Design temp. and pressure 2

[opinion1055]

Hello friends,

Myself is a beginner in stress analysis.
I have a doubt.
In most cases i happen to work for very OLD Plant expansion projects.
The input from client will be certain OLD existing documents. Most cases from scratch.

Suppose i have a line list that show only Operating pressure and temperature and not Design temp. and pressure,

then how can i judge its design temp and pressure.
Is it a multiplication factor like 1.2 times or 1.5 times etc ?

Please advice..

Thanks in advance

 

[dcasto]

I would start with the installed material specs and recalculate the maximum allowable pressure at say 20% over the normal operating temperature to get an initial design point. Then, use mechanical integrity things like ultra sonic and some random X-rays to prove up that design point over the next few years. Remember to look at valves , flanges and other materials that could put limits besides the piping!

 

[opinion1055]

Dear dcasto,

Thank you for your reply.
Let me clarify what u stated ..
Suppose the Existing Matrl. spec = A106 GrB
Operating Temp = 150 Deg C
Thickness = Sch 40

So, u mean to say you will find "P".
Now that P = design pressure and " 20% over the normal operating temperature" = Design temperature...
Am i ryt ?

Thanking you

 

[gjmorin]

Design pressure is not necessarily what the pipe can withstand - it's what they system will see.

If the line has a pressure limiting device, such as a relief valve or rupture disk, pressure is a pretty straightforward - the design pressure must be above that value. This might be 20% above the operating pressure, but is not necessarily so. You would also have to account for head effects - If it's a water line, with a PSV at the high point, but the line then drops down, you'll have more pressure at the low point & the design pressure should account for that.

If the line is pressurized by a centrifugal pump, then you can look at the pump curve and find the shutoff head (probably take the shutoff head from the max impeller size for the casing instead of the installed - and add a margin). You can do the same kind of thing for other self limiting features in the system, such as an atmospheric tank.

Design Temperature can be more straightforward. If you're certain the operating temperature is a true maximum, add a margin and be done with it, but consider all possible heat sources when doing so.

In short, you need to look at the whole system as if you were designing it from scratch. Find the limitations and assign an appropriate design pressure and temperature. Then a condition assessment (UT, RT, etc) as indicated above would be appropriate to check to see if the installed line can handle the pressure.

 

[desertfox]

Hi

I think the first thing I would do is find the code the vessel was designed too, for instance; if say the vessel is manufactured to BS5500, then knowing the material the vessel is manufactured from, you can look up within the code a table which gives the design stress for various service temperatures for particular steel grades and I assume that the other codes like ASME etc. do similar.

 

[opinion1055]

gjmorin "Then a condition assessment (UT, RT, etc) as indicated above would be appropriate to check to see if the installed line can handle the pressure"

How to do this conditional assessment with UT, RT etc ?

 

[SeanB]

If I remember this correctly...
You conduct the UT or RT to find the minimum wall thickness remaining in the pipe. Then using the minimum wall thickness remaining calculate the maximum allowable operating pressure.

 

[opinion1055]

SeanB

Thanking you.. You made the point clear to me.

 

[desertfox]

calculate the maximum operating pressure but based on what code?

 

[opinion1055]

What I could infer from the above posts is that add a margin to operating temp and pressure. Say 20% and that will be the design parameter.
But earlier in a forum I read that, based on the fluid category , the margin varies. For ex.
Category D = 1.1, 1.2 max
Category M = 1.4 to 1.6 etc…


gjmorin said , study the system and put margin accordingly which i think is correct.

Why I’m asking this is bcoz,

I have an iso of system 8”-P- line with Op.temp = 134 deg C, & Design temp = 38 deg C (The line is connected to Stripper Column).
Op.Press = 1.01 Bar, & Design Press = 19.64 Bar


In the isometric(23 yrs old ), design parameter specified is actually the B16.5, 150# flange rating temp-press.
Client is asking to use this as design value.

But how can I use this in CAESAR Stress Analysis ?
It not logical.

That is why I raised this question.

 

[desertfox]

If you bothered to read my earlier posts then you would realise that if you find the code the vessel was designed too, then you would have a base to work from knowing what the original design stress was based on and taking measurements of the vessel in its current state you can calculate the safe working pressure based on the original code.
the construction code should be on the vessel nameplate.

 

[LittleInch]

I think that you can only work with what you are given. If all you have is operating pressure and temperature then use this in the stress analysis with your known wall thickness and see what comes out.

to re-engineer an OLD system is a complex procedure and there is no simple percent or multiplier that you can safely use to determine the "design pressure" as it varies depending on all the connected systems, safety devices etc etc. A particular spool or piece of pipe may have a higher design pressure based on flange rating or wall thickness, but unless you are told the code used (prob 31.3, 16.5 ) and all other parameters you are putting youself in a dangerous place to say something is good or not. That is without the key issue of is what was there originally in terms or wall thickness there now??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way