GENERAL METHODS OF PROTECTION OF INSTRUMENTS
The general methods of protection of instruments shall include but are not limited to:
(1) Insulation of the instrument including tubing and piping to prevent heat loss or gain.
(2) Tracing the instrument including tubing and piping
• All tracing of instruments shall account for Manufacturer recommendations regarding maximum temperature of operation.
• Where tracing is to be used, care shall be taken to ensure that overheating of the process fluid does not occur.
(3) Purging or flushing the instrument including tubing and piping to prevent undesirable liquids, solids or gases from entering the instrument or impacting its performance.
(4) Isolating the instrument from the process fluid through the use of a diaphragm seal.
(5) Isolating the instrument including tubing and piping from the process fluid through the use of a sealing liquid, noting the following:
• Sealing liquids shall be immiscible/ compatible with the process fluid.
• Water may be used as a sealing liquid where the only purpose is to protect against the entry of the process fluid.
• An instrument may be sealed with a liquid having a low freezing point (e.g.: dibutyl phthalate at –35°C or 50%, ethylene
glycol in water at –36 °C).
• Instruments with a low (negligible) displacement may be sealed without the use of seal pots.
Instruments with appreciable displacement require seal pots with special piping arrangements for control of the seal liquid level in order to prevent hydrostatic errors.
• Instruments that have sealed leg installations should only have legs filled by qualified personnel. Warning labels should be attached to vent/drain connections.
(6) Isolating the instrument and piping from entrained fluids by use of vapor/liquid separators.
Special consideration shall be given to protection method(s) used, where hazardous/toxic fluids are handled. Protection philosophy in these cases shall be to contain the process fluid as close to the process (e.g. piping/vessels) as possible, even when the instrument system materials do not necessarily require protection.
(8) Design and installation of systems shall be such that pockets, crevices, etc. are minimised in order to prevent undesirable build up of hazardous/toxic material.
(9) In some cases, a suitable means of flushing or purging may be required (either continuous or intermittent).
(10) Enclosing the instrument in an insulated housing (where possible equipment shall be specified for the appropriate climatic conditions).
(11) Care shall be taken to ensure temperatures at the instrument to be kept within manufacturer’s tolerances.
(12) Where necessary, personnel protection shall be applied to instrument piping.
(13) Sunshades shall be provided where levels of solar radiation could cause excessive heat within an instrument and affect
its operation or accuracy. Other methods of protection may be used.
(14) The use of NACE compliant trims for corrosive services where applicable.
FLUID TYPES OF PROCESS USED IN INSTRUMENTATION
Fluid Types Description
1) Water and water solution with freezing point approx. 0 Deg C.
2) Liquids that are subject to gravity separation which may cause water to accumulate in impulse lines. (Note. Excluding steam out operations).
3) Corrosive liquids (Where corrosive to the wetted parts of instruments or instrument impulse lines).
4) Liquid whose pour points are above the minimum design (or lowest average) ambient temperature.
5) Liquids that may change in chemical composition due to a decrease in temperature from operating to minimum design temperature.
6) Liquids that may be subject to vaporisation at operating pressure and ambient temperature conditions.
7) Liquids which contain solids.
8) Steam
9) Corrosive vapours and gases
10) Air, vapours and gases containing solids
11) Wet gas
12) Dowtherm or other proprietary vapour heat transfer fluid
13) Liquid heat transfer fluid
14) No protection required. (i.e. inert gases, dry gases, non-corrosive fluids).The term dry shall mean free from condensable at theminimum ambient design temperature.
15) Process fluids that require minimum maintained temperatures (MMT) above –44 Deg C other than 4 above.
16) Light hydrocarbons (C2-C5) which will be in a vapour state at operating pressure and maximum ambient temperatures.
17) Light hydrocarbons (C2-C5) which will be in a liquid state at operating pressure and all ambient temperatures
18) Hydrocarbon vapors which can condense at operating pressure and any ambient temperature.
19) Pyrophoric substances e.g. DEAC/TEAL.
HAZARDOUS AND TOXIC SERVICE IDENTIFICATION
Identification Fluid
A Innocuous
B Wet sour
C Chloride
D L.P.G. or F.G.
E Auto-ignition
F Caustic
G Other corrosive services including acids
H Hydrogen
J Toxic
K Cryogenic
DEFINITIONS OF SERVICE
Innocuous Service (A)
Any substance, which when released to the atmosphere in small quantities, does not cause a hazard to health, e.g. general purpose fluids, air, N2, water, etc.
Wet Sour Service (B)
Process streams containing an aqueous phase with a partial pressure of H2S in the gas phase greater than 0.3 kPa absolute (0.05 psia). Wetted parts of instrument shall meet the requirements of NACE MR-0103.
Chloride Service (C)
Any substance which contains a chloride concentration greater than defined in the graph below:
LPG and FG Service (D)
Liquid hydrocarbon streams having an atmospheric boiling point below 20 deg C (typically containing butane and propane) which upon leakage to atmosphere will form a substantial vapor cloud.
Auto-Ignition Service (E)
Hydrocarbons during plant operation at or above their auto-ignition temperatures.
Caustic Service (F)
All sodium hydroxide at all temperature and concentrations wetted parts of instrument shall be of stainless steel.
Other Corrosive Services (G)
Any substance not defined above, where SCH.80 carbon steel or stainless steel pipe has insufficient corrosion allowance for instrument process piping. In these cases, instruments will be fitted with diaphragm seals made of suitable material.
Note: Instrument process pipes are invariably dead legs and hence operate at lower temperature and do not experience main pipe corrosion.
Hydrogen Service (H)
Process streams containing hydrogen with a partial pressure of 350 kPa-abs and higher. Excess flow shut-off values shall be provided for instrument connection e.g. pressure gauge.
Toxic Service (J)
Fluid containing more than 5% of a chemical with Health Category Rating 3 of NFPA 704 and CO/H2S/NH3>IDLH or toxic fluid specified by Licensors. Excess flow shut-off value shall be provided for instrument connection e.g. pressure gauge.
Cryogenic Service (K)
Cryogenic service shall be defined as min. design temperature of service below -46 deg. C. All pressure containing parts shall be of austenitic stainless steel. Valves shall be subject to cryogenic test.