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Using Heat Transfer Fluids Safely

http://www.process-heating.com/articles/90436-using-heat-transfer-fluids-safely

August 5, 2014

National Fire Protection Association 30 (NFPA 30) is a code of guidelines for the use, storage and handling of flammable and combustible liquids. NFPA 30 was first published in 2003, with a revision published in 2008 containing formatting and technical changes. The most recent revision was published in 2012 and contains further technical changes. The guidelines found in NFPA 30 are important to maintain a safe environment in which a heat transfer system can function effectively. Users of heat transfer systems should use it as a guide not only for the proper handling, storage and use of heat transfer fluid, but also for ensuring that the plant inherently mitigates hazards and protects against fires and explosions. Equipment design, proper ventilation and electrical area classification are three important categories for safeguarding a heat transfer system.

NFPA 30 categorizes fluids by flashpoint and boiling point and places them into one of three classes by which safeguards and guidelines are applied. The flashpoint of a fluid is defined as the minimum temperature at which sufficient vapor is given off to form an ignitable mixture with the air. The boiling point of a fluid is defined as the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.

NFPA 30 requires that the flashpoint of a liquid be determined by ASTM D93 Pensky-Marten Closed Cup. Many heat transfer fluids fall into Class IIIB with flashpoints greater than 200°F (93°C). However, it is important to note that the classification of a liquid can change if it is contaminated with a fluid of a different class. One of the revisions made in 2012 requires that Class II and Class III liquids that are stored, handled, processed or used above their flashpoints follow all applicable requirements for Class I liquids. This is due to the fact that at temperatures above their flashpoint, Class II and Class III liquids can produce ignitable vapors if the fluid is released or the system is vented. An all-encompassing revision to NFPA 30 maintains that a system using a Class II or Class III liquid above its flashpoint must integrate safety design features to handle the release of ignitable vapors. It is very common for heat transfer systems to operate at temperatures above the flashpoint of the heat transfer fluid. Unless an alternative design is evaluated and approved by an engineer, these systems must follow the guidelines stated in NFPA 30. The following paragraphs discuss some of the major features required of systems operating with a liquid above its flashpoint.

Equipment Design

Any equipment used in the processing of Class II or Class III liquids above their flashpoint must be designed to prevent the accidental release of vapors and to minimize the amount of vapor released should an accident occur. Any processing vessel should vent outside of the building and should not be open to the room in which it is located. Liquid processing equipment must be located no closer than 25’ from property lines to mitigate the risk of damage to neighboring property should a fire occur. The equipment should be routinely inspected and maintained in order to prevent leakage or spillage. NFPA 30 has more specific guidelines for different types of liquid processing systems.

Ventilation

One of the best ways to prevent the ignition of vapors is to incorporate proper ventilation in the system. NFPA 30 states that enclosed processing areas using Class II or Class III liquids above their flashpoint should have a ventilation rate high enough to retain the vapor concentration in the area at or below 25% of the lower flammability limit of the fluid. The lower flammability limit (LFL) is defined as the lower end of the concentration range over which a flammable mixture of vapor in air can ignite at a given temperature and pressure. This can be determined through sampling or calculation, methods for both of which are outlined in NFPA 30. Exhaust ventilation must be outside of the building to a safe location and must be by mechanical or natural methods. If applicable, ventilation must also be included on the ground and in corners where vapors can collect or build up. It is advised that a “safe location” be chosen for vent discharge to prevent vapors from traveling to an ignition source. The location should be chosen based on the surrounding businesses and neighborhoods, the characteristics of the vapor being vented, its proximity to ignition sources, how likely the vapor is to build up outside of the venting location and how likely it is that a discharge volume with an ignitable concentration actually reach an ignition source.

Electrical Systems

In the event that ignitable vapors do escape, it is important that sources of ignition be controlled as to mitigate the risk of fire. Smoking should be prohibited in areas containing processing equipment using Class II or Class III liquids above their flashpoint. All vessels and equipment in the processing area should be designed to prevent electrostatic ignition. Hand tools used in the processing area are a huge source of ignition so spark-resistant hand tools should always be used. All electrical utilization equipment should be specified and installed according NFPA 70, National Electric Code, in order to mitigate the risk of ignition. Electrical equipment should be installed in classified areas determined by NFPA 30. The three zones are defined as areas where, under normal operating conditions, ignitable concentrations are present for long periods of time (Zone 0), are frequently present (Zone 1) and not likely to be present or only present for a short amount of time (Zone 2).

In conclusion, it is quite common for heat transfer fluids to be used above their flashpoint, meaning that vapors escaping from the system could cause a fire or explosion if ignited. Because they are Class III fluids being used above their flashpoint, the new guidelines set in place by NFPA 30 require that they be used with the same safety precautions as Class I fluids. These guidelines require specific equipment design, proper ventilation to prevent the escape of ignitable vapors into an unsafe area and electrical area classification to ensure that there are no ignition sources to come in contact with ignitable vapors. As with all processes, it is important to carry out drills, educate employees and post shutdown and emergency procedures in order for a rapid response to fires or explosions. Job safety and activity hazard analyses should be carried out before new tasks are started in order to mitigate possible hazards. For additional safety guidelines, heat transfer system users should consult NFPA 30.