Purpose of Shaft Seals
Shaft seals are installed where the shaft extends through the pump casing to prevent leakage. On the pump suction side the seal prevents air ingress that would disrupt suction, while on the discharge side the seal prevents high-pressure liquid from leaking out.
Packing Seal (gland packing)
Structure
- Shaft sleeve: Protects the shaft from corrosion by the liquid and prevents direct contact and wear between the shaft and the packing.
- Stuffing box and packing: The stuffing box together with the packing separates the exterior from the pump interior and reduces leakage.
- Water seal: A water seal ring is installed inside the stuffing box and connected to an external water-seal pipe. Small holes and grooves around the ring form a water ring in operation, which prevents air ingress and provides lubrication and cooling to the packing and shaft sleeve to reduce wear.
Leakage Control and Installation
To minimize leakage, packing must be installed correctly and maintained during operation. Key steps:
- Thoroughly clean the stuffing box and inspect the shaft sleeve and stuffing box surfaces for damage or wear.
- Select packing of the specified type and size suitable for the conveyed fluid. Packing that is too small will leak.
- Use a sharp cutter when cutting packing. Cut the ends to form oblique joints of about 30 to 45 degrees, and ensure the cut faces are smooth. After installation the packing rings must form a complete circle without gaps or excessive overlap.
- When installing multiple rings, stagger adjacent ring joints by at least 90 degrees. For designs with water cooling, avoid aligning packing joints with the cooling water inlet and position the water-seal ring chamber opposite the inlet.
- After placing the last ring, fit the gland follower and tighten evenly until the packing is seated. Then loosen the follower and retighten to the appropriate torque. It is often best to initially leave the follower slightly loose, add water to the pump, then tighten the follower to allow a small controlled leakage. After startup adjust based on packing temperature and leakage rate to avoid excessive leakage or overheating.
- After tightening, verify that the clearance between the gland follower and shaft is uniform around the shaft, and that the follower applies even pressure to avoid rubbing on the shaft.
Inspection After Packing Installation
Check that the gland follower nuts are tightened to the proper force. Excessive tightening reduces leakage but increases friction between the packing and shaft sleeve, which can cause overheating, smoke, or destruction of packing and sleeve. Insufficient tightening results in excessive leakage. The tightening must be such that the liquid passing through the packing and sleeve forms a thin film to provide lubrication and cooling. Ideally a small continuous drip from the stuffing box should be maintained after startup. Adjust the follower after pump startup as needed.
Mechanical Seal
Forms and Working Principle
A mechanical seal is a non-packing, face-type seal that limits fluid leakage along a rotating shaft. It typically consists of a stationary ring, a rotating ring, an elastic (or magnetic) element, drive elements, and auxiliary sealing rings.
Sealing is achieved by press-fitting the rotating ring to the shaft and the stationary ring to the pump casing, using the elastic element and fluid pressure to press the ring faces together. A thin, flowing lubrication film maintained between the rotating and stationary faces by the seal fluid prevents direct contact and results in very low leakage. Auxiliary O-rings or seals address leakage paths between the stationary ring and the gland, between the rotating ring and shaft, and between the gland and casing.
Operation
The stationary ring is fixed in the gland by an O-ring and retained against rotation by an anti-rotation pin. The rotating ring is pressed against the stationary ring by the elastic element. The rotating ring is sealed to the shaft by a rotary O-ring and is driven by drive pins connected to the elastic element so it rotates with the shaft. When the shaft rotates, the driven rotating ring and stationary ring create relative rotational motion while maintaining tight face contact to achieve sealing.
Types of Mechanical Seals
- By seal faces: Single-face and double-face seals. Single-face seals are simple and used when the fluid is lubricating and small leakage is acceptable. Double-face seals use two pairs of faces and are applied when the fluid is non-lubricating, toxic, flammable, or when very low leakage is required. A barrier or buffer fluid at a pressure higher than the process is introduced between the faces for cooling and sealing. Double-face seals can be axial or radial.
- By balance: Balanced and unbalanced seals. Balanced seals reduce the pressure load on the faces, lowering friction and wear. They may be partially balanced or fully balanced and require a shaft or sleeve step, increasing cost. Unbalanced seals are simpler and commonly used when fluid pressure is below about 0.7 MPa.
- By spring arrangement: Internal-spring and external-spring seals. Internal-spring types place the spring in contact with the fluid and are more prone to corrosion and blockage; they are not recommended for high-viscosity fluids if the spring rotates with the shaft. External-spring types isolate the spring from the fluid and are suitable for corrosive, high-viscosity, or crystallizing fluids.
- By number of springs: Single-spring and multi-spring seals. Single-spring designs use thicker, corrosion-resistant wire and are less prone to particle buildup, but face loading may be uneven. Multi-spring designs provide more uniform face loading and easier adjustment of spring force, though individual spring wires are thinner and potentially less corrosion-resistant.
- By elastic element form: Rotary and stationary seals. Rotary seals have the elastic element rotate with the shaft and are widely used but can be affected by centrifugal forces at high speed. Stationary seals keep the elastic element stationary relative to the casing and are better suited to high-speed applications.
- By leakage direction relative to centrifugal force: Inward-flow and outward-flow seals. Inward-flow seals have leakage direction opposite to centrifugal force, resulting in smaller leakage and reliable sealing. Outward-flow seals leak in the same direction as centrifugal force and are sometimes used at very high speeds to improve face lubrication, but they are limited to moderate pressures, typically 1–2 MPa.
- By face contact mode: Contact and non-contact seals. Contact seals operate in boundary or mixed lubrication; they are simple with low leakage but higher wear, friction, and heat, which limits use at high speed and pressure. Non-contact seals operate with full fluid film lubrication; they have low heat and wear and can handle more severe conditions, though they leak more. Non-contact designs include hydrostatic and hydrodynamic seals. Hydrostatic non-contact seals use externally introduced pressure fluid or the process fluid itself to generate a supporting film. Hydrodynamic non-contact seals rely on relative rotation and face geometry, such as spiral-grooved faces, to generate the hydrodynamic film.
Other variants include bellows seals and single-, double-, or multi-stage seals.
