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PIPE SYSTEM CURVE

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Relationship between Head and Capacity 

The head capacity curve can be used to illustrate two i   i   f    if l 

HEAD

important properties of a centrifugal pump:

1. The  discharge  from  a centrifugal pump may be throttled without causing damage to the pump.

shut‐off 

Capacity

H

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Relationship between Head and Capacity 

The head capacity curve can be used to illustrate two i   i   f    if l 

HEAD

important properties of a centrifugal pump:

2. The  total head developed  is not  affected  by  the  specific gravity  of  the  liquid  being pumped.

WaterOil 

Capacity

H

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H

Efficiency

Hth

PFriction loss

H

Actual H‐Q characteristics of radial pumps 

Q

Shock loss

leakage loss

Design flow rate

oss

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H

pump characteristics operating point

Statichead

friction head

Q

Pump and System Performance3/4/2013 6

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H

pump characteristics operating point

friction h d

Statichead

friction head

friction head

Q

Effect of Throttling on System Performance3/4/2013 7

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Total dynamic head (Ht or TDH) The total dynamic head is the head against which the pump must

work. It is determined by adding the static suction and discharge head(with respect to signs), the frictional head losses, the velocity heads,( p g ), , y ,and the fitting and valve head losses.

2 KQHH statt

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Where K is the system constant that depends on the system components and theircharacteristics such as the pipe length, pipe diameter, coefficient of friction, andfittings coefficients.

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Pump efficiency is defined as the ration between the output power and input power which

is usually range from 20 to 85% and increase with the size of the pump.

Energy losses in a pump are volumetric, mechanical, and hydraulic.Volumetric losses are those of leakage through the small clearancesbetween wearing rings in the pump casing and the rotating element.

Mechanical losses are caused by mechanical friction in the stuffing boxesand bearings, by internal disc friction, and by fluid shear. Frictional andeddy losses within the flow passages account for the hydraulic losses.

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Pump Specific Speed

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The following figure shows a typical pump curve as furnished by a manufacturer. It is acomposite curve which tells at a glance what the pump will do at a given speed with variousimpeller diameters from maximum to minimum. Constant horsepower, efficiency, lines aresuperimposed over the various head curves. It is made up from individual test curves atvarious diameters

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Matching a Pump to a Piping System

Steady operating point:

Energy equation:

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For a specified impeller diameter and speed, a centrifugal pump has afixed and predictable performance curve.

The point where the pump operates on its curve is dependent upon thecharacteristics of the system in which it is operating commonly called thecharacteristics of the system in which it is operating, commonly called theSystem Head Curve or, the relationship between flow and hydraulic lossesin a system.

This representation is in a graphic form and, since friction losses vary as asquare of the flow rate, the system curve is parabolic in shape.

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Construction of system total-head curve

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No static head all friction, As the levels in the suction and dischargeare the same (Figure 11), there is no static head and therefore, the systemcurve starts at zero flow and zero head and its shape is determined solelyfrom pipeline losses.

The point of operation is at the intersection of the system head curveand the pump curve. The flow rate may be reduced by throttling valve.

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Positive static head, The parabolic shape of the system curve isdetermined by the friction losses through the system including allbends and valves. However, in this case, there is a positive static headinvolved. This static head does not affect the shape of the system curveor its "steepness", but it does dictate the head of the system curve atp , yzero flow rates.

The operating point is at the intersection of the system curve andpump curve. Again, the flow rate can be reduced by throttling thedischarge valve.

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Negative (Gravity) head, In the illustration below, a certain flow ratewill occur by gravity head alone. However, to obtain higher flows, apump is required to overcome the pipe friction losses in excess of "H"the head of the suction above the level of the discharge. In otherwords, the system curve is plotted exactly as for any other caseinvolving a static head and friction head, except the static head is nownegative. The system curve begins at a negative value and shows thelimited flow rate obtained by gravity alone. More capacity requiresextra work.

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Most lift – Little friction head, The system head curve in theillustration below starts at the static head "H" and zero flow. Since thefriction losses are relatively small (possibly due to the large diameterpipe), the system curve is "flat". In this case, the pump is required toovercome the comparatively large static head before it will deliver anyovercome the comparatively large static head before it will deliver anyflow at all.

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For a fixed set of conditions in a pumping system, there is just one totalhead for each flow rate. Consequently, a centrifugal pump operating at aconstant speed can deliver just one flow.

In practice, however, conditions in a system vary as a result of eithercontrollable or uncontrollable changes. Changes in the valve opening inthe pump discharge or bypass line, changes in the suction or dischargeliquid level, changes in the pressures at these levels, the aging of pipes,changes in the process, changes in the number of pumps pumping into acommon header, changes in the size, length, or number of pipes are allexamples of either controllable or uncontrollable system changesexamples of either controllable or uncontrollable system changes.

These changes in system conditions alter the shape of the system‐headcurve and, in turn, affect pump flow.

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Construction of system total‐head curve to determine gravity flow and centrifugal pump flow

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In a system where a pump is taking suction from one reservoir and fillinganother, the capacity of a centrifugal pump will decrease with an increasein static head. The system‐head curve is constructed by plotting thevariable system friction head versus flow for the piping.

To this is added the anticipated minimum and maximum static heads(difference in discharge and suction levels). The resulting two curves arethe total system heads for each condition.

The flow rate of the pump is the point of intersection of the pump head‐capacity curve with either one of the latter two system‐head curves or withany intermediate system‐head curve for other level conditions. A typicalhead versus flow curve for a varying static head system is shown in thefollowing Figure.

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Construction of system total‐head curves for a pumping system having variable static head

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If it is desired to maintain a constant pump flow for different static headconditions, the pump speed can be varied to adjust for an increase ordecrease in the total system head.

A typical variable‐speed centrifugal pump operating in a varying statichead system can have a constant flow, as shown in the following figure.

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Varying centrifugal pump speed to maintain constant flow for the different reservoir levels .

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Variable System Resistance

A valve or valves in the discharge line of a centrifugal pump alter thevariable frictional head portion of the total system‐head curve andconsequently the pump flow.

The following figure illustrates the use of a discharge valve to change thesystem head for the purpose of varying pump flow during a shopperformance test. The maximum flow is obtained with a completely openvalve, and the only resistance to flow is the friction in the piping, fittings,and flow meter.

A closed valve results in the pump’s operating at shutoff conditions andproduces maximum head. Any flow between maximum and shutoff canbe obtained by proper adjustment of the valve opening.

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Construction of system total‐head curves for various valve openings.

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BRANCH-LINE PUMPING SYSTEMS

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In some systems the liquid leaving the pump or pumps will divide into anetwork of pipes. If the pump is of the centrifugal type, the total pumpFlow is dependent on the combined system resistance. The total pumpflow and flow through each branch can be determined by the following

th dmethods.

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The following figure illustrates a pump and network of piping consistingof three parallel branches in series with common supply and returnheaders.

Junction points 1 and 2 need not be at the same elevation (provided theliquid density remains constant and the pipes flow full and free of vapor)because, in a closed‐loop system, the net change in elevation is zero.

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The following figure shows the system total‐head curves for eachbranch line and header considered independent of the others. Thesecurves are constructed for several flow rates by adding the frictionalresistances of the pipes, fittings, and head losses through theequipment serviced from point 1 to point 2. Curves A, B, C, and Dtherefore represent the variation in system resistance in feet (meters)versus flow through each branch and header.

If the valves are open in all branches, the total system resistance, totalpump flow, and individual branch flows are found by the followingmethod. First observe that (a) the total flow must be equal to the sumof the branch flows, (b) the head loss or pressure drop across eachbranch from junction I to junction 2 is identical, and (c) the flowdivides to produce these identical head losses.

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The following figure shows the construction of the curves required todetermine pump flow point X'.

Obviously the pump flow and branch A flow are the same. Note thath l fl l h h ll l lthe total flow at point X is less than when all valves are open as a resultof an increase in system head.

If all valves were open and the total flow were obtained by a positivedisplacement pump having a constant capacity curve F. closing valvesB and C would not change the flow.

The system head would, however, increase to point X” and the headwould be greater than for a centrifugal pump having curve E.

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BRANCHES IN OPEN-LOOPED SYSTEMS

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The following figure illustrates a pump supplying three branch lineswhich are open‐ended and terminate at different elevations.

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The following figure shows the system total‐head curve for each branchline and main supply line considered independently of each other.

These curves are constructed by starting at elevation heads ZA, ZB, ZC,and ZD at zero flow. To each of these heads is added the frictionalresistances in each line for several flow rates. Frictional losses from thesuction tank to junction 1 are included in curve D. Curves A, B, C, and Dtherefore represent the variation in system resistance in feet (meters)versus flow through each branch and supply line. Note that ZD isnegative because in lineD there is a decrease in elevation to point 1.

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CENTRIFUGAL PUMP CENTRIFUGAL PUMP BYPASS

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Bypass orifices around centrifugal pumps are often used to maintain aminimum flow recommended by the pump manufacturer because ofone or more of the following reasons:

h d/ Limit the temperature rise to prevent seizing and/or cavitation.

Reduce shaft and bearing loads.

Prevent excessive re‐circulation in the impeller and casing.

Prevent overloading of driver if pump power increases with decrease inflow.

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The following figure shows the separate system‐head curves for flow totank A and for flow to tank B. Curve E is the head‐capacitycharacteristics of the centrifugal pump.

Individual flow rates to each tank are shown as QA and QB. Therecommended minimum flow is QR, which is greater than QB by theamount shown. In order to maintain the minimum flow, a bypass orificewith necessary pipe, valves, and fittings is required to pass flow Qc attotal head H when the pump discharges to tank B only.

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The following figure, shows the construction necessary to determinethe required bypass head versus flow characteristics of the orifice andpipe. The bypass system‐head curve C includes the pipe, valve, andfitting losses from the pump connection between the suction tank

d th d f th b i i b l th ti t l l Thand the end of the bypass piping below the suction water level. Theselosses must be deducted from the total bypass losses to determine therequired orifice head.

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The following figure illustrates the resultant pump Flow with the bypassin operation. Curve C is added to curve 5 to obtain curve B +C bycombining flows through each system at the same heads. Note that flowthrough the piping from the suction tank to junction I is the total fromboth systems, Therefore, the combined system‐head curve B + C shouldtake this into consideration. Similarly, curve C is added to curve A toobtain curve A + C. Note that when the flow is directed to tank B withthe bypass open. pump flow is increased from QB to Qn and tank flowis decreased from OB to QB. When the Flow is directed to tank A withthe bypass open, pump flow is increased from QA to Qy aad Flow isdecreased from QA to Q~. if it is desired that there be no reduction inflow and/or that there be no waste of pumping power when flow is totank A the bypass can be closed either manually or automatically Iftank A, the bypass can be closed either manually or automatically. Ifpump flow is monitored, this measurement can be used to open, close,or modulate the bypass valve automatically to maintain desired flow.

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Design Conditions?‐What if Off The treatment of Off design operation of centrifugalg p gpumps starts with the recognition of the various eventsthat take place within such pumps as there is variedfrom BEP.

Once the events are understood, it is then necessary tof h ff h d f hquantify their effect on the pump, and from that

determine the allowable flow range of the pump for agiven set of operating conditions

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OPERATION AT HIGH FLOWS There are two circumstances that might lead to the operation of a

pump at flows in excess of its best efficiency or even of its design

point. The first of these occurs when a pump has been

oversized by specifying an excessive margin on total head.

Under this circumstance, the pump performance and its relation tothe system‐head curve might look as in the following figure.

The head‐capacity curve intersects the system‐head curve at acapacity much in excess of the real required flow.

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Pressure reduction in the external suction system of the pump

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Events at off‐design operation of low and medium specific speed centrifugal pumps.

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Flow path of fluid inside the pumpThe internal suction system is comprised of the pump’s suction nozzle andimpeller. Figures 5 and 6 depict the internal parts in detail.

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