Anal. Chem. 1998, 70, 1652-1653
A Safety Pressure Relief Valve for Glass Systems C. A. M. Brenninkmeijer* and P. Bergamaschi
Air Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany
By fitting a low-friction vacuum O-ring valve with a stopping ring, which prevents the valve knob from engaging the thread on the glass barrel, a pressure relief valve for glass systems is realized. Glass systems are widely used for the separation of gases and vapors and for treating air-sensitive compounds. Several circumstances can lead to the occurrence of excess pressures in such systems. For instance, high pressure may be supplied via a gas bottle with a regulator, of which the outlet pressure is set too high, or by evaporation of a condensed phase, which can be particularly serious when cryogenic trapping has been used. Also, absorbents like molecular sieves and activated charcoal can lead to inadvertently building up high pressures. Furthermore, a leak in any system in which liquid nitrogen is used for trapping may lead to the liquification of air, which will boil off rather rapidly when the liquid nitrogen is removed. Over the years, the method of construction of glass systems has changed. Ground stopcocks have been phased out and replaced by O-ring taps. Also, tapered ground glass joints mostly have been replaced with O-ring seals in a ball joint or flange-type connection. As a consequence, today’s glass systems that the authors know to exist in many laboratories can withstand considerable excess pressures, estimated to be of the order of tens of bar. Glass, however, tends to break unpredictably due to its brittle nature and the frequent occurrence of tension inside the material. The latter can be due to mounting supports, incomplete annealing, or sudden temperature changes. For instance, the lowering of a dewar filled with liquid nitrogen from a trap will lead to increased tension and vulnerability to catastrophic breakage. Yet, probably, most systems would not break and release pressure at moderate levels but only yield when considerable pressures have been built up, with potentially serious consequences for those present. Usually, the operator will be close to the system when removing dewars with liquid nitrogen and may try to intervene when an unexpectedly large or sudden pressure rise occurs, thus being at risk. In metal systems, so-called bursting disks are sometimes incorporated. For glass systems, however, these disks, being in a metal housing, are not always suitable. Moreover, for glass vacuum systems, we would ideally have a device that yields at a low overpressure inside the system, without the risk that, during evacuation, the disk yields, letting air flow into the system. One option to make glass systems safer is to incorporate a ground glass joint. Nevertheless, even these may tend to stick under static conditions. A partial solution has been offered by the use 1652 Analytical Chemistry, Vol. 70, No. 8, April 15, 1998
Figure 1. A 9-mm-bore O-ring glass valve of a type with very low sliding friction (Louwers Hapert, Hapert, The Netherlands, part no. 402001), modified to act as safety pressure release valve. A stopping ring is screwed onto the glass barrel to prevent the knob from engaging the thread. With the lowermost Viton O-ring removed as shown, optimal rapid pressure release is assured. With the lowermost ring still present, the valve can be used as a normal valve by temporarily turning the stopping ring downward.
of spring-loaded valves. Automated valves1 that are presently available use vacuum or pressure for opening, while closing is achieved with a spring. Thus, these valves can help to release gases from one part of the system to another part, or to the outside. Concerned about the lack of intrinsic safety in some of our glass systems, we have devised a simple, satisfactory safety valve (Figure 1). It is based on an O-ring glass valve, of which the piston, being fitted with 2 Viton O-rings as seals to the outside air, can move with little resistance (Louwers Hapert, Hapert, The Netherlands). By cutting a ring from a spare plastic valve knob and screwing it on the glass thread, we can prevent the existing knob from engaging the thread on the glass barrel. When the pressure inside the system rises above 1 bar, the piston moves out of the barrel to release pressure. When the system is under partial or total vacuum, the piston is pushed into the barrel. We have tested this concept by applying excess pressure in a system and found that the valve opens readily when excess (1) Brenninkmeijer, C. A. M. Anal. Chem. 1985, 57, 960. S0003-2700(97)01262-6 CCC: $15.00
© 1998 American Chemical Society Published on Web 02/20/1998
pressures of typically 0.1 bar occur. Because of the small mass of the knob and plunger, also more sudden pressure bursts are released adequately. It is advisable to incorporate a bracket preventing the piston from totally ejecting from the barrel. In this way, the piston does not turn into a potentially dangerous projectile, and it returns into the barrel after the overpressure event. Because with the glass valves used here the barrel diameter increases slightly toward its end, pressure release already occurs during partial ejection. To further improve the safety valve, the lowermost O-ring can be removed, as shown in the figure. As inert high-vacuum grease for moving seals involving Viton (Dupont) O-rings, we have had the best experience with Fomblin greases (Monsanto RT 15), which provide, even after (2) Board on Chemical Science and Technology, National Research Council. Prudent Practices in the Laboratory; National Academy Press: Washington, DC, 1995.
long times, minimal friction. In case vacuum grease cannot be tolerated, low-friction O-rings made of Kalrez (Dupont) can be used. If the lowermost O-ring is not removed, the safety valve can still be used as a common valve at any time by turning the stopping ring fully downward. We have experimented with O-ring valves from other manufacturers with various success. We suggest testing valves for their suitability. In some cases, there was insufficient thread on the glass barrel to hold the stopping ring; in other instances, friction was rather high. For an extremely useful comprehensive guide to safe laboratory practices, the reader is referred to the National Research Council book, Prudent Practices in the Laboratory”.2 Received for review November 18, 1997. January 13, 1998.
Accepted
AC971262V
Analytical Chemistry, Vol. 70, No. 8, April 15, 1998
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