That is a bit vague, what is the context.
In relation to a boiler a blow down is to remove solids and reduce concentration of TDS in the boiler water.
For a pipe during commissioning it can be to blow out debris from the pipe.
There are probably some other applications for blow down that depend upon the industry/situation.
In a recirculating fluid system, contaminants will build up over time as the fluid is reused. Blow down is simply bleeding the system down to reduce the concentration of contaminants.
For example, if you kept adding oil to your car engine without ever changing the oil, the contaminants would concentrate until your engine oil was a sludge.
With a boiler, the boiler system is closed and salts will accumulate in the blower over time as water is heated and turns to steam (leaving the salts behind). If the concentration of contaminants is too high, then the contaminants will plate out inside the boiler on the heat transfer surfaces and destroy the boiler. That is why boiler blowdown is used to control the salt concentration within a set operating range. You can do the blowdown continuously or as a batch process.
In a cooling tower system, water is evaporated in the cooling tower which also leaves the salts behind. A blowdown process is also used to control the salt concentration in a cooling tower recirculation system.
For some cases blow down means emergency depressurization. In oil field which i am working on there is special blow down system which depressurizes oil plant from initial pressure to pressure of 4 bar with in 4 minutes. Bolw down is initiated by detection of gas or fire in the area. This safety control is applied to oil production offshore installation. Every main equipment has blow down valve which opens and deprssurizes system to flare system. Blow down valve is the same as shut down valve but usualy it is closed. In chemical plant i assume that there should be different blow down systems according to compatabilitiy of streams.
The problem here is that the lack of context makes the search for the proper answer more difficult. If you give more information perhaps the answer will be better.
There are several kinds of blowdowns:
1. Steam Boiler. two blowdonwns here
continious blowdown. the goal is to reduce the conductivity of the boiler water (and related contaminats)
Bottom blowdown. the goal is to eliminate the solids that accumulate in the boilers.
2. Cooling tower. the goal is here to reduce the conductivity (and related contaminants) from the system.
3. Safety valves. Blowdown is in this context value of pressure drop to reseal after discharge.
In your second comment you wrote "....blow out the contaminants inside system? by steam ...". This sounds more like you are trying to clean (aka flush out).
This comprehends a series of measures to clean process systems which may start with water flushing which may be a lot of water running down the system to clean it or blowout the pipes with steam which is also because afterwards you do not have tons of dirty water. some times you do both other only air etc. this depends on your application
Our "blowdown" lines are for blowing the water out of a reactor jacket. We apply compressed air to the top half of the jacket or coils and the water there within is pushed back to the cooling tower surge tank.
We also "blowdown" our boiler as others mentioned above, to help remove any build up from the boiler.
Lastly we are installing a "blowdown tank" to which we are piping the rupture disk from a couple of large reactors. Should the rupture disk go, the contents of the reactor will go to the tank, rather than spraying out in the process area.
myrdale, I was wondering about your "blowdown" tank. Currently, we have these types of tanks in our plant, which recieve spent chemicals, oils/waxes, etc that get "blown down" to this type of tank. I was wondering, how to calculate the blow down rate to this tank?
