Centrifugal Pump BasicsTerms commonly used in the pumping industry

Centrifugal Pump BasicsCentrifugal, (Sen-trif-u-gal)Tending to fly from a centerThrown from a centerCentrifugal pump impeller does just thatWater drawn into impeller eye, accelerated down the blades and thrown against pump casing that directs flow out of discharge

Centrifugal Pump BasicsCentrifugal Pump

Centrifugal Pump BasicsCentrifugal Pump Curve

Centrifugal Pump BasicsCentrifugal Pump Curve

Centrifugal Pump Basics1 Gallon of water (U.S. @ 60DegF) weighs 8.33 lbs1 cubic foot of water contains 7.48 gallonsSpecific Gravity (Sp Gr) of a fluid is the relationship of the density (lbs/ft3) to that of water (62.3 lbs/ft3) WATER COLUMN1 psig2.31 ft11Basic Fluid Facts

Centrifugal Pump BasicsPump Shut-off Head 231ftSpecific Gravity

Centrifugal Pump Basics1 inch square column of air, the height of the earths atmosphere (From Sea Level) weights 14.7 lbs Air Column14.7 psia0 psig11BASIC FLUID FACTS

Centrifugal Pump BasicsBasic Centrifugal Pump Equations

Centrifugal Pump BasicsFriction Loss for Water in Commercial Steel Pipe (Schedule 40)System Resistance

Centrifugal Pump BasicsA Closed hydronic system is one that has only one interface with air or flexible membrane An Open hydronic system is one that has more than one interface with air or a flexible membraneOpen & Closed Systems

Centrifugal Pump BasicsLOADLOADLOADSOURCEEXPANSIONBUILDINGHEIGHTBalancingValve3-Way Control ValveIsolating ValveLOADClosed System

Centrifugal Pump BasicsClosed System Resistance Curve

Centrifugal Pump BasicsOpen System

Centrifugal Pump BasicsOpen System Resistance Curve

Centrifugal Pump BasicsClosed System with Minimum Maintained Pressure

Centrifugal Pump BasicsHEADSystem Head CurveCAPACITYFriction LossesMinimumMaintainedPressureClosed System Resistance Curve with Minimum Maintained Pressure

Centrifugal Pump BasicsTerms commonly used in the pumping industry

2Centrifugal action is demonstrated well when a dog shakes the skin rotates around the body and everyone is aware of the results. Liquid is flung from the animal with some force.Same effect is evident if you observe a party-goer with a drink in one hand. Someone taps them on the shoulder and they turn around too fast. Their drink is shared by the people around them.Each is a result of centrifugal action ...

3As liquid enters the pump impeller eye, the blades accelerate the liquid along the convex surfaces of the impeller blades, or vanes. The liquid is thrown from the impeller blade tips into the casing.

The casing is designed to accept the flow efficiently and channel the liquid to the casing discharge port, converting the liquid velocity to pressure energy.

The pump discharge pressure or pump head is determined ONLY by the impeller tip speed. Just as a sling shot with long strings can throw a stone further than a sling shot with short strings, if rotated at the same speed (a la David and Goliath); the larger the impeller diameter, the higher the pump head. The only other way to increase head is to increase the impeller tip speed by increasing pump speed (rpm)You will not obtain more head (or more flow) by installing a larger motor (At the same speed)4If a pump were sized for 231 feet shut off head, the impeller tip speed would be sufficient to raise (Throw) the water to a 231 feet height. There would be no flow at that height and the height would never increase. The discharge gauge on the pump would read 100 psig, for water. The pump is reacting similar to a discharge valve being closed while running. This pump would develop 100 psig and no more. If the pump continued to run, the pressure would not increase but the energy imparted into the water by the impeller would be wasted and the liquid would rapidly heat up to a temperature greater perhaps than the seal and pump is designed for. Catastrophic effects have resulted in pumps running at no flow conditions.If a graph were drawn with flow (gpm, for example) on the X axis and column of water head (In feet) on the Y axis. A point may be marked at 231 feet head at 0 gpm.If the pipe were now cut at a lower height, say 150 feet, and the rate the water overflowed was measured, perhaps by weighing the overflow, another point at that measured flow could be marked at 150 feet head on the Y axis.The pipe could be cut at several descending heights, the overflowing volumes measured, and the points made and joined together to form a pump performance curve.This is how pump manufactures assemble pump curves. Not by cutting pipe, obviously, but by getting similar effects by utilizing approved flow measuring and pressure reading devices and by taking exact power readings with torque shafts or with calibrated motors to establish pump efficiency.5Impellers are trimmed from full size and the pumping unit is retested at each size, until finally the fully assembled pump performance curves are completed. Pump impellers may be trimmed to any diameter in the range, but the normal tests and display is indicated in 1/2 to 1 trim increments, depending on the pump size.A vacuum may be drawn on the suction tank in the pump test assembly, enabling the suction capabilities or NPSH requirements to be established. (More information on NPSH is available in the another Pump Basics presentation on Cavitation)All the tested information, including flow and head curves, efficiency information, NPSH requirements and power requirements shown at standard available motor sizes, may be assembled into a published pump performance curve, as indicated on this slide. A typical pump curve shows flow along the bottom (x axis), head in linear measure on the vertical (y) axis. A user friendly curve will incorporate the hp lines and the efficiency lines directly on the performance curves, as on this slide and in Armstrongs catalog. It is easier to obtain the whole picture in this manner, at a glance.Note that the single NPSH curve, shown on published performance curves, is for the design (Maximum) impeller only. NPSH requirements will change as the impeller is trimmed. (May increase) 6From the figures on the slide it is obvious that the weight of a cubic foot (ft3) of water at 60 DegF is 8.33 lbs (lbs/US gallon) x 7.48 (Gallons in ft3) = 62.3 lbs.This is a stable measure against which all other liquid can now be measured.Many liquids have different densities. 20% solution of Calcium Chloride (Brine) for instance, which is commonly used in the refrigeration industry and pumped around ice-making piping in a great many arenas by Armstrong pumps, weighs in at about 75 lbs/ft3The sp gr of Brine is 75/62.3 = 1.2If the water column is heavier by 20%, it makes sense that it takes 20% more energy for the pump to throw that column of liquid to the same height. Power is increased by 20% so motor size must be increased accordingly.If 1 ft3 of water weighs 62.34 lbs, its easy to determine what the weight of a 1 in2 column would weight by dividing the top of the cube into square inches (12x12). The water column would weigh: 62.3/144 = 0.43 lbs/foot. To obtain 1 psig or the pounds per square inch registered on a pressure gauge, the column of water would need to be 1/0.43 = 2.31 feet high.Any water column of cold water may then be expressed in psig by dividing the height by 2.31It is not advisable to express pump head in psig (Or kPa). One can only convert pressure units directly to the feet (Or Meter) Y axis shown on the pump curve if the liquid density is known. 7As the liquid is, literally, thrown from the pump to a specific height (Pump head in feet of water, as an example), the pump would throw any liquid to the same height. There is some sacrifice with viscous liquids, but as this is uncommon in hvac applications well save that for another presentation.For liquids of different density, or specific gravity, the pressure effects will be different for the same pump. The illustration on this slide shows the same pump pumping water, gasoline and brine (The 20% Calcium Chloride Soln mentioned in the previous slide. Typically used for freezing water by circulating under the skating rink floor)Compared to water: Gasoline is 80% as heavy (0.8 sp gr) and Brine is 120% the weight of water (1.2 sp gr). Sp Gr always compares the weight of the liquids to the weight of water at 60DegF (62.34 lbs/ft3)The gauge reading for the gasoline will be 80% of the water reading; the brine 120%. The head, in feet, will be the same for all liquids.This is important when selecting pumps for liquids other than water.Power requirements vary directly with Sp Gr as:Power=Flow*head*sp gr*100/constant*pump efficiency (Constant is 3960 for flow and head expressed in gpm & ft as 1 hp =33,000 ft lbs. Head is already in feet, so we must get the gpm into lbs. Constant converts the ft lbs into hp by dividing the 33,000 by the weight of US gallon of water [8.33 lbs]. 33,000/8.33=3960)Pump heads do not change UNLESS THE REQUIREMENT IS EXPRESSED IN WEIGHT PER UNIT AREA (psig or kPa). If pressure units are specified, the pump head, for pump selection, is divided by the sp gr, as a heavier liquid will produce more psig/foot and a lighter one less. 8This may not be a fluid fact but is relates directly to many pumping related issues, particularly in open systems.On a different presentation (Cavitation), we mention that pumps dont suck; they merely lower the pressure of the liquid in the impeller eye and the liquid, if at a higher pressure, moves in to fill the void.In an open system, the liquid pressure on the pump is the static height of the liquid plus the atmospheric pressure acting on the surface of the liquid. Atmospheric pressure will not register on a regular pressure gauge (Atmospheric pressure is 0 psig) but does exert 14.7 psia (psi absolute above a prefect vacuum, if you like) on the surface of the liquid that will drive the liquid into the pump, in an open system, if the pressure in the pump falls below atmospheric pressure.See another Pump Basics presentation on Cavitation for more details on effects of varying liquids and temperatures on pump performance as it relates to atmospheric pressure.9The pump equations shown on this slide are part of the Pump Affinity Laws and in essence state that, for the same operating point on a pump performance curve, the flow changes directly as the impeller diameter or pump speed and the pump head capability changes to the square of the impeller diameter or speed change.Examples: (Note: Where speed is shown: Read speed or impeller diameter)If the speed of pump is reduced by 10% then the flow capability of the pump is reduced by 10% At the same time the head capability is reduced by the square of the speed change or (0.9*0.9) 81% of the original head.As we have seen from the power equations earlier: The power requirement varies directly to both the flow and head, (hp = gpm*hd*sp gr*100/3960*Eff) then if the flow changes directly and the head changes to the square, then the power changes to the power 3. That is: The power would be reduced by (0.9*0.9*0.9)73% of the original power.If the reduction is because of impeller diameter changes, the full power reduction may not be seen as the pump efficiency is likely to reduce with an impeller trim. 10Similarly, resistance to flow in piping and fittings varies to the square of the flow. That is: Friction loss in valves, piping in fact the hvac system as a whole, may be thought of as varying to the square of the flow change.Example: If the flow through a system is increased by 25%. The head to force the design flow through the same system would increase by (1.25*1.25) 56%. If the original flow through the system was 100 gpm at 100 ft head. The new conditions would be 125 gpm at 156 ft head.The power would be (1.25*1.25*1.25) 95% greater than the original power requirement.The chart in this slide is borrowed from the 1981 ASHRAE Fundamentals handbook, because of the Imperial units. A more recent handbook may be referenced for metric units.The ASHRAE general recommendations have been marked. Basically that hvac piping may be sized for 4 fps up to 2 diameter piping and sized on 4 feet friction loss per 100 feet of pipe for larger piping.One may follow the 500 gpm line vertical and see it intersect the 4 ft/100 ft line at just below 5 diameter piping. 5 diameter piping is the general recommendation for 500 gpm and would carry an 8 fps velocity.To prove the square law, one may follow the 1000 gpm up to the 5 diameter pipe. For the law to be proven, the friction loss needs to be a little below (1000/500)2*4=(2)2*4=4*4=16. QED! Very close! 11It is important to know the difference in open and closed systems as the knowledge impacts the pump selection.

By interface with air or flexible membrane we mean where the liquid is free to the open air or meets air through a flexible membrane such as a diaphragm or bladder in an expansion tank.12As mentioned on the previous slide: A closed system would, typically, have only the expansion tank as the interface with air (Or flexible membrane, as in a diaphragm or bladder tank). In a closed system the height of the system has no bearing on the pump head. The system is filled from some external source and the pump is sized only to circulate the design flow against the resistance of the piping. There is little resistance at close to 0 flow (90% reduction in flow would need [0.10*0.10]1% of the original head) so the system resistance curve begins at 0 head for 0 flow. The liquid in the system will not flow due to gravity (Unless there is a temperature change).The pump flow is based on the total flow of all the components in the system, added together. As the piping to each load branches off, the remainder of the flow must go to the remaining loads.The head on the other hand, must be sized for the worst system branch only. The resistance through the common piping (Piping carrying flow for more than one load) takes care of the cumulative flow issues but only the worse dedicated load piping is added to this to get the pump head.In the slide above, only the worst friction loss of the (4) load/valve combinations is added to the common piping losses to calculate the pump head.13The closed system friction loss curve starts at 0 flow and head and is determined by friction loss in the piping and fittings only.The curve is a quadratic curve as the head varies to the square of the flow change.14Example of open system is shown in this slide. An open system has more than one interface with air (Several expansion tanks may be installed in a closed system as this is the same as installing one large tank). This condenser water system has (2) interfaces with air:Surface of tower reservoir and return piping outlet.Friction loss in piping varies to the square of the flow change, so the system friction head varies as the square of the flow change.One thing that does not vary with the flow change is the static height.With no pump running the water in the system levels at the height of the tower reservoir level. So the static suction head (Height above pump) does not figure into the pump head calculations.Only the difference in fill height (Reservoir level) and Outlet height (Entrance to tower) is added to system friction loss to calculate pump head. In an open system, the pump fills the part of the system between the fill level and the final operating elevation. Assuming all (check) valving is ignored, the liquid in an open system will return to the fill level once the pump is stopped. The static difference in fill and operating levels is constant, regardless of the flow, so the system resistance curve starts at this point. 15An open system friction loss curve begins at the static height difference between the fill level (Liquid level when the system is first filled or the pump is stopped) and the highest operating level.That is: The static height that must be overcome by the pump only. This height must be added to the friction loss head to determine the pump head and is constant. It does not vary with flow.This could prove important to pump selections. Particularly parallel pump selections. See Parallel Pumping presentation for more parallel pumping selection information.12As mentioned on the previous slide: A closed system would, typically, have only the expansion tank as the interface with air (Or flexible membrane, as in a diaphragm or bladder tank). In a closed system the height of the system has no bearing on the pump head. The system is filled from some external source and the pump is sized only to circulate the design flow against the resistance of the piping. There is little resistance at close to 0 flow (90% reduction in flow would need [0.10*0.10]1% of the original head) so the system resistance curve begins at 0 head for 0 flow. The liquid in the system will not flow due to gravity (Unless there is a temperature change).The pump flow is based on the total flow of all the components in the system, added together. As the piping to each load branches off, the remainder of the flow must go to the remaining loads.The head on the other hand, must be sized for the worst system branch only. The resistance through the common piping (Piping carrying flow for more than one load) takes care of the cumulative flow issues but only the worse dedicated load piping is added to this to get the pump head.In the slide above, only the worst friction loss of the (4) load/valve combinations is added to the common piping losses to calculate the pump head.15An open system friction loss curve begins at the static height difference between the fill level (Liquid level when the system is first filled or the pump is stopped) and the highest operating level.That is: The static height that must be overcome by the pump only. This height must be added to the friction loss head to determine the pump head and is constant. It does not vary with flow.This could prove important to pump selections. Particularly parallel pump selections. See Parallel Pumping presentation for more parallel pumping selection information.