Refrigeration piping refers to the piping network that operates below -290 degrees Celsius. This temperature indicates the fragility boundary for carbon steel materials. Various literatures, however, consider piping systems operating at less than -1500 ° C (-3000F) as cryogenic piping systems in the true sense of the word. Industrial processing and transportation of propane, butane (LPG), methane (LNG), ethylene, nitrogen, ammonia, oxygen, etc. requires extensive use of the refrigeration piping system. These piping systems must be carefully designed to operate at such low temperatures. In this article, we will look in more detail about refrigeration plumbing.
Properties of common refrigerants
Refrigerants are odorless, tasteless and colorless. Refrigerant liquids should be used with caution as they may cause skin burns and frostbite. Table 1 lists the following liquid temperature and liquid to gas expansion ratios of some common refrigerants:
Table 1: Properties of common refrigeration materials
Why is cryogenic plumbing challenging?
Because the temperature drops too low during operating conditions, pipe materials face a variety of corrosion and deterioration issues as the chemical and physical properties of the material change. Conventional piping systems cannot hold process gas in liquid form. Also, as shown in Table 1, refrigerants produce large volumes of gases as they evaporate. Therefore, if they evaporate inside a closed container, the container may explode due to excessive pressure. As a result, this refrigeration piping requires a special type of material, supports and valves different from conventional piping systems, which makes the design of refrigeration piping systems very vital and challenging. Some of the requirements of the refrigeration piping system are:
- Sufficient flexibility because at lower temperatures, the material shrinks and creates a lot of heat stress that must be compensated for with proper flexibility.
- All refrigeration lines are insulated to prevent overheating of the environment and for safety. All this increases the weight of the pipe and tightens the refrigeration systems.
- Long-stemmed valves with special design are used as refrigeration valves.
- The use of expensive materials increases the cost of the project. Therefore, every chance of optimization should be used to minimize project cost.
Refrigeration piping materials
As the temperature decreases, the materials become brittle and according to the code, there is a need for impact testing. Different parameters must be considered for the selection of refrigeration piping materials.
- Suitability for different manufacturing techniques
- Corrosion resistance
- Resistance to oxidation and sulfidation
- Strength and ductility
- Suitable for cleaning process
- Toughness, wear resistance, erosion, damage and size.
- Properties Physical properties
- Impact resistance and so on
The materials that have introduced themselves as suitable refrigeration piping materials are presented in Table 2 below:
Table 2: Common refrigeration piping materials
Several non-metallic materials such as graphol, mineral wool, fiberglass, polyurethane, polyurethane, styrofoam, perlite, viton, glass-reinforced Teflon, etc. act as various components in refrigeration piping applications.
Refrigeration piping standards and refrigeration piping design guide
ASME B31.3 is the main standard for the design of refrigeration piping systems. Common considerations in the design of refrigeration pipes are:
Pipe size is done using standard pressure drop criteria. A drop in pressure can cause part of the liquid to blink, which may lead to a two-phase flow. Therefore, if a similar situation occurs, a two-phase current must be considered for measurement. However, for oxygen gas piping, fluid velocity is also taken into account when measuring the pipe.
Because the ambient temperature is warmer than the temperature of the refrigerant, there will be continuous heat leakage into the pipeline and the refrigeration piping system that must be considered during design.
Long stem valves are used to keep the operator at room temperature.
Refrigeration plumbing insulation
Repeated reusable piping systems and pipelines are insulated using any of the following types of refrigeration insulation:
- Expanded foams (eg, foam glass, polyurethane)
- Powder insulation (eg perlite)
- Vacuum insulation
- Drain powder and fiber insulation
- Matte powder insulation
The main purpose of the refrigeration piping insulation system is to create a vapor barrier to prevent atmospheric moisture from leaking into the insulation space. This moisture penetrates the insulation and then condenses. Which significantly increases the corrosion changes in the lines. Also, accumulation of water or ice may occur, which in turn reduces performance. Whenever insulation is compromised, thermal efficiency is lost and energy consumption is increased. Therefore, using sufficient insulation materials can reduce high energy consumption. The vapor barrier system must prevent atmospheric moisture from entering the insulation space and freezing against the refrigeration lines.
Whenever a cold system is required, the entire system must be fully insulated, including plumbing components, pipe / tool pipe, drainage, equipment nozzles and supports. Cryogenic insulation is applied in several layers.
Support for refrigeration pipes
Due to their very low temperature, excellent insulation properties, durability and stable performance are required for refrigeration pipe support devices. When designing refrigeration supports, we must consider the structural features, design load, other requirements, and economic aspects of each shoe, guide, stop, and trannion. We need to clarify the behavior of refrigeration piping, including pipe support, during normal operation they must also consider heating and cooling conditions. There are problems such as further displacement due to thermal expansion and contraction, insulation of pipes, brittleness of materials, freezing around or between supports, rapid phase changes due to high heat fluxes in the system.
Refrigeration pipe supports must meet the following conditions:
- Lighter weight
- High reliability in water and resistance to oil and corrosion
- High air tightness
- They must be physically strong against pressure, bending and cutting
- Suitable for mass production
- Low water absorption
- Resistance to heat and flame
- It must have a heavy-duty layer molded with stainless steel.
Cold insulation supports are usually made of the following:
- High density polyurethane foam
- Phenolic foam insulation
- Polyisocyanurate or PIR
The bearings must meet the design requirements in terms of compressive strength under stable load, thermal conductivity, friction service temperature and flammability. Even taking into account unexpected thermal bending and fluctuations in pipe flow velocities, the support opening for refrigeration piping should be much shorter than hot insulation piping, the abutment should be immediately adjacent to any change in piping direction.
Conventional refrigeration piping support
Cryogenic supports will be equipped with advanced temperature-resistant technology that protects the pipes in extreme cold. Cold weather is critical for pipe supports that are not designed to withstand the elements. Worse, the pipes are brittle in cold environments and ice formation can wear down both the pipes and the supports. It must also be designed to support the pipes at -320 degrees Fahrenheit. They enclose the fragile insulation used in these piping systems.
They must be non-conductive to prevent heat transfer from inside the pipes to the surrounding structures. Foam insulation cores are given to some shoes to naturally protect the tubes from sudden temperature changes due to heat transfer. By keeping the heat inside the pipes, energy can be saved and the formation of ice can be stopped, which can destroy the pipes. Cold shoes are a support used for refrigeration applications where the heat is not transferred to the relevant surface and can be used for temperatures up to -300 degrees Fahrenheit.
Refrigeration piping stress analysis
Refrigeration pipelines are a special case where the operating temperature used is much lower than the ambient temperature installed. There must be a degree of accuracy that is relevant to the safety of the operation and potential hazards, and therefore accurate flexibility analysis is crucial for managing thermal forces, stresses, and displacements. Some of the considerations of refrigeration piping stress analysis are presented below:
- Refrigeration piping systems may have a thermal arc effect. So it must be considered. Click here for more information on using the tube heat arc effect.
- If the pipe is a vacuum insulator, two pipes with different displacements must be considered and connected. The main tube, which is at refrigeration temperature, contracts, and the jacket tube usually expands at a slightly higher temperature than the environment.
- Equipment nozzle loads can usually be assessed using finite element analysis or FEA.
- Flexible or expandable hoses may be required for analysis.
- Refrigeration pipe systems work in stark contrast to high temperature piping systems. Due to the contraction of the refrigeration piping system, the supports that are usually lifted in high temperature piping carry the load in the refrigeration piping.
LNG piping systems and cold box piping systems are common examples of refrigeration piping systems.
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