Cara Memilih Material Pipa

"PIPA SEAMLESS" Material pipa ada berbagai jenis. Bagaimana cara untuk menentukan material yang harus digunakan? Untuk dapat menentukan material, terutama untuk bidang industri, faktor yang paling penting adalah fluida apa yang akan mengalir didalamnya. Selain itu, kondisi luar dari pipa juga akan mempengaruhi. Dan terakhir, tentu saja dalam sisi ekonomi juga akan menjadi dasar pemilihan material.
Pipa juga dapat dibagi menjadi 2 bagian besar. Pipa dari logam dan non-logam. Logam telah terdiri dari carbon steel, stainless steel, aluminium, nickel dan lainnya. Berikut ini adalah contoh dalam desain pipa untuk pabrik industri gas alam, minyak, atau pabrik kimia lainnya.
Pertama, insinyur proses harus bisa menghitung apa dan berapa banyak macam kandungan yang akan melewati pipa. Pada dasarnya, semua pipa untuk proses biasanya harus memakai pipa logam dan dimulai dari material carbon steel yang paling murah.
Akibat aliran fluida, bagian dalam pipa akan mengalami korosi, dan salah satu cara untuk dapat menetapkan kecepatan korosi adalah memakai grafik de Waard – Milliams nomograph. Grafik ini membantu untuk dapat menentukan berapa kecepatan korosi (mm/tahun) yang telah disebabkan adanya kandungan CO2 dalam fluida.
Problem disebabkan korosi juga dapat diatasi dengan menambah ketebalan pipa sebesar kecepatan korosi dikali tahun lamanya pabrik didesain. Tetapi, jika total ketebalan yang dibutuhkan untuk dapat mengatasi korosi itu terlalu tebal, pipa akan menjadi sangat tebal dan tidak efektif dalam pembangunannya. Untuk keadaan ini, pipa dari stainless steel telah menjadi pilihan selanjutnya.
Selain korosi, suhu fluida juga dapat menentukan material pipa. Semakin rendah suhu, logam akan menjadi mudah mengalami retakan. Ini karena sifat brittle (getas)  logam bertambah pada suhu rendah . Stainless steel merupakan salah satu yang tahan akan suhu rendah. Karena itu, untuk cryogenic service (fluida dengan suhu operasi dibawah -196 degC) stainless steel adalah material yang cocok dibandingkan dengan carbon steel.
Stainless steel sering disebut juga corrosion resistance alloy (campuran logam tahan korosi) dan tentunya akan lebih mahal jika dibandingkan carbon steel. Stainless steel bisa dibagi menjadi beberapa jenis, contohnya austenitic, feritic, martenistic, duplex dan high alloy stainless steel (campuran tinggi logam stainless steel). Sayangnya, stainless steel tidak tahan terhadap semua jenis korosi, terutama korosi yang disebabkan oleh klorida, sulfida serta fluida asam (sour fluid) lainnya.
Untuk sistem pipa yang mengalirkan fluida asam (piping system for sour service) biasanya di desain berdasarkan standar NACE (National Association of Corrosion Engineers) MR0175. Mulai tahun 2003, standar NACE MR0175 bersatu dengan ISO 15156 dan yang telah memiliki syarat desain yang sulit dibandingkan edisi tahun sebelumnya.

Berdasarkan NACE MR0175/ISO 15156, penggunaan austenitic stainless steel telah dibatasi oleh kombinasi dari kadar khlorida, H2S (hydrogen sulfide) dan suhu fluida. Jika austenitic stainless steel tidak dapat digunakan, maka penggunaan duplex atau high alloy stainless steel merupakan pilihan selanjutnya. Jika duplex atau high alloy stainless steel juga tidak dapat digunakan, maka pilihan selanjutnya adalah menggunakan logam campuran nikel seperti incoloy dan inconel.
Semakin tahan terhadap berbagai korosi, semakin mahal harga material tersebut. Untuk dapat mengurangi biaya, pengaplikasian cladding atau overlay juga merupakan salah satu alternatif. Misalnya dengan menggunakan pipa dari carbon steel dengan dilapisi logam mahal pada bagian dalamnya saja yang bersentuhan langsung dengan fluida sumber korosi akan bisa menekan biaya tanpa mengurangi ketahanan terhadap korosi.
Pemilihan material ini bukan hanya untuk pipa, tetapi juga berlaku untuk bejana (vessel), katup (valve) dan elemen pipa lainnya. Untuk katup, walaupun material dari badan katup bisa memakai carbon steel, tetapi bagian dimana korosi tidak diperbolehkan untuk dapat menjaga kemampuan katup untuk menyekat (sering disebut sebagai trim, seperti bagian valve seat, stem dan lainnya), maka penggunaan stainless steel atau logam tahan korosi lainnya menjadi keharusan.
Pada saat melakukan pemilihan material yang sebenarnya, mungkin tidak akan semudah yang dijabarkan diatas, tetapi secara umum, begitulah proses pemilihan material pada saat mendesain pabrik industri.

Editor : Dian Sukmawati


Hockey is not exactly known as a city game, but played on roller skates, it once held sway as the sport of choice in many New York neighborhoods.

“City kids had no rinks, no ice, but they would do anything to play hockey,” said Edward Moffett, former director of the Long Island City Y.M.C.A. Roller Hockey League, in Queens, whose games were played in city playgrounds going back to the 1940s.

From the 1960s through the 1980s, the league had more than 60 teams, he said. Players included the Mullen brothers of Hell’s Kitchen and Dan Dorion of Astoria, Queens, who would later play on ice for the National Hockey League.

One street legend from the heyday of New York roller hockey was Craig Allen, who lived in the Woodside Houses projects and became one of the city’s hardest hitters and top scorers.

“Craig was a warrior, one of the best roller hockey players in the city in the ’70s,” said Dave Garmendia, 60, a retired New York police officer who grew up playing with Mr. Allen. “His teammates loved him and his opponents feared him.”

Young Craig took up hockey on the streets of Queens in the 1960s, playing pickup games between sewer covers, wearing steel-wheeled skates clamped onto school shoes and using a roll of electrical tape as the puck.

His skill and ferocity drew attention, Mr. Garmendia said, but so did his skin color. He was black, in a sport made up almost entirely by white players.

“Roller hockey was a white kid’s game, plain and simple, but Craig broke the color barrier,” Mr. Garmendia said. “We used to say Craig did more for race relations than the N.A.A.C.P.”

Mr. Allen went on to coach and referee roller hockey in New York before moving several years ago to South Carolina. But he continued to organize an annual alumni game at Dutch Kills Playground in Long Island City, the same site that held the local championship games.

The reunion this year was on Saturday, but Mr. Allen never made it. On April 26, just before boarding the bus to New York, he died of an asthma attack at age 61.

Word of his death spread rapidly among hundreds of his old hockey colleagues who resolved to continue with the event, now renamed the Craig Allen Memorial Roller Hockey Reunion.

The turnout on Saturday was the largest ever, with players pulling on their old equipment, choosing sides and taking once again to the rink of cracked blacktop with faded lines and circles. They wore no helmets, although one player wore a fedora.

Another, Vinnie Juliano, 77, of Long Island City, wore his hearing aids, along with his 50-year-old taped-up quads, or four-wheeled skates with a leather boot. Many players here never converted to in-line skates, and neither did Mr. Allen, whose photograph appeared on a poster hanging behind the players’ bench.

“I’m seeing people walking by wondering why all these rusty, grizzly old guys are here playing hockey,” one player, Tommy Dominguez, said. “We’re here for Craig, and let me tell you, these old guys still play hard.”

Everyone seemed to have a Craig Allen story, from his earliest teams at Public School 151 to the Bryant Rangers, the Woodside Wings, the Woodside Blues and more.

Mr. Allen, who became a yellow-cab driver, was always recruiting new talent. He gained the nickname Cabby for his habit of stopping at playgrounds all over the city to scout players.

Teams were organized around neighborhoods and churches, and often sponsored by local bars. Mr. Allen, for one, played for bars, including Garry Owen’s and on the Fiddler’s Green Jokers team in Inwood, Manhattan.

Play was tough and fights were frequent.

“We were basically street gangs on skates,” said Steve Rogg, 56, a mail clerk who grew up in Jackson Heights, Queens, and who on Saturday wore his Riedell Classic quads from 1972. “If another team caught up with you the night before a game, they tossed you a beating so you couldn’t play the next day.”

Mr. Garmendia said Mr. Allen’s skin color provoked many fights.

“When we’d go to some ignorant neighborhoods, a lot of players would use slurs,” Mr. Garmendia said, recalling a game in Ozone Park, Queens, where local fans parked motorcycles in a lineup next to the blacktop and taunted Mr. Allen. Mr. Garmendia said he checked a player into the motorcycles, “and the bikes went down like dominoes, which started a serious brawl.”

A group of fans at a game in Brooklyn once stuck a pole through the rink fence as Mr. Allen skated by and broke his jaw, Mr. Garmendia said, adding that carloads of reinforcements soon arrived to defend Mr. Allen.

And at another racially incited brawl, the police responded with six patrol cars and a helicopter.

Before play began on Saturday, the players gathered at center rink to honor Mr. Allen. Billy Barnwell, 59, of Woodside, recalled once how an all-white, all-star squad snubbed Mr. Allen by playing him third string. He scored seven goals in the first game and made first string immediately.

“He’d always hear racial stuff before the game, and I’d ask him, ‘How do you put up with that?’” Mr. Barnwell recalled. “Craig would say, ‘We’ll take care of it,’ and by the end of the game, he’d win guys over. They’d say, ‘This guy’s good.’”

Tribute for a Roller Hockey Warrior

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