Valves frozen

Most reported accidents with hydrogen cyanide involve operators inhaling or being splashed witli liquid hydrogen cyanide. Some of tliese accidents are due to equipment failure (blocked lines, frozen valves, etc.), and some arc due to operator error. 

Feyzin, France 1966 Propane Frozen valve during sampling from storage sphere allowed a vapor cloud to form and explode killing 16, injuring 63 UVCE ... 

Figure 3 Thermal effect of a gas leak, a frozen valve...

No No flow Valve V-19 CKiSCsj 111- V. ipori/cT ml. , header jslijL Ckl/ frozen Loss of HF to B-1 process unkiuivi.li consequences No knotvn protection 1 Action item level of prot availaldc. m conseqiK- ... 

Less l ow flow Valve V-19 partially closed HF Vaporizer inlet header partially phijiv.ed/frozen Insufficient HF supply to B-1 process, consequences unknown No known protection 3 Same as Scenam) i... 

An operator had to drain water from a 1,200-m spherical storage vessel nearly full of propane (Figure 8-1). He opened valves A and B. When traces of oil showed that the draining was nearly complete, he shut A and then cracked it to complete the draining. No flow came. He opened A fully. The choke—presumably hydrate, a compound of water and a light hydrocarbon with a melting point above 0°C—cleared suddenly, and the operator and two other men were splashed with liquid. The handle came off valve A, and they could not get it back on. Valve B was frozen and could not be moved. Access was poor because the drain valves were immediately below the tank, which was only 1.4 m above the ground. 

No No flow Valve V-19 closed HF Vaporizer inlet header plugged/frozen Loss of HF to B-l process consequences unknown. No known protection. 1 Action Item Determine die level of protection available and potential consequences in B-l Wing. 

Frozen product 2, vial or the end of a shelf 3, open surface (FI) for the water vapor flow between 2 and 4 4, chamber wall 5, valve with an open area F2 6, condenser chamber 7, cooling and condensing surface in the condenser chamber having a surface of F3 8, vacuum pipe with the diameter d 9, stop valve 10, vacuum pipe with the length 1 (from 8 to II) 11, vacuum pump pjce, water vapor pressure at the sublimation front of the ice /, pressure in the vial pco, pressure in the condenser. 

The freeze valve Another simple valve which is extremely effective and especially useful when one needs to seal off a unit containing a volatile liquid from the vacuum line, is the freeze-valve. This is a glass U-tube of not more than 5 mm i.d. in the duct connecting the two parts which need to be separated. If some of the liquid is frozen into the U so that the tube is blocked, the low pressure side of the U can be sealed off without the necessity of freezing the bulk of the liquid on the other side. This technique can be especially useful when the flask containing the bulk liquid is very large or if... 

Freeze-Dried Samples. Solid Materials and Tissues. These are first cut into approximately 1-inch cubes, frozen on a Teflon cookie sheet in a freezer, and placed in 1200-ml. freeze-dry flasks to capacity. The flasks are attached to the freeze-dried (lyophilizer) manifold, the valves are opened to vacuum, and the flasks are evacuated. The water from the tissues is trapped on a condenser. The dry tissues (drying time about 2-3 days) are removed from the lyophilizer and compressed into thin-walled aluminum cans with a Carver Laboratory press fitted with a special die, at about 24,000 lb. pressure (total). From 150-250 grams of the dry material, representing 500-1000 grams of fresh tissue, can be packed into a single can. The cans are sealed with a hand sealer and set aside for counting. Samples can be removed from the cans at a later date for chemical analysis or beta-emitter analyses. 

Valve I is opened and the trimethylindium is sublimed, under static vacuum, into the 2-L flask (trap-trap distilled). A hot-air gun is used to assist this sublimation process and to ensure that all the trimethylindium is collected on, or near, the surface of the cold, or frozen, amine/petroleum spirit suspension. When all the trimethylindium is trapped into the 2-L flask, valve D is closed to seal the 2-L flask and the rest of the system is filled with air through valve H (make sure valve D is closed ). The liquid nitrogen Dewar is removed and the 2-L flask is disengaged from the rest of the system by careful disconnection between the adaptor housing valve D and the U tube. The top of this adaptor is plugged with a stopper. 

With the Teflon vacuum valve of the solvent container closed (and the vacuum line under dynamic vacuum), the contents are thoroughly frozen by placing a dewar holding liquid nitrogen around the solvent storage container. The liquid nitrogen level should not reach the Teflon vacuum valve (it should be at least 1 in below the valve). 

Fig. 9.8. Apparatus for the determination of molecular weights by vapor-pressure depression. The bottom of the differential manometer (D) may he frozen with Dry Ice to keep the mercury from sloshing when solvent is added or removed, or when the sample tube is attached. The solenoid used to stir the sample tube is described in Fig. 9.6. V and V are grease-free valves.

E. Vapor Pressures above Room Temperature. Since a volatile liquid will distill to the coldest point in an apparatus, it is necessary to thermostat the entire tensimeter system when vapor pressures are determined above room temperature. Two different designs are presented in Fig. 9.7 which meet this requirement alternatively an immersible glass Bourdon pressure transducer may be used. The apparatus in Fig. 9.7.b is suitable for the measurement of gas-phase equilibria as well as vapor pressures. The first and simplest design of the two (Fig. 9.7.a), called an isoteniscope,3 is operated in the following manner On a vacuum system, liquid is condensed into the terminal bulb. A few hundred torr of an inert gas is introduced, the valve is turned off, the apparatus removed from the vacuum system, and the frozen liquid is allowed to melt. The part of the liquid in the terminal bulb is now tipped into the lower U, and inert gas in the region between the bulb and the U is removed by gentle pumping on the system. 

Fig. 4 d. Valve type D Restriction is formed by frozen part of the liquid... 

Jannasch, H.W., Mcgill, P.R., Zdeblick, M., Erickson, J., Integrated micro-analyzers with frozen plug valves. Micro Total Analysis Systems, Proceedings 5th p7AS Symposium, Monterey, CA, Oct. 21-25, 2001, 529-530. 

Overpressure in cryogenic systems can also occur in a more subtle way. Vent lines without appropriate rain traps can collect rainwater. Which when frozen can block the line. Exhaust tubes on relief valves and burst disks likewise can become inoperable. Small-necked, open-mouth dewars can collect moisture from the air and freeze closed. Entrapment of cold liquids or gases can occur by freezing water or other condensables in some portion of the cold system. If this occurs in an unanticipated location, the relief valve or burst disk may be isolated and afford no protection. Such a situation usually arises from improper operating procedures and emphasizes the importance of good operating practices. 

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