Any pressure below atmospheric pressure.
Absolute Vacuum is the absence of pressure.
The concept of Vacuum is relative (and therefore can be confusing).
- The Los Angeles beach is at sea level where atmospheric pressure is 14.7 psi. Therefore, any pressure at that location, which is lower than 14.7 psi, is a vacuum.
- Denver is 5,280 feet above sea level where atmospheric pressure is 12.2 psi. Therefore, in Denver any pressure lower than 12.2 psi is a vacuum.
In the pump industry vacuum gauges are often calibrated in inches of Mercury (Hg) Gauge Pressure, (as opposed to Absolute Pressure). This method of measurement causes much confusion for three reasons.
- First, the value reported indicates how far BELOW atmospheric pressure at sea level the measured pressure is, and therefore this value is NOT a direct measure of pressure above zero pressure.
- Second, the scale is in "inches of Hg", a scale not normally used to express NPSH calculations, pump pressures, or system pressures.
- NPSH calculations are made in Absolute Pressure, usually in feet of head in the United States, and in Meters in the CGS system (used in Europe, Russia, Asia, Latin America, etc). Therefore, all vacuum gauge readings using the inches of Hg scale require two conversions if those readings are to be used in NPSH calculations: first inches of Hg must be converted to feet of head, then the vacuum reading must be subtracted from atmospheric pressure at sea level to obtain the absolute pressure, which is then useful in NPSH calculations.
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One of the four causes of cavitation in pumps. Vane Passing Syndrome Cavitation is caused by impeller vane tips passing too close to the Cutwater or Tongue. If the pump manufacturer provides a pump with an impeller in which the impeller vane tips pass too close to the cutwater, excessive turbulence occurs each time a vane tip passes the cutwater, in turn causing cavitation and increased pulsation at the discharge of the pump. Depending on system conditions the pulsation may or may not be noticeable, but the cavitation damage will occur regardless of system conditions.
The damage caused by cavitation from vane passing syndrome is diagnostic and may be observed in the center of the cutwater, the tips of the vanes, and to the pump casing downstream of the cutwater possibly on the back side of the cutwater.
Engineering specifications may attempt to preclude this problem by specifying that pump manufacturers not supply pumps with the largest impeller diameter available for that pump family. This policy is unfortunate but understandable because the closer vane tips are to the cutwater, the higher the efficiency of the pump. This fact would tempt many to provide pumps with maximum efficiency at the cost of degrading performance over time due to Vane Passing Syndrome.
It is more accurate to specify minimum cutwater to vane tip clearances. Authorities such as The Pump Handbook and McNally have firm opinions about vane tip to cutwater clearances based on research.
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Variable Frequency Drive (VFD)
and
The two terms VFD and VSD refer to the same device when speaking of AC Induction motors. For this article, aimed primarily at the irrigation and pressure boosting industries, the term VFD is used most often by convention.
Variable Frequency Drives
A solid state electrical device capable of regulating the speed of AC Induction motors by changing the frequency of the AC circuit.
In the United States AC line power is 60 Hertz (Hz), meaning that the positive and negative polarity of the circuit is reversed 60 times per second. AC induction motors are constant speed machines. Since motor speed is determined by the line frequency, changing that frequency changes the motor speed. The higher the frequency, the faster the AC induction motor turns.
IMPORTANT NOTE:
The question asked most often is whether or not a VFD should be used in a specific application. The answer to that question requires system analyses by a knowledgeable and careful person who understands pumps and pump controls. Blanket statements that VFDs are "the answer to all your problems", are useful because they tell you something quickly, the person or company making the statement is not a reliable source of information.
Irrigation Craft considers VFDs to be a tool. This tool is a great thing when applied correctly and for the correct reasons. VFDs are not the answer to all problems, and furthermore, VFDs are definitely not the correct solution in many applications, irrigation systems and high static head applications such as pressure boosting to tall buildings are two areas where VFDs are often not suitable.
The measurement of how much a fluid resists flow.
In general, most common liquids such as water have a decreasing viscosity as temperature rises, (they become thinner and less resistant to flow as they get hotter).
The units of measurement for viscosity are:
Poise (P, but also Po and Ps) - The measure of dynamic (also referred to as absolute) viscosity.
Stoke or Stokes (St & cSt for centiStokes) - A measure of kinematic viscosity, defined as the dynamic velocity (poise) divided by the density of the fluid. The most common unit used for liquids is centistoke (cSt).
Seconds Saybolt Universal (SSU) - Another measure of kinematic viscosity.
There are other scales for viscosity, and there also scales showing how much a liquid changes viscosity in relation to temperature.
Water is the primary liquid that Irrigation Craft works with. The dynamic viscosity of clean fresh lake water at 60° F is about 1.13 cSt (centiStokes).
Low viscosity fluids such as water flow easily, while high viscosity fluids such as SAE 90W gear oil (14-25 cSt @ 2100 F) resist flow. Irrigation Craft only works with low viscosity fluids such as water.
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Glossary - Compressible and Non-Compressible Fluids
Glossary - Newtonian and Non-Newtonian Fluids
1 Volt is that force producing 1 ampere of current flow when applied continuously to a circuit with 1 ohm resistance.
The Volt is a unit of measure describing the difference in potential between two points.
Potential
The word potential means "possible" or "could be". Therefore, when used in reference to electricity the concept is - "if the two points were connected by a conductor". Potential then expresses the degree of difference between two points informing us whether or not current will flow if the two points are connected by a conductor, and also allowing us to predict the behavior of the circuit including how much current will flow.
If there is no difference in potential, we say there is no potential or no voltage between those two points. If there is a difference in potential we say there is potential with the result reported as volts.
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Volumetric Flow Rate (Q) - See Flow Rate
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