Cavitation in centrifugal pumps

Yedidiah, S., Effect of Scale and Si>eed on Cavitation in Centrifugal Pumps, ASME Symposium on Fluid Mechanics in the Petroleum Industry, Dec. 1975. 

To avoid cavitation in centrifugal pumps, the pressure of the fluid at all points within the pump must remain above saturation pressure. 

There are three causes of cavitation in centrifugal pumps change of phase, entrained air or gas, and turbulent flow. 

Stahl and Stepanoff, Thermodynamic Aspects of Cavitation in Centrifugal Pumps. 

The study of cavitation in cryogenic fluids and various hydrocarbons is being applied to research in centrifugal pumps, turbines, marine propellers, etc. The method of applying this to pumps is shown in the enclosed Figure 15, in which sensing strips are mounted on the pump rotor and connected through electrical leads to a miniaturized electronic control circuit located in the hollow pump drive shaft. The output of this circuit is used to vary the frequency of an F-M oscillator. The... 

With this type of instrumentation, it is possible to determine the point at which cavitation occurs in centrifugal pumps or other types of rotary equipment and to record this data with normal laboratory recording equipment. 

The apparatus consists of six components pump, heating system, test section, condensing systems, receiver, and instrumentation, A conventional centrifugal water pump is used to establish and maintain flow through the apparatus. The receiver is cooled with water to prevent cavitation in the pump. The flow rate through the pump is measured by means of a rotameter. 

According to Bernoulli s Law, when velocity goes up, pressure goes down. This was explained in Chapter 1. A centrifugal pump works by acceleration and imparting velocity to the liquid in the eye of the impeller. Under the right conditions, the liquid can boil or vaporize in the eye of the impeller. When this happens we say that the pump is suffering from vaporization cavitation. 

Cavitation of a centrifugal pump, or any pump, develops when there is insufficient NPSH for the liquid to flow into the inlet of the pump, allowing flashing or bubble formation in the suction system and entrance to the pump. Each pump design or family of dimensional features related to the inlet and impeller eye area and entrance pattern requires a specific minimum value of NPSH to operate satisfactorily without flashing, cavitating, and loss of suction flowt... 

For the centrifugal pump, two terms are in common use, the Thoma cavitation number cr and the suction specific speed Sn ... 

Garbers, A. W. and A. K Wasfi, Preventing Cavitation in High Energy Centrifugal Pumps, Hydrocarbon Processing, V. 69, No. 7, 1990. 

A centrifugal pump is to be used to extract water from a condenser in which the vacuum is 640 mm of mercury. At the rated discharge the net positive suction head must be at least 3 m above the cavitation vapour pressure of 710 mm mercury vacuum. If losses in the suction pipe account for a head of 1.5 m. what must be the least height of the liquid level in the condenser above the pump inlet ... 

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/veloc-ity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion. 

You want to pump water at 70°F from an open well, 200 ft deep, at a rate of 30 gpm through a 1 in. sch 40 pipe, using a centrifugal pump having an NPSH of 8 ft. What is the maximum distance above the water level in the well that the pump can be located without cavitating (Vapor pressure of water at 60°F = 18.7 mmHg.)... 

You must install a centrifugal pump to transfer a volatile liquid from a remote tank to a point in the plant 500 ft from the tank. To minimize the distance that the power line to the pump must be strung, it is desirable to locate the pump as close to the plant as possible. If the liquid has a vapor pressure of 20 psia, the pressure in the tank is 30 psia, the level in the tank is 30 ft above the pump inlet, and the required pump NPSH is 15 ft, what is the closest that the pump can be located to the plant without the possibility of cavitation The line is 2 in. sch 40, the flow rate is 100 gpm and the fluid properties are p = 45 lbm/ft3 and (j, = 5 cP. 

A centrifugal pump will operate normally at a point on its total head against capacity characteristic curve until the available NPSH falls below the required NPSH curve. Beyond this point, the total head generated by a centrifugal pump falls drastically as shown in Figure 4.6 as the pump begins to operate in cavitation conditions. 

For transferring water, a positive displacement, screw type pump is necessary centrifugal pumps will introduce cavitation when they have either a restricted imput or output stream. Even with a screw type pump, it is advisable to insert a bubble trap between the pump and the tube. Such a trap consists of a steel cylinder, perhaps a foot high, in which the water is pumped into near the top and extracted from the bottom. Bubbles will rise to the top where the air they carry can be removed by a valve on the cylinder. In addition, perhaps due to the wall thickness of 1/2 , we have found it necessary to pressurize the tube to 100 psi to obtain accurate data. Bath clear (VWR Scientific, Inc.) or iodine can be added to the water to control bacterial growth. 

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