As part of our “let’s talk about” series looking at Cryogenics we talk to Suretank's expert on the subject, Group Quality and Technical Manager, Donal Duggan.
Donal, in simple terms, what is a cryogenic liquid?
Cryogenic liquids are gases kept in their liquid state at very low temperatures.
All cryogenic liquids have boiling points below -150°C (- 238°F). This means that all cryogenic liquids are gases at normal temperatures and pressures.
Different cryogens become liquids under different conditions of temperature and pressure, but all are very cold, and minimal amounts of liquid can expand into very large volumes of gas.
The vapours and gases released from cryogenic liquids also remain very cold. They often condense the moisture in air, creating a highly visible fog.
Because of their complex and dangerous properties it's vital that they are stored and transported correctly and safely.
Most cryogenic liquids can be placed into several groups:
Inert gases, for example nitrogen, helium, neon, argon and krypton; flammable gases, such as hydrogen, methane and liquefied natural gas and oxygen.
Who uses cryogenic liquids in the offshore environment and why?
Offshore cryogenic tanks are designed to transport bulk cryogenic liquids to offshore rigs.
You may have come across a cryogenic material if you've visited a fancy restaurant recently. Chefs use liquid nitrogen to freeze food sometimes at the table as a wow factor in a meal. A quick search on youtube will bring up some interesting examples.
Why are cryogenic gases put into a cryogenic liquid state?
Small amounts of cryogenic liquid can expand into very large volumes of gas so it's more cost effective to transport them in their liquid state.
While conventionally used in blanketing and inert offshore applications, on-site gaseous nitrogen - supplied at high flow rates and pressures - is finding increased use in drilling, injection for reservoir and wellbore pressure maintenance, and in coiled tubing operations.
How are cryogenic liquids transported and stored?
Cryogenic liquids are shipped and stored in highly engineered thermally insulated containers. These cryogenic liquid containers are specifically designed to withstand rapid temperature changes and extreme differences in temperature.
Suretank's offshore cryogenic tanks are designed and approved to transport bulk cryogenic liquids to offshore platforms and rigs via rail, road and sea. Their compact design is constructed to fit in confined spaces on the oil rigs and to withstand the harsh offshore environment.
Our design is fully compliant with all international safety and quality standards, including DNV 2.7-1, ADR, RID, IMDG, US DOT, ASME U-stamp, CSC, providing optimum access and storage, long life and low maintenance.
The storage frames are designed for safe and secure stacking during transport and are available in bolt-down versions, which has a fully removable tank and pipe work, for easy maintenance of both the frame and vessel. Customers also have the option of specifying a stainless steel or carbon steel outer vessel, depending on their requirements.
What are the main challenges when manufacturing a tank that will contain cryogenic liquids?
The key design criteria for a Cryogenic tank is to minimize heat transfer from the atmosphere to the inner vessel. When a large temperature differential exists, this is a significant challenge. Any heat ingress will cause the pressure to rise. When the pressure reaches the pressure relief valve setting, some product will be discharged, and the pressure is reduced and the cycle continues.
The product literally goes up in smoke!
The more often this happens, the quicker the tank will empty. The tank will eventually empty due to this phenomenon, there is no possibility of having zero heat transfer.
The key is to keep heat transfer to an absolute minimum. This has always been the primary focus for Suretank and we know our tanks are designed to have minimal heat ingress and therefore the product lasts longer in our tanks.
This is not always of huge importance to customers on a relatively short trip, but if a product is moved over a longer period of time this becomes a significant metric.
Another challenge is to achieve minimal heat transfer, with a robust transport frame and vessel assembly design which is suitable for road, rail and sea (including offshore conditions). Suretank has years or experience designing and building transport equipment, so this was a bit easier!
Are there any special processes that differ when producing a cryogenic tank compared to a tank for other liquids?
The most effective way to reduce heat transfer is to use a vacuum between inner and outer tanks (like a flask).
The quality of vacuum is of extreme importance and Suretank pay particular attention to all steps along the way.
How are finished tanks tested by Suretank before they are shipped to the customer?
There are many tests during the construction of a cryogenic tank, some of which are mandatory code tests and some of which are additional tests which Suretank insist upon to guarantee absolute quality of construction.
Each design is tested with Liquid Nitrogen to check functionality and verify the hold time.
A Cryogenic tank holding time is a measure of how long it takes for the pressure to rise to the pressure relief valve setting. The pressure rises due to heat in-leak and a longer hold time means a superior tank.
This is why we are confident we have the best hold time in the market!