5 replies to this topic

[awesome]

Hello,


What is depressurisation? WHy is it done? Where I can read about the basics about this issue?

Is there any calculation sheets available and can somebody help me with some examples? I am very new and hence please elaborate with patience.

[kkala]

What is depressurisation? WHy is it done? Where I can read about the basics about this issue?
Is there any calculation sheets available and can somebody help me with some examples? I am very new and hence please elaborate with patience.

I have met depressuring as an emergency operation on two towers in a hydrodesufphurization unit. Little area of them was wetted in case of fire. So their vapor load should be discharged to flare through two remotely operated valves, one for each tower. This was manually in that case, before PSVs opening; there are more sophisticated cases where valves are automatically operated through a programmable software.
http://boonchoo.org/... Philosophy.pdf is an introduction to depressuring and its intents. Placing "depressuring" or "depressurizing" in the forum search engine will reveal a lot of information to select the interesting part, e.g. http://www.cheresour...h__1#entry43685

Edited by kkala, 16 August 2011 - 09:09 AM.

[tarafdar]

Depressurisation is reduction of pressure.It may be gradual or abrupt.It may be planned or it may happen due to an emergency.
Depressurisation is required for the following reasons:
1)To bring a system to a safe condition.Say a compressure is running at a high pressure(for process requirement)and suddenly the process tripped.There may be progamm to depressurise it to lower pressure level and the comressure will continue to run on recycle or it may be depressrise to a further lower pressure level and go for shut-down.
Or say a vessel pressure suddenly increased due to process abnormality.Then you have to deprssurise it(there may be auto venting system).
In case of fire, vessel pressure will increase and process vessels are fitted with thermal safety valves.
2)To make a vessel or equipment free of unwanted gas or vapour .Say we like to make a vessel free of some gas and fill it with N2.We first reduce the vessel pressure and then pressurise it with N2.We do it several times until the goal is achieved.Pressurisation/depressurisation is better than continuous purging.
3)For preservation purpose.We depessurise high pressure equipments to a just positive pressure to preserve it during shut-down condition.
4)For some tests.Say during hydro-test of a boiler,we have to pessurise it and later depressurise it in a planned way.
(There may be many other reasons where depressurisation is necessary)

[AZIZ_MN]

Hello,


What is depressurisation? WHy is it done? Where I can read about the basics about this issue?

Is there any calculation sheets available and can somebody help me with some examples? I am very new and hence please elaborate with patience.

Hi awesome,
and addition to above two examples, the moleseive dryer or adsorbers operation are best example,these are working on automatic sequence, in that out of two moleseive one will be in line and other will be in regenration mode, to change online molseive to regeneration mode first molsieve will go on depressurisation step in that inlet and uotlet XV'S of molesieve will close and depressurising XV will open to reduce pressure of molesieve safely to low pressure system and further it will proceed for regenration step i.e purging, heating,cooling and back to in line.
hope this will help thanks and regards
azzu

[awesome]

Thanks people,



It helped. Isnt it Engineering fascinating, you keep discovering so much every day..

Back to depressurisation. I am been reading fair bit about this, however it will further help if

someone can share with me an example with calculation behind as well I read one needs some dynamic tools

to measure the temperature drop. It will help.

Hello,


What is depressurisation? WHy is it done? Where I can read about the basics about this issue?

Is there any calculation sheets available and can somebody help me with some examples? I am very new and hence please elaborate with patience.

Hi awesome,
and addition to above two examples, the moleseive dryer or adsorbers operation are best example,these are working on automatic sequence, in that out of two moleseive one will be in line and other will be in regenration mode, to change online molseive to regeneration mode first molsieve will go on depressurisation step in that inlet and uotlet XV'S of molesieve will close and depressurising XV will open to reduce pressure of molesieve safely to low pressure system and further it will proceed for regenration step i.e purging, heating,cooling and back to in line.
hope this will help thanks and regards
azzu

[S.AHMAD]

Hi Awesome

Let me give you a more specific example - compressed air system.

In most compressed air system, we have a pressure vessel called air receiver.Let say the operating pressure is 10 barG. Let say that the air receiver is due for inspection and maintenance. The pressure of the receiver must be emptied by 'depressurization' where the pressure is reduced from 10 barG to 0 barG (atmospheric) so that the vessel can be opened up inspection and maintenance. The depressurization process, in this case is simply venting the air inside the vessel into the atmosphere.

The calculation involved is normally to determine the time taken to empty the vessel or to depressurize from 10 barG to 0 barG.

1. Calculate sonic mass flowrate - refer to any fluid mechanic textbook for the sonic flow equation.
2. Calculate the mass of air inside the vessel using the Universal Gas Law (PV=nRT)
3. Calculate the time by dividing the total mass (step 2) with the mass flowrate (step 1) to arrive at the depressurization duration.

For temperature drop, you need to do enthalpy balance or use the adiabatic expansion process PV^k = constant and PV = nRT to express the temperature in terms of pressure ratio.

To gain more valuable experience, you need to do the calculation by yourself.



I hope the above example helps.

Edited by S.AHMAD, 21 August 2011 - 10:45 PM.