow varies with the amount of head generated. As a general rule with centrifugal pumps, an increase in flow causes a decrease in head.
Head
The left side of the performance curve indicates the amount of total head a pump is capable of generating.
Trimmed Impeller Curves
Notice that on some there are several curves, which slope generally downward as they move from left to right on the curve. These curves show that actual performance of the pump at various impeller diameters.
Duty Point
The point on the curve where the flow and head match the application's requirement is known as the duly point. A centrifugal pump always operates at the point on its performance curve where its head matches the resistance in the pipeline.
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
It is important to understand that a centrifugal pump is not limited to a single flow at a given speed. Its flow depends on the amount of resistance it encounters in the pipeline. To control the flow of a centrifugal pump it is normally necessary to restrict the discharge pipeline, usually with a valve, and thus set the flow at the desired rate. Note: Generally speaking, do not restrict a pump's flow by putting a valve on the suction line. This can cause damage to the pump!
Brake Horsepower
Brake Horsepower (BHP)
Along the bottom of the typical performance curve are brake horsepower lines sloping upward from left to right. These lines correspond to the performance curves above them (the top performance curve corresponds to the top BHP line and so on). These lines indicate the amount of driver horsepower, which is required at different points of the performance curve. The lines correspond to a BHP horsepower scale on the lower right hand corner of the page. In our example operating point at 120 gpm and 150 feet of head we observe that the corresponding BHP line equals about 6.8 horsepower. See the chart below.
End of Curve Horsepower
When sizing a motor driver to fit an application it is necessary to consider whether the pump will ever be required to operate at a flow higher than the duty point. The motor will need to be sized accordingly. If the pump may flow out to the end of the curve (if someone opens the restriction valve all the way, for example) it is important that the motor does not become overloaded as a result. Therefore it is normal practice to size the motor not for the duty point, but for the end of curve (EOC) horsepower requirements.
Cavitation
Cavitation
Cavitation is a phenomenon, which occurs when a liquid vaporizes as it passes through a pump and then quickly turns back into a liquid. The collapse of the vapor bubbles creates destructive microjets of liquid strong enough to damage the pump. Vaporization occurs if the pumped liquid drops below its vapor pressure. As a liquid accelerates through a pump it loses pressure (Bernoulli's Principle). If the pressure drops below the vapor pressure of the liquid then gas bubbles will instantly form as the liquid vaporizes. These bubbles just as quickly collapse, causing cavitation to occur. To prevent cavitation the pressure (more correctly the head) of the liquid entering the pump must be high enough to prevent the subsequent liquid pressure drop from reaching liquid vapor pressure. Cavitation is the vaporization of the liquid caused by the pressure dropping below the vapor pressure of the liquid at the flowing temperature.
NPSH
A minimum amount of suction pressure (head) is needed for a pump to operate without cavitating. The term used to describe this suction pressure is Net Positive Suction Head (NPSH). The amount of NPSH the pump requires to avoid cavitation is called NPSHr. The amount of NPSH available to the pump from the suction line is termed NPSHa. When selecting a pump it is necessary to see how much NPSH it requires at the duty point and make sure the NPSH available exceeds that amount. It is normal practice to have at least 2 feet of extra NPSH available at the suction flange to avoid any problems at the duty point. Also, if the pump were inadvertently operated at a flow higher than the rating point then a higher NPSH would be required to avoid cavitation.
NET POSITIVE SUCTION HEAD
Flooded suction:
NPSH = ha - hv + hs - hf
Suction lift:
NPSH = ha – hv - hs - hf
ha = the absolute pressure in feet of liquid on the surface of the supply liquid.
hv = the vapor pressure of the liquid being pumped expressed in feet of head.
Hs = the height in feet of the supply liquid surface with respect to the pump inlet.
Hf = suction line friction losses expressed in feet of head.
These calculations yield the available net positive suction head for a given system. This must be compared to the required net positive suction head NPSHr calculated by the manufacturer. NPSHa must exceed NPSHr.
Specific speed
Specific speed (NS) is calculated from:
NS = [N x FAQ^0.50] / [H^0.75]
N = pump speed, RPM
FAQ = capacity at best efficiency point (BEP) at maximum impeller diameter, GPM
H = head at BEP at maximum impeller diameter, FT
Specific speed identifies the type of pump according to its design and flow pattern. According to this criteria a pump can be classified as radial flow, mixed flow, or axial flow type. A radial flow pump is one where the impeller discharges the liquid in the radial direction from the pump shaft centerline, an axial flow pump discharges the liquid in the axial direction and a mixed flow pump is one that is a cross between a radial and an axial flow pump design.
Specific speed identifies the approximate acceptable ratio of the impeller eye diameter (D1) to the impeller maximum diameter (D2) in designing a good impeller.
NS: 500 to 5000; D1/D2 > 1.5 - radial flow pump
NS: 5000 to 10000; D1/D2 < 1.5 - mixed flow pump
NS:10000 to 15000; D1/D2 = 1 - axial flow pump
Time, Temperature and Pressure effects
Time, Temperature and Pressure effects.
A general rule of thumb for engineering plastics, thermoplastics, is that their mechanical properties are impacted, some rather dramatically. These factors affect the creep of the plastic, much differently than is the case with metals. Due to this fact and for certain processing benefits, many thermoplastics are reinforced with fiber (glass, carbon). Be sure to understand these relationships for the material of the product you are using.
UHMW PE (ultra high molecular weight polyethylene) is nearly chemically inert, has great impact strength, abrasion resistance (sliding), toughness, lubricity. It is a highbred of the standard PE.
PP (polypropylene) is a crystalline polymer, light weight (sg=0.91), excellent in chemical resistance with caustics, solvents and acids, and other organic chemicals, it is not recommended for use with oxidizing type acids, detergents, low-boiling point hydrocarbons, alcohol's and some chlorinated organic materials. Upper practical use temperature limit is 160 – 180 F, depending on the load. Natural PP is white.
PVDF (polyvinylidene fluoride) is a thermoplastic fluoropolymer and is similar to PTFE. It is has high chemical resistance like the fluoropolymer group with the exception of not be being suitable with strong acids, fuming acids, polar solvents, amines, ketones, and esters. It has a high tensile strength and heat distortion temp (HDT) of around 300F. It is sold under the trade names of Kynar (Atochem) and Solef (Solvay). PVDF is naturally an off white / cream color.
PFA (perfluoroalkoxy) is a thermoplastic fluoropolymer and is very similar to PTFE but lacks some of the chemical resistance and mechanical properties. However, due to its better processibility and near PTFE chemical resistance it is a PTFE alternative often. PFA is often used in roto-molding of metal parts.
PTFE (polytetrafluorethylene) is resistant to practically every known chemical or solvent and highest use temperature of all the thermoplastic fluoropolymers. PTFE has a low coefficient of friction. Due to its physical properties it is not readily processed and there for does not lend itself to wide use in injection molded products. Components tend to be machined only. It is sold under the trade name Teflon (DuPont). PTFE is bright white in color.
ETFE (ethylene tetrafluoroetheylene) is a tough plastic with good tensile strength, high impact resistance. It lacks the full chemical resistance of PTFE, but again is easier to process. It has us useful temperature limit around 350F. It has excellent resistance to solvents, caustics, chlorides, and most corrosive chemicals. It is sold under the trade name Tefzel (DuPont). It is a creamy-white color.
ECTFE (ethylene chlorotrifluoroethyhlene) is a tough plastic with good tensile strength, high impact resistance. It lacks the full chemical resistance of PTFE, but again is easier to process. It has excellent resistance to solvents, caustics, chlorides, and most corrosive chemicals. It has a useful temperature limit of around 300F. It is sold under the trade name Halar (Ausimont). It is a shade of white in color.
PVC (polyvinyl chloride) is the most popular of the vinyl thermoplastics, making its way into almost every day life in some form of a product. It is easily processed and joined together, stock shapes and extrusions are common. It has a useful temperature of around 140F, high impact strength and is amorphous. It has broad general chemical resistance. It is gray in color
CPVC (chlorinated PVC) is very similar to PVC but it has a higher operating temperature of around 212F.
Chemical resistance and impact properties are the same as PVC. It is gray to gray-blue in color.
Santoprene is a competitive diaphragm material. It is similar to EPDM.
Geolast is similar to that of Buna-N (nitrile). Non-aggressive fluids only
FLOW RATES IN PIPES - Normal to peak
1" = 16-30 GPM 3" = 120-270 GPM
1-1/4" = 30-35 GPM 4" = 250-500 GPM
1-1/2" = 40-70 GPM 6" = 500-1100 GPM
2" = 65-120 GPM 8" = 1000-2000 GPM
2-1/2" = 80-170 GPM 10" = 1500-3000 GPM
QUICK CALCULATE - FLOW RATE
Normal Flow Rate of a Pipe = D 2 x 20
(Diameter Squared x 20)
Twice the Diameter = 4 times the flow
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