When the gears are disengaged, they create an enlarged […]
When the gears are disengaged, they create an enlarged volume on the suction side of the gear pump. Liquid flows into the gear tooth cavity and is captured by the gear teeth as it rotates. The liquid may also travel around the inside of the casing in a pocket between the tooth and the casing. This small flow does not pass between the gears. The engagement of the gears forces the liquid to pass through the discharge port under pressure.
In a gear pump, the running clearance between the gear face, the gear tip and the housing produces a relatively constant loss in any pumping amount at a fixed pressure. This means that the volumetric efficiency at low speeds and low flow rates may be poor, so the gear pump should be operated close to its maximum rated speed.
Although the losses caused by running gaps or "slips" increase with pressure, the losses are almost constant at different speeds and flow rates and vary linearly with changes in pressure. When operating at higher speeds and outputs, changes in slip with pressure typically have little effect on performance.
Many viscous liquid pumping applications require adjustment of the flow rate regardless of the discharge pressure, and the pressure-independent volumetric efficiency is also independent. Some gear pumps consist of pressure compensated sealing elements that reduce end face clearance and tip clearance to reduce internal leakage and increase volumetric efficiency. The design of the sealing element is usually based on theoretical predictions combined with practical experience. The geometry and design of the seal should be optimized in several stages. Operating experience with gear pumps using appropriately designed pressure compensated sealing elements has shown that when the critical pressure differential (eg, about 6-10 Barg) is exceeded, the ideal characteristics and almost no pressure independent volumetric efficiency are about 74% to 88%. .
In addition, pressure pulsations caused by unstable discharge of the gear pump should be measured to verify trouble-free operation of the gear pump. Pressure pulsations or fluctuations (inhalation or discharge) may be due to the interaction between pumping dynamics and the dynamic behavior of the intake and exhaust piping systems. The presence of pressure pulsations will result in fluctuating pressure differences and thus cause fluctuations to enter the inter-tooth space. If the minimum pressure pulsation point coincides with the expansion phase when the sidestream area is open, it may cause some failure or performance degradation.
In gear pumps, the friction torque and the resulting pump operation and the required power are affected by the liquid temperature as well as the operating pressure and pump speed. When the pressure difference is large, the friction torque first decreases and then increases as the pump speed increases. For large differential pressures, in the low pump speed region, the friction torque may become higher as the liquid temperature increases, but in the high pump speed region, the friction torque may have an opposite tendency.
Transient operation and cavitation
When the gear pump is operated at a relatively low suction pressure (for example, when the liquid comes from a tank with a lower liquid level), the pressure in the suction duct and the chamber is close to the vapor pressure, and cavitation occurs upstream of the gear meshing. region.
Another common operational problem is cavitation in the case of transient operation. A common cause of cavitation is that there is not enough flow into the expanding teeth. In many theoretical or operational studies on these topics, the interdental volume formed at the roots of the drive and driven gears should be considered. Compressible inflows and outflows of these volumes play an important role in cavitation and transient operation.
To investigate the effects of operating parameters such as pumping pressure on pump operation, in one case study, the gear pump was operated at 1,200 rpm and 3,400 rpm with an exhaust pressure of approximately 20 Barg. The suction of the pump comes from the atmospheric tank. When the pump was operated at 3,400 rpm, the suction port pressure dropped by 0.8 bar. In other words, at approximately 3,400 rpm, the average absolute suction pressure of the gear pump should be 0.2 bar absolute (Bara), which is relatively close to the pump limit and cavitation should be expected. At 1,500 rpm, the same situation indicates a small suction pressure drop of only 0.5 bar. This results in an average absolute suction pressure of approximately 0.5 Bara and good cavitation resistance.
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