Jenis-jenis Valve dan Fungsinya

Jenis-jenis valve dan fungsinya :
1. Gate Valve
Gate Valve adalah valve yang paling sering dipakai pada sistem perpipaan. Fungsinya untuk dapat membuka dan menutup aliran (on-off), tetapi tidak bisa untuk mengatur besar kecil aliran (throttling). Kelebihan Gate Valve, minimnya halangan/ resistan saat valve ini telah dibuka penuh, sehingga aliran bisa
maksimal. Gate Valve telah mengontrol aliran melalui badan valve yang berbentuk pipa, dengan sebuah lempengan atau baji vertikal  yang bisa bergeser naik turun saat handel valve diputar. Valve ini telah didesain untuk dapat mengatur posisi terbuka penuh, atau tertutup penuh. Jika valve ini dalam keadaan setengah terbuka, maka akan dapat menyebabkan pengikisan pada badan valve, dan turbulensi aliran zat bisa dapat menyebabkan getaran pada baji valve sehingga dapat menghasilkan suara gemeretak.

2. Globe Valve
Globe Valve biasanya akan digunakan pada situasi dimana pengaturan besar kecil aliran (throttling) sangat diperlukan. Dengan mudah memutar handel valve, besarnya aliran zat yang telah melewati valve bisa diatur. Dudukan valve yang sejajar dengan aliran, telah membuat globe valve efisien ketika dapat mengatur besar kecilnya aliran dengan minimum erosi piringan dan dudukan. Namun demikian tahanan didalam valve cukup besar. Desain globe valve yang sedemikian rupa, telah memaksa adanya perubahan arah aliran zat didalam valve, sehingga tekanan menurun drastis dan dapat menyebabkan turbulensi di dalam valve itu sendiri. Dengan demikian, Globe Valve tidak disarankan diinstal pada sistem yang menghindari penurunan tekanan, dan sistem yang menghindari tahanan pada aliran.

3. Angle Valve  
Sama seperti globe valve, angle valve juga akan digunakan pada situasi dimana pengaturan besar kecil aliran telah diperlukan (throttling). Namun angle valve telah di buat dengan sudut 90°, hal ini untuk dapat mengurangi pemakaian elbow 90° dan fitting tambahan.
4. Check Valve
Check Valve telah memiliki perbedaan yang sangat signifikan dari Gate Valve dan Globe Valve. Valve ini telah di disain untuk dapat mencegah aliran balik. Ada beberapa jenis check valve, tapi ada 2 jenis yang paling umum yaitu Swing Check dan Lift Check. Swing Check Valve biasanya telah dipasangkan dengan Gate Valve, sedangkan Lift Check Valve oleh beberapa pabrikan digunakan untuk dapat menggantikan fungsi Ball Valve sebagai Ball Check Valve. Check Valve tidak menggunakan handel untuk dapat mengatur aliran, tapi dengan menggunakan gravitasi dan tekanan dari aliran fluida itu sendiri. Karena fungsinya yang juga dapat mencegah aliran balik (backflow). Check Valve juga sering digunakan sebagai pengaman dari sebuah equipment dalam sistem perpipaan.
5. Ball Valve
Ball Valve adalah alternatif murah dari jenis valve-valve yang lain. Ball valve dengan menggunakan bola logam yang tengahnya ada lubang tembus, diapit oleh dudukan valve untuk dapat mengontrol aliran. Sering dipakai pada proses hydrocarbon, ball valve mampu untuk dapat mengatur besar kecil aliran gas dan uap terutama untuk tekanan rendah. Valve ini juga dapat dengan cepat ditutup dan cukup kedap untuk menahan fluida/ zat cair. Ball valve tidak menggunakan handwheel, tetapi dengan menggunakan ankle untuk dapat membuka atau menutup valve dengan sudut 90°.
6. Butterfly Valve
Butterfly Valve telah memiliki bentuk yang sangat unik jika dibandingkan dengan valve-valve yang lain. Butterfly dengan menggunakan plat bundar atau wafer yang dioperasikan dengan ankel untuk posisi membuka penuh atau menutup penuh dengan sudut 90°. Wafer ini tetap berada ditengah aliran, dan dihubungkan ke ankel melalui shaft. Saat valve dalam keadaan tertutup, wafer tersebut tegak lurus dengan arah aliran, sehingga aliran terbendung, dan saat valve terbuka wafer sejajar/ segaris dengan aliran, sehingga zat dapat mengalir melalui valve. Butterfly valve telah memiliki turbulensi dan penurunan tekanan (pressure drop) yang minimal. Valve ini sangat bagus untuk pengoperasian on-off ataupun throttling, dan bagus untuk dapat mengontrol aliran zat cair atau gas dalam jumlah yang besar. Namun demikian valve ini biasanya tidak memiliki kekedapan yang bagus, dan harus digunakan pada situasi/ sistem yang memiliki tekanan rendah (low-pressure).

7. Relief Valve
Relief valve telah memiliki fungsi yang sangat berbeda dari valve-valve yang lain. Valve ini telah didisain khusus untuk dapat melepas tekanan berlebih yang ada di equipment dan sistem perpipaan. Untuk dapat mencegah kerusakan pada equipment, dan lebih penting lagi cedera pada pekerja, relief valve dapat melepas kenaikan tekanan sebelum menjadi lebih ekstrim. Relief valve menggunakan pegas baja, yang secara otomatis akan terbuka jika tekanan mencapai level yang tidak aman. Level tekanan pada valve ini bisa diatur, sehingga bisa ditentukan pada level tekanan berapa valve ini akan terbuka. Ketika tekanan kembali normal, relief valve secara otomatis akan tertutup kembali.

Editor : Dian Sukmawati


HOBART, Tasmania — Few places seem out of reach for China’s leader, Xi Jinping, who has traveled from European capitals to obscure Pacific and Caribbean islands in pursuit of his nation’s strategic interests.

So perhaps it was not surprising when he turned up last fall in this city on the edge of the Southern Ocean to put down a long-distance marker in another faraway region, Antarctica, 2,000 miles south of this Australian port.

Standing on the deck of an icebreaker that ferries Chinese scientists from this last stop before the frozen continent, Mr. Xi pledged that China would continue to expand in one of the few places on earth that remain unexploited by humans.

He signed a five-year accord with the Australian government that allows Chinese vessels and, in the future, aircraft to resupply for fuel and food before heading south. That will help secure easier access to a region that is believed to have vast oil and mineral resources; huge quantities of high-protein sea life; and for times of possible future dire need, fresh water contained in icebergs.

It was not until 1985, about seven decades after Robert Scott and Roald Amundsen raced to the South Pole, that a team representing Beijing hoisted the Chinese flag over the nation’s first Antarctic research base, the Great Wall Station on King George Island.

But now China seems determined to catch up. As it has bolstered spending on Antarctic research, and as the early explorers, especially the United States and Australia, confront stagnant budgets, there is growing concern about its intentions.

China’s operations on the continent — it opened its fourth research station last year, chose a site for a fifth, and is investing in a second icebreaker and new ice-capable planes and helicopters — are already the fastest growing of the 52 signatories to the Antarctic Treaty. That gentlemen’s agreement reached in 1959 bans military activity on the continent and aims to preserve it as one of the world’s last wildernesses; a related pact prohibits mining.


But Mr. Xi’s visit was another sign that China is positioning itself to take advantage of the continent’s resource potential when the treaty expires in 2048 — or in the event that it is ripped up before, Chinese and Australian experts say.

“So far, our research is natural-science based, but we know there is more and more concern about resource security,” said Yang Huigen, director general of the Polar Research Institute of China, who accompanied Mr. Xi last November on his visit to Hobart and stood with him on the icebreaker, Xue Long, or Snow Dragon.

With that in mind, the polar institute recently opened a new division devoted to the study of resources, law, geopolitics and governance in Antarctica and the Arctic, Mr. Yang said.

Australia, a strategic ally of the United States that has strong economic relations with China, is watching China’s buildup in the Antarctic with a mix of gratitude — China’s presence offers support for Australia’s Antarctic science program, which is short of cash — and wariness.

“We should have no illusions about the deeper agenda — one that has not even been agreed to by Chinese scientists but is driven by Xi, and most likely his successors,” said Peter Jennings, executive director of the Australian Strategic Policy Institute and a former senior official in the Australian Department of Defense.

“This is part of a broader pattern of a mercantilist approach all around the world,” Mr. Jennings added. “A big driver of Chinese policy is to secure long-term energy supply and food supply.”

That approach was evident last month when a large Chinese agriculture enterprise announced an expansion of its fishing operations around Antarctica to catch more krill — small, protein-rich crustaceans that are abundant in Antarctic waters.

“The Antarctic is a treasure house for all human beings, and China should go there and share,” Liu Shenli, the chairman of the China National Agricultural Development Group, told China Daily, a state-owned newspaper. China would aim to fish up to two million tons of krill a year, he said, a substantial increase from what it currently harvests.

Because sovereignty over Antarctica is unclear, nations have sought to strengthen their claims over the ice-covered land by building research bases and naming geographic features. China’s fifth station will put it within reach of the six American facilities, and ahead of Australia’s three.

Chinese mappers have also given Chinese names to more than 300 sites, compared with the thousands of locations on the continent with English names.

In the unspoken competition for Antarctica’s future, scientific achievement can also translate into influence. Chinese scientists are driving to be the first to drill and recover an ice core containing tiny air bubbles that provide a record of climate change stretching as far back as 1.5 million years. It is an expensive and delicate effort at which others, including the European Union and Australia, have failed.

In a breakthrough a decade ago, European scientists extracted an ice core nearly two miles long that revealed 800,000 years of climate history. But finding an ice core going back further would allow scientists to examine a change in the earth’s climate cycles believed to have occurred 900,000 to 1.2 million years ago.

China is betting it has found the best location to drill, at an area called Dome A, or Dome Argus, the highest point on the East Antarctic Ice Sheet. Though it is considered one of the coldest places on the planet, with temperatures of 130 degrees below zero Fahrenheit, a Chinese expedition explored the area in 2005 and established a research station in 2009.

“The international community has drilled in lots of places, but no luck so far,” said Xiao Cunde, a member of the first party to reach the site and the deputy director of the Institute for Climate Change at the Chinese Academy of Meteorological Sciences. “We think at Dome A we will have a straight shot at the one-million-year ice core.”

Mr. Xiao said China had already begun drilling and hoped to find what scientists are looking for in four to five years.

To support its Antarctic aspirations, China is building a sophisticated $300 million icebreaker that is expected to be ready in a few years, said Xia Limin, deputy director of the Chinese Arctic and Antarctic Administration in Beijing. It has also bought a high-tech fixed-wing aircraft, outfitted in the United States, for taking sensitive scientific soundings from the ice.

China has chosen the site for its fifth research station at Inexpressible Island, named by a group of British explorers who were stranded at the desolate site in 1912 and survived the winter by excavating a small ice cave.

Mr. Xia said the inhospitable spot was ideal because China did not have a presence in that part of Antarctica, and because the rocky site did not have much snow, making it relatively cheap to build there.

Anne-Marie Brady, a professor of political science at the University of Canterbury in New Zealand and the author of a soon-to-be-released book, “China as a Polar Great Power,” said Chinese scientists also believed they had a good chance of finding mineral and energy resources near the site.

“China is playing a long game in Antarctica and keeping other states guessing about its true intentions and interests are part of its poker hand,” she said. But she noted that China’s interest in finding minerals was presented “loud and clear to domestic audiences” as the main reason it was investing in Antarctica.

Because commercial drilling is banned, estimates of energy and mineral resources in Antarctica rely on remote sensing data and comparisons with similar geological environments elsewhere, said Millard F. Coffin, executive director of the Institute for Marine and Antarctic Studies in Hobart.

But the difficulty of extraction in such severe conditions and uncertainty about future commodity prices make it unlikely that China or any country would defy the ban on mining anytime soon.

Tourism, however, is already booming. Travelers from China are still a relatively small contingent in the Antarctic compared with the more than 13,000 Americans who visited in 2013, and as yet there are no licensed Chinese tour operators.

But that is about to change, said Anthony Bergin, deputy director of the Australian Strategic Policy Institute. “I understand very soon there will be Chinese tourists on Chinese vessels with all-Chinese crew in the Antarctic,” he said.


Top News China’s Intents Are Questioned as It Builds in Antarctica

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