Pumps/pumping total system head

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). 

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. 

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. 

If the static head is 20 metres, and the pipeline is 100 metres long, one would expect a total system head requirement in the order of 25 metres, so this should be well within the range of available centrifugal pumps, without resorting to 2-pole speed. 

Whereas the total dynamic head developed by a centrifugal, mixed-flow, or axial-flow pump is uniquely determined for any given flow by the speed at whicdi it rotates, positive-displacement pumps and those which approach positive displacement will ideally produce whatever head is impressed upon them by the system restrictions to flow. Actually with slippage neglecTed, the maximum head attainable is determined by the power available in the drive and the strength of the pump parts. An automatic relief valve set to open at a safe pressure... 

The Total Dynamic Head (TDH) of each and every pumping system is composed of up to four heads or pressures. Not all systems contain all four heads. Some contain less than four. They are ... 

The goal is to apply the formulas, the K values, and the pipe and connections friction values to determine the Hf and Hv, plus the Hs and Hp, and then the TDH, total dynamic head in the system. Then we can specify a pump for this application. 

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. 

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. 

We have seen how to determine the driving force (e.g., pumping requirement) for a given pipe size and specified flow as well as how to determine the proper pipe size for a given driving force (e.g., pump head) and specified flow. However, when we install a pipeline or piping system we are usually free to select both the best pipe and the best pump. The term best in this case refers to that combination of pipe and pump that will minimize the total system cost. 

System and pump total head against capacity curves. The intersection of the two curves defines the operating point... 

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... 

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... 

The total static head, as computed above, refers to the head on the pump without liquid flow. To determine the total head on the pump, the friction losses in the piping system during liquid flow must also be determined. 

Plot the low-friction, high-head system-head curve. The system-head curve for the vertical pump installation in Fig. 6.25 starts at the total static head, 15 ft (4.6 m), and zero flow. Compute the friction head for 15,000 gal/min (946.4 L/s) as follows ... 

The literature has used two names for this subject total dynamic head or total developed head (H or TDH). Let us derive TDH first by considering the system connected in parallel between points 1 and 2. Since the connection is parallel, the head losses across each of the pumps are equal and the head given to the fluid in each of the pumps are also equal. Thus, for our analysis, let us choose any pump such as the one with inlet g. From fluid mechanics, the energy equation between the points is... 

Example 4.1 It is desired to pump a wastewater to an elevation of 30 m above a sump. Friction losses and velocity at the discharge side of the pump system are estimated to be 20 m and 1.30 m/s, respectively. The operating drive is to be 1200 rpm. Suction friction loss is 1.03 m the diameter of the suction and discharge lines are 250 and 225 mm, respectively. The vertical distance from the sump pool level to the pump centerline is 2 m. (a) If the temperature is 20°C, has cavitation occurred (b) What are the inlet and outlet manometric heads (c) What are the inlet and outlet total dynamic heads From the values of the idh and odh, calculate TDH. 

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 flow rate and the total head is the sum of the heads developed by the individual pumps at the prevailing flow rate, and equal to the... 

---