12 replies to this topic

[xyz168]

I am lost in the 2/3 rule (or may have already been known as 10/13 rule).
I've read the article written by PLeckner but I think I am still not too clear on that.

Which one is the right statement for the 2/3 rule?
1. Pshell/Ptube > 2/3
2. Phigh side/Plow side > 2/3 (which always gives an answer greater than 2/3).

Please advise. Thank you.

DJ

[JoeWong]

DJ,
Welcome to this forum.

This issue is has been in previous post. Please read an EXCELENCE discussion by Phil in HERE. In future, you may search previous post for similar question...

Which one is the right statement for the 2/3 rule?
1. Pshell/Ptube > 2/3

Not clear with P. Design pressure, operating pressure, test pressure...?
Assuming P is design pressure, "Shell side design pressure / tubeside design pressure > 2/3", this is a wrong statement.

2. Phigh side/Plow side > 2/3 (which always gives an answer greater than 2/3).

Not clear with P.
Assuming P is design pressure, "Design pressure of high pressure side / Design pressure of low pressure side > 2/3", this is a wrong statement.


A Pressure Relief Device for tube rupture may NOT required if the following criteria are MET :

• Corrected Test pressure of low pressure side > Design pressure of high pressure side
• Transient peak pressure of low pressure side < (100%-X%) * Corrected Test pressure of low pressure side

Note :
- Refer API Std 521, section 5.19.2
- X% being a margin between transient peak pressure and corrected test pressure of low pressure side
- Designer shall address overpressure by other scenario. Example, pressure relief device is required for overpressure protection of low pressure side which cause by FIRE attack, even though above scenario are MET.


Thus,
Assuming test pressure is 150% (3/2) of design pressure

API Std 521, section 3.21,
==> Corrected Test Pressure = (Stress at Design Temperature / Stress as Test Temperature) * Test pressure
==> Corrected Test Pressure = (Stress at Design Temperature / Stress as Test Temperature) * 3/2 * Design pressure (all for low pressure side)

From above criteria,
==> Corrected Test pressure of low pressure side > Design pressure of high pressure side

==> (Stress at Design Temperature / Stress as Test Temperature) of low pressure side * 3/2 * Design pressure of low pressure side > Design pressure of high pressure side

==> (Design pressure of low pressure side / Design pressure of high pressure side)*(Stress at Design Temperature / Stress as Test Temperature) of low pressure side > 2/3

In low design temperature (< ?? degC), (Stress at Design Temperature / Stress as Test Temperature) is close to 1. Thus (Stress at Design Temperature / Stress as Test Temperature) is ignored.

==> Design pressure of low pressure side / Design pressure of high pressure side) >= 2/3



Similar derivation for case where test pressure is 130% (13/10) of design pressure
==> Design pressure of low pressure side / Design pressure of high pressure side) >= 10/13

First time raise the stress ratio factor in the derivation, hoping others (Compulsory for Phil ) can provide input / comments.

[gvdlans]

It think you are confusing operating and design pressures. These 'rules' are based on design pressures, not operating pressures!

Note that the latest revision of API Std 521 does not mention 2/3 or 10/13 rule anymore. It has been replaced by:

Pressure relief for tube rupture is not required where the high-pressure exchanger side design pressure does not exceed the low-pressure exchanger side (including upstream and downstream systems) test pressure.

See API Std 521 (5th edition), section 5.19.2

Only when test pressure = 1.5 times design pressure this reduces to:

Pressure relief for tube rupture is not required where Pdesign (low pressure side)/Pdesign (high pressure side)>=2/3

However, my recommendation is not to speak about 2/3 or 10/13 rules or whatever, but to remember the text that is shown in bold font above.

[xyz168]

Thank you Sirs.

[xyz168]

"The 2/3 rule of old which is now 10/13 rule in the latest API RP for new services, refers to the MAWP of the vessel or exchanger. Design pressures are usually at or less than the MAWP of the vessel or exchanger. The set pressure of the PSV would need to be at the MAWP or less and thus the design pressure should adhere to this as well.
All this being said, the MAWP being used for the design pressure of the exchanger tube side should be fine. "

Could anyone comment on that, please? I am confused. People have different opinion. Thanks.

[JoeWong]

"The 2/3 rule of old which is now 10/13 rule in the latest API RP for new services, refers to the MAWP of the vessel or exchanger. Design pressures are usually at or less than the MAWP of the vessel or exchanger. The set pressure of the PSV would need to be at the MAWP or less and thus the design pressure should adhere to this as well.
All this being said, the MAWP being used for the design pressure of the exchanger tube side should be fine. "

Could anyone comment on that, please? I am confused. People have different opinion. Thanks.

Above statement is pretty clear.
PSV set pressure <= Design pressure <= MAWP of vessel

Please read API 520 or 521 for the definition of set pressure, design pressure and MAWP.

Can you please advise what the opinion of other people ?
Or ask specific question ?

[gvdlans]

The latest API Standard 521 (5th edition 2007, note that it is not a RP anymore...) does refer to design pressure (or maximum possible system pressure as alternative on case by case basis) and test pressure in relation to tube rupture. It does not contain any reference to MAWP or to 10/13 (or 2/3) rules!!

So it seems to me that the person you are quoting does not have the latest API Std 521 but the previous edition (1997). Please read for yourself in API Std 521 section 5.19.2...

[pleckner]

"Design" pressure may very well be interpreted as being "MAWP" per the definition given for design pressure, Section 3.23, so @xyz168 is not incorrect in using the term in this instance. I think the authors of the new issue just didn't want to continually write, "design pressure (or MAWP)" every time they needed the verbiage. However, I also feel that the choice of using "design pressure" is more appropriate than "MAWP" because the design pressure may very well be less than MAWP.

I agree that we can't blatently use 2/3 or 10/13 as a rule anymore because of the new verbiage, even though it will probably come out to the same thing more times than not.

This new verbiage opens a whole new world for process engineers as we now need to be very aware of the design and test pressures of the units we specify. We should tell the vendors to provide the calculations as they build the unit and test it before it leaves the shop. But remember that your exchanger will be inserted within pipes (for the most part) and the pipes are still tested to 1.5 times the design pressure.

[gvdlans]

In his latest post, xyz168 gives a quote from some unknown person and asks why there is a discrepancy between the quoted text and the answers in this forum. What I tried to show in my post yesterday was that the quoted text seems to be based on API RP 521 (1997 edition) and not on API Std 521 (2007 edition).

[fallah]

1-Let me interprete the below API 521 point:

Pressure relief for tube rupture is not required where the high-pressure exchanger side design pressure does not exceed the low-pressure exchanger side (including upstream and downstream systems) test pressure.as follow:

-In the case of being tubes as low pressure side: For no need to pressure relief due to tube rupture it is required that design pressure of shell side does not exceed the "design pressure of tubes*1.5".In this case it is assumed that if said condition does not exist it is needed to pressure relief in tube side system(for example psv on main header of steam flowing into tubes as heating medium).

-In the case of being shell as low pressure side: For no need to pressure relief due to tube rupture it is required that design pressure of tube side does not exceed the "design pressure of shell*1.3(or 1.5 depending on given factor for hydrotest).In this case it is assumed that if said condition does not exist it is needed to pressure relief in shell side system.

Please correct me if I am wrong.

2-In a case which I am faced there is a Depropaniser reboiler into which Hydrocarbon(in shell) is heated by LP steam.The design and operating conditions are as follow:
- TEMA Type:BKU
-Design Temp. Tube side: 270˚ C Shell side: 160 ˚C
-Desigs pressure of steam system: 8 barg
-Operating pressure of steam system: 4 barg
-Design pressure of shell side: 28.5 barg
The engineering contractor has considered one PSV on tube side (SP=28.5 barg) whose Tail Pipe conducted to safe location(not to Flare).He also ignoring design pressure of steam system, considered the design pressure of tue side equal to 28.5 barg(for external pressure) and FV(Full Vacuum for internal pressure).

My questions are :1-In the cases same as above (steam in low pressure side) we must consider DP of the tube side equal to shell side?
2-Is the reason that: ΔP=28.5(ext. press.)-FV(int. press.)=28.5 barg for the tube side in worst case?
3-Must the value of DP(28.5 barg) of tube side be extended to entire relevant LP steam system (piping and header)?
4-Why the tale pipe of PSV conducted to safe location,while we know in the case of PSV action on 28.5 barg certainly HC released along with steam?
5-Can we conclude in the cases using steam in tube and given as low pressure side , the 2/3 or 10/13 rules are ignored and DP of two sides are considered rqual?

REGARDS

[JoeWong]

Fallah,
I have difficulties in understand your "understanding". Suggest you to read above responses again.

As for second question, there are many points may not correspondent to the topic. Suggest you open your own thread.

[xyz168]

JoeWong:

I should've made myself clear.

Basically what this person says is that we can actually use the design P on the HP side / the design P on the LP side because design pressure goes in line with the MAWP of vessel, ie. if design P goes up, the MAWP goes up as well.

However, what I was told is that we have to use the MAWP.

DJ

"The 2/3 rule of old which is now 10/13 rule in the latest API RP for new services, refers to the MAWP of the vessel or exchanger. Design pressures are usually at or less than the MAWP of the vessel or exchanger. The set pressure of the PSV would need to be at the MAWP or less and thus the design pressure should adhere to this as well.
All this being said, the MAWP being used for the design pressure of the exchanger tube side should be fine. "

Could anyone comment on that, please? I am confused. People have different opinion. Thanks.

Above statement is pretty clear.
PSV set pressure <= Design pressure <= MAWP of vessel

Please read API 520 or 521 for the definition of set pressure, design pressure and MAWP.

Can you please advise what the opinion of other people ?
Or ask specific question ?

[JoeWong]

JoeWong:

I should've made myself clear.

Basically what this person says is that we can actually use the design P on the HP side / the design P on the LP side because design pressure goes in line with the MAWP of vessel, ie. if design P goes up, the MAWP goes up as well.

However, what I was told is that we have to use the MAWP.

DJ,
From our explanation, i hope you have pretty clear my on the criteria on condition where tube rupture is not considered credible.

The "design P on the HP side / the design P on the LP side" obviously does not inline with API STD 521 and common understanding. Please use the correct definition.

Good luck.