Pump Cavitation and Air Entrainment at the inlet line Lab questions: 1) why is t
ID: 1849457 • Letter: P
Question
Pump Cavitation and Air Entrainment at the inlet line Lab questions: 1) why is the initial inlet line pressure below atmospheric pressure in both the cavitation and air entrainment tests? 2) Explain the reason the pressure drops in the pump inlet line as the cavitation test value is closed. 3) why must the inlet line pressure be seversl psi below normal operting pressure before cavitation begins? 4) what causes the noise when a pump cavitates? 5) Describe what can mechanically happen to a pump if it continues to be operated while cavitating. 6) ehat happens to the inlet line pressure as the air entrainment valve is opened? Explain why this occurs. 7) what causes noisy pump operation when air enters the inlet line? 8) List several factors that could cause pump cavitation. 9) list several factors that could cause air entrainment at the pump inlet line. 10) what procedures would you use to identify the cause of noisy pump operation that may be caused by cavitation or air entrainment at hte inlet line?Explanation / Answer
ANSWER:
The requirement forces the selection of turbomachinery as the only pumping machinery capable of delivering the large volume of fluid required with the desired head output yet remaining within the limitations of the weight and space requirements, In addition, the use of the impulse turbine.as the driving element demands that the pump machinery operate at high rotative speeds to eliminate or reduce heavy gear trains between the turbine and the pump. These high rotative speeds, coupled with a head rise in the order of twenty atmospheres require extreme local fluid velocities with respect to the impeller blade. From Bernoulli's Law, the static pressure of the uid must be reduced, and the reduction is generally to such a degree that the static pressure becomes less than the vapor pressure of fluid.In this state the fluid is thermodynamically unstable, and the condition is relieved by the formation of vapor bubbles on the impeller blading, This phenomenon, known as cavitation, has, as an immediate conse quence the reduction of through flow area within the pump causing a decrease in the volume output of the unit.
A further consequence of the cavitating flow is impeller damage caused by the collapse of the vapor bubbles. Cavity col- lapse is usually rapid, permitting the liquid boundary of the cavity to strike the impeller surface with a high velocity. The resultant impact stresses, though lower than the failure stress of the ma-terials commonly used in impeller construction, occurrepeatedly, and may rapidlylreach the fatigue limit of the material. To aggravate cavitation further, the missile utilizing rocket propulsion operates most efficiently in the outer reaches of the atmosphere where atmospheric pressure is negligible, further reducing the static pressure available at the pump entry. Pumps operating in a highly cavitating flow field are not peculiar to rocket engines; other typical applications are the con- densate pumps in steam power plants and aircraft fuel pumps. The problems encountered have been solved by extreme over- design of the pumps. In a missile, however, the penalty that must be paid in loss of missile performance for the decrease in loading density of the missile is too severe to perrnit extensive over-design of any component . One solution of the cavitation problem in missile fuel pump system has been the conversion of the fuel and oxidizer storage tanks to pressure vessels, and the use of an inert gas to pres- surize the tanks to maintain the local static pressure at the im- peller above the vapor pressure of the fluid.
The earth's atmosphere however, the decrease in loading density of the missile caused by the increased weight of the fuel storage tanks and the inert gas does not make this generally the optimum solution for rocket motors producing large thrusts for a long period of time. Another solution is the use of a booster pump or inducer capable of operating in a highly cavitating flow field to supercharge the inlet of the main pump sufficiently to eliminate cavitation. The inducer "may be mounted integrally with the main pump may be a separate unit in the flow system. The proper design of such an inducer is dependent upon a knowledge of the internal flow in the inducer when operating in a cavitating regime. This information is not yet generally available, Thus, it is of some importance to obtain knowledge of internal flow in hydrodynamic machinery when cavitation is present and to interpret this information in terms of the performance param- eters of the machinery, The performance of hydrodynamic machinery is usually given in terms of dimensionless head, flow and torque coefficients.From the principles of dynamic and kine- matic similarity, the non-cavitating performance of geometrically similar machines will be the same at corresponding flow rate coefficients if the Reynolds number is the same.
The mechanics of cavity formations _-is» very complex; however, if the Reynolds number is not too low, it has been found that the extenttand effect of cavitation is governed by a pressure coefficient introduced by Prandtl, This coefficient is called the cavitation number, and is defined as the difference between the undisturbed stream pressure and cavity pressure divided by the free stream dynamic pressure. In many experiments on hydrofoils and pumps, the per formance at a given angle of attack or flow coefficient has been found to be only a function of the cavitation number, water was the working fluid, and, since the ratio of the vapor to liquid density is negligible, the cavity pressure in experiment was considered constant. However, the cavity pressure is usually unknown, and is frequently assumed to be the vapor pressure of the fluid at the liquid bulk temperature. This assumption can introduce considerable error into "cavita- tion number" scaling. In the hydraulic machinery literature, the cavity pressure is assumed to be the vapor pressure; two additional similarity parameters (related to each other and to the cavitation number) are used in the interpretation of cavitation tests. These are the Thoma cavitation parameter and the "suction specific speed“ introduced by Bergernon.
However, these tests were conducted over a small rangeof rather high cavitation nurnber and throughout a limited speed range. Now it seems clear that the effects of air diffusion into the cavity and the evaporation of vapor into the cavity and the subsequent entrainment of the cavity gases into the fluid stream are important factors in determining the cavity pressure; Thus, the adequacy of the usually used cavitation similarity parameters is open to question, particularly for those situations in whichthe free stream pressure is only slightly above the vapor pres- sure. In the sections that follow, the objectives of the present work will be outlined, and the experimental program described. A simple model for cavitation "breakdown" is put forward and is compared with the results of the experiments.
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