One of the problems recognised early on with the storage of gases as liquids at cryogenic temperatures (typical -150oC) is boil-off gas (BOG). This occurs when heat from a variety of sources transfers into the tank causing the liquid to boil and the pressure in the tank to rise.
What causes boil-off?
A number of conditions can lead to boil-off gas within a cryogenic vessel, this includes
- heat being absorbed from ambient air by refrigerated storage tanks
- heat absorbed from ambient air by lines
- heat produced by the operation of pumps
- vapour displacement due to liquid inlet in the tank
- rapid variation of barometric pressure
For obvious reasons, cryogenic gases are stored in tanks at temperatures below their boiling point. This means that when heat enters a cryogenic tank during storage or transportation, some of the product in the tank continuously evaporates and boils off.
If there is a large temperature differential between the media being stored and its environment, heat ingress can occur through the floor, wall or roof of storage tanks by conduction, convection and radiation. When boil-off occurs it can lead to a great deal of product wastage – and once started, it can be continuous throughout the tank.
Heat and pressure control
To minimise the risk of BOG, it is important to first understand that temperature and pressure are directly related. Inside the tanks, the product exists in an equilibrium between a thermodynamic liquid and vapour, depending on the given pressure and temperature. As temperature increases in an industrial gas storage tank, so does the pressure.
Controlling heat and pressure are the most important concerns of the cryogenic industry. Here the aim is to limit the transfer of heat into the cryogenic vessel when it needs to be stored or transported. By doing this it prevents boil off from occurring and liquid turning to vapour and venting from the tank.
Choosing the right cryogenic valve
A great deal of innovation has gone into both the design of storage tanks and valves to minimise the risk of boil off. The boil-off effects of a very cold liquid hitting much warmer metal (the tank) need to be minimised. Innovation within Parker Bestobell has resulted in valves that are designed with minimum material mass to ensure cryogenic operating temperatures are reached as quickly as possible. This improves cool down times and reduces boil off when the product is in contact with the valve, therefore lowering risk.
Cryogenic storage tanks are also designed to minimise the heat that is transferred from the warm external environment into the cryogenic liquid so that vaporisation is less than 0.05% of the total tank content per day. Choice of cryogenic valve is essential to ensure the design is effective in reducing the risk of boil-off gas under cryogenic conditions.
Steve Fidler, product manager, Parker Bestobell Industrial, Instrumentation Products Division, Europe.