Multistage High-Pressure & Boiler Feed Pumps

Multistage High-Pressure & Boiler Feed Pumps — Sealing Overview

Multistage pumps build pressure through several impellers in series. Horizontal designs include axially split (API 610 BB3), ring-section (BB4) and double-casing barrel (BB5) pumps used for boiler feed, pipeline, water injection and high-pressure process duty. Vertical in-line multistage pumps serve pressure boosting, RO feed and small boiler feed.

The seal does not normally see discharge pressure. On horizontal pumps the suction-end seal sits at about suction pressure, and the discharge-end seal sits behind a balance drum or disc whose leak-off is piped back to suction, so it too runs close to suction pressure. Select the seal for that seal-chamber pressure and confirm it from the pump maker. Vertical in-line pumps are the exception: the seal is in the top chamber at or near discharge pressure.

Key Sealing Challenges

• Hot feedwater: feedwater has little lubricity and flashes easily. API Plan 23 uses a pumping ring to circulate seal-chamber water through a cooler, so only the small seal-chamber volume is cooled rather than a continuous flush.
• Speed and face loading: high-speed boiler feed pumps raise face surface speed; balanced seals with stationary springs keep face loading and heat generation under control.
• Thermal transients: starts, trips and warm-up cycles thermally shock faces and elastomers; SiC vs carbon faces tolerate this better than hard-vs-hard pairs.
• Balance-line problems: a worn balance drum raises leak-off flow and pressure, which raises the discharge-end seal-chamber pressure.

Fault Diagnosis Example

Symptom: Discharge-end seal on a boiler feed pump runs hot and fails; the suction-end seal is healthy.
  ├── Incorrect Response: "Fit a seal rated for full discharge pressure."
  ├── Diagnostic Evidence Required:
  │     ├── Measure balance leak-off pressure and flow; have they risen since the last overhaul?
  │     ├── Check the Plan 23 loop: was it vented at its high point, and is the cooler fouled?
  │     └── Inspect faces for heat checking and the elastomers for hardening.
  └── Engineered Solution:
        ├── Restore balance drum clearances to bring seal-chamber pressure back to design.
        ├── Vent and clean the Plan 23 loop; keep cooling water on during standby.
        └── Use SiC vs carbon faces with peroxide-cured EPDM or FEPM.

Frequently Asked Questions

Why does a boiler feed pump seal not need a discharge pressure rating?

Both seals sit in chambers connected to suction, one directly and one through the balance drum leak-off. Their pressure is close to suction pressure, which is far below discharge. The seal still needs the right rating for that pressure, temperature and speed.

Why is Plan 23 preferred over Plan 21 on hot water?

Plan 21 cools a continuous flush taken from discharge, which wastes heat and needs a large cooler. Plan 23 recirculates only the seal-chamber water through the cooler, so the cooler duty is much smaller and the seal runs cooler.

What is the main cause of seal failure on vertical multistage pumps?

Dry running, usually because the top chamber was not vented before start-up or the pump ran with a closed suction valve. The faces can be damaged within seconds. Vent the pump before every start after draining.

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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.