Pump Cavitations

The formation and subsequent collapse of vapor filled cavities in a liquid due to dynamic action are called pump cavitation. The cavities may be bubbles, vapor filled pocket or a combination of both. The local pressure must be at or below the vapor pressure of the liquid for cavitation to begin, and the cavities must encounter a region of pressure higher than the vapor pressure in order to collapse. Dissolved gasses often are liberated shortly before vaporization begins. This may be an indication of impending cavitation, but true cavitation requires vaporization of the liquid. Boiling accomplished by the addition of heat or the reduction of static pressure without dynamic action of the liquid is arbitrarily extruded from the definition of cavitation.

When a liquid flows over a surface having convex curvature, the pressure near the surface is however and the flow tends to separate from the surface. Separation and cavitation are completely different phenomena. Without cavitation, a separated region contain turbulent eddying liquid at pressures higher than the vapor pressure. When the pressure is low enough, the separated region may contain a vapor pocket that fills from the downsteam end, collapses, and forms again many times each second. This causes noise and, if severe enough, vibration. Vapor filled bubbles usually are present and collapse very rapidly in any region where the pressure is above the vapor pressure.

Bubble that collapse on a solid boundary may cause severe mechanical damage. Shutler and Mesler photographed bubbles that distorted into toroidal shaped rings during collapse and produced ring shaped indentation in a soft metal boundary. The bubbles rebounded following the initial collapse and caused pitting of the boundary. Pressure on the order of 104 atm have been estimated during collapse of a bubble. All known material can be damaged by exposure to bubble collapse for a sufficiently long time. This is properly called cavitation erosion, or pitting.

It has been postulated that high temperature and chemical action may be present at bubbles collapse, but any damaging effects due to them appear to be secondary to the mechanical action. It seems possible that erosion by foreign material in the liquid and cavitation pitting may augment each other. Cavitation pitting, as measure by weight of of the boundary material removed per unit time, frequently increase with time. Cast iron and steel boundaries are particularly vulnerable. Controlled experiment have shown that cavitation pitting in metal such as aluminum, steel and stainless steel depends strongly on the velocity of the fluid in the disturbed flow past the surface.

Variable Flow Control

Rather than by speed control (changes in the number of displacements in a given period of time), flow may be varied by changing the volume displace par stroke. Variable stroke are generally limited to small mattering or “controlled volume” pumps. Although large capacity variable strokes pumps have been built, the complexity of the mechanism involved and the development of other effective and economical means of flow control have precluded their use.

Changing Delivery to the System
Another means of capacity control is not to vary the pump capacity but to alter the amount of pumpage delivered to the system. There are two popular means of during thin.

Suction valve uploading:
Suction valve unloaders cause the displacement to the ineffective during the stroke for which the unloaders are activated. They command either full or zero delivery while the pump is kept running. Thus, current inrush problem from frequent motor starts are avoided. Because discharge pressure is not developed when suction valve are unloaded, energy consumption is held to a minimum.

A pump valve is unloaded by mechanically preventing the valve plate from returning to its sent. If the suction valve is held away from its sent, liquid will ebb and flow through the valve from suction header to pump cylinder and from cylinder to header during the stroking of the plunger or piston.

The unloader mechanism does not move the valve. The valve is lifted off its seat by the pressure differential created by normal pump operation. In synchronically unloading the suction valve spring that had already been compressed and moves away from the valve while the spring are still compressed. This permit the valve to close when the plunger begin its discharge stroke.

For smooth transition between full to zero, the unloading and loading are accomplished in an immediate and identical pumping order of the plunger. It is the function of the synchronized suction valve unloader distributor to control unloading event in that proper and immediate sequence. Similarly can be drawn in the gasoline engine distributor in its function of ensuring the proper and immediate sequence of firing of the spark plug.

The distributor driven directly by the crankshaft on its position is positively indexed in the stroking of the plungers. It provided an electrical signal to a solenoid valve, which admit motive fluid pressure to the unloader chamber when the suction valve to the particular cylinder has been opened. Then in sequence, each of the remaining suction valve is retained so, in the first revolution of the pump after a control signals the distributor to initiate so, in the first revolution of the pump after a control signals the distributor to initiate unloading, all suction valves are unloaded and the pump capacity is reduced to zero. When the control against to signal for capacity, such suction valve is released, in sequence during a single revolution of the crank.

GBC Couplings

Couplings are use to joint of pumping system and the pump driver. Pump with high load need a high power should use a good perform coupling. Pump system should align all together and should be test by maintenance people. Design of GBC coupling has been optimized in order the power capacities balanced to the appropriate shaft diameter.

Load on the coupling to move the part like pump impeller can reduce by a flexible element, the deflection of which is a prime design consideration. Transient peak load also can reduce by this element.

Misalignment incidentally on the parallel angular and axial displacement of the connected shafts can be accommodated. Taper Bushes are fitted to the complete standard coupling range, bored to size flanges also available.

Selection using this kind of coupling is may either of two ways:

1. Where the prime mover is an electric motor and demand power or demand torque unknown, select the coupling using data from couple tables.
2. Where the driven machine demand power (or torque) and operating duty are known, select the coupling using the following procedure.
   
   • Service Factor
   • Design Power
   • Coupling Size
   • Bore Size