What a Cryogenic Globe Valve Is
A cryogenic globe valve is a globe valve whose closure member moves along the centreline of the seat, with the port opening in proportion to the disc travel. It is intended for service at very low temperature, typically for liquefied gases such as liquid nitrogen, liquid oxygen, liquefied natural gas and liquid argon. Because the stem stroke is short and the seat can be closed against high differential pressure, the design works well as a shutoff, control or throttling device on cryogenic transfer lines, storage tank connections and vaporiser circuits.
The difference between an ordinary globe valve and a cryogenic one is not the basic flow geometry. It is the set of design changes that keep the operating mechanism, the stem seal and the materials functional while the wetted parts sit at a temperature far below ambient.
Working Principle
The valve uses a spring loaded seating arrangement. Rotation of the handwheel or actuator stem moves the disc along the seat axis; when the valve closes, the disc is pressed onto the seat and blocks the medium, and when it opens, the disc lifts and the flow passes through the annular gap between disc and seat. That geometry gives a long, controllable travel, which is why a globe valve is preferred over a quarter turn valve where throttling is required.
In cryogenic duty the spring loaded seating is important for a second reason: the spring maintains seat contact as the components contract at low temperature, so the sealing force does not fall away when the trim shrinks.
Structural Features and Materials
Extended bonnet. The bonnet is lengthened so that the packing and the operating mechanism stay at a safe distance from the cold body and remain above the frost line. The extension length is selected according to the service temperature and the insulation thickness of the line.
Body and trim material. Austenitic stainless steel grades such as CF8M, CF3M and forged equivalents are standard, because austenitic steels retain toughness at low temperature while carbon steel becomes brittle. Aluminium and selected copper alloys are used for some components; low temperature carbon steel grades such as LC3 are used only where the service temperature stays within their tested range.
Sealing elements. Gaskets, seats and packing are selected for low temperature elasticity, and the seat design may combine a soft insert with metal backup to hold tightness through the whole temperature cycle.
Body form. Straight pattern, angle pattern and Y-pattern bodies cover normal flow, right angle pipe routing and reduced pressure loss respectively.
Cleaning. Valves for oxygen service require an oxygen compatible clean condition, with all components degreased before assembly.
| Design element | Cryogenic requirement |
|---|---|
| Bonnet | Extended construction to raise the stem seal above the frost line |
| Body material | Austenitic stainless steel with documented low temperature impact toughness |
| Seat and packing | Materials that seal at liquid gas temperature and through thermal cycling |
| Testing | Shell and seat tests at ambient, plus cryogenic verification to the agreed standard |
Standards and Testing
General body pressure temperature ratings are read from ASME B16.34, and face to face dimensions follow the applicable ASME B16.10 series. For cryogenic service specifically, BS 6364 is the reference standard for valves used at low temperature: it defines the low temperature test, requires the test medium and temperature to represent the service condition, and requires the valve to be tested while cold and then checked again after it returns to ambient. MSS SP-134 covers the extended bonnet and the insulation requirements for low temperature valves. Shell and seat pressure tests are normally performed to API 598, and a helium mass spectrometer check is commonly used where a very low leakage rate is specified.
Applications
LNG storage, loading arms, transfer lines and vaporiser circuits, where tight shutoff of a two phase medium is required.
Air separation plants handling liquid nitrogen, liquid oxygen and liquid argon.
Liquid hydrogen and liquid helium facilities, where the lowest temperature classes and the strictest cleanliness apply.
Petrochemical and chemical plants with cryogenic recovery or liquefaction sections.
Research installations, cold boxes and test rigs that cycle repeatedly between ambient and low temperature.
In all of these duties the valve must do more than seal when cold: it must also open and close reliably after a long period of standing at temperature, and it must not build an ice plug at the stem.
Installation and Operating Points
Orientation and bonnet. The valve is normally installed with the bonnet upright or at an angle that allows liquid to run away from the packing area. On cryogenic lines the bonnet is often insulated together with the pipe; the extension length must be chosen so that the packing still sits clear of the insulation.
Thermal contraction. Piping and supports should allow for contraction of the cold line, and the valve should not be used as an anchor. Long cold lines are commonly fitted with flexible loops or bellows adjacent to the valve.
Operation. Cryogenic valves should be cycled through their full travel at commissioning and during maintenance checks, because polymer seats and packing can take a set at low temperature. Throttling duty should stay within the range where the disc is stable, to avoid vibration damage to the seat.
Purge and drain. Where a cryogenic line is taken out of service, provision must be made to depressurise, purge with dry gas and allow safe warming, so that liquid trapped in the body does not vaporise and over pressurise it.
FAQ
Q: Why does a cryogenic valve need an extended bonnet?
The extension keeps the stem seal and the operating mechanism away from the cold body, so that the packing stays above the frost line and remains elastic, and so that the cold pipe can be insulated without burying the packing in the insulation.
Q: Which body materials are used for cryogenic globe valves?
Austenitic stainless steel grades such as CF8M, CF3M and their forged equivalents, because they retain impact toughness at low temperature. Carbon steel becomes brittle and is limited to services within its tested low temperature range.
Q: What does BS 6364 require?
It specifies the low temperature testing of valves for cryogenic service, including the test medium and the requirement that the valve is proved tight while cold and then re-checked after returning to ambient temperature.
Q: Can a cryogenic globe valve be used for throttling?
Yes. The long stem travel and the disc to seat geometry give a controllable characteristic, which is one of the main reasons a globe valve is chosen over a quarter turn valve in cryogenic control duty.
Q: What has to be done before shipping a valve for oxygen service?
All components must be cleaned and degreased to an oxygen compatible standard, since residual hydrocarbon in an oxygen line is a fire risk. Documentation of the cleaning should accompany the valve.
Q: How is a cryogenic valve tested for leakage?
The shell and seat tests follow API 598 at ambient, and low temperature verification follows BS 6364. Where very low leakage is required, a helium mass spectrometer test is used in place of a water or air test.