Liquefied gases, such as nitrogen, oxygen and helium, are used in many cryogenic applications. To measure pressure in these situations, your pressure instruments must be able to stand up to very low temperatures.
So, how can you effectively measure process pressure when your pressure gauge is not rated for such a low cryogenic temperature?
In this article, I will discuss cryogenic applications, and the different mounting options and accessories available that can help protect your pressure instruments from the effects of very low temperatures.
What is Cryogenics?
Cryogenics is the production of, and behavior of, materials at very low temperatures. The National Institute of Standards and Technology (NIST) defines cryogenic as anything involving temperatures below -153 °C (-243.4 °F). A very common application involves liquid Nitrogen, where the temperature is -195 °C (-320 °F).
At such low temperatures, elastomers and plastics perform very poorly. Carbon steel or alloy steel are also poor choices. The aforementioned will get brittle at such low temperatures, but 316 stainless steel is a good choice for wetted parts when measuring cryogenic processes.
Direct mounting a pressure instrument directly to a cryogenic process is not recommended. While wetted parts of a pressure instrument may be 316 stainless steel, an instrument such as a pressure gauge will have gaskets and other components not designed for exposure to very low temperatures.
This can lead to instrument failure very rapidly. Protecting the instrument from these very low temperatures is critical to properly measuring such applications.
Provided the media is compatible, stainless steel is the preferred material for cryogenic and low-temperature service. Bronze and Monel are acceptable. Steel tube gauges should be avoided for low or cryogenic service because the ductility of the metal is reduced at low temperatures.
Cryogenic Applications
Applications that involve ultra-low cryogenic temperatures include:
- Freezing and transportation of food products
- Cryopreservation of organic goods
- Coolant for industrial equipment
- Storage of gases for high-volume applications such as inert gas cooling during heat treatment
The use of liquid oxygen in aerospace, medical, and pharmaceutical applications is also a common cryogenic application. In an oxygen-enriched atmosphere, materials that are combustible and flammable in air ignite more easily, burn more vigorously and produce a higher temperature.
Instruments used in oxygen applications must be properly cleaned for oxygen service to remove any contaminants such as oil or grease that can easily ignite.
Measuring a Cryogenic Process
Many cryogenic applications do not use pumps because the fluid is self-pressurizing (as the liquid boils, gas pressure builds up in a closed container). Additional piping or use of a capillary line allows the liquid to vaporize to a gas and warm up over the ambient dewpoint.
In these applications, the cryogenic liquid itself is not directly being measured, but the pressure of the gas above the cryogenic liquid is being measured. This allows the pressure instrument itself to not be directly exposed to low temperatures.
Instrument internals that are elastomers will be protected, and not become brittle. They will maintain their elasticity allowing for proper pressure measurement.
A capillary line may also be used in cryogenic applications. A capillary has a small internal diameter and is available in lengths of 1 foot up to 100 feet. The small internal diameter along with the line length allows for the temperature to be raised to an acceptable temperature for the pressure instrument to function properly.
Ashcroft’s 1115A capillary line and 2198 MicroTube™ siphon are both excellent and cost-effective choices for protecting pressure gauges in applications where low temperatures are involved. Both are available in stainless steel wetted parts, which is preferred for such low temperatures.
We don’t like to pressure you, but we have more information.
I hope this article cleared up any confusion you may have about cryogenic applications. As I stated before, it is critical to properly protect your pressure instrument from extreme low temperatures. Proper installation and mounting will allow your pressure instruments to properly monitor the process and require limited maintenance.
If you want to learn more about temperature protection, you can read some related articles:
- Heat Dissipation: Pressure Gauge Accessories for Non-Steam Applications
- How Does Temperature Affect Pressure Gauge Performance?
- How Does Media Temperature Affect Pressure Transducer Performance?
- What’s the Right Pressure Instrument Mounting Assembly for Me?
You can also contact us today to talk to one of our industry experts and get all of your questions answered. Let us be your one-stop-shop for pressure and temperature measurement!