1 Scope ……………………………………………………………………………………………………………………2 References…………………………………………………………………………………………………………….3 General ………………………………………………………………………………………………………………..4 Instrument Cases and Enclosures …………………………………………………………………………….5 Seals (Isolation of Process from Instruments) …………………………………………………………….6 Steam Tracing ……………………………………………………………………………………………………….7 Electrical Tracing ……………………………………………………………………………………………………8 Prefabricated Traced Bundles ………………………………………………………………………………….9 Packaged Instrument Heating System ………………………………………………………………………10 Instrument Air Supply System ………………………………………………………………………………….11 Transmittal of Design Information ……………………………………………………………………………..12 Revision History ……………………………………………………………………………………………………..FIGURE 1 – Single Stud, Steam Heat, Insulated Enclosure (O’brien System) (Body Cavity
Temperature Vs. Steam Pressure for Selected Ambient Temperatures and Wind Velocities) …..FIGURE 2 – Instrument Enclosures …………………………………………………………………………………..FIGURE 3 – Steam Tracing and Heating ………………………………………………………………………….FIGURE 4 – Steam Tracing Supply Diagram ……………………………………………………………………FIGURE 5 – Tracing of Level Transmitters and Controllers Displacement Type ……………………FIGURE 6 – Tracing of Typical Differential-pressure Transmitter ………………………………………..FIGURE 7 – Tracing of Typical Pressure Transmitters ……………………………………………………….FIGURE 8 – Tracing of Transmitting Rotameter Metal Tube ……………………………………………….FIGURE 9 – Tracing Of Indicating Rotameter …………………………………………………………………..FIGURE 10 – Tracing of Horizontal Control Valve ……………………………………………………………..FIGURE 11 – Tracing of Typical Indicating Pressure Gauge Installations …………………………….FIGURE 12 – Hollow Stud, Steam Heating (O’brien System Shown) …………………………………..FIGURE 13 – Hollow Stud, Electrical Heating (O’brien System Shown) ……………………………….FIGURE 14 – Orifice Tap Winterization Using Prefabricated Bundle With Self-regulating HeatingCable, General Purpose Area ………………………………………………………………………………………..FIGURE 15 – Orifice Tap Winterization Using Field-installed And Insulated Self-regulating
Heating Cable, General Purpose Area …………………………………………………………………………….FIGURE 16 – Orifice Tap Winterization Using Prefabricated Bundle With Steam Heating and
Pre-insulated Single Tube Assembly ………………………………………………………………………………FIGURE 17 – Orifice Tap Winterization Using Field-installed And Insulated Steam Tracing ……
1 Scope
This standard describes the methods and materials used to prevent instrument systems from
freezing, and to assure reliable performance under adverse weather conditions or when
handling high freeze point materials.
2 References
Reference is made in this standard to the following documents. The latest issues,
amendments, and supplements to these documents shall apply unless otherwise indicated.
SABIC Engineering Standards (SES)
R01-E02
Hazardous Area Instrumentation Protection -Explosion Proof
National Fire Protection Association (NFPA)
NEC (NFPA 70) National Electrical Code (NEC)
3 General
3.1 All installation requirements to assure continuous operation must be specified to the
installer and be included in the job definition. Insufficient protection causes
maintenance to be performed under adverse conditions. Overprotection increases
cost and should be avoided.
3.2 Freeze protection cautions
a.
Instrumentation which should never be traced. For instruments in hazardous
service areas where the surface temperature of the heating device could
ignite the process, SES-R01-E02 requirements shall be complied.
b. Precautions for instrument heating: Diaphragm seals should be traced on the
process side and insulated on both the process and the seal fluid sides, it is
also necessary to consider the effect on process, or seal fill fluids where
uncontrolled temperatures will cause degradation. The effects of enclosures
should be carefully thought through. A box which contains the complete
instrument will heat the entire instrument to one temperature. This box should
have some form of temperature control to protect the electronic or pneumatic
transmitter portion of the instrument to a maximum of 150 deg F. Some
controls or operating procedures are required to ensure that freeze protection
heat is shut off in warm weather. Partial enclosures which include only the
wetted parts of the instrument will leave the top-works unheated. In
applications where high-freeze point process temperatures are required the
partial enclosures may provide an advantage; the heat for these is usually left
on year-round. Analyzers are an example of the sort of instrument where
careful design is required for temperature protection.
3.3 Freeze protection may be described as:
a. Winterization. Protection to prevent failure of instrument systems due to
temperatures of 32 deg F (0 deg C) and below.
b.
Heating high freezing point materials. Protection to prevent a solidification of
process material brought to an instrument through a process lead line
(impulse line) at temperatures above 32 deg F (0 deg C) or specific design
minimum ambient.
3.4 The methods for winterizing instruments vary with the minimum temperatures to be
expected. The amount of heating needed to keep high freeze point materials from
freezing in impulse lines and instruments varies with the material handled. Lines
containing low viscosity and non-freezing fluids, with pour points below the minimum
ambient temperatures encountered, need not be freeze protected. Lines containing
liquids that freeze or carry traces of moisture must be protected by steam, electrical
tracing, or other form of heating, and must be insulated.
3.5 For critical applications, an alarm system should be considered which will indicate the
failure of either the steam or electricity used for heating.
3.6 All water due to steaming or wash-out of piping and equipment during start-up or
shutdown periods should be removed completely from instruments and process
impulse (lead) lines before instruments are placed in operation.
4 Instrument Cases and Enclosures
4.1 An instrument case is the covering which protects the internal mechanism.
Some cases with hinged doors are not weatherproof. If an instrument with a hinged
door is to be located outdoors, an enclosure may be required. Instruments mounted
outdoors may require a weatherproof enclosure.
4.2
Instrument enclosures are housings covering all or part of an instrument case and are
used for weather and freeze protection. The features to be considered when selecting
enclosures are:
a. Working space and access. The space inside the enclosure and around the
instrument should be adequate for routine maintenance and for removal of
the instrument. Properly sized and positioned access doors are necessary
and observation windows (safety glass) may be required.
b. Piping and services. Entry is preferred through the sides or the bottom of the
enclosure (never through the top) and should be located to reduce piping.
c. Mounting. Enclosures may be self-supporting, wall-mounted, or attached to
an instrument support. Instruments may be flush-mounted, located behind
windows in the door, or mounted on the side.
d.
Insulation. The inside of an enclosure may be lined with foil faced fiberglass
or other suitable insulating materials. Polyurethane enclosures are also
available and provide insulation requirements
e. Weather protection. Enclosures should be weather protected with line entries
sealed. Metal enclosures should be galvanized or painted. The hardware,
assembly bolts, and screws should be corrosion resistant.
f.
Type of heating. Enclosures may be heated with steam (tracing or finned
radiator), or with an electrical heater or other available heating media.
5
Seals (Isolation of Process from Instruments)
5.1
Diaphragm seals are protective attachments for pressure sensing instruments. A thin
flexible diaphragm separates the process material from the instrument element. The
volume between the diaphragm and the instrument element is filled with a liquid. The
liquid should not:
a.
Contaminate or react with the process material,
b.
Decompose at the highest process temperature, or
c.
Freeze or become viscous at the lowest ambient temperature. Also, it should
have a low coefficient of cubical expansion. Fill liquids commonly used
include silicone fluids and mineral oils which are suitable for operating
temperatures ranging from about -40 to 600 deg F (-40 to 315 deg C).
5.2
Diaphragm-sealed systems are sensitive to ambient temperature changes. A change
in ambient temperature of about 40 to 50 deg F (22 to 28 deg C) can cause a shift in
calibration of 1/2 to 1 percent.
6 Steam Tracing
6.1 Steam is used for raising or maintaining the temperature of material in instrument
systems. Steam heating is effective and normally trouble free; however, it can cause
an instrument error or erratic operation if overheating, boiling, or uneven heating
result from improper application. Calibration shifts may occur if instruments are
calibrated prior to application of heat.
6.2 Tracing is the tubing carrying heating or cooling fluid, and is attached to instrument
tubing, piping, and other instrument equipment. Copper tubing tracer systems are
limited to temperatures of 400 deg F (205 deg C) maximum. Aluminum tubing and
wire are not recommended. Where external corrosive conditions or temperature
requires a stainless steel tracer, the same type stainless steel is used for straps and
wire to prevent galvanic corrosion.
6.3 Steam branches are taken from the top of the header to assure that dry steam will be
supplied. Each branch must have an individual shutoff valve; and to eliminate
bypassing and recycling within a tracer system, each tracer shall be individually
trapped. Select supply for tracer systems so that steam is available during process
shutdowns. Tracer systems should be installed for gravity drain to traps. Condensate
lines from traps should be installed horizontally or, if possible, with a downward slope
and with no pockets. Tracers shut down in freezing weather must be blown free of
condensate (see Figure 4 for typical supply diagram).
6.4 When the process fluid in the instrument line is temperature sensitive or has a boiling
point lower than the steam temperature, the tracer should be separated from the line
by using a spacer or insulation to prevent hot spots in the heated line. This may be
accomplished through the use of self-regulating preassembled cables such as
Chemelex TracePac or Unitherm 2256 electrical-traced assemblies or Unitherm 2202
steam traced assembly (see 8.1). Where expediency requires it, similar results can
be achieved by laying a double thickness of glass tape between the heated line and
the tracer [SN900M, Code 992, listing tape (glass cloth) 0.060 inch thick].
6.5 For maximum heat transfer, the tracer is held in direct contact with the instrument line
or equipment. Steam pressure should be selected for desired heating; however,
consideration must be given to boiling or process degradation.
6.6 Typical installations are shown on Figures 3, and 5 through 11.
Maintenance joints are installed in tracers for the removal of instruments without
distorting or cutting the tracer. Insulated tubing should be used across the gap on
instrument removal to keep the tracing in service when necessary.
6.7 Maximum tracer length
Steam condenses along the length of the tracer and creates a temperature profile.
After all the steam has condensed, only hot water will remain in the tracer line,
therefore, the length of the tracer used with various steam pressures is limited.
6.8 Traps
Steam traps are automatic valves that release condensate, air, and gases, and
prevent the loss of steam. Traps should be located below and close to the end of the
tracer and be accessible for inspection and maintenance. Each tracer shall be
provided with a separate trap.
6.9 Strainers
Steam traps are sensitive to dirt. Each trap must be protected by a strainer which
should be line size.
6.10 Insulation
Traced instrument lines operating at temperatures of 400 deg F (205 deg C) or less
may be insulated with fiberglass listing tape 2 inches wide by 1/8 inch thick. Insulate
assembly by wrapping longitudinally with two layers of listing tape, overlapping the
tape 1 inch, and securing with corrosion-resistant wire at 4-foot intervals, or as
required. Bond the overlap with sodium silicate.
6.10.1 Exposed short runs of pipe and tubing may be insulated with flexible braided
fiberglass tubing where required for personnel protection.
6.10.2 When the tracer or impulse lines are austenitic (300 series) stainless steel,
the inner layer of listing tape shall be treated to provide protection against
stress corrosion cracking.
6.10.3 Cover the outer layer of listing tape with PVA mastic for weather and spill
protection.
6.10.4 Syphons, flanges, etc, are to be wrapped completely with fiberglass tape to
enclose tracer and instrument piping in the same enclosure, even though the
tracer may not completely follow the syphon or full flange surface. Do not
insulate condensate pots.
6.10.5 When special conditions require other types of insulation, the type and
thickness will be specified.
7
Electrical Tracing
7.1 Electrical power can provide higher temperatures, better control, and sometimes is
available more readily than steam for tracing applications. Heat is produced by an
electric current flowing through resistance wires. The installation must comply with
the electrical classification of the area. Emergency backup power should be available
to protect the installation in case of power failure.
7.2 One thermostat measuring air temperature may be used to control several heating
circuits, but, when close temperature control is required, individual controllers are
used for each circuit.
7.3 Precautions must be taken during the installation of ordinary heating cables to avoid
mechanical damage. The installation must be checked for continuity, shorts or
grounds before energizing. Type MI (mineral insulated, metal-sheathed) cable is
recommended where mechanical protection is required.
7.4 When the process fluid in the instrument line is temperature sensitive or has a boiling
point lower than the steam temperature, the tracer should be separated from the line
by using a spacer or insulation to prevent hot spots in the heated line. This may be
accomplished through the use of self-regulating preassembled cables such as
Chemelex TracePac or Unitherm 2256 electrical-traced assemblies or Unitherm 2202
steam traced assembly (see 8.1) or equivalents. Where expediency requires it,
similar results can be achieved by laying a double thickness of glass tape between
the heated line and the tracer [SN900M, Code 992, listing tape (glass cloth) 0.060
inch thick].
7.5 Physical disconnects are required in tracers to permit removal of instruments without
distorting or cutting the wire.
7.6 Caution: All electrically traced instrument lines shall be marked or properly tagged in
conspicuous locations to warn mechanics against the danger of sawing into
energized cables when removing the thermal insulation. A 2-inch wide by 60-yard
long adhesive yellow tape with black-printed “Danger Electrical Tracing” is suitable.
7.7 Electrical installations shall meet the requirements of the NEC.
8
Prefabricated Traced Bundles
8.1 Prefabricated installations produce the lowest installed cost for runs greater than 10
feet. Prefabricated traced bundles using either steam or electrical heating are
available.
8.2 Bundle ends must be protected from moisture and fumes to prevent chloride stress
corrosion (see 6.11.3 and vendor instructions).
9 Packaged Instrument Heating System
9.1 One or more instrument body bolts of differential-pressure transmitters may be
removed and replaced with hollow studs for heating with steam or electricity. This
system is more efficient than tracing with tubing (see Figures 12 and 13).
9.2 One stud with a minimum of insulation is adequate for most installations. Insulation is
not required where moderate temperatures are experienced.
9.3 The temperatures to which electronic instruments are subjected must not exceed
manufacturers’ limitations. Special component parts and special insulation for
internals may be required for higher temperatures. Before heating, obtain
temperature limitations from manufacturer.
10 Instrument Air Supply System
The instrument air supply system does not require winterizing downstream of an air dryer
provided the dew point is reduced to a level which will prevent freezing.
11 Transmittal of Design Information
11.1 The following information should be provided:
a.
Identity of instruments and impulse lines requiring tracing
b. Seals, static fill medium
c. Source of heat, electricity or steam.
d.
Hollow studs when needed
e. Size and type of enclosure
f.
Packaged systems when needed
g. Special considerations
Steam tracer piping diagram:
a.
Header identification
b.
Reducing station
c. Valves, sizes and code
d.
Tracer size, material, and maximum length
e.
Insulation type, thickness, weatherproofing
f.
Traps, size and type
g.
Condensate disposal
h. Steam supply pressure.
Electrical tracing diagram:
a. Electric circuit identification
b.
Connection, series or parallel
c.
Tracer size, type, and length
d.
Current setting
e.
Thermostat location and setting
f.
Insulation specification
FIGURE 1 – Single Stud, Steam Heat, Insulated Enclosure (O’brien
System) (Body Cavity Temperature Vs. Steam Pressure for Selected
Ambient Temperatures and Wind Velocities)
FIGURE 2 – Instrument Enclosures
FIGURE 3 – Steam Tracing and Heating
FIGURE 4 – Steam Tracing Supply Diagram
FIGURE 5 – Tracing of Level Transmitters and Controllers Displacement
Type
FIGURE 6 – Tracing of Typical Differential-pressure Transmitter
FIGURE 7 – Tracing of Typical Pressure Transmitters
FIGURE 8 – Tracing of Transmitting Rotameter Metal Tube
FIGURE 9 – Tracing Of Indicating Rotameter
FIGURE 10 – Tracing of Horizontal Control Valve
FIGURE 11 – Tracing of Typical Indicating Pressure Gauge Installations
(Typical for Flanged or Screwed Valves)
FIGURE 12 – Hollow Stud, Steam Heating (O’brien System Shown)
FIGURE 13 – Hollow Stud, Electrical Heating (O’brien System Shown)
FIGURE 14 – Orifice Tap Winterization Using Prefabricated Bundle With
Self-regulating Heating Cable, General Purpose Area
FIGURE 15 – Orifice Tap Winterization Using Field-installed And
Insulated Self-regulating Heating Cable, General Purpose Area
FIGURE 16 – Orifice Tap Winterization Using Prefabricated Bundle With
Steam Heating and Pre-insulated Single Tube Assembly
FIGURE 17 – Orifice Tap Winterization Using Field-installed And
Insulated Steam Tracing
