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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

From sea to shining sea, or at least from one side of the Hudson to the other, politicians you have barely heard of are being accused of wrongdoing. There were so many court proceedings involving public officials on Monday that it was hard to keep up.

In Newark, two underlings of Gov. Chris Christie were arraigned on charges that they were in on the truly deranged plot to block traffic leading onto the George Washington Bridge.

Ten miles away, in Lower Manhattan, Dean G. Skelos, the leader of the New York State Senate, and his son, Adam B. Skelos, were arrested by the Federal Bureau of Investigation on accusations of far more conventional political larceny, involving a job with a sewer company for the son and commissions on title insurance and bond work.

The younger man managed to receive a 150 percent pay increase from the sewer company even though, as he said on tape, he “literally knew nothing about water or, you know, any of that stuff,” according to a criminal complaint the United States attorney’s office filed.

The success of Adam Skelos, 32, was attributed by prosecutors to his father’s influence as the leader of the Senate and as a potentate among state Republicans. The indictment can also be read as one of those unfailingly sad tales of a father who cannot stop indulging a grown son. The senator himself is not alleged to have profited from the schemes, except by being relieved of the burden of underwriting Adam.

The bridge traffic caper is its own species of crazy; what distinguishes the charges against the two Skeloses is the apparent absence of a survival instinct. It is one thing not to know anything about water or that stuff. More remarkable, if true, is the fact that the sewer machinations continued even after the former New York Assembly speaker, Sheldon Silver, was charged in January with taking bribes disguised as fees.

It was by then common gossip in political and news media circles that Senator Skelos, a Republican, the counterpart in the Senate to Mr. Silver, a Democrat, in the Assembly, could be next in line for the criminal dock. “Stay tuned,” the United States attorney, Preet Bharara said, leaving not much to the imagination.

Even though the cat had been unmistakably belled, Skelos father and son continued to talk about how to advance the interests of the sewer company, though the son did begin to use a burner cellphone, the kind people pay for in cash, with no traceable contracts.

That was indeed prudent, as prosecutors had been wiretapping the cellphones of both men. But it would seem that the burner was of limited value, because by then the prosecutors had managed to secure the help of a business executive who agreed to record calls with the Skeloses. It would further seem that the business executive was more attentive to the perils of pending investigations than the politician.

Through the end of the New York State budget negotiations in March, the hopes of the younger Skelos rested on his father’s ability to devise legislation that would benefit the sewer company. That did not pan out. But Senator Skelos did boast that he had haggled with Gov. Andrew M. Cuomo, a Democrat, in a successful effort to raise a $150 million allocation for Long Island to $550 million, for what the budget called “transformative economic development projects.” It included money for the kind of work done by the sewer company.

The lawyer for Adam Skelos said he was not guilty and would win in court. Senator Skelos issued a ringing declaration that he was unequivocally innocent.

THIS was also the approach taken in New Jersey by Bill Baroni, a man of great presence and eloquence who stopped outside the federal courthouse to note that he had taken risks as a Republican by bucking his party to support paid family leave, medical marijuana and marriage equality. “I would never risk my career, my job, my reputation for something like this,” Mr. Baroni said. “I am an innocent man.”

The lawyer for his co-defendant, Bridget Anne Kelly, the former deputy chief of staff to Mr. Christie, a Republican, said that she would strongly rebut the charges.

Perhaps they had nothing to do with the lane closings. But neither Mr. Baroni nor Ms. Kelly addressed the question of why they did not return repeated calls from the mayor of Fort Lee, N.J., begging them to stop the traffic tie-ups, over three days.

That silence was a low moment. But perhaps New York hit bottom faster. Senator Skelos, the prosecutors charged, arranged to meet Long Island politicians at the wake of Wenjian Liu, a New York City police officer shot dead in December, to press for payments to the company employing his son.

Sometimes it seems as though for some people, the only thing to be ashamed of is shame itself.