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Instrument Safety

  • Intrinsically Safe is the state of a device, instrument or component that will not produce any spark or thermal effects under any conditions that are normal or abnormal that will ignite a specified gas mixture. Electrical and thermal energy limits are at levels incapable of causing ignition. It is common practice to use external barriers with intrinsically safe installations
  • Ingress Protection (IP) is derived from the IEC 60529 classification standard defining the state of an enclosure. IP has 3 digits, though 2 digit notation is the most commonly seen. The first digit is about solid contact, either 0(total contact, anything can get through) to 6 (not even dust can get through). Usually, 6 is used for process plants instruments since we do not want any dust to enter the electronic boards. The second digit is for protection against water ingress valued from 0 to 8. Usually 6 is used. In our plant, most of the instruments are IP65

IP First number - Protection against solid objects

0 No protection.
1 Protected against solid objects up to 50mm, e.g. accidental touch by hands.
2 Protected against solid objects up to 12mm, e.g. fingers.
3 Protected against solid objects over 2.5mm (tools and wires).
4 Protected against solid objects over 1mm (tools, wire, and small wires).
5 Protected against dust limited ingress (no harmful deposit).
6 Totally protected against dust.

IP Second number - Protection against liquids

0 No protection.
1 Protection against vertically falling drops of water e.g. condensation.
2 Protection against direct sprays of water up to 15o from the vertical.
3 Protected against direct sprays of water up to 60o from the vertical.
4 Protection against water sprayed from all directions o limited ingress permitted.
5 Protected against low pressure jets of water from all directions o limited ingress.
6 Protected against low pressure jets of water, e.g. for use on ship decks - limited ingress permitted.
7 Protected against the effect of immersion

IP Third number - Protection against mechanical impacts (commonly omitted)

0 No protection.
1 Protects against impact of 0.225 joule (e.g. 150g weight falling from 15cm height).
2 Protected against impact of 0.375 joule (e.g. 250g weight falling from 15cm height).
3 Protected against impact of 0.5 joule (e.g. 250g weight falling from 20cm height).
4 Protected against impact of 2.0 joule (e.g. 500g weight falling from 40cm height).
5 Protected
  • Fuel can ignite when the concentration of combustibles in air is between LEL(Lower Explosive Limit) and UEL(Upper Explosive Limit). This is because a correct Air to Fuel Ratio is required for a combustion.
Substance LEL UEL
Acetone 3% 13%
Acetylene 2.5% 82%
Benzene 1.2% 7.8%
Butane 1.8% 8.4%
Ethanol 3% 19%
Ethylbenzene 1.0% 7.1%
Ethylene 2.7% 36%
Diethyl ether 1.9% 36%
Diesel fuel 0.6% 7.5%
Gasoline 1.4% 7.6%
Hexane 1.1% 7.5%
Heptane 1.05% 6.7%
Hydrogen 4% 75%
Hydrogen sulfide 4.3% 46%
Kerosene 0.6% 4.9%
Methane 4.4% 17%
Octane 1% 7%
Pentane 1.5% 7.8%
Propane 2.1% 9.5%
Propylene 2.0% 11.1%
Styrene 1.1% 6.1%
Toluene 1.2% 7.1%
Xylene 1.0% 7.0%
  • An LEL meter detects the concentration of all combustible gases and translates it into percentage of an LEL limit. Typically, LEL meters are calibrated to Methane(CH4). Methane LEL limit is used as a reference for all other gasses. Most plants will set %LEL limit at 10% for hot works. This means that since methane LEL is 4.4%, if the gas detected is methane, the LEL limit will be 0.44%. From the view of an LEL meter, it will give 10%. 10% LEL is used as a safe level so that whatever the gas is, it is always safe to work since the LEL is still far away.



Equipment Protection

  • Area in a plant is usually divided into

· Classes or Zone

· Zone is used for Europe

· Zone 0 = Flammable material always present

· Zone 1 = Intermittent available and for long hours (>10 hours)

· Zone 2 = Not always available and for short durations only (<10 hours)

· Class is used in US

· Rosemount transmitters use

· Gas group

· Refers to a group of gas which has similar propertie as the group gas. The main property is the ignition energy

· Group I = Methane

· Group IIA = Propane

· Group IIB = Ethylene

· Group IIC = Hydrogen

· Temperature Classes

· Each gas has an auto ignition temperature

· An area can be classified from T1 and T6. T1 has the highest auto ignition temperature hence it is safest.

· An equipment can be certified as T1 to T6. This is the worst case temperature the equipment can get hot

  • Typically, for zone 1 it shall be 3m away from zone 0,
  • Ex is a certification chop for explosive protection equipment, i.e. equipments that protect explosion from happening such as JB and instrument enclosures. It is stated on the requirements from ATEX directive. In Europe, every Ex marked equipment shall have a CE mark
  • CE is also a mark like Ex. It was introduced by the European commission. With the CE sign, the product complies to the safety standards to be sold in Europe.
  • Protection types

· Ex ‘d’ = Flame proof – Not designed to be gas tight, energy is released through flame path, Large design to prevent explosion.

· Ex ‘o’ = oil immersion – used for transformers, Zone 1 and 2

· Ex ‘p’ = Pressurized – used for analyzers, Zone 1 and 2 – Equipment needs to be powered off if the enclosure is opened. This is why purging is ‘’not usually recommended

· Ex ‘q’ = Powder field – weighing machines, Zone 1 and 2

· Ex ‘ma’ = encapsulation – Zone 1 and 2

· Ex ‘d’ = Flame proof , explosion is confined within box– DC motors, Zone 1 and 2

· Ex ‘e’ = Increased safety, it is not possible for explosion to happen. The box is designed in such a way to prevent

· Connections oversized

· Terminations more robust

– Induction motors, instrument JB, Zone 1 and 2

· Ex ‘ia’ = instrinsic safety – if 1 component fails, IS can still retain - Zone 0,1,2

· Ex ‘ib’ = intrinsically safe, Zone 1 and 2 only

  • For zone 0


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