Fire crews say maintenance work was being done on the valve when it failed.
Officials quickly capped the leak, but a second leak developed and ammonia fumes filled the plant. It took crews several hours to cap the second leak.
Employees were evacuated and emergency officials sent out reverse 911 phone calls to about 800 residents alerting them about the situation.
Officials say 30 people who live around the plant went to a temporary shelter. The shelter was closed around 11 p.m. after authorities gave the all clear.
One person inside the plant was treated for minor injuries.
Crews are ventilating the building. They say once air quality is within safe limits, they'll allow workers back inside the plant.
Roads around the House of Raeford plant, located about 90 miles southwest of Raleigh, are expected to be re-opened by Thursday morning.