Demister Pad Materials Selection

Material choice drives both lifetime and cost. Start from the process fluid, not from a catalogue default of “304 stainless” — and start from the mechanism that will attack the part, not from a list of alloy names.

The arithmetic that makes demisters different

This is an important mechanism behind premature demister failures and is easy to miss when vessel-wall corrosion practice is copied directly to fine wire.

Corrosion data are often reported as penetration rate, but whether a rate is acceptable depends on component geometry, required strength, service life, inspection interval and any stated corrosion allowance. A rate used for a pressure-retaining wall cannot be transferred directly to fine wire.

A knitted demister wire normally has no separately specified corrosion allowance. HG/T 21586-1998 Annex A specifies round wire of 0.19 mm and 0.23 mm for its mesh styles. For scale only, a 0.23 mm wire has a radius of about 0.115 mm; if it experienced uniform radial metal loss of 0.05 mm/year, the idealised time to consume that radius would be about 2.3 years. This is illustrative arithmetic, not an acceptable-rate criterion or a life prediction. Localised attack can fail a wire sooner, and strength can become inadequate before the section disappears.

So: an alloy that is entirely correct for the vessel can be the wrong choice for the internals. If the vessel material was chosen with an allowance, do not simply copy it into the demister without checking the rate against the wire diameter. This is also why a thicker pad is not a corrosion remedy — it adds wires of the same diameter, and they corrode at the same rate.

Selection sequence

1. List temperature, pressure and full chemistry, including credible upset cases and known contamination history.
2. Identify the mechanism you are defending against — pitting, stress corrosion cracking, general acid attack, crevice attack, galvanic coupling, thermal limit or erosion. Each points to a different remedy.
3. Check the expected rate against the wire diameter, not against a plate corrosion allowance.
4. Screen candidate materials against compatible corrosion data, service experience or testing, including the worst credible case and the uncertainty in the evidence. Then compare cost and confirm the grid, hardware and support material alongside the mesh.
5. Freeze the decision in the RFQ so it enters the quotation or PI as the manufacturing basis.

Mechanisms, not just materials

Chloride pitting and stress corrosion cracking (SCC). The classic failure of austenitic stainless in chloride-bearing hot vapour or brine mist. Pitting perforates fine wire quickly, and cracking can appear at knit crossings and clamped edges where residual stress concentrates. Duplex grades are commonly considered where chloride and temperature exceed comfortable 316L margins, at the cost of stricter fabrication discipline. See demisters for desalination and evaporator mist eliminators.

General acid attack. Concentration and temperature govern, and the relationship is often not monotonic — an alloy adequate at high concentration can fail at intermediate dilution. Sulphuric service is a category of its own; see sulfuric acid mist eliminators.

Crevice corrosion. A knitted pad contains many wire-to-wire contact points, plus wire-to-grid and wire-to-tie-wire contacts. Where the geometry retains a stagnant, oxygen-depleted or concentrated liquid film, those contacts can act as crevices. A material screened only on general corrosion data may still fail locally.

Galvanic coupling. Dissimilar mesh, grid, tie-wire and hold-down materials can form a galvanic couple when they are electrically connected through a conductive liquid film. Which member becomes anodic depends on the materials’ corrosion potentials in that actual environment; area ratio does not decide polarity. If the fine wire is the small anode coupled to a larger cathode, however, the area ratio can accelerate its section loss sharply. Specify the complete bill of materials and verify compatibility for the service rather than assuming the wire is always the sacrificial member.

Thermal and time-dependent deformation. For polymer internals, temperature interacts with sustained load, exposure time, chemistry and the named material grade. Loss of geometry can accelerate near a material’s application limit, but there is no universal single “cliff” for PP, PTFE or ETFE as families. See the polymer section below.

Erosion and mechanical fatigue. High velocity, slugging, vibration and repeated thermal cycling can break wire independently of chemistry. Coarser wire or a more open construction may improve mechanical tolerance in some designs, but the complete mesh geometry, supports and required separation duty must be checked together. Slugging duty upstream of rotating equipment is one example; see compressor suction scrubber demisters.

Common metal options

Galvanized and coated wire deserve a specific warning: coatings can be damaged by knitting, handling and installation, and a breached coating can concentrate attack at the exposed area. Suitability therefore depends on coating continuity after fabrication and on the service environment.

Polymers and fluoropolymers

Polymer internals are not simply cheap substitutes. A suitable named grade and support arrangement can outlast stainless in a defined chemistry, while sustained load or an upset outside the established design envelope can cause permanent deformation and loss of separation geometry.

• PP (polypropylene) — commonly considered in wet scrubbers and flue gas desulfurization (FGD) related duty. Do not convert a family-level melting point, Vicat value or heat-deflection temperature into a continuous demister service limit. Use the named raw-material grade’s application guidance and check sustained load, duration, chemistry, UV, oxidants and upset or cleaning temperatures.
• PTFE — for aggressive chemistry where PP and common stainless grades are unsuitable. Higher cost, and support grids and hold-downs must be designed for its mechanical behaviour, including creep.
• ETFE — available for specialty monofilament and drawing-based replacement requirements, with better mechanical properties than PTFE over a useful temperature range.
• Co-knit metal and fibre — can improve fine-mist capture relative to a specified plain-metal construction when the fibre, loading and operating point are suitable. The fibre must match temperature and chemistry independently of the wire. It is not automatically equivalent to a fibre-bed element. See co-knit high efficiency demisters.

For monofilament products, the polymer type is a family and not a specification. What has to be named is the polymer type together with the raw material producer, its commercial designation and the applicable temperature limits — plus the monofilament diameter, the support-grid material and the traceability record required. Where the monofilament producer or monofilament product carries its own separate identification, record that as well. Detail on the three polymers is in PP, PTFE and ETFE plastic demisters.

Read the corrosion morphology before you upgrade

If you are replacing a failed pad, photograph how it failed. The pattern points to different remedies, and an upgrade aimed at the wrong mechanism wastes money:

• Discrete pits or perforation with sound metal between — localised attack, which may be associated with chlorides, deposits, crevices or other service-specific conditions. Confirm the cause before selecting a higher-alloy, duplex or chemistry-control route.
• Uniform thinning across the wire — general corrosion. Rate is the issue; check whether the rate was ever acceptable for this wire diameter.
• Cracking, often at knit crossings or clamped edges — investigate stress-corrosion cracking, fatigue and embrittlement; any alloy or clamping change should follow the diagnosis.
• Attack concentrated at grid contacts and tie wires — investigate crevice and galvanic effects and verify the complete bill of materials rather than upgrading the mesh alone.
• Heavy deposits with apparently sound wire — confirms fouling is present but does not rule out under-deposit or crevice corrosion. Inspect again after controlled cleaning and use testing where the remaining section or alloy is uncertain. See demister fouling and clogging.

More on upgrade paths in demister corrosion upgrades, and on evidence collection in mist eliminator replacement without drawings.

HF and sour service notes

Do not select a demister material from the word “HF” alone. Concentration, water content, temperature, oxidizing impurities and upset cases can change the alloy decision completely, and the wrong choice in HF service fails quickly.

If the project specifies NACE MR0175 / ISO 15156, state the applicable edition and scope in the RFQ so that wire, grid and hardware materials and documentation can be matched to it. ISO 15156 addresses H₂S-containing environments in oil and gas production and natural-gas sweetening and is not necessarily applicable to refining or other downstream service. Requirements applied to the vessel do not automatically become contractual requirements for internals unless the order documents say so.

Specify now, prove at shipment

Two different things protect you and they happen at different times.

Specifying is your job at the RFQ stage: the alloy grade for metal wire and hardware, or the polymer type with raw material producer, commercial designation and temperature limit for monofilament products. It is then frozen in the quotation or PI as the manufacturing basis.

Proving happens at shipment, through the ordinary material or batch traceability record named in that PI, delivered as part of the quoted supply.

EN 10204 Type 3.1 or Type 3.2 documentation for metallic products, positive material identification and additional testing are optional upgrades. Raise them before material procurement and production so that traceability, mill availability, cost and lead time are confirmed in advance. Do not assume they can be added afterwards: the required source records or inspection participation may be unavailable, and Meshium does not promise retrospective certification.

Common mistakes

• Copying the vessel alloy into the internals without rechecking the rate against wire diameter.
• Treating a thicker or denser pad as a corrosion remedy.
• Specifying the mesh alloy and leaving grid, tie wires and hold-downs to the fabricator.
• Screening on general corrosion data alone when the real mechanism is crevice or galvanic attack.
• Selecting a polymer on normal operating temperature without checking the upset case.
• Naming only a polymer family such as “PTFE” or “ETFE” and calling it a specification.
• Deciding documentation requirements after production.

If a metal pad is corroding within weeks inside a scrubber, send the chemistry and temperature before defaulting to “thicker 316L”. If you are replacing a failed pad, send photographs of the corrosion morphology — pitting, uniform thinning and cracking each point to a different upgrade path.