Magellan Midstream Partners Chief Executive Aaron Milford said he is confident the pipeline operator's planned sale to larger peer ONEOK Inc for $18.8 billion will gain approval from its investors despite some opposition.

Energy Income Partners, the fourth-largest unitholder in Magellan with a 3.1% stake, has said it intends to vote against the proposed deal, concerned about the loss of tax benefits.

Magellan is a master limited partnership (MLP), a publicly traded corporate structure that enjoys the benefits of paying no tax at the company level.

Investors in MLPs, known as unitholders, are also shielded from ongoing taxes but when either the units, or the whole company, are sold, the accumulated taxes become due for payment.

Milford countered that the financial benefits to Magellan's investors will outweigh the tax bills.

"We expect to get the vote, as the value proposition is too compelling", Milford told Reuters in an interview. "The benefits far outweigh that (tax) cost," he said

Under the deal in which ONEOK will also assume Magellan's debt, ONEOK will pay $25 and 0.6670 shares of ONEOK common stock for each outstanding Magellan common unit, representing a premium of 22% to Magellan's closing price on May 12.

Milford said Magellan investors would also benefit from the combined company's increased scale, product diversification, and annual cost savings of up to $400 million.

The vote on the deal is scheduled for Sept. 21.

MLPs were designed to promote the production of commodities such as oil and natural gas and the building out of infrastructure facilities, but in recent years they have fallen out of favor with investors in part due to their exclusion from major indexes.

Magellan's pipelines move refined petroleum products and crude oil, while ONEOK is focused on transporting natural gas liquids and natural gas.