Gland Packing Technical Guide: Frequently Asked Questions
Q: What are the standard stuffing box dimensions for pump applications?
A: The stuffing box dimensions follow guidelines from ANSI B73 and ISO 3069 standards. The packing cross-section (W) is determined by the shaft outer diameter (OD) as follows:
- 16 mm < Shaft OD ≤ 29 mm: W = 8 mm
- 29 mm < Shaft OD ≤ 48 mm: W = 10 mm
- 48 mm < Shaft OD ≤ 75 mm: W = 13 mm
- 75 mm < Shaft OD ≤ 120 mm: W = 16 mm
- 120 mm < Shaft OD ≤ 300 mm: W = 19 mm
Note: These dimensions serve as guidelines. Always consult the packing manufacturer for specific requirements.
Q: What is the recommended total packing depth for rotating shafts?
A: Dimension A represents the total depth of packing including the lantern ring. When a lantern ring is used, the standard depth is 7W (7 times the packing cross-section). When the lantern ring is omitted, a depth of 5W is recommended.
Q: How should the lantern ring be positioned in the stuffing box?
A: The common practice is to position the lantern ring with 2W on the pressure side and 3W on the gland end. However, the reverse configuration (3W on the pressure side and 2W on the gland end) can also be used depending on the application. Consult your packing manufacturer for the optimal setup for your specific needs.
Q: What is the recommended gland take-up length?
A: The gland take-up or gland follower length (Dimension C) should be limited to 2W (2 cross-sections of packing). This limitation serves two purposes: it compensates for packings with the largest volume loss, and it prevents galling of the shafts. When complete take-up occurs before equipment damage, it indicates that packing replacement is needed. This design philosophy is based on the observation that most shaft damage occurs during the late stages of packing life.
Q: What are the specifications for the lantern ring (seal cage)?
A: The suggested depth or length of the lantern ring is set at 2W.
Q: What chamfer specifications are recommended for the stuffing box?
A: For easy entrance of packing into the stuffing box, the chamfer depth should follow standard machined dimensions for the given chamfer angle. The optimal wedging or guiding action is achieved with a chamfer angle between 15 and 30 degrees.
Q: What is the minimum gland entrance requirement?
A: A minimum of 1W should be maintained to minimize the probability of gland cocking and to accommodate general variations in packings, such as molded or soft packings.
Q: Are there special considerations for small shaft diameters?
A: Yes. For equipment with shaft diameters below 16 mm (5/8 inch), consult the individual packing manufacturer regarding the required packing space (W).
Q: What surface finish requirements apply to packing contact surfaces?
A: Surface finish quality directly impacts packing life expectancy. The key requirements are:
- Rotating elements: The finest economically feasible finish should be used — smoother finishes extend packing life.
- Valve stem surfaces: Must be free of scratches, pits, or voids deeper than 0.05 mm (0.002 inches). Finish should be 0.80 Ra μm (32 Ra μin) or smoother.
- Stuffing box bore: Finish should be 3.2 Ra μm (125 Ra μin) or smoother. Must be free of scratches, pits, or voids deeper than 0.015 mm (0.006 inches).
- Bore requirements: Must meet squareness to the bonnet gasket surface, circularity, taper, straightness, and concentricity with the bonnet stem hole within acceptable positional tolerance.
- Packing gland: Must also meet dimensional tolerances.
Q: What pressure range do these standard dimensions cover?
A: The standard dimensions provided are intended for use up to approximately 103 bars (1500 psi).
Q: How does high-speed operation affect packing performance?
A: Performance at high speeds (m/sec or FPM) is primarily a function of the packing material used, rather than the stuffing box dimensions. High-speed applications require special consideration and should be referred to individual packing manufacturers for specific recommendations.
Q: What clearance and finish standards should be followed?
A: Clearances should follow accepted machining practices, taking into account thermal expansion and contraction of metals. For valves specifically, ensure all dimensional and surface smoothness measurements for the stem and stuffing box are within specification, including stem OD tolerances, circularity, taper, and straightness requirements.