Test Pressure Selection 2

[AhmadFahmi]

I am in the mist of understanding the test pressure to be selected during the pressure test

Wall thickness calculation
Pipe material A106-B
Flange material A105
Design pressure 127 bar (set by process engineer)
Operating pressure 65 bar
Design temperature 75 degree C
Operating temperature 33 degree C
Corrosion Allowance 3mm
Pipe Size 24 inches
304.1.2 Straight Pipe Under Internal Pressure

PD scope:
t = ----------- (3a) t < D/6 : 27.05 < 101.60 OK
2(SEW+PY) P/SE <= 0.385 : 0.092 <= 0.385 OK

12.7 * 609.60
t = ---------------------------------------------------------------------------- = 27.05 mm
2*( 138 * 1.00 + 12.7 * 0.4 )

t = 27.05 mm Pressure design thickness.
P = 127 Barg Internal design gage pressure.
D = 609.60 mm Outside diameter of pipe
Y = 0.4 - Coefficient from Table 304.1.1
S = 138 N/mm2 Stress value for material from Table A-1
E = 1.00 - Quality factor from Table A-1A or A-1B
PIPE SMLS A106-B
with:
Yield strength min. 241.0 N/mm2 @ 20.0 °C
Allowable stress 138.0 N/mm2 @ 75.0 °C
Tensile strength 414.0 N/mm2 @ 20.0 °C
W = 1.0 Weld joint strength reduction factor below 510°C (para.302.3.5e)

The required thickness

tm = t + c + tol t nom
tm = 27.05 + 3.00 + 5.75 = 35.81 mm < 46.02 mm OK

tm = 35.81 mm minimum required thickness, including mechanical,
corrosion and erosion allowances.
t = 27.05 mm pressure design thickness, as calculated in (3a)
c = 3.00 mm the sum of the mech. Allowances (thread) +
corrosion and erosion allowances.
tol = 5.75 mm fabrication tolerance (12.5%)

t nom = 46.02 mm wallthickness nominal



Hydro test pressure

From B16.5 table 2-1.1 Maximum working pressure for flange 900LB at 38 degree C

153.2 bar

So hydro test pressure 1.5x153.2 bar = 230 bar


Question

1) Why we take 1.5x153.2 bar as internal gauge pressure and not 1.5x127 bar for the hydro test pressure as per calculate in the wall thickness calculation? Is it because the piping system has flange?

2) Is there any clause in the code said that we have to use the Maximum working pressure for flange rating in ASME B31.3?

3) Is it wrong if I use 1.5x127 bar as hydro test pressure? Just because the operating pressure is so much lower (1/2).

 

[LittleInch]

1) I don't know and as you've described it not correct. It is however quite common for piping systems to be classified and designed in design pressures that equal the flange rating. Whilst this makes the wall thickness of the pipe thicker than it needs to be as the actual design pressure for any particular system is lower than the maximum class pressure, it makes ordering pipes simpler and less chance of someone making a mistake or believing that just because a section of pipe has a class 900 (note "900lb" officially disappeared in the 1970's) flange then automatically believing the whole system is good for that pressure. I'm not saying I agree, but you can see the logic behind this decision.

However you have stated this system / line number has a design pressure of 127 bar so it is the one you need to use, not the flange maximum design pressure. Often the line list will have a column designating the test pressure of each line / system which can all be different.

2) Not as far as I am aware, but as said the piping specification document used by your company may work that way, but not the code.

3) No it's not wrong, it is correct. ASME B 31.3 requires the test pressure to be based on design pressure (345.4.2 (a)

There must be a good reason why your process engineer has chosen the design pressure so much higher than the normal operating pressure. Why don't you ask him or her? Either way it would appear that at some point the pressure in this system could rise to the design pressure, therefore you need to test at a pressure higher than that in order to give everyone confidence that the system has been designed and constructed correctly in order to withstand that pressure.

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

 

[mk3223]

The hydrotest of the piping may be at 1.5x of either the process design pressure or the maximum flange pressure rating, which normally specified on the Client Spec.

As tested per the design pressure, i.e 1.5x127, the piping can be run at the operation pressure (65 bar), or okay at any pressure just less than 127bar. However, if the operation is required to be higher than 127bar, the system needs to be retested per new pressure. Of course, the maximum pressure is limited by the 900# flange rating pressure of 153.2bar.

IMO, as an Engineer focus on the term of the "design pressure" per B31.3, the Operation may want the system with the max flange rating pressure, which can be benefit for the future process improvement or expansion.

 

[GD2]

Sorry, there is no either Or or when it comes to hydrotesting. Hydrotesting can be expensive and must be specified correctly. In a piping system, the system will comprise of different piping elements manufactured to different standards. In the example above, pipe's manufacturing standard is ASME B36.10 whereas Flange is ASME B16.5.

Pipe hydrotest pressure is validated against the yield strength of the material at test temp. The simple hoop stress equation can be used (make sure to use the mill tolerance in inputing value of t). Flange test pressure which is ASME B16.5 is limited to 1.5 x MAWP at test temp. Two different criteria for two different elements.

AhmadFahmi, Can you present this calculation and show us which component (pipe or flange) will limit the test pressure? Consider 46.02mm, 120 sch and pressurizing up to 90% of yield strength for the pipe at test temp.

ASME 31.3 states that test pressure for each elements should be calculated and the minimum will set the test pressure. Bend and elbows usually follows the pipe schedule.

One of the things I also noticed in AhmadFahmi is that the Test Pressure calc is incorrect. The design temp is 75C. h
He must show in his calc the temp compensation factor ST/S (basic allowable stress at test temp divided by the basic allowable stress at design temp).

Design pressure and design temp setting is a joint responsibility of Process and Mechanical Engineers and is captured in the Line List. when setting the design pressure, the process will consider other factors like potential back pressure, surge in pressure or temp control system fluctuations, start-up shut-down conditions in addition to normal hydraulic conditions.

GDD
Canada

 

[LittleInch]

mk3223,

I agree with GD2 - It's a bit dangerous to state either/or without also stating that testing to 1.5 x max flange rating can ONLY be used if the rest of the system, including the pipe, has been designed for the maximum flange rating. Sometimes it is, sometimes it isn't...

In this case it is clear that the lower design pressure of the pipe (127) takes precedence over the flange rating as the pressure to use for the hydrotest pressure.

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

 

[Aub_Engineer]

I will agree with the statement that there is no "OR" in specifying hydrostatic pressure. Per ASME B31.3 the piping shall be subjected to a hydrostatic test at no less than 1.5X the design pressure. Your piping code will dictate what the minimum hydrostatic test pressure shall be.

However, in my experience it is a common practice to specify hydrostatic test pressure based on the limiting component in the piping system. If line classes have been put together so that the flanges are the weakest component then you may consider testing at the appropriate multiple of the B16.10 flange rating. This is done so that existing piping does not have to be retested if design pressure changes in the future, and hydrostatic test pressures are standardized throughout the plant i.e. 450 psig, 1125 psig, 2225 psig, etc.

To your questions:

1) Why we take 1.5x153.2 bar as internal gauge pressure and not 1.5x127 bar for the hydro test pressure as per calculate in the wall thickness calculation? Is it because the piping system has flange?
Your piping code and design pressure will tell you what the minimum hydrostatic test pressure shall be. You should choose components to be within that design pressure.
2) Is there any clause in the code said that we have to use the Maximum working pressure for flange rating in ASME B31.3? I have not seen any clause. User preference may dictate that flange rating sets the hydrostatic pressure.
3) Is it wrong if I use 1.5x127 bar as hydro test pressure? Just because the operating pressure is so much lower (1/2).
It is not wrong if 127 bar is your design pressure and building to ASME 31.3 code. You should verify all components meet this design pressure and that temperature is accounted for. You may elect to use a higher hydrostatic pressure if the piping design allows it as I mentioned above.

 

[mk3223]

Just to clarify my response above, It's correct that you need only one test with a hydrotesting pressure for your system.
As based on the "design pressure" case by the Code, the test pressure of 1.5x127 is acceptable by the Engineer and also by the Client.
As mentioned by Aub_Engineer with the rationale above, some Clients request to test up to the Flange rating per B16.5, Which it's 1.5x153.2. Of course, the proper "components" should be used for the piping system to be tested per the flange rating pressure.
Both cases have their points and both are happened in the real practices.