201442 Thread mismatch impairs fire extinguisher
Edited from SCG Safety Alert 03-14
During a fire investigation onboard a vessel it was discovered that a portable carbon dioxide (CO2) extinguisher had failed to properly discharge during the fire-fighting event. CO2 only seeped from the neck of the extinguisher when it was used, rather than sprayed. The fire was subsequently extinguished using a dry-chemical fire extinguisher.
During examination it was found that the hose and discharge horn had been replaced. The end of the hose screws on to a diffuser on the side of the discharge valve/handle assembly of the extinguisher. The diffuser is a ported protrusion on the male end of a 90 degree fitting. On the side of the protrusion are orifices through which the CO2 flows. The examination revealed that the spherical end of the protrusion, which contains no orifices, ‘bottomed out’ against the orifice in the connection fitting that leads to the hose and horn assembly. The flow of CO2 was thus completely blocked.
It was further noted that the male threads of the diffuser were tapered US national pipe threads, while the female threads of the hose connection were straight. This thread mis-match likely allowed the hose connection to be tightened to a greater extent than intended on the diffuser threads. This permitted the spherical end of the diffuser to ‘bottom out’ against the orifice in the tube. This may have also resulted in the reported leakage from the neck of the extinguisher due to back pressure.
There are newer types of diffusers in which the orifice follows the length of the protrusion and the end is not spherical. However, the issues regarding the tightening of the two components and the importance of ensuring proper lengths and compatibility of the threaded and machined surfaces remain. Binding or ‘bottoming out’ should not occur except at the threaded surfaces. Replacement parts should be as specified by the equipment manufacturer and servicing should always be done by qualified fire safety equipment technicians.
