Winterization is a crucial aspect of processing plant design as it protects equipment and piping from solidifying or coagulating contents. To achieve winterization, different methods are employed, including steam tracing, steam jacketing, electrical tracing, and process heating.
This article focuses on the basic design requirements for winterization systems and highlights the essential data sources for the design process. The design data includes the Process Flow Diagram (PFD), Basic Engineering Design Data (BEDD), Piping & Instrument Diagram (P&ID), Plot plan, Equipment datasheet, and Piping material specification.
All process fluids should be considered for winterization, and the design should be tailored to suit the local ambient conditions. Additionally, water and steam condensate piping should also be winterized to ensure optimal performance of the processing plant.
The following table gives criteria for the requirement of winterization.
Winterization of Process Piping: Basic Principles
Process piping needs to be winterized when the internal fluid’s pour point or solidifying point is higher than the lowest ambient temperature. In such cases, the fluid temperature should be maintained above the solidifying point or at least 10°C above the pour point, unless otherwise specified.
For liquid sulfur lines, steam jacket piping or electric heat tracing is used to maintain the fluid temperature between 118°C and 158°C.
For highly viscous fluids such as asphalt and bitumen, steam tracer piping or steam jacket piping is used to maintain the fluid temperature at temperatures exceeding the pour point +10°C or temperatures giving a kinetic viscosity of 300 CST, whichever is higher.
When the plant is operating and the fluids are always flowing (on-stream), appropriate measures are taken to prevent temperature drop. Winterization needs to be studied for cases where the plant stops operating.
Tank yards have many piping items in which fluids are not always flowing (not on-stream), so care should be taken to prevent temperature drop in such cases.
Winterization Requirements for Liquid Lines
In order to prevent coagulation or solidification of fluids with higher pour point or solidifying point than the lowest ambient temperature, the following winterization requirements should be applied to liquid lines:
Winterization Philosophy for Lines always on-stream:
(a) Bare pipelines likely to coagulate within 12 hours after liquid stops flowing should be hot insulated.
(b) Hot-insulated lines likely to coagulate within 12 hours after liquid stops flowing should be steam traced, even if operating temperature is high.
Winterization Philosophy for Lines not always on-stream (liquid-filled lines):
All piping sizes should be steam traced and hot insulated regardless of liquid temperatures. The same criteria should also apply to bypass lines for startup and shutdown, liquid blowdown lines, slop oil lines, standby lines around the pump, lines to remove small amounts of water, instruments, liquid relief valves and their inlet/outlet lines, control valve bypass lines, chemical injection lines, makeup water lines, sampling lines, and vents and drains. Steam tracing requirements for vents and drains should be according to line conditions.
Winterization requirements for Lines not always on-stream (usually empty):
Such lines should be sloped to avoid forming pockets and should have steam purge connections to completely empty the inside. Otherwise, they should be hot insulated.
Other requirements for Winterization:
Lines with highly viscous fluids such as heavy fuel oil should be steam traced. Caustic and amine solution lines should be steam traced if their freezing point is higher than the lowest ambient temperature. Steam tracing of these lines should have insulating spacers to prevent alkali embrittlement.
Winterization Requirements for Vapor Lines Saturated with Steam
(1) Winterization for Lines always on-stream (a) The upstream side of lines containing orifice plates or control valves, where steam could possibly condense, should be hot insulated. The amount of condensate generated from the gas line has to be calculated by estimating the temperature drop and consequent partial pressure decrease of steam. (b) Lines where freezing of condensed water can cause trouble during continuous operation should be steam traced. (c) Lines should be properly sloped to avoid condensate accumulation. (d) Lines where ice or hydrate formation is possible on depressurizing should be steam traced.
(2) Winterization for Lines not always on-stream Piping should preferably be free draining. The following items should be steam traced:
- Instruments such as LG, LT, PG, and lead pipes of PT.
- Bypass lines for control valves.
- Inlet line of a relief valve.
In some cases, lines should only be hot insulated depending on the pipe size and length, considering heat loss.
Winterization Requirements for Vapor Lines with Higher Dew Point Fluid
Winterization requirements for vapor lines with higher dew point fluid are crucial to prevent condensation and potential problems associated with low temperatures. The following guidelines should be applied to vapor lines containing fluid with a dew point higher than the lowest ambient temperature.
(1) Lines always on-stream:
(a) The upstream side of the lines of orifice plates or control valves, in which vapor could possibly condense, should be hot insulated. This is necessary to prevent the formation of condensate that can occur due to a decrease in temperature and pressure, especially in areas where the ambient temperature is low. Hot insulation maintains the temperature of the fluid above the dew point, preventing the formation of condensate.
(b) Lines in which condensate is likely to solidify or is corrosive should be steam traced. Steam tracing is necessary to maintain the temperature of the fluid above the dew point, preventing the formation of condensate that can solidify and cause blockages in the line or damage to the piping due to corrosion.
(c) Lines in which condensate is likely to freeze or coagulate due to depressurization during shutdown operation should be steam traced. Steam tracing is necessary to maintain the temperature of the fluid above the dew point and prevent the formation of condensate that can freeze or coagulate during shutdown, potentially causing blockages or damage to the piping.
(d) Piping should preferably be free draining to prevent the accumulation of condensate in pockets, which can have adverse effects on the indications of instruments such as pressure gauges and lead pipes of pressure transmitters.
(e) Pockets where condensate accumulates, which may have adverse effects on the indications of instruments (such as PG, and lead pipes of PT), should be steam traced to prevent the accumulation of condensate that can cause damage or interfere with instrument readings.
(f) Lines that are likely to have adverse effects on continuous operation due to the condensing of the fluid should be hot insulated to prevent the formation of condensate that can disrupt operations.
Winterization requirements for vapor lines with higher dew point fluids are essential to maintain the proper functioning of the system and prevent potential problems associated with low temperatures. Proper insulation and steam tracing help maintain the fluid temperature above the dew point, preventing the formation of condensate that can cause damage to the piping and other equipment.
Winterization of Utility Piping
When it comes to winterizing utility piping, such as water piping, it’s important to take certain precautions to prevent issues such as freezing and interruptions in flow.
Main water pipes should ideally be buried below the freezing depth, but aboveground or underground pipes buried above the freezing depth should follow the same requirements as process piping for winterization.
For piping that is 2″ or less in size, it should be heat traced and hot insulated to prevent freezing. For larger piping that is always on-stream, measures should be taken to ensure water flow is not interrupted as much as possible. Additionally, temperature drops in winter should be calculated, and the piping should be hot-insulated for freeze-proofing as necessary.
Piping that is not always on-stream should also be heat traced and hot insulated. A circulation line should be provided at the terminal of each header to prevent the water from stopping flowing.
For pump coolers, it’s recommended to circulate water into spare standby pumps to minimize freezing trouble.
Winterization for Air and Nitrogen Piping
During winterization of air and nitrogen piping, it is important to consider the moisture content in the air or nitrogen being transported. Instrument air and nitrogen typically contain little moisture and therefore may not require hot insulation. However, the piping should be constructed using materials suitable for low-temperature services, taking into account the lowest ambient temperature.
On the other hand, plant air piping may require steam tracing and insulation if the air is not dry. If the air is dry, then the piping may be left bare. Proper attention should be paid to the moisture content in the air being transported to determine if steam tracing and insulation are necessary.
Winterization for Steam Piping
When it comes to winterizing steam piping, special attention should be given to preventing condensate accumulation, which can freeze and cause blockages or damage to equipment. One important measure is to install steam traps in lines where condensate is likely to accumulate, such as pockets or control valve bypass lines. Additionally, even for lines that are not frequently used, it is important to install a steam trap at the inlet of each block valve to prevent freezing. This will help ensure that steam can flow freely and that equipment remains protected from the effects of freezing temperatures.
Winterization for Steam Condensate Piping
To prevent freezing and ensure smooth operation of steam condensate piping during winter, the following precautions should be taken:
- Steam condensate piping of 1½” or smaller diameter should be heat traced and hot insulated.
- Steam condensate piping of 2″ or larger diameter, which is continuously in use during plant operation, should be hot insulated.
- For steam condensate piping of 2″ or larger diameter, which suffers from intermittent flow or condensate accumulation for prolonged periods, heat tracing and hot insulation should be applied.
- Steam traps should be installed at low points and pockets where condensate is likely to accumulate to prevent damage due to freezing.
- The condensate piping should be properly sloped to facilitate drainage and prevent condensate accumulation.
Winterization of Equipment
- Equipment requiring winterization: (a) Equipment containing water that accumulates for a long period, such as separators, flash drums, and receiver boots, should have water removed. (b) Equipment containing fluids with high pour points, high solidifying points, or high viscosity should be addressed to prevent coagulation or hard-to-flow conditions. (c) Equipment handling hydrocarbons in gas, such as fuel gas drums, which may cause partial condensation leading to adverse effects on the entire unit. (d) Equipment handling chemicals, such as caustic soda solution drums and inhibitor drums.
- Winterization of static equipment: (a) Towers, vessels, and heat exchangers that handle fluids that may freeze should have a drain valve at a position to allow fluids to be drained completely during plant operation suspension. (b) Parts of vessels (boots, etc.) that come into contact with water, nozzles, valves, and piping should be heat traced and hot insulated. (c) No winterization is required for equipment that can be heated by internal or external heating coils or similar facilities, even if the equipment contains liquid during plant shutdown.
- Winterization of air-cooled heat exchangers: (a) Winterization should comply with API standard 632. (b) Louvers should be installed to prevent excessive cooling when the inside tube skin temperature in winter decreases to lower than the freezing point or pour point of the fluid passing through the tube. (c) For air-cooled heat exchangers handling heavy oils with high pour points or viscosity, steam coils should be provided to prevent the plugging of tubes due to excessive cooling. The use of a hot-air circulation system may be considered necessary. Data on the steam consumption by air-cooled heat exchanger steam coils should be obtained from the manufacturer, together with the criteria for use.
Operation Mode Change
In order to save energy, winterization is designed on a seasonal basis. This means that different winterization measures are implemented based on the season and the lowest expected temperature.
For water or moisture freezing prevention, heat tracing is typically not necessary during seasons in which the lowest temperature is above 0°C. However, experience has shown that when the temperature drops to around -5°C, water can freeze and cause bare piping to break at pockets. Therefore, winterization measures may need to be implemented when the lowest expected temperature falls below this threshold.
For maintaining process fluid temperature higher than its pour point/solidifying point, heat tracing may be suspended during seasons when the lowest temperature exceeds a certain threshold based on the pour/solidifying points of the process fluid. For instance, if the process fluid has a pour point of 10°C or lower, heat tracing can be suspended during seasons in which the lowest temperature exceeds 15°C. Seasons may be divided into two groups, such as summer and winter seasons, to simplify the operation.
Selection of Heat Tracing Method for Winterization
When it comes to selecting a heat tracing method for winterization, steam tracing is generally recommended. However, there are some exceptions to this rule. For instance, electric tracing can be used in cases where the equipment or piping is located far away from the steam supply source or in positions where it is difficult to supply steam. Additionally, electric tracing may be considered when the fluid needs to be maintained at temperatures of 200°C or higher.
If steam cannot be used due to the properties of the internal fluids, hot water tracing may be used instead. It’s important to carefully consider the properties of the fluid being transported, as well as the location and accessibility of the equipment and piping, in order to select the most appropriate heat tracing method for winterization.