STORAGE, HANDLING AND PROCESSING OF DANGEROUS SUBSTANCES - ELEMENT C4

C4.1 - Industrial chemical processes Unlike other branches of engineering (civil, mechanical, electrical, aerospace), which are concerned with mainly applied physics, chemical engineering is unique in integrating chemistry systematically into industrial chemical production.

Most chemicals produced in industrial processes do not reach consumers, but are used as intermediates in manufacturing processes, such as bleaching agents in the textile and paper industries. Industrial processes fall into two main classes: inorganic and organic. Examples of inorganic include heavy chemicals such as acid or alkalis, which are consumed by industry in vast quantities. Organic includes fine chemicals such as dyes, pharmaceuticals and polymers, which are made from the raw material of hydrocarbons found in crude oil. Most plastics, resins, synthetic fibres, ammonia, methanol, and organic chemicals are manufactured from oil or natural gas. They are called petrochemicals, and there are hundreds of thousands of substances produced worldwide.

Factors affecting rate of chemical reaction A chemical reaction is a process that leads to the transformation of one set of chemical substances (reactants) to another (products) with different properties to the reactants. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, requiring some form of energy such as heat, light or electricity. Chemical reactions involve the movement of electrons in the breaking and forming of new chemical bonds; the rate of reaction can be affected by temperature, pressure and substances called catalysts.

EFFECT OF TEMPERATURE The rate of reaction of any chemical process will increase with increases in the temperature of the reactants. The rate of reaction is exponential with temperature and increases by a factor of at least two for every 10oC rise. The reaction rate will also increase with increasing concentration of the reactant(s).

For chemical reactions that are exothermic (generate heat energy) caution has to be taken to keep the reaction rate (which is exponential) within control, because the removal of heat, for example, by the use of cooling water, acts at a much slower transfer rate (linear process). Therefore it is essential to maintain the reaction temperature within a narrow band to prevent uncontrollable (run-away) conditions occurring.

Consider a common organic polymerisation manufacturing process, involving the production of urea formaldehyde adhesive resins for the manufacture of chip-board.

The process is relatively simple. Urea is mixed with 36% formaldehyde at a controlled pH and held for approximately 4 hours at 60oC. Polymerisation occurs and the viscosity of the material increases to a pre-determined value, which represents the increased molecular chain length required. If the temperature is not controlled carefully and is allowed to rise above 70oC, the exothermic reaction rate will become uncontrollable, resulting in high temperature, solidification of the reactants and release of volatile toxic materials, i.e. formaldehyde (WEL 2 ppm).

Reaction temperature control is a very important issue when considering scale-up processes from laboratory sized experiments (5 kg), increased through pilot plant (1,000 kg), to final production batch quantities (20,000 kg). The temperature control, particularly cooling, is far easier with small quantities of reactants than with increasing larger quantities, because the ratio of cooling surface decreases with increasing volume of reactants. See section: “Methods of control of temperature and pressure”.

EFFECT OF PRESSURE The most significant use of pressure is the production of compressed gases, the most common being compressed oxygen, liquid nitrogen, and carbon dioxide produced from the fractional distillation of compressed air.

Figure C4-1: Heat production/volume. Source: RMS.

Other gases are produced either directly or indirectly from chemical processes and for convenience are stored under pressure. Common gases are chlorine, liquefied petroleum gas (propane), sulphur dioxide, hydrogen, ammonia and acetylene.

Some organic processes require reaction at relatively high pressure to prevent degradation of the products. One such reaction is the hydrogenation of castor oil in the manufacture of margarine. The reaction process is designed to be continuous and the reactants are combined at pressures of 200 atmospheres, which allows the products to be produced at much lower temperatures than at atmospheric pressure, below temperatures that would cause degradation.

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ELEMENT C6 - WORK EQUIPMENT (WORKPLACE MACHINERY)

222 © RMS

Barriers

Where it is not practicable to use enclosing guards, barriers may be used to prevent people reaching the danger zone or point. These rely on a combination of height and distance to achieve their purpose. A guide figure of 1.8 metres height is suggested for perimeter fencing. With the body upright and standing at full height, the safety distance when reaching upward is determined to be 2,500 mm.

Barriers are not foolproof and they cannot prevent access to person’s intent on gaining access. Therefore, as a person’s intent on reaching a dangerous part increases, for example by climbing on chairs, ladders or the barrier itself, the protection provided by a barrier will decrease.

When reaching down over an edge, the safety distance is dependent on:

Distance of danger point from floor. Height of edge of barrier. Horizontal distance from the edge of the barrier to the danger point.

Figure C6-39: Interlocking guard for positive clutch power press.

Source: BS PD 5304.

INTERLOCKED GUARDS An interlocking guard is a guard, similar to a fixed guard, but which has a movable (usually hinged) part so connected to the machine controls that if the movable part is in the open position, the dangerous moving parts at the work point cannot operate. This can be arranged so that the act of closing the guard activates the working part (to speed up work) for example the front panel of a photocopier.

Functions of an interlock An interlock provides the connection between a guard and the control or power system of the machinery to which the guard is fitted.

The interlock and the guard with which it operates should be designed, installed and adjusted so that:

Until the guard is closed the interlock prevents the machinery from operating by interrupting the power medium.

Either the guard remains locked closed until the risk of injury from the hazard has passed, or opening the guard causes the hazard to be eliminated before access is possible.

Interlocking media

The four media most commonly encountered in interlocking are electrical, mechanical, hydraulic and pneumatic. Electrical interlocking, particularly in control systems, is the most common and electrical components are often incorporated in hydraulic and pneumatic circuitry, for example solenoid operated valves.

The principles of interlocking apply equally to all media. Each has advantages and disadvantages, and the choice of interlocking medium will depend on the type of machinery and the method of actuation of its dangerous parts.

Some interlocking systems have more than one control channel, for example dual control systems. It is often advantageous to design these systems so that the similar failures in both channels from the same cause (common cause failures) are minimised. One way of achieving this is by using a different control medium for each channel, for example one hydraulic and one electrical.

Figure C6-40: Schematic representation of power and control interlocking.

Source: Ambiguous.

Interlocking methods

Methods of interlocking that ensure the power medium is interrupted when a guard is open fall into two groups:

1) Power interlocking. 2) Control interlocking - illustrated by the following schematic diagrams (note: not actual circuit diagrams).

ELEMENT C7 - WORK EQUIPMENT (MOBILE, LIFTING AND ACCESS)

Figure C7-3: Rough-terrain lift trucks. Source: RMS.

Figure C7-4: Rough-terrain lift trucks. Source: RMS.

Telescopic materials handlers These lift trucks are often referred to as multi-tool carriers. This versatility makes them particularly useful for agricultural, maintenance and construction work where lift trucks would be unsuitable.

When fitted with forks they can be used as a front-end materials loader and when fitted with a jib they can be used for crane duties. Instead of a vertical mast to raise or lower loads, this type of lift truck is fitted with a telescopic boom. This allows both vertical and forward movement of the forks, enabling the effects of uneven ground to be compensated for and the placement of loads in difficult positions that other lift trucks could not reach.

Figure C7-5: Telescopic handler. Source: Ambiguous.

Figure C7-6: Side loading trucks. Source: Ambiguous.

Side loading trucks This type of lift truck is used for staking and moving long loads, such as timber or pipes.

The operator of a side loading truck is positioned at the front and to one side of the lift truck. Side loading lift trucks have a mast and load carriage that moves sideways from the operator’s position to pick up or deposit a load.

This enables the centre of gravity of the load to be within the wheelbase of the truck during travelling. During travelling, the load is usually resting on the truck structure.

Pedestrian controlled trucks Pedestrian controlled trucks usually have a more restricted lifting height compared with other trucks, some may only have just enough to lift a pallet from the floor and enable it to be moved a relatively short distance, they are used extensively in chemical process operations.

Figure C7-7: Pedestrian controlled trucks.

Source: HSE, HSG6 - Safety in working with lift trucks.

252 © RMS

ELEMENT C8 - ELECTRICAL SAFETY AND EWR 1989

Control measures SELECTION AND SUITABILITY OF EQUIPMENT The British Standard BS 7671:2008, “Requirements for Electrical Installations”/Institution of Engineering and Technology IEE 17th edition requirements specifies the need for good workmanship and that proper materials shall be used. Construction, installation, inspection, testing and maintenance shall be such as to prevent danger. Equipment must be suitable for the power demanded and the conditions in which it is installed. Additions and alterations to installations should comply with these regulations.

The Low Voltage Electrical Equipment (Safety) Regulations (LVEESR) 1989, that relate to the supply of equipment, refer to equipment using between 50 and 1,000 volts AC and require it to be safe. Construction, including flexible cables and cords, must be to EU accepted good engineering practice standards. The EWR 1989 are deemed satisfied if the equipment bears a recognised standard mark, certificate or other acceptable authorisation. Supply of unsafe equipment or components is prohibited.

PROTECTIVE SYSTEMS Fuses This is a device designed to automatically cut off the power supply to a circuit, within a given time, when the current flow in that circuit exceeds a given value. In effect, it is a weak link in the circuit that melts when heat is created by too high a current passing through the thin wire in the fuse case. When this happens the circuit is broken and no more current flows. They tend to have a rating in the order of amperes rather than milliamperes, which means they have limited usefulness in protecting people from electric shock. They may also act slowly if the current is just above the fuse rating. Using the wrong fuse causes many electrical problems.

The following formula should be used to calculate the correct rating for a fuse:

(Volts) Voltage

(Watts) Power(Amperes) Current

For example, the correct fuse current rating for a 2-kilowatt kettle on a 230-volt supply would be:

A8.7230

2,000(Amperes) Rating Current Fuse

Fuses are available for appliances as 2, 5, 7, 10 and 13 Ampere ratings. The nearest fuse just above this current level is 10 A.

Typical examples of power ratings are:

Computer processor 350 Watts. Electric kettle 1,850 - 2 200 Watts. Dishwasher 1,380 Watts. Refrigerator 90 Watts.

In summary a fuse is:

A weak link in the circuit that melts slowly when heat is created by a fault condition. However, usually too slowly to protect people.

Easy to replace with wrong rating. Sometimes needs tools to replace. Easy to override.

Figure C8-24: Plug-foil fuse, no earth. Source: RMS. Figure C8-25: Earthing. Source: RMS.

Reduced voltage systems One of the best ways to reduce the risk from electricity is to reduce the voltage. This is achieved by the use of a transformer (step down), which will reduce the voltage. A common reduction of voltage from 230V is to 110V.

296 © RMS