The environment inside a wet FGD tower

Inside a limestone-gypsum wet FGD tower the flue gas is humid and close to saturation at tens of degrees Celsius, and the slurry contains limestone, gypsum, sulphate and chloride salts. Internal components see alternating mild acid and mild alkali with a high solids content, and erosion and deposition occur together.

Grating in an FGD system is mainly used for maintenance platforms, spray-level access, mist eliminator walkways and tower-top platforms. It does not sit in hot flue gas directly, but it meets slurry splash, condensed acid and wash water, and the concentration of those media varies with operation, so selection should use the worst case.

Compared with an ordinary chemical platform, the difficulty in FGD is change: pH moves with load, chloride moves with makeup water and coal, and components dry in air during shutdowns. The material must tolerate these swings, not just one design point.

Sulfuric acid dew point and condensed acid

SO3 in flue gas combines with water vapour to form sulfuric acid vapour, and when a surface is below the acid dew point the acid condenses on it. The dew point varies with SO3 and water vapour partial pressure and commonly falls above one hundred degrees Celsius; the actual value must be calculated from the gas composition, not replaced by a fixed temperature.

Inside the tower the saturated gas is usually below the inlet acid dew point, so condensed acid on walls, supports and grating is normal. The concentration depends on the condensation conditions and may range from very dilute to fairly strong; dilute and concentrated acid attack resin by different mechanisms, and both must be considered.

The grating itself sits close to the tower environment and is rarely the coldest surface, but metal supports, fasteners and penetrations can form cold bridges. Keep the grating away from cold surfaces, or insulate the cold bridge, so that acid condenses less near the grating.

Chloride and pitting risk

Chloride in an FGD system comes from makeup water and coal and concentrates in the slurry. Austenitic stainless steel is sensitive to pitting and crevice corrosion in chloride, 316L may not be enough at high temperature and high chloride, and duplex or higher grades cost more. One advantage of FRP is that it does not rely on a passive film and does not pit.

FRP is not immune to everything, however. Chloride itself has little effect on the resin; the real issue is the low pH and oxidising media that come with it, which speed up resin hydrolysis and fibre attack. Read the full medium composition, not the chloride figure alone.

Metal fasteners remain the weak point. The grating may be FRP while the clamps and bolts are stainless; if the fastener grade is too low, it fails first. Match fasteners to the corrosion class of the grating, or use FRP fasteners and isolating pads. Selection options are compared in the chemical-resistant grating range

Resin and barrier layer

The usual resins inside an FGD tower are isophthalic polyester and vinyl ester, with vinyl ester generally used where chloride is high, pH is low or temperature is high. Orthophthalic polyester is cheaper but has limited hydrolysis and acid resistance and does not suit long-term slurry contact.

The barrier layer matters more than the resin grade. A surface veil, chopped-strand mat and resin-rich layer form a barrier that keeps the medium out of the structural laminate; its thickness, resin content and cure decide whether it works. Hand-laid grating needs particular care against bubbles, dry spots and delamination.

Fillers and flame retardants affect resistance. Calcium carbonate filler reacts in acid, and aluminium hydroxide is unstable in strong acid; where acid resistance is needed, choose an acid-resistant filler or reduce filler content. Where fire performance is required, assess the flame-retardant system and the chemical-resistant system together. Resin and medium matching is covered in the FRP grating chemical resistance guide

Platforms, access and maintenance design

FGD platforms usually wrap around the tower, are narrow, and must avoid spray headers, mist eliminator wash pipes and manways. Walkway width, headroom and handrail height follow the project specification, and grating cuts and openings must stay clear of load bars.

Maintenance brings mist eliminator modules, spray nozzles and tools, so the platform should be checked for maintenance point loads, not just a uniform crowd. A point load acts on a small area and raises local deflection and panel stress; add a support or a bearing plate under the load where needed.

Tower platforms are wet and collect slurry, so slip resistance and drainage go together. Fall the platform, keep the grating free of water traps, and fit toe boards at handrail bases to stop tools and debris falling. Platform arrangements are shown in the industrial maintenance work platforms

Supports, fasteners and cut-edge sealing

Set support spacing from load and deflection, and in an FGD tower also allow for corrosion allowance and maintenance access. Steel supports need protection, FRP supports should match the grating system, and dissimilar materials must be isolated against galvanic corrosion.

Choose fastener grades for the chloride environment and install to the specified torque. Over-tightening crushes the grating; under-tightening lets the platform work loose underfoot, and both open a path for the medium. Check every fastener after installation and mark the checked ones.

Seal every cut, drilled hole and opening. Acid and chloride in the FGD environment wick along exposed fibre and cause delamination and loss of strength; the sealing resin must match the panel system and be fully cured before service. Factory sealing is usually better than site sealing, so prefabricate wherever possible.

Manufacture, inspection and installation

The manufacturing method affects chemical resistance. Moulded grating distributes fibre and resin fairly evenly and suits complex shapes; the barrier quality of hand-lay-up or pultruded grating depends more on process control. In every case, confirm resin content, cure and barrier thickness.

Inspection should cover appearance, dimensions, thickness and cure; where chemical resistance is critical, add a coupon check or Barcol hardness test as agreed. Panels with bubbles, dry spots, delamination or exposed fibre should not go into critical locations.

Install from the inside out and from the bottom up so that finished grating is not damaged by later welding and cutting. Protect the surface from welding sparks and dust, and after installation remove oil and metal particles so that local corrosion does not start.

Operation, maintenance and replacement

After commissioning, inspect the grating periodically for blistering, cracks, whitening and exposed fibre, check fasteners for corrosion and looseness, and check supports for distortion. Tower shutdowns are the best inspection window.

Local damage can be repaired with matching resin, but assess the depth and extent first; where fibre is broken or the delaminated area is large, replace the panel rather than hiding a structural problem with a patch.

Keep spares for vulnerable locations, in the same resin system and colour as the originals. Record the batch and installed location when replacing, so that later faults can be traced. Long-term operating records say more than a single inspection, so keep a log.

• Select resin for the worst medium combination, not the normal one.
• Keep the barrier intact and seal every cut and drilled hole.
• Match fasteners to the grating class and isolate contact faces.
• Inspect surfaces and supports at shutdown; replace damaged panels whole.