prevention and control of ignition sources in the explosives industry

8/11/2019 Prevention and Control of Ignition Sources in the Explosives Industry

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Prevention and Control of Ignition Sources in the Explosives

Industry BOS (Basis of Safety)

Dr. Martin Held

Austin International, Inc.

25800 Science Park Drive

Cleveland, OH 44122

[email protected]

Prepared for Presentation atAmerican Institute of Chemical Engineers

2013 Spring Meeting

9th Global Congress on Process SafetySan Antonio, Texas

April 28 May 1, 2013

UNPUBLISHED

AIChE shall not be responsible for statements or opinions contained in papers or printed in its

publications.


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Prevention and Control of Ignition Sources in the Explosives

Industry BOS (Basis of Safety)

Dr. Martin Held

Austin International, Inc.

25800 Science Park Drive

Cleveland, OH 44122

[email protected]

Keywords: Control of Ignition Sources, Explosives, Basis of Safety

Abstract

Prevention and control of ignition sources is the key element in manufacturing and processing ofexplosives. The fundamental approach of BOS (Basis of Safety) and its application principles

will be described.

Examples will be given to demonstrate how safe operation principles based on BOS are

implemented in inherent plant design for manufacturing of explosives of varying sensitivity and

also describe trends in modern plant processes.

1. Introduction

As with other industries, the explosives manufacturing industry has gotten safety policies and

procedures, regular safety audits and tries and usually does comply with any available safety

standard. However, despite all this paperwork, time and attention the explosives industry

continues to carry a fair number of safety incidents, the vast majority of which could have been

anticipated. Conclusively, it can be said that for a whole variety of reasons there is in many cases

a lack of attention to basic explosive safety, or so called BOS (Basis of Safety) in the explosives

industry. In simple words, BOS is a set of simple established principles to do just two things:

1) prevent unintentional explosions (by preventing ignition) and

2) manage the consequences of a potential explosion.

The consequent application of BOS principles ensures that all (process) controls are appropriate

and in place.

With general principles on prevention and control of ignition sources from the chemical process

industry in place as applicable, this paper is putting a focus on principles for materials and


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substances with an immanent explosion hazard that does not require explosive atmospheric

conditions as reactive oxygen and fuel are chemically bound in molecules.

2. How does an unintended explosion occur?

2.1 Explosive definition

In the environment of explosives manufacturing an explosive or a substance with explosive

properties is defined as a substance/mixture that under the conditions in which it is presentcan

react violently to produce an explosive effect. These substances include (non-exhaustive)

-

Detonating explosives lead azide, lead styphnate, PETN (pentaerythrite tetranitrate),

nitroglycol, nitroglycerine, emulsion and watergel explosives

- Pyrotechnics delay composition, fuse head dips (in detonators)

- Reactive mixtures wastes, raw materials

-

Ammonium nitrate and ammonium nitrate emulsion matrix

Explosives are designed to rapidly liberate vast amounts of energy. This energy is released by a

shock wave which can travel up to 8,000 meters per second through the explosive material.

To avoid an explosion and with this the release of the stored energy, we need to understand what

is happening and required to lead to it.

Application of BOS principles is not a separate exercise, but does complement existing safety

management and QC systems.

2.2Sequence of events - BOS hierarchy

To characterize and be able to control ignition sources and with this an explosion hazard, we

need to understand what is happening. These are the conditions for an ignition or the sequence of

events.

2.2.1 Presence of explosive material

All explosives have the oxygen and the fuel necessary for a reaction contained either on a

molecular level or in a chemical mixture. An explosion once initiated will travel through all the

material present leaving the products of explosion and a destructive and expanding high pressure

wave or air blast.

Thus, looking at the sequence of events, this means, if there is no explosive material present then

there won't be an explosion. So, if these materials can be prevented at early stages in a


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production process (technology) or be kept to a minimum (inventory, batch size, reactor size) it

should be done. From a plant design perspective, this could be a change from a batch to a

continuous process.

2.2.2 Sources of ignition

In the sequence of events, adding any energy to explosive materials can potentially trigger an

initiation leading to ignition. Preventing of ignition is the best place to intervene to stop the chain

of events leading to an explosion. Ignition sources in the explosives industry can be summarized

as F.I.S.H. which are Friction (between surfaces), Impact (between impacting or colliding

surfaces, increase of pressure and temperature), Static (electrostatic discharge) and Heat (fire,

decomposition, adiabatic compression, radiant heat, prolonged friction) as illustrated in Figure 1.

Most explosive substances are well characterized regarding their sensitivity towards ignition

sources.

Figure 1: Sources of Ignition (Friction, Impact, Static, Heat)

However, they can behave differently under slightly changing process or operating conditions

and may become unpredictable. Therefore, the process design has to take a very conservative

approach.


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2.2.3 Decomposition

Once an ignition triggered by one of the above mentioned ignition sources has started, the

decomposition of an explosive material may be local and only consume a small quantity of

material.

But it could propagate and spread rapidly (this may be immediately for highly sensitive

explosives up to minutes for less sensitive material). Eventually, this may lead to deflagration or

burning or at worst case to an explosion.

The actual behavior of an explosive material after an ignition depends on several factors that

include the chemical properties of the material, the temperature and pressure, the confinement,

the amount of solvents, the shape (solid material), impurities etc.

Figure 2: Sequence of events leading to an explosion[1]

3. Hazards with explosives

Explosives are designed to explode when used in a specific application such as breaking rocks ina mining operation. Most of the explosives in todays world are very well characterized in their

physical and chemical properties, thermo-chemistry, stability, minimum ignition sources, and

compatibilities. However, it needs to be kept in mind that the properties have been determined

under exactly defined conditions that in general are not entirely transferable to manufacturing

process conditions.


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The actual processing environment from design of equipment, control systems integrity over

maintenance conditions, raw material quality towards people at any given time is not

characterized as quite as exactly and much smaller deviations (lower threshold levels) as

compared to the non-explosives chemical process industry can trigger a step change in system

entropy and make it impossible to regain containment and control (normal operation) at a very

early stage.

Figure 3: Step change in system entropy[2]

The explosives behavior under even small deviations may become unpredictable. Therefore, the

safety systems applied have to be conservative.

As a consequence, process design and safeguards as applied in a wide area of the chemical

process industry may not be sufficient for the explosives manufacturing industry. Consequences

of a loss event (explosion) are massive very often.

Shift


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Picture 1: Explosion aftermath

4. BOS principles

BOS is based on the principles of using lines of defences to block hazards from turning into

incidents.

Figure 4: Line of defences[3]

These lines could be:

- Learning from the past

-Inherent Design (e.g. remote operation, process equipment), plant layout (e.g. inventory,

separation of processes, safety barriers), Hazard Studies


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- Workplace controls

- Individual operator controls

- Organizational controls

BOS principles are intended to assist all personnel involved in designing explosives

manufacturing operations, working in these and handling explosives and sensitive (raw)

materials to understand:

- The hazards associated with those materials

- How hazardous conditions can arise under normal and abnormal situations/process conditions

- How to avoid those hazardous conditions arising (prevention of ignition sources)

- How to minimize the consequences in the event of an incident (not covered in this paper)

4.1 Principles for safe operation

The principles for safe explosives operation to warrant a manufacturing process without sources

of ignition can be summarized as follows:

- Slow

- Soft

-

Light- Low

- Gentle

- Cool

- Grounded

- In the correct place

- Under control

4.2Inherent design

4.2.1 Quantity/Distance (Q/D) approach

From a historical perspective, one of the first principles applied in the explosives manufacturing

industry is the Q/D approach, i.e. quantity and distance.


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The explosives industry had observed a shift to apply less stringent Q/D principles inside an

explosives operation after the introduction of safer industrial explosives such as watergels and

emulsions while keeping those principles to protect third parties and the public. However,

incidents in the past recent years have changed this trend to become more conservative and/or to

apply the approach of QRA (quantified risk assessment) by applying mathematical methods.

4.2.2 Remotely controlled operations

There are operational processes where despite of all measures taken rare events of explosions are

unavoidable depending on ambient conditions (e.g. detonator manufacturing). In this case,

processes have been designed to be operated remotely, i.e. the operator is protected by blast

walls that can withstand the effect of an explosion and, in addition, the amounts capable to

detonate are limited to one piece (detonator) or to a small amount of explosives.

Another reason for remote or robotic controlled operations is to avoid human failure in

mechanical operations. As an example, operating a drying oven with trays carrying pyrotechnicmixtures sensitive to impact and friction to be moved in and out under robotic control would

prevent an incident if an operator slipped and dropped a tray with the consequence of the mixture

to ignite when the tray hits the floor.

4.2.3 Type of process, conveying by gravity and desensitizing prior to transfer

Batch nitration processes in manufacturing of nitroglycol and nitroglycerin (NG), where

operators had to attend the nitration process and monitor temperature, have been replaced by

continuous nitration processes under remote control.

Figure 6: Batch nitrator for nitroglycerine[4]


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Another principle is to not apply mechanical energy to convey sensitive explosive material, but

to apply gravity. This today is always the case in the manufacturing of NG where flow induced

by gravity is used to convey the material and density effect are used to separate the product from

the reaction mixture. This is the reason why dynamite plants have always been placed in a region

with mountains or hills and the process following the slope of a valley.

Furthermore, in todays manufacturing processes NG is transferred as an emulsion mixed with

water which is much less sensitive to effects of friction and impact. The NG/water emulsion is

then separated for further processing of the NG. In addition, mixing processes for dynamite

down the line to cartridging the final product are more and more transferred to remote

operations.

4.2.4 Inventory control, confinement, pump selection, sealings and bearings, stage of process

where explosives are present, detonation traps

Emulsion explosives (water-in-oil type emulsion containing an oversaturated, highlyconcentrated (ammonium) nitrate solution dispersed in a fuel phase consisting of oil/paraffin and

an emulsifier)) are insensitive to static, friction and impact during normal process conditions

compared to classical explosives. Emulsion explosives are sensitive to heat (overheating, fire,

thermal effect from prolonged friction), especially under confinement.

The reduced sensitivity in manufacturing processes lead to the introduction of more standard

equipment as being used in the chemical process industry (pumps, mixers etc.), higher

inventories and different process steps taking place in one building. However, still having a

material with explosives properties in the process was sometimes neglected and the explosives

industry learnt this lesson after a number of severe incidents.

After these incidents until recently, much more attention has been paid towards plant design

(separation of processes) and the selection of equipment from the chemical process industry.

New emulsion plant designs are almost all continuous processes where the amount of hazardous

material present is kept to a minimum (e.g. size of hoppers, volume of emulsion mixers,

explosives limits in packaging areas) for inventory control. A similarity from the chemical

process industry would be minimizing the amount of hazardous process intermediates such as the

very toxic methyl isocyanate (MIC) in the production of carbamate pestizides.

Even though an ammonium nitrate based emulsion is a hazardous material (class 5.1, UN 3375),it is not an explosive until it is sensitized (this is done by introducing tiny gas bubbles either by

chemical reaction or with glass or plastic microspheres). The stage of sensitization can be taken

to the very end of material confinement in pipework during processing just before cartridging

(into plastic film) or filling into containers.


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Another option is to build in detonation traps (these could be reducing the pipe diameter into an

array of (non-metal) pipes of much smaller diameter below the so-called critical diameter for

detonation (a detonation would not be sustainable below this diameter and stop) or to convey

explosives cartridges on belts at high speed to generate a large distance between those cartridges)

to prevent the transmission of a potential detonation from the main process area to further

process steps. An analogy in the chemical process industry would be flame arrestors.

Very important is the selection and the control of pumps to convey emulsion and emulsion

explosives. Progressive cavity pumps (PC pumps) are very common because of an almost

pulsation free flow. However, in a dry running or dead heading (blocked flow in line) PC pump

heat induced to continuous input of energy by friction can lead to high temperatures with the

emulsion matrix starting to decompose with the potential of a pump explosion. There have been

a number of incidents in the past with unattended PC pumps continuing to operate and eventually

exploding. Depending on the application the safety devices for PC pumps comprise pressure and

temperature trips and alarms with automatic shutoff, rupture discs, no flow detection and timeout

devices (dead man handling).

Figure 7: Progressive cavity pump (operating principle)

For transfer only application where no accuracy of flow is required, diaphragm are in use very

often as the maximum pressure is limited to the air pressure applied to the pump and dry running

conditions with heat generated by friction as with a PC pump do not exist.

In todays world, packing glands for pump and mixer shaft sealing are almost entirely out of use

and have been replaced with lip seals. The contact surface between the rotating shaft and the seal

is under permanent friction. With a lip seal worn out, the emulsion matrix would leak to the

outside and the seal would be replaced. In case of packing glands that are often retightened

within maintenance intervals, the emulsion starts to migrate into the sealing material and to

crystallize between the shaft and the packing glands. The crystals start to groove the shaft and


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with the enhanced friction the temperature in that contact area starts to rise and eventually may

lead to a decomposition of the ammonium nitrate with the risk of fire and explosion.

In addition, it is good practice and very often standard to keep a certain distance between sealing

and bearing that explosive material cannot migrate into the bearing and mix with grease and be

exposed to friction. Also, the space between sealing and bearing should not be entirely

encapsuled to allow visual control for leaking material and to avoid it to accumulate and reach

the bearing.

Figure 8: Lip seal and packing glands

It is also common to limit the number of revolutions (rpm) when using a PC type pump and not

to operate the pump towards its limit to avoid the entry of too much energy into friction.

4.2.5 Other controls of ignition sources

Static mixers are common in continuous nitration processes and in mixing and sensitizing of

water-based explosives (emulsions and watergels) versus the application of motor driven mixers

that are transferring dynamic energy into the system.

When agitators and impellers are used in batch mixers or crystallizers, it has to be ensured that a

safe clearance between the moving equipment and the inner walls of the tank/kettle is warranted

to avoid metal/metal contact and with this friction and impact. Precautions have also to be taken

to prevent distortion and the agitator or parts of it to become loose and drop into the tank when

explosive material sensitive to friction and impact is present (primary explosives such as lead

azide (PbN3) are very sensitive to any kind of friction and impact).

Crystallized ammonium nitrate here can start

to decompose under enhanced friction and

rising temperature

Emulsion would visibly leak prior to considerable

crystallization due to reduced contact surface

between seal and shaft and less friction


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Ball valves are a source for friction when the valve is actuated and this can be can ignition source

when processing dissolved explosives. Pinch valves in this case are a safer alternative, but it has

to be taken care that explosive material cannot migrate into fissures aging over time and

becoming more sensitive than the original substance. Friction/impact effects when closing a

valve could then also trigger decomposition that propagates into an explosion.

Screws, nuts and bolts becoming loose with the potential to fall into openings of equipment

where explosive material is processed are a major concern when explosive material sensitive to

friction and impact is present. Therefore, drilling and lacing of screws and bolts, gluing or self-

securing nuts are very suitable in prevention of ignition sources and this can be taken care of at

the design phase (definition of zones for securing).

Explosives dust and sublimate accumulate in threads (e.g. sublimed TNT in cast booster melting)

over time even when good housekeeping is applied. When loosening a thread by unscrewing,

friction is applied to entrapped material and could start an ignition. Control devices plugged into

a lid rather than screwed in or caps on nuts after equipment has been fixed are good explosives

practice.

4.3 Safe operation principles

The use of shadow boards to identify and control permitted articles and lanyards to prevent tools

from falling into the process environment are very common in the explosives manufacturing

industry. The correct use of permitted tools only (e.g. spatula from soft material instead of metal,

brass vs stainless steel to prevent sparks, conductive shovels) is a main factor to prevent ignition

sources from manual operation.

Picture 2: Shadow board for control of permitted articles

Consequent sealing of wall joints and cracks prevents explosives material to become entrapped

and accumulate and also to migrate behind light wall constructions. This eases and prevents


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hidden material to be ignited especially during maintenance work and dismantling and when hot

work is applied.

Picture 3: Sealing of cracks and wall joints

Clean floor policies, wet mats in entry areas and soft floors prevent spilt explosives material and

dust to mix with grit and the risk of ignition by friction when personnel is walking over floors.

Soft bumpers on trolleys and around corners at room entries significantly reduce the effect from

impact (shock) when material is transported on walk-/driveways from one location to another.

Picture 4: Soft bumpers on trolleys and containers

5. References

[1] Begg A.H., Principles of BOS and GEP, Lecture, XVII SAFEX Congress 2011,

Istanbul/Turkey.

[2] Johnson R.W., The Anatomy of Process Safety Incidents, Unit 1, Lecture 2, AIChE

eLearning Course ELS 104 (accessed Aug. 23, 2012).

[3] Swiss Cheese Model by James Reason published in 2000 (Human error models and

management,British Medical Journal, 320, pp. 768 770).

[4] Picture from ICI.

prevention and control of ignition sources in the explosives industry

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