Fluid Mechanics

Topics: Fluid dynamics, Pump, Pressure Pages: 9 (836 words) Published: November 11, 2014
1. Using diagrams and/or graphs, explain the following terms:

a. Pressure Head
pressure head [′presh·ər ‚hed]
(fluid mechanics)
Also known as head.
The height of a column of fluid necessary to develop a specific pressure. The pressure of water at a given point in a pipe arising from the pressure in it.

b. Total Discharge Head
Total discharge head refers to the actual physical difference in height between the liquid level in the pit and the highest point of the discharge pipe or water level in the outlet.

c. NPSH
Net Positive Suction Head (NPSH). The measurement of liquid pressure at the pump end of the suction system, including the design of the pump.

d. Suction Lift

Pump Performance Curve
The pump characteristic is normally described graphically by the manufacturer as a pump performance curve. The pump curve describes the relation between flow rate and head for the actual pump. Other important information for proper pump selection is also included – efficiency curves, NPSHr curve, pump curves for several impeller diameters and different speeds, and power consumption.

Increasing the impeller diameter or speed increases the head and flow rate capacity - and the pump curve moves upwards. The head capacity can be increased by connecting two or more pumps in series, or the flow rate capacity can be increased by connecting two or more

e. Pump Efficiency
Pump Efficiency
The term pump efficiency is used on all types of pumps to describe the ratio of power supply to the drive shaft against water horsepower. Pump Efficiency ηp = Water horse power x 100%
Required power or
Pump Efficiency ηp = Q x H x ρ x g
ω x T
Where: Q = capacity (m³/s)
H = total head/pressure (m)

ρ = fluid density (kg/m³)
g = acceleration due to gravity (m/s²)
ω = shaft angular velocity (rad/s)
T = shaft torque (Nm)

f. Available NPSH
The difference between standard atmospheric pressure and the combination of atmospheric pressure at elevation, total dynamic suction lift, vapor pressure, and safety factor. The result must be equal to or greater than NPSHr.

g. Required NPSH
NPSH Required (NPSHR): The minimum pressure required at the suction port of the pump to keep the pump from cavitating. NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer. NPSHA MUST be greater than NPSHR for the pump system to operate without cavitating. Put another way, you must have more suction side pressure available than the pump requires.

h. Cavitation
Cavitation is the formation of empty cavities in a liquid, followed by their immediate and sudden implosion. Cavitation is a common problem encountered in pumps and control valves; one that causes serious wear and tear and can reduce a component's time-in-service dramatically. The primary causes for their formation include vaporization at low pressure, air ingestion, flow turbulence, and internal re-circulation.

i. Hydraulic Noise
Noise in hydraulic systems is generated primarily by the mechanical workings of the pump and fluid pulsations exiting the pump as it supplies the flow for the system. It can also be created by any element that causes turbulence or fluid velocity change. Noise is additive, so small amounts of noise from many components can be effectively amplified resulting in a significant noise problem. The transmission of the noise to the operators or the equipment can cause fatigue, nerve damage and require operators to wear additional hearing protection.

Hydraulic Efficiency
Hydraulic Efficiency
The term hydraulic efficiency is used on centrifugal and liquid ring type pumps to describe one of the three elements of centrifugal and liquid ring pump total efficiency as described above. Where Hydraulic Efficiency (ηh) = Pump head loss (m) x 100% Total head (m)...
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