This design guide provides the basic philosophy and an engineering methodology for developing and implementing Heat Tracing and Winterization design in Oil and Gas projects.
This guide covers the protection of equipment, piping and instruments from the effects of low ambient temperature. Failure to protect against low ambient temperatures may cause operational upsets and interruption of production.
This document presents all measures to be taken to prevent the effects of ambient condition causing loss of production.
Winterizing includes such procedures as:
- Protective external heating by electric or thermic fluid;
- Application of insulation;
- Elimination of dormant sections of piping;
- Maintaining a partial flow in dormant sections of piping;
- Draining and flushing;
- Providing suitable housing (enclosure), generally accompanied by heating from internal heating element or by a heating/ventilation system;
- Antifreeze solutions such as hydrate formation inhibitor injection, and pour point depressant;
- Hot Air Recirculation (for Air Cooler only);
- Use of Wind Wall to reduce rate of heat loss;
- Equipment and Structural design to allow for wind and snow loads.
If project specific criteria are available in FEED documents or Licensor package, the same should be followed in design.
This guide is applicable to process design requirements during the Conceptual Design, FEED, and detailed design phase of a Project.
Safeguards for personnel protection are not within the scope of this document.
Winterization is required for piping components, instruments and equipment to prevent interruptions of plant operations due to the effects of varying ambient temperature conditions and prevention of personnel to the combined effect of wind, low ambient temperatures, hot and cold surfaces, falling frozen debris which may be hazardous.
All components shall be designed for the minimum ambient temperature as a minimum, and their integrity shall not be compromised by loss of winterization.
Safeguards for personnel protection, not within scope of this document, should be considered as appropriate.
It is always the intent, wherever possible and economically justified, to design and select equipment and materials suitable for the site and environmental conditions, in order to minimize or eliminate the need for investment in winterization.
Consideration should be given to all stages of installation, commissioning and start-up, operation and shutdown periods, as well as to winterization of storage facilities and items that will be subject to long-term storage such as spare parts.
The winterization methods are generally based on experience and engineering judgment.
When considering the selection of the appropriate form of winterization for the equipment and systems, the following factors form the basis of selection:
- Environmental considerations
- Availability of electric power or heating medium.
- Economics, including:
- Cost of energy (electric power/steam/thermic fluid system)
- Capital cost and installation costs
- Maintenance Costs
- Required accuracy of temperature control and operating temperature range
- Reliability and ease of control
Often Client specifies the winter design temperature to be used for air coolers and winterization conditions. Where this is not specified by the Client, these parameters should be defined by analyzing climatic conditions for the particular geographic area.
3.1 Application of Winterization
Winterization may be required where:
- The fluid has a pour point above the minimum ambient temperature;
- The fluid is water contaminated with sub-zero minimum ambient temperature.
- The fluid can freeze or form hydrates including chemicals which congeal/solidify;
- The fluid contains a solution where the solute could precipitate at an undesirable location;
- The fluid contains a vapour which upon condensation forms a corrosive liquid or may cause operational problems. This occurs specifically when CO2, SO2 or SO3 are present with water vapour in the gas stream;
- The fluid has normal operating temperature below the dew point of the surrounding air and condensation of water vapour may form on the external surface of pipes and equipment. Dew point varies with relative humidity & atmospheric condition;
- Crude oil and heavy condensate will require heat tracing of equipment, piping and instrumentation when pour point is higher than minimum ambient temperature. The extent of tracing requirement can be minimized if the system is equipped with draining and flushing facilities and the same is expected to be positively functional at the time of shutdown;
- Crude oil and heavy condensate will require heat tracing of equipment, piping and instrumentation, where wax deposition temperature is higher than minimum ambient temperature;
- There could be fluid stagnation in pipe segments for long time causing drop in fluid temperature leading to any of the above mentioned situations.
3.2 Winterization Temperature
Tracing temperature for winterization is the minimum maintained temperature and depends on the physical properties of the fluid, such as pour point, freezing point, cloud point and maximum tolerable viscosity, whichever gives the higher temperature. In general:
- 5°C will be used as winterization temperature for aqueous systems;
- For process fluids where wax or hydrates might form, 7-10°C higher than the hydrate formation temperature or wax appearance temperature;
- Glycol Systems can be winterized for the cases highlighted in section 5.4.1;
- Minimum ambient temperature to be used for heat loss calculation.
Some of the process and utility systems that are influenced by extreme low ambient temperature conditions are outlined below:
Liquid Typical Freezing Point (oC) Water 0 Rich MEG – 74.2 wt% Does not crystallize Lean MEG – 80 wt% -47 Lean MEG – 90 wt% -31 Lean MEG – 100 wt% -13 Diethanolamine 28 TriEthylene Glycol -5 Diesel Oil -18 (Pour Point) Arctic Grade Diesel -40 (Pour Point) Aviation Fuel <-30
The approach to apply winterization to piping and equipment is as follows:
- Heat Tracing;
- Heated flow bypass/ Recirculation;
- Heated enclosures;
- Insulated enclosures (unheated);
- Heat retention;
- Maintenance draining;
- Addition of anti-freeze solutions;
- Burying the pipe.
The type of winterization is selected based on operating range, accuracy of temperature control, characteristics of process fluid, ease of maintenance, availability of heating system and economic evaluation of available alternatives. Wherever practical, winterization methods which don’t require protective heating should be used.
Methods of heat tracing, heat enclosure and heat flow bypass are described in this section.
Winterization techniques should be indicated on P&IDs.
3.3.1 Heat Tracing
The requirement of heat tracing for all equipment, piping, instruments, etc. is to be determined based on process fluid properties.
Tracing may be employed where isolated items of piping and equipment require protection.
It should be used as a method for temperature conservation and not as a method for raising equipment temperature.
The heat input for tracing may be from electrical tapes or a heated fluid (thermic fluid).
Heat tracing by these methods should be accompanied by heat conservation insulation.
Electric trace heating systems are the most commonly used and provide more accurate temperature control than thermic fluid systems.
Certain types cannot be used in all classified areas and therefore careful selection should ensure that the system adopted can be certified for all area classifications specified for both onshore and offshore systems.
Electrical tracing within the process and utility units is normally supplied from the area heat tracing distribution boards.
Heat pads may be used to protect the low sections of vessels containing water which may freeze.
In case required as per project philosophies/specifications, winterization of pressure relief valves’ inlet flange and tail pipes, emergency shutdown valves, blowdown valves, firefighting systems, etc using electric tracing should be duplicated and supplied from area trace heating emergency distribution boards. Electric tracing on piping and instruments in Safety Critical service should consider the application of surface temperature monitoring, in addition to ‘smart’ tracing, to ensure the correct functioning of the tracing.
Heating medium (thermic fluid) may be economical where plant heat tracing requirement is large. The number of tracer runs installed on a piping is limited by the pipe diameter, which is provided as per the applicable engineering standards. Where heat loss is large and is beyond the capacity of tube tracers, it will be necessary to use jackets on the piping.
Use of heating medium is normally limited to applications that permit relatively wide temperature variations. From an operational and maintenance point of view, heating medium is not the preferred method because it requires high maintenance to keep it operational and is dangerous when it leaks.
Consideration should be given to the prevention of corrosion under insulation.
Internal heating coils can distribute heat effectively over the area of large storage tanks. In comparison, electric heaters only provide heat at the periphery and often require the use of agitators.
Heating medium should not be used for piping to safety showers and should not be employed when accurate temperature control is required.
When heating medium is applied, the lines should have an appropriate design temperature for pipe stress calculation. This is to prevent pipes experiencing excess stress when the operating temperature of heat traced pipes, under no flow condition, exceeds the normal flowing conditions.
For offshore production facilities, steam tracing is not preferred, mainly due to the following reasons:
(a) it would entail a dedicated utilities system making the plant more complex for plot area and weight,
(b) Insufficient water available, and
(c) Safety and operability problems.
Steam tracing is therefore not considered for winterization of offshore facilities.
3.3.2 Heated Flow Bypass/ Recirculation
Maintaining a heated flow at all times through bypass lines, if practical, may be used to avoid heat tracing, e.g. maintaining stream flow in lines to isolated equipment by provision of a bypass local to the equipment, flow controlled using manual globe in bypass line.
Similarly, long headers which may at times have no flow may be provided with a bypass line with manual globe valve to maintain a minimum flow and avoid the requirement for tracing. Back flow through off-line equipment may also be used, e.g. back flow through standby pump by means of bypass around discharge check valve. One globe valve and a Restriction Orifice (RO) should be provided in the bypass to restrict the flow.
When a heated flow bypass is specified, heat conservation insulation must be specified. If the heated flow cannot be maintained for any operational reason, e.g. process unit shutdown, the isolated equipment must be drained to prevent freezing. Heated flow bypasses should not be used where the plant safety is critical.
3.3.3 Heated Enclosures
Only instrumentation and small items of equipment which require periodic attention during the normal course of operations, e.g. regular calibration, should be provided with a heated enclosure, if not already located within a building. A means of space heating should be provided to maintain the inside air temperature. Similarly cooling may be required in the summer period. Insulation should be provided to the enclosure walls and roof to mitigate heat loss in the winter months and heat gain in the summer months. The temperature within the enclosures should be maintained at a temperature suitable for equipment operation and maintenance.
3.4 Application and extent of winterization
In general, winterization should be provided to:
- Critical services, such as saturated fuel gas lines and associated valves and instruments, in exposed locations;
- Excessive cooling and icing up due to auto-refrigeration by Joule-Thompson effect coupled with low temperature at start-up;
- Wet sections of fire water;
- Dead legs.
In case required as per project philosophies/specifications, winterization for safety critical instrumentation and equipment should have power and control duplicated with back-up power from the Emergency Generator.
3.4.1 Process Piping Containing Liquid
Winterization in certain situations may be applied to piping in continuous service where there is an operational risk of freezing or condensation due to long lines and gradual temperature loss without heat tracing. Heat tracing should be provided for all stagnant lines and instruments in water service where subzero ambient temperatures are encountered.
No heat tracing is required for glycol systems in general as there is no concern of freezing of glycol solution. Viscosity increase due to situations causing stagnation such as start-up should be addressed by maintaining appropriate flow rates.
However, heat tracing should be considered for following cases:
- Stagnant TEG lines, such as liquid PSV lines, instruments impulse lines etc.
- TEG drain lines 3” and below.
3.4.2 Process Gas Lines
Primary protection against condensation in gas lines should be by piping design that eliminates pockets where water may collect and freeze or form hydrates.
Gas lines where hydrate or condensate formation may give rise to operational problems should be insulated. In specific cases they may also be heat traced to maintain the temperature above the dew point temperature of the gas and prevent corrosion problems.
In particular, compressor suction lines between the knockout drum and the compressor should be heat traced and insulated where ambient temperature can be below the dew point of the gas at compressor suction.
Winterization by heat tracing should be provided for low points/pockets in wet gas lines where ambient cooling may cause condensed water accumulation during a shutdown.
3.4.3 Pressure Relieving Systems
The inlet lines of PSVs and Flare PCVs are expected to flow back to the source vessel / line as these lines are free draining towards the source. Though this prevents liquid accumulation and subsequent freezing/congealing of liquids in these stagnant PSV inlet lines, the possibility of:
- hydrate formation resulting from a combination of high operating pressure and low ambient temperature; and
- liquids freezing/congealing in a liquid full system cannot be ruled out. Hence, PSV inlet lines should be reviewed for possibility of hydrate formation or liquid freezing/congealing and heat tracing provided accordingly.
All the PSV/BDV/PCV outlet lines (to flare) are sloped towards main flare header which is sloped towards flare KOD, no stagnation of liquid is envisaged in these lines. Thus there should be no liquid accumulation in these lines. With the relatively low operating pressure (flare back-pressure), hydrates are not likely to form even at very low ambient temperatures. Nonetheless, these lines should be reviewed for hydrate formation and heat tracing to be provided if hydrate formation is a possibility. In some cases Client specifications may require the lower half of the flare header/sub-header(s) in wet service to be heat traced to prevent water freezing.
When the pressure relief is to the atmosphere, then the discharge line should be heat traced up to 600 mm after the first vertical bend to prevent freezing of water resulting from atmospheric condensation.
3.4.4 Water and Drain Systems
In areas with sub-zero minimum ambient temperatures, water (fresh, potable and service water) and drain systems should be heat traced and drains should be provided at all low points.
Care must be taken when specifying heat tracing on potable water lines to eyewashes and safety showers to avoid overheating. Thermostatically controlled electric trace heating should be used. Potable water to be maintained at 24°C (+ or – 5°C) and must not exceed 30°C.
Closed drain headers should be heat traced and insulated where piping may become plugged due to wax deposits, congealed viscous fluids.
3.4.5 Sewerage System
In cold climates with sub-zero minimum ambient temperatures, sewage lines should be heat traced and insulated.
3.4.6 Instrument & Utility Air and Nitrogen Systems
Instrument/utility air and nitrogen systems are generally designed to have a dew point of (-) 40°C at dryer outlet. These systems generally do not require winterization.
Where required, water, oil and drain circuits should be heat traced and all electric motors should be equipped with electric anti-condensation heaters. Drain valves should be provided in air system strainers, traps, and low points in the piping where moisture may collect.
The air intake system for the air compressor should be provided with an anti-ice system
(electrically heated) in order to control within limits entering air temperature.
3.4.7 Storage Tanks and Vessels
Where tanks and vessels contain liquids that may become too viscous or solidify at ambient conditions, and there is no sufficient circulation of fluid in the system to prevent the liquid inside the vessel from cooling to ambient conditions, heat tracing, internal/external thermic fluid heating coil should be provided.
3.4.8 Heat Exchangers
Design should provide gravity drainage facility for heat exchangers (shell & tube, plate & frame or air cooled) containing liquids which might congeal or freeze at the low ambient design temperature when not in service.
For the Air Cooled Heat Exchangers wherein there is a possibility of hydrate formation, winterization cabins along with thermic fluid coils (for start-up) should be provided. The winterization cabin air temperature should be maintained during operations and start-up to prevent the possibility of process fluid freezing in the tube bundles.
Hot Service Pumps
Where the pumped fluid is >150°C, a bypass line with valves should be installed around the pump discharge check valve to avoid thermal shock. When the pump is on standby with the suction valve open, this by-pass valve should be opened allowing hot process fluid to back flow through the standby pump.
When back flow of hot process fluid to the standby pump is not an option, such as PD pump, heat tracing should be provided to the pump and its associated components.
Cold Service Pumps
If the service requires the pump to be continuously available, then the pump should be insulated (cold conservation/anti-condensation). A 1” bypass line complete with valve should be installed around the pump discharge check valve. When the pump is on standby with suction valve open, this by-pass valve should be opened allowing process fluid to back flow through the standby pump.
3.4.10 Major Machinery
Turbo-expander and re-compressors, turbines, engines and other major machinery which require frequent routine maintenance and on-site inspection (including auxiliaries) should be installed in heated buildings.
Air intakes to turbines, engines and HVAC systems should have anti-icing provisions. When machineries require liquid fuel as the fuel source, ‘Arctic grade’ diesel can be specified during winter months. Use of stabilized condensate product can also be considered as alternate liquid fuel.
3.4.11 Water Seal Legs and Seal Drums
Seal legs and seal drums should be heat traced and insulated to prevent freezing of the water seal. Alternatively, anti-freeze may be added to the water to lower the freezing point of the solution below the minimum ambient temperature; typically 60% to 65% ethylene glycol is required. Note that the static head held by the seal should account for the change in liquid density if anti-freeze is used.
If lines contain fluids that meet the criteria stated in Section 5.1, the lines together with their associated instrumentation (including impulse lines) should be traced. This applies to flow, pressure and analyzer instrumentation. If instruments are chemically sealed/diaphragm sealed, heat tracing is not required.
If fluids in vessels meet the criteria stated in Section 5.1, level and pressure instruments (and corresponding connections) connected to the vessels should be winterized.
Special instruments such as analyzers should be located in heated enclosures as appropriate if required to suit the ambient conditions.
Advice from Instrumentation discipline should be taken with regard to winterization of instrumentation.
The extent of heat tracing on instrument may be reduced if the precautionary measure of using chemical seals or non-invasive measurement techniques to prevent instruments
3.4.13 Fuel Gas Systems
All fuel gas lines to the gas turbines, furnaces and purge systems should be reviewed for requirement of heat tracing. Factors to be considered include hydrate formation and condensation of heavy hydrocarbons.
By maintaining the fuel gas temperature sufficiently above the dew point temperature of the gas, heat tracing can be avoided.
3.4.14 Diesel Fuel Systems
Consideration should be given to using Arctic grade diesel for very cold climatic condition.
An insulated and heat traced distribution system may be required for removing the possibility of the diesel system freezing.
Consideration should be given to the requirement for a heating coil in the Diesel Storage Tank for viscosity control, if required.
3.4.15 Chemical Injection and Chlorination Systems
The Chemical Injection and Chlorination Packages, including the injection lines should be heat traced and insulated where the chemical is water based and/or has a high pour point, high viscosity, etc.
3.4.16 Flare Systems
Relief headers should be heat traced and insulated based on fluid properties and ambient condition requirements; although consideration should be given to avoid heat tracing and insulation on dry flare headers.
The lower half of HP and LP Flare Knockout Drums should have internal electric heaters to prevent freezing of any accumulated water, and the liquid outlet lines should be heat traced and insulated.
Blow down pipe work where hydrates can form and relief valve inlet lines which contain oil exhibiting waxy characteristics should be traced and insulated.
Flare drum level instrumentation and drum pump suction and discharge lines should be heat traced and insulated as for water systems.
3.4.17 Helifuel System
A helifuel package should include winterization features as per the package specification.
Any piping between package skids should be winterized accordingly.
3.4.18 Service Oils
Seal oil and lube oil systems should be suitable for operation in the environmental conditions present. This may include the use of heating coils for start-up conditions. The equipment supplier should identify equipment piping for insulation and tracing if it is considered necessary, including off skid piping which should be insulated and heat traced in line with supplier requirements.
3.4.19 Fans and Blowers
3.5 General Considerations for Heat Tracing
- Process gas lines with dew points higher than the minimum ambient temperature can be subject to condensation in the piping. Wellhead gas piping coming out of Separators, Compressor suction piping can be subject to liquid carry over issues. Hydrocarbon gas under pressure can be subject to retrograde condensation over a range of operating pressures if not adequately superheated.
- Similar to above, the plant fuel gas lines can be subject to condensation in the KO Drums, filters, piping, pressure letdown stations and instrumentation and can carry condensate to burners.
- Gas heater should be as close as practical to users; thus, eliminating long runs of supply lines (> 100m). Calculations should be done for estimation of heat losses from long insulated supply lines.
- The condensate knock out pot should be located immediately upstream of gas heater.
- Heat tracing is meant to maintain fluid temperature, not to elevate it.
Winterization: Protection Against Low Ambient Temperature
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