Which is the Hydrotest Pressure with alternative design conditions?

[ChriChri81]

Good morning,
i've a pressure vessel with a design pressure of 5 bar @ 200°C, the steam out condition for the vessel need an alternative design condition of 10Bar/F.V. @ 260°C; The vessel works in steam out condition few days every three years c.ca. Which value of design pressure should i use to calculate the hydrotest pressure?
Thanks
Christian

 

[JoeTank]

10 Bar

Joe Tank

 

[jamesl]

The hydrotest pressure shall be based on MAWP stamped on your vessel, not the design pressure, except when MAWP is the same as D.P.
Usually the steam pressure and temperature in steam out is not used to design a low pressure vessel.

In this case if your vessel MAWP is 5 bar and you do the hydrotest based on 10 bar, you could ruin the vessel.

 

[vpl]

but if the vessel has to go to 10 bar for a few days every 3 years and it is designed only for 5 bar and tested at 6.5 bar, then it might very well fail in service.

So which is worse -- ruining the vessel during the hydro or having it fail in service?

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[jamesl]

The original post is asking for hydrotest pressure, so apparently the design pressure was already set at 5 bar.

Steam out with 10 bar steam does not mean the vessel is going to experience 10 bar pressure. There are thousands of vessel designed for 5 bars pressure or under that has steam out nozzles on the vessel. If this vessel itself is going to have 10 bar pressure for several days every three years, then the process engineer should specify 10 bar as design pressure.

 

[jehan17718]

ChriChri81,

You need to compute the hydrotest pressure and check the vessel based on 10 bar.

Another way to view this situation is to consider the 10 bar pressure as the "primary" design condition with the alternate design condition set at 5 barg. Now would'nt you without a tinge of hesitation compute the hydero pressure based on 10 bar?

In your above case, considering that the metallurgy is carbon steel like SA-516 Gr. 60/70, the allowable stress is constant till about 260 deg C. In other words and in your case the 10 bar/FV at 260 deg C will be the governing condition.

The problem would be come more pronounced if the pressure difference is not too much but temperature difference and therby allowable stresses are appreciably differnt in the design conditions. For example, consider a SS vessel with design condition as 5 bar @ 200 deg C and alterbnate condition at 4 bar @ 300 deg C. Obviously, one cannot decide in such cases the governing condition and it is reccomended to perform caclculations for both the condition and then deside the governing one and governing hydrotest pressure.

-jehan

 

[gr2vessels]

Chris,
The nominated steam out condition is a theoretical only condition which could only occur if you pressurize the vessel with steam and forget to open the vents. It will never reach 10 bar with open vents, as required for the steam out operation. Even such, the temperature will not raise to 260 deg without flow of steam. That means, you may use the mentioned 5 bar design figures, but check the stress level at 10 bar and full vacuum, not to exceed say 90% of the yield. If exceeds the 90% of yield, then you might need to look at the vessel thicknesses.
However, never operate a vessel above the hydrotest pressure under any circumstance.
cheers,
gr2vessels

 

[roca]

Chris
The steam out condition is specified as an “alternative design” condition – not theoretical as stated above.

In my opinion it does does not matter if the steam out is done for just 10 minutes each year - it is a specified design condition and as such shall be designed for.

The steam out is specified an alternative design condition of 10barg at 260°C which is more severe than the 5barg at 200°C.

This alternative steam out design conditions will design the vessel and as such shall be used to determine the hydrotest pressure.

Opening of vents is something that is forgotten regularly….!!

 

[mbt22]

My (process engineering) perspective is that if I'm specifying a vessel for steam out, it must be rated to operate at steam pressure and full vacuum. As Roca says, vents do get left closed or blocked. The fact that the vessel is rated for this will probably have been used as a mitigation during the HAZOP or similar.

Not being a mechanical engineer, I wouldn't want to guess whether 200C/5 bar or 260C/10 bar was the worst case for design. I would just put down all the possible operating regimes and let the designer do his work. It sounds like whoever wrote the spec sheet is giving you all the information so you can make sure the vessel is suitable for all services.

 

[jamesl]

As I said in my original post, the hydrotest shall be based on stamped MAWP, not design pressure.
If steam out at 10 bar/260 C is specified as a design condition, then 5 bar/200 C design pressure should not matter any more because both the pressure and temperature are lower than steam out condition. The vessel should be stamped at a MAWP no less than 10 bar at 260C, and hydrotest pressure is to be based on this MAWP.

 

[MJCronin]

Christian...

I do not understand why your "steam out" cannot occur at a lower pressure.

It is my understanding that it is only the heat of the steamout that is useful in cleaning vessels, not the pressure.

Can't you simply install a pressure reducing valve set at 5 bar in the steam system and all of your problems go away ?

No more concerns about blocked vents...etc... as stated above ?

Why can't this work ?

-MJC

 

[ChriChri81]

MJCronin, i know what you mean; but is not thescope of my question, i don't have any problem to do the hydrotest, but i want to know if my hydrotest' pressure calculation should be based on 5 or 10 bar. The thickness of the shell is calculated with 10 bar. So i'm sure that i can do the test, for example, using 14.3 bar; but my question is: eventually can i avoid to use this pressure? because 10 bar is not the real operating (or process design) pressure.
Thank you for your collaboration.
Chri.

 

[gr2vessels]

The hydrotest is the proving operation of the selected design pressure. Unless you hydrotest the vessel designed for 10 bar at a suitable pressure required by the design code, any lower hydrotest pressure is useless. It does not proove that the vessel can be safely operated at 10 bar. Regardless of you are operating or not the vessel at 10 bar, the 10 bar is the rating of the vessel (probably the MAWP) and the hydrotest records must produce evidence of suitable test pressure.
cheers,
gr2vessels

 

[roca]

Chris
As advised earlier the steam out is an "alternative design" condition which is the ruling case for internal pressure and as such the test pressure shall be based on this.

Note that the "alternative design" also includes FULL VACUUM - which depending on your vessel dimensions may govern the design (increased shell wall thickness and / or addition of external stiffeners).
Are you also looking to avoid the FULL VACUUM design as well.....???????

 

[ChriChri81]

don't worry roca....the vessel is adequately reinforced with stiffening rings, but the hydrotest pressure is another thing probably if the mawp based on used shell thk was required, then the full vacuum condition may be the worst condition for hydrotest.

 

[knstar]

quote
"So i'm sure that i can do the test, for example, using 14.3 bar; "

i think you may use the 14.3 bar as hydrotest pressure,and shouldn't less than the 13 bar (base on 10 x 1.3).

so your nameplate can sign a MAWP 11(@14.3) or 10(@13).

it is suit for the worst condition.

 

[ChriChri81]

knstar, i didn't say this: the vessel is designed using asme code but also PED, so according with the client specification we must choose the max value for hydrotest pressure between asme & ped.
PED : - 1.25 * Pdes * (stress ratio as asme) or
- 1.43 * Pdes
the max value is the second, so 10 bar * 1.43 = 14.3
sorry for my oversight

 

[knstar]

i see. i think you should find the MAWP_or(owner require) first.

if the MAWP_or is input by designer as owner require, you shoul design the MAWP_or equal the design pressure.And then you should calculate the MAWP(the realy MAWP) base on your design.

1.25*MAWP>1.43*MAWP_or

the MAWP_or should be sign on the nameplate.

if the owner require the MAWP must be based on calculation

i don't know the higer multiple(1.43>1.25) is accepted by ASME