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The fluid handled is very rarely a pure liquid (see also Gas separation). Both solid and gaseous substances are usually present in the fluid (see Two-phase flow). Gases can be present in a liquid in dissolved and undissolved form. In the dissolved form, the gas molecules are distributed throughout the liquid and attached to the liquid molecules as a result of physical forces; undissolved gas is present in a liquid in the form of bubbles.

In most cases, the amount of a particular type of dissolved gas cannot exceed the specific limit defined by the properties and condition of the liquid (excluding unstable supersaturated conditions), however, there is practically no upper limit to the amount of undissolved gas a liquid may contain. This is largely dependent on the nature of the fluid's motion. If the fluid is at rest, degassing usually commences rapidly (e. g. the bubbles rise to the surface of the liquid), and the dissolved gas content approaches its maximum value.
A measuring device invented by van Sylke is used to determine the gas content in a liquid. It separates the dissolved gas under vacuum and measures its volume.

Dissolved gases in fluids have an effect on the suction characteristics of centrifugal pumps in so far as a high gas content results in an increase in the required NPSH and thus leads to earlier instances of cavitation.

See Fig. 1 Gas content of fluid handled

Undissolved gas in bubble form influences the overall operating behaviour, and the extent to which gas can be entrained varies widely and continuously according to the design, size and mode of operation of the pump concerned. Fig. 2 Gas content in the fluid handled illustrates the changes in the characteristic curve H(Q), η(Q) of a non-clogging impeller pump (impeller) as a function of the percentage of air entrained.

Gas separation describes the process of gas separating from the fluid handled due to changes in pressure.
The solubility of air in water depends on the water's temperature and pressure (see Gas content of fluid handled). As solubility decreases in line with decreasing pressure and increasing temperature, air comes out of solution in the pumps' suction lines or in siphoning lines (see siphoning lines  (see Formation of air pockets). See Fig. 1 Gas separation

The gases which have come out of solution accumulate at the apex of the line and have to be removed to prevent separation of the water column.

Relative to the suction conditions, the average volume (Q_SL) removed per hour in the case of suction lines is:

X      Volume percent of air dissolved in water at the absolute pressure of 1 bar
Q     Flow rate of pump or in siphoning line in m³/h
p₁     Absolute water pressure in bar at the beginning of the evacuation process.
p₂    Absolute water pressure in bar at the end of the evacuation process
       (for pumps operating on suction lift, the absolute suction pressure at flow rate (Q)
       should be entered in the equation)

The following applies to siphoning lines: See Fig. 1 Siphoning line

e     Height difference of both water levels in m 

e_s   Height difference between apex and inlet side water level in m
v     Flow velocity in m/s
Σζ   Sum of resistance coefficients (see Pressure loss)
p_b   Atmospheric pressure in bar

If the apex lies close to the collecting tank, the following equation applies:

The volume of gas which comes out of solution is removed by an automatic evacuation system (see Venting). Gassy liquids such as waste water and sewage also liberate gas in the low pressure zones of the impeller which impairs the pumping performance.

Open impellers with large clearance gap widths or free-flow impellers (see Torque flow pump) have achieved the best results in separating and removing the gas from the pump casing.

Applications in the food, paper and pulp industries require gas to be pumped alongside the fluid handled. A pump should therefore be designed in such a way that gas separation and fluid transport can take place simultaneously. See Fig. 2 Gas separation

The pump is provided with a device which separates the gas collected at the impeller inlet and presses it to the impeller outlet. From there it is discharged via a separate pipe. This device enables the efficient pumping of fluids with a gas content of up to 60 %. See Fig. 3 Gas separation

Gaskets are sealing elements, used where the parts to be sealed are in contact with each other and do not perform a relative movement (static).

They are mostly employed in apparatus engineering and piping construction. Effective static sealing requires relatively high forces to be applied to the axially sealing gasket.

The gate valve is a valve which fully opens or closes a pipeline (see Valve).

Gauge pressure is the pressure, inside a pressure gauge.

Gear drives are ideal for achieving an optimal type of pump (in terms of design effort and operating data) where higher power is involved. In this context, the rotational speed of the pump must be selected independently of the rotational speed of the drive.

Depending on the transmission ratio required, either a single-stage or multi-stage type of gear unit can be used. See Figs. 1 and 2 Gear drive

If the pump's impellers (or the individual impeller) are attached directly to the gear drive output shaft, the latter and the driven machine form a compact set as with the close-coupled pump and geared pump. 

If high transmission ratios are required with the driven and the drive machines in a line, it is advisable to use planetary gear units, see Fig. 3 Gear drive, and star gears, see Fig. 4 Gear drive, because the rotationally symmetrical casing geometry is ideal for turbomachinery as well as electric motors. The same applies to similarly high gear reduction ratios as with pumps for use in low-lift pumping stations.

In the case of high gear circumferential speeds, a high degree of production accuracy is required from the perspective of noise emissions (see noise in pumps and systems).

A gear pump is a rotating positive displacement pump, whose meshing teeth of the two gear wheels simultaneously serve as displacement bodies and separating elements. This self-priming pump generates high static pressures, e.g. in lubricating oil systems.

A gear unit includes moving connections with revolving parts for transmitting or converting rotary motion (e.g. gear drive, hydraulic torque converter).

Geared pumps are centrifugal pumps with a gear drive fitted between the pump shaft and the drive. A special variant is the high-pressure gear pump, a high-speed pump which is used to generate large heads (up to 1000 m) at small flow rates and high efficiencies. If used with a normal 2 or 4 pole electric motor a transmission gear is required to achieve a rotational speed of approximately 15,000 rpm, which is unusually high for a centrifugal pump. Such pumps comprise of a pump casing (in-line version), an open impeller with radial vanes and inducer, two mechanical seals for sealing off the fluid handled and the gear oil, and a radial and axial bearing supporting the sun gear (central gear) of the transmission gear (power split transmission) arranged above.

The motor is firmly flanged to the gear housing and fitted with a coupling, at its shaft end. The coupling meshes with the annulus, which drives the planet gears of the gear drive Decisive factors for the wear resistance of gears are the manufacturing quality, the lubrication of bearings and gear wheels, and the correct lubricant maintenance for cooling and cleaning purposes. See Fig.1 Geared pump

The single-stage back-pull out design, enables the complete rotating assembly to be pulled out towards the top and minimises any downtimes required for assembly work.

The in-line design (see In-line pump) only requires a minimum foundation size and allows for straightforward installation. Gentle speed changes can be achieved by selecting sufficiently small gear steps from the modular system. Each casing design is usually assigned a variety of impeller diameters and a corresponding diffuser design.

Thanks to the variation options by changing the input speed, transmission ratio, inducer, diffuser size, impeller size and trimmed impeller diameter any required operating point in the characteristic curves selection chart depending on the suction-side inlet conditions can be attained economically for a high-pressure geared pump.

The geodetic head refers to the difference in elevation, or height, between a pump system's outlet and inlet cross-sections.

The geodetic height refers to the elevation (z) of a chosen point of a centrifugal pump or pump system with regard to a specified datum level. See Fig. 1 Geodetic height and Fig. 1 NPSH

The geodetic positive suction head (H_z.geo) is the difference between the liquid level in the inlet tank and the impeller's centreline. If the geodetic positive suction head chosen is sufficient, it will reduce the risk of cavitation. See Fig. 1 Geodetic positive suction head

The geodetic suction lift (H_s.geo) is the difference between the suction-side liquid level and the impeller's centreline.
Depending on the atmospheric pressure, water temperature and the location's altitude (measured in metres from the sea level), the theoretical maximum geodetic suction lift under ideal conditions is 10.33 m. Especially in the case of centrifugal pumps, however, cavitation will occur earlier at a suction lift of approx. 8 m due to both friction in the suction line and vapour formation. See Fig.1 Geodetic suction lift

The German Equipment and Product Safety Act [Geräte- und Produktsicherheitsgesetz] regulates how products are placed on the market and exhibited within the context of an independent commercial undertaking.

The gland packing is a contact-type sealing element. Compared with other types of shaft seals, they are adjustable and suitable for use with pressures and circumferential speeds higher than those radial shaft seal rings are used for (see also Shaft seal).

A globe valve is a valve used to control the flow of fluids.