Autor: Georgii Feodoridi

Place: NANIO CCVE, Zavod ECOMASH, VUGI Settlement Lyubertsy

Date: June 2011

Abstract: The influence of polyethylene-based ballistic materials on the reduction of explosion pressure inside flameproof enclosures has been studied.

Key Words: Flameproof enclosure, polyethylene, ballistic material

DOI: 10.13140/RG.2.2.29194.68809

Research disclosure. 

Limiting the effects of blast waves in flameproof enclosures using polyethylene-based ballistic materials

The latest high-strength synthetic materials, including polyethylene, have unique properties; the modulus of elasticity and tensile strength of polyethylene fibres are significantly higher and the elongation at fibre break is lower. Synthetic fibres are inherently resistant to chemicals, solvents and lubricants used in industry.

After the invention of the technology of production of fibres from ultra-high molecular weight polyethylene (UHMWPE), Honeywell (Allied Fibers) launched fibres that were 10 times stronger than steel, but lighter than water and had non-linear viscoelastic properties. Due to their chemical composition, UHMWPE fibres are virtually unaffected by a variety of chemical reagents [1]. UHMWPE fibres are used in ballistics for the production of armour, helmets, lightweight body armour.

Properties of the developed newest ballistic materials can be used in technologies of explosion protection of equipment to reduce the impact of shock wave and expand the temperature range of equipment use, to reduce the weight and dimensions of enclosures used for the type of explosion protection "flameproof enclosure".

Ballistic materials can be used to create a protective layer on the inner surface of the enclosure to help reduce the pressure created by the shock wave when an explosive mixture detonates inside the enclosure.

During operation, when the mixture explodes due to sparking, the walls of the flameproof enclosure are subjected to the pressure of the blast wave, which is evenly distributed in all directions, the pressure value depending on the geometrical parameters of the enclosure ranges from 5 to 20 atm (approximately 0.5-2 MPa). This pressure can lead to a breach of the integrity of the enclosure, covers, hatches and to the propagation of the explosion outside the flameproof enclosure. When applying the ballistic layer, part of the explosion energy is absorbed, thus significantly reducing the pressure inside the flameproof enclosure.

Depending on the various types of combustion processes (slow deflagration or fast turbulent flame or detonation), the pressure history may be different.

During deflagration, the peak pressure in a closed tank for most hydrocarbon mixtures is on the order of 0.8 MPa, for a hydrocarbon-oxygen mixture it is even 1.6 MPa. The hydrogen-air mixture, initially at NTP, will reach a pressure of 0.815 MPa; its volume will increase by 6.89 times [2]. Peak overpressures during detonation are usually in the range of 1.5-2 MPa [3].

The ballistic material can be fabrics, hybrid or mesh materials, plasticine, composites based on glass, carbon or polyethylene fibres that do not support combustion.

To test the protective properties of enclosures with an internal protective layer, this study used a high-density polyethylene film material made of flat film material with a welded layer of polyethylene with bubbles, which has ballistic properties. The layer of polyethylene with bubbles is a damping layer, on which outer layers made of aluminum foil were applied, which reflect the light and IR radiation of the explosion.

Literary sources do not reveal the use of ballistic material based on air bubble polyethylene film protected with aluminum foil to contain the shock wave, but there is data on polyethylene as a material from which light armor is made, which in its characteristics is not inferior to armor made from aramid fibers and PBO fibers [1].

Ballistic material in the form of air bubble film is created from high-density polyethylene. Bubble film is covered with bubbles that are filled with dry air. The technology for creating a bubble coating consists of welding a layer of polyethylene to a substrate (flat film), which is obtained by molding heated polyethylene into bubbles on an extrusion production line. This method ensures the strength of the connection between each bubble and the substrate, while the bubbles are located at a certain distance from each other. The structure of the film allows you to maintain the overall damping properties of the coating even if some bubbles are damaged.

The main objective of the tests below was to test the ability of a ballistic material based on air bubble polyethylene film protected by aluminum foil to reduce explosion pressure.

As part of the study, the enclosure was tested to determine the explosion pressure (reference pressure) in accordance with IEC 60079-1 [4].

The first series of tests was carried out at normal temperature, the second series of tests for the application of enclosures in conditions of negative temperatures down to -60°C.

For the tests were used flameproof enclosures of rectangular shape with flanged connection of the cover and the body of the enclosure made of aluminium-silicon alloy of two sizes, with the volume of 0.0054 m3 (5.4 l) and 0.0092 m3 (9.2 l) respectively.

For the first series of experiments, a mixture of hydrogen at a concentration of (31.0 ±1.0) % with air for equipment subgroup IIC and a mixture of ethylene at a concentration of (8.0 ±0.5) % with air for equipment subgroup IIB was used as the test gas-air mixture.

For the second series of experiments, a mixture of hydrogen at a concentration of (40.0 ±1.0) % with oxygen at a concentration of (20.0 ±1.0) % and nitrogen was used as the test gas-air mixture for the IIC equipment subgroup. This choice of test mixture was due to the fact that for hydrogen-air mixtures the minimum energy sufficient for ignition is 0.017 MJ [5], and for hydrogen-oxygen mixtures it is even lower - 0.0012 MJ [6], which simulates the situation of explosion pressure at subzero temperatures down to -60 °C. For equipment subgroup IIB, a test mixture containing ethylene at a concentration of (8.0 ±0.5) % with air.

The tests consisted of igniting an explosive mixture inside the enclosure and measuring the pressure caused by the explosion. The mixture was ignited by a spark discharge with a voltage of 24V and a current of 0.5A. The width of the pressurised slots was selected within tolerances, with a slot length of 12.5 mm and a maximum gap of 0.15 mm. The tests were carried out at normal atmospheric pressure (743 mmHg) and normal ambient temperature (21 °C).

The explosive mixture inside the enclosure was ignited by one or more high-voltage spark plugs or other low-energy ignition sources. The location of the spark plug or spark plugs and the pressure measuring instrument or instruments was determined by the testing organisation.

The maximum value established by pressure sensors located in several places of the enclosure was taken as the design explosion pressure. Samples from a typical batch of equipment were used for the tests. The inner volume of the enclosure was not filled with electrical equipment.

Several tests were scheduled for each equipment subgroup and enclosure size and the explosion pressure was measured and recorded during each test, the results of which can be found below:

Test Series No. 1

Enclosure size 1

The explosion pressure without ballistic material was 7.9 bar

Combustible gas hydrogen at a concentration of (31.0 ±1.0) %. Explosion pressure with ballistic material 3.61 bar, 3.3 bar, 3.35 bar.

Combustible gas Ethylene at a concentration of (8.0 ±0.5) %. Explosion pressure with ballistic material 3.33 bar, 3.31 bar, 3.28 bar.

Test Series No. 2

Enclosure size 1

The explosion pressure without ballistic material was 9.8 bar

The flammable gas was hydrogen at a concentration of (40.0 ±1.0) %. Explosion pressure with ballistic material was 6.6 bar, 6.8 bar, 6.85 bar.

Combustible gas Ethylene at a concentration of (8.0 ±0.5) %. It was decided to cancel the tests due to the fact that the explosive mixture with ethylene is quite linear in behaviour due to the low detonation burning rate.

Enclosure size 2

The explosion pressure without ballistic material was 10.2 bar

Combustible gas hydrogen at a concentration of (40.0 ±1.0) %. Explosion pressure with ballistic material 7.3 bar*.

* Further tests were discontinued due to the increase in pressure.

Combustible gas Ethylene at a concentration of (8.0 ±0.5) %. It was decided to cancel the tests due to the fact that the explosive mixture with ethylene is quite linear in behaviour due to the low detonation burning rate.

As can be seen from the results of the first series of tests, the use of ballistic material at normal ambient temperature reduces the explosion pressure by more than a factor of 2 (7.9 ÷ (3.61 + 3.3 + 3.35)=2.3).

According to the results of the second series of tests we can conclude that at negative temperature up to -60°C the pressure decreases only 1.5 times (9.8÷(6.6 + 6.8 + 6.85)=1.45). And at increase of the case size it starts to tend to 1 (10,2÷7,3=1,39).

In the presented research work, tests of flameproof enclosures using polyethylene-based ballistic material have been carried out.

Results:

When analysing the ability of material in the form of air-bubble film based on high-pressure polyethylene to reduce the explosion pressure inside the enclosure, it was found that the pressure is reduced by more than a factor of 2 at normal ambient temperature.

The use of various ballistic materials in explosion protection technologies is a very promising direction.

Coating the inner surface of the enclosure and lid with a ballistic material that reduces the pressure created by the shock wave can improve mass-dimensional performance by reducing the thickness of the shell and eliminating the use of reinforcing stiffeners and spacers.

The results of this research will be used to create a method of limiting the impact of the shock wave during the explosion of hydrogen-air medium inside flameproof enclosures "d" by creating a protective coating formed on the inner surface of the explosion-proof enclosure "d" made of promising ballistic materials. This method will make it possible to create a flameproof enclosure "d" with high reliability, improved mass and dimensional characteristics, as well as with reduced impact of an explosion on the equipment placed in it.

Reference list:

1. Bhatnagar A, editor. Lightweight ballistic composites: Military and law-enforccement applications. 1st ed.  Cambridge England: Woodhead Publishing Limited; 2006.

2. Baker WE, Cox PA, Westine PS, Kulesz JJ, Strehlow RA. Explosion Hazards and Evaluation. New York: Elsevier Scientific Publishing Company; 1983.

3. van Wingerden C.J.M.  Detonations in pipes and in the open. Bergen Norway: CMR internal report, Christian Michelsen Research; 1999.

4. IEC 60079-1 Equipment protection by flameproof enclosures «d».

5. ISO/TR15916 Basic considerations for the safety of hydrogen systems.

6. Kuchta JM. Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries - A Manual. Bulletin/U.S. Dept. of the Interior,  Bureau of Mines, 680; 1985.