Coal-Fired Thermal Power Plant

Coal-Fired Thermal Power Plant — Process Overview

In a coal-fired (Rankine-cycle) station, demineralised feedwater is pumped to boiler pressure, converted to superheated steam, expanded through the turbine and condensed under vacuum before returning through the heaters to the deaerator. Auxiliary circuits handle cooling water, flue gas desulphurisation (FGD) slurry, ash disposal and start-up fuel oil. Each circuit sets a different sealing problem: low-lubricity hot water close to saturation, vacuum suction, abrasive slurry and heavy oil.

This page covers subcritical and supercritical pulverised-coal units. For bagasse and biomass boilers see Co-Generation & Biomass; for gas-turbine plants with heat recovery steam generators see Combined Cycle (CCGT).

Key Sealing Challenges

• Boiler feed pumps: feedwater at 145–185 °C close to its saturation temperature. The seal is sized for seal-chamber pressure and cooled with API Plan 23.
• Condensate extraction pumps: suction under vacuum, so air must not be drawn across the seal faces into the condenser.
• FGD and ash slurry: abrasive solids need hard-face pairs, external-spring designs and a clean flush or pressurised barrier.
• Heavy fuel oil: coking on the atmospheric side is controlled with an API Plan 62 steam quench.

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Root-Cause Failure Analysis — Field Example

Symptom: Boiler Feed Pump (BFP) seal shows intermittent carbon face blistering and popping sound during unit load changes.
  ├── Incorrect Response: "Install harder tungsten carbide faces."
  ├── Diagnostic Evidence Required:
  │     ├── Inspect carbon face under 50x magnification: look for crater-like pits (vapour trapped in carbon pores).
  │     ├── Review API Plan 23 cooler delta-T: is the cooler fouled or air-bound, letting seal-chamber temperature rise above the seal datasheet limit?
  │     └── Check deaerator pressure during turbine trip: did suction pressure fall towards vapour pressure?
  └── Engineered Solution:
        ├── Fine-grain, low-porosity electro-graphite carbon paired with reaction-bonded SiC.
        ├── Clean, vent and re-rate the Plan 23 loop; keep cooler outlet within the seal datasheet limit.
        └── Maintain the NPSH margin (NPSHa − NPSHr) recommended by the pump OEM during load transients.

Operational & Environmental Benefits

1. Lower heat loss with API Plan 23 vs. Plan 21: Plan 21 continuously cools a stream taken from pump discharge; Plan 23 cools only the small closed volume in the seal chamber. It removes far less heat from the feedwater cycle and needs a smaller cooler, which is why API 682 identifies Plan 23 as the preferred plan for hot-water services. Calculate site savings from the actual cooler duty.
2. Reduced demineralised-water loss: replacing packed glands on hot-water auxiliaries with mechanical seals cuts continuous leakage of treated water. Quantify only from site measurements.

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Frequently Asked Questions

Why is API Plan 23 preferred over API Plan 21 for boiler feed pumps?

Plan 23 recirculates only the water inside the seal chamber through a cooler using an integral pumping ring, while Plan 21 cools a continuous flow of hot discharge feedwater. Plan 23 therefore needs less cooling water and removes less heat from the cycle while keeping the seal faces below their vaporisation limit.

How is air ingress prevented in condensate extraction pumps (CEP)?

Where the seal chamber is under vacuum, a balanced cartridge seal is supplied with an API Plan 32 condensate injection to hold the chamber above atmospheric pressure, with a Plan 62 quench on the atmospheric side. Where air ingress is critical, a pressurised double seal with a Plan 53A barrier is used.

Which seal faces suit FGD limestone and gypsum slurry?

Sintered silicon carbide against sintered silicon carbide in a slurry cartridge with external springs, so the springs stay out of the slurry. Single seals use a Plan 32 clean water flush where dilution is acceptable; otherwise a pressurised double seal with a Plan 53A barrier at least 1.4 bar above seal-chamber pressure.

Related: Co-Generation & Biomass · Combined Cycle (CCGT) · Mining Slurry.

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Request a Seal Selection

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.

Equipment solutions: Boiler Feed & Multistage Pumps · Vertical & Sump Pumps · Slurry Pumps · Turbines.