Pumps system curve

System Curves In addition to the pump design, the operational performance of a pump depends upon factors such as the downstream load characteristics, pipe friction, and valve performance. Typically, head and flow follow the following relationship ... 

Upon developing the. system eurve, the pump eurve will always intersect the. system curve, it doesn t matter about the pump design. We ll. see this later in Chapter 8. 

Nowadays,. some pump companies publish their lamily curves on the Internet. You can request a copy with an e-mail, phone call, fax, or letter. The curves and gauges are the difference between life and death of your pumps. The pump family curve goes hand in hand with the system curve, which we ll cover in the next chapter. 

Of the four elements of the TDH, the Hs and the Hp (elevation and pressure) exist whether the pump is running or not. The Hf and the Hv (frietion and velocity losses) can only exist when the pump is running. This being the ease, we can show the Hs and the Hp on the vertical line of the system curve at 0 gpm flow. The Hs is represented as a T on the graph below. For example, if the pump has to elevate the liquid 50 feet, the Hs is seen in Figure 8-2. 

Let s continue with system curves. Up to this point, all elevations, temperatures, pressures and resistances in the drawings and graphs of systems and tanks have been static. This is not reality. Let s now consider the dynamic system curve and how it coordinates with the pump curve. 

Let s say that the needs of the system require X tlovv. Now we seareh for a pump with a BKP at X gpm, at a head falling right between Hpi and Hp2 on the system curve. See the next graph (Figure 8-16). 

As mentioned earlier, the system curve with the clean and dirty filters should coincide within the sweet zone of the pump on its curve. (Figure 8-20 and Figure 8-21). 

On superimposing the curve of, i single pump over this system curve, we see that the system extremes are too wide for the pump to cover on its curve (Figure 8-22). 

Recause the system is designed for both pumps running together in parallel, the. system curve appears as shown in Figure 8-27. 

The same previously mentioned critical tips apply, plus one more. Upon observing the system curve, with the pump curves, it appears that the operator can operate any one pump, or any two, or any three or four pumps. Actually there is no option to run three pumps in this... 

Consider the following graph. Figure 9-14. Radial loading on the shaft rises if the pump is operated too far to the left or right of the be.st efficiency zone. Another interpretation of the. same concept is to. say that the maintenance and problems rise when the pump is operated away from its BEP. Many pumps have a rather narrow operational window. These pumps can be very efficient if they are correctly specified and operated. This is discussed completely in Chapters 7 and 8 Pump Curves and System Curves. 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

System curves The graphical representation of the resistance (static pressure) that occurs in a ventilation or pump system at different flow rates. 

Calibration curves for PS and PMMA are shown in Figs. 15.3-15.5. The slight differences in courses of calibration curves for PS in THF, chloroform, and toluene, as well as the curve for PMMA in THF (Fig. 15.3), can be explained by the flow rate variations for different pumping systems and by the hydrodynamic volume effects, respectively. The calibration curves for PMMA in mixed eluents THF/toluene are shown in Fig. 15.4. Three percent of THF in toluene assured a reasonable SEC elution of PMMA. However, more chloroform was needed to obtain a good SEC elution of PMMA in mixed eluent chloroform/toluene... 

Pumps are operated in parallel to divide the load between two (or more) smaller pumps rather than a single large one, or to provide additional capacity in a system on short nodce, or for many other related reasons. Figure 3-35 illustrates the operational curve of two identical pumps in parallel, each pump handling one half the capacity at the system head conditions. In the parallel arrangement of two or more pumps of the same or different characteristic curves, the capacities of each pump are added, at the head of the system, to obtain the delivery flow of the pump system. Each pump does not have to carry the same How but it will operate on its own characteristic curve, and must deliver the required head. At a common tie point on the discharge of all the pumps, the head will be the same for each pump, regardless of its flow. 

The flow loss in a system varies as (velocity) or (flow rate), so the total loss imposed by a system on a pump can be shown to vary with flow rate in the way shown in Figure 32.38(a). Figures 32.38(b) and 32.38(c) illustrate how system curves vary with the proportions of static and dynamic losses. 

If the constant speed characteristic of a pump is superimposed on a system curve, there is usually one intersection point, shown in Figure 32.39. If a flat system curve is being matched with a mixed or axial flow machine there can be flow instability, as illustrated in Figure 32.40, which is only corrected by changing pump speed or the static lift, or selecting a different pump. 

Figure 32.40 Combination of a flat system curve and an inflected pump curve...

The intersection of the system curve with the pump impeller characteristic curve is the operating point corresponding to the total head, H. This point will change only if the external system changes. This may be accomplished by adding resistance by partially clos-... 

In actual experiments we do not usually observe directly the desorbed amount, but rather the derived read-out quantities, as is the time dependence of the pressure in most cases. In a closed system, this pressure is obviously a monotonously increasing function of time. In a flow or pumped system, the pressure-time dependence can exert a maximum, which is a function of the maximum desorption rate, but need not necessarily occur at the same time due to the effect of the pumping speed S. If there are particles on the surface which require different activation energies Ed for their desorption, several maxima (peaks) appear on the time curve of the recorded quantity reflecting the desorption process (total or partial pressure, weight loss). Thereby, the so-called desorption spectrum arises. It is naturally advantageous to evaluate the required kinetic parameters of the desorption processes from the primarily registered read-out curves, particularly from their maxima which are the best defined points. 

The static pressure difference will be independent of the fluid flow-rate. The dynamic loss will increase as the flow-rate is increased. It will be roughly proportional to the flow-rate squared, see equation 5.3. The system curve, or operating line, is a plot of the total pressure head versus the liquid flow-rate. The operating point of a centrifugal pump can be found by plotting the system curve on the pump s characteristic curve, see Example 5.3. 

When selecting a centrifugal pump for a given duty, it is important to match the pump characteristic with system curve. The operating point should be as close as is practical to the point of maximum pump efficiency, allowing for the range of flow-rate over which the pump may be required to operate. 

Plot the system curve on the pump characteristic given in Figure A and determine the operating point and pump efficiency. 

Most pump manufacturers provide composite curves, such as those shown in Fig. 8-3, that show the operating range of various pumps. For each pump that provides the required flow rate and head, the individual pump characteristics (such as those shown in Fig. 8-2 and Appendix H) are then consulted. The intersection of the system curve with the pump characteristic curve for a given impeller determines the pump operating point. The impeller diameter is selected that will produce the required head (or greater at the specified flow rate). This is repeated for all possible pump, impeller, and speed combinations to determine the combination that results in the highest efficiency (i.e., least power requirement). Note that if the operating point (Hp, Q) does not fall exactly on one of the (impeller) curves, then the... 

The point where the flow rate of 275 gpm intersects the system curve in Fig. 8-2 (at 219 ft of head) falls between impeller diameters of l and 7 in., as indicated by the O on the line. Thus, the P in. diameter would be too small, so we would need the 7 in. diameter impeller. However, if the pump with this impeller is installed in the system, the operating point would move to the point indicated by the X in Fig. 8-2. This corresponds to a head of almost 250 ft and a flow rate of about 290 gpm (i.e., the excess head provided by the larger impeller results in a higher flow rate than desired, all other things being equal). 

One way to achieve the desired flow rate of 275 gpm would obviously be to close down on the valve until this value is achieved. This is equivalent to increasing the resistance (i.e., the loss coefficient) for the system, which will shift the system curve upward until it intersects the 7 in. impeller curve at the desired flow rate of 275 gpm. The pump will still provide 250 ft of head, but about 30 ft of this head is lost (dissipated) due to the additional... 

An oil with a 32.6° API gravity at 60°F is to be transferred from a storage tank to a process unit that is 10 ft above the tank, at a rate of 200 gpm. The piping system contains 200 ft of 3 in. sch 40 pipe, 25 90° screwed elbows, six stub-in tees used as elbows, two lift check valves, and four standard globe valves. From the pump performance curves in Appendix H, select the best pump to do this job. Specify the pump size, motor speed, impeller diameter, operating head and efficiency and the horsepower of the motor required to drive the pump. 

The range of flow rates possible with the control valve can be estimated by inserting the linear valve trim [i.e., Cv max/(T) = 64X] into Eq. (10-33) and calculating the system curves for the valve open, half open, and one-fourth open (X = 1, 0.5, 0.25). The intersection of these system curves with the pump curve shows that the operating range with this valve is approximately 150-450 gpm, as shown in Fig. 10-17. 

System total heads should be estimated as accurately as possible. Safety factors should never be added to these estimated total head values. This is illustrated by Figure 4.8. Suppose that OAi is the correct curve and that the centrifugal pump is required to operate at point A. Let a safety factor be added to the total head values to give a system curve OA2. On the basis of curve OA2, the manufacturer will supply a pump to operate at point A2. However, since the true system curve is OA, the pump will operate at point Ai. Not only is the capacity higher than that specified, but the pump motor may be overloaded. 

Draw a horizontal constant total head line in Figure 4.9 which intersects the two pump curves at capacities Q and Q2 respectively, and the system curve at capacity Qs. 

Two centrifugal pumps are connected in series in a given pumping system. Plot total head Ah against capacity Q pump and system curves and determine the operating points for... 

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