Chloride induced stress corrosion cracking of stainless steel thermowells: Potential for ingress of atmospheric moisture

Health and Safety Executive - Safety alert

Introduction:

This safety notice describes a specific degradation mechanism found inside stainless steel thermowells operating where the external atmosphere contains halides, as is typical in coastal locations or near to cooling towers. Thermowells can 'breathe' during normal operation as vessels heat up and cool down, drawing in the external atmosphere through non gas tight fittings. If the atmosphere contains halides this can leave any stainless steel susceptible to Chloride Stress Corrosion Cracking (CISCC). The HSL Research Report 902 (Reference 1) covers the susceptibility of stainless steel to CISCC in some detail and links to other research papers and published documents.

Background:

Figure 1: Typical layout of thermowell or thermopipe

During routine maintenance on a refinery, a crack was noted on a thermowell pipe extension above a reactor vessel. Closer examination found other thermowells to be cracked. The thermowells were several metres long and hung inside the reactor vessel; they carried an array of thermocouples to measure temperature at varying depths inside the reaction vessel. The thermowell extensions, supporting the instrumentation boxes, were exposed to atmosphere and had either screwed or flanged connections to the instrument boxes. A typical layout is illustrated in Figure 1.

Figure 2: Typical position of CISCC on thermowell with screwed connection to instrument box

Close examination of the full length of a number of the thermowell pipes revealed stress corrosion cracking from the inside of the thermowell that was moving towards the outside surface. The majority of the cracking found was in the extension of the thermowell tube above the top of the reactor vessel (See figure 2).

In addition, cracking had started to propagate in some of the tubes just below the top of the reactor vessel flange (see figure 3); had this cracking proceeded to full thickness it would have allowed a release of hazardous substances from the reactor through the tube and then to atmosphere.

Figure 3: CISCC in thermowell below reactor head

The thermowell breathes during normal operation as the reactor heats and cools and this allows the atmosphere outside the reactor to be drawn inside the thermowell tube during cooling and expelled during heating. If the external atmosphere contains halides, as is typical in coastal locations or near to water cooling towers, then over time the halides will concentrate in crevices inside the thermowell and can initiate cracking.

The thermowell connections to instruments are typically not gas tight over the pressure range that can be seen inside the thermowell tube during normal use and so breathing through the connection can be expected. The temperature within the reactor vessel will influence the degree of pressure change and hence the amount of atmosphere drawn in each time the thermowell cools. This in turn will influence the halide concentration inside the thermowell tube. A thermowell tube heated from 20 to 420°C would experience a pressure rise (assuming constant volume) in the region of 2.4bar. Assuming this pressure dissipates through the connections over time; when the thermowell cools a negative pressure is formed that draws in the atmosphere.

Action required:

Operators of plants containing hazardous substances should identify and register all stainless steel thermowells on site where failure of the thermowell could lead to a release of a hazardous substance. Operators should then set out an appropriate maintenance/inspection programme, taking into account the susceptibility of stainless steel thermowells to CISCC, as described, the consequences of failure and potential release of hazardous substances to the environment.

References:

HSL RR 902 Chloride stress corrosion cracking in austenitic stainless steel. Assessing susceptibility and structural integrity

Further information:

Hazardous Installations Directorate,
CEMHD Unit 1
Redgrave Court
Merton Road
Bootle
L20 7HS

General note:

Please pass this information to a colleague who may have this product/equipment or operate this type of system/process.