Activist hedge fund Sandell Asset Management Corp is ratcheting up pressure on pipeline operator SemGroup Corp. with PR help from Sloane & Co to push for a sale of the Oklahoma-based company.

semgroupSandell went public with its plan in an open letter on Jan. 26 criticizing SemGroup's dismissal of the hedge fund's suggestion of a sale.

"We have reviewed your response to our letter dated January 26th and, frankly, are disappointed (but perhaps not surprised) that you have chosen to answer our concerns not with a thoughtful, considered response but instead seemingly with what your lawyers have advised you to say to 'thwart' an activist investor," Sandell said in a press release that knocked SemGroup's "canned reply" and "status quo" approach.

Sloane CEO Elliot Sloane and managing director Dan Zacchei rep Sandell.

SemGroup, advised by Joele Frank partners Andrew Siegel and James Golden on the PR front, said it remains open to constructive input and added that its board "will carefully consider the recent letter from Sandell."

Sandell says SemGroup is worth up to $104 per share. SemGroup shares were down nearly three percent to $68.38 in trading early Feb. 3.