Flue gas desulfurization (FGD) systems remove SOโ from coal-fired power plant exhaust. The limestone slurry that absorbs SOโ is one of the most abrasive, corrosive, and scale-prone fluids in any power plant. FGD pumps fail frequently if not properly specified. This guide explains the FGD process, what makes slurry pumping difficult, and how to select pumps that deliver 5+ years of reliable service.
The FGD Process and Pumping Points
A typical wet limestone FGD system has multiple pump applications:
- Absorber recirculation pumps โ move 10-30% limestone slurry through spray headers. Largest pumps in the system, 5000-50000 mยณ/h.
- Slurry feed pumps โ transfer limestone slurry from ball mill to absorber. 15-25% solids.
- Gypsum slurry pumps โ move reaction product to dewatering filters. 20-30% solids, abrasive gypsum crystals.
- Wastewater pumps โ handle blowdown with chlorides, heavy metals, and suspended solids.
- Seal water and flush pumps โ provide clean water for mechanical seal flushes.
Why FGD Slurry Destroys Pumps
| Challenge | Cause | Effect on Pump |
|---|---|---|
| Abrasion | Limestone and gypsum particles (50-200 ฮผm) | Impeller and volute wear, efficiency loss, leakage |
| Corrosion | Chlorides (up to 50,000 ppm), low pH (4.5-5.5) | Pitting corrosion, stress corrosion cracking |
| Scaling | Gypsum precipitation on wetted surfaces | Flow reduction, imbalance, seal failure |
| Gas entrainment | Air in absorber sump, COโ release | Cavitation, vibration, loss of prime |
| Large particle size | Unmilled limestone, scale spalls | Impeller clogging, shaft breakage |
Pump Type Selection
Absorber Recirculation Pumps
- Type: Horizontal centrifugal with rubber-lined casing or high-chrome iron wet end
- Impeller: Closed or semi-open with 28% chrome iron (A532 Class III)
- Shaft: Duplex stainless steel (2205 or 2507) for corrosion resistance
- Seal: Mechanical seal with Plan 32 flush (clean water, 2 bar above seal chamber pressure)
- Motor: Medium voltage (6-10 kV) for large pumps, variable speed for turndown
For high-chloride systems (seawater FGD), specify heavy-duty slurry pump with super duplex wet end and rubber-lined casing.
Gypsum Slurry Pumps
- Type: Horizontal centrifugal slurry pump or vertical sump pump
- Material: 28% chrome iron or natural rubber lining
- Impeller: Open design to pass large particles and reduce clogging
- For filter feed: positive displacement pump (piston or hose) for precise dosing
Material Selection Guide
| Component | Material | Expected Life |
|---|---|---|
| Impeller | A532 Class III (28% Cr) | 18-36 months |
| Casing liner | Natural rubber or 28% Cr | 24-48 months |
| Shaft sleeve | 2205 duplex or ceramic | 36-60 months |
| Mechanical seal faces | Silicon carbide vs silicon carbide | 12-24 months |
| Wear plates | 28% Cr or rubber | 12-24 months |
Design Considerations
- Low speed operation: FGD slurry pumps should run at 600-900 RPM to reduce wear. Higher speed = exponentially higher abrasion rate.
- Oversized shaft: Deflection at seal face causes premature seal failure. Specify L3/D4 ratio below 60 (in inches).
- Seal flush: Plan 32 with clean water is mandatory. Flush water must be filtered to 50 ฮผm and regulated to 2 bar above seal pressure.
- Suction design: Minimum 5D straight pipe before pump suction to reduce swirl and gas entrainment.
- Standby pump: Every absorber recirculation pump should have a standby unit. FGD systems cannot operate with reduced spray flow for more than hours.
Procurement Recommendations
- Require wear life guarantee: minimum 18 months for impeller, 24 months for casing liner
- Specify material test reports (MTR) for all wetted components
- Require performance test on water and slurry (if possible) before shipment
- Include complete spare parts package: impeller, casing liner, wear plates, seal assembly, shaft sleeve
- Verify supplier has FGD reference installations with 5+ years operating history
- Specify baseplate with integral drain pan and grouting holes
Maintenance Best Practices
- Monitor vibration weekly โ rising vibration indicates impeller wear or imbalance from scaling
- Check seal flush flow daily โ reduced flow causes seal failure within hours
- Inspect impeller during planned outages โ measure vane thickness, replace when 50% worn
- Flush pump thoroughly after shutdown โ slurry left in pump sets like concrete
- Rotate standby pumps monthly to prevent shaft bowing and seal sticking
Related Industries
FGD slurry pumping technology applies to other abrasive slurry applications: mining tailings, phosphate processing, cement kiln dust scrubbing, and industrial wastewater with high solids content.