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Tube rupture sizing case not calculated for TS/SS same MAWP 1

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jamesbanda

Chemical
Sep 21, 2004
223
Dear all,

I am working as the client engineering lead for a new plant. We are completing the FEP or FEED (front end package) shortly. I am checking RV calculations as client.

On one case I found that the tube rupture case was not calculated becuase each side has the same MAWP.. I trust the EPC involved but they only stated references (ie it is in this code) as a reason. I want to understand why you dont calculate it for this case..

is it to do with if the Heat exchanger has differnt MAWP's the tubes can rupture due to differential pressure stress ? and this is common (or common enough to require a RV to prevent overpressure from this event ?)




 
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jamesbanda,

In general my understanding is as follows, but there are variations on these generalities:

1) Both sides of an exchanger is a pressure vessel and must be protected by a PSV (or relief path).

2) The PSV does not have to be directly on the exchanger provided the requirements of UG-125 and UG-135 are met.

3) If the low pressure side test pressure is not less than the high pressure side design pressure, then you do not need a PSV SPECIFICALLY for the tube failure case (API 521).

4) Further to (3), a tube failure may still be possible (vibration, corrosion, etc), so you still have to calculate the relief load from a tube failure case and ensure whatever PSV (or relief path) is protecting the LP side has sufficient capacity.


 
A great discussion on this very topic took place on the Cheresource message board:

(
If this link doesn't work, go to click on "message board" (menu on left side of page), click on "Relief Device Forum", and scroll down to the topic "Relieving Pressure". This particular post started 16, October 2006.
 
The logical about the "test pressure/design pressure rule", formerly known as the "2/3 rule" is that:
1) Full tube rupture is a rare event
2) In case a full tube rupture does happen, it is unlikely that it will result in loss of containment (release of the contents to atmosphere) when the low pressure side test pressure is equal to or higher than the high pressure side design pressure.

Therefore a relief valve is not required for the full tube rupture case when low pressure side test pressure is equal to or higher than the high pressure side design pressure.
 
Guidoo,

The methodology you mention is what we did for quite some time, and is based largely on API521. However, ASME muddied the waters with Interpretation VIII-1-04-38.

 
I just read the referenced interpretation. Don't you just love it when the way the question is worded creates more questions than answers?

Anyways, there were three questions in this series of interpretations. Question #1 posed to ASME asks if internal failure should be considered and the answer is yes. This interpretation still doesn't say one needs to protect for internal failure, just that it needs to be considered for a credible scenario.

Question #2 asks if ASME provides any requirements regarding the criteria or methods to be used in determining the internal failure scenarios to be considered. ASME replys that it is the user’s or the user’s designated agent’s responsibility to define the criteria or
methods to be used in determining the internal failure scenarios to be considered in the design of the heat
exchanger. I interpret this as they leave it up to us and by default, the standards, better known as API RP520/RP521.

Question #3 assumes an internal failure can occur, so it is asked if a pressure relief device must be provided on the low pressure side if the maximum operating pressure that results on the low pressure side does not exceed the test pressure? ASME replies Yes; but again puts the burden on the user or the user’s designated agent’s to define
the internal failure scenarios to be used in determining the pressure relief capacity requirements for the
heat exchanger.

So, I go back to what we've all been doing in the past, check with the standards.
 
Thank you Phil, I was getting curious after CJKrugers reply...

From what you write I understand that the referenced interpretation can mean that the user states that the internal failure scenario "tube leakage" (e.g. leakage through a 1 mm diameter hole) is considered a credible scenario, while the internal failure scenario "full tube rupture resulting in loss of containment" is not.
 
Unfortunately, it appears ASME allows us to interpret the scenario, or potential for a scenario almost anyway we want. I would most definitely stick with API RP521 (since this is at the very least considered to be good engineering practice) and the two main design pressure/test pressure ratio rules of thumb, 2/3 or 10/13, to determine if there is a credible scenario. If you decide there is one, I would use the typical calculation for a single tube break as the flow through two orifices, one from each side of the broken tube.
 
Thankyou for your responces..

This is how I will proceed. Ask the EPC to validate if the RV is sized for this senerio.

1. Outcome 1 RV adequatly sized on issue
2. Ontcome 2 RV inadequatly sized. I will need to assess the potential for tube rupture. Industry suggests a 1 in 1000 to 1 in 100 frequency for this. I will line this up with the likley pressure risk and how high the pressure could get to determine the consequence to dicide on the level of protection required.


 
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