A cold box is a highly engineered large rectangular or round box that encloses the major cryogenic equipment in an air separation unit. It is comprised of steel panels that are welded onto the cold box frame and the space between the cold box wall and the equipment is filled with insulating material. The frame and foundation are designed to support the weight of all process equipment, as well as piping and valves containing the maximum operating level of liquid. It must also withstand high wind loadings as well as being suitable for the earthquake zone in which it is installed.
Insulating Material Used in Cold Box
The insulation materials used in a cold box are typically perlite, which is a fine powder, or slag wool, which is similar to glass wool but denser. Perlite is used for insulating the bulk of the cold box where access is unlikely to be needed and slag wool is used in areas where access may be necessary as it can be removed more easily than perlite. Slag wool is typically used in turbine, pump, and valve boxes.
Equipment Housed in Cold Box
The equipment housed within the cold box typically includes distillation columns, heat exchangers, separators, cryogenic adsorbers, piping and valves, and the cold end of the turbine. The equipment associated with the cold box typically includes adsorbers, cryogenic pumps, and expansion turbines. Access to the internal equipment of a cold box is not possible during operation so all equipment functions are controlled through valves with extended stems that pass through the cold box wall.
Why Does Water Cause a Hazard to the Cold Box?
Water entering a cold box will freeze and can become a hazard by preventing piping from free movement or causing excessive weight loads. Ice may greatly extend the time required when a cold box is thawed, so it is extremely important to maintain the integrity of the seal at an opening (for example, at the valve and piping seal boots and manways). Water should be prevented from pooling on any horizontal surface.
Cold Box Purging
The space inside the cold box is filled with nitrogen to prevent heat transfer and oxygen condensation on liquid nitrogen pipework. This is known as the cold box purge and it must be maintained whenever the cold box equipment is at cryogenic temperatures. It is usually maintained at other times also, except during internal work on the cold box equipment.
Different Types of Piping Involved in Cold Box
The four types of piping involved in a cryogenic cold box are process piping, relief valve piping, purge, vent, and drain piping, and instrument piping. Process piping carries fluids between the equipment items and relief valve piping connects pressure vessels to external pressure relief valves. Purge, vent, and drain piping connects process equipment and process piping to the outside of the cold box for the supply of services and to allow venting of process gases or purge gases to the atmosphere. Instrument piping connects instruments from outside the cold box through the cold box face to the equipment items and the process piping.
Why Warm Seal is Provided in the Cold Box Safety Valve Line?
A warm gas seal is created on the piping to prevent relief valves from being permanently cold. The piping runs vertically upwards from the box penetration by at least 750 mm (2.5 ft) and where feasible, the line within the cold box is routed close to the box face prior to the box penetration.
What Material is Used for Piping?
Piping is usually manufactured from aluminum, although stainless steel and copper alloys are used in some services. All three materials remain ductile at cryogenic temperatures. Carbon steel is not used because it becomes brittle and may fail at temperatures below –29°C.
Why Does a Cold Box Require Flexibility? How is the Flexibility Provided?
Equipment in a cold box shrinks when in operation. All the piping attached to the equipment must accommodate this shrinkage. Flexibility is provided by a series of turns in the piping, usually by a sequence of vertical and horizontal sections.
Supporting Cold Box Pipes
The long lengths of piping in a cold box require support at regular intervals. Various types of support are used which either provide a fixed anchor or allow controlled movement. Horizontal runs of large pipes may have spring-assisted supports to allow controlled movement and vertical pipe supports may allow sliding movement.
What Safeguards Follow to Minimize Leaks Inside the Cold Boxes?
To minimize piping leaks, all piping joints in the perlite region of a box are welded. Special transitions are required for any unavoidable transition joints between incompatible metals and where it becomes necessary to use flanges connections, these are placed in a separate internal box which is insulated with slag wool.
How Does Cold Box Accommodate Shrinkage?
Shrinkage of vessels and piping in operation causes movement of piping and valves relative to the cold box surface. To accommodate this movement, a flexible rubber boot is placed between the box face and the valve stem.
Valves in Cryogenic Service
Valves used in cryogenic service have the same functions and duties as in any other process service but have some additional requirements for operation at low temperatures. The valve body is contained within the cold box, with the stem extending through the box wall. Both stem and bonnet are usually manufactured from type 304 or 316 stainless steel, which has the necessary strength coupled with low thermal conductivity. Cryogenic globe valves are typically used to a diameter of 150 to 200 mm (6 to 8 inches) and butterfly valves are used for larger services.