Mechanism of cavitation
The phenomenon of cavitation is a stepwise process as shown in Figure 2.
Figure 2: Phenomenon of Cavitation
Step One, Formation of bubbles inside the liquid being pumped.
The bubbles form inside the liquid when it vaporises i.e. phase change from liquid to vapor. But how does vaporization of the liquid occur during a pumping operation?
Vaporization of any liquid inside a closed container can occur if either pressure on the liquid surface decreases such that it becomes equal to or less than the liquid vapor pressure at the operating temperature, or the temperature of the liquid rises, raising the vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. For example, if water at room temperature (about 77 °F) is kept in a closed container and the system pressure is reduced to its vapor pressure (about 0.52 psia), the water quickly changes to a vapor. Also, if the operating pressure is to remain constant at about 0.52 psia and the temperature is allowed to rise above 77 °F, then the water quickly changes to a vapor.
Just like in a closed container, vaporization of the liquid can occur in centrifugal pumps when the local static pressure reduces below that of the vapor pressure of the liquid at the pumping temperature.
NOTE: The vaporisation accomplished by addition of heat or the reduction of static pressure without dynamic action of the liquid is excluded from the definition of cavitation. For the purposes of this article, only pressure variations that cause cavitation shall be explored. Temperature changes must be considered only when dealing with systems that introduce or remove heat from the fluid being pumped.
To understand vaporization, two important points to remember are:
1. We consider only the static pressure and not the total pressure when determining if the system pressure is less than or greater than the liquid vapor pressure. The total pressure is the sum of the static pressure and dynamic pressure (due to velocity).
2. The terms pressure and head have different meanings and they should not be confused. As a convention in this article, the term “pressure” shall be used to understand the concept of cavitation whereas the term “head” shall be used in equations.
Thus, the key concept is - vapor bubbles form due to vaporization of the liquid being pumped when the local static pressure at any point inside the pump becomes equal to or less than the vapor pressure of the liquid at the pumping temperature.
How does pressure reduction occur in a pump system?
The reduction in local static pressure at any point inside the pump can occur under two conditions:
1. The actual pressure drop in the external suction system is greater than that considered during design. As a result, the pressure available at pump suction is not sufficiently high enough to overcome the design pressure drop inside the pump.
2. The actual pressure drop inside the pump is greater than that considered during the pump design.
The mechanism of pressure reduction in the external and internal suction system of a pump system is explored next.
· Pressure reduction in the external suction system of the pump
A simple sketch of a pump ‘external suction system’ is shown in Figure 3.
Figure 3: External Suction System
Nomenclature used for Figure 3 r - Liquid density in lbm / ft3 G - Acceleration due to gravity in ft / s2 Psn - p refers to local static pressure (absolute). Subscript s refers to suction and subscript n refers to the point of measurement. The pressure at any point can be converted to the head term by division with the factor - r g ps1 - Static pressure (absolute) of the suction vessel in psia hps1 - Static pressure head i.e. absolute static pressure on the liquid surface in the suction vessel, converted to feet of head (ps1/ r g/gc). If the system is open, hps1 equals the atmospheric pressure head. vs1 - Liquid velocity on the surface in the suction vessel in ft/s hvs1 - Velocity head i.e. the energy of a liquid as a result of its motion at some velocity ‘vs1’. (v2s1 / 2g). It is the equivalent head in feet through which the liquid would have to fall to acquire the same velocity, or the head necessary to accelerate the liquid to velocity vs1. In a large suction vessel, the velocity head is practically zero and is typically ignored in calculations. hs - Static suction head. . . . i.e. head resulting from elevation of the liquid relative to the pump centerline. If the liquid level is above pump centerline, hS is positive. If the liquid level is below pump centerline, hS is negative. A negative hS condition is commonly referred to as “suction lift”. hfs - Friction head i.e. the head required to overcome the resistance to flow in the pipe, valves and fittings between points A and B, inclusive of the entrance losses at the point of connection of suction piping to the suction vessel (point A in Figure 1). The friction head is dependent upon the size, condition and type of pipe, number and type of fittings, valves, flow rate and the nature of the liquid. The friction head varies as the square of the average velocity of the flowing fluid. ps2 - Absolute static pressure at the suction flange in psia hps2 - Static pressure head at the suction flange i.e. absolute pressure of the liquid at the suction flange, converted to feet of head - ps2 / r g/gc vs2 - Velocity of the moving liquid at the suction flange in ft/s. The pump suction piping is sized such that the velocity at the suction remains low. hvs2 - Velocity head at suction flange i.e. the energy of a liquid as a result of its motion at average velocity ‘vs2’ equal to v2s2 / 2g. pv - Absolute vapor pressure of the liquid at operating temperature in psia. hpv - Vapor Pressure head i.e. absolute vapor pressure converted to feet of head (pv / r. g/gc). Hs - Total Suction Head available at the suction flange in ft. Note: As pressure is measured in absolute, total head is also in absolute. |
The pump takes suction from a vessel having a certain liquid level. The vessel can be pressurised (as shown in the Figure 3) or can be at atmospheric pressure or under vacuum.
Calculation of the Total Suction Head, Hs
The external suction system of the pump provides a certain amount of head at the suction flange. This is referred to as Total Suction Head (TSH), Hs.
TSH can be calculated by application of the energy balance. The incompressible liquid can have energy in the form of velocity, pressure or elevation. Energy in various forms is either added to or subtracted from the liquid as it passes through the suction piping. The head term in feet (or meters) is used as an expression of the energy of the liquid at any given point in the flow stream.
As shown in Figure 3, the total suction head, Hs, available at the suction flange is given by the equation,
Hs = hps1 + hvs1 + hs - hfs + hvs2 | (1) |
For an existing system, Hs can also be calculated from the pressure gauge reading at pump suction flange,
Hs = hps2 + hvs2 | (2) |
Equations 1 and 2 above include the velocity head terms hvs1 and hvs2, respectively.
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