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Pentair to spin off its electrical business

* Electrical unit had sales of $2.1 bln in 2016

* Water unit to retain the Pentair name

* Separation expected to close in Q2 2018

* Shares up 2.1 pct premarket (Adds background, shares)

May 9 (Reuters) - Manufacturer Pentair Plc said on Tuesday it would spin off its electrical business into a separate publicly traded company to focus on its core business, which makes water treatment equipment.

The company's shares were up 2.1 percent at $67.49 in trading before the bell, and were set to open at a nearly two-year high.

Pentair, which was under pressure from activist hedge fund Trian Fund Management LP, its second-biggest shareholder, has shed non-core businesses to improve its performance.

The UK-based company last month sold its valves and controls business for $3.15 billion in cash to U.S. factory automation equipment maker Emerson Electric Co.

Pentair's electrical business, which makes stainless steel, aluminum and non-metallic enclosures that guard sensitive electrical and electronic equipment, reported sales of $2.1 billion in 2016.

This company, to be separated in a tax-free spin-off to Pentair shareholders, would be named at a later date, Pentair said on Tuesday.

Pentair would be left with its water business, which reported sales of $2.8 billion in 2016.

The company's water treatment equipment is used in swimming pools and spas, aquaculture farms and laboratories, and the food and beverage industry.

The business also makes pumps and filtration systems used in flood control and the oil and gas industry.

Pentair expects one-time separation costs of about $100 million-$125 million related to the spin-off, which is estimated to close in the second quarter of 2018.

Pentair Chief Financial Officer John Stauch will become the company's chief executive after the spin-off, and Karl Frykman, president of Pentair's water unit, will become the company's chief operating officer.

Beth Wozniak, current president of Pentair's electrical unit, will become CEO of the newly spun-off company.

Randall Hogan, current chairman and chief executive of Pentair, will retire and serve as chairman of the electrical business.

Goldman Sachs & Co LLC served as financial adviser to Pentair, while Foley & Lardner LLP acted as legal adviser. (Reporting by Ankit Ajmera in Bengaluru; Editing by Martina D'Couto)