Overhung End-Suction Pumps

Overhung End-Suction Pumps — Sealing Overview

Overhung pumps carry the impeller on the end of a shaft that projects beyond the bearings. They are the most common process pump: ISO 2858 / ISO 5199 chemical pumps, EN 733 water pumps, ANSI B73.1 pumps and API 610 OH1, OH2 and OH3 pumps. Because the impeller is cantilevered, shaft deflection at the seal faces rises sharply when the pump runs away from its best efficiency point, and that deflection, not the fluid alone, is a leading cause of seal failure on these pumps.

The seal chamber sits directly behind the impeller. Its pressure is set by the impeller back vanes or balance holes and is usually close to suction pressure plus a fraction of the differential head. Size the seal for that seal-chamber pressure, not the discharge pressure, and confirm it from the pump maker or by measurement.

Key Sealing Challenges

• Shaft deflection: API 610 limits deflection at the primary seal faces to 50 µm (0.002 in). ANSI B73.1 pumps use a shaft flexibility index L³/D⁴ below 60 in⁻¹ (about 2.4 mm⁻¹). Running at low flow pushes the shaft beyond these values.
• Seal-chamber geometry: older pumps have narrow packing boxes; modern pumps have large cylindrical or tapered bores. Cartridge seals must match the bore, gland stud pattern and the distance to the first obstruction.
• Vapour and solids: a cylindrical chamber needs a flush to remove heat and vapour; a tapered bore helps solids and vapour leave the faces but should not be combined with a flush that disturbs its flow pattern.
• Packing conversion: many older pumps still run gland packing. A cartridge seal on its own sleeve removes shaft-sleeve wear and leakage.

Fault Diagnosis Example

Symptom: Seal on an end-suction chemical pump leaks within weeks; wear track on the stationary face is wide and off-centre.
  ├── Incorrect Response: "Fit a harder face material."
  ├── Diagnostic Evidence Required:
  │     ├── Where does the pump run on its curve? Long periods at low flow raise radial load and shaft deflection.
  │     ├── Measure shaft runout and end play at the seal chamber with a dial gauge.
  │     └── Check bearing condition and pipe strain on the casing.
  └── Engineered Solution:
        ├── Return the pump to its operating range (minimum-flow bypass or a trimmed impeller).
        ├── Correct runout, bearings and pipe strain before refitting.
        └── Fit a balanced cartridge seal with its own sleeve, set to the pump maker's dimensions.

Frequently Asked Questions

Can a packed overhung pump be converted to a mechanical seal without machining?

Usually yes. A cartridge seal is designed to the existing stuffing-box bore, depth and gland studs. Where the bore is too narrow for a cartridge, a component seal or a pump-specific design is used. Send the stuffing-box dimensions or the pump make and model.

Why does seal life drop when the pump runs at low flow?

Away from its best efficiency point the impeller sees an uneven radial load, which bends the cantilevered shaft. The faces then open and close every revolution, which causes leakage and face damage. The remedy is to restore the operating point, not only to change the seal.

When is a double seal needed on an end-suction pump?

When a leak would be toxic, flammable or environmentally unacceptable, or when the fluid cannot lubricate the faces. A pressurised double seal on Plan 53A keeps clean barrier fluid between the faces; an unpressurised tandem seal on Plan 52 suits flashing hydrocarbons.

Related: Between-Bearings Split-Case Pumps · Multistage & Boiler Feed Pumps · Chemical Plants · 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.