The concrete rubble and twisted debris of a dockside grain elevator, devastated by a powerful explosion Tuesday night and some of it still on fire yielded a 13th victim today and officials feared that untold other may still lie dead in the wreckage.

Rescuers tried to press their search for the remaining victims while officials prepared to investigate the cause of the 8:35 p.m. blast. It was the second major explosion at a Gulf Coast grain elevator in five days and the fourth U.S. grain facility to explode in a week. The series of deaths - now at least 50 - brought heightened concern among federal officials and others over safety conditions at grain elevators, where volatile grain dust may ignite from static electricity or a spark.

Workers on the scene pointed out that the Farmers Export Co. terminal was hardly an old one - having been built less than two years ago at a cost of $26 million. They also added that crews had been at work for months to improve the dust collection and ventilation at the facility that erupted in flames Tuesday night. The federal Occupational Safety and Health Administration, which is charged with assuring safety in the workplace, today acknowledged that it had never inspected the Farmers Export elevator.

Both it and the Continental Grain elevator that blew up near New Orleans last Thursday, killing 354, are in an OSHA administrative region whose top official was removed two months ago after complaints by organized labor of alleged laxities in the enforcement of safety standards.

An OSHA spokesman said in Washington today that the agency and the U.S. Department of Agriculture - the latter of which has now lost 13 federal grain inspectors in the two major explosions - are now working on "crash basis" to develop safety guidelines for grain-handling facilities. In addition to Tuesday night's explosion here and last Thursday's in New Orleans, two people were killed and 17 injured, also last Thursday, at the Sunshine Mills and Grain, Inc., elevator at Tupelo, Miss.

A fire last Wednesday at the J. & R. Grain Co. in Courtland, Kan., destroyed the main portion of the grain elevator there, but no one was injured.

Tuesday night's explosion here at the 3.8 million bushel facility apparently bagan in an unloading area where grain was being transferred from rail cars to conveyors that take the grain to the storage elevators. At the time, a Yugoslavian ship was being loaded with wheat nearby.

So powerful was the blast that it rocked the 55,000-ton ship and blew the train's diesel engine into the 40-foot-deep conveyor tunnel. Other rail cars were lifted off the tracks.

The thirteenth known victim was pulled from the debris as night fell, and officials expressed the fear that others might still be unaccounted for. The confusion over the possibility of more victims lay in the belief that some dayshift workers may not have checked out at the end of their workday.

Cranes, bulldozeis and tractors, meanwhile, worked in a cold drizzle to clear the roadway and railroad tracks near the elevator. Automobiles parked nearby were knocked over, their windows shattered.

Tattered strips of tin that were once a part of the rail unloading facility had been blown every which way. Some store windows in downtown Galveston, which is about a mile from the elevator, were shattered by the blast.

Smoke continued to drift from the upper floors of the heavily damaged, 300-foot "headhouse" of the elevator, from which grain is loaded onto ships, but there was little fear of new explosions because of the rain.

L. E. Bartelt, administrator of the Federal Grain Inspection Service, which only this year took over inspection from private concerns because of scandals involving shortweighting and misgrading, said both the New Orleans and the Galveston explosions apparently occurred during times of high dust and low humidity. "Handling grain in this way is not too different from handling gasoline," he said. "and we've got to learn how to do it better."

Hundreds of grain elevator fires and explosions occur every year, many of them in remote "country elevators" where grain first finds its way into the marketing systems. Dust from the grain gathers naturally in the surrounding air and can become an imminent hazard unless cleared.

"These are just powder kegs," said Richard Ginnold, an associate professor at the University of Wisconsin, who has studied grain explosions. Overheated machinery, a hot light bulb, a spark, lightning or a careless outsider may touch off the volatile dust.

For the past two years, the National Academy of Sciences has been studying grain explosions while OSHA has been working on guidelines aimed at reducing hazards. An OSHA spokesman said today that because of the week's events "we escalated" that effort.

Until the guidelines are finished, grain elevators are covered by general OSHA regulations, and, indeed, last Oct. 27 OSHA inspectors cited Continental for two violations at the New Orleans elevator - lack of electrical grounding and poorly marked exits, a spokesman said - which had been corrected in time for a reinspection.

But the Farmers Export Elevator, which is owned by nine regional farmer cooperatives seeking to enter the grain export market, has never been inspected, according to OSHA. One employee, Alcide Marquer, 55, who was slightly injured in Tuesday night's explosion, said today there had "been some complaints' from workers of dust levels, which, he said, "had been exceptionally bad."

OSHA has been criticized for drawing up standards for minor safety mattes - toilet facilities and knotholes in ladders - while allowing more important issues to go unaddressed. Dr. Eula Bingham, appointed by the Carter administration to head OSHA, recently announced that the agency was abandoning 1,100 nitpicking regulations to concentrate on serious hazards.

Last May, OSHA moved to include grain handling facilities in its priorities list for inspection.

But last September, the Oil, Chemical and Atomic Workers Union asked Bingham to oust Dallas regional administrator Robert Tice for failure to vigorously pursue violations. Tice denied the allegations, but Bingham removed him the following month.

"What happened in Galveston shows all the catching up they have to do down there," said Steven Wodka, international representative for the OCAW in Washington.

Officials said that sabotage had been ruled out in the New Orleans and Galveston explosions.

Officials are also trying to determine what role abnormally low humidity rates may have played, if any.

Environmental restrictions forbid grain elevators from merely venting grain dust into the air and instead require grain collection systems that generally function like an oversized vacuum cleaner, One official with Cargill, a major grain exporter with 175 grain elevators in the United States, said dust gathering equipment can run into the millions at each elevator.