Machinery's Handbook, 31st Edition
2800
PUMPS
For a tank at gauge pressure p , v o
p γ -- h +
=
C V 2 g
The time required to drain a tank can be calculated using the following equation if the tank has a constant cross-sectional area and is not replenished. Strategies to reduce turbu lence as a fluid exiting a tank into a mouthpiece or discharge tube include employing a re-entrant mouthpiece that protrudes into the tank, or using a convergent-divergent mouthpiece.
2 A t z 1 (
)
z 2 –
C d A o 2 g = -------------------
t
where t is time in seconds, A t is the cross-sectional area of the tank, A o is the cross-sec- tional area of the discharge orifice, z 1 and z 2 are the initial and final liquid heights in the tank, and C d is the coefficient of discharge for the type of orifice used (see Table 33). Table 33. Discharge and Velocity Coefficients Orifice Type C d C V Sharp Edged 0.62 0.98 Round Edged 0.98 0.98 Short Tube (Fluids separates from walls) 0.61 0.61 Sharp Tube (No fluid separation) 0.82 0.82 Sharp Tube with Rounded Entrance 0.97 0.98 Re-entrant Tube, L < 0.5 d 0.54 0.99 Re-entrant Tube, 2 ≤ L ≤ 3 0.72 0.72 Smooth, Well-Tapered Nozzle 0.98 0.99 Pumps.— Pumps are used to move fluid. More generally, pumps add pressure energy to fluid through a variety of means. Pumps fall into two general classifications: positive dis placement pumps and kinetic (dynamic) pumps. Some pumps are meant to be submerged completely in the fluid being pumped, while others are not. Positive Displacement Pumps: Positive displacement pumps move a fixed amount of fluid during each discharge stroke or revolution. Flow rate is directly related to the speed of the moving parts of the pump, and for many of these pumps flow pulsates as the cavity or cavities discharge and refill. These pumps are suitable for low to moderate flow rates, for moving highly viscous fluids, and generating very high pressures. Because of the high pressure capability of these pumps, they tend to be sturdily built and the use of a pressure relief valve is recommended in fluid circuits employing these pumps. Positive displace ment pumps can be further broken down into reciprocating and rotary types. The most common types of reciprocating pumps are: bladder, diaphragm, peristaltic, and piston or plunger pumps. Commonly encountered rotary types are: gear, lobe, vane, screw, and progressive cavity pumps. Metering pumps are positive displacement pumps that have a precisely controlled discharge volume. This volume is normally adjustable through stroke adjustment. Bladder pumps deliver pulsing flow, usually by inflating and deflating a bladder ar - ranged axially along a pumping chamber. Diaphragm pumps deliver pulsing flow from a single cavity, but can be arranged in multiples to provide smoother flow. Peristaltic pumps deliver pulsing flow, and have the advantage of completely containing the fluid within a section of tubing that runs through the pump mechanism. The pump uses a series of rollers to progressively pinch the tube and move the fluid ahead of the pinch point.
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