From the Latin word "cavus", meaning cavity or hole. Cavitation is a two-step process, beginning with molecules changing phases from the liquid phase to the gas phase, and ending when the molecules change phases from a gas back to the liquid phase. These phase changes are very fast, resulting in intense energy releases that damage equipment and makes audible sounds.
Cavitation causes variable sounds depending on the source and intensity. Some of the noises caused by cavitation can be described as: a hiss, static on a radio, crackling and popping, and more. The cavitation sound is distinctive and diagnostic to the experienced practitioner. Entrained gas bubbles are for the most part not related to cavitation, do not cause damage like cavitation, and gas bubbles have a very different sound than cavitation.
Cavitation destroys and/or prematurely ages pumps and valves. Cavitation causes efficiency loss both by the immediate presence of cavitation interfering with fluid flow, and also by damaging the equipment. Efficiency loss continuously increases as the equipment degrades over time from cavitation damage. It is therefore important to understand the cavitation process including: how fluids react to pumping, how cavitation damages equipment, and then how to diagnose, predict, and most of all prevent or reduce cavitation.
Learn about Cavitation In Depth on this Website
Includes Explanation of the Process with Video and Sound
Related Subjects on this Website:
Glossary - Dissolved Substances & Gasses
Glossary - Suction Specific Speed
This is the "center fleeing" force described as the outward push against a body in a rotating system. Centrifugal Force pushes against mass in a rotating system, such that mass attempts to "flee" from the center of rotation. Centrifugal Force is called a "fictitious force", because it is described by means of Newton's First and Second Laws of Motion as applied to a dynamic rotating system.
Centrifugal Force in Pumps
The term "Centrifugal Force", describes how energy is transferred from a rotating impeller into the fluid contained between the vanes of the impeller. At the center of the impeller the vanes push against the body of fluid entering the impeller eye, increasing the velocity of that fluid but also changing the angular momentum of the body of fluid, forcing the fluid continuously outwards away from the center of rotation. Moving away from the center of the impeller, vane speed increases, therefore fluid velocity increases. Maximum fluid velocity is eventually achieved as the fluid exits the outermost diameter of the impeller where vane tip speed is greatest.
The amount of energy transferred into the fluid by means of centrifugal force is indicated by the Specific Speed of a pump. Energy transfer in low Specific Speed pumps (Ns = ~400), is dominated by centrifugal force with little diffusion component in the energy transfer process. As Specific Speed increases, energy transfer is by ever decreasing amounts of centrifugal force and ever greater amounts of diffusion force. When Specific Speed reaches Ns = 8000 - 10000, diffusion force energy transfer is dominant with little centrifugal component.
Related Subjects on this Website:
Article - How Fluids Move Through Impellers
A Rotodynamic Machine that continuously imparts velocity into a fluid by means of a spinning impeller. The term "centrifugal" refers to the fact that centrifugal force is an important component of the energy transfer from the impeller into the fluid.
Centrifugal Pumps are higher head lower flow pumps compared to Axial Flow Pumps. The power curve for centrifugal pumps is either directly proportional to flow rate, or flat (little change in power requirement over the flow range). The operating range of Centrifugal Pumps is generally larger than for Axial Flow pumps, especially at flow rates below BEP.
Centrifugal Pumps have a Specific Speed of approximately Ns = 400 -10,000. Within that range of Ns = 400 - 10000, the following descriptions apply:
Radial Vane Impellers - Ns = 400-1800
Francis Vane Impellers - Ns = 1800 - 5000
Mixed Flow Impellers - Ns = 5000-10000
When specific Speed exceeds Ns>10000 the pump is no longer a centrifugal pump, but now is called an Axial Flow pump.
Centrifugal pumps are used for any application requiring the movement of fluid with pressures higher than axial flow pumps can provide such as: irrigation systems with pressurized pipe systems, boosting pressure to buildings, municipal water systems, water features with spray effect nozzles, filtration systems, swimming pool pumps, cold water circulation systems (HVAC), and fluid transfer systems.
Related Subjects on this Website:
Article - How Fluids Move Through Impellers
Compressible & Non-Compressible Fluids
Compressible fluids change volume when pressure is increased or decreased, while temperature is kept constant. Gasses are compressible Newtonian fluids.
Non-Compressible fluid volume is unaffected by pressure at a constant temperature. A good example is water. Non-Compressible fluids are sometimes called Liquids.
Non-Compressible fluids can be slightly compressed by extreme pressures found in laboratories, however for most engineering and common use, those fluids are considered non-compressible.
Related Subjects on this Website:
Glossary - Newtonian and Non-Newtonian Fluids
Including: Relay, Power Relay, Starter, Overload
The terms Relay, Power Relay, Contactor, and Starter, all relate to similar components, and all three are actually relays. Industry convention defines them differently as follows:
Relay - Completes one or more circuits allowing current to flow. In general use, the word relay refers to an electromechanical device, as opposed to a solid state device, See "Relay (Solid State)" below. Relays use wire coils (magnetic induction) to move an arm with contacts attached to the arm, to close or open circuits.
Relay (Smart) - A programmable or digitally controlled relay, however typically they have multiple I/O operated by the program. In reality, these are small PLCs (Programmable Logic Controllers). Each I/O point can be programmed to act like any of the common relays and time delay relays available on the market such as: On-Delay, Off-Delay, Interval, Flashing, Cycle, Latching, and more.
Relay (Solid State) Also Called SSRs - Silicon based switch, with no moving parts, as opposed to electromechanical relay action of point closure and opening.
Power Relay - Capable of passing larger amounts of current than other relays, as opposed to smaller relays that may be used for logical circuit control.
Contactor - A relay that conducts higher currents, and often inductive type currents such as motors. Contactors are similar to Power Relays.
Starter - A Contactor with an Overload or Over-Current function, used to control and protect circuits with motors and other types of heavy inductive properties. The Contactor and Overload may be built as one component, or as two components, in which case the two components together make up a Starter.
Overload - In this context, a protective device attached or built into a Contactor or Power Relay. This component protects circuits and devices from small or low level over-current conditions, where the wiring and/or devices may develop sufficient heat to cause fires and of course electrocution from heat damaged components. Large over-current conditions will also be detected by Overloads, however UL 489 protection (required upstream of the Starter), provides high current overload protection by responding faster to high current faults (short circuits and ground faults).
Contactors are constructed to comply with standards established by either NEMA or IEC. These two standards have very different criteria resulting in very different products, each with its own advantages.
IEC Contactor with Overload Relay Attached on Bottom
Related Subjects on this Website:
Article - Overloading and Non-Overloading Pumps
A valve that reduces, limits, sustains, or otherwise controls pressure or flow in a system. The number of possible types of valves and their multiple functions appear limitless, but the most common features found in the irrigation industry are (valves often have more than one function):
Pressure Reducing
Pressure Sustaining
Pressure Relief
Surge Anticipator
Check Valve
Altitude Control
Rate of Flow Control
There are two types of Control Valves : Pilot Controlled & Direct Acting.
Direct Acting valves contain a rubber diaphragm or piston with hydraulic forces (from the system) on one side and a spring on the other side (usually adjustable) to achieve the desired function. Direct Acting valves are not as accurate as Pilot Operated valves in: stability, repeatability, they have a small range of flow rates, and they have a much larger hysteresis than Pilot Operated valves.
Pilot Operated Valves utilize Direct Acting "pilot control" valves to control the diaphragm position on a main valve. Pilot controlled Valves have advantages over Direct Acting valves as follows: higher accuracy, greater stability and repeatability. Pilot Operated Valves also have a wider operating range, and they have a much smaller hysteresis than Pilot Operated valves.
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