REPORT INVESTIGATION OF 3M CLEAN AND STRIP DISKS IN EXPLOSIVE ATMOSPHERES. Client: 3M Norge A/S. Authors: Bjørnar A. Johnsen Geir H.

Transcription

1 REPORT INVESTIGATION OF 3M CLEAN AND STRIP DISKS IN EXPLOSIVE ATMOSPHERES Client: 3M Norge A/S Authors: Bjørnar A. Johnsen Geir H. Pedersen Bergen, June 2005 Ref. no.: GexCon-05-A Rev. no.: 01

2 GexCon AS Fantoftvegen 38, Postbok 6015 Postterminalen N-5892 Bergen Telephone Telefax Internet: Bank A company in the CMR -group REFERENCE PAGE Author(s)) REPORT Bjørnar A. Johnsen Geir H. Pedersen Doc. no.: GexCon-05-A Security Classification (A) Rev. no.: 01 Title INVESTIGATION OF 3M CLEAN AND STRIP DISKS IN EXPLOSIVE ATMOSPHERES Summary Different 3M Clean and Strip disks have been tested in order to investigate their ignition capability of explosive atmospheres. During the first part of the investigation, spark formation and temperature development was studied. Based on these results, tests were performed in explosive atmosphere in order to find out whether or not use of the disks could lead to ignition of the atmospheres. Index terms Norwegian Gasseksplosjon Tennkilde Slipeverktøy Eksplosjonssikkert verktøy English Gas explosion Ignition source Grinding tool Explosion proof tool Client 3M Norge A/S Client ref. Knut A. Mo Project Equipment testing Project no Rev. No Date Prepared by Controlled by Approved by Reason for revision Bjørnar A. Johnsen Bjørnar A. Johnsen Geir H. Pedersen Brian Wilkins Draft rapport Geir H. Pedersen Brian Wilkins Final rapport

3 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres i DISCLAIMER GexCon shall not be liable for damages, which the assignor suffers when applying or using the results of GexCon s work, unless there is misconduct or gross negligence on the part of GexCon or on the part of the persons used by GexCon to carry out the work.

4 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres ii TABLE OF CONTENTS 1. INTRODUCTION AND BACKGROUND EXPERIMENTAL EQUIPMENT Phase 1, Measurement of temperature development Phase 2, experiments with flammable liquids Phase 3, experiments with explosive gas mixture RESULTS AND DISCUSSION Measurements of temperature development during grinding Influence of grinding force, type of material and grinding disks for straight- and angular- grinder Experiments with flammable liquids Experiments in explosive atmospheres CONCLUSIONS...14

5 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 1 of INTRODUCTION AND BACKGROUND The objective of the project was to evaluate the use of 3M new purple and blue disks for use in explosive atmospheres. Tests with Radial Bristle Brush P36 were also preformed. In the first phase of the project, the temperature of the rotating disks and the temperature of the surface where the tool was used was measured. In the second phase of the project, ignition tests were performed in the presence of heated flammable liquid hydrocarbons. In order to increase the vapour pressure of the flammable liquid and to ensure that the liquid temperature was above the flash point, experiments were performed at temperatures up to 113 ºC. In the final part of the project, similar tests were performed in explosive atmospheres of propane/ air. The tests were mainly performed at conditions similar those giving the highest temperatures obtained in Phase 1. In order to study the igniting ability of hot surfaces and mechanically produced sparks, the tests were performed under extreme working with respect to the force between tool and the surface being grinded. The experiments were performed at GexCon s explosion laboratories at Fantoft, Bergen.

6 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 2 of EXPERIMENTAL EQUIPMENT 2.1. Phase 1, Measurement of temperature development The experiments for measurement of temperature development were performed using a remotely operated apparatus where the different grinding machines with the 3M disks, could be mounted. This apparatus was designed in such a way that the force between the grinding machine and the surface where the grinding took place could be varied. The apparatus is shown in Figure 1 where a Dynabrade P/N straight grinder is mounted with purple disk running at max 3400 rpm, blue and bristle brush. Figure 3 and 4 shows an angular grinding machine from Georges Renault, Type KA F running at max rpm. The temperature development was measured using thermocouples, type K and by an optical temperature sensor. The thermoelements were inserted into holes in the steel plates, approximately 3-4 mm below the surface where the grinding took place. Normally four thermocouples with a spacing of 20 x 5 mm were used. See Figure 5. After grinding for a certain time, the temperature stabilizes. In some of the experiments so much steel was removed that the grinding disks came into contact with the thermocouples. When this occurred the experiment was stopped. The grinding force was varied typically from 1.5 kg to 11 kg, depending on the surface material. Using a higher force, the grinding machines stopped rotating. The signals from the different sensors were measured continuously using computerised data acquisition equipment from National Instruments. The temperature development versus time was recorded. Figure 1 Grinding against a surface of stainless steel with the straight. The four temperature sensors are mounted from below in holes in the steel plate.

7 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 3 of 14 The optical sensor was used for the measurement of the temperature of the rotating disks perpendicular and tangent close really to the contact area between the disk and the surface. See Figure 2. Figure 2 Grinding against a surface of stainless steel with the straight grinder. The optical temperature sensors are measuring the temperature perpendicular close to the contact area between the disk and the surface Phase 2, experiments with flammable liquids The experiments in the presence of flammable liquids were performed in three different ways. First, the grinding occurred against a stainless steel plate 316, with heptane dripping on to a preheated, 76? C, surface. See Figure 3 and Figure 4.

8 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 4 of 14 Figure 3 Grinding with purple disk in contact with flammable liquid Figure 4 Grinding with purple disk in contact with flammable liquid

9 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 5 of 14 In the second case, the grinding occurred at a higher surface temperature, 110? C. The position of the heptane source was also changed. In both cases the liquid was heated to temperatures well above the flash point of the liquid. At temperatures above the flash point, the gas concentration above the liquid is higher than the lower explosion limit. In the third case, the grinding occurred with higher surface temperature, 113? C. And the liquid was changed to diesel. In every case the liquid was heated to temperatures well above the flash point of the liquid. Figure 5 The positions of the thermocouples, 20 mm in distance in x-axis and 5 mm in y- axis Phase 3, experiments with explosive gas mixture The experiments with explosive gas mixtures were performed in an explosion chamber having a volume of 1.5 m 3. The remotely operated apparatus described in chapter 2.1 was mounted in the bottom of the chamber. The experiments in explosive atmospheres were performed under similar conditions as those giving the highest temperatures in phase 1 of the project. The force between the grinder and the surface varied between 2 kg and 12 kg for 10 minutes. Different gas mixtures were prepared by the addition of pure hydrocarbon gas (propane) to a re-circulated flow of the gas mixture. This enabled a homogeneous gas concentration within the test chamber. The gas concentration was measured at five different locations using a gas analyser. During the experiments, the gas concentration was measured directly. The gas concentrations were between 3.9 and 5.0 vol%. For propane stoichiometric mixtures is 4.05 vol%. The stoichiometric mixture corresponds to the mixture where the fuel concentrations correspond to oxygen contents of the air in such a way that all the fuel and oxygen is consumed in the combustion. The experimental set up is shown in Figure 6 to Figure 8.

10 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 6 of 14 Figure 6 The explosion chamber used during the experiments with explosive gas mixtures. Figure 7 The test rig with the angular grinder mounted inside the test chamber. During the first tests, due to the design of the grinder, some air exhaust was blown into the explosion chamber resulting in some dilution of the explosive atmosphere near the grinder. Adding extra hydrocarbon gas into the re-circulated flow compensated this. In the following tests, the holes in the grinding machine were blocked to prevent air entering the test vessel. Tests were performed with the purple disk and the blue disk on stainless steel 316 and St52.

11 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 7 of 14 Tests with the Radial Bristle Brush P36 Ø 150 x 13 mm were also preformed in the chamber with explosive gas. The force between the rotating brush and the surface varied between 0.5 kg and 5 kg. Tests were performed continuously for 10 minutes. The Radial Bristle Brush is shown in Figure 8. Figure 8 The test rig with the Radial Bristle Brush mounted inside the test chamber.

12 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 8 of RESULTS AND DISCUSSION 3.1. Measurements of temperature development during grinding During the first phase of the project a comprehensive series of tests were performed to study the temperature development during the use of the disks. The influence of different parameters influencing the temperature development was studied. These included: * Type of grinder * Type of material (carbon steel (St52), stainless steel and aluminium) * Grinding force * Spark production Influence of grinding force, type of material and grinding disks for straight- and angular- grinder The first tests were performed to find out what kind of material produced the most sparks and produced the highest temperatures. Initially, tests were performed with St37 and St52 to find out which of these that produced most visual sparks and produced the highest temperatures. The material producing most sparks and the highest temperatures were used in the rest of the tests. The experiments were performed by varying the grinding force from 1,5 kg to 11 kg. At maximum force, the grinding machine worked very heavily and the speed was reduced. At higher force, the machine stopped. See Figure 9, Figure 10 and Figure 11.

13 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 9 of Temperature Time (sec) Figure 9 The graph shows the temperature development as function of time with a grinding force of approximately 5.5 kg pressure on aluminium with the purple disk Temperature Time (sec) Figure 10 The graph shows the temperature development as function of time with a grinding force of approximately 10 kg on the St52 surface with the purple disk.

14 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 10 of Temperature Time (sec) Figure 11 The graph shows the temperature development as function of time with a grinding force of approximately 10 kg on the stainless steel (316) surface with the purple disk. The results show a clear relationship between the grinding force, material and the temperature development. The maximum temperatures were obtained at high grinding force. The maximum temperature, 330?C, was measured for 316 on the material where the grinding occurred. The maximum temperature on the tool was 95?C. 320 Straight Grinder 280 Temperature St52 Alu Load in kg Figure 12 Temperature development as a function of grinding force using the straight grinder on stainless steel.

15 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 11 of 14 The following graph shows the maximum disk temperature measured as a function of the grinding force (with the straight grinder). 100 Tool temperature for Straight Grinder 316 Perpendicular Tangent 80 Temperature Load in kg Figure 13 The maximum disk temperature measured as a function of the grinding force. Grinding on stainless steal with a straight grinder. 350 Angular Grinder 300 Temperature St52 Alu Load in kg Figure 14 The maximum disk temperature measured as a function of the grinding force. Grinding on the three different surfaces with the angular grinder.

16 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 12 of Experiments with flammable liquids The experiments with flammable liquids were performed as described in Chapter 2.3. To ensure that the experiments were performed under conservative conditions, grinding at full speed, 7-8 bar pressure to the angular grinder, against stainless steel was used as basis for the tests. The grinding force was 8-10 kg. Only purple disks were used, since these produced the highest temperatures. Experiments were performed with two flammable solvents, heptane and diesel. Different physical properties, such as flash point, ignition temperature and boiling point, of the two solvents are shown in Table 1. (the values are taken from datasheets showing occupational health aspects). Liquid Flash point (?C) Ignition temperature (?C) Boiling point (?C) Heptane Diesel > Comment Test temp 76 ºC and 110 ºC Test temp 113 ºC Tabell 1 Different physical properties for heptane and diesel. By definition the atmosphere above a flammable liquid is explosive if the temperature is higher than the flash point. If the liquid temperature is identical to the flash point, equilibrium between the fuel concentration above the liquid and the liquid will establish. This concentration is equal to the lower explosion limit (LEL). The use of rotating grinding tools will produce heat and explosive fuel concentrations well above the lower explosion limit may occur, even if the liquid temperature is below the flash point. In order to produce high fuel concentrations above the flammable liquids, the liquids used in the experiments were heated up. The temperature varied from typically from ca 76?C to 113?C. During the tests, diesel produced a strong smoke or vapour clouds at the maximum test temperatures. A strong boiling process was observed when heptane was in contact with a surface having a temperature above the boiling point. During the experiments the grinding disk was in contact with the flammable liquid. Experiments lasted typically 3 to 5 minutes. Due to the evaporation of the liquid, additional liquid was constantly added during the experiments. The flammable liquids DID NOT ignite in any of the tests.

17 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 13 of Experiments in explosive atmospheres Based on the results from the temperature measurements, experiments with the grinding disks were performed in explosive gas atmospheres in order to find out whether the tools would ignite the mixtures. The experiments were performed with stoichiometric atmospheres of propane in air (propane concentration 4,05 vol%) in a 1,5 m 3 test cabinet at GexCon s explosion laboratories at Fantoft in Bergen. The results from the temperature measurements showed that the maximum temperatures were obtained when grinding against stainless steel. The results from the previous experiments had shown that the test material could reach a maximum surface temperature of 330?C. The tests in explosive atmospheres were preformed both with the purple and the blue disk and grinding occurred on stainless steel and St52. Some tests were also performed with the Radial Bristle Brush P36 Ø 150 x 13 mm in the explosive gas atmosphere. During these tests grinding was performed on St52. The first experiments were performed in mixtures of propane and air. The concentration varied from vol%, i.e. the fuel concentration was close to a stoichiometric mixture. For such composition of fuel and air, the mixture has the maximum sensitivity of ignition, i.e. the ignition energy and the ignition temperature are at minimum values. For propane, the minimum ignition energy is 0.26 mj and the minimum ignition temperature is 493?C. (Corresponding values for natural gas are 0.28 mj and approx 480?C, - these values depend on the composition of the natural gas). A comprehensive series of experiments were performed and the experiments lasted at least as long as during the temperature measurement experiments. Typical duration of a test was 10 minutes. The mixtures of propane and air did NOT ignite during any of the tests. The different grinding disks are designed to be used in areas with a potential explosion risk related to accidental releases of natural gas. The present investigation was performed with mixtures of propane in air, which are slightly more sensitive to ignition than natural gas. Hence, it is considered to be very unlikely that potential ignition sources produced by the disks, when used under similar conditions as during the present investigation, will ignite explosive atmospheres of propane and air. It is also considered to be very unlikely that potential ignition sources produced by the disks, when used under similar conditions as during the present investigation, will ignite explosive atmospheres of vapours of heptane or diesel and air.

18 Investigation of 3M Clean and Strip Disks in Explosive Atmospheres 14 of CONCLUSIONS 3M s purple and blue grinding disks and Radial Bristle Brush P36 Ø 150 x 13 mm in combination with pneumatic grinding machines from Dynabrade P/N and Georges Renault Type KA F, have been investigated with respect to use in explosive atmospheres. Different tests have been performed in order to find the maximum temperature that can be produced under realistic use of the tool. The experiments have been performed in a conservative way. During maximum load when grinding against stainless steel, a maximum temperature of 330?C was measured on the steel. The maximum temperature on the rotating disk was ca 95?C. The maximum temperature measured during grinding increased with increasing grinding force. If the grinding force was increased above certain limits, the pneumatic grinding machines stopped due to excessive friction. Experiments performed in explosive atmospheres of propane and air showed that the grinding disks did not produce strong enough ignition sources (i.e. hot surfaces and for mechanical sparks) to ignite the explosive atmospheres. Since a stoichiometric mixture of propane and air is more sensitive to ignition than natural gas, it is considered to be unlikely that natural gas will ignite if it is brought in contact with the grinding disks during a grinding operation. The experiments in and above flammable liquids have shown that the use of the disks will not cause ignition of the liquids. The present investigations have been performed in a conservative, but realistic way. During normal operation, an operator of the tool will not be able to expose it to the same load as in the tests. Provided that the grinding disks are used with similar grinding machines as those used in the present investigation, and also within the limitations of this investigation, it is considered to be very unlikely that the tool will cause ignition of accidental leakages of propane or natural gas.