External vs. Internal Pressure
Because of their proximity to the nuclear reactor, the project required the doors and their locksets to provide a protective barrier against an external pressure event and hold the door closed. At the same time, if a specified internal pressure was reached, such as in the event of a catastrophic failure of the steam pressure relief systems, the door systems would open as a relief mechanism. The doors would open to 90 degrees—and stay open after coming to a full stop from a final velocity of 33 miles per hour.
Krieger partnered with Surelock McGill, a leading manufacturer of high-performance door hardware and accessories, and engineered the locking mechanism that would meet these seemingly contradictory specifications. It was determined that essentially the lock needed to be designed to fail and allow the door to freely swing open if the internal pressure ever reached a specified amount—and stay open allowing the full door area to act as a steam relief vent. “Surelock McGill has been protecting doors from opening under wind, air, and water pressure for decades, but this was the first time we were asked to hold the door against an external threat and then open at a specific amount of internal pressure,” explains James Stewart, VP of Sales and Marketing at Surelock McGill.
Additional performance properties and features of Krieger’s exit door system included: airborne missile (a large mass-accelerating projectile) resistance; blast resistance; thermal resistance; fire protection; and seismic safety under extreme loading conditions with pressure-release shear strikes. The doors included single leaf (roughly 48” x 88”) and double-leaf configurations (roughly 38” x 88” with a 3” mullion). Each door weighed 1,200 pounds.
Rigorous Research, Development, and Testing
A custom strike for the lockset needed to be designed with one connection point that does not move under the load, and the other connection point that releases under the load. “Typically blast assemblies are designed to be simple protective barriers, but this application called for it to be mechanically calibrated to both stay closed or swing open depending on the external or internal pressure,” explains Aaron Lewis, Engineering Manager at Krieger.
In developing the locking assembly, the engineers and technicians used a hydraulic press and a custom fixture to repeatedly force the strike to fail. “They devised various methods to secure the strike, raise the desired external pressure, and then make sure it failed at the specified internal pressure,” says Lewis. “This meant that when a pressure of 2.5 psi +/- 10% was applied to the door from inside, the shear pin in the strike would break and allow the door to swing open at 33 mph, stopping at a 90-degree angle with absolutely no bounce back.”
The frame, door, and hardware were subjected to over a dozen tests until the proper pressure levels were achieved. Engineers took video at multiple angles from inside and outside the pressure chamber so they could study what was happening on both sides. “These views were crucial in helping them understand how the lock interacted with both the door and the strike, along with how the forces were distributed from the bolt to the pivot side of the strike,” says Lewis.
Testing of the lock’s pressure relief strike was conducted at Ellis and Watts. Keeps were fitted to a press and loads applied until failure. The force applied was measured using a calibrated load cell. Initial tests were conducted to determine that the design works in principle and to refine the design. Once a final design was confirmed, tests were conducted on various shear groove sizes to determine the correct shear groove size to meet the requirement.
Krieger engineered the door frame to stay attached to the wall and the doors to stay rigid under the load. The mullions in the double-leaf door assemblies were designed to not warp under the load. “We made sure that the same tensile force that allowed the strike to release under a certain amount of pressure would simultaneously act on the panic device’s guide fasteners that are mounted into the Krieger doors,” explains Lewis. “If the door wasn’t designed properly, the threading on the fasteners would fail before the shear pin broke.” To make sure everything worked properly, Krieger’s engineers reinforced the doors and mullions so the threading on the fasteners would hold under the intense 2.5 psi unseated pressure.
The performance specifications at the nuclear facility required all of the door and lockset components to work together in harmony. Krieger Specialty Products worked to get the job done, meeting an unusual challenge to create a custom product that keeps the power plant and its workers safe.