Testing Lubricants For Spray Flammability

The test materials consisted of five phosphate esters, two halogenated hydrocarbons, two water-based, two petroleum-based and two silicone-type lubricants and hydraulic fluids. The samples were obtained from their respective manufacturers except for standard Navy stock items.

 

Description of test apparatus

The test apparatus was developed for the purpose of making a practical evaluation of the flammability of fire resistant fluids in such a manner that it could be applied directly to existing possibilities of hazards occurring aboard ship due to mists and sprays flaming. The apparatus was constructed in such a way that anyone could set up a similar apparatus very simply and at a relatively low cost. In this way it was hoped that a standard apparatus for testing fire-resistant fluids could be established to replace the various methods which each individual company now uses. As it presently stands, no correlation of the flammability of the various fire-resistant fluids can be made since they have all been tested using different methods.

 

It is believed that this apparatus has at least two points of superiority to some other measured temperature surface contact methods; (1) The contact surface is generous so that if one small area will not ignite a portion of the spray there are additional surface areas that will be certain to; (2) there is such a high ratio of igniting surface element mass to flowing air and spray mass that the surface temperature is not lowered by the flow of air and spray.

 

The testing apparatus consisted of a specially designed furnace assembly with electrical power supply and temperature recorder, a paint spray gun, and a hood. The furnace was comprised of a Hoskins Electric Muffle Furnace heating unit with platinumrhodium thermocouples imbedded in the hottest spot in each of the four walls. The heating unit was surrounded by Babcock and Wilcox K26 insulating brick and calcine block and was open at both ends to the atmosphere. The furnace was controlled by a G. E. induction voltage regulator and the thermocouples were connected to a 16-channel Leeds and Northrup recorder capable of recording temperatures from 0 to 3000°F.

 

The furnace was placed on a metal table about 3 1/2 feet high, and a moveable target, which rested in slotted angle irons, was positioned directly in front of the furnace. The target could be moved as close as 4 inches to the furnace or as far away as 16 inches.

 

The target had a 3-inch square opening at its center. Slots were provided on either side of this opening and plates with circular openings ranging from inch to 3 inches, in increments of inch, were made to fit the slots. Thus, the amount of fluid from the spray gun passing through the target could be regulated even without varying the spray pattern.

 

The spray gun used was a Binks paint spray gun with a maximum capacity of 50 psig. It was mounted on a special stand in line with the center line of the opening in the target in such a way that it could be positioned at any distance from the target up to 3 feet when the target was as close to the furnace as possible. The spray gun stand support was about 3 1/2 feet from the furnace. The support was slotted and the spray gun was mounted on the end of a bar which rode back and forth in the slot, thus allowing for positioning the gun at different distances from the target. The gun was fired using a remote control wire enabling the operator to stand far enough away to avoid any danger from flashback. The entire assembly, including the spray gun, was under a hood which used air jets to help remove the smoke and toxic gases generated by thermal breakdown or combustion.

 

Method and results of test

 

In the test method, the furnace was heated to a temperature somewhat above what was believed to be the spray flammability temperature of the fluid to be tested. A piece of insulating brick 4 1/2x2 1/2xl 1/2" deep, was placed in the furnace. The position of the target was 4 inches from the furnace with the gun flush against the target and a plate with a 1/4-inch circular opening was placed in the slots at the center of the target. With the fluid in the gun and the air pressure set at 40 psig, the gun trigger was held open for about one second allowing the fluid-air mixture to pass into the furnace. The following data was taken from each fluid injection: temperature of the hottest internal furnace surface (in this case, the top wall), ignition delay in seconds, smoke observations, degree and nature of burning, room temperature, and relative humidity. The ignition delay in this case was defined as that elapsed time from the pulling of the trigger until ignition occurred. If burning occurred, the temperature of the furnace was lowered in 50°F. intervals until there was no ignition. The furnace temperature was then raised in small increments until burning again occurred. This temperature was recorded as the spray flammability temperature. The above procedure was repeated for each of the 14 fluids tested.

 

The spray flammability temperatures of the fluids tested were generally reproducible within 20-50°F. This is considered sufficiently accurate for the temperature involved and the desired results. Relative spray flammability temperatures were desired rather than the absolute temperatures.

 

With a spray gun pressure of 40 psig, and a target distance of four inches from the furnace, with the gun flush against the target, optimum conditions for fluid ignition in the furnace were created. At this distance, variations in pressure from 10 to 40 psig had little effect on the spray flammability characteristics of the fluid tested. However, at a target distance of 16 inches with the gun flush against the target, and 40 psig, the slower moving outer periphery of the spray pattern which settled on the hot inner furnace walls at 4 inches now landed outside the furnace. The center portion of the spray passed through the furnace at too great a velocity for evaporation and ignition of the droplets to take place. At 16 inches and gun pressure of 5 and 10 psig, insufficient quantities of air-fuel mixture entered the furnace. From these observations, it was concluded that optimum ignition probabilities were achieved at a target distance of 4 inches from the furnace with the gun flush against the target and a spray pressure of 40 psig. This pressure was selected to assure that viscous fluids would pass through the spray gun nozzle. The 1/4-inch diameter plate hole was used to permit all fluid from the gun to enter the furnace and to minimize the possibility of a flashback.

 

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The reason for using air in the final test apparatus was that this test was designed to be as practical as possible in nature rather than scientific. In almost all cases where danger could arise from a lubricant or hydraulic fluid spraying out and flaming, normal atmospheric conditions would prevail.

 

Because of the wide disparity in viscosities of the fluids tested, the air-fluid ratio and the spray velocity entering the furnace varied somewhat with each fluid tested. With some fluids, the spray velocity was sufficiently high to cause the mixture to pass through the furnace before evaporation of the fluid droplets could occur. For these fluids, the true spray flammability temperature ranges involved, this variable is not considered to greatly affect these results.

 

Other variables such as room temperature and relative humidity seemed to have little effect on the final results, due mainly to the very large differences in temperature between the various types of fluids tested. The authors believe that the differences produced by varying these conditions are not significant.

 

With several fluids, the smoke produced by spraying was extremely dense and choking whether or not ignition actually occurred. The smoke and/or vapor produced throat irritation and nausea in one case. Products of combustion were not analyzed for toxicity. It is highly probable that toxic gases are formed due to the initial ingredients of the hydraulic compounds. The spray flammability temperature of all fluids tested agreed reasonably well with their upper ignition temperatures found by using the Jentzsch tester with the exception of the petroleum-based fluids. The Jentzsch tester is commonly used to determine ignition properties of fuels under certain conditions of temperature and oxygen concentration.

 

Conclusions:

The method developed for determining spray flammability temperature gives consistent and reasonable results.

The simplicity of equipment needed in addition to the non-sensitivity of the method to atmospheric conditions makes it feasible as a standard method for determining spray flammability temperature.

Halogenated hydrocarbons with spray flammability temperatures above 1300°F. and phosphate esters with spray flammability temperatures above 1250°F. were far superior to other types tested. Petroleum-based fluids exhibited relatively inferior flame-resistant properties.

The phosphate esters in general were the most disagreeable fluids with regard to odor, smoke density and irritation.

Water-based fluids will burn at a relatively low temperature after the water has flashed off.

There is no such thing as a flameproof hydraulic fluid or lubricant other than water. Any available fluid will bum at some temperature in the test apparatus used.

 

It is recommended that, pending the development of more refined testing techniques, future Navy lubricants and hydraulic fluids be subject to this type of standard spray flammability test to ascertain the safety of the fluids with regard to fire retardance. Since temperatures in the range of 950°F. are encountered in modern ships’ steam lines, the spray flammability temperature of fluids contemplated for use should exceed 1100°F. It is also recommended that additional work be . done to assess the effect of spray droplet size on the spray flammability temperature and to determine if any toxicity problem exists.