System curves

The pressure-volume curve of a proposed centrifugal fan has a different shape. This fan curve must be drawn for the anticipated fan inlet density expected at its location in the system. The point of intersection of these two curves locates the flow rate and pressure rise at which the fan and system operate. This intersection represents a desirable operating combination for fan and system. The system curve intersects the fan curve in the middle of its maximum efficiency range and also at a point where the fan pressure produced varies smoothly but distinctly in a constant trend with flow rate which is desirable for flow control. 

Fig. 4. Selection of fan size where the soHd line represents a typical setting and the dashed lines the operating extremes, (a) Desirable sizing. The system resistance curve intersects the fan curve near its maximum efficiency. Changes in system resistance from a flow-control element also intersect the fan curve at desirable points for good flow control. The dashed curves also intersect system resistance curves at desirable locations, (b) A fan essentially too large for the system. The intersection of the system curve near the peak of the fan curve results in poor system flow control and perhaps surging.

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. 

The system curve, once again, is the visual graph of the four elements of the TDFd. The Hp is stacked on top of the Hs. The Hp changes with a change in temperature in the reactor. If the reactor were cold, the Hp would be minimum or zero. We ll call this Hpi. When the tank and fluid are heated, the Hp rises to its maximum. This is represented as Hp2 Oil the graph (Figure 8-15). 

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. 

The Nichols chart for the uncompensated and compensated system (curve (a)) is shown in Figure 6.34 (see also Appendix, fig634.m). From Figure 6.34, curve (a)... 

Figure 6.34 is generated usmgfig634.m and shows the Nichols Chart for the uncompensated system. Curve (a) is when the compensator gain K =, and curve (b) is when K = 0.537 (a gain reduction of 5.4dB). 

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

Considering Figure 3-39 as one situation which might apply to the system curve of Figure 3-51 the total head of this system is ... 

The system curves are the summation of the appropriate friction curves plus the static head, a, required to reach the base point. Note that the suction side friction is represented as a part of B-P-C in this example. It could be handled separately, but must be added in for any total... 

Below the loading region, the pressure drop can be read from appropriate system curves if they are a ailable, as Figure 9-20. However, for general use the data have been well correlated. Figures 9-21B-9-21F. The slope of most of the curves for pressure drop indicate a proportionality of 1,8 to 2.8 power of the superficial gas mass... 

Figure 12-84A illustrates a system with all line friction. Here, the operations would follow the system curve and operate at the intersection with the speed curve. For example, if the speed is cut 10%, the flow decreases 8%. Figure 12-84B shows a system with essentially constant back pressure. Following the operating system curve shows that a small speed cut back of say 10% results in a flow drop of 40%. 

At any constant or steady speed of operation of a compressor, the head-capacity and efficiency curves are characteristic of the impeller and casing design only. These curves that are determined by test can be translated to other reasonable speeds and conditions of operation of the wheel-casing combination of the affinity laws. The operation of the compressor must meet or establish the desired point on the head-capacity-system curve, which requires a combination of controls. 

---