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How Conservative Mechanical Engineering was? October 19, 2009

Posted by nzafee in DUNIA MIGAS.

Rangkuman Diskusi

Pembahasan – Cahyo Hardo

To cover uncertainity in the real world, design engineer always add factor to cover it. We call “safety factor”.

Adding 20% capacity for dehydration tower is make sense, to cover uncertainity in gas feed condition. Water content, which is the target to be reduced is really function of gas feed temp. & pressure. Higher temp. & lower pressure are condition that should be anticipated. That’s why, gas in saturated condition with water, always be an assumption during design the dehydration system.

Adding 20% excess area in heat exchanger (HE) is also still make sense since what we calculated in HE, is just a prediction, eventhough sophisticated process calculation was used. Another argumentation was, adding excess area will safe your heat transfer performance, eventhough fouling occurs inside of HE.

For gas turbine engineer, specify inlet air temperature at average condition is a not-make sense assumption, since the performance of the machine is strongly function of inlet air temp. So, to cover uncertainity, the engineer always assumed inlet air at highest temp. condition, which give us a bigger size unit (in energy term).

But, when you buy a pressure vessel, a repeat question always come back again in my foolish brain.

ASME (American Society of Mechanical Engineer) Section VIII define thickness of the cylindrical pressure vessel under internal pressure as:

t = P Di/(2SE-1.2P) + C

For Div I, ASME applied S as 1/3.5 of Tensile Strength (TS)

For Div II, applied 1/3 of TS

British Standard (BS) 5500 define thickness for the same vessel as

t = PDi/(2SE-P) + C, when S = (2/3) of specific minimum yield strength (SYMS)


P = design pressure

Di = inside design pressure

S = max. allowable stress

C = corrosion allowance

F = joint efficiency (1 for fully radiograph, 0.65 for

no radiograph).

TS, according to the stress vs elongation diagram is the point when the vessel start to broken. The question are, why both standards applied different value?

The next question probably more interesting, why ASME applied safety factor for S only 1/3.5 or 1/3 of TS value. These values cause thickness of the vessel is thicker. If we compare with 100% of TS, the reduction of wall thickness could be 3 times.Meanwhile for BS, choosing 3/3 rather than 2/3 of SYMS will saving wall thickness by 1.5 times.

I try to correlate this with hidrotest pressure, which is usually equal with 1.5 times of design pressure. By re-calculate the S factor use the same thickness but use 1.5 times of design pressure, give us S about 50% of TS. And for BS, give us 100% of SYMS. Therefore, both of them are not make sense to me.

I just think, if we can bargain the design, probably cost will be decrease significantly, because cost of vessel is linear with weight (linear with wall thickness). For existing vessel, probably we can take out PSV block discharge-type forever without diluting safety. And probably, we can alco take out our existing HIPPS system. Why? because our vessel inherently self protected. Our vessel met “passive protection” concept.

But, wait the minute, those just my opinion. Why mechanical engineering is very conservative? Are there any correlation with welding of accessories attached to the vessel? I really do not know.

Let’s expert to explain these…!

Tanggapan 1 – A. Jamal

I’d like to start this discussion with my thesis “The most sincere proffesion is engineer, because engineer use safety factor simply because he admits his lack of knowledge”.

To explain this problem, we should ask a material specialist rather than mechanical engineer likes me.

However I hope, with my lack of knowledge about material, I can release a little bit curriosity why mechanical engineer so conservative.

BTW, ASME Section VIII div 1, pre-1999 edition use Safety factor 4 instead of 3.5, This SF against tensile strength (TS). The allowable stress S according to Div. 1 pre 99, is minimum value of 1/3 TS or 0.63 YS (Yield Strength) After 1999, the American Mechanical Engineer became less conservative by lowering SF 4 to 3, so this may cheer up Mr Cahyo, that even mechanical engineer are getting smarter. Another interesting point is actually why the American (ASME) has two standards for presuure vessel( i.e. ASME VIII DIv 1 for conservative design and Div. 2 for less conservative design). Although I know the Yankee always has double standard in everything. :), It is up to end users which design code they want to use. In general, British (and most other European) are less conservative than american.

Now stop talking about other people, talking more technically, not intending to teach expert in this milist. material (especially metal) has two weak points, namely, yield strength (maximum stress for metal to behave elastically) and Tensile Strength (maximum stress for metal not to break).

There are another limit stresses like fatigue strength (dealing with repetitive loading), and Creep strength (dealing with high temperature and time). These last two weak points are seldom reffered by ASME, because it is more difficult to be understood. In short, ASME Code is a “simple rule” to make mechanical engineer’s live “easy” and “happy” and most important thing IF they follow the code, their design will be “SAFE”. But I agree with Mr. Cahyo this code is far from “ECONOMIST”, but we will be faced with a question whether we build a space shuttle, where one extra kilogram means many time payload (dollars), or a pressure vessel, where price and weight has linear relation. FYI, SF less than two is very common in aerospace engineering.On more thing about hydrotest, the rule said, no permanent deformation may be detected after test, so, 1.5 times 2/3 YS is 100% yield strength, the maximum stress you may apply on the vessel, unless you prefer a nice-bass-guitar form for your column after a test….

Back to my thesis in the begining, if Indonesian engineers have sufficient knowledge about their work, why we stilll use foreign code in stead of develop our own code and rule and use it in our country as mandatory rule???

Tanggapan 2 – Sukandar, Erman H.

Saya tertarik dengan pernyataan pak Abu Kamil:

“…yield stress bukan harga mati yang tidak boleh dilampaui…padahal ada di ASME/ANSI B31”. Sudikah anda memberitahukan tepatnya di pasal berapa dari ASME B31 yang mendukung pernyataan anda.

Tanggapan 3 – Hari, Suprayitno

Pak Cahyo ini sudah jadi orang “Londo” rupanya.

Sepengetahuan saya koefisien, allowance, safety factor atau konstanta2 yang lainnya itu adalah empiris jadi berdasarkan experimen2. Nanti mungkin akan tambah banyak lagi kalau ditambah dari JIS atau DIN misalnya. Nah karena lembaga penelitian kita belum diberdayakan untuk tujuan2 seperti itu, ya sampai sekarang kita akan ngikut terus apa yang sudah ada. Entah sampai kapan.




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