Photo Gallery F10: Fire Sprinkler Valve Trim Pipe
As is often the case, some of the smallest of issues have been responsible for the greatest failures and tragedies – such as the lack of a bolt washer to a jet aircraft’s engine, or an O-ring for the space shuttle. For fire protection systems, accumulated corrosion product inside the steel pipe has the potential to reduce or stop all water flow through the fire sprinkler heads; rendering the entire system useless in a time of greatest need. For that reason, regular inspections are required. Yet often overlooked, far less corrosion product is required to alter the function of the main valve providing water into the system in the first place.
For dry and pre-action fire systems, vastly complicated piping arrangements control the main water supply valve both to open and close. This “trim” pipe is typically small diameter 3/8 in. or 1/4 in. schedule 40 threaded galvanized steel pipe. Aside from its small inside diameter, the galvanized steel pipe used in its assembly is the same lower quality galvanized steel so quickly failing at the much larger main supply runs. When the smaller trim pipe corrodes, however, only a small volume of rust product is required to prevent proper functioning of the valve it controls – a condition which is well known to have occurred.
Initially, the zinc protective finish begins failing; creating what is called “white rust.” This is generally not sufficient to produce an obstruction or failure, but once the zinc finish totally fails, red rust corrosion develops at the carbon steel substrate below. This condition generates a larger volume of rust capable of obstructing orifices, interfering with solenoids, valves, etc. Given that this issue is directly related to the corrosion resistance of the galvanized steel pipe, we find far few issues with much older systems. Those systems installed using brass trim pipe are at virtually no risk in terms of corrosion.
Separately, and an almost equal threat, is where the trim pipe has been assembled on site, and where another common finding is the presence of excess pipe thread sealant liquid or tape. Far less likely at pre-manufactured equipment, excess tape sealant wrapped beyond the first threads can be cut while assembled; the loose tape then capable of moving about during each flow test cycle. As this photo gallery demonstrates, some installation practices themselves can also cause a failure of the main valve to open.
