PIPE SYSTEM CURVE3/4/2013 1 3/4/2013 2 1 Relationship between Head and Capacity The head capacity curve can be used to illustrate two important i properties i of f a centrifugal if l pump: shut‐off 1. The discharge from a centrifugal pump may be throttled without causing damage to the pump. H HEAD Capacity 3/4/2013 3 Relationship between Head and Capacity The head capacity curve can be used to illustrate two important i properties i of f a centrifugal if l pump: Water Oil 2. The total head developed is not affected by the specific gravity of the liquid being pumped. H HEAD Capacity 3/4/2013 4 2 Efficiency H Hth H P Design flow rate leakage loss Friction loss oss Shock loss Q 3/4/2013 Actual H‐Q characteristics of radial pumps 5 pump characteristics H operating point friction head Static head Q 3/4/2013 Pump and System Performance 6 3 pump characteristics H friction head h d operating point friction head Static head Q 3/4/2013 Effect of Throttling on System Performance 7 3/4/2013 8 4 . the frictional head losses. g ). 3/4/2013 10 5 . Ht HstatKQ 2 Where K is the system constant that depends on the system components and their characteristics such as the pipe length. . and fittings coefficients.3/4/2013 9 Total dynamic head (Ht or TDH) The total dynamic head is the head against which the pump must work. and the fitting and valve head losses. the velocity y heads. . coefficient of friction. pipe diameter. It is determined by adding the static suction and discharge head ( (with respect p to signs). and hydraulic. Frictional and eddy losses within the flow passages account for the hydraulic losses. 3/4/2013 11 Pump Specific Speed 6 . mechanical. Energy losses in a pump are volumetric.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. by internal disc friction. Volumetric losses are those of leakage through the small clearances between wearing rings in the pump casing and the rotating element. Mechanical losses are caused by mechanical friction in the stuffing boxes and bearings. and by fluid shear. lines are superimposed over the various head curves.The following figure shows a typical pump curve as furnished by a manufacturer. It is made up from individual test curves at various diameters 3/4/2013 13 Matching a Pump to a Piping System Steady operating point: Energy equation: 7 . Constant horsepower. It is a composite curve which tells at a glance what the pump will do at a given speed with various impeller diameters from maximum to minimum. efficiency. since friction losses vary as a square of the flow rate.For a specified impeller diameter and speed. 3/4/2013 15 Construction of system total-head curve 3/4/2013 16 8 . the relationship between flow and hydraulic losses in a system. This representation is in a graphic form and. operating commonly called the System Head Curve or. a centrifugal pump has a fixed and predictable performance curve. the system curve is parabolic in shape. The point where the pump operates on its curve is dependent upon the characteristics of the system in which it is operating. No static head all friction. the system curve starts at zero flow and zero head and its shape is determined solely from pipeline losses. 3/4/2013 17 3/4/2013 18 9 . As the levels in the suction and discharge are the same (Figure 11). there is no static head and therefore. The flow rate may be reduced by throttling valve. The point of operation is at the intersection of the system head curve and the pump curve. in this case. This static head does not affect the shape of the system curve or its "steepness". The parabolic shape of the system curve is determined by the friction losses through the system including all bends and valves. However. Positive static head. 3/4/2013 19 3/4/2013 20 10 . but it does dictate the head of the system y curve at zero flow rates. p . the flow rate can be reduced by throttling the discharge valve. there is a positive static head involved. Again. The operating point is at the intersection of the system curve and pump curve. In the illustration below. 3/4/2013 21 3/4/2013 22 11 . The system curve begins at a negative value and shows the limited flow rate obtained by gravity alone. to obtain higher flows. Negative (Gravity) head. a certain flow rate will occur by gravity head alone. the system curve is plotted exactly as for any other case involving a static head and friction head. However. More capacity requires extra work. a pump is required to overcome the pipe friction losses in excess of "H" the head of the suction above the level of the discharge. In other words. except the static head is now negative. the pump is required to overcome the comparatively large static head before it will deliver any flow at all. 3/4/2013 23 3/4/2013 24 12 . In this case. Most lift – Little friction head. Since the friction losses are relatively small (possibly due to the large diameter pipe). The system head curve in the illustration below starts at the static head "H" and zero flow. the system curve is "flat". the aging of pipes. however. changes in the pressures at these levels. in turn. changes in the size. 3/4/2013 25 Construction of system total‐head curve to determine gravity flow and centrifugal pump flow 3/4/2013 26 13 . changes These changes in system conditions alter the shape of the system‐head curve and. In practice. length. Consequently. Changes in the valve opening in the pump discharge or bypass line. changes in the process. For a fixed set of conditions in a pumping system. changes in the suction or discharge liquid level. conditions in a system vary as a result of either controllable or uncontrollable changes. there is just one total head for each flow rate. affect pump flow. changes in the number of pumps pumping into a common header. a centrifugal pump operating at a constant speed can deliver just one flow. or number of pipes are all examples of either controllable or uncontrollable system changes. 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 with any intermediate system‐head curve for other level conditions. The system‐head curve is constructed by plotting the variable system friction head versus flow for the piping. The resulting two curves are the total system heads for each condition. 3/4/2013 27 Construction of system total‐head curves for a pumping system having variable static head 3/4/2013 28 14 . To this is added the anticipated minimum and maximum static heads (difference in discharge and suction levels). In a system where a pump is taking suction from one reservoir and filling another. the capacity of a centrifugal pump will decrease with an increase in static head. A typical head versus flow curve for a varying static head system is shown in the following Figure. 3/4/2013 29 Varying centrifugal pump speed to maintain constant flow for the different reservoir levels . as shown in the following figure. If it is desired to maintain a constant pump flow for different static head conditions. 3/4/2013 30 15 . the pump speed can be varied to adjust for an increase or decrease in the total system head. A typical variable‐speed centrifugal pump operating in a varying static head system can have a constant flow. The maximum flow is obtained with a completely open valve. and flow meter. The following figure illustrates the use of a discharge valve to change the system head for the purpose of varying pump flow during a shop performance test. 3/4/2013 31 Construction of system total‐head curves for various valve openings. and the only resistance to flow is the friction in the piping. Any flow between maximum and shutoff can be obtained by proper adjustment of the valve opening.Variable System Resistance A valve or valves in the discharge line of a centrifugal pump alter the variable frictional head portion of the total system‐head curve and consequently the pump flow. 3/4/2013 32 16 . fittings. A closed valve results in the pump’s operating at shutoff conditions and produces maximum head. BRANCH-LINE PUMPING SYSTEMS 3/4/2013 33 In some systems the liquid leaving the pump or pumps will divide into a network of pipes. the total pump Flow is dependent on the combined system resistance. th d 3/4/2013 34 17 . If the pump is of the centrifugal type. The total pump flow and flow through each branch can be determined by the following methods. The following figure illustrates a pump and network of piping consisting of three parallel branches in series with common supply and return headers. the net change in elevation is zero. 3/4/2013 35 3/4/2013 36 18 . Junction points 1 and 2 need not be at the same elevation (provided the liquid density remains constant and the pipes flow full and free of vapor) because. in a closed‐loop system. and D therefore represent the variation in system resistance in feet (meters) versus flow through each branch and header. B. 3/4/2013 37 3/4/2013 38 19 . Curves A. and (c) the flow divides to produce these identical head losses. total pump flow. The following figure shows the system total‐head curves for each branch line and header considered independent of the others. the total system resistance. (b) the head loss or pressure drop across each branch from junction I to junction 2 is identical. fittings. and head losses through the equipment serviced from point 1 to point 2. If the valves are open in all branches. C. These curves are constructed for several flow rates by adding the frictional resistances of the pipes. and individual branch flows are found by the following method. First observe that (a) the total flow must be equal to the sum of the branch flows. 3/4/2013 39 3/4/2013 40 20 . however. The following figure shows the construction of the curves required to determine pump flow point X'. increase to point X” and the head would be greater than for a centrifugal pump having curve E. Note that the h total l flow fl at point X is less l than h when h all ll valves l are open as a result l of an increase in system head. The system head would. closing valves B and C would not change the flow. Obviously the pump flow and branch A flow are the same. If all valves were open and the total flow were obtained by a positive displacement pump having a constant capacity curve F. 3/4/2013 42 21 .BRANCHES IN OPEN-LOOPED SYSTEMS 3/4/2013 41 The following figure illustrates a pump supplying three branch lines which are open‐ended and terminate at different elevations. ZC. Note that ZD is negative because in line D there is a decrease in elevation to point 1. B.3/4/2013 43 The following figure shows the system total‐head curve for each branch line and main supply line considered independently of each other. C. 3/4/2013 44 22 . To each of these heads is added the frictional resistances in each line for several flow rates. ZB. and D therefore represent the variation in system resistance in feet (meters) versus flow through each branch and supply line. These curves are constructed by starting at elevation heads ZA. and ZD at zero flow. Curves A. Frictional losses from the suction tank to junction 1 are included in curve D. 3/4/2013 45 CENTRIFUGAL PUMP BYPASS 3/4/2013 46 23 . Prevent excessive re‐circulation in the impeller and casing. 3/4/2013 47 3/4/2013 48 24 . Prevent overloading of driver if pump power increases with decrease in flow. Bypass orifices around centrifugal pumps are often used to maintain a minimum flow recommended by the pump manufacturer because of one or more of the following reasons: Limit the h temperature rise to prevent seizing and/or d/ cavitation. Reduce shaft and bearing loads. The recommended minimum flow is QR. Curve E is the head‐capacity characteristics of the centrifugal pump. a bypass orifice with necessary pipe. In order to maintain the minimum flow. and fittings is required to pass flow Qc at total head H when the pump discharges to tank B only. Individual flow rates to each tank are shown as QA and QB. which is greater than QB by the amount shown. The following figure shows the separate system‐head curves for flow to tank A and for flow to tank B. 3/4/2013 49 3/4/2013 50 25 . valves. shows the construction necessary to determine the required bypass head versus flow characteristics of the orifice and pipe. 3/4/2013 51 3/4/2013 52 26 . and fitting losses from the pump connection between the suction tank and d the th end d of f the th bypass b piping i i below b l the th suction ti water t level. The bypass system‐head curve C includes the pipe. valve. The following figure. l l These Th losses must be deducted from the total bypass losses to determine the required orifice head. The following figure illustrates the resultant pump Flow with the bypass in operation. or modulate the bypass valve automatically to maintain desired flow. Therefore. pump flow is increased from QA to Qy aad Flow is decreased from QA to Q~. curve C is added to curve A to obtain curve A + C. if it is desired that there be no reduction in flow and/or that there be no waste of pumping power when flow is to tank A. the combined system‐head curve B + C should take this into consideration. automatically If pump flow is monitored. Curve C is added to curve 5 to obtain curve B +C by combining flows through each system at the same heads. When the Flow is directed to tank A with the bypass open. Similarly. Note that flow through the piping from the suction tank to junction I is the total from both systems. this measurement can be used to open. Note that when the flow is directed to tank B with the bypass open. A the bypass can be closed either manually or automatically. pump flow is increased from QB to Qn and tank flow is decreased from OB to QB. close. 3/4/2013 53 3/4/2013 54 27 . 3/4/2013 55 3/4/2013 56 28 . 3/4/2013 57 3/4/2013 58 29 . and d from f that h determine the allowable flow range of the pump for a given set of operating conditions 3/4/2013 59 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. it is then necessary to quantify f their h effect ff on the h pump. 3/4/2013 60 30 . Once the events are understood. Under this circumstance. The head‐capacity curve intersects the system‐head curve at a capacity much in excess of the real required flow. The first of these occurs when a pump has been oversized by specifying an excessive margin on total head. the pump performance and its relation to the system‐head curve might look as in the following figure.What if Off‐ Design Conditions? The treatment of Off design g operation p of centrifugal g pumps starts with the recognition of the various events that take place within such pumps as there is varied from BEP. 3/4/2013 62 31 .Pressure reduction in the external suction system of the pump 3/4/2013 61 Events at off‐design operation of low and medium specific speed centrifugal pumps. 3/4/2013 63 Flow path of fluid inside the pump The internal suction system is comprised of the pump’s suction nozzle and impeller. Figures 5 and 6 depict the internal parts in detail. 3/4/2013 64 32 .
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