Thermal Fluid Heater - Vapor Explosion

[WallyH]

I recently responded to a "explosion" that resulted from a thermal fluid heater at a large bakery. The thermal fluid used in this heater is an oil; Exceltherm 600 with a ignition point of 660 degrees F. The oil is pumped from the heater and through heat exchangers into an oven that is used to heat the clients product.

The heater is not a pressure vessel; it is opened to the atmosphere. Their is an expansion tank located above the heater used as the ballast for the system. The expansion tank then vents into a dump tank located at the floor level. The dump tank vents into the mechanical room; directly to the floor. The vent is located about 8 feet from two air compressors and about 10 feet away from a gas-fired domestic hot water heater.

What happened was an occurence caused the thermal fluid (oil) to form a vapor which was vented from the dump tank into the room. The mechanic noticed white smoke which was most likely the vapor going through a condensing phase. After a few minutes of this white smoke an explosion occurred. Most likely from the ignition of the gas-fired domestic hot water heater or the start up of the air compressors, possible causing a spark.

If I am reading the code correctly, as per NEC 500.5(B)(2), this mechanical room should be classified as a Class I Division 2. Obviously one of my recommendations to the client is to re-pipe the vent to an area outside the room. That being said, what are your thoughts regarding the air compressors and the hot water heater being in the same room?

 

[rbalex]

What you described sounds more like Class I, Division 1. See Section 500.5(B)(1)(1).

 

[WallyH]

Section 500.5(B)(1)(1) states "....flammable gase or vapors can exist under normal operating conditions". The Class I, Division 2 addresses vapor escaping in the case of abnormal operation of equipment. TThis oil causes a vapor when the system is not operating correctly, this is why I considered it to be Class I, Division 2.

Do you still feel that this should be classified as Division 1? By the way, thanks for your input.

Walter Haupt
CNA

 

[rbalex]

I may have misunderstood your original post. It is important to realize ?normal? is not necessarily ?common? though. If the ?occurrence,? as you described it in the OP, can happen during the situation when the equipment is operating within its design parameters, it would generally be considered ?normal.? There is no formal definition of normal in the NEC as it relates to hazardous locations.

It could be the situation is more in line with 500.5(B)(1)(2): In which ignitible concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage, ?

I don?t understand the process you described well enough to give conclusive advice; however "frequency? of a condition is part of the overall evaluation, ?normal? or not.

I do note I didn?t address your direct question in the last paragraph of the OP. The domestic water heater will generally have no bearing on the electrical area classification.

 

[weressl]

I may have misunderstood your original post. It is important to realize ?normal? is not necessarily ?common? though. If the ?occurrence,? as you described it in the OP, can happen during the situation when the equipment is operating within its design parameters, it would generally be considered ?normal.? There is no formal definition of normal in the NEC as it relates to hazardous locations.

It could be the situation is more in line with 500.5(B)(1)(2): In which ignitible concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage, ?

I don?t understand the process you described well enough to give conclusive advice; however "frequency? of a condition is part of the overall evaluation, ?normal? or not.

I do note I didn?t address your direct question in the last paragraph of the OP. The domestic water heater will generally have no bearing on the electrical area classification.

rbalex,

You are correctly identified the installation deficiency. The dump area should be Division 1 and the extent of that and the Div. 2 area surrounding it should be defined by the expected ventilation/dilution rate, if any is applicable.

The expansion vent should have been terminated outside with a mist condenser in a quenched tank.

OSHA will have a hayday.....