The System Curve

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.

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. 

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). 

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... 

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. 

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... 

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%. 

Figure 12-86 shows the effect of changing speed on the compressor characteristic curve. The speed can be adjusted to meet a desired point on the system curve. This is the most popular form of control. 

Figure 12-143 shows the individual static pressure curve Pf and total pressure curve Pff If pressure losses between the two fans are neglected (and they should he very low for good design), the combined total pressure curve is twice the value of curve Pft, 2 Pff The new operating static pressure also should be twice the individual total pressure value minus the velocity pressure, 2 p — p for identical fans, the new operating static pressure is equal to 2 p + Pf. The operation of the series fans will be along the system resistance curve, and the resultant point of operation will be at the intersection of the system curve with the curve for (2 pa — Pfv). 

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-... 

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. 

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

To find the system curve the calculations were repeated for the velocities shown in the table below ... 

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... 

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