Fire Testing for Grooved Piping Systems
Authors: Patterson B.H. 
Published: December 2007

Fire tests and evaluation of a well-known, resilient gasket-sealed pipe coupling have led to the development of a new fire-resistant gasket. it had been assumed, from limited fire experience, that resilient gasket- sealed couplings in hydrocarbon service posed an unacceptable fire-loss risk. A modified API Standard 607 fire test was run on a standard rubber gasket and coupling. The test proved that, if exposed, the gasket could leak sufficiently to spread a fire. Further research and testing resulted in a new fire-resistant gasket that offers a cost-effective way to reduce the risk of loss.

Introduction

The common use of soft sealing materials (i.e., synthetic rubber or other elastomers) in the petrochemical industry has long caused concern about the durability of the soft material when exposed to the severe conditions of a petroleum fire. While attention has been directed toward soft-seated valves, another area has recently drawn some attention: soft materials used in the couplings of mechanically grooved piping systems. Will the ductile-iron coupling and rubber gasket that hold mechanically grooved pipe and fittings together fail in a fire? if they do, will the leaking hydrocarbon from the failed coupling spread the fire and cause increased damage? Is the risk of the increased damage acceptable? If not, what can be done to lower the risk to an acceptable level?

Background

The coupling in question is used primarily in producing, pipeline. and marketing applications. The grooved piping system basically consists of pipe and fittings that have grooved ends, produced by cutting or rolling, clamped together with a ductile-iron, rubber-gasket-sealed coupling (see Fig. 1). The coupling leaves a A-in. [6.4-mm] gap between the pipe ends. The rubber gasket fits into the coupling and gap to create a leak-tight seal. The coupling and gasket are typically rated for 2,000-psig [14-MPa) working pressure. The actual gasket compound tested is a high-modulus nitrile rubber. But for this paper, the gasket compound will simply be referred to as rubber. There was a great deal of uncertainty about the true risk of these couplings. Although some fires have involved grooved systems, no actual test data were available; therefore, the evidence was inconclusive.

One major manufacturer of such couplings volunteered to provide fire testing and, if necessary, to develop a gasket with acceptable fire resistance. As the end user, we were asked to provide technical support to define a specific test procedure and to establish the specifications for an acceptable fire-resistant gasket. This manufacturer's coupling and gasket were thought to be those most commonly used in the oil field. Only one model from this one manufacturer was tested. The product is referred to here as the standard coupling and gasket; the improved design is referred to as the fire-resistant coupling and gasket.

The testing program was designed to answer several questions.

Will the Standard Coupling Leak Significantly To Spread a Fire and To Increase the Risk of Loss?

On average, the standard rubber gasket failed in 12 minutes when exposed to a fire in preliminary testing.

Considering a producing facility as an example, the leak rate will vary directly with the pressure, and the actual burn rate will depend on the amount of light ends in the crude and the overall composition of the fluid, specifically the water cut. Fig. 2 shows a typical installation.