liquid level

[picasa]

What does ASME SEction VIII Div 1 code say about the liquid level in pressure vessels? When fabricators design the pressure vessel, do they just assume liquid full to make sure that all the thicknesses calculated are safe? or do they depend of level values given in purchaser's datasheet? with revisions in P&ID's the level numbers do change. So, I am a little concerned. What is the normal practice?

 

[codeeng]

The Code says you have to take all loads into account, including liquid head. I usually make a judgement call. If the vessel is small in height then designing for full-of-liquid has no real impact. Larger vessels obviously need to be looked at in more detail (additional pressure, seismic, etc).

 

[codeeng]

Sorry, I didn't answer your other question. The Purchaser specifies the design parameters including design liquid level and density and the manufacturer will follow those same numbers unless you tell him otherwise. If, say, you had a 5' dia horizontal vessel with a max liquid level of 2' then I would say design for full-of-liquid. On the other hand if you had a 100' high tower with 5' of liquid then I would say design for 5' liquid. I wouldn't go anywhere in between, either full or max liquid level.

 

[SnTMan]

Often, in practice, if your vessel is good for a field hydrotest (full of water, corrorded thickness) from a structural standpoint, it is good for any level of hydrocarbons which often have lesser density.

As for static head, I prefer not to add to the customer's design pressure. For instance I have a 5' dia. vessel. How do I know it's not connected to 500' vertical piping run? I don't, so why add 5' head? It has 150# flgs, and now my extra 5' head design pressure is above the flange rating at the design temp. Another problem.

Naturally if you KNOW, you have to add it.

I doubt if the average fabricator gets anything like that kind of detailed information.

 

[codeeng]

SnTMan
Field hydro is a completely different case than service. For starters allowable stresses are higher and you don't combine seismic with hydro. So service conditions can govern.

You should be adding static head to your design pressure and its the Purchaser's responsibility to provide this. Granted, it doesn't make much difference on small vessels but what about tall vessels that operate fully flooded ? You can bet your boots that this will be the governing case.

 

[SnTMan]

Codeng, yeah you're right, I just meant that in many cases, if a vessel will withstand the extra static head due to being full of water, it will withstand the extra static head due to being full of a fluid of lower density. Did not intend to confuse that with the actual hydro.

Also you are right about towers, it could very well be a consideration. I mostly do horizontal vessels of a moderate diameter, it's usually not much of a factor.

We have had some jobs where the purchaser spec required static head added (it came to 1 psig on, say, 250 design pr.) but most never require it.

Actually, there is kind of a convention in this industry to add static head to vertical exchangers but not horizontal ones. So, you add static head to a 3 ft long vertical, but not to a 6 ft dia. horizontal. Makes sense, no?

As far as getting something like that from our customers, forget it, the job will be long overdue before you get anything useful out of 'em.

 

[jte]

SnTMan-

I hope I'm not one of your customers! I buy plenty of S&T exchangers. I even buy some from Shell and Tube Inc. But I'll have to agree with you on the generality...

jt

 

[SnTMan]

jte, yes it was a generality, based on a class of work. Not to be taken TOO literally.

As far as being one of my customers, above remarks aside, I'm pretty detailed, I think you'd be satisfied.

You buy lots of S&T, how many of your fabricators add static head above the specified design pressure for moderately sized horizontal units, when not required by spec? I'm curious.

 

[waskillywabbit]

I don't ever "assume" anything that I do not know when designing a pressure vessel, I find out and ask the customer. My boss told me that there was nothing wrong w/ saying "I don't know", but he said I'd better always add to the end of that phrase "but I will find out."

Like was said above, no need to design a 100' tower to operate "full" when it will only see a maximum liquid level of several feet.

On many vessels we fabricate the customer requires different weights (for which they must provide liquid levels)and they must provide operating conditions to determine these weights for hydrotest, lug/leg calculations, support steel, etc. i.e., empty weight, operating weight, hydrotest weight and some even maloperation weight, which then might govern you specifically overdesigning a vessel for operator/equipment error/failure.

You know what they say about what happens when you "assume"...

Brian

 

[jte]

SnTMan-

Unless I specify a liquid level on moderately sized exchangers or vessels, I tend to get about a 50:50 split on including hydrostatic head. One very frequently used fabricator always includes a full static head on its horizontal vessels/exchanger calc's.

I agree with the trend of this thread, though, that liquid levels should be (but too often are not) specified by the buyer. Not only the liquid levels, but density (specific gravity) and type of loading also. Full of liquid can mean either "operating, with a liquid s.g. of 0.8 with a concurrent wind or seismic load" or "field hydrotest with no concurrent wind or seismic."

jt

 

[SnTMan]

jte, agree 100%. These things should be specified, however they often are not or are incomplete. Getting complete information can be a lengthy process, especially when you do not deal with the owner. Do your fabricators calculate complete MAWP's? If so, do they go back and take the static head off? Your fabricators, in adding static head based on diameter are implicitly assuming no vertical piping, etc. Realistic? Yes, no, maybe.

picasa, sorry for hijacking your thread!

If you SPECIFY a vessel you are required by Code to include the effects of static head and external loading. In my opinion, when designing a vessel SPECIFIED by others it does not always make sense to add to the specified design conditions, due to a lack of complete information.

Life is just messy sometimes.