President Reagan's ban on the sale of U.S. equipment and technology for the Siberian natural gas pipeline is unlikely to result in a significant delay in construction of the pipeline or natural gas deliveries from the Soviet Union to western Europe, according to an American expert on Soviet economics.

Edward A. Hewett, a senior economist for the Brookings Institution, said the Soviet Union had sufficient technology to circumvent the purpose of the embargo, which seeks to penalize the Soviets for the martial-law crackdown in Poland in part by denying use of American-designed turbine rotor blades needed for massive compressors to pump the gas through the 3,000-mile pipeline.

Administration officials have predicted that the embargo could delay the project, scheduled for completion by 1986, by up to two years. But Hewett told the Senate subcommittee on international economic policy that the Soviets could use their own smaller turbines for the project, possibly in combination with larger American-designed ones built by European firms.

The result, he predicted, was that, "It is not likely the embargo will impose a significant delay." He also said the Soviets had enough excess natural gas capacity to begin scheduled delivery of supplies to western Europe in 1984.

Hewett gave his analysis at a subcommittee session at which two administration officials defended the embargo, which was strongly criticized by several senators and businessmen.

James L. Buckley, undersecretary for security assistance, science and technology at the State Department, and Lionel Olmer, undersecretary for international trade at the Commerce Department, both suggested that the embargo had been a factor in the easing of martial-law restrictions announced last week by the Polish government.

But Buckley added that those moves were not adequate to "begin to meet our minimum requirements" for lifting U.S. trade sanctions.

Olmer said that, despite announcements by the French and Italian governments that they would defy the president, European companies were reluctant to violate the embargo because of the possibility they would be banned from future trade with U.S. firms.

Senate Foreign Relations Chairman Charles H. Percy (R-Ill.) led the attack on the ban, saying the policy would cost hundreds of American jobs and deeply split the United States from its European allies without having any significant impact on the Soviet Union.

Percy, who said 75 percent of the products affected by the ban are manufactured in his home state, said Soviet leaders should be celebrating with "champagne parties" the impact the embargo would have on U.S. relations with western Europe. "If the Kremlin had plotted this policy they could not have done a better job," he said.

Buckley said the president's original ban last December on exports from the United States had cost U.S. firms $800 million. Olmer said last month's extension of the ban to include foreign subsidiaries and licensees would cause European losses of $1.6 billion and additional U.S. losses of between $300 million and $600 million over the next three years.