Safety Aspects of Instruments

Instrument failure can also cause accidents. Therefore while designing and manufacturing it is necessary to consider safety features of instruments also. For examples, ordinary glass or plastic parts may not withstand high pressure or temperature; corrosive metal parts are not suitable for corrosive chemicals coating with stainless steel is required for material like lead is preferred to make film badges for shielding against ionising radiation, electrical parts must conform to Indian standard for better performance and special alloys are required for special purpose. F.P Lees has listed following features:

o    Instrument may fail due to a functional fault or loss of containment from the instrument.

o    Slight glasses or rotometers, made of glass can break and leak, if leak are dangerous glass should be avoided.

o    Material of construction should be proper. After considering the properties of the material to be handled and working parameters, material to be handled and working parameter, material for instrument should be selected.

o    Pressure transmitters and regulators can be damaged by overpressure. If they break, their pressure should be safely released.

·         Protection against corrosive fluid is necessary. E.g. use of inert liquid in impulse lines on pressure transmitters or of chemical diaphragm seals on pressure gauges.

·         Sampling and impulse line should be checked for blockage. Steam tracing is useful against freezing (e.g. Phthalic anhydride).

·         A thermo well to hold thermometer should be carefully designed to take into account corrosion/erosion effect.

·         Pulsating flow in flow meters can cause inaccuracies. Design of orifice plate should consider this factor.

·         Complex instruments like analyser, speed controllers, vibration monitors and solids weighers are generally less reliable. Consequence of their failure should be carefully analysed.

·         Out of different of pressure regulators (e.g. PRV, NRV, EFV, SV, RD), the right one should be selected at right place. No bypass should be provided between pressure regulators.

·         A control valve should be selected properly. Nominal capacity, range ability, control characteristics, flow limit when fully open, cooling fins for temperature protection, bellows seals to prevent leaks and mechanical balancing (capacity) against process pressure are the factors to be considered for selection.

·         Instruments should not become a source of ignition. Electrical area classification should be considered. They should be flameproof as per requirement.