System heads

The required shaft work for the pump, wx, when expressed as a head, is known as the system head, Ahsysteni. This is the head required from the pump to achieve the required flow through the pipe system. 

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FIG. 8-82 Installed flow characteristic as a function of percent of total system head allocated to the control valve (assuming constant head pump, no elevation head loss, and an R equal 30 equal-percentage inherent characteristic). 

Certain pump designs use an internal bypass orifice port to alter head-flow curve. High liquid velocities often erode the orifice, causing the pump to go farther out on the pump curve. The system head curve increase corrects the flow back up the curve. 

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. 

Note that the equivalent suction lift must be added to the total discharge head for the pump system to obtain the total system head. Keep in mind that the work the pump must accomplish is overcoming the suction losses (-f or —) plus the discharge losses, that is, + discharge loss (all) — (-f if head, or — iflift on suction losses, all). Thus, the suction lift becomes a ( — )( — ) or a ( + ) to obtain the total system head. Keep in mind that a vacuum condition on the suction of a pump never helps the pump, but in effect is a condition that the pump must work to overcome. 

It is important to recognize that a cenlrijugal pump will operate only along its performance curve [10, II]. External conditions will adjust themselves, or must be adjusted in order to obtain stable operation. Each pump operates within a system, and the conditions can be anticipated if each component part is properly examined. The system consists of the friction losses of the suction and the discharge piping plus the total static head from suction to final discharge point. Figure 3-51 represents a typical system head curve superimposed on the characteristic curve for a 10 by 8-inch pump with a 12-inch diameter impeller. 

Figure 3-51. System head curves for single pump installation.

Example 3-13 System Head Using Two Different Pipe Sizes in Same Line... 

Example 3-14 System Head for Branch Piping with Different Static Lifts... 

In some cases, friction losses are difficult to quantify. If the pumped liquid is delivered to an intermediate storage tank, the configuration of the tank s inlet determines if it adds to the system pressure. If the inlet is on or near the top, the tank will add no back pressure. However, if the inlet is below the normal liquid level, the total height of liquid above the inlet must be added to the total system head. 

In applications where the liquid is used directly by one or more system components, the contribution of these components to the total system head may be difficult to calculate. In some cases, the vendor s manual or the original design documentation will provide this information. If these data are not available, then the friction losses and back pressure need to be measured or an over-capacity pump selected for service based on a conservative estimate. 

The operating point is where the pump head [Eq. 10-38] matches the system head requirement [Eq. 10-33]. Thus if Eq. (10-33) for the system head is set equal to Eq. (10-38) for the pump head, the result can be solved for 1//P02 to give... 

Nose and Tail MT Fuzes (older as well as newer designs), consist of a body which contains the time element the expl train, and a head which contains the mechanical arming firing system. Head and body... 

The operating point may be found as the intersection of plots of the pump and system heads as functions of the flow rate. Or an equation may be fitted to the pump characteristic and then solved simultaneously with Eq. (7.16). Figure 7.17 has such plots, and Example 7.2 employs the algebraic method. 

When needed, greater head or greater capacity may be obtained by operating several pumps in series or parallel. In parallel operation, each pump develops the same head (equal to the system head), and the flow is the sum of the flows that each pump delivers at the common head. In series operation, each pump has the same... 

When using a draft tube, the back flow possibility in the center of the impeller requires the use of a large-diameter hub. This is not normally desirable in fluidfoil impellers used in open tanks. The system head for a draft tube circulator is a function primarily of the design of the entrance and exit of the draft tube, and considerable work has been... 

The shape of the system curve determines the saving potentials of using variable-speed pumps. All system head curves are parabolas, but they differ in steepness and in the ratio of their static head to friction drop. The value of variable-speed pumping increases as the system head curve becomes steeper. Therefore, in mostly friction systems, the savings will be greater. 

When two or more pumps operate in parallel, the combined head-capacity curve is obtained by adding up their individual capacities at each discharge head, as illustrated in Figure 2.122. The total capacity of the pump station is found at the intersection of the combined head-capacity curve with the system head curve. When constant-speed pumps are used in parallel, the added increments of pumping can be started and stopped automatically on the basis of flow. 

When two or more pumps operate in series, the total head-capacity curve is obtained by summing up the pump heads at each capacity. Series pumping is most effective when the system head curve is steep. Multiple pumps in... 

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