EXPLOSION HAZARDS AND PROTECTION IN THE USE OF INTERMEDIATE BULK CONTAINERS (part 1)
Many flammable dusts are stored and handled using IBC, but they are weak containers and if an explosion occurs without adequate explosion protection, they may rupture due to the high explosion pressure. The consequence may be external fires and injury to personnel. Some explosions that have occurred during the filling/emptying of FIBC have been attributed to static electricity and so called “anti-static” FIBC have been developed. However, they do not provide protection against other sources of ignition e.g. burning powder from upstream units. IBC are normally closely linked to other process equipment e.g. mills, blenders, dryers, in which combustion of bulk powder may be initiated. This may increase the possibility of burning material initiating a dust explosion in the IBC. Explosions have also been attributed to ignitions outside the dust containment system. In a recent incident, an ignition started outside an FIBC, at a filter associated with a pneumatic conveying system. An adjacent aluminium hopper was fed from above by the FIBC. The fire at the filter spread to the aluminium hopper, which failed, and then spread to the FIBC, melted the fabric and caused the contents to spill into the fire. A dust cloud formed in the hopper which then resulted in an explosion. he increase in container volume can markedly change the characteristics of the dust cloud in the container. Where relatively small drums are used, filling times are relatively short, the powder concentration within the filling stream is normally above the Upper Explosion Limit, and little, if any, separation of “fines” occurs in the ullage space. All these factors mitigate against the initiation of a propagating explosion. However, in IBC, the filling stream forms a smaller part of the volume and a dispersed cloud can be formed around it. The dust cloud characteristics will depend on the physical form of the powder and the filling procedure but in many cases there will be an increased tendency for a dust cloud within the explosion limits to be formed. Furthermore, if the filling procedure is such that the “fines” can separate from the coarser material, then a dust cloud requiring a lower energy for ignition may be formed.
This paper considers the use of IBC and the explosion prevention and protection techniques that are currently used. This paper does not consider the storage or handling of liquids, however any potentially explosive vapour or gas external to the IBC does require consideration.
There are a number of standards that are relevant to intermediate bulk containers including:
- BS EN 1898:2001 Specification for flexible intermediate bulk containers (FIBC) for non-dangerous goods. This standard superseded the British Standard BS6382: 1983 Part 1 Recommendations for FIBC for non-dangerous goods in January 2001.
- The UN Recommendations on the transport of dangerous goods has replaced British Standard 6939 (7 parts). There is also a draft European standard, ISO/DRAFT 16467:2000, Transport packages for dangerous goods — test methods for IBC, which has similar requirements to the UN Recommendations.
- Electrostatic safety of flexible intermediate bulk containers is assessed in accordance with BS 5958 Part 1 & 2 1991, Control of undesirable static electricity. The CENELEC report PD R044-001, based on BS 5958 and other documents, has been updated and issued as PD CLC/TR 50404:2003. Reference is made to powder storage in containers including type A, B, C and D FIBC; these are discussed later.
- Draft European Standard IEC 61340-4-4 Electrostatic safety of flexible intermediate bulk containers (FIBC) — Test method and requirements. The draft standard gives the test requirements for electrostatically safe FIBC that are intended for use in the presence of flammable material or in a hazardous atmosphere. A new draft standard IEC 61340-4-6 is also applicable.
DESIGN AND OPERATION
RIGID INTERMEDIATE BULK CONTAINERS
Rigid intermediate bulk containers are often referred to as IBC. Because of the robust construction of rigid IBC, when compared with FIBC, they are classed as multi-trip containers. Manufacturers offer a wide range of rigid intermediate bulk containers each having different design features to suit a given application.
With capacities in the order of 300 to 3000 litres they are generally four sided with a coned shaped bottom to aid discharge. Other shapes include cylindrical containers, which use less material in their construction, are easier to clean and can better withstand internal pressure.
Most commonly, the materials of construction are carbon steel, stainless steel and aluminium. Grounding is relatively easy and earthing straps are often fitted as a matter of course. Rigid plastic IBC are manufactured with a rigid plastic body which is designed to be in contact with the contents (either directly or through an inner liner or lining). Polyethylene containers are normally mounted in a mild steel or stainless steel frame. However, some polyethylene containers are moulded such that a steel frame is not required. Carbon impregnated black polyethylene containers to dissipate charge buildup are manufactured as a conductive IBC and are classed as anti-static.
Other less common materials of construction are fibreboard, wood and composite. Some rigid IBC are UN certified for the transportation of certain hazardous goods. For the transport of dangerous goods by sea, rail and road throughout Europe, IBC are generally only used when they are permitted for specific contents and when the IBC design has been officially tested and bears the appropriate marking.
Rigid IBC filling
Filling of metal or rigid plastic IBC is generally via a central access hatch. Automated filling can be achieved using a containment transfer system (CTS). The operator does not have to remove or replace the lid nor operate a valve. The CTS typically consists of a flanged containment unit that connects to the feed silo, a lift table to locate and lift the IBC and a control panel. With the IBC placed under the CTS unit and the controls initiated, the IBC is lifted and docks with the CTS, the CTS opens the IBC lid and filling takes place. When filling is completed, the lid is automatically replaced and the IBC is separated from the CTS.
Rigid IBC discharging
Conventional valves such as butterfly, slide, gate, and so on, create an opening at the silo outlet, bin or IBC but the discharge of some products may be difficult. Many powders may not flow consistently and may bridge over even very large outlets. It may be impossible to close the valve in some cases. One method of overcoming this problem is to install a specialized IBC cone discharge system. The product within the IBC is sealed by the cone to ensure a dust-tight seal. To discharge the product, the IBC is placed on a purpose designed IBC discharge station. Once located on the station, the IBC outlet engages a lip seal at the discharge station to create a dust-tight seal. When the open signal is received, a probe rises within the discharge station and locates into the cone valve. The probe expands to seal and lock tightly inside the cone and then lifts the cone valve into the IBC. The probe/cone then pulses and/or vibrates to discharge the product.
FLEXIBLE INTERMEDIATE BULK CONTAINERS
FIBC offer many advantages. They are lightweight, weighing a small percentage of the weight of the contents and are much less expensive than rigid IBC. It is easier to discharge difficult products and the containers are hygienic since plastic liners protect the product. They are low maintenance since replaceable liners eliminate cleaning costs. The main disadvantage when compared to a rigid IBC is that they are susceptible to damage, for example from the fork of a forklift truck and they therefore have a more limited life span.
The capacites of FIBC are generally no greater than 3 m3 and are designed to be lifted from above by integral or detachable devices. An FIBC, designed and intended for one filling and one discharge only, is designated as a “single-trip FIBC”. An FIBC intended to be used for more than one filling and more than one discharge is designated as a “multi-trip FIBC”.
FIBC are manufactured largely from UV stabilized woven polypropylene with most commonly one, two or four lifting points. Sufficient suspension points should be attached to the top corners or the rim to give stability. Traditionally, the more usual shape is approximately cubic, fitted with a filling chute and an emptying cone. The containers are usually stacked on pallets or lifted by four loop straps at each corner. The FIBC is stitched with man-made fibre or twine and some heavy duty FIBC have welded seams. The fabric can also be woven as a circular tube.
FIBC can be coated with thin film to keep the product from leaking out of the weave, whilst some FIBC have disposable polythene or polyethylene liners that are used to prevent product seepage through the fabric and to improve watertightness. Electrically conductive liners are also available for use in antistatic bags. Where liners are used in multi-trip FIBC, the need for cleaning is avoided and the life of the FIBC is extended.
The National Engineering Laboratory (NEL) provides an independent testing and evaluation of FIBC. However, many of the major FIBC manufactures carry out their own testing. In common with rigid IBC, some FIBC are UN certified for the transport of certain hazardous goods. For FIBC that are intended to contain non-dangerous solid materials in powder, granular or paste form, and designed to be lifted from above by integral or detachable devices, BS EN 1898:2001 specifies the materials, construction and design requirements, type test, certification and marking requirements.
Electrostatic safety of flexible intermediate bulk containers is assessed in accordance with BS 5958 Part 1 & 2 1991, Control of undesirable static electricity. The CENELEC report PD CLC/TR 50404:2003 is also used and this is based on BS 5958 and other documents. A new document is currently in draft form, IEC document 61340- 4-4 IEC:2001, and deals exclusively with the electrostatic safety of FIBC. This draft document describes procedures for evaluating the electrostatic safety of all types of FIBC that are intended for use in flammable or explosive environments with ignition energies of more than 0.15 mJ. The procedure uses a re-circulating FIBC filling rig, an ignition probe, a gas control system, mixing apparatus and calibration apparatus. The FIBC is suspended in the recirculation rig initially with the top filling spout open and the emptying spout closed. Polypropylene pellets (nominally 3 mm) are loaded into the recirculation system and begin to fill the FIBC until it is approximately half full, at which point the bottom spout is partially opened to maintain the fill level so that the out-flow matches the in-flow. The pellets become charged by triboelectric action but if necessary, additional charge may be injected by the incorporation of high voltage corona points inside the filling chute. A series of ignition tests is carried out by bringing the ignition probe up to the side of the charged FIBC with a calibrated flammable propane gas mixture flowing through the probe having gas mixture volume concentrations of approximately 4% propane, 33.6% oxygen and 62.4% nitrogen. The mixture has an ignition energy of 0.15 mJ. The sequence of ignition attempts is made at various points on each of the four sides of the FIBC, making a total of 200 ignition attempts. No ignition should occur in any attempt.
Heavy duty FIBC
These are expensive and constitute a small portion of the market. They are made from PVC coated polyester cloth and usually have complex metal lifting devices fixed to, or detached from, the top of the bag. They have the most demanding test specification which includes an 8:1 safety factor. They are intended for multiple use and are repairable.
Standard duty FIBC
Standard duty or multi trip FIBC are usually made from woven polypropylene cloth and may contain a polyethylene liner, to protect the contents and prevent leakage. They have a safety factor of 6:1 and the most usual load is 1000 kg. A discharge spout at the base has ties to seal the bag and can be used to close a partially discharged bag or to regulate the rate of discharge and they are retied before the bag is refilled. The life of this bag is between 15 and 20 trips and is dependent on the nature of the contents and on the handling and transport methods used.
Single trip FIBC
The single trip FIBC is intended for only one filling operation. They usually have a flat bottom and discharge takes place by cutting open either the bottom to allow full discharge or by cutting a T-shaped slit at the side and controlling the discharge by inserting a board vertically downwards. They have a safety factor of 5:1.
FIBC bag filling and discharging methods
Filling and discharging is generally done through spouted top and bottom openings, where the bottom spout requires only unfolding/untying to begin gravity unloading. Some FIBC require a cut to be made in the fabric. However, industries such as the pharmaceutical industry require sophisticated filling and discharge systems where hygiene and containment are of paramount importance.
The main objective during filling is to ensure that the entire product enters the bag and does not spill. In the most basic filling system, the FIBC is located on a pallet or platform beneath the filling spout with a support frame supporting the lifting straps. The filling spout is connected to a filling point and the product is fed into the bag. This could be via a wide range of devices including: gravity feed, a rotary valve or screw feeder. However, filling methods can be more sophisticated with features designed to enable filling to take place more safely, cleanly and efficiently. Minimizing dust leakage is an important feature. Inflatable seals are often used to connect the filling spout of the FIBC to the filling head to ensure a dust tight connection. The seal is generally a rubber tube which encircles the spout and is lightly inflated suffi- ciently to seal the bag or liner. For bags with liners, inflation of the bag is necessary before filling. Inflating the FIBC using air is commonly used to help square the bag and eliminate any creases. In some cases, inflating the collapsed bag with nitrogen is done for mitigation of the explosion risk by inerting, and for product quality reasons.
FIBC are generally loaded onto a discharge station by hoist or forklift truck. Typically, a discharger will comprise an open topped, four sided hopper. One side of the hopper incorporates an access door that allows the operator access to the discharge spout to untie the tying-off strings. When the tying-off strings of the discharge spout are released without proper control, and depending upon the product characteristics, the product could either discharge rapidly or could fail to discharge at all. A correctly designed discharger should be able to cope with the extremes. This is achieved by use of design elements such as product flow control systems, clamps and seals to prevent dust leakage. A flow choke can be used to hold back the product whilst the ties are released. This seals the liner above the ties using sliding clamp plates, or wire rope, which can be released in a controlled manner after the strings have been released. Flow can be stopped and the bag retied when only part of a bag needs to be discharged. When a bag is discharged, the inner liner can become an operational problem. A liner tensioner is used to wind-up the liner which otherwise would slacken and elongate as it empties. Bag tensioners are used to gradually raise and tension the bag as its weight decreases during discharge. Flat-bottomed single trip FIBC are emptied by cutting the bag. A simple and efficient method is to use a discharge hopper incorporating knife-edge. As the bag is lowered onto the hopper the bag is split and also makes contact with a rubber membrane, creating a seal to prevent dust leakage. FIBC can be vacuum unloaded. The hose is inserted into the product via the top of the FIBC and a vacuum pump extracts the material. The hose is antistatic or conductive with grounding of metal connectors.
to be continued...
Source: P Holbrow Health and Safety Laboratory, Harpur Hill, Buxton, SK17 9JN SYMPOSIUM SERIES No. 150