Pak Tengku

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Pak tengku, Coba dicek di ASME B31 code, saya lupa section berapa, kalo gak salah ada tertulis di situ, test pressure tidak boleh menyebabkan yielding. Jadi sebelum menuliskan spek, sebaiknya salah satu algoritma saat perhitungan wall-thickness juga mengecek berapa hoop nya. Max. 90% dari SMYS. Salam, teddy susanto psn-melbourne > Rekan2 yg terhormat, > Saya ada pertanyaan mendasar yaitu: Menurut ASME B31.3 Hydrotest > Pressure adalah tidak boleh kurang dari 1.5 x Design Pressure dan > berdasarkan ASME B31.4 Hydrotest Pressure tidak boleh kurang dari 1.25 > x Design Pressure. Pertanyaan saya kenapa justru kita melakukan Test > di atas besaran Design Pressure? Hydrotest Pressure adalah salah satu > tujuannya untuk mengecek Leakage/kebocoran, apakah pipa tidak akan > langsung bocor bila di test diatas design pressure? > Mohon pencerahannya dari rekan2 sekalian. > > Tengku Syahdilan SePakat dgn pk Teddy, B31 code mensyaratkan hydrotest pressure tidak boleh menyebabkan stress > 90%SMYS. JAdi Pak,stress akibat design pressure drancang tidak d perbatasan SMYS. Seringkali dibatasi 72% SMYS bahkan 60%SMYS. Please refer to code B31. Yakinlah, bila udah dilakukan stress analysis, de el el. tekanan hydrotest jarang bikin bocor pipa/vessel kecuali bilamana materialnya abal2an or analysisny kurang mumpuni. Terima kasih atas masukan dari rekan2 semuanya. Namun saya sampai sekarang terkadang masih bingung antara penggunaan rumus P = 2(SE)t/Do-2Yt (ASME B31.1) dan P = 2(0.72 x E x Sy)t/Do (ASME B31.4)

Transcript of Pak Tengku

Page 1: Pak Tengku

Pak tengku,

Coba dicek di ASME B31 code, saya lupa section berapa, kalo gak salah adatertulis di situ, test pressure tidak boleh menyebabkan yielding. Jadisebelum menuliskan spek, sebaiknya salah satu algoritma saat perhitunganwall-thickness juga mengecek berapa hoop nya. Max. 90% dari SMYS.

Salam,teddy susantopsn-melbourne

> Rekan2 yg terhormat,> Saya ada pertanyaan mendasar yaitu: Menurut ASME B31.3 Hydrotest> Pressure adalah tidak boleh kurang dari 1.5 x Design Pressure dan> berdasarkan ASME B31.4 Hydrotest Pressure tidak boleh kurang dari 1.25> x Design Pressure. Pertanyaan saya kenapa justru kita melakukan Test> di atas besaran Design Pressure? Hydrotest Pressure adalah salah satu> tujuannya untuk mengecek Leakage/kebocoran, apakah pipa tidak akan> langsung bocor bila di test diatas design pressure?> Mohon pencerahannya dari rekan2 sekalian.>> Tengku Syahdilan

SePakat dgn pk Teddy, B31 code mensyaratkan hydrotest pressure tidak bolehmenyebabkan stress > 90%SMYS. JAdi Pak,stress akibat design pressure drancangtidak d perbatasan SMYS. Seringkali dibatasi 72% SMYS bahkan 60%SMYS. Pleaserefer to code B31. Yakinlah, bila udah dilakukan stress analysis, de el el.tekanan hydrotest jarang bikin bocor pipa/vessel kecuali bilamana materialnyaabal2an or analysisny kurang mumpuni.

Terima kasih atas masukan dari rekan2 semuanya.Namun saya sampai sekarang terkadang masih bingung antara penggunaanrumus P = 2(SE)t/Do-2Yt (ASME B31.1)dan P = 2(0.72 x E x Sy)t/Do (ASME B31.4)bukankah rumus diatas untuk menentukan MAOP, kemudian bagaimana kitamenentukan Design Pressure dari pipa ataupun proses itu sendiri?sbg contoh saat ini saya sedang mengerjakan project pipeline dan sayamelanjutkan pekerjaan piping engineer terdahulu yg baru saja dimulai,namun tidak ada tercatat berapa design pressure dan Design Temp. hanyatekanan dari pompa di jetty 100 Psi. apakah dari operating pressure inikita bisa menentukan angka pasti Design pressure dari system? malahsaya lihat material sudah dipilih pada rating 150# namun berapa designpressurenya tidak tercatat?bukankah seharusnya kita tahu angka pastidari design pressure baru kita bisa menentukan rating material dan lalu

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membuat spec? Material pipa A106 Gr B Sch 40, t= 0.280 inchMohon pencerahannya......

Design pressure dari suatu pressurized system (entah itu Pressure Vessel, PipingSystem, ataupun Pipeline) selalu ditentukan sejak awal desain dibuat. Dasarpenentuannya bisa bervariasi, tapi pada prinsipnya selalu memperhitungkankemungkinan tekanan (pressure) tertinggi yang mungkin terjadi pada pressurizedsystem dimaksud (untuk B31.3, bisa dibaca pada paragraf 301.2.2., B31.4 padapara 401.2, B31.8 lupa, ASME VIII D1 pada UG-21).Itu dulu yang harus dipegang mengenai design pressure.

Mengenai Hydrotest Pressure, ini lain lagi ceritanya, lebih berkaitan ke berapakekuatan aktual pressurized system tersebut (MAWP or MAOP) yg resmi (dgn katalain tersertifikasi/terbukti).Tapi intinya, untuk mematok berapa nilai MAWP (untuk Piping & Pressure Vessel)atau MAOP (untuk pipeline), maka perlu ditest untuk membuktikan kekuatan sipressurized system tersebut.Mengapa lebih tinggi X kali dari MAWP/MAOP? tujuannya adalah untuk memberi bebanlebih pada finished form dari pressurized system tersebut (setelah selesaipengelasan dan NDT test clear). JIka lolos uji (biasanya ditandai dengan tidakadaadanya kebocoran DAN tidak adanya deformasi permanent), maka pressurized systemtersebut layak diberi sertifikat "teruji" untuk MAWP/MAOP yang ditentukan itu.

DP menjadi MAWP/MAOP ini pun ada ceritanya tersendiri...

okterimakasih atas pencerahan nya pak..kalau boleh saya ingin tahu lagi, kira2 waktu penahanan (mgkn istilah teknik nyaretention time ya pak? ) untuk hydrotest itu gimana cara menentukan nyaPak?apakah umum nya 3 jam gitu atau bgmn? mohon pencerahannya

Pak Andreas,

Merefer B31.3, hydrotest 1.5x, pneumatic 1.1x.

Menambahkan komentar lainnya, tujuan hydrostatic test, (saya bicara pipa,untuk pipeline, ahli pipeline bisa menambahkan), sbenearnya dan yangterutama "BUKANLAH UNTUK MENGUJI JOINTS DAN WELDS BAKAL BOCOR ATAU TIDAK",tapi untuk "memberi semacam 'pre-stress' terutama pada flange-gasket" padamargin integirty di atas design pressure. Sehingga bila terjadi upsetpressure, pressure containtment masih tersedia.

Dalam stress analisis, pressure containment/hoop stress adalah primarystress, yang bersifat non-self limiting, potensial energi pressure akanmembesar ekivalen dengan pressure integrity si pipa, sampai akhirnyamelewati yield-UTS, failure point-dan bursting sesuai hukum hook.

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Menjawab pertanyaan Pak Tengku, mengapa perhitungan pressure bisa berbedadari satu code ke code yang lainnya adalah karena tiap code memberipenekanan dan esensi yang berbeda terhadap safety factor yang diberikan,B31.3 menjadi sangat konservative, karena dia tidak mengenal faktor regionplus pertimbangan ekonomi karena jarak pipeline yang berskalakilometer, apakah dia melewati remote location, jalan raya yang ramaidilewati orang, dsb seperti pada codes punyanya pipeline. B31.3 memandangperlu memberikan faktor yang lebih besar, karena lokasi piping berada didalam plant sehingga efek kerugian dan resiko jika piping kehilanganpressure containtment terhadap fasilitas juga lebih besar.

> hmmm,,> mungkin mau samaain persepsi dulu ..>> Hydrotest itu bukan nya 1.3 * MAWP (Maximum Allowable Pressure ) ya?> dan bukannya Pneumatic Test itu yang baru 1.5 * MAWP ?>> Design pressure mah sependek yang saya tahu hubungan nya dengan operating> pressure.>> besarya 10 % diatas OP atau sekitar 30 KPa diatas OP..?

> Pak Tengku Yth.>> Angka design pressure, seperti angka design engineering lainnya adalah> batas maksimum yang masih dapat ditolerir dalam pengoperasian suatu> komponen. Batas design ini pada umumnya mencakup tambahan faktor toleransi> yang disebut dengan "nilai margin".>> Jadi contoh suatu jembatan didisain untuk menahan beban operasi 10 ton.> Maka dalam analisis dimaksukkan faktor keamanan misal dengan margi 5 ton> sehingga dikatakan batas design 15 ton.>> Nah terkait dengan uji kekuatan termasuk untuk pressure vessel, yang lebih> tinggi dari nilai design itu e.g. 1,5 x saya kira sekedar untuk lebih> meyakinkan bahwa kondisi itu sangat aman untuk beban operasi normal yang> direncanakan (nilai konservativ)>> Demikian pendapat saya,> Salam

Hydrotest itu bukan nya 1.3 * MAWP (Maximum Allowable Pressure ) ya?dan bukannya Pneumatic Test itu yang baru 1.5 * MAWP ?

Design pressure mah sependek yang saya tahu hubungan nya dengan operatingpressure.besarya 10 % diatas OP atau sekitar 30 KPa diatas OP..?

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Pak Andreas,

Kalau yang di refer Pak Tengku adalah ASME B31.4, maka hydrotest pressure adalah1.25 x internal design pressure dengan durasi not less than 4 hours (untukpiping system yang akan dioperasikan at hoop stress of more than 20% of SMYS).

Menambahkan yang di informasikan Pak Teddy sebelumnya, pada saat piping systemdi test pada pressure yang dapat menyebabkan hoop stress melebihi 90% of SMYS,maka harus ada special care untuk prevent overstrain. Lebih detailnya silakanrefer ke ASME B31.4 Chapter VI Inspection and Testing, paragraph 437.4 TestPressure.

Salam,Didik

1.19.2010LEAVE A COMMENT

Maximum Allowable Working Pressure (MAWP)

Oleh Administrator

Tanya - iwan febrianto

Mohon pencerahan :

Sesuai rumus ketebalan pipa :

t =tc + th + Pi Do /(2(SE+Pi Y)

dimana Pi adalah design pressure

1. Jika kita ada data kondisi operasi maka kondisi design pressurenya berapa?

2. MAWP pipa dari table API RP 14 E apa bedanya dengan design pressure? bagaimana hubungannya

dengan thickness hasil rumus diatas.

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Terimakasih

Tanggapan 1 - Teguh Santoso

MAWP adalah kondisi maksimum operasi yang diperbolehkan. Design pressure selalu lebih besar dari MAWP.

Ini untuk mengakomodasi ketidak sempurnaan pengerjaan atau cacat material.. biasanya setting PSV akan

berada di bawah nilai MAWP. Ini sebagai proteksi juga. FYI, dalam disain ada parameter lain yaitu corrosion

allowance..CMIIW.

Tanggapan 2 - Rifai, Boorham (Jakarta)

Maaf Mas Teguh, AFAIK, design pressure adalah tekanan maksimum yg mungkin akan dicapai selama

pengoperasian equipment (vessel/piping?) dan digunakan sebagai basis utk perancangan equipment

(vessel/piping?). Ketebalan vessel (/piping?) dihitung dgn menggunakan design pressure, dan hasil

perhitungannya dicocokkan dgn standar ketebalan dan jenis material vessel (/piping?). Utk tiap2 jenis material

dan ketebalannya, memiliki rentang operasi pressure - temperature maksimum yg berbeda2. Maksimum

pressure ini yg dinamakan MAWP. Karena itu, design pressure < t=" Pi" mawp =" design">0<10>10 110%

MOP

Tanggapan 6 – asoulisa

MAWP bisa lebih besar daripada design P?

Mungkin maksudnya MAP.

Tanggapan 7 – Allan

Ya, coba baca Di buku "Pressure Safety Design Practices for Refinery and Chemical Operations" cukup detail

di jelaskan.

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Berikut kutipan nya

"......The design pressure is never greater than maximum allowable working pressure. In the case where the

actual metal thickness available for strenght is not known, the design pressure is assumed to be equal to the

maximum allowable working pressure......".

Tanggapan 8 - Bakti Kumoro

quote : (design pressure tidak akan lebih besar dari MAWP)"

beneran nih mas Allan ??

bisa jebol semua tuh mas kalau design pressure < MAWP.......

Tanggapan 9 – MahadiCapa

Betul mass, beberapa code/rules hampir sama tentang MAWP : 1.1 DP untuk pneumatic test dan 1.3 DP untuk

hydrostatic test.

Tanggapan 10 - muhammad rifai

yang ini hubungannya bagaimana ya?... kok nyambungnya MAWP : 1.1 DP dan 1.3 DP...

Tanggapan 11 - Bakti Kumoro

maaf, saya yang salah.......

thanks atas koreksinya mas......

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Tanggapan 12 - Harry Satriadi

Jelasnya bisa dilihat di API 520 P1 Figure. 1

Tanggapan 13 - Rifai, Boorham (Jakarta)

Mas Bakti, sepertinya Mas Allan bener deh.

Beberapa temuan yg menarik tentang design pressure vs MAWP dari Mas Google. Semoga berguna...

Max allowable working pressure is always equal to or higher than the design pressure.The design pressure is

normally the PRV set pressure of a vessel.This pressure is used to determine the minimum wall thickness. Say

a calculated thickness is 9.2 mm based on a des press. However, a standard material may have athickness of

10 mm. If the press is now back calculated based on this thickness this will be the MAWP.

http://www.eng-tips.com/faqs.cfm?fid=133

MAWP and Design Pressure

In paragraph 1.2.3.2 (b), API 520 defines maximum allowable working pressure (MAWP) as

"... the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position

at the designated coincident temperature specified for that pressure."

The operative word here is "completed". The vessel is completed when a fabricator, according to the code laid

down by ASME, has designed it. The vessel's fabricator, not the Process Engineer, determines MAWP. (Some

may try to stretch my definition of "completed" to mean that the vessel is also erected in place. Not quite

because the certified vessel drawings, which are delivered way before the vessel is, contains this information).

In the same paragraph, API 520 says that the MAWP is normally greater than design pressure. The Process

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Engineer usually sets the design pressure at the time the vessel specification is being written. The design

pressure is the value obtained after adding a margin to the most severe pressure expected during normal

operation at a coincident temperature. Depending upon the company the engineer works for, this margin is

typically the maximum of 25 psig or 10%. The vessel specification sheet contains the design pressure, along

with the design temperature, size, normal operating conditions and material of construction among others. It is

this document that will eventually end up in a fabricator's lap and from which the mechanical design is made.

http://www.cheresources.com/asiseeit1.shtml

The "Design" pressure is that pressure that the engineer decides is the value of the pressure at which the

vessel will normally operate (or which it must withstand under operating conditions). This value must include

any normal excess pressure that can occur during the vessel's operation. This is a discretionary value that

depends on the background and experience of the design engineer. Sometimes the design value can be 10%

over the pressure calculated (as in a simulation) or as much as 25% more. Good engineering judgment is

employed in arriving at this design figure.

Once the Specification Sheet is received by the vessel fabricator, mechanical fabrication design takes place in

which alloys, fabrication techniques, available materials, and other factors are taken into consideration to

generate a fabrication drawing. Although the design pressure given is employed to generate the required

vessel physical characteristics, some practical factors - such as available materials, fabrication efficiency

factors, and alloys employed - will result in a vessel that not only meets the required design pressure, but often

EXCEEDS it. This is a fortunate and conservative procedure because it ensures that the vessel will meet

pressure safety expectations. The Maximum Allowable Working Pressure (MAWP) is a result of back-

calculating the ultimate resulting fabricated vessel and is the prime factor in setting the pressure at which the

corresponding vessel Safety Relief Devices will be activated. I consider the MAWP the most important pressure

value attached to a vessel and one that should be clearly understood and stamped on the vessel for all to

clearly read. The MAWP will change with time (as will the related design value) due to wear, corrosion, and

vessel fatigue. This is why it is so important to religiously keep and maintain current and accurate data sheets

and calculations on all pressure vessels as they are inspected and repaired through the years of service.

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http://www.cheresources.com/invision/lofiversion/index.php/t2544.html

Tanggapan 14 - muhammad rifai

lebih bagus lagi kalau merefernya ke prasasti yang asli, misalnya ASME VIII... kalau nggak salah, PRV

dihubungkan dengan MAWP... soal MAWP bisa sama dengan PRV (atau disamakan), itu masalah lain.. ini

juga pernah dibahas panjang lebar...

jadi, kalimat "The design pressure is normally the PRV set pressure of a vessel" jangan ditelan mentah

mentah...

gimana kabare mas Rifai? WP jakarta aman?

Mas Andreas,

Kalau rajin2 mengikuti perkembangan Code, maka sebenarnya nilai hydrostatictest di ASME BP&V itu berubah dari 1.5x ke 1.3x. Salah satu alasannya adalahuntuk menghindari overstressed pada PV akibat hydrotest. Pada saat yang samajuga allowable stress dari beberapa material dinaikan dari 1/4 UTS ke 1/3.5UTS, meskipun angka 2/3 Yield itu tidak berubah. Jadi ASME BP&V codessebenernya semakin progresif mengikuti european codes.

Hal lain adalah PV umunnya mendapat proteksi yang cukup dari over-pressuresaat operasi dengan adanya PSV , mengingat juga kemajuan teknologi produksimaterial, quality control, daannn mungkin saja perubahan diatas akibat lobi2vendor dan kontraktor kelas kakap di US sana untuk menurunkan costproduksi??? who knows....

Sewaktu training di Abu Dhabi dulu, hal yang sama pernah ditanyakan ke PakGeorge Antaki, dukunnya ASME-PCC, API-579 dan chairman ASME-API, kenapauntuk pipa tidak ikut diturunkan saja. Dia bilang committe memandang bahwasecara "keekonomian" tidak lah terlalu perlu menurunkan batasan 1.5x sepertipada PV. Dalam hal ini committe memandang perlu untuk tetap mempertahankanconservativism pada piping code.

Artikel dibawah mungkin bisa dijadikan bahan bacaan, selain chapter B14kemarin.

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always confuse with design pressure (DP) and maximum allowable working pressure (MAWP). Do they carry same meaning? What about maximum operating working pressure (MOWP)? Can anybody clarify me on this? TQ.

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#2 Art Montemayor

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Posted 28 June 2006 - 07:27 AM

naihusna:

It is not unusual for engineers to get confused with the terms "design pressure" and "Maximum Allowable Working Pressure" (MAWP). The two terms are not related mathematically; rather, they are related in a practical procedure that takes place during the actual fabrication of a pressure vessel.

Every Chemical Engineer - sooner or later - will have to deal with a pressure vessel fabrication or operation and it is sound and good advice that he/she should familiarize themselves with engineering terms employed and the logic of their application. In dealing with or specifying a pressure vessel, an engineer must resort to filling in or using a Vessel Specification Sheet - much like the one I have included in the attached Excel Workbook that gives you a host of Specification Sheets that you should be familiar with and employ in the course of your work.

The "Design" pressure is that pressure that the engineer decides is the value of the pressure at which the vessel will normally operate (or which it must withstand under operating conditions). This value must include any normal excess pressure that can occur during the vessel's operation. This is a discretionary value that depends on the background and experience of the design engineer. Sometimes the design value can be 10% over the pressure calculated (as in a simulation) or as much as 25% more. Good engineering judgment is employed in arriving at this design figure.

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Once the Specification Sheet is received by the vessel fabricator, mechanical fabrication design takes place in which alloys, fabrication techniques, available materials, and other factors are taken into consideration to generate a fabrication drawing. Although the design pressure given is employed to generate the required vessel physical characteristics, some practical factors - such as available materials, fabrication efficiency factors, and alloys employed - will result in a vessel that not only meets the required design pressure, but often EXCEEDS it. This is a fortunate and conservative procedure because it ensures that the vessel will meet pressure safety expectations. The Maximum Allowable Working Pressure (MAWP) is a result of back-calculating the ultimate resulting fabricated vessel and is the prime factor in setting the pressure at which the corresponding vessel Safety Relief Devices will be activated. I consider the MAWP the most important pressure value attached to a vessel and one that should be clearly understood and stamped on the vessel for all to clearly read. The MAWP will change with time (as will the related design value) due to wear, corrosion, and vessel fatigue. This is why it is so important to religiously keep and maintain current and accurate data sheets and calculations on all pressure vessels as they are inspected and repaired through the years of service.

When you have a need to set a PSV on a vessel and you don't have its MAWP figure, you can employ the "design" pressure value - as long as it can be proven that the vessel is in as good a physical condition as the day it was fabricated. Note that I'm going to lengths to define the physical condition of the vessel. We often neglect to mention that we are ASSUMING that the physical condition of the vessel doesn't change from the day it was fabricated. This can be a dangerous assumption that doesn't necessarily apply. A vessel can undergo corrosion and wear as well as other chemical attacks through its use and lifetime. Physical and meticulous inspections and reports are essential to ensure that the vessel can be safely applied to a process --- especially to a high-pressure application. And I consider any pressure over 50 psig as HIGH PRESSURE. When a vessel explodes, it isn't the pressure that kills you; it's the amount of shrapnel and steel pieces that are blown about that do the damage. And even 50 psig can cause a considerable amount of serious damage if allowed to trigger a vessel failure.

I have never come across the term "Maximum Operating Working Pressure" and can only presume it means the same thing as MAWP. People are forever changing the writing of terms in order to suit their own likes and dislikes. MAWP was first described and is still employed by ASME in the USA and is the term I have always used to define what I have described in the above.

I hope this helps you understand the terms you have been confused with.

Attached Files

Art__s_Process_Spec_Sheets.zip 191.47K 6968 downloads

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Page 12: Pak Tengku

#3 djack77494

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Posted 28 June 2006 - 12:27 PM

naihusna:

Art has provided a very useful and complete picture of the meaning and significance of these two terms. I want to take a moment to stand on a sopbox and say to all that you should REQUIRE vessel manufacturers to indicate the MAWP in their documentation. They should always do so, but I have seen a very disturbing trend wherein the design pressure is represented as being the MAWP. They are not the same and should not be represented as the same. If only one MAWP is listed, I like to see what is called the "hot and corroded" MAWP. This means merely the MAWP when the vessel is at design temperature and after the corrosion allowance you specified has been taken into account. I have seen alternative numbers represented as "the MAWP", though I'm not sure of the legality of doing this.Doug

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#4 pleckner

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Page 13: Pak Tengku

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Posted 29 June 2006 - 11:42 AM

Adding:

The reason the MAWP, as stamped on the vessel nameplate, is more commonly the specified design pressure is cost and nothing more. By selecting the next standard plate thickness after calculating the minimum required to meet the design pressure at the coincidental design temperature, they are assured of meeting the desgin requirements and may stamp the vessel as such. But to get the true MAWP requires the manufacturer to do some more calculations (back calcualte the value as Art stated) and then offer that guarantee. If you, the owner want the true MAWP, then ask for it on your specification and expect to pay some more.

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#5 naihusna

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Page 14: Pak Tengku

Posted 01 July 2006 - 10:53 PM

A lot of thanks to Art and others for that lengthy explanations. Also, thanks for the process spec sheet. I'll try to utilise it in my workplace.

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#6 jerald04

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17 posts

Posted 21 September 2007 - 10:06 PM

Thank you Art and everyone here for the clear explanations on the difference between these two terms. I was having difficulties trying to digest the definitions provided in ASME codes for a young engineer like me, and this forum made the explanations in so much easier to swallow.

Appreciate it.

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#7 jerald04

jerald04

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Page 15: Pak Tengku

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Posted 02 October 2007 - 07:49 AM

I apologise if my following post is going to go a little off topic, but it is somewhat related to the MAWP.

I came across a pressure vessel recently, with a design pressure of 43kgf/cm2G and a MAWP of 43.95kgf/cm2G. Obviously, the penumatic and hydrostatic test pressure for the vessel would be at 47.3k and 55.9k respectively.

May I find out if the test pressure values is supposed/ intended to be always higher than its MAWP? If the test pressures have to be higher than MAWP, wouldnt it exceed the max pressures that the vessel is designed for?

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#8 djack77494

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Page 16: Pak Tengku

Posted 02 October 2007 - 09:17 AM

jerald,You raise some interesting points and seem to suffer from some confusion about this topic. Concerning pneumatic and hydrostatic testing, it's "either/or". You either hydrostatically test the vessel OR (if absolutely necessary) you pneumatically test the vessel. A discussion on these two types of testing was recently active in this forum, and I suggest you review what is said there.

You correctly note that the test pressure, whether hydrostatic or pneumatic, is greater than the MAWP. If you think about it a bit, you'll note that the vessel is not designed so that it will explode just above the MAWP (duh). In fact, you want to test it at a somewhat elevated pressure using an innocuous fluid - e.g. water - so that if its integrity is found lacking you have not released some toxic or flammable fluid. You don't want to do that "test" with a hot nasty operating fluid filling the vessel, so you test with water or air. Codes may vary some, but I'm accustomed to the hydrostatic test pressure being 1.3 x the MAWP. (It was formerly 1.5 x the MAWP.) There are correction factors for the lower temperature conditions during testing when the design temperature is elevated. I'm not sure how corrosion allowances are handled. Test pressures are different (lower) if you test pneumatically. In any case, the vessel is actually designed to be able to fully withstand the test pressure, providing a bit of a "buffer". Never try to infringe into this buffer; it is both dangerous and illegal.

I hope this provides somewhat of an explanation of how these terms interrelate.Doug

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#9 JoeWong

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Page 17: Pak Tengku

Posted 03 October 2007 - 02:19 AM

Doug,Good explanation...

QUOTE I'm not sure how corrosion allowances are handled.

I guess short term exposure will not be considered.

I believe normally there are requirement to testing medium e.g. water with Chloride content less than 250 mg/l (in some area and some project) to be used...some client may have even more stringent requirement (demin water for reactor) on this aspect.

JoeWong

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#10 fallah

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Posted 03 October 2007 - 06:19 AM

QUOTE (jerald04 @ Oct 2 2007, 07:49 AM) <{POST_SNAPBACK}>.

I came across a pressure vessel recently, with a design pressure of 43kgf/cm2G and a MAWP of

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43.95kgf/cm2G. Obviously, the penumatic and hydrostatic test pressure for the vessel would be at 47.3k and 55.9k respectively.

I think, because of conservative recommended values of allowable stress for material in standards(for example in B31.3 , max. recommended allow. stress is two third or 2/3 of min. yield strength) the short therm exposure to higher pressure(1.3 times that still lower than yield point) results in any problem.

Fallah

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#11 jerald04

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Posted 05 October 2007 - 10:19 AM

Hi Doug,

Thank you for your detailed explanation. Now I can confirmed that things are in order from the observation I made for the pressure vessel MAWP and its test pressures.

I'm not sure if its part of the ASME code to do both types of pressure testings for pressure vessels. Its a requirement for my company though.

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#12 pleckner

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Posted 05 October 2007 - 11:09 AM

It is NOT a requirement of ASME to pressure test using both methods. I would go back and double check if this is indeed your company standard because it makes no sense to me at all.

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#13 djack77494

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Posted 05 October 2007 - 02:28 PM

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I have to weigh in on this topic again. Phil is absolutely correct to note that it makes no sense to "double test". The hydrostatic test is a good vessel integrity test; the pneumatic test is a lousy test.

Besides the great potential for disaster, it is very difficult to detect a small leak in a pneumatic test. Think about it. If you have a small leak, a small amount of air escapes. What happens? A very small drop in pressure that is very difficult to detect. Switch to a hydrostatic test of a vessel with a small leak. If you've done a good job of filling the vessel with water, a small leak is readily detectable as a LARGE drop in pressure. That's what you want - amplification of problem detection.Doug

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#14 trailbarge

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Posted 17 October 2007 - 08:52 AM

I registered to this forum just so I could reply to this thread that I ran across in search for something else. Not long ago, I worked as a mechanical engineer in a pressure vessel company. We dealt in very high pressure stuff (30 ksi?... that's midrange). I'm no P.E. (yet) and I was the most junior engineer on the staff, but I can address this dual testing thingy that some folks are soapboxing about.

We ran both a pneumatic and a hydro test. The hydro test was always done per ASME code and is required by that code. Water is fairly safe for the test per comments earlier in the thread... mainly because it has few toxins (so spills and ruptures don't require NBC gear), is relatively

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inert to most construction materials (chlorides might be a sticky point to some because spots of precipitates are thought by some to be sites for stress corrosion cracking later on) and is relatively incompressible (especially when compared to ANY gas). You can think of pressure as storing energy in a fluid. A gas is like a springboard... a springboard (or diving board) deflects quite a bit and will return it when released, flinging the diver high. A liquid like water will be less compressible... more like a diving platform. The diver must use shear muscle to launch with no springs to help out (there is still spring... but almost immeasurable in comparision to the springboard).

Puttting the asides aside (sic)... the hydrotest is supposed to be a test of the structural integrity of the vessel. If it should fail, water will spray and spill, but will not explode. The stored energy content is too small. A gas will explode, so it needs to be heavily barricaded and is all sorts of trouble and expense... so it is avoided if at all possible.

After a vessel passed the hydro test, we usually did a pneumatic test. Most of the time with air sometimes with nitrogen. Nitrogen is fairly inert so leaks don't cause secondary problems (just ventilate enough so no one gets suffocated). The purpose of the pneumatic/gas test was to test for leaks. Granted to some of the posters, a leak of an "incompressible" fluid will show on a pressure guage faster. On the other hand, gases are generally smaller and more slippery and will sneak out around seals and migrate through elastomers where a liquid will not. Some process fluids will accumulate in a pit until they reach some critical density and then something tragic will happen, so we test for smaller leaks than would be caught on a pressure guage. We gas test at 1 or 1.1 of MAWP to see if the little buggers are getting out. Usually, a bubble test is enough. In some cases, we actually used a sniffer to detect really small leaks. The costs go up as you add more test but none of them are useless. You have to balance these costs against the criticality of the system.

I hope that this helps straighten things out.Paul

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Change in ASME B&PV Code Allowable Stresses

The allowable stress in the pressure vessel code has changed for SectionVIII, Div 1 Pressure Vessels and Section I Boilers. The change is already ineffect for most materials via Code Cases, and will be changed in Section IIof the Code for all materials soon. This creates the opportunity for thinnerwall vessels and rerating of existing pressure vessels for increased designconditions or greater corrosion allowance.

The factor on the tensile strength of the material that is used inestablishing the allowable stress has been changed from 1/4 tensile strengthto 1/3.5. This change brings the ASME Boiler and Pressure Vessel Code designallowable stresses closer to those of most European Pressure Vessel Codes,and was done after a careful study concluded that this change could be madewithout impacting the safety of the affected pressure equipment. Note that

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the last change was made over forty years ago, and was from 1/5 to 1/4.

Not all materials will be affected since some are limited by the existing2/3 yield strength limit which remains unchanged. Also, the allowable stressis not changed at elevated temperatures where it is controlled by creepproperties of the material.

The following Code Cases have been approved. Note that Code Cases can beused as soon as they are approved; it is not required to wait forpublication.

Code Case 2284 on Alternative Allowable Stresses for Section I ConstructionBased on a Factor of 3.5 on Tensile Strength, Section I and Section II

Code Case 2290 on Alternative Maximum Allowable Stresses Based on a Factorof 3.5 on Tensile Strength; Section II and Section VIII, Division 1

Code Case 2278 on Alternative Method of Calculating Alternative MaximumAllowable Stresses Based on a Factor of 3.5 on Tensile Strength Section IIand Section VIII, Division 1

The first two Code Cases provide new allowable stress tables for selectedalloys. The Section I Code Case contains a more complete listing ofmaterials, and Code Case 2278 provides, for Section VIII, Division 1, ameans for the user to calculate the allowable stresses for materials thathave tensile and yield strength values provided in Section II. Note thatconservative temperature limits are provided in Code Case 2278 to preventthe designer from calculating allowable stresses in the creep regime for thematerials.

The new Code Cases also include the latest data analysis for the materials,so they contain some additional changes based on reevaluation of thematerial property data. These tables also show where the allowable stress isbased on creep properties of the material. This is how future tables inSection II will appear.

In addition to changing the allowable stress, the factor used in calculatingthe hydrotest pressure is reduced from 1.5 to 1.3. This is intended to avoidoverstress conditions during the hydrotest; the stresses during hydrotestremain essentially the same as with the prior code allowable stresses.

While rules for rerating of components (e.g., in API 510) using these higherallowable stresses are not yet issued, the following considerations apply.

Reanalysis of the vessel for the new design conditions considering allcomponents and loads (e.g., wind).

Conformance with all essential technical requirements in the current code(e.g., toughness, welding PWHT, NDE, etc.).

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Inspection and evaluation of present condition of the vessel; review ofinspection records.

Adequate hydrotest for the new condition; re-hydrotest if the originalhydrotest is not sufficient.

AI and/or jurisdictional acceptance, as applicable.

A complete reevaluation of the vessel, considering current conditions (e.g.,present corrosion and anticipated future corrosion) should be performed.Note that one very important consideration is material toughness; the vesselmust comply with the current impact test requirements.

In addition to a change in allowable tensile stresses, Code Case 2290 onAlternative Rules for Determining Allowable Compressive Stresses forCylinders, Cones, Spheres and Formed Heads; Section VIII, Divisions 1 and 2,was approved. It provides more modern rules that generally permit a greaterexternal pressure than the present rules.

Be sure to check with the appropriate jurisdiction as to whether they willaccept these Code Cases. Per the Foreword of the Code, "Manufacturers andusers of components are cautioned against making use of revisions and Casesthat are less restrictive than former requirements without having assurancethat they have been accepted by the proper authorities having jurisdictionwhere the component is to be installed."

*This article published in Becht Engineering Mechanics Division NewsletterVolume 8/Number 1 (1999).*