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System head demand is a method of evaluating the Total Dynamic Head requirements of a pumping system. This method evaluates the entire pumping system.
The first portion of the system to consider is the elevation head requirement. This would be the greatest increase in elevation from the datum plane of the pump or the least amount of decrease in elevation from the datum plane of the pump. Think of a center pivot sprinkler system being used in a field. There can be changes in elevation as the system is moved around the field. The largest increase of elevation needs to be considered so the output pressure is adequate to operate the sprinklers. When you are dealing with decreases in elevation, all of the discharge points are lower than the pump datum plane. The least amount of decrease should be used for the calculations. If the maximum decrease is used, any discharge point higher than that will have less output pressure available. The same consideration needs to occur for the suction side as well. Elevation head will be a constant of the job site. The elevation head is added as feet for this part of the system head demand equation.
The second portion of the system to consider is the required operating pressure. This will need to be the maximum pressure needed for the job. Pressure requirements need to be converted to feet of head and added with the elevation head for this part of the system head demand equation.
The final portion of the system to consider is the head loss due to friction throughout the piping system (this includes all pipe, fittings, and valves on the suction and discharge sides of the pump). This is called the equivalent length of pipe and must be determined. The effective length of pipe will be the actual length of pipe plus the equivalent length of pipe. The effective length of pipe would be divided by 100. The result would be multiplied by the friction loss per 100’ from the friction loss charts giving you the head loss in feet due to friction for the piping system. This head loss in feet would be added to the elevation head and the pressure requirement. The results would allow you to calculate the system head demand at different flow rates and to plot a system head demand curve.
By plotting this information a system head demand curve can be generated. By overlaying a pump performance curve on the system head demand curve we can predict where the pump will operate within the system as it is designed. Where the two lines intersect is where the pump should operate in this system. The system head demand will determine where the pump will operate on the performance curve (when installed in the system).
Pipe friction loss rates increase as flow rates (GPM) increase. Discharge piping systems with a flow velocity rate of less than 7 ft/sec. will typically perform satisfactorily. On the suction side the flow velocity rate needs to be less than 5 ft/sec for satisfactory performance. This has a profound impact on which pump is used.
Calculating and plotting the system head demand can be very helpful in avoiding problems with pump installation and operation.