Understanding the Maximum Allowable Pressure 1

[Packermanfan]

From my understanding, the Maximum Allowable Pressure cannot be exceeded without some sort of yielding of the materials.
1. Is this interpretation of the MAP true?
2. Can the hydrotest pressure exceed the calculated MAP?

If it can, what is the actual highest pressure it could be tested at?
If it can't, does the design need to be changed to allow MAP/1.3 (since hydro pressure is typically MAWP*1.3)?​

3. The COMPRESS software I use lists the MAWP and MAP as the same values for a particular design. How is this possible? Does it even make sense?

I've found plenty of resources for understanding the MAWP, but very little for understanding the MAP, so any information is appreciated.

 

[david339933]

See Note 7 of Fig UCS-66.2 for a definition of MAP...and remember you are using stress values, so you shouldn't yield.

 

[GD2]

Correct. What David had said is absolutely correct. MAP is calculated based on allowable stress, S, from ASME Sec II Part D, Subpart 1, table 1A which is based on 2/3 of Yield Strength or 1/3 of Ultimate Tensile strength. So, MAP will never see the Yield strength of the material.

Hydrotest pressure is based on MAWP (not MAP)= Design Pressure (if MAWP is not calculated).

Hydrotest Pressure = 1.3 x MAWP x LSR. This Test Pressure is the Minimum Test Pressure. Code doesn't provide the max value but the Inspector shall reject the test for any visible permanent deformation. This is where you should be concerned that the test doesn't exceed the yield strength of any pressure part.

Now, you may ask, what is MAP and MAWP. In simple terms, MAP is calculated at MDMT/Room temperature whereas MAWP at Design Temperature as per ASME Sec VIII Div 1.

GDD
Canada

 

[XL83NL]

Also bear that MAP is usually MAPnc, where nc stands for non corroded. So it’s only present just after new-built, prior to start/in-service.

Huub
- You never get what you expect, you only get what you inspect.

 

[Christine74]

I've heard MAP_N&C referred to as "new and cold" meaning uncorroded condition at ambient temperature, sometimes used for determining the hydrotest pressure for a new piece of equipment. It's not clear if the OP is referring to the UCS-66 definition of MAP or to the "industry standard" New and Cold MAP sometimes used for hydrotesting.


-Christine

 

[r6155]

New and cold is the concept used in UG-99 c)

Regards

 

[XL83NL]

Correct, that’s what nc stands for

Huub
- You never get what you expect, you only get what you inspect.

 

[LittleInch]

Interesting question. Hopefully I've got this right, but open to challenge. To answer the specific points

"From my understanding, the Maximum Allowable Pressure cannot be exceeded without some sort of yielding of the materials.
1. Is this interpretation of the MAP true?
NO.
MAP appears to be the maximum pressure when you don't include CA and based on allowable stress at ambient "cold" temperature when empty
MAWP is the max temp in a fully corroded condition at the max operating temperature when full of liquid
Both of these have a margin between the stress at this pressure and yield stress

2. Can the hydrotest pressure exceed the calculated MAP?
Yes and sometimes the vessel is tested this way

If it can, what is the actual highest pressure it could be tested at?
It appears to be 1.3 x MAP

If it can't, does the design need to be changed to allow MAP/1.3 (since hydro pressure is typically MAWP*1.3)? Not required

3. The COMPRESS software I use lists the MAWP and MAP as the same values for a particular design. How is this possible? Does it even make sense?
It is possible if there is no CA and the allowable stress does not change between ambient temp and the max operating temperature.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[r6155]

See UG-99
(d) The requirements of (b) above represent the minimum standard hydrostatic test pressure required by this
Division. The requirements of (c) above represent a special test based on calculations. Any intermediate value of
pressure may be used. This Division does not specify an upper limit for hydrostatic test pressure.

Regards

 

[Christine74]

Back to the OP, in Section VIII Div. 1 a vessel can have localized yielding/strain-hardening at discontinuities at the MAWP, so certainly that could happen at a hydrotest of 1.3X MAWP or 1.3X MAP_N&C. Some would argue that this localized yielding is desirable and is a major justification for conducting a hydrostatic test.

Of course it's a different situation altogether when it comes to primary membrane stresses which have a much lower margin with respect to the material yield strength.


-Christine

 

[r6155]

See
UG-101 PROOF TESTS TO ESTABLISH MAXIMUM
ALLOWABLE WORKING PRESSURE.

Regards