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[gimenz]

hi,

i have a trouble in understanding steam out condition.
my current comprehension is this,

as the LP steam (3.5kg/cm2 at 175C) enters the vessel will be come saturated (3.5kg/cm2 at 147C)
since the vessel is vented to atmosphere during steam out.
at the end of steam out, steam will be expected to be 0.5kg/cm2 at 135C (same enthalpy of 3.5 at 147C)
thus, steam out condition specified for vessel will be 0.5kg/cm2 at 150C (15C is additional margin to 135C)

However, the compay received a letter from client and stated as follows,

as LP steam (3.5kg/cm2 at 175C) enters the vessel, it will flash across the valve to 0.5 kg/cm2 and 166C (isenthalpic expansion).
steam out condition for vessel will be specified as 0.5kg/cm2 and 181C (15C additional margin to 166C)

Now, i began to doubt my understanding about steam out.

Will anyone enlighten me, what happens as you open the valve to let steam in inside the vessel during steam out. will it really flash to 0.5 kg/cm2?

thank you for your time.

sincerely,
gimenz

[Art Montemayor]

Raymundo:
I have written extensively in our Forums on the subject of steam-out and the hazards related to such an operation. This information is available through our SEARCH engine – which I strongly recommend you use to access all this information. I have submitted Excel workbooks illustrating actual effects and photos of steam-out failures and have given the method to calculate the correct nozzle dimensions to mitigate a “sucked-in” event.

Now to your specific questions:
You fail to state exactly what it is you are proposing to do. What is the pressure inside the tank (or vessel) you are steaming-out? This is important information you omit. You state, “Since the vessel is vented to atmosphere during steam out.” If that is so, then it is impossible to have “LP steam (3.5 kg/cm² at 147 C)” inside the vessel while it is vented to atmosphere. Someone has failed to understand what is happening when the steam control valve is opened. All this could easily be averted if a simple, detailed schematic drawing of the operation were generated and shared with this forum. An engineering solution to a process problem is easily resolved when a drawing is generated because it is worth a thousand words. Other pieces of advice:
ALWAYS DESIGNATE IF YOUR PRESSURES ARE ABSOLUTE OR GAUGE. This information is needed for subsequent calculations.
ALWAYS DESIGNATE IF YOUR STEAM IS SATURATED OR SUPERHEATED. This can be figured out, but why do you subject the people who try to help you to additional work effort? Be specific and concise!

The free expansion of steam to atmospheric pressure is an adiabatic, irreversible expansion and, as such, is an isenthalpic process (Delta H = 0). If that is so, then the end conditions of the expanded steam can be identified – either with a database or with a Mollier (or T-S) Diagram. By using your data, I find that you have SUPERHEATED STEAM expanding down to atmospheric pressure (NOT 3.5 kg/cm2).

Your client has done the correct thing in correcting you and the wrong process results presented by your company. If you are correct in saying that the vessel is at atmospheric pressure during steam-out, then even your client has a small error in their resulting calculations. They use 0.5 kg/cm²(g?) as the vessel pressure and they come up with 166 ^oC; I arrive at 161 ^oC (as can be checked in the attached workbook). In any event, your company has entirely “missed the boat” in analyzing the process of steam-out.

To address your specific question: “Will anyone enlighten me, what happens as you open the valve to let steam in inside the vessel during steam out. Will it really flash to 0.5 kg/cm²?
The superheated steam (that you have defined) will expand isenthalpically – as I have explained above. If you have problems visualizing or understanding this process, always refer to a Mollier (or T-S) Diagram and trace out the expansion on a constant enthalpy line. You will see that if you expand to 0.0 barg, you will get 161 ^oC superheated steam. If you allow for some resistance in the tank’s vent nozzle (creating a backpressure of 0.5 kg/cm²g), then you probably will obtain 166 ^oC superheated steam. You should do the calculations or use a Mollier Diagram. The bottom line to all this is that the superheated steam source WILL REALLY FLASH (expand) to the pressure inside the vessel.

I would caution you on being careful on designing the proper size of the steam venting nozzle(s) used on this vessel – especially if the vessel is outside and exposed a sudden rain shower. If you use our SEARCH Forum feature, you will find examples of what I mean.

Attached Files

•  Steam Out of Vessels.xls   892.5KB   1296 downloads

[gimenz]

Dear Mr. Art,

Thank you so much for your reply.
I have dowloaded your zip file and understand what you mean by the danger imposed by vacuum condition because of improper sizing
of vent nozzles.
I was having the trouble on understanding what happens to the superheated steam as you let it inside the vessel.
This will help me comprehends how the "steam out condition" is specified.
I apologise for myself, and i really want thank you for sharing your thoughts above.


sincerely,
gimenz