What is the meaning of Steam-out condition?

[AravindSujay]

Hi Guys,

This is a tall equipment of about 45m high which consists of two columns with different diameters placed one above the another. The below column is welded to the head of the above column by a cone structure.

Now the entire column is designed for the given condition, But one more condition is that it has to be designed for "Steam out condition". Moreover in its normal design condition, vacuum pressure of 0.175 kgf/cm2 has to be considered. Would you tell me what is the meaning of steam out condition and what are the design conditions are to considered?

Need all your expertise on this.

Thanks in advance,
AravindSujay

 

[SnTMan]

AravindSujay, in my experience a steamout is used for interior cleaning, using steam at 300 - 350 deg. F. A half or full vacuum is generally associated with the steamout, such that the steamout condition might be something like FV @ 300 F.

This can be a controlling condition for large diameter thin shells.

I think I would get clarification as to the exact conditions and verify the design.

Regards

Mike

 

[doct9960]

AravindSujay,

From what I learned from our process engineer, vessel steamout is done to:
1. Remove harmful chemicals left in the vessel prior to entering it for maintenance or inspection.
2. Remove air in the vessel prior to start-up

If your vessel is subjected to steamout, it should be able to withstand 2 pressure conditions @ the associated steam temperature: (plus other applicable loadings in UG-22)
1. steam pressure (i.e. internal pressure) introduced in the vessel
2. vacuum (as a result of steam condensing inside the vessel)

Thus a steamout condition should have a design internal pressure, design external pressure and a design temperature associated with it. Sometimes the steamout external pressure governs over the normal vacuum condition, which in your case is just 0.175 kgf/cm2 or 2.5 psi. In my experience, vessels subjected to steamout is designed for a MINIMUM of half vacuum (7.5 psi). Most of the time, process engineers or Clients specifies FV for steamout.

As Mike has already said, you better get the exact steamout conditions and verify your vessel design.

 

[vesselfab]

We find that most all vessels are being asked to be designed for full vac(15 psi external pressure) @ 300-350*F

This has become standard, and yes, it can be the controlling design on longer large diameter thin wall (low pressure) applications.

 

[jte]

doct9960 explained steamout conditions well. One of the first things you'll see at a refinery during a turnaround, even before "oil out" is the steam lines (hoses and temporary pipe headers) being pre-positioned so that the turnover to the mechanical crews will be a bit shorter. The reverse happens with startup to air free the equipment. The concern with steamout is that the operators open a vent and drain, hook up the steam, and start blowing. A few hours later, they stop the steam and close the vent/drain. Of course, stuff happens... probably more often on shutdown. Sometimes the vent/drain get closed with the vessel hot and full of steam and with its process connections blinded or valves closed. The steam will condense and pull a vacuum. This has happened often enough that most refineries will require some sort of vacuum condition design for steamout conditions. As mentioned above, check with the client. Some require full vac, some half vac, some 300°F some as high as 450°F. If you have to make an assumption, I'd go with FV at 450°F just to be safe, though you may well find that this governs the design and costs you more.

jt

 

[AravindSujay]

Guys,
Should I do the design for 2 cases and check the thickness for both the cases and consider the maximum?

1)Internal design conditions with external press of 0.175 kg/cm2g and
2)Full vacuum with 300 or 400 deg F

What should I do?

 

[vesselfab]

yes, of course you should design for both cases.

each element should be checked in all design cases.

 

[AravindSujay]

ok, I got it. Thanks for all your valuable suggestions. I will check the design.

Aravind Sujay