CAVITATION ON CHEMICAL TANKERS– CAPT AJIT VADAKAYIL
Cavitation erosion is very harmful on a chemical tanker, as it causes causes contamination.
Eliminating cavitaion is a major field in the study of fluid dynamics. Cavitation is a major problem in the chemical industry because it affects a basic tool, the centrifugal pump.
Cavitation happens when the liquid pressure drops below the vapor pressure causing liquid boiling.
The physical process of cavitation inception is similar to boiling..
Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas.
Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas.
Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid.
Vapor bubbles may reduce liquid flow or stop it completely due to pump casing getting vapour bound. It occurs when the pump is driven to over capacity.
You can hear the crackle or gravel sound. Too high suction pressure causes “flashing into vapour”and bubbles implode on the impeller thus eroding it or causing pitting damage..
You can hear the crackle or gravel sound. Too high suction pressure causes “flashing into vapour”and bubbles implode on the impeller thus eroding it or causing pitting damage..
Cavitation mainly happens when pumping ballast—the pumps are run with too little back pressure and too small head because there is nothing for the pump to wok against.
To avoid cavitation –throttle delivery valve to get a good head of atleast 50 metres at appropriate speed. When ballasting the difference between the gauges on the suction/ dischg side should be around 5 bars.
By superimposing ship and shore HQ curves the optimum head the pump has to work against can be determined. The time for dischg can be calculated. The intersection of the system H-Q and the pump H-Q curves defines the actual duty point.
When centrifugal pumps are run in parallel they must be run at the same speed/ same volumetric flow-- –lest one becomes a sleeping pump.
When pumping bulk, the dischg valves should be fully opened. At the dischg side the pumps should be connected as far away from the pump as possible. At the suction side the pump must draw liquid from the shortest and simplest way.
Connect lines at the suction side only when necessary a—this problem does not exist on framo powered chemical tankers . Never open a closed valve suddenly to a pump running empty with delivery valve open at high speed.
When shore terminal has extra frictional losses avoid running many pumps.
To prevent cavitation from happening, suction pressure must be increased above the vapor pressure.
Vapor pressure is a dependent on temperature.
A pipes diameter becomes a main factor because it directly reflects the velocity of the pipe. Velocity is an important factor in determining cavitation in a pipe since it effects the velocity head for a piping system.
If the velocity head is too great it could decrease the pressure inside of the pipe causing cavitation.
The velocity in the pipe is dependent on its diameter. velocity is based on the ratio of flow to area. Higher velocities will be at areas where the diameter is small and slower at areas where the diameter is larger.
When the staticpressure of the liquid for one reason or another is reduced below the vapour pressure of theliquid in current temperature. When cavities are carried to higher-pressure region they implode violently and very high pressures can occur.
When the pressure difference across an orifice is increased sufficiently, cavitation occurs in the exit flow. Cavitation starts when the inlet corner of an orifice is sufficiently sharp and the flow detaches from the orifice walls, say of a DPT transmitter orifice. Cavities travelling along the flow collapse when they enter to higher pressure region of flow.
In order for cavitation inception to occur, the cavitation "bubbles" generally need a surface on which they can nucleate. The cavitation pits increase the turbulence of the fluid flow and create crevasses that act as nucleation sites for additional cavitation bubbles.
The bubble is trying to collapse from all sides, but if the bubble is lying against a piece of metal such as the impeller or volute it cannot collapse from that side.
So the fluid comes in from the opposite side at this high velocity and bangs against the metal creating the impression that the metal was hit with a "ball pin hammer". The resulting long-term material damage begins to become visible by so called pits.
Suction cavitationoccurs when the pump suction is under a low-pressure/high-vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump, where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure.
This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look sponge like.
Both cases will cause premature failure of the pump, often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.
Discharge cavitationoccurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge.
As the liquid flows around the impeller, it must pass through the small clearance between the impeller and the pump housing at extremely high velocity. This velocity causes a vacuum to develop at the housing wall (similar to what occurs in a venturi), which turns the liquid into a vapor.
A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump housing. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.
The formation of bubbles causes a volume increase decreasing the space available for the liquid and thus diminish pumping capacity.
For example, when water changes state from liquid to gas its volume increases by approximately 1,700 times. If the bubbles get big enough at the eye of the impeller, the pump “chokes” i.e. loses all suction resulting in a total reduction in flow.
The unequal and uneven formation and collapse of bubbles causes fluctuations in the flow and the pumping of liquid occurs in spurts. This symptom is common to all types of cavitations.
The sound of cavitation can be described as similar to small hard particles or gravel rapidly striking or bouncing off the interior parts of a pump or valve.
People can easily mistake cavitation for a bad bearing in a pump motor. To distinguish between the noise due to a bad bearing or cavitation, operate the pump with no flow. The disappearance of noise will be an indication of cavitation.
Cavitation is usually divided into two classes of behavior: inertial cavitation, and noninertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave.
Such cavitation often occurs in pumps, impellers, and in the Noninertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is observed in pumps.
NPSHr/ NPSHa
The minimum head required to prevent cavitation with a given liquid at a given flowrate is called the net positive suction head required (NPSHr).
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In a pumping system, the difference between the actual pressure or head of the liquid available (measured at the suction flange) and the vapor pressure of that liquid is called the net positive suction head available (NPSHa).
In a pumping system, the difference between the actual pressure or head of the liquid available (measured at the suction flange) and the vapor pressure of that liquid is called the net positive suction head available (NPSHa).
The pump manufacturer determines the NPSHr of an impeller pattern by conducting a suppression test using water as the pumped fluid.
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These tests are usually only made on the first casting for an impeller pattern, not on individual pumps.
These tests are usually only made on the first casting for an impeller pattern, not on individual pumps.
Keep in mind that very few process applications call for a pump to handle a pure liquid such as water. Most services handle a mixture of various components (e.g., crude oil, blended gasoline or even paint). As such, they will have a range of vapor pressures or boiling points, which depend on the volatility of each component.
The available NPSH of the system must be equal to or greater than the NPSH required in order to avoid pump cavitation difficulties.
Keep available NPSH as high as possible by:----
Keeping the maximum available suction head on the impeller eye
Keeping the liquid temperature as low as possible
Keeping the suction pipe size as large as economics will allow
Avoiding any unnecessary elbows or other fittings and valves on the suction line to keep suction line losses as low as possible
Selecting the pump which gives the most favourable NPSH requirements within the bounds of economy
Specifying the pump discharge head as near as possible to actual operating conditions, the most common cause of cavitation in pump installations is pump discharge requirements specified for above the actual requirements of the installation. From the performance curve data, it’s obvious that the tendency toward cavitation increases as the pump head drops or as the capacity increases.
Keep the pump discharge head as near the specified head as possible and correct systems which are at fault in this respect by:---
Throttling discharge gate or other valves until pump cavitation noise disappears
Installing orifice or venturi in discharge line
Cutting pump impeller to a predetermined smaller diameter
Reducing pump speed if possible and where practical
Decreasing discharge pipe size if practical
Where increasing the discharge head will not correct trouble:----
Lower pump nearer to water level where lifts occur if possible
Lower pump or raise water level if possible where suction head occurs
Increase suction pipe size and remove any unnecessary elbows, other fittings and valves, even if suction lines and pump have to be reoriented.
To reduce cavitation use a pump of larger size and run it at lower speed because the required suction head would then be reduced.
SonoLuminescence is a display of ZPE in action.
In this action high intensity ultrasound (>20 Kh )cavitation is used on a bath of water with Argon added. The water glows a bright blue (blue shifted) which is found to be the ZPE spectrum, then bubbles violently.
ZPE gathers on the 100 micron bubble surfaces and when the bubbles implode, massive energy is released creating temperatures of 5500 degrees C greater than the surface of the sun, and pressures greater than 2000 bars, for less than 1 millionth of a second ..
You can see this in the Hollywood thriller Chain Reaction starring Morgan Freeman.
Lord Shiva is a cosmic allegory.
Shiva is NOT an aghori ( corpse eater ) from a grave yard
People like Amish Tripati have been catapulted by R media and our own Benami media to international celebrity status for converting Lord Shiva into a ordinary mortal , a recent ( 1900 BC ) sex starved tribal who can be bled and wounded .
Forbes has ranked Amish Tripati among the top 100 celebrities in India thrice in a row, in 2012, 2013 and 2014.
There are R Zionist prizes to be won if you run down ancient India and our culture..
Amish was also selected as an Eisenhower Fellow, an exclusive programme for outstanding leaders from around the world.
ALL THOSE TURDS WHO THINK OUR SANATANA DHARMA GODS ARE OF RECENT 1900 BC, ORIGIN, THINK AGAIN.
- hi sk,Delete
indeed- 10th july is nikola tesla's birthday.
my first blog was about nikola tesla.
while writing the post NO 1 i told my wife - - i have started well, within 4 years i will india's no 1 blogger.
cristiano ronaldo with 244 million views is now behind -- i have overtaken him with 251 million
https://plus.google.com/+CristianoRonaldo/posts
https://plus.google.com/109255865130996771184/posts/C9izbxcDdKy
HAPPY BIRTHDAY NIKOLA TESLA !
http://ajitvadakayil.blogspot.in/2009/05/indigo-adult.html
capt ajit vadakayil
.. - https://plus.google.com/102666943676707972768/posts
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CHECK OUT THE PATHETIC VIEW COUNTS OF THESE MAHA MERUS -- DARLINGS OF R MEDIA.
THE PROOF OF ZE PUDDING LIES IN THE EATING -- NOT DISPLAY AT PRIME TIME ON BENAMI MEDIA TV
capt ajit vadakayil
.
Grace and peace !
ॐत्र्यम्बकंयजामहेसुगन्धिंम्पुष्टिवर्धनम्।
उर्वारुकमिवबन्धनान्मृत्योर्मुक्षीयमामृतात्।।
oṁ tryambakaṁ yajāmahe sugandhiṁ puṣṭi-vardhanam ǀ
urvārukam-iva bandhanān mṛtyormukṣīya māmṛtāat
We pray to Lord Shiva whose eyes are the Sun, Moon and Fire
May He protect us from all disease, poverty and fear
And bless us with prosperity, longevity and good health.
CAPT AJIT VADAKAYIL
..
Today is 'Science for peace' day.
Tributes to super genius Nikola Tesla!!