Case Study - Fire Hose Failure

Smithers Rapra were asked to identify the failure behind a jammed spool valve in a fire hose.

History

A wall mounted fire hose located in a municipal swimming pool failed during a statutory fire drill. The manually operated spool valve had apparently jammed in the closed position. As similar fire fighting systems were known to be employed extensively in other public places the failure stimulated much concern & immediate action was required.

Inspection & Analysis

The hose valve was carefully disassembled by our consultants & they discovered the plastic spool had fragmented as shown in the photograph. The spool remnants were found to be fragile. Other parts included rubber o-rings & a brass rod.

A process known as Fourier transform infrared (FTIR) spectroscopy confirmed that Nylon 6 had been used to injection mould the spool. Gel permeation chromatography (GPC) revealed only minor differences between the molecular weight distributions of a new spool & the failed spool.

The brass rod was visibly corroded & caked in a white surface deposit. The deposit was analysed & found to be zinc chloride.

Failure Diagnosis

The pool attendants were interviewed & it was revealed that the fire hose had been used occasionally for cleaning the pool floor & sides. During this operation the hose end would be submerged from time to time in the pool water. This would allow access of chlorinated water to the spool valve assembly. It was possible therefore that for a substantial period of its service life about 5 years the Nylon spool & the brass rod had been in contact with heavily chlorinated water. During this time the solution, by attacking the brass rod, would convert to a salt solution of predominantly zinc chloride. It is well known that zinc chloride is a powerful stress cracking agent for Nylons.

A high level of cohesion in Nylons relies upon intermolecular hydrogen bonding between amide groups. Bond strengths are typically 33 kJ/mol. Zinc chlorides & other metal halides with high cation charge density form metal complexes with the amide carbonyl thus weakening the hydrogen bonds. The reduction in intermolecular cohesion renders the amorphous phase of the material in particular susceptible to localised swelling & plasticisation in contact with water. In the presence of stress these transform into cracks & crazes.

The rate of attack is known to be rapid even at ambient temperatures. The suppliers of Nylon usually clearly admit incompatibility but only at high salt concentrations. In this case the concentration was not known but could, by slow evaporation over long periods of time, have eventually reached a critical level.

Lessons & Consequences

1. It was a relief to all concerned that the failure was proved to be the result of a rare service history, & therefore hoses in most public places were unlikely to suffer from the same problem.
2. The supply chain cannot be held responsible for failing to predict & therefore avoid the problem. If anyone was to blame it was the pool attendant who broke the rules.
3. The supply chain should however be sensitised to the potential dangers of hybrid metal/plastic assemblies. Lithium, calcium, zinc, magnesium in contact with Nylons (or acetals) in a wet environment should be considered very carefully.