7.4 Pump Maintenance, Packing & Mechanical Seals

Compression Packing Installation & Leakage Control

Compression packing (gland packing) is a traditional dynamic sealing method used to seal the rotating shaft sleeve passing through a pump stuffing box. Packing consists of flexible braided synthetic or natural fibers compressed axially by a gland follower, forcing the packing rings to expand radially outward against the stuffing box bore and radially inward against the shaft sleeve.

Packing Material Selection

• Synthetic/PTFE Impregnated Braids: Excellent chemical resistance for acids and alkalis at moderate shaft speeds.
• Flexible Graphite Braids: High thermal conductivity and self-lubricating capability for high-temperature condensate and hot oil applications (up to 500°C / 930°F).
• Aramid/Kevlar Braids: High tensile strength for heavy abrasive slurries and solids handling.

Step-by-Step Packing Installation Procedure

1. Stuffing Box Inspection & Cleaning: Remove all old packing rings using a flexible packing extractor hook. Inspect the renewable shaft sleeve for scoring, pitting, or grooves. Sleeve surface roughness must be smoothed to 0.4 µm (16 µin) Ra; replace scored sleeves to prevent rapid packing destruction.
2. Precision Ring Cutting: Wrap packing stock tightly around a spare mandrel or shaft sleeve of exact matching diameter. Cut individual rings using a razor-sharp packing cutter at a 45° skive joint (diagonal cut). Skive joints provide a superior overlapping seal compared to straight 90° butt joints under pressure.
3. Staggered Ring Insertion: Install packing rings into the stuffing box one ring at a time using a split tamping bushing. Seat each ring firmly against the bottom of the box. Stagger adjacent ring joints by 90° (for 4-ring boxes) or 120° (for 3 or 6-ring boxes) to prevent creating a continuous axial leak path.
4. Lantern Ring (Seal Cage) Alignment: Position the metallic H-shaped lantern ring directly beneath the external flush inlet port on the stuffing box. The lantern ring distributes clean external flush water evenly around the shaft, lubricating packing fibers and flushing abrasive solids away from the box.
5. Break-in Adjustment & Cooling Leakage Rate: Tighten gland follower nuts finger-tight initially. Start the pump driver and allow generous initial leakage. Adjust gland nuts gradually—turning nuts 1/6 turn (one flat) every 10 to 15 minutes—until establishing a controlled cooling leak rate of 40 to 60 drops per minute (2 -- 3 mL/min). Caution: Never tighten packing to zero leakage; unlubricated packing will scorch, score shaft sleeves, and seize the driver.

Mechanical Seal Principles & Construction

Mechanical seals are precision engineered sealing devices that replace compression packing, eliminating shaft sleeve wear and reducing process liquid leakage to unmeasurable levels (< 1 drop per hour).

Four Primary Components of a Mechanical Seal

1. Primary Seal Faces (Rotating & Stationary Rings): Two optically flat, lapped ring faces running perpendicular to the shaft axis. One face rotates with the shaft; the other remains stationary in the gland plate. Sealing occurs across a microscopic fluid film (1.0 -- 2.0 µm thick) separating the two faces.
2. Secondary Seals (Elastomers): O-rings, V-rings, or elastomeric bellows that prevent fluid bypass between the rotating face and shaft sleeve, and between the stationary face and gland plate. Common materials include Nitrile (NBR), Viton (FKM), EPDM, and Kalrez (FFKM).
3. Mechanical Face Loading (Springs): Single central coil springs, multiple small coil springs, or wave springs that exert continuous mechanical axial force on the primary faces to keep them closed when the pump is shut down.
4. Drive Linkage: Drive collar set screws, drive pins, or keys that lock the rotating seal components firmly to the shaft sleeve.

Primary Face Material Selection

Primary seal performance depends on matching rotating and stationary face metallurgy:

• Carbon-Graphite vs Silicon Carbide (SiC): Standard versatile combination for clean liquids. Resin-impregnated Carbon provides self-lubricating low-friction qualities; Silicon Carbide provides extreme hardness and thermal conductivity.
• Silicon Carbide vs Silicon Carbide (SiC vs SiC): Hard-on-hard combination specified for abrasive slurries, high pressures, and corrosive chemicals to resist particle embedding and face scratching.
• Tungsten Carbide (TC): Heavy, tough metallic carbide used where extreme shock loads or thermal stress would fracture silicon carbide.

Seal Configurations & API Flush Piping Plans

Mechanical Seal Arrangements

• Single Mechanical Seal: Consists of one set of primary faces. Suitable for benign, non-hazardous process fluids.
• Dual Pressurized (Double) Seal: Features two seal sets arranged back-to-back or face-to-face enclosing an intermediate barrier fluid chamber pressurized at 0.15 -- 0.20 MPa (15 -- 30 PSI) higher than stuffing box pressure. Barrier fluid leaks inward into process fluid, preventing toxic, flammable, or hazardous chemicals from escaping to atmosphere.
• Dual Unpressurized (Tandem) Seal: Features two seal sets arranged in series with a buffer fluid chamber maintained at atmospheric pressure. The outer seal acts as an automatic safety backup if the primary inner seal fails.

Standard API Flush Piping Plans (API 682)

Mechanical seal faces require continuous fluid circulation for cooling and heat dissipation:

API Plan 11 (Discharge Bypass)       API Plan 32 (External Clean Flush)    API Plan 53A (Pressurized Barrier)
   [Pump Discharge]                     [External Clean Source]                [Pressurized Reservoir]
          |                                        |                                      |
    (Orifice Restrictor)                 (Regulated Flush Line)                 (Barrier Fluid Lines)
          |                                        |                                      |
   [Seal Chamber Inlet]                 [Seal Chamber Inlet]                  [Dual Seal Barrier Port]

• API Plan 11 (Internal Discharge Bypass): Fluid is routed from pump discharge, through a flow-restricting orifice, into the seal chamber to cool faces, returning to the pump interior through the impeller back balance holes. Standard setup for clean liquids.
• API Plan 32 (External Clean Liquid Flush): Clean liquid injected into the seal chamber from an external supply at higher pressure than the stuffing box, flushing abrasive slurries away from seal faces.
• API Plan 53 (53A / 53B / 53C Pressurized Barrier): External barrier fluid reservoir continuously pressurized by nitrogen or a bladder, circulating barrier fluid through dual mechanical seals for toxic or hazardous fluid containment.

Pump Overhaul Precision Tolerances & Cartridge Seals

Mechanical seals are precision devices with primary faces lapped flat to within 2 to 3 helium light bands (0.6 µm / 0.000024"). Excessive shaft deflection or misalignment will instantly unseat seal faces, causing immediate leakage failure.

Mandatory Overhaul Precision Limits

Before installing a mechanical seal, millwrights must verify five mechanical tolerances using dial indicators:

1. Shaft Radial Runout: Mount dial indicator on pump casing with stylus resting on shaft sleeve surface; rotate shaft 360°. Total Indicator Reading (TIR) must not exceed 0.05 mm (0.002"). Higher runout causes dynamic face orbit and fluid leakage.
2. Shaft Axial Endplay (Float): Mount indicator on casing against shaft shoulder; push/pull shaft axially. Total float must not exceed 0.08 mm (0.003") to prevent spring over-compression or face unloading.
3. Stuffing Box Face Squareness: Mount indicator on rotating shaft with stylus resting on stuffing box mating face. TIR must be within 0.05 mm (0.002") per inch of bore diameter.
4. Stuffing Box Concentricity: Measure stuffing box bore eccentricity relative to shaft axis; TIR must be within 0.12 mm (0.005").
5. Shaft Surface Finish: Surface finish under elastomeric secondary seal O-rings must be smooth—0.4 µm (16 µin) Ra maximum.

Cartridge Seal Installation Rules

Cartridge mechanical seals assemble the rotating face, stationary face, sleeve, gland plate, and springs into a pre-set factory unit, eliminating field measurement errors.

• Critical Installation Rule: Cartridge seals feature brightly colored brass or stainless steel setting clips (spacers) that hold seal faces at exact working length during installation. Mechanics must bolt the gland plate securely to the pump housing and lock drive collar set screws onto the shaft sleeve BEFORE removing setting clips. Removing clips prematurely or forgetting to remove clips before shaft rotation will cause immediate face overload or clip destruction.