Between-Bearings Split-Case Pumps

Between-Bearings Split-Case Pumps — Sealing Overview

Between-bearings pumps support the impeller on a shaft carried by bearings at both ends. Single- and two-stage designs include axially split (API 610 BB1) and radially split (BB2) pumps, most often with a double-suction impeller. They move large flows of raw water, cooling water, seawater, fire water and pipeline products.

Each pump has two seals, one at the drive end and one at the non-drive end. With a double-suction impeller both seal chambers sit close to suction pressure, so the seals see far less pressure than the discharge gauge suggests. Because the shaft is supported at both ends, deflection at the faces is low, and seal life is usually limited by the water quality, by bearing condition or by how the seals are fitted.

Key Sealing Challenges

• Two seals per pump: both must be replaced and set correctly; a mis-set non-drive-end seal is a common cause of repeat failures.
• Downtime: replacing a solid seal on a large split-case pump means uncoupling the motor and often lifting the top casing. A split seal can be fitted around the shaft instead.
• Sand and silt: raw water and seawater carry abrasives into the seal chamber; hard faces and a clean external flush protect them.
• Bearing contamination: leakage and wash-down water reach the bearing housings. Non-contacting bearing isolators keep the oil clean.

Fault Diagnosis Example

Symptom: Non-drive-end seal on a split-case cooling-water pump fails repeatedly; the drive-end seal lasts.
  ├── Incorrect Response: "Change the seal supplier."
  ├── Diagnostic Evidence Required:
  │     ├── Check the seal setting length against the drawing; was the sleeve located after the impeller was centred?
  │     ├── Measure axial float at the thrust bearing.
  │     └── Inspect the faces for sand scoring and the flush line for blockage.
  └── Engineered Solution:
        ├── Centre the impeller first, then set both cartridge seals with their setting clips.
        ├── Replace worn thrust bearings to remove excess axial float.
        └── Use SiC vs SiC faces and a filtered Plan 32 flush on sandy water.

Frequently Asked Questions

What pressure does a split-case pump seal see?

With a double-suction impeller, both seal chambers are connected to the suction side, so each seal sees roughly suction pressure plus a small margin. Confirm the value from the pump maker before selecting the seal.

When is a split mechanical seal worth the extra cost?

When the time and labour to uncouple the motor, lift the casing or re-align the drive outweigh the seal price. This is typical on large cooling-water, circulating-water and fire-water pumps.

Do both seals on a between-bearings pump need the same arrangement?

Usually yes, because both chambers see similar conditions. Rotation is opposite relative to each seal, so seals with direction-sensitive pumping features must be ordered for the correct hand.

Related: Overhung End-Suction Pumps · Multistage & Boiler Feed Pumps · Municipal Water & Sewage · Pipelines, Terminals & LPG · All process pump solutions.

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Send your pump make and model, fluid, temperature, pressure, shaft size and speed to info@gipfelengg.com or use our enquiry form. Gipfel Engineering (ISO 9001:2015, NSIC-registered MSME, Mumbai) designs and manufactures mechanical seals, seal support systems and process pumps.