3.2Stopcocks and Valves

Fig. 3.17 Plugs of the wrong size can prevent flow in a stopcock.

just as rocks are ground in lapidary work. The finer the grade of grinding compound, the smoother the finish on the ground (lapped) surface. The smoother the finish on the glass parts, the easier they will slide past each other when rotated. Joints can be used for high-vacuum purposes without the same degree of grinding because they are not rotated.

Vacuum stopcocks are not meant to be interchangeable. Each plug and barrel receive their final grind together, and therefore they are mated for life. To prevent mismatching a plug and barrel set, numbers and/or letters are inscribed for each stopcock on the handles of the plug and on the side of the barrels (see Sec. 3.3.1). Aside from alignment problems, cross-matched plugs and barrels of high-vacuum stopcocks may leak or jam when rotated.

A simple, basic two-way glass stopcock (shown in Fig. 3.17) will have a single hole drilled through a solid plug. The arms are straight and placed 180° from each other. The plug is typically held in place by a rubber washer or metal clip at the small end. This stopcock design is easy to make and is inexpensive. The usual problems that arise from this type of stopcock are as follows:

1. Dirt can collect on the grease and work its way into the plug's hole. From there the dirt can eventually wear a groove on the inside of the barrel. This groove, leading from one arm to another, can eventually become great enough to prevent a complete seal when the plug is rotated to the "off' position.

2. As stopcock grease becomes old and/or cold, it becomes brittle, and rotation can cause a "shear"-type break, allowing the plug to be free from the barrel.

Fig. 3.18 Standard and vacuum designs of an oblique two-way stopcock.

By setting the two arms oblique to each other on the barrel and having a hole drilled diagonally in the plug (Fig. 3.18), two advantages are achieved:

1.If the first of the above problems arises, the grooves will not line up with the opposite arm and potential leakage is reduced considerably.

2.This configuration allows only one position (in 360° of rotation) for material flow.

As mentioned in Sec. 1.1.5, which discussed different types of glass, glass expands or contracts with temperature. Thin glass does not expand (or contract) the same relative amount as thick glass under the same conditions. Because of this difference, all vacuum stopcocks have hollow plugs so that the plugs may expand and contract at the same relative amount as the barrels. Thus, when there is a drop in temperature, the plug will not loosen by contracting a greater amount than the barrel. Likewise, if there is a sharp rise in temperature, the plug will not jam within the barrel by expanding a greater amount within the barrel. This quality

Fig. 3.19 How to evacuate the vacuum bulb of a vacuum stopcock.

Offset design L design

Fig. 3.20 The offset and "L" design of two-way vacuum stopcocks.

can be critical if an experiment or environment has rapid temperature changes. Figure 3.18(a) shows a solid plug. The holes in solid plugs are drilled. Figure 3.18(b) shows a hollow plug. The hole in a hollow stopcock plug is a glass tube sealed obliquely within and later ground to fit the barrel. This inner tube provides the passageway for gases* through the hollow plug.

Oblique arm stopcocks are not immune from "shear" breaks caused by cold or old stopcock grease. Because an accidental rush of air into a vacuum system can cause damage to the system or destroy oxygen sensitive materials within the system, vacuum stopcocks have a design feature to prevent accidental removal of the plug from the barrel. The stopcock design shown in Fig. 3.19 demonstrates how a vacuum stopcock uses a vacuum to secure its plug from slipping out of the barrel. The bottom of the barrel is closed with a bulb [see Fig. 3.19(a)]. A hole is bored opposite the low opening of the oblique tube [see Fig. 3.19(b)]. Aligning the hole with the lower oblique arm of the stopcock [see Fig. 3.19(c)] creates a passage for the vacuum that holds the plug securely in the barrel. A 180° rotation from this position is the "open" position for this stopcock [see Fig. 3.19(d)]. The only way to separate the plug from the barrel is to release the vacuum from inside the stopcock by rotating the bored hole toward the lower oblique arm when no vacuum is present.

Figure 3.20 shows two different alignments of the same type of stopcock. This plug design is simpler, and therefore it is somewhat less expensive. There is no advantage of the "offset" design to the "L" design. The choice depends on how your system is built—that is, whether the "offset" design or the "L" design stopcock physically fits better into your given system or apparatus. With these stopcocks, the vacuum of the system holds the plug in continuously. However, once the system loses its vacuum, its ability to hold the plug is lost as well.

3.2.2 Teflon Stopcocks

Teflon stopcocks provide excellent alternatives to standard glass stopcocks because no grease is required. They can be used in distillation systems where organic solvents, UV radiation, or oxidizing gases would normally make using a glass stopcock impossible. This advantage does not come without some cost because Teflon stopcocks are generally more expensive than standard glass stop-

Vacuum stopcocks are not intended for liquid transport use.

Fig. 3.21 Proper orientation of the parts of a Teflon stopcock.

cocks. The Teflon stopcock design looks essentially the same as its glass counterpart. However, it is not possible to take a Teflon plug and use it with a standard stopcock because Teflon stopcocks are made with a 1:5 taper rather than the 1:10 taper of glass. Because Teflon flows, or "creeps," it is likely to stick at a 1:10 taper by flowing into the arm holes, but far less likely to stick at a 1:5 taper. Teflon stopcocks with a 1:5 taper follow ASTM guidelines and are known as Product Standard.

There was a period of time when some Teflon stopcocks were made with 1:7 tapers, but this line was discontinued. I mention this point because some of you may have found one of these stopcocks and are having difficulty finding a replacement plug or are frustrated by trying to fit an old 1:7 plug in a new 1:5 barrel. The taper variations are small and subtle, so it is easy to confuse the two. Because the 1:7 taper design is no longer made, it is recommended that you phase any pieces with that design out of your lab.

The Teflon stopcock has more pieces (see Fig. 3.21) than the glass stopcock, and often the pieces are assembled in the wrong order. Inserting the plug into the barrel provides no challenges, but the white and black washers invariably get inverted. Each piece serves a specific function for the successful operation of the Teflon stopcock: The white washer helps everything on the plug shaft rotate together when the plug is turned; the colored locking nut helps maintain a certain amount of tension on the plug; and finally, the black washer (O-ring) serves as a lock washer to prevent the locking nut from rotating itself off the threaded section of the plug.

If the lock washer is placed on the plug shaft before the white washer, the washer's friction will grab the surface of the barrel and tend to make everything past the black washer not rotate while the plug is turned, causing the plug to tighten when rotated clockwise and loosen when rotated counterclockwise.

The white washer has a flat side on the hole through the center to maintain its nonslip position on the plug. Occasionally, the flat spot on the white washer