A pump creates a pressure difference between the outgoing and incoming flows; if this difference - known as prevalence - is proportional to the thrust that the pump imparts to the fluid and therefore to the energy used for this work, it is clear that the greater the flow rate, or the quantity of fluid on which to distribute the effort, the lower the head. The correlation between flow rate and head can be represented in a graph in which the heads are placed in ordinate and the abscissa in flow; keeping the pump rotation speed constant, for each flow rate value, the head has valueswhich, when combined, represent a curve, called the characteristic curve.
The ideal curve of a pump for a fire extinguishing system should be as flat as possible, i.e. a situation in which the head remains relatively insensitive to changes in flow rate. The reason is easy to understand when considering the fact that the supply request, during the operation of a fire extinguishing system, can vary significantly - for example, depending on how many sprinkler heads open - and this cannot happen at the expense of pressure and of the quantity of water that is supplied, which could result in an unpredictable action on the flames (the flow rate and pressure required by the fire system must be guaranteed even when the water supply is at its minimum level). Although the pump operates along all the points of the curve just described, there is an area in which the efficiency - that is, the ratio between the power made available and the power usefully employed to carry out the work of pushing the fluid - is maximum.
There, the hydraulic forces inside the pump balance. Imbalanced forces can cause the pump shaft to bend or flex. Some factors, in fact, such as the degree of opening of valves along the pipes, the increase in the roughness of the pipeline, the variation of the pressure in the tank or the level of the liquid in the case of an open tank, etc., can cause variations in the operating point. The pump rotation speed (the shape of the characteristic curve essentially depends on the type of impeller) and the deposits in the pipes can also affect the pump operating point.
The cause and effect of an imbalanced pump are also the cavitation phenomena - dependent on the energy that the liquid must have in the inlet section of the impeller, to continue its journey inside the impeller itself. In case of an overly low discharge pressure, bubbles formed by the gases dissolved in the water can be released, creating cavities in the liquid. These gases, if dragged through the pump, can reach areas of greater pressure, undergoing condensation/dissolving phenomena with energy release.
The gas particles implode and create micro-jets against the pump walls. The damage that phenomena of this type can cause is the fall in the characteristic curve of flow-rate and head of the pump which can result in non-priming of sufficient liquid. The vibrations transmitted to the pump and from this to the piping can exceed the structural resistance of the system (wear of the bearings and thrust bearing, with consequent breaking of the motor shaft and of the impeller) or can wear it (causing corrosive phenomena against blades and impeller discs, of the shaft liner and other parts of the pump). Such damage increases the inefficiency of the pump and the problems can self-propagate until the pump is no longer usable.
Water temperature, altitude and pressure drops created by suction butterfly valves, curves and flow direction changes are variables that affect the delivery flow characteristics and with respect to which an incorrect system calibration can be derived from the analysis of pump performance curves. For this type of analysis, a volumetric ultrasonic flow meter is used. This is a non-invasive, ultrasonic investigation technique. Each fluid has its own characteristic speed with which sound propagates inside it.
By applying transducers to the outside of the duct where the fluid passes, and noting their relative position, an ultrasound train is emitted which passes between the two points. The path that the sound takes depends on the position of the sound input points, the material and the geometric characteristics of the duct into which the fluid passes, the type and density of the fluid, all known variables upstream. The transit time also depends on the direction of the flow and its speed, variables that make it possible to derive the characteristic curve.