ZGB 200 Slurry Pump: Effect of Chloride Concentration in FGD Slurry on Mechanical Seal Life – Material Selection and Flush Plan Optimization

Introduction

In power plant wet flue gas desulfurization (FGD) systems, the ZGB 200 slurry pump handles limestone/gypsum slurry containing high chloride concentrations – the primary cause of premature mechanical seal failure. Chloride ions, being highly reducing and penetrating, continuously attack metal components of the mechanical seal, causing pitting, crevice corrosion, and even stress corrosion cracking. As environmental regulations become stricter, chloride concentrations in FGD slurry are increasing, presenting unprecedented challenges to sealing systems.

At one power plant, the FGD circulation pump mechanical seal had a service life of only 2‑3 months, far below expectations. After optimization and retrofit, its life was extended to 2‑3 years. Based on extensive field service experience, Hebei Xingou Machinery Equipment Co., Ltd. analyzes the mechanism by which chloride concentration affects mechanical seal life and provides systematic upgrade solutions from the perspectives of material selection and flush plan optimization.

1. Source and Concentration Range of Chlorides in FGD Slurry

Wet FGD systems use limestone slurry to absorb sulfur dioxide from flue gas. During recirculation, chlorides continuously accumulate. Chlorides can inhibit chemical reactions in the absorption tower, alter pH, and reduce desulfurization efficiency. At high concentrations, they not only affect desulfurization performance but also cause severe equipment corrosion. Special anti‑corrosion measures are required for high‑chloride environments to ensure long‑term reliable operation.

Typical chloride concentration ranges in FGD slurry and material tolerance are as follows:

In typical FGD service for the ZGB 200 slurry pump, chloride concentration is around 20,000 ppm; at some plants, it can reach 60,000 ppm. At these levels, standard stainless steel is already severely threatened.

In FGD systems, solids content is typically 30%‑40%, operating temperature below 70°C, pressure below 0.8 MPa, and pH between 4 and 8. Chloride ions, being highly reducing, strongly attack mechanical seal materials, causing surface corrosion that can even penetrate the entire seal structure.

Under such severe conditions, the target design life for a mechanical seal should be 2‑3 years.

2. Mechanisms and Hazards of Chloride Corrosion on Mechanical Seals

2.1 Pitting – The Most Insidious “Internal Injury”

When chloride concentration exceeds the critical value for a stainless steel material, chlorides destroy the passive film on the steel surface, forming small localized corrosion pits. Once initiated, pitting rapidly develops inward, creating holes in metal components and leading to leakage through the seal chamber.

2.2 Crevice Corrosion – “Deadly Gaps” on Seal Faces

Tiny crevices exist between the rotating and stationary seal faces, in O‑ring grooves, etc. Chlorides accumulate in these crevices, causing localized acidification and accelerated corrosion. Crevice corrosion is difficult to detect during routine inspections and is often the direct cause of seal failure.

2.3 Stress Corrosion Cracking (SCC) – The Invisible “Crack Killer”

Under the combined effect of cyclic stress and a chloride environment, metal components may suffer SCC. Cracks initiate at the surface and propagate inward, eventually causing sudden fracture of the component, leading to abrupt seal failure and high downtime risk. For example, 22Cr duplex stainless steel effectively avoids chloride‑induced SCC below 150°C.

2.4 Galvanic Corrosion – “Electrochemical Attack” Between Dissimilar Metals

When different metals are used in the mechanical seal (e.g., a stainless steel rotating ring and a carbon steel gland), the conductive slurry medium creates a galvanic cell, accelerating corrosion of the less noble metal. When designing the seal chamber, direct contact between dissimilar metals should be avoided.

2.5 Thermal Cracking and Aging – “Synergistic Effect” of High Temperature and Corrosion

When the mechanical seal experiences interrupted cooling water, dry friction, or particle ingress, the seal face temperature rises sharply, causing radial cracks on the ring surface. At the same time, high temperature accelerates aging of elastomers such as O‑rings, causing them to harden and lose elasticity, ultimately leading to seal failure.

3. Material Selection Optimization

For the operating conditions of the ZGB 200 FGD pump, material selection for each mechanical seal component must consider chloride concentration, temperature, pH, and economics. Material selection, seal arrangement, flush plan, and installation procedure must be treated as an integrated system to ensure long‑term stable seal operation.

3.1 Seal Face Materials (Rotating and Stationary Rings)

For solid‑containing FGD slurry, the seal face material must have both high hardness and good corrosion resistance.

Recommendation: For ZGB 200 FGD pumps, use sintered silicon carbide (SSiC) for both rotating and stationary rings. SSiC has extremely high hardness and excellent corrosion resistance, capable of resisting both chloride attack and abrasive wear from solid particles.

3.2 Metal Component Materials (Springs, Gland, Shaft Sleeve, etc.)

Metal parts of the mechanical seal are the most vulnerable to chloride corrosion. Corrosion resistance in increasing order: 304 SS → 316L SS → Duplex SS (2205/2507) → Hastelloy C‑276.

Duplex stainless steel 2205 offers high mechanical strength and superior corrosion resistance. It outperforms 316L and is a cost‑effective choice for FGD slurry at typical chloride concentrations. Duplex 2205 can handle chloride levels up to 20,000 mg/L. For higher concentrations, higher‑grade materials like Hastelloy are required.

3.3 Secondary Seals (O‑Rings)

Standard ZGB 200 seals use FKM. When chloride exceeds 30,000 ppm or temperature exceeds 80°C, upgrade to FFKM. FFKM has demonstrated service life exceeding 3 years in acidic/alkaline chloride‑containing FGD slurry, making it a preferred upgrade over FKM.

3.4 Metal‑Free (All‑Polymer) Seals

For extremely severe, high‑chloride environments, consider completely metal‑free perfluoroelastomer seals. These contain no metal components (including springs), thus eliminating corrosion concerns entirely, with no degradation of sealing function. Although such seals have a higher initial cost, their long‑term total cost of ownership in extreme conditions is very favorable.

• Metal‑free construction: Eliminates galvanic corrosion and chloride pitting risk, and prevents metal ion contamination of the FGD slurry.

• All‑polymer seal components: Extend service life from 2‑3 months to 2‑3 years, reduce maintenance costs by over 80%, and significantly reduce unplanned downtime.

4. Flush Plan Optimization – Application of API Plan 54

High‑chloride slurry itself is unsuitable for directly flushing seal faces. Therefore, pressurized double mechanical seals + API Plan 54 barrier fluid system is the best choice for isolating corrosive slurry and extending seal life.

4.1 Why Choose API Plan 54?

API Plan 54 uses an externally pressurized barrier fluid system. It injects clean barrier fluid (e.g., demineralized water + glycol + corrosion inhibitor) into the double mechanical seal cavity via an external pressurized tank, maintaining barrier fluid pressure higher than the pump casing pressure. In ZGB 200 FGD pumps, Plan 54 helps solve both chloride corrosion and solid particle abrasion problems. The flush plan improves the seal environment by dissipating heat and adjusting pressure in the seal cavity.

Core advantage: API Plan 54 completely isolates high‑chloride slurry from mechanical seal metal parts, eliminating chloride attack on sensitive components such as springs and metal bellows, and preventing slurry crystallization or solid particles from clogging springs.

4.2 Plan 54 System Configuration Points

A high‑quality Plan 54 system requires attention to the following core components.

• Barrier fluid reservoir and accumulator: Use bladder‑ or piston‑type accumulators to maintain stable system pressure.

• Barrier fluid selection: Use deionized water or a dedicated seal fluid to prevent scaling and avoid galvanic corrosion due to abnormal conductivity. For zero‑leakage environmental compliance, biodegradable seal fluids are also available.

• Instrumentation and alarms (pressure/level switches): Immediately generate an alarm when system pressure or level deviates from allowable ranges.

• System materials: Since barrier fluid can also be contaminated by migrating chlorides, Plan 54 piping and valves should be at least 316L stainless steel. For extremely high chloride environments, upgrade to duplex stainless steel to avoid corrosion of the auxiliary system.

When external instrumentation or DCS detects a drop in barrier fluid pressure or level, trend analysis can provide early warning of inner or outer seal leakage, enabling predictive maintenance.

5. ZGB 200 Seal Upgrade Case Study

Background: A 2×600 MW power plant FGD system used a ZGB 200 absorber recirculation pump with a single mechanical seal and external flush water. Chloride concentration in the slurry was as high as 35,000 ppm. Seal life was less than 3 months, with frequent leakage causing equipment outages and environmental pollution.

Retrofit plan:

• Upgraded from single‑ to double mechanical seal

• Added an API Plan 54 pressurized barrier fluid system (external bladder accumulator, 316L piping)

• Upgraded seal faces to sintered silicon carbide (SSiC)

• Metal components upgraded to 2205 duplex stainless steel; springs to Hastelloy C‑276

• Barrier fluid: deionized water + glycol + corrosion inhibitor; nitrogen pre‑charge 0.6 MPa (casing pressure 0.35 MPa)

Results after retrofit: Zero leakage operation; no slurry crystallization in the seal cavity. Payback period: only 6 months.

6. Daily Monitoring and Maintenance

6.1 Chloride Concentration Control

In addition to equipment upgrades, controlling chloride concentration at the source is fundamental to extending equipment life. Key measures include:

• Increase wastewater discharge to keep chloride concentration within design limits (typically below 20,000 ppm).

• Perform chemical analysis of the slurry weekly.

• When chloride exceeds the threshold, increase blowdown (wastewater discharge) to reduce Cl⁻ and inert material content in the absorption tower slurry.

6.2 Routine Plan 54 Barrier Fluid System Checks

• Daily: Check pressure gauge on the barrier fluid reservoir; ensure pressure is stable at 2‑3 bar above casing pressure.

• Weekly: Observe barrier fluid level; a drop may indicate internal or external leakage.

• Monthly: Sample and analyze barrier fluid conductivity and pH; an increase in conductivity suggests slurry contamination.

• Quarterly: Calibrate pressure and level switches to confirm alarm functionality.

7. Conclusion

The service life of mechanical seals on ZGB 200 slurry pumps in FGD service is determined by two key factors: material selection and flush plan optimization. In the face of high‑chloride corrosion challenges, traditional single mechanical seals with external water flush are no longer adequate.

Through extensive FGD pump retrofit experience, Hebei Xingou Machinery Equipment Co., Ltd. has demonstrated that adopting a double mechanical seal with API Plan 54 pressurized barrier fluid system, combined with sintered silicon carbide faces, Hastelloy springs, and 2205 duplex stainless steel metal components, can extend mechanical seal life from 2‑3 months to 2‑3 years, while achieving zero‑leakage operation.

For users seeking a balance between environmental compliance and long‑term equipment reliability, this integrated solution is well worth considering.