A Cryogenic Valve Primer

by Phil Redenbarger, VP of Engineering, Technifab Products, Inc.,

It is remarkable but true that we’ve used valves to control the flow of liquids for thousands of years. The city of Pompeii’s water system, for example, utilized quarter-turn style valves that were in principle very similar to those in use today. Cryogenic valves, while unique, are similar to these age-old designs, only modified to accept extremely low temperatures. Many of the modifications in low temperature cryogenic valves resulted from work done by NASA when the US began space exploration in the 1960s.

NASA improved upon this research in the last decade, developing an improved gate valve for cryogenic service, and many manufacturers have also recently received patents for cryogenic valves, particularly in the medical field. Cryogenic valves are available from a number of quality suppliers, a list of which can be found in the Cold Facts Buyer’s Guide (https://csabg.org). This article discusses best practices and describes what to look for when selecting valves currently available for cryogenic fluid control, but first let’s refresh ourselves with regard to the basic valves used to control fluid flow.

Figure 1 Schematic of BS6364 Low temperature Valve Leak test. Image: Technifab

Figure 1 Schematic of BS6364 Low temperature Valve Leak test. Image: Technifab

Typical cryogenic valve styles include gate, ball and globe valves. Gate valves operate as the name implies, by raising and lowering a gate within the valve body so that flow of fluid can be turned on, slightly metered and turned off. The gate valve has the advantage of providing straight through flow when fully open, resulting in very low restriction and low pressure drops. Gate valves also typically open by means of a threaded linear actuator mechanism that slowly raises the gate as a handle is turned. This slow movement mitigates a rapid change in fluid flow and the rapid change in fluid momentum that can result in “water hammer.”

Water hammer involves the sudden stop of fluid flow resulting in inertial energy that is absorbed by mechanical deflections. The deflections can cause large acoustic disturbances (loud noise and vibration of the pipe system). At low pressure, gate valves tend to leak due to lack of force on sealing surfaces. Gate valve seals harden at low temperature, making them a poor choice for cryogenic applications when zero leakage is needed. Gate valves also have bolted joints that promote outgassing and small leaks at the very low pressures existing within a vacuum space.

Ball valves are designed to open quickly due to a quarter-turn, full open to full close action. This feature, however, promotes water hammer. Some of the valve’s positive features include low pressure drop when fully opened and trouble-free simplicity. Like the gate valve, they are nearly impossible to vacuum insulate.

Globe valves are generally a good choice for cryogenic applications since high forces can be placed on the seals as the seals are pushed into seats. This feature, when used with the right seal material, allows the globe valve to achieve zero leak under most cryogenic temperature conditions. In a similar fashion to the gate valve, water hammer is usually not a problem due to the globe valve’s slow close and open rate. Globe valves are also easy to vacuum insulate since the valve body can be cast as a single homogeneous part.

Best Practices when Selecting a Cryogenic Valve
Safety: Cryogenic valves are generally used to control the flow of cryogenic fluids such as liquid nitrogen, oxygen, argon, methane, helium, CO2 and hydrogen. Fluid temperature can be as low as -452°F for liquid helium and -320°F for liquid nitrogen, the most commonly used of the group. Safety is a big issue with cryogenic valves because of these low temperatures. The release of even small amounts of cryogenic fluid can result in the rapid freezing of human tissue, while the release of large amounts of liquid can result in the potential for asphyxiation as the fluid vaporizes and expands at room temperature, pushing oxygen out of the area.

Many manufacturers sell valves that are ASME Code compliant because of the safety issues associated with cryogenic valves. The ASME (American Society of Mechanical Engineers) publishes worldwide safety codes that outline requirements for the design, test, manufacture and installation of cryogenic valves, piping and pressure vessels. ASME introduced its first pressure vessel Code in 1914 to address the rising number of deaths resulting from pressure vessel explosions (steam boilers). The result of this initial code was a dramatic reduction in boiler failures despite increasing boiler pressures. It has been updated many times and adopted into law by many states, even though it is not mandatory. Kentucky, for example, requires all piping systems—and therein valves—to be ASME Code compliant.

The current ASME Code requires manufacturers to X-ray products to check for voids and to ensure that minimum wall thickness is maintained. Each valve must also be pressure/leak tested to minimize risk of in-service failure. A class 300 valve, for example, is pressure tested to 720 psig. The pressure containment materials, from which the valve is built, must have all source records placed on file with the valve manufacturer for full traceability and accountability by each supplier. Users who purchase an ASME Code compliant cryogenic valve receive a valve marked with the ASME Code B16.34 designation and a class number that defines its pressure rating. Such valves are also marked u with the manufacturer’s name, material type and valve size.

ASME compliant cryogenic valves additionally require the internal stem and components to have a redundant retaining mechanism so that the stem cannot be expelled from internal pressure when the stem packing is replaced.

Internal Leak Performance Assurance: Beyond safety, there are a number of application-specific secondary issues to consider. These vary in importance but the cold leak rate of a closed cryogenic valve is a prominent concern.

Closed valves are especially susceptible to leaks as cryogenic fluids are generally quite low on the viscosity scale. Liquid helium, for example, has a dynamic viscosity of .0031 while water at 68°F is 1.02. Many cryogenic valves also rely on a soft seal compressed against a metal seat to close the valve. This design depends upon deformation of the soft seal to fill in minor irregularities of the metal seat and thus close off all possible leak paths. In this case, however, a substantially large leak can form through a very small opening, when high pressure is combined with low viscosity fluid.

Soft seals are extremely important at cryogenic temperatures, but few polymers or metals work to form them. Many manufacturers use PCTFE (original trade name Kel-F) for cryogenic valve seals as it retains a small amount—about 1 percent—of ductility at cryogenic temperatures all the way down to near absolute zero. It also has the benefit of being LOX compatible.

Cryogenic valves may also require pneumatic actuation. Globe valves are usually the best choice under such circumstances as these valves are more forgiving of material shrink due to large temperature changes. A spring-to-close/air-to-open valve, for example, is closed when a spring exerts force on a globe valve seal, forcing it into a tapered seat. As the valve cools—and the materials shrink—the spring is able to move the seal into the seat by spring force without additional need for adjustment. The spring actuator can also compensate for the creep often generated under load. Globe valves do, however, create more pressure drop than many other valve styles but the ability to remain tightly closed usually offsets this deficiency. The issue here becomes the application: Do you want zero leak or do you want small pressure drop?

Users specifying a cryogenic valve for a low internal leak application can use the British Standard BS6364 cryogenic valve leak test to separate good and lesser valves (Figure 1, page 12). The test measures internal leak rate across the valve seat at cryogenic temperatures, using helium as the leak medium. In general, a user cools down the valve by immersing it in liquid nitrogen to a depth such that the level of the liquid covers at least the top of the valve body/bonnet joint. The valve is then cold soaked at the test temperature for at least 30 minutes before it is subjected to a seat pressure test in the normal flow direction at rated pressure. The pass/fail leakage rate measured at the flow meter should not exceed .1 in3/sec per inch of valve diameter (1.6mL/sec per inch of valve diameter).

Flow Capability: After determining a valve is safe, meets pressure requirements at operating temperature and has an acceptable internal leak rate, the last major issue is to determine how the valve will affect the overall pressure drop of the plumbing system. Nearly all valve manufacturers publish a flow coefficient (Cv value) for each offered valve. The Cv is most often a measured value that defines the flow rate vs. pressure drop for a given fluid through the valve. The Cv is the equivalent number of gallons of water at standard condition that will move through a passage with a 1 psig pressure drop.

This equation provides a method of calculation for pressure drop through a valve given the Cv and the specific gravity of the fluid passing through the valve where:CHART_Web

DP= pressure drop across the valve during flow condition in PSIG
Q = flow rate in gallons per minute
Cv = flow coefficient of the valve
G = specific gravity of fluid being flowed

Cryogenic Valve Insulation: Thermal insulation is a very important—yet often overlooked—consideration in the search for a good cryogenic valve. Some manufacturers offer cryogenic valves without insulation and rely on an extended stem to prevent freezing of the upper seals and packing. Freezing of the upper packing can cause the valve to become inoperable, and the handle can even become so cold it can harm an operator’s hand. The extended stem valve mitigates this by creating a longer than normal conduction path whereby less heat is transferred from the valve packing and handle into the cryogen flowing in the valve. The negative of this style valve is that while the handle may remain warm, heat is absorbed all along the bonnet and transferred into the cryogen, thus creating some two-phase fluid flow. Simply put, the valve has a relatively high heat leak.

Vacuum jacketed cryogenic valves are fully encased within an outer steel jacket and the space between the valve and jacket is vacuum evacuated, thus removing air, eliminating air conduction and convection and terminating heat transfer into the cryogen by these means. This style valve is usually wrapped in the vacuum space with a few layers of reflective aluminum foil interspersed with spacer material to prevent conduction through the foil layers.
The foil layers reflect infrared radiant heat waves and further improve the insulation of the cryogenic fluid. In this design, thin-wall, stainless steel valve stems minimize conductive heat leak into the cryogen. The annular space between the valve stem spacers and the valve stem enclosure is typically .002 inch or less to prevent convection and associated heat leak internal to the valve. This design provides the best thermal insulation and effectively limits the formation of two-phase flow in the cryogen due to heat leak.

To understand the importance of using a vacuum insulated valve in a cryogenic pipe system, let’s examine the arithmetic of heat leak. A typical one inch vacuum insulated pipe will have a heat leak rate of about .4 Btu per hour, per foot of pipe length when flowing cryogenic fluid. A non-insulated pipe will have a heat leak of about 575 Btu per hour per foot of pipe. Therefore, a 50 foot length of vacuum insulated pipe will have a heat leak of 20 Btu/hr. Let’s assume we insert a non-vacuum insulated valve that is the equivalent of two feet of non-vacuum insulated pipe into the pipe system. The heat leak rises to 20 Btu/hr plus 1150 Btu/hr = 1170 Btu per hour. While a vacuum insulated valve does little to increase overall heat leak, the insertion of the non-vacuum insulated valve raises the pipe system heat leak 59 times!

Other Features of Interest: Cryogenic valves may require optional features dictated by application. This includes, for example, flow plugs, which are in the most basic sense tapered seats that gradually increase the flow area of the valve as it opens, resulting in the ability to modulate flow.

I-to-P (current-to-pressure) and P-to-P (pressure-to-pressure) transducers can be installed to control pneumatic actuated cryogenic valves. The I-to-P controller converts an electronic signal to a pneumatic one, and thereafter sends it to the pneumatic valve actuator to control valve flow. This is useful in cryogenic devices such as phase separators that have a capacitance probe to monitor and a pneumatic actuated cryogenic inlet valve to control liquid level. The probe sends a 4 to 20 mA signal to the I-to-P positioner that in turn signals the pneumatic actuator to open and close the cryogenic valve. The P-to-P controller works in the same fashion except its input signal is pneumatic. A number of other options include limit switches and electropneumatic positioners, all similar to commonly used hydraulic valve accessories. www.technifab.com