What the paper mill environment is like

From stock preparation through pulping to the paper machine, almost every section of a paper mill has water. Walkways and platforms stay wet, pulp, filler and chemicals leave a slippery film when spilled, and wash water keeps washing it away, so a steel grating coating is hard to maintain.

The corrosive media are not of one kind either. Pulping uses alkali, bleaching uses chlorine and chlorine dioxide, the paper machine uses fillers and sizing agents, and cleaning often uses hot water and alkaline detergent. One panel may meet alkali, oxidising agents and abrasion at the same time.

FRP grating resists corrosion, is light and does not conduct, which suits wet-area platforms and walkways. Its tolerance has limits, though, and the wrong resin will fail early no matter how good the structure is.

Another characteristic of a paper mill is continuous production. Stops are expensive, and much maintenance can only be done during a planned or partial shutdown, so how quickly grating can be removed and whether spares fit directly affects the schedule.

Alkali, chlorine and bleaching media

Sodium hydroxide solution is common in pulping and cleaning, and the higher the concentration and temperature, the more clearly it attacks the resin. Sodium hypochlorite, chlorine dioxide and hydrogen peroxide are oxidising media that dull and chalk the surface of some polyester resins, exposing fibre over long exposure.

Vinyl ester resin usually tolerates alkali and oxidising media over a wider range than polyester and is common in bleaching and caustic recovery areas. Isophthalic polyester suits weak alkali and ordinary wash water, but needs care in hot concentrated caustic.

Combined media are harder to judge than a single one. Alternating alkali and oxidant, repeated temperature swings and wet-dry cycling all accelerate resin ageing. Provide the section, medium, concentration and temperature list to the resin supplier for confirmation rather than writing alkali resistant.

Pulp fibre and filler also abrade the grating surface, and where flow velocity is high, abrasion may appear before chemical attack.Resin selection for wet paper mill duty

Wet slip and biofilm

Slippery floors are a long-term problem in paper mills. Pulp falls on the floor or grating and forms a lubricating film with water, and an ordinary anti-slip pattern can be smoothed over within weeks.

Concave grating drains through grooves in the bearing bars and suits wet locations with less debris. A gritted surface bonds hard aggregate to the top and gives stronger slip resistance, suiting the paper machine, press section and areas where stock spills, but the grit traps dirt and needs more frequent washing.

Slip design must be read with the cleaning method. A high-pressure lance clears stock from the gaps but can also strip sealant and edge coating, and the wrong detergent leaves a soapy layer that is more slippery than before.

Ramps, stairs and platform edges carry the highest slip risk, and their slip class should be higher than that of a general walkway.Slip-resistant FRP grating surface options

Washdown, hot water and temperature

Paper mills wash frequently, and hot water and steam raise the grating surface temperature. Both mechanical properties and corrosion resistance fall as temperature rises, and near the resin heat distortion temperature the load capacity drops clearly.

Distinguish continuous service temperature from short washdown temperature. A short hot wash can be checked against the peak, but the peak is not the normal condition; where grating sits in a hot wet area for long periods, use a higher temperature resin and reduce design stress.

Repeated temperature change also causes expansion and contraction. Large areas fixed too rigidly tend to crack at edges and around fastener holes, and once the edge seal is broken corrosion starts there.

The chemistry of the wash water belongs in the selection conditions too. Acid and alkaline cleaners affect resin differently, and alternating them can neutralise on the surface and leave salt deposits. When the cleaning programme changes, recheck the resin tolerance.

Platforms, walkways and machine access

Walkways along a paper machine must let people pass, observe, and reach across. Width and grating opening positions follow the equipment layout, and enough maintenance space must be left near rolls, showers and drives.

Platforms in the wet end drip continuously, and where electrical cabinets or drives sit below the grating, the cover must shed water. A solid cover does that but traps pulp and condensation, so cleaning openings are needed.

Maintenance involves lifting and roll removal, so removable grating panels should be numbered and marked with their correct orientation. A panel replaced the wrong way round does not match the support beams and loses capacity.

The space under a paper machine is often full of piping and cable, and grating support beams have to coordinate with them. A support landing on a pipe or trunking may look fine at first, but over time it loads and distorts the pipe.

Stairs, guardrails and openings

Paper mill stairs are often steep and narrow, with water and stock on the treads. Tread slip resistance should be stronger than the walkway, nosings should be clearly marked, and handrails should run continuously to the platform.

Guardrails and toe boards matter just as much in wet areas. Toe boards stop tools and pulp lumps falling onto equipment or into the stock pit, and post fixings should avoid the open edge of the grating.

Platform openings and lifting holes need covers or guardrails, not a loose piece of grating laid over them. Grating at an opening edge needs a support beam and must not rely on the neighbouring panel to carry it.

Supports, fastening and installation

FRP grating in a paper mill usually sits on steel or concrete beams, with spacing from the load table. Steel beams in wet areas corrode too, and water collects between beam top and grating, so drainage and protection belong in the design.

Use 316 stainless fasteners in the number the maker requires. Prevent galvanic corrosion where they meet carbon steel supports, with insulating washers if needed, and do not over-tighten, since a crushed fastener hole is harder to find than a loose one.

• Process section, medium list and temperature range.
• Design load, span and cleaning method.
• Slip class, colour and cover or opening positions.
• Fastener material, edge sealing and acceptance criteria.

Seal every cut edge on site, especially where washing is frequent. The sealant must be compatible with the grating resin, and the surface must be dry and clean before application, or the coating will peel off in one piece.Grating applications in pulp and paper

One-line rule

For a paper mill, choose the resin from the medium and temperature, choose the surface from wet slip and cleaning, and protect service life with edge sealing and fasteners. What you save on material is usually paid back at the first shutdown wash.

For selection, a one-page note covering process section, medium, temperature and cleaning method is more useful than comparing prices in a catalogue. Clear information is what lets a maker propose a solution that survives washing and shutdowns.