Vapor stops (also known as vapor dams) are the part of an insulation system constructed from vapor retarding joint sealant or mastic, usually reinforced with a coarse fiber mesh. They adhere to the pipe, go up the face of the insulation at a circumferential joint, and then attach to the primary vapor retarder on the outside surface of the insulation. The purpose of a vapor stop is to greatly reduce the movement of water and water vapor movement axially (in a direction lengthwise down the pipe) through the insulation system. Entry of water and water vapor into a cold insulation system, especially a cryogenic insulation system, is a key source of problems with the insulation system. It can greatly reduce the insulating performance of the insulation, lead to excess condensation on the insulation system outer surface, contribute to corrosion of the underlying pipe or vessel, and, in particularly bad situations, lead to ice formation in and on the insulation system. Such ice formation can physically damage the insulation system, the pipe, or nearby gauges and equipment.
Vapor stops are used at three types of locations for three specific reasons:
a. At the point where the insulation system terminates. This could be on a dead leg of pipe, on a vessel or equipment support leg that is insulated for some distance away from the primary vessel insulation, or on some sort of internal pipe support such as those used on vertical pipes where the support is insulated for some distance away from the primary pipe insulation. In all these cases, the insulation ends. This end point is a location where water and water vapor could readily enter the insulation system and travel through the insulation via various paths. A vapor stop is installed at this location to prevent or, at least greatly reduce, the movement of water through it and hence into the insulation system.
b. On either side of insulation system locations that are known problem areas for water intrusion. One example of these is a valve. The valve stem or some part of the valve almost always protrudes through the insulation system thus penetrating the vapor retarder. Great care is taken to seal the insulation system at this penetration location using sealants and caulks, but this seal is never as good as the vapor retarder on the rest of the insulation system and is subject to degradation from sunlight. If the seal is around an actual valve stem, when this stem is turned or moved, it will break the seal. Another example of a location that is a known problem area for water intrusion is a portion of the pipe system that requires repeated maintenance and so is covered with removable insulation to facilitate that maintenance. The vapor retarder on a removable insulation system is seldom very good so it will more readily allow water and water vapor movement through the removable insulation and into the insulation system.
In both of these examples, a vapor stop should be installed on either side of the problem area. Another way of viewing this sort of vapor stop is that it is a subset of item a., above. The main insulation system terminates on either side of the problem area, and a vapor stop is installed. Regardless, the purpose of vapor stops is to prevent water that enters the insulation system at the problem location from moving into the rest of the insulation system. The vapor stop isolates and limits the potential for damage caused by the water entering at such problem locations.
c. Periodically on a run of pipe as a protective measure. Even in the absence of a known problem location, there is the possibility that the insulation system could be damaged or installed improperly. Either of these scenarios could allow water or water vapor to enter the system. Vapor stops are located periodically along a run of pipe to limit the extent of the water movement in such a case. The purpose of this vapor stop location is to isolate the damage from water entry to a smaller portion of the insulation system. If the system can be shut down and the pipe allowed to warm, this reduces the amount of the insulation system that must be repaired. If repairs are not possible due to the undesirability of turning off the system and allowing it to warm, then periodic vapor stops can limit the increase in condensation and heat gain to only a small portion of the total pipe length. Bear in mind, in this case, there is no single correct spacing distance for vapor stops on a run of pipe. This spacing depends on many factors including quality/cost of the original insulation system design, quality/cost of the installation, and frequency of inspection and maintenance. Typical spacing for vapor stops on a run of pipe would range from every 20 to every 100 feet.
If you have questions about where to install vapor stops on your insulation system, please contact our technical team for more information.