The British company that has already defied United States sanctions by shipping six 25-megawatt turbines to Leningrad now has high hopes of completing its entire order of 21 turbines.
John Brown Engineering Ltd. is apparently attempting to persuade Moscow to divert to company headquarters 15 of the 40 rotor blade sets - the heart of the turbines - originally ordered as spare parts by the Soviets last November from a nationalized French engineering company.
If the Soviets agree, John Brown would be able to fit the ''spare'' rotors in the remaining turbines destined for Moscow, and thus complete its entire contract about one year late.
The Soviets apparently ordered the 40 extra rotor sets - made by France's Alsthom-Atlantique under license from General Electric in the US - because they feared that the US might try to block the flow of pipeline technology from Europe.
John Brown was able to ship the first six turbines because it had received six rotors from General Electric in the US before President Reagan's sanctions were clamped into place Dec. 29, 1981.
The Reagan sanctions now forbid GE from sending rotors to any company providing technology for the Siberian pipeline. Hence John Brown's urgent need for 15 extra rotors.
The Soviet project will use 125 turbines (and thus 125 rotors) in 41 separate stations along its 3,500-mile length from Siberia to Western Europe. The Soviet order of 40 spare rotors from France's Alsthom-Atlantique is said here to exceed by a wide margin the normal number of spare rotors needed for the entire pipeline.
The French government became the first European nation to openly defy the Reagan sanctions when it ordered Alsthom-Atlantique to begin production of the rotors in July. France later ordered other French firms to fulfill their pipeline contracts with the Soviet Union and the first French compressors have been delivered. But whether Alsthom-Atlantique would technically break the US embargo by supplying Soviet-ordered rotors to John Brown is not known.
John Brown officials here on the banks of the Clyde near Glasgow will not talk openly, since their lawyers are still combing the Reagan sanctions order. The company is also holding talks with other nations providing pipeline technology - West Germany, Italy, and France.
But it can be reported that they expect that Soviet-ordered rotors from France will indeed find their way to Scotland, and that John Brown will indeed fulfill its entire contract for seven stations containing three pumping turbines each.
The pipeline order, totaling $181 million, is the largest turbine order in the company's history. It is helping to keep 1,400 workers busy in western Scotland, an area hard-hit by recession and unemployment.
The company also has another reason for relief. As it reads the sanctions, GE is forbidden to send only rotors to be used in turbines that drive pumps. Yet 80 percent of the John Brown turbine business is in turbines that generate electricity. The company believes rotors for generator turbines are unaffected.
John Brown itself would continue to defy the US embargo, at least in spirit, by using French rotors in their remaining 15 turbines.
If the John Brown move does succeed, the British government would also be pleased.
In September, 352,400 workers were jobless in Scotland, almost 16 percent of the work force. That is why Prime Minister Margaret Thatcher, known as the ''iron lady'' for her anti-Soviet stands on other issues, recently said on a visit here that the sanctions meant ''we have been wounded by a friend.''
The British object to the US ordering its companies to break existing contracts. And theyask why the US can sell grain to Moscow while telling its allies how to trade.
John Brown does face at least two drawbacks because of the sanctions.
Since it must start work on ancillary equipment 21/2 years before delivery and because most orders specify delivery in six months to a year, workshops are always assembling parts for turbines as yet unsold.
The delay in getting rotors from France will mean more John Brown workers being diverted to unsold equipment instead of working on the Soviet machines already sold. Company planning will suffer as a result.
Moreover, John Brown's competitors, it is learned, are trying to take advantage of the publicity around the Reagan sanction order.
''Don't order spare parts from John Brown,'' some competitors urge potential customers. ''They're embargoed, you know.''