Total head

When the energy terms are expressed as energy per unit weight, the term head is often used. Therefore, the total head, h, is equal to the elevation... 

Fig. 2. (a) Schematic of a tensiometer and (b) a hydraulic analogy showing the negative value of the total head, /. 

Closed Vessels. Liquid level can be measured by the static pressure method also at non atmospheric pressures. However, ia such cases the pressure above the Hquid must be subtracted from the total head measurement. Differential pressure measuriag instmments that measure only the difference ia pressure between the pressure tap at the bottom of the tank and the pressure ia the vapor space are used for this purpose. At each tap, the pressure detected equals the Hquid head pressure plus the vapor pressure above the Hquid. Siace the pressure above the Hquid is identical ia both cases, it cancels out. Therefore, the change ia differential pressure measured by the instmment is due only to the change ia head of Hquid ia the vessel. It is iadependent of the pressure within the tank and is an accurate measure of the level. 

Capacity. Pumps deHver a certain capacity, Q, sometimes referred to as flow, which can be measured directly by venturi, orifice plate (11), or magnetic meters (12) (see Flow measurement). The indirect way to determine capacity is often used. Whereas this method is less accurate than applying a flow meter, it often is the only method available in the field. The total head is measured and the capacity found from the pump head—capacity (H— curve (Fig. 2). More recently, sonic flow meters (13) have been used, which can be installed on the piping without the need for pipe disassembly. These meters are simple to use, but require relatively clean single-phase Hquid for reHable measurements. 

Suction Limitations of a Pump Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating pump, the vapor bubbles will be carried along to a point of higher pressure, where they suddenly collapse. This phenomenon is known as cavitation. Cavitation in a pump should be avoided, as it is accompanied by metal removal, vibration, reduced flow, loss in efficiency, and noise. When the absolute suction pressure is low, cavitation may occur in the pump inlet and damage result in the pump suction and on the impeller vanes near the inlet edges. To avoid this phenomenon, it is necessary to maintain a required net positive suction head (NPSH)r, which is the equivalent total head of liquid at the pump centerline less the vapor pressure p. Each pump manufacturer publishes curves relating (NPSH)r to capacity and speed for each pump. 

Practically, the NPSH required for operation without cavitation and vibration in the pump is somewhat greater than the theoretical. The actual (NPSH)r depends on the characteristics of the liquid, the total head, the pump speed, the capacity, and impeller design. Any suction condition which reduces (NPSH ) below that required to prevent cavitation at the desired capacity will produce an unsatisfactoiy installation and can lead to mechanical dimculty. 

FIG. 10-32 Variation of total head versus flow rate to overcome friction. 

FIG. 10-33 Variation of total head as a function of flow rate to overcome both friction and static head. 

FIG. 10-57 Simplified sketch of an air lift, showing submergence and total head. 

This pump (Figure 1-6) is raising the liquid from the level in the suction ve.s.sel to the level in the di.scharge vessel. This distance is called the Total Head. 

In this case, the pump must aspirate or lift the liquid up from the suction vessel into the pump and then push the liquid up into the discharge vessel. In this case the total head is the discharge head plus the suction lift. In all cases the total head is the work being performed by the pump. 

According to the Standards of the Hydraulic Institute, a suction lift test is performed on the pump and the pressure in the suction vessel is lowered to the point where the pump suffers a 3% loss in total head. This point is called the NPSHr of the pump. Some pump manufacturers perform a similar test by closing a suction valve on a test pump and other manufacturers lower the suction elevation. 

The definition of NPSHr may change in the future. A pump is in a definite state of cavitation with the 3% total head loss definition. Many pump users want a more explicit definition of NPSHr, and higher NPSHa safety margins to avoid inadequate NPSHa and cavitation altogether. 

A loss is usually expressed as a loss of heat or enthalpy. A eonvenient way to express them is in a nondimensional manner with referenee to the exit blade speed. The theoretieal total head available (i/ioi) is equal to the head available from the energy equation... 

For a positive power output, the blade tip speed and whirl veloeity eombination at the inlet must be greater than at the exit. From Equation (8-2), the flow must be radially inward so that eentrifugal effeets may be used. The veloeity exiting from a turbine is eonsidered to be unreeoverable therefore, the utilization faetor is defined as the ratio of the total head to the total head plus the absolute exit veloeity. 

Reaction is defined as the ratio of the static head converted in the impeller to the total head produced by the stage. Restating in a more philosophical sense, the object of the compressor stage is to increase pressure of the gas stream, and reaction gives the relationship of the divi-L.. fort between the impeller and the diffuser. 

Divide the total head per section by the allowable head per stage to develop the number of stages required in each section. 

In this case the outlet of the first fan is connected to the inlet of the second fan. With this arrangement, the total head developed at a given volume is equal to the sum of the total heads developed by the individual fans. Sometimes more than one fan is used in a duct arrangement. In the series connection (Fig. 9.49), the flow through the fans has the same volume flow, and the leaving flow from the first fan is connected to the suction side of the second fan. 

The head losses calculated using K coefficients by these figures can be added directly to the total friction head loss for the straight pipe portions of a system. When equivalent lengths are determined, they must be added to the straight pipe before determining the total head loss, as shown in the example calculations for a water system. 

Figures 3-36A, 3-36B, and 3-36C represent typical and actual performance curves showing discharge total head (head pressure at pump outlet connection for any fluid), required minimum water horsepow er (for pumping water), and capacity or pumping volume of the pump (for any fluid) for several impeller diameters that would fit the same case (housing). In addition the important NPSHr (net positive suction head required by the pump) charac-...

Total Head the pressure available at the discharge of a pump as a result of the change of mechanical input energy into kinetic and potential energy. This represents the total energy given to the liquid by the pump. Head, previously known as total dynamic head, is expressed as feet of fluid being pumped. 

The total head developed by a pump is composed of the difference between the static, pressure and velocity heads plus the friction entrance and exit head losses for... 

This applies because the total head for a pump is total discharge head a( + ), minus ( —) the [suction head, a( + )J, or [suction lift, a( —)]. 

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 ... 

Tbe intersection of tbe system curt e 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 maybe accomplished by adding resistance by partially clos-... 

For a system made up of the suction side as shown in Figure 3-41 (a) and the discharge as shown in Figure 3-42(a), the total head is... 

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