AS4041 Section 3.10.3 and B31.3 section on pressure variations

[dsg1985]

In AS4041 (Australian Pressure Piping standard) there is a section and clause that I am trying to understand. It is also (I've been told) in the one of the B31 codes. It is 3.10.3 (variations of pressure, mainly point (iii) and onwards).

It reads as follows:

3.10.3 Variations in normal operating conditions

Occasional variations in pressure and temperature during the design life of ferrous piping are acceptable within the following limits:

(a) Where the fluid is steam... shall not exceed those specifications in Clause 3.9.5.
(b) Where the fluid is boiler feed water. . . shall not exceed those specified in Clause 3.9.6

Where the fluid is other than steam or boiler feed water, a piping system shall be considered safe during those variations when all of the following conditions are fulfilled:

(i) The piping does not contain pressurized components made from cast iron or other non-ductile materials.
(ii) For piping not in the creep range, the hoop stress is not more than the hot yield strength at the highest temperature occurring during the variation.
(iii) The number of significant variations, or cycles of pressure during the design life, is not more than 7000. In this requirement a pressure variation of greater than +/-20% of the design is significant.
(iv) The highest pressure occurring during the variation is less than the hydrostatic test pressure.
(v) Occassional variations above the design pressure and temperature comply with the following:
(A) The period of the variation is less than 10 h at any one time and the sum of the periods is less than 1000 h in a year. The pressure rating of a component of the yield strength at the highest temperature during any of the variations may be increased by not more than 33 percent.
(B) The period of the variation is less than 50 h at any one time and the sum of the periods is less than 500 h in a year. The pressure rating of a component or the yield strength at the highest temperature during any of the variations may be increased by not more than 20 percent.

(vi) An evaluation of the combined effects of the sustained and cylcic variations on the design life of all components shall be made. The results of the evaluation are the subject of agreement between the parties concerned.



It's 3.10.3 (iii) and onwards that bugs me. If my design pressure is 1000 kPa, does this mean a variation of more than 400 kPa cannot occur more than 7000 times during the design life?

What is the intent of this clause? In the applications I deal with typically, there will be 1-2 pump starts per day (minimum). Take a 1000kPa system, starting from 0kPa and achieving full working pressure. Does this mean this system is unsafe if operated for more than 10 years (7300 starts at 2 per day) even if the design pressure of the pipe is, say, 1200kPa? (assuming no other variations). Even if a pump start only raises the pressure by 400kPa? It seems absurd.

Or does it mean it can tolerate 7000 pump starts were the starts to go above 1200kPa? I'm speculating too much now. Any guidance or references on this clause would be very helpful.

(I had to type this out from a hardcopy, please forgive typos)

 

[BigInch]

This section is to limit fatigue of the pipeline material. A 40% increase in pressure over design pressure is significant, as it is close to yield stress ranges, so it contributes towards reaching the material's fatigue life limit.

The result is, if you want to operate your system with 2 starts/day for more than 10 years, you must increase the base pipe design pressure such that the new overpressure range/design pressure falls within the code's prescribed time of event limits. If you increase the design pressure to 1400 kPa, you will be well below yield stress excursions and theoretically you could operate forever with little concern for fatigue failure.

From "BigInch's Extremely simple theory of everything."

 

[dsg1985]

OK so the clause (iii) means no more than 7000 at 120% or above the design pressure (and I assume -20% below the minimum allowable pressure).

Read literally, the clause can be taken to mean variations below the design pressure, if they are of 40% of the design pressure, are 'significant'. e.g. In a system with a design pressure of 1000kPa, pressure variations, even if well below the design pressure, are unacceptable if they are greater than 400kPa in range - a pump start producing a 0kPa to 500kPa rise is unacceptable more than 7000 times.

 

[BigInch]

I don't believe that clause applies in the literal sense, since negative deviations from an acceptable design pressure would not produce excursions into the fatigue danger zone and other specified loading provisions would cover any system load conditions outside of the fatigue ranges, such as vacuum pressures, or pressures lower than the fluid's vapor pressure. Provided that the return to above vacuum, or pressure's higher than Vp were not also accompanied by high transient pressures during the colapse of vapor pockets, etc.

From "BigInch's Extremely simple theory of everything."

 

[stanier]

You could change the levels in the tank or tank size to reduce the number of pump starts.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[stanier]

"Where the fluid is other than steam or boiler feed water, a piping system shall be considered safe during those variations when all of the following conditions are fulfilled:"

"(iii) The number of significant variations, or cycles of pressure during the design life, is not more than 7000."

The number of cycles in your case is > 7000 and hence you cannot use the dispensation of "Occasional variations in pressure and temperature during the design life of ferrous piping are acceptable within the following limits:"

Occassional variations in pressure are NOT acceptable and you need a higher class of pipe, surge mitigation measures or other means of reducing your pressure.

In respect of "Does this mean this system is unsafe if operated for more than 10 years (7300 starts at 2 per day) even if the design pressure of the pipe is, say, 1200kPa? " No it does not. If your number of cycles did not exceed 10hr with a total per year of 1000h then provided all the other criteria are met the pressure can exceed 1.33P alternatively 50 hr, 500r and 1.20P applies.

The consideration for fatigue relates to 3.10.3 iii) condition only which is but one criteria. ie the pressure and temperature should NOT have exceeded the normal conditions.

You cannot compare one clause from one standard with another as the standard embraces design, fabrication, installation, testing and other matters. A lower requirement in one clause may be augmented by a more severe requirement in another.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/