Page 79 - Fluid mechanics, heat transfer, and mass transfer

PIPING, SEALS, AND VALVES
56

FIGURE 3.16 Different types of valves for solids flow.

. Name and illustrate different types of valves used for
solids flow.
& Typical solids flow valves are shown in Figure 3.16.
. What are the different problems associated with valves?
& A high DP through a valve creates a number of
problems, such as cavitation, hammer effects, flash-
ing, choked flow, high noise levels and vibration,
leading to erosion or cavitation damage, malfunction,
or poor performance.
Collapse of a bubble. (Source: Samson AG,
& Blockages and leakages. FIGURE 3.17
Frankfurt.)
& Seizure, that is, getting stuck.
. Describe cavitation in valves.
boundary surfaces roughened until large cavities are
& When liquid passes through partially closed valve,
formed.
static pressure in the regionofincreasing velocityand
& Figure 3.17 illustrates the collapse of a bubble.
in the wake of the closure member, drops and may
. What is cavitation index with respect to valves? What is
reach vapor pressure of the liquid in the low pressure
its significance?
region may vaporize and form vapor-filled cavities
that grow around minute gas bubbles and impurities
C ¼ðP d P v Þ=ðP u P d Þ; ð3:17Þ
in the liquid.
& When the liquid reaches a region of high static
where C is the cavitation index,P d is the pressure in pipe
pressure, the bubbles collapse suddenly. 12 pipe diameters downstream of the valve seat, P v is
& Impingement of opposing liquid elements in the the vapor pressure relative to atmospheric pressure
form of microjets on the collapsing vapor bubbles (negative), and P u is the pressure in pipe 3 pipe dia-
produces locally very high pressures that are short meters upstream of the valve seat.
lived. The microjets can reach velocities of & Cavitation index indicates the degree of cavitation or
50–100 m/s. the tendency of the valve to cavitate.
& For water, pressure surges with amplitudes between . How is cavitation minimized in valves?
2
750 and 1500 N/mm are reached with the jet veloc- & The development of cavitation can be minimized by
ities mentioned above. letting the pressure drop occur in stages.
& The surge lasts between several microseconds and
& Injection of compressed air immediately downstream
several milliseconds.
of the valve minimizes the formation of vapor bub-
& The effect of one single surge is limited to an area of bles by raising the ambient pressure.
only a few micrometers in diameter. & A disadvantage of this method is that the entrained air
& If the collapse occurs near the boundaries of thevalve may interfere with the reading of any downstream
body/pipe wall, local fatigue failures that can cause instrumentation.