A fuel gas pipeline in a pipe trench went under a road bridge inside a factory. A flange joint, fitted with a compressed asbestos fiber (caf) gasket, was located only 2.5 m from the roadway.
According to the area classification code used by the company there was a Division 2 area for a radius of 3 m around a caf joint and road vehicles should not be allowed unrestricted access to Division 2 areas. The factory wanted to open the bridge to unrestricted traffic.
Use secondary containment with a sensor
My recommendation is to leave the gasket alone for the reasons mentioned by the operations department. I would add a jacket around the flange and a sensor to detect if there is any leakage. The jacket would provide a small amount of secondary containment but would also concentrate any vapors so that the leak can be detected more readily. The signal should be transmitted to the control room and to a traffic light located at the bridge that signaled if travel on the bridge was safe or not. Since the flange has not leaked to date is a good reason to leave it alone but it does not insure that a leak won't develop in the future.
Properly dispose of the CAF gasket
The gasket may not develop a leak, but why play the odds? SAFETY FIRST! Replace the asbestos gasket with the spiral wound gasket. If the binding material is intact (not friable) remove it, clean and inspect flange surfaces, and dispose of the asbestos gasket according to regulations. If the gasket is friable it may be required to glove bag during removal by a supervised asbestos worker. Replace the studs and nuts with ferilium. Then do a wire wrap with injector cap nuts and valves attached. Inject sealant through the valves into the space between the wire wrap and the gasket. This will insure the flanges integrity and prevent a major spill in the off chance the gasket should fail. If the area is able to trap the gas in the event of a leak, a lower explosive limit (LEL) gas monitor should be installed with a warning signal if area gets above the LEL. This may seem a bit extreme, however all data are not available and more measures may be needed.
Rather than replacing the gasket, use one of the proprietary "gasket bands" that several vendors make. Some have the option with a grease zert to fill the flange gap with grease or other thixotropic material that would be pushed out under pressure. After applying this, a six-month visual inspection would tell if there was leakage from the pipe because the grease would be pushed out from under the flange seal. Provided the safety department, plant operators, and engineering personnel are all happy with the flange the classification of the area could be changed. The gasket replacement could then be deferred until a future "opportunity."
If the flange is under the bridge and it is a confined space, gas testing may also be required prior to entering under the roadway bridge; the safety department should be able to make this rule clear.
Chuck Stewart, sr. process engineer
BP America, Houston, TX
Which is the safest route?
We can shift the flange joint away from the road bridge, remove the existing flange joint, and make it a welded joint. This will require hot work to be done. But this is a one time job and it is the permanent solution. By doing this we will obey the codes and safety.
