School of Physics Safety Manual

7. MECHANICAL SAFETY

Lifting Equipment

There is a statutory requirement for the periodic testing of lifting equipment. The School Safety Officer must be informed of all such equipment brought into the School. No mechanical lifting is to be undertaken by anyone other than those members of staff specifically qualified to do so, who are responsible for conducting the operation safely.

Mechanical Hazards

Personal Protective Equipment (PPE). Suitable PPE must be worn at all times where risk assessment dictates. Goggles must be worn for any operation which is hazardous or potentially hazardous to the eyes. Goggles are recommended particularly for use when working with bromine or volatile bromine compounds, strong ammonia, any irritating dust, acids and alkalis. They should also be used when making fusions or cutting sodium, breaking up solidified melts, chipping any hard materials, opening cylinders containing gas under pressure, when using grinding apparatus, and when carrying out any operation with a danger of liquid splashing.

Centrifuges. Centrifuges can be very easily damaged and are a source of danger when improperly used. At 5,000 rpm, the periphery of a 10 cm radius rotor is travelling at over 110 mph: never try to stop a centrifuge with the hand. The load on a centrifuge should always be balanced accurately before switching on to avoid putting undue stress on the bearings. Centrifuges should be kept scrupulously clean as corrosion can seriously weaken the rotor. Only centrifuges specifically designed for low temperature work should be kept in cold rooms.

Vacuum Systems. Although much vacuum apparatus is constructed of glass, it is worthwhile remembering that an increasing range of equipment made of plastic is now available, including vacuum desiccators. Unsuitable glassware, e.g. conical flasks other than thick-walled Buchner flasks should never be even partially evacuated. Considerable damage can be caused by the collapse of large glass vessels. They should be exhausted behind safety screens. Do not use a large vessel if a small one will do.

When vacuum apparatus is assembled. The equipment should always be examined for stresses and strains, both before and after filling, (a kilogram of reagents, e.g. 75 mls mercury, can introduce severe strains in glass apparatus). Any glass vessel that is seriously scratched should be discarded. Sudden changes of temperature should be avoided and the vessel should be placed on a rubber mat, or even surface, to avoid any irregular pressure on the base.

Metal or plastic tubing should be used wherever possible, and flexible items, e.g. bellows couplings, should be included in the apparatus. Where ground glass unions are used, ball and socket joints are to be preferred to cone and socket joints.

As far as possible, vacuum apparatus should be screened. Equipment should be prominently labelled when left under vacuum. The risk of implosion with wide bore tubing, bulbs and items up to about 1 litre capacity can be reduced by strapping with cloth adhesive tape, cellophane tape, or varnished cloth mesh. Larger items should be encased in a stout gauge wire screen. Where such precautions are impracticable the entire apparatus should be placed behind Perspex or stout wire screens. It is a wise precaution to wear safety goggles when operating glass vacuum apparatus.

It is important to ensure that rubber bungs are large enough to resist being sucked into a vacuum vessel. Stopcocks should always be properly lubricated, operated slowly and never forced.

There is serious risk of implosion (and consequently a flying glass hazard) with certain commercially available equipment. Dewars, for example, the large vessels present as considerable hazard. With these implosion is most likely as a result of careless handling. Thermal shock is produced while filling the vessel with liquefied gas, and allowing it to evaporate before filling. All Dewar vessels should be in a protective case or at least made shatter-proof by plastic adhesive tape stuck on the outside. Vacuum tubes of all kinds should be handled with care. Because of their size the hazard is particularly great with cathode ray tubes, and special care should be taken not to scratch any part of the bulb, especially the face.

Vacuum desiccators and bell jars are frequently treated with contempt. These should always be screened and never moved whilst under vacuum. They should always be labelled if left evacuated.

High Pressure Systems (Including Autoclaves)

Most of the safety precautions required for vacuum work apply to high pressure apparatus but the following points also apply.

Pressure vessels should be examined by a competent person at least once every period of 26 months or at such intervals as the competent person dictates. The safe working pressure of each vessel should be marked on it and any pressure relieving safety devices that are fitted should be so designed as to ensure that in the event of equipment failure leading to a rapid rise in temperature within the pressure vessel under test, the safe working pressure will not be exceeded. If it is not possible to achieve this standard of safety protection then the whole apparatus must be encased within an enclosure of such strength as will be able to resist the effect of explosion of the pressure vessel.

Safety valves and other methods of pressure release should be sited so as to minimise the chance of injury to people or damage to equipment if they should operate.

Great care should be taken to use only materials and equipment designed to withstand high pressures, e.g. seamless tubing etc. Regular inspections should be made to discover blocked filters, leaking valves, weld crack etc. If glass apparatus is to be pressurised it should be screened and safety goggles worn by the operator.

At very high pressures, serious accidents have occurred due to unexpected chemical reactions taking place and producing much higher pressure than those for which the apparatus was designed.

Some commercially available light sources, e.g. the xenon arc lamps used in spectrofluorometers are filled under very high pressures. Such lamps should never be handled without safety goggles.

Workshop hazards

The principal hazards in workshops include injuries caused by moving parts, failure of equipment, incorrect or careless use of hand tools, faulty or damaged tools, etc. Many accidents in this category can be avoided merely by the correct use of equipment that is well maintained and inspected. In general, the wearing of rings, necklaces and bracelets is discouraged and for some work, their removal will be required.

All machines, whether hand or power operated should be fitted with appropriate guards or other safety devices. These should always be used. Protective clothing, appropriate to the work in hand, should always be worn and long hair should be tied back and protected by suitable head gear.

Only qualified technicians or staff/students acting under the supervision of suitably qualified technicians as authorised by the Workshops Supervisor may use the workshops facilities.

Welding

Welding should always be carried out in well ventilated booths since toxic gases can be generated at the high temperatures involved. Special care is necessary with painted or plated metals. Appropriate eye protection should be worn by anyone working in or visiting a welding booth while work is in progress. Only technicians trained and qualified in welding procedures and associated safety precautions are authorized to operate the School’s welding equipment.