PP, PTFE and ETFE Plastic Demisters

Plastic demisters are not simply “cheap substitutes.” PP, PTFE and ETFE are not subject to metallic chloride pitting or chloride stress-corrosion cracking, but they have their own chemical, thermal and mechanical degradation modes. A suitable polymer construction can outlast stainless in a defined service; an unsuitable grade or support arrangement can lose geometry under sustained load or an upset.

The temperature, time and load envelope

Neither metal nor polymer internals can be assumed to give reliable visual warning before functional failure. Polymer deformation is time-, load-, temperature- and chemistry-dependent: sustained self-weight, liquid holdup and drag can produce creep below a short-term test temperature, while an upset can accelerate softening or permanent distortion. A pad may look intact after cooling while no longer sealing at the wall.

That is why the temperature question for polymer internals is never “what is the normal operating temperature.” It is:

• Normal continuous temperature
• Design temperature
• Credible upset temperature, and for how long it can persist
• Whether steam-out, regeneration or hot-water washing will ever contact the internals

Steam-out, hot-water washing and start-up or regeneration cases are easy to omit. A pad selected only against normal scrubber temperature can be unsuitable for a hotter cleaning or upset case that was not stated in the enquiry.

PP (polypropylene)

PP is commonly considered in wet scrubbers and flue gas desulfurization (FGD) related service for corrosion resistance, low weight and cost. 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 and the complete temperature envelope.

Beyond temperature, review:

• Oxidant exposure. Strong oxidizers and some radical-forming conditions attack polyolefins; stabilizer packages differ between raw material grades.
• UV. Relevant for outdoor installations, open scrubber tops or long outdoor storage before installation.
• Mechanical loading. PP monofilament has lower stiffness and strength than typical steel wire. The support and hold-down arrangement must therefore limit unsupported spans and accommodate pad self-weight, liquid holdup and drag.

PP is a common candidate for wet FGD and scrubber service, not an automatic default; see demisters for FGD and power and wet scrubber mist eliminators.

PTFE

PTFE is considered for aggressive chemistry where PP and common stainless grades are unsuitable. It generally has broader chemical resistance than PP or ETFE and can retain useful properties at higher temperature than PP, but suitability still depends on the named grade, stress and environment.

The trade-offs are mechanical and commercial:

• Creep. PTFE deforms under sustained load, with rate depending on stress, time and temperature. A support and hold-down arrangement copied unchanged from a metal pad cannot be assumed adequate; spans, grid spacing and clamping need to be checked for the material and duty.
• Low strength and stiffness relative to metal and to ETFE, which limits section size and handling robustness.
• Cost and lead time are materially higher, and fabrication detail matters more.

ETFE

ETFE is another fluoropolymer option. Compared with PTFE grades it commonly offers higher strength, stiffness and abrasion resistance, and compared with PP grades it commonly offers a broader chemical and temperature range. The named grades and test basis still control. It can be considered for specialty monofilament requirements and for drawing-based replacement where the original internals specified ETFE.

ETFE enquiries may carry a named raw-material requirement, particularly for drawing-based replacement. Whether named or not, the specification discipline below is necessary because “ETFE” alone does not define the grade or monofilament.

How to specify a polymer monofilament so it is actually defined

“PP”, “PTFE” and “ETFE” are polymer families. None of them is a specification, and the property spread within a family is wide enough that a temperature limit quoted for the family is not a limit you can design to. Two pads both honestly described as “ETFE mesh” can differ in monofilament diameter, strength and service temperature.

A defined polymer monofilament specification names:

• The polymer type
• The raw material producer and its commercial designation
• The applicable continuous and upset design limits, established for the named raw material and finished assembly from supplier data, load, exposure duration and engineering assessment rather than copied from the polymer family
• The monofilament diameter, and any separate identification carried by the monofilament producer or the monofilament product itself
• Pad density and thickness, noting that polymer compressibility differs from metal
• The support grid and hardware material
• The traceability record required at shipment

Specifying and proving are separate stages. The specification is set in the RFQ and frozen in the quotation or PI as the manufacturing basis; the traceability record named in that PI is what proves it at shipment. Where a named raw material has been locked, a substitution is not a like-for-like swap — it requires agreement in advance, because price, availability, suitability and traceability all move with it. The same discipline is described for metals in demister pad materials selection.

Support design is different, not just weaker

Three differences can require drawing changes:

Creep under sustained load. Polymer mesh can accumulate time-dependent deformation under stress. Grid spacing that is suitable for a metal pad may allow sag in a polymer pad over time, and sag at the perimeter can create a bypass path.

Thermal expansion. Polymers expand substantially more than steel. A polymer pad in a steel vessel with steel grids sees differential movement across every thermal cycle, so wall clearance, section joints and clamping must accommodate it instead of fighting it.

Mixed-material assemblies. Polymer mesh does not undergo metallic corrosion, but metal grids, tie wires, clips and hold-downs remain exposed to the wet service and may control assembly life. Specify the whole bill of materials and check each exposed material against the chemistry and any electrical contact between dissimilar metals. See demister installation types for the support and segmentation arrangements.

Density and thickness do not transfer from metal

Do not carry a metal pad’s density target across to a polymer pad. Monofilament diameter, stiffness and compressibility are different, so the same nominal density describes a different structure with different open area, pressure drop and drainage behaviour. Either specify the polymer pad’s parameters on their own terms or state the duty and ask for them to be proposed. Background on the parameters themselves is in mesh density, thickness and HG/T 21618 basics.

Site rules that can rule a polymer out

Hot-work, fire-load and material-approval rules at the plant may restrict polymer internals regardless of process suitability, and some end users prohibit them in specific services. Confirm this with the plant before engineering around a polymer solution, because it is a cheap question that invalidates an otherwise good selection.

When not to use plastic

• The continuous, upset or cleaning case falls outside the established time-temperature-load design envelope for the named grade and assembly.
• Mechanical duty is severe: high velocity, slugging, vibration or heavy solids.
• The chemistry is oxidizing in a way that attacks the specific polymer.
• Site rules prohibit it.
• Fine-mist capture requirements point to a co-knit or fibre-bed solution rather than a plain polymer mesh; see co-knit high efficiency demisters.

Common mistakes

• Selecting on normal operating temperature and never asking about steam-out or start-up.
• Treating a polymer family name as a complete specification.
• Reusing a metal pad’s support and hold-down design unchanged.
• Specifying polymer mesh and leaving the metal grid material unexamined.
• Copying a metal density target onto a polymer pad.
• Deciding traceability requirements after the raw material has been purchased.

If a metal pad is corroding within weeks inside a scrubber, send the chemistry and full temperature envelope before defaulting to “thicker 316L” — but send the upset and cleaning cases with it, because those decide whether a polymer is available to you at all.