Keep heat-transfer-system repairs uneventful

By Conrad E. Gamble, Solutia Inc.

A good plan is the key to avoiding risks in the maintenance of systems using high-temperature organic fluids.

Traditional heat transfer fluids (HTFs) and systems have been around for many decades. While much has been learned and written about the safe operation of these systems, less has been shared about the proper approach to making uneventful repairs. The safe execution of repairs depends upon effective planning to protect against potential hazards. This article focuses on mitigating potential HTF-related hazards that could be encountered when repairing high-temperature organic HTF systems (those normally operating above 500°F). It is not intended to supersede any fluid-specific risk information available from the manufacturer, nor process or equipment-specific risk details associated with individual processes involved.

Every process has unique hazards associated with the chemical(s) being handled. Effective job planning and execution takes into consideration the hazards of both the processes and HTFs involved. The risks associated with HTFs can be divided into three primary areas: fire, human exposure and environmental exposure. The single, best source for fluid safety information is the material safety data sheet (MSDS) provided by the manufacturer.

Table 1. HTF chemistries and fire properties
HTF type	Fire properties (ASTM D92 & D445)
	Flash point, °F	Fire point, °F	AIT, °F
DP/DPO Eutectic	235-255	245-265	1,100-1,150
Alkylbenzenes	350-375	410-425	650-710
Mineral oils	340-445	380-500	640-715
Dibenzyltoluenes	390-420	445-475	840-880
Diaryl alkyls	245-380	255-310+	725-805

Fire potential is typically assessed using flash point, fire point and autoignition temperature (AIT) data (Table 1). Most of the commonly used HTFs operating from 500°F to 750°F, including diphenyl/diphenyl oxide (DP/DPO) eutectic fluid and the others listed in the table, have hydrocarbon chemistries. As such, they are typically classified as Class IIIB combustible liquids capable of igniting under the right conditions (fuel/heat/oxygen).

Depending upon the system design, operating environment, and fluid age and maintenance, some fire properties might become depressed below the values shown, which can increase fire risk. With awareness of these conditions, proper job planning can interrupt the hazard mechanisms and greatly reduce or eliminate the fire risk. A safe approach to fire prevention should include minimizing spark potential, inerting, conducting work while equipment is near ambient temperature, and insuring the area is cleared of hydrocarbon residues before beginning hot work.

Limiting the potential for human exposure to chemicals should be at the forefront of job planning considerations. Specific chemistries of HTFs will help define certain personal protective equipment (PPE) requirements, materials selection for fluid handling, etc. Review of the MSDS and technical literature will aid in the proper selection of PPE to avoid permeation of gloves or protective aprons, and could prevent failure of polymer components and other equipment resulting in a loss of containment.

The most common HTF-related injury is thermal burns. Whenever possible the system should be allowed to cool sufficiently to prevent burns from hot fluids or equipment contact when hands-on work begins. An additional benefit of cooling the fluids is reduced vapor pressure, thereby lowering potential exposure to fluid vapors.

Should non-routine material transfers be required, selection of hose materials, gaskets, containers, and O-rings/seals should all conform to manufacturer guidelines to avoid unexpected leaks, releases and physical contact. Ideally, job plans will incorporate these considerations, plus knowledge from industrial hygiene monitoring, to best determine the right combination of splash, face, eye, thermal burn, and/or respiratory protection required for each situation.

Exposure to the environment can also be safely managed by proper planning. Chemicals should be responsibly handled, but certain fluids might have regulatory restrictions that place stricter emphasis on environmental protection. This information should be provided in the MSDS.