Valves, cryogenic

DATA BOUNDARY 21 system/component categories such as cryogenic valves,... 

The gas chromatograph has an oven with sufficient volume to hold the column easily and which can be heated to the desired temperature (between 40 and 450 °C, stabilised to within 0.1 °C). The atmosphere inside the oven, which usually has a very small thermal inertia, is constantly agitated by forced ventilation. Using a cryogenic valve through which nitrogen or carbon dioxide can be introduced, the oven can be regulated at low temperatures. 

For noncold-box applications, a cryogenic valve with an extension bonnet is used (Figure 2.76), allowing the packing box area of the control valve to be warmed by ambient temperatures, thus preventing frost from forming on the steam and packing box areas. 

When testing cryogenic valves for shutoff or in hydrostatic tests, the use of water-based tests should be avoided. If water tests were conducted, it is possible that moisture can be trapped inside the body or extension bonnet, which could ultimately form ice in the valve. For these types of applications, in order to prevent freezing, the proper test medium is usually helium. 

The construction of a cryogenic valve with extended bonnet. (Courtesy of Fisher Controls.)... 

The gas chromatograph comprises an oven with sufficient volume to hold one or two columns easily and which can heat up to more than 400 °C. A weak thermal inertia permits a rapid but controlled temperature climb (gradient able to attain 100 °C/min). The temperature must be controlled to within 0.1 °C in order to get reproducible separations in isothermal or temperature programmed modes. By installation of a cryogenic valve fed with N2 or CO2 in the liquid state, the oven can be regulated at low temperature. 

Austenitic Fe-Mn alloys are also under study at the Central Research Laboratories of the Daido Steel Company in a project whose objective is the development of castable alloys for cryogenic valves Basic research portions of this project have been published ]. The results published to date suggest that Mn content should be high to ensure low-temperature toughness and that the Cr content should be high to stabilize austenite and improve ancillary properties, but not so high (—13% Cr) that 5-ferrite is retained. Alloys of composition near 25 Mn-10 Cr are currently under investigation. 

All valves are pneumatic ball valves, and special cryogenic valves (Meca-inox) were used at those positions where low temperatures were prevailing. Signal processing and process control was carried out by NI Labview. A picture of the experimental set-up is shown in Figure 2.18. 

In modem gas chromatographs the temperature controller of a column oven is a microprocessor incorporated into a feedback loop, allowing both temperature programming ramp profiles and isothermal heating to be accomplished accurately and reproducibly. Under microprocessor control, a flap or door movement permits the blending of the proper amount of ambient lab air with oven air in the control of oven temperature. In addition, a cryogenic valve can be opened by a microprocessor for delivery of carbon dioxide or liquid nitrogen in the column oven. 

Most gas ehromatographs have the capability to operate the column oven at subambient temperatures. An accessory kit is available for either liquid nitrogen (—99°C) or carbon dioxide (—40°C) as a coolant and includes a cryogenic valve that is mieroprocessor controlled. The valve opens and closes, depending on the demand for coolant. In the open position, coolant is sprayed into the oven, where it chills the oven down with assistance from forced-air convection. 

The atmosphere is drawn from the conditioned compartment by the blower. It then passes through the simulated equipment heat load, where its temperature is Increased to a maximum of about 200 F. The air then passes the point where the cryogenic liquid enters the recirculation duct. At this time it is cooled by the cryogenic mixture to the temperature required to keep the chamber at the desired temperature. Thepercentopeningof the cryogenic valve is determined by the control thermocouple which operates through the controller to nm the valve servomotor. If the compartment temperature increases above the set point, the valve opens. It closes on tempeature decrease. 

The principal uses of PCTFE plastics remain in the areas of aeronautical and space, electrical/electronics, cryogenic, chemical, and medical instmmentation industries. AppHcations include chemically resistant electrical insulation and components cryogenic seals, gaskets, valve seats (56,57) and liners instmment parts for medical and chemical equipment (58), and medical packaging fiber optic appHcations (see Fiber optics) seals for the petrochemical /oil industry and electrodes, sample containers, and column packing in analytical chemistry and equipment (59). 

W. Broadway, "A Pressure Sensitivity and Temperature Response Butterfly Valve for Cryogenic Service," paper presented at Energy Technology Conference and Exhibition, Houston, Nov. 5—9,1978. 

W. Broadway, "Development of Cryogenic Butterfly Valve Seat," paper presented atMJOE 71 st Mnnual Meeting Cyogenic Equipment Session, Miami, Nov. 12-16,1978. 

The Gifford-McMahon cryocooler consists of displacer, regenerator, compressor and intake/exhaust valves that can be staged to reach cryogenic temperatures. 

Expanders The primary function of cryogenic expansion equipment is the reduction of the temperature of the gas being expanded to provide needed refrigeration. The expansion of a fluid to produce refrigeration may be carried out in two distinct ways (1) in an expander where mechanical work is produced, and (2) in a Joule-Thomson valve where no work is produced. 

Jotile-Thomson Valves The principal function of a J-T valve is to obtain isenthalpic coohng of the gas flowing through the valve. These valves generally are needle-type valves modified for cryogenic operation. They are an important component in most refrigeration systems, particularly in the last stage of the liquefac tion process. Joule-Thomson valves also offer an attractive alternative to turboexpanders for small-scale gas-recovery applications. 

Carbon steel piping, valves, structurals and vessels operating at cryogenic and ambient temperatures. 

Figure 9-3 shows a typical cryogenic plant where the gas is cooled to -100°F to -150°F by expansion through a turbine or Joule-Thompson (J-T) valve. In this example liquids are separated from the iniei gas at 100 F and 1,000 psig. It is then dehydrated to less than I ppm water vapor to assure that hydrates will not form at the low temperatures encountered in the plant. Typically, a mole sieve dehydrator is used. 

Figure 4.1 Schematic diagram of a coupled column system. The first column (ID) is connected to the second column (2D) tlirough the interface or valve system. The interface can be a diiect coupling, a live T-union, a complex multiport valve, or a thermal or cryogenic modulation system. The stimulus can be the switching of the valve, abalancing pressure to divert flow towards 2D, an added flow that is used in pressure tuning, or the drive mechanism for the modulator. The line to detector 1 will normally be a non-retaining section of column. In a two-oven system, ID and 2D will be in different ovens the dotted line indicates separately heated zones.

Figure 13.3 Schematic diagram of the parallel cryogenic trap MDGC-IR-MS system A, splitless injection port B, RC-5 non-polar first-stage separation column C, HP 5970B MSD D, HP 5965B IRD E, four-poit two-way valve (300 °C maximum temperature) F, external auxiliary earner gas G, six-poit selection valve (300 °C maximum temperature) H, stainless-steel cryogenic caps I, tliree-poit two- way valve (300 °C maximum temperature) ...

There are many design features and styles of safety relief valves, such as flanged ends, screwed ends, valves fitted internally for corrosive service, high temperature service, cryogenic service/low temperatures, with bonnet or without, nozzle entrance or orifice entrance, and resistance to discharge piping strains on body. Yet most of these variations have little, if anything to do with the actual performance to relieve overpressure in a system/vessel. 

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