A stuffing box provides a seal around a rotating shaft and also around a shaft that moves axially.
In some applications, the shaft rotates and moves axially (along its axis) in response to changes in operating conditions. A common example is the piston rod in a reciprocating pump or a hydraulic cylinder. In such cases, the stuffing box is designed to accommodate both the axial movement and the rotation of the shaft.
This differs from mechanical seals, which are good only with rotating members.
The “box” is a chamber cut into the stationary member surrounding the shaft or pipe. Often, a boss is provided on the casing or vessel wall to give a deeper chamber. A “boss” as shown in Figure 1 refers to a protrusion or raised area on the casing or vessel wall of a piece of equipment. This boss is intentionally added to create a deeper chamber within the equipment.
Simple Stuffing Box
The diagram of the simple stuffing box is shown in Figure 2. The annular space between the shaft and the wall of the chamber is filled with packing, consisting of a rope or rings of inert material containing a lubricant such as graphite. The packing, when compressed tightly around the shaft, keeps the fluid from passing out through the stuffing box and yet permits the shaft to turn or move back and forth. The packing is compressed by a follower ring, or gland, pressed into the box by a flanged cap or packing nut. The shaft must have a smooth surface so that it does not wear away the packing; even so, the pressure of the packing considerably increases the force required to move the shaft.
A stuffing box, even under ideal conditions, does not completely stop fluid from leaking out; in fact, when the box is operating properly, there should be small leakage. Otherwise, the wear on the packing and the power loss in the unlubricated stuffing box are excessive.
Stuffing Box with Lantern Gland
When the fluid is toxic or corrosive, means must be provided to prevent it from escaping from the equipment. This can be done by using a lantern gland, which may be looked upon as two stuffing boxes on the same shaft, with two sets of packing separated by a lantern ring as shown in Figure 3. The ring is H-shaped in cross-section, with holes drilled through the bar of the H in the direction perpendicular to the axis of the shaft. The wall of the chamber of the stuffing box carries a pipe that takes fluid to or away from the lantern ring. By applying vacuum to this pipe, any dangerous fluid that leaks through one set of packing rings is removed to a safe place before it can get to the second set. Or by forcing a harmless fluid such as water under high pressure into the lantern gland, it is possible to ensure that no dangerous fluid leaks out the exposed end of the stuffing box.
Summary
A stuffing box seals around a rotating or axially moving shaft, as seen in reciprocating pumps. It uses packing material to prevent fluid leakage while allowing shaft movement. Minor leakage is necessary to avoid wear. For toxic or corrosive fluids, a lantern gland design adds two sets of packing and a lantern ring to safely manage leaks, either by vacuum removal or injecting a harmless fluid.
FAQ’s
What are stuffing boxes?
Stuffing boxes are sealing devices used around rotating or axially moving shafts to prevent fluid leakage. They consist of a chamber filled with packing material, which can be compressed to create a seal while allowing the shaft to turn or move. Stuffing boxes are common in applications like pumps and hydraulic cylinders. There are variations, such as lantern glands, which are designed for toxic or corrosive fluids, providing additional safety by managing potential leaks.
What are the function of stuffing box?
Sealing: Prevents fluid leakage around a rotating or axially moving shaft.
Accommodating Movement: Allows for rotational and axial movement of the shaft without compromising the seal.
Pressure Maintenance: Maintains internal pressure within the equipment by preventing fluid escape.
Reducing Wear: Minimizes wear on the shaft and packing material through controlled leakage, which helps with lubrication.
Safety: In applications involving toxic or corrosive fluids, it prevents dangerous substances from escaping, ensuring safe operation.
Ease of Maintenance: Provides access for packing replacement and adjustments without needing to remove the entire assembly.
How stuffing box works?
A stuffing box works by creating a seal around a rotating or axially moving shaft. Here’s how it functions:
Chamber Structure: The stuffing box consists of a cylindrical chamber cut into a stationary housing surrounding the shaft.
Packing Material: The space between the shaft and the chamber wall is filled with packing material (like rope or rings) that may be lubricated with substances such as graphite.
Compression: A gland or follower ring is pressed against the packing material by a flanged cap or packing nut. This compression creates a tight seal around the shaft.
Movement Accommodation: As the shaft rotates or moves axially, the packing material deforms slightly, maintaining contact and preventing fluid from leaking out.
Controlled Leakage: Some leakage is typically allowed to ensure proper lubrication and reduce wear on the packing. This controlled leakage helps maintain a balance between sealing effectiveness and shaft movement.
Safety Features: In cases of toxic or corrosive fluids, additional features like a lantern gland may be used to manage leaks safely.
Overall, the stuffing box ensures efficient operation while minimizing fluid loss and maintaining safety in various applications.
