An explosion at 10 a.m. Monday injured four workers at the BP Amoco Cooper River plant in Wando, S.C., located about 17 miles north of Charleston on the edge of Francis Marion National Forest.

The plant does not make the same product or use the same production process as the BP Amoco Polymers plant off Tobacco Road, where three men died after a pressure tank blew March 13. However, some aspects of the two incidents are chillingly similar.

``It makes you pause when you see things like this happen so close together,'' said John Curry, public affairs director of BP Amoco's Baltimore regional office. ``I can promise you it will be pored over with a fine-tooth comb to figure out if there's something that we could do better.''

One of the men in Monday's accident was in critical condition; another was in fair condition. The other two were treated at the scene.

Monday's explosion occurred within one of two manufacturing units at the 5,000-acre site, which according to the company's Web site houses the world's largest producer of purified terephthalic acid. The product, known as PTA, is a material used in the manufacture of plastic, carpet and film.

The unit was being shut down for routine maintenance when the explosion occurred, said Pam Johnson, the plant's human resources area manager. Workers might have been adding a chemical catalyst to the unit at the time, Mr. Curry said.

Ms. Johnson said the explosion might have involved the release of pressurized steam, or the ignition of a substance such as hydrogen.

According to company officials, victims of the fatal blast in south Augusta early March 13 also were shutting down their unit for maintenance. Something went wrong when they tried to open a vessel used to contain byproducts in the production of Amodel, a heat-resistant plastic, federal and state investigators have said. After a failed start-up attempt, materials inside the tank might have continued to react together, generating pressurized steam.

Operators Heinrich ``Heins'' Kohl, 25; George Sanders, 42; and John Rowland, 35, were killed in the explosion.

The Cooper River plant had gone more than a year without a mishap requiring medical attention for an injured worker, Mr. Curry said.

``It doesn't do much good now to say, `Gee, we had a good safety record,''' he said. ``I would not be surprised if something comes out from a corporate level that asks everyone to refocus and redouble their efforts to working and operating safely so that we can prevent these type of things.''

Reach Johnny Edwards at (706) 823-3225.