Disc-Stem Connections

Disc-Stem Connections... important factor in valve selection

In Gate Valves

In a gate valve, the sole function of the stem is to raise and lower the disc. In doing its job, the stem should not be subject to corollary stresses and strains of service conditions on the disc.

Thus, with gate valves, especially those used for higher pressure installations, a relatively loose disc-stem connection is desired.

If the connection were rigid, any side thrust on the disc caused by pressure and flow, would readily be transmitted to the stem, and tend to strain and possibly bend it.

A properly fitted loose connection relieves strain on the stem due to any lateral movement of the disc.

In Globe Valves

The stem in a globe valve not only raises and lowers the disc, but also must help guide it squarely to its seat.

Thus, unlike a gate valve, the globe valve disc-stem connection must be relatively close fitting to prevent any extreme lateral motion of the disc that would cause it to cock and seat improperly.

But, once the disc and seat are joined, the disc must stop turning while seating is completed by the stem. This will avoid metal-to-metal friction between disc and seat that would be destructive to seating surfaces.

The solution to this need is a swivel action in globe valve disc-stem connections, which permits true and tight seating without damage to seating surfaces.
Variations in Stem Operations

Although in many valve applications the type of stem operation makes little or no difference, in other cases it can be important. A simple example of the latter is the need for a self-indicator to show open or closed position, as in the case of rising stem valve, or, conversely, the need for a non-rising stem valve because of lack of head room. This shows how stem operating designs are adapted to service needs.

Rising Stem with Outside Screw

On both valves shown here, whether opened or closed, the stem threads always remain outside the valve body. They are not subjected to corrosion, erosion, sediment, or any elements in the line fluid that might damage stem threads inside the valve body. Being outside, they can be lubricated easily when necessary.

Rising Stem with Inside Screw

This is the simplest and most common stem construction for gate, globe, and angle valves in the smaller sizes. Stem turns and rises on threads inside the valve. Position of handwheel indicates position of disc - opened or closed.

Non-rising Stem with Inside Screw

Generally used on gate valves only, this stem does not rise, but merely turns with handwheel. In turning, the stem threads raise or lower the disc. Since stem only rotates, packing wear is less. Ideal where head room is limited.

Sliding Stem is Often Useful

The sliding stem valve is useful where quick opening and closing are wanted. A lever takes the place of the handwheel, and stem threads are eliminated. Available in both gate and globe valves.

Stuffing Box Designs Featured on Valves

Stuffing box must effect a tight seal around the stem to retain pressure inside piping system. Stem must be tight without binding. Packing is subject to wear and must be periodically compressed and eventually replaced.

1. Packing Nut without Gland

Used on low-pressure and small-size valves. With wheel and packing nut removed, this type is easier to repack than ordinary gland type on valves with small diameter stem.

2. Packing Nut with Gland

Conventional type packing nut with loose gland. Gland has small lip at top edge so that it can be pried out with screwdriver tip if jammed all the way down.

3. Bolted Gland

Deep stuffing box with two-piece ball-type gland and flange with swing-type eye bolts. Construction maintains an even load on the packing and prevents binding on the stem even when the gland bolt nuts are pulled up unevenly.

4. Injection Type

Add new packing with the twist of a wrench, even under full rated line pressure, and with the disc in any position! No need to backseat the disc. The specially designed ball check valve eliminates possibility of packing extrusion. When the packing reservoir is empty, simply back out the adjustment screw and insert a new pack stick.

5. Lantern Type

Superior construction for larger-size high pressure-temperature valves. Cooling chamber with lantern spacer and three rings of packing below to wipe stem clean before it passes into the sealing rings above. Two-piece ball-type gland and flange with swing-type eye bolts.