PP Duct Installation: Support Spacing, Welding and Leak Testing

Key takeaways

1. Most PP duct failures in the first year are installation failures, not material failures: sag between supports, a cracked flange, a weld pinhole. The duct itself is usually the last thing at fault.
2. Support spacing for PP duct is set by deflection, not by a steel-duct table. It gets shorter as temperature rises, and it is never a straight look-up by diameter alone.
3. A PP duct that is clamped rigidly at every support cannot expand, and the force it develops is large — for a φ315 mm run the fully-restrained load reaches roughly 37 kN before creep relaxation. Expansion joints come first; the fixed point is positioned around them.
4. Hot gas welding has two temperatures that are constantly confused: the hot air leaving the gun (around 300 °C) and the temperature the material actually reaches (PP melts near 165 °C). Both figures are correct and they are not the same number.
5. Every welded and flanged joint must be leak tested before the duct is insulated, boxed in or hidden above a ceiling. The same repair costs several times more after concealment.
6. PP duct is notch-sensitive and expands about ten times as much as steel, so handling, storage and the installation datum temperature all belong in the method statement, not in a fitter’s judgement on the day.
7. The installation parameters on this page are our own fabrication and site data, published by diameter so a specification can be checked rather than guessed.

PP duct installation decides whether a ventilation system delivers its designed airflow for its whole service life or starts sagging, leaking and cracking inside the first year. The duct is rarely the weak point. On the systems we are called out to inspect, the material is almost always still sound: what has failed is a support that was spaced for steel, a flange that was bolted to a steel mating ring, or a weld that was laid down in a draught.

Polypropylene behaves unlike the materials most duct crews work with every day. It is roughly 60% lighter than steel at the same section, it moves about ten times as far when it warms up, it cannot be solvent-welded, and it is notch-sensitive, which means a dragged spool piece or a sharp-edged hanger can start a crack that only appears months later. None of that is difficult to manage. All of it has to be planned for before the first cut rather than corrected afterwards.

This guide runs through the full installation sequence in the order a site actually executes it: pre-installation checks, support spacing and hanger loads, thermal expansion and fixed points, welding and flanged connections, then leak testing and commissioning. The spacing, expansion and load figures are our own fabrication and installation data, given by diameter. If you need the material background first, start with our guide to polypropylene duct and come back.

What Makes PP Duct Installation Different

Where the material choice is still open when the run is being designed, the comparison that usually settles it is against galvanized steel, and it turns on condensation rather than on the installation itself. That is covered in our article on PP duct vs galvanized duct, and the support and expansion figures above apply once the answer is PP.

A crew that has spent years on galvanised steel duct arrives with three habits that are correct on metal and wrong on polypropylene. Each one is cheap to avoid and expensive to correct, and all three cause the failures that get reported as material problems.

Three steel habits that fail on PP

The first is rigid clamping. Steel duct is restrained hard at every hanger because its expansion is negligible and its stiffness is high. Clamp a PP duct the same way and it has nowhere to go when it warms up, so the movement turns into force. The duct buckles between supports or cracks around a flange bolt, and the crack appears at the stiffest point in the run, which is usually the connection to a fan or a scrubber.

The second is spacing by look-up table. Steel duct hanger spacing comes from a table indexed on diameter and gauge. PP needs the same nominal support density but the controlling variable is deflection under a material whose stiffness falls as it heats, so a run that is stable at 20 °C can sag visibly at 60 °C with the supports unchanged. Spacing has to come from a deflection limit at the operating temperature, not from a diameter alone.

The third is joining by mechanical fastening. Steel duct is stitched, bolted, or sealed with tape and mastic. PP duct cannot be solvent-cemented, has no room-temperature adhesive that works, and cannot be taped to a gas-tight standard. It is welded, or it is flanged with a gasket. Self-adhesive tape and sealant have no place in the specification of a chemical exhaust run.

The three things the installer actually controls

Everything that determines whether a PP duct installation lasts comes down to three decisions made on site. The datum temperature at which the duct is fixed is the first: expansion is measured from the temperature the duct was installed at, not from zero, so a run installed in a 35 °C afternoon behaves differently from the same run installed at 18 °C, and the method statement has to record which one applies.

The second is the restraint pattern: where the fixed points sit, which supports slide, and where the expansion joints land. This is a design output that the installer executes, and it cannot be improvised as the duct goes up. The third is weld quality, which is a function of surface preparation, air temperature, travel speed and whether the joint was shielded from draughts while it was laid down.

Temperature is the factor that drives most of this, and it is the same axis that separates PP from other duct materials. Our comparison of PP duct against PVC duct works through where each material’s ceiling sits and why expansion, not chemical resistance, is usually the deciding design input in a hot exhaust line.

Before the First Cut: Site Checks and Staging

The most expensive PP duct installation problems are created before any duct reaches the ceiling. Factory-fabricated spool pieces are cut and welded to an approved shop drawing; if the as-built building does not match the drawing the spools will not fit, and correcting a misalignment on site costs several times what a walk-down costs before the duct is unloaded.

Check the shop drawing against the as-built route

Walk the complete duct route with the drawing in hand before anything is lifted. Measure ceiling height to structural steel, column centrelines, the position of every equipment nozzle, and the clearance at each doorway or beam the run crosses. Two specific traps are worth naming. A ceiling height measured from slab rather than finished floor is consistently 50–150 mm optimistic, which is enough to lose the duct above a beam. And equipment nozzles are often positioned from the equipment centreline rather than from a building grid, so they drift when the equipment is finally set.

Mark the support positions on the structure at the same time, while the route is still accessible. Every support point needs an existing structural member to clamp to, or a bracket that has been designed and anchored. Deciding support positions later means drilling overhead next to a duct that is already in place.

Set the installation datum temperature

Expansion is calculated from the temperature at which the duct is fixed, so the datum has to be recorded rather than assumed. On an outdoor or semi-open installation the datum is the ambient temperature during fixing; indoors, it is the room temperature at the time. The number matters because the movement the system has to absorb is the difference between datum and operating temperature, not the operating temperature alone.

A duct installed at 34 °C in an unshaded yard and then exposed to a 60 °C exhaust stream moves through a 26 K range. The same duct installed at 18 °C in a shaded bay moves through 42 K. Same duct, same process, roughly 60% more movement to absorb, purely because of when it was installed. Record the datum in the installation record and size the expansion joints against the worst-case range the site can produce.

Handling and storage of PP spool pieces

Polypropylene is notch-sensitive: a sharp point loading the surface starts a crack that propagates later under stress. That changes handling rules in a way that catches crews used to steel. Do not drag spool pieces across a floor or over an edge; lift them with fabric slings or wide webbing rather than wire rope or chain; do not use the inside of the duct as a load-bearing surface; and do not stack heavy material on top of a spool.

Storage matters as much as handling. PP duct stored outdoors degrades under ultraviolet light, and thin-wall sections stored in summer sun can distort before they are ever installed. Keep spools under cover or under an opaque tarp, off the ground on timber bearers, with the ends open to air so condensation does not sit inside a sealed section. Wipe any contamination off mating surfaces before welding rather than after.

Tools and consumables the crew must have

PP duct installation uses a short list of equipment that a steel duct crew does not normally carry. The essentials are a hot gas welding gun with controllable air temperature in the 250–320 °C band, welding rod of the same grade and colour as the duct, a scraper or deburring tool for edge preparation, a fine-tooth saw or plastic-cutting blade, a soap solution and spray bottle for leak testing, and a torque wrench for flanged connections. Rod is stored sealed and dry: moisture absorbed by the rod vaporises in the weld and leaves porosity behind.

Before production welding starts, the gun should be checked against a surface thermometer rather than against its own dial. A gun that drifts 30 K low produces unmelted rod and a joint that looks welded but is not fused; one that drifts high scorches the material and destroys the surface chemistry that the weld depends on. The full picture of what a PP duct system is and how its parts fit together is set out in our guide to what a PP duct is.

Support Spacing and Hanger Loads for PP Duct

Support spacing is where the material choice shows up on site. FRP’s greater stiffness allows fewer supports over a longer span, but each one carries roughly three times the load, and the joint count moves the other way. Those figures are worked through in our article on PP duct vs FRP duct.

Support spacing is the single PP duct installation decision that most often separates a run that stays straight for fifteen years from one that sags visibly within a season. The rule is not a diameter look-up. Supports are spaced so the duct does not deflect beyond an acceptable limit under its own weight plus condensate, evaluated at the operating temperature rather than at ambient, because polypropylene loses stiffness as it warms.

Support spacing by diameter — our installation data

The table below is our own fabrication and site data, given as outer diameter, wall thickness, mass per metre, the maximum support spacing we specify, and the resulting hanger load at that spacing. The load column already includes a factor of 1.5 on the static weight to cover dynamic effects and any condensate or dust carry-over, so it can be used directly for hanger selection.

Two points about reading this table. The masses are calculated from the section geometry at a polypropylene density of 0.91 g/cm³, which is why they sit slightly above the nominal figures on a product page that has been rounded down. And the spacing column is a maximum for a straight horizontal run at the stated duty; it shortens where the duct carries a fitting, where it is exposed to higher temperature, or where a branch adds weight at one point. Spacers are never lengthened to save hangers on a chemical exhaust line, because the failure mode is a duct that bellies down and pools condensate at the low point.

How the hanger load is arrived at

The load on a hanger is the mass per metre multiplied by the spacing on either side of it, plus the factors above. A support carrying a straight run evenly is loaded by roughly one full span of duct, which is why the figures in the table read as they do: at 315 mm, 3.78 kg/m over a 2.5 m span gives 9.45 kg, and the 1.5 factor lifts it to about 14.2 kg. A hanger immediately under an elbow carries twice that because the run turns and both legs load it, and a hanger beside a heavy fitting should be checked against the fitting’s own mass.

That number is what selects the hanger hardware, and it is small. A 347 N load at 500 mm is comfortably within an M8 threaded rod, which means the sizing problem on a PP duct run is almost never the hanger, it is the structural attachment and the bearing surface against the duct. That distinction matters: an undersized hanger would fail visibly, whereas a hanger that is strong enough but bears on the duct over a narrow strip will cut into the wall over months of thermal cycling and vibration.

Fixed points, guides and sliding supports

A duct run needs exactly one fixed point per straight segment between expansion joints, and that point is the anchor from which thermal movement is measured. Everywhere else the duct must be free to move axially while being held laterally. A support that clamps the duct hard in both directions gives the run nowhere to go, and the movement converts to force against the fixed point and the nearest weld.

The pattern that works is: one fixed point, sliding or guide supports on the rest, and expansion joints positioned so each straight run between anchors moves into its own joint. Support saddles should be wide enough to spread the bearing load and should be lined or smooth so the duct can slide without abrasion, and they should never present a sharp edge to the wall. The components that go with a run of straight duct — our PP duct range including couplings, reducers and transitions — are made to the same wall thicknesses as the pipe they connect to, so the bearing and welding details carry across.

Thermal Expansion and Where the Fixed Points Go

Polypropylene expands roughly ten times as much as steel over the same temperature rise, and this single property drives most of the detail that differs between a PP duct installation and a metal one. The movement is predictable and the formula is simple, so there is no reason to guess at it. What catches people out is not the arithmetic, it is the datum the arithmetic starts from and the consequence of refusing the duct the freedom the arithmetic assumes.

Calculating thermal movement

The relationship is the standard one for thermal expansion: the coefficient of expansion multiplied by the original length multiplied by the temperature change, or ΔL = α · L · ΔT. For polypropylene we use a design value of α = 0.15 mm per metre per kelvin, which is on the conservative side of published figures and covers the range of polypropylene homopolymer grades we fabricate. Steel sits at about 0.012 mm/m·K, so the ratio between them is roughly twelve to one, and that ratio is the whole reason the two materials cannot share a support and restraint detail.

Put numbers on it. Over a 40 K rise, which is what a duct installed at 20 °C faces when it carries a 60 °C exhaust, PP moves 6.0 mm for every metre of run, or 60 mm in a 10 m straight length. The same 10 m of steel duct moves 4.8 mm. A run of a few tens of metres therefore has to absorb movement measured in centimetres, not millimetres, and that movement has to go somewhere.

What happens if the duct cannot move

If a run is restrained so it cannot expand, the movement converts into an axial force rather than displacement. That force is the elastic modulus times the cross-sectional area times the coefficient times the temperature change: F = E · A · α · ΔT. Because the modulus of polypropylene is low but the areas involved are not, the result is larger than most people expect. A φ315 mm duct with a 4.2 mm wall has a section area of about 4,156 mm², and over 40 K it develops roughly 37 kN of axial force at the short-term modulus.

That is about 3.8 tonnes pushing along a plastic duct. Creep relaxation over time reduces the sustained force substantially — at 40–60 °C the long-term figure is nearer 12 kN — but the initial force is what the first heat-up applies, and it is what cracks a flange that was bolted rigidly to a steel mating ring, or buckles a run that was clamped at every support. The lesson is not that PP is weak; it is that a fully restrained PP duct is a design error, not a material limit.

Sizing and placing expansion joints

An expansion joint absorbs a fixed amount of travel, and the distance it can serve is that travel divided by the movement per metre. At 6.0 mm/m, a joint with 60 mm of travel serves a 10 m straight run, and one with 40 mm serves 6.7 m. That is why joints are placed by calculating the run length each one covers, not by putting one at every building bay. A run that is split into segments longer than the joint can serve will simply pull the joint apart or push it closed.

The fixed point is positioned at the centre of the segment the joint serves, so each half of the run moves outward into a joint at each end. That is why the fixed point and the joints are designed together: placing a fixed point first and then finding room for joints gets the pattern backwards. Where a run leaves a fixed point and turns, the flexible element should sit close to the turn so the leg that moves is short. For runs where a lateral offset or a change of section is part of the layout, a custom fabricated PP duct section can be made to suit the geometry rather than forcing the layout to fit standard parts.

Welding and Flanged Connections

PP duct cannot be solvent-welded, has no room-temperature adhesive that holds under chemical duty, and cannot be taped to a gas-tight standard. Every permanent joint is a hot gas weld, and every serviceable joint is a flange. The quality of a welded joint is decided by three variables the welder controls — surface preparation, air temperature and travel speed — and one the site controls, which is whether the joint was shielded from draughts while it was laid down.

The two temperatures that constantly get confused

Ask three people for the welding temperature of polypropylene and you will get 230 °C, 300 °C and 165 °C, and all three are correct because they describe different things. The polymer itself melts at around 165 °C. The air leaving a hot gas welding gun is set between roughly 250 °C and 320 °C, and the common working figure is about 300 °C; that air is hotter than the melt point because it has to transfer heat to the joint faster than the surrounding material conducts it away. The temperature the material actually reaches at the weld interface is between those two, and it is the one that decides whether the rod and the parent material fuse.

This is why a temperature quoted without saying which thing it refers to causes arguments. A specification demanding “weld at 300 °C” is asking for the gun air temperature. A note saying “do not exceed 165 °C” is guarding the bulk material against distortion. Both can appear on the same drawing, and a welder who reads them as contradictory will change the wrong setting. The practical test is not the dial, it is the bead: a correctly welded PP bead is smooth, fully fused to both sides, and shows no line of un-melted rod at its root.

Welding methods for PP duct

Two methods cover almost all duct work, and the European guidance that most specifications reference is the DVS 2207 series. DVS 2207-3 covers hot gas welding, where a welding rod of the same grade is fed into a heated joint and the two materials fuse. DVS 2207-4 covers extrusion welding, where a plasticised filler is extruded into a prepared groove; it is faster on long seams and heavier sections and is the usual choice for thick-wall duct and for joints that will be tested. The parent material and the filler must match in grade and colour: a mismatched filler is a joint that fails at the interface rather than in the material.

Preparation matters as much as the method. The edges are cut square, deburred and wiped clean, and the weld zone is scraped back just before welding so the surface is fresh rather than oxidised. Material absorbs moisture from the air, so thick sections are sometimes pre-dried before welding to stop steam voids forming in the joint. Travel speed sets the bead shape as much as temperature does: too fast and the rod does not fuse at the root, too slow and the material scorches and loses the surface chemistry the weld relies on.

Flanged connections and gaskets

Where a joint has to come apart — at a fan, a scrubber, a damper or a section that will be inspected — a flange pair with a gasket is the connection. The PP flange is backed by a metal flange plate to give the bolt load somewhere to go, and the gasket sits between the two PP faces. A full-face gasket is the safe choice: it spreads the bolt load and does not rely on the PP face sustaining a localised crush on its own. Over-torquing is a common error and the opposite of what works; the bolts are tightened evenly in a crossing pattern to the gasket’s rated compression, because uneven tightening distorts the flange and causes a leak at the point of highest load rather than sealing the joint.

The components for this are standard rather than special: PP duct flanges carry the gasket and bolt pattern, and duct couplings provide the clamped or bolted joint where a run has to be assembled on site. Both are made in the same diameter and wall range as the duct so the joint geometry stays consistent along the run.

Laying down a weld in the field

Wind is the enemy of a field weld. A draught pulling heat out of the joint as it is laid changes the fusion and leaves a weld that looked fine when it was made and leaks when tested. Welding outdoors or near an open doorway calls for a screen, and welding near a running extraction system calls for it to be isolated. The joint should be welded in one continuous pass where the geometry allows, because a restart is a cold joint and a likely leak path.

Welders are qualified on the joint and the position they will actually work in, not just on the material, and the qualification is documented. On a chemical exhaust line the welds carry the leak-tightness of the whole system, so the pipe fitter’s habit of welding from the easiest side is replaced by welding the position the drawing calls for. Where a run passes through a wall or floor, the penetration is sleeved and the duct passes through without being welded to the structure, so movement can still occur.

Leak Testing and Commissioning

A chemical exhaust duct exists to move a stream from where it is produced to where it is treated without releasing it into the space in between. That makes leak-tightness the acceptance criterion for a PP duct installation, not airflow alone, and it is tested before the duct is insulated, boxed in or closed above a ceiling. A pinhole in a weld found before concealment is a five-minute repair; the same pinhole found after a ceiling goes back up is a demolition and rebuild.

The leak test

The standard field check is a soap-bubble test on the completed run. The duct is blanked at the ends, pressurised to a low positive pressure — a few hundred pascals is enough for a low-pressure exhaust system and is well within what the duct can take — and every weld, flange and joint is sprayed with a soap solution and watched. Bubbles mark a leak. The test pressure is deliberately low because the goal is to find leaks, not to proof the duct; over-pressurising a plastic exhaust run proves nothing except the duct’s ability to burst.

On larger systems a ductwork leakage class test is specified instead, and the acceptable leakage is quoted as a percentage of design flow at a stated test pressure. The figure should come from the ductwork specification and be agreed before the test, because a contractor who tests first and then looks for the acceptance number will find one that passes. The joints to watch are the field welds, the flanged connections, and the first metre of duct downstream of a fan where vibration loosens a connection over time.

Commissioning checks after the duct is tight

Leak-tightness is necessary but not sufficient. Once the duct holds, the system is balanced so the airflow at each branch matches the design, and the dampers are set and locked so the balance cannot drift. The fan is checked against its curve, the current draw is recorded, and the system is run through its temperature range so the expansion joints are seen to move — a joint that never moves during heat-up is a joint that is not working, and the run it serves is being loaded somewhere else. Where the duct feeds a scrubber or another treatment device, the commissioning is done jointly with the wet scrubber so the pressure drops of the two halves add up to what the fan was selected for.

The record from commissioning is what makes the installation maintainable. Note the test pressure and result, the balanced airflow at each branch, the fan current, and the datum temperature the duct was fixed at. That last figure is the one nobody records and everybody needs later, because it is the only way to know how much movement the run is actually working through.

What to do with a leak found early

A leak in a welded joint is repaired by cutting back the weld and re-welding, not by smearing sealant over it. Sealant on a chemical duct is a temporary fix that fails at the first thermal cycle and lets the stream attack the parent material at the leak, which converts a small defect into a large one. A leaking flange is re-gasketed and re-torqued evenly rather than over-tightened, and a leak at a coupling is inspected for alignment before it is tightened, because a coupling that leaks is often a coupling that is under strain from a misaligned run.

Every repair is re-tested, and the re-test covers the repair and the joints on either side of it, since the heat of the repair can disturb a neighbouring weld. The system is only handed over when the whole run holds at the test pressure in a single pass, with nothing left marked as a follow-up item.

Six Installation Mistakes We See Repeated

These are the faults that come back on inspection, and none of them is a material limitation. Each one is a decision that was reasonable on a steel job and wrong on this one, or a step that was skipped because the duct looked finished without it.

1. Hanging other services from the duct or its supports

The duct supports are sized for the duct and its condensate, and the margins in the table above are for that duty. When a cable tray, a small pipe or a light fitting is hung off the same rod, the load is added to a hanger whose bearing surface against the duct was never chosen for it, and the duct carries a point load it was not designed for. The deflection that follows is slow and easy to blame on the material. Other services get their own supports, anchored to the structure, with a gap between them and the duct so movement does not transfer either way.

2. Setting the run out at the wrong datum

A run assembled on a cold morning and lined up true will not be true at operating temperature unless it was set out for that condition. The alignment is done at the datum temperature recorded in the method statement, and the expansion joints are set with the gap that corresponds to that datum, so they have travel available in both directions when the duct heats. Setting a joint with no gap on a cold day means the first heat-up closes it and the run has nowhere to go. Setting it wide open on a hot day means the first cold shutdown pulls it apart.

3. Using a welding rod of the wrong grade

The filler rod must match the parent material in grade and colour. A rod of a different grade fuses to the surface but does not fully blend, and the joint becomes a boundary between two materials rather than a single piece — it may pass a pressure test and then fail in service at the interface. Rod of the correct grade but the wrong colour is not a cosmetic problem either; colour is how the grade is identified, so a mismatched bead is a joint nobody can verify later. Rod is also stored sealed and dry: absorbed moisture turns to steam in the weld and leaves porosity behind.

4. Skipping the deburr and the fresh scrape

Polypropylene is notch-sensitive, so a rough cut edge or a burr is not just untidy, it is a stress raiser where a crack starts. Edges are cut square with a fine-tooth blade, deburred, and the weld zone is scraped back just before welding so the surface is fresh rather than oxidised. A joint welded over a dirty or oxidised surface looks the same as a good one from the outside and behaves differently under load. This step takes a couple of minutes per joint and is the one most often dropped when the programme is tight.

5. Passing a run through a wall without a sleeve

Where a duct crosses a wall, floor or roof, it passes through a sleeve that is larger than the duct, and the duct is not welded or clamped to the structure at that point. The penetration is not a support: it is a hole the duct moves through. A duct packed hard against a masonry edge has both its movement and its bearing concentrated at one line, and it will eventually wear or crack there. The sleeve also lets the gap be sealed with a flexible fire or weather seal that allows movement, rather than a rigid mortar fill that cracks and leaks.

6. Concealing the duct before it is tested

Insulation, cladding, ceiling tile and box-in all happen after the leak test, never before. A joint that is covered is a joint that cannot be sprayed and watched, and the cost of finding and repairing it afterwards is many times the cost of testing it first. This is the mistake with the worst ratio of saving to consequence, because the work that hides the duct is the work that has to be undone to reach it. The same rule applies to the final weld inspection: the run is checked and signed off while every joint is still visible and reachable.

Safety During PP Duct Installation

The hazards on a PP duct installation are a mix of the ordinary ones that apply to any duct work and a few that are specific to working with hot tools and a combustible material. None of them is unusual, and all of them belong in the method statement rather than in a toolbox talk held after the first incident.

Hot work and fire

Hot gas welding is hot work. The gun runs at a nozzle temperature in the region of 300 °C, welding rod and offcuts are combustible, and the duct itself will burn. That combination means a hot work permit, a fire watch during welding and for a period after it stops, and the removal of combustible material from below and around the work area before welding starts. A fire blanket or extinguisher is present at the point of work, not at the far end of the building. Where the duct passes near a heat source or through a fire compartment, the penetration details are those of the building’s fire strategy, not a site improvisation.

Fume and ventilation

Welding polypropylene releases decomposition products, and the concentration depends on the temperature and how long the material is held hot. The risk rises sharply when the material is overheated or scorched, which is another reason to weld within the specified window rather than at the high end of the dial. Work in a ventilated area, use local extraction at the weld where the space allows, and do not weld inside a closed duct or tank without forced ventilation and a person outside. A welder who can smell acrid decomposition products is welding too hot or without enough airflow.

Working at height and lifting

Light sections are an advantage for handling and a trap for judgement: a spool that one person can carry is still a spool that has to be lifted overhead and held while it is aligned. Spools are lifted with slings rather than by hand where they go above shoulder height, and the lifting point is not the duct wall. Scaffold or a working platform is provided for overhead welding so the welder is not welding one-handed while holding a spool with the other. Waste and offcuts are cleared from the platform as the work proceeds, since a dropped offcut is both a trip hazard and a fall hazard.

Chemical exposure at tie-in

Commissioning a duct that has carried process gas means breaking into a system that may still contain the stream. Before any tie-in or inspection, the duct is isolated, purged and tested for the contaminants it carried, and the result is recorded. This is standard practice on chemical plant and it is the step most often assumed rather than checked when a duct is being inspected for the first time. The person who opens a duct is not the person who decides it is safe; the decision is made against a measured result.

Worked Example: A 24 m Run at 315 mm

Pulling the pieces together on a single run shows how the numbers interact and where the design decisions actually sit. Take a 24 m straight horizontal run of φ315 mm duct, 4.2 mm wall, carrying a 60 °C exhaust stream in a plant where it is installed at a datum of 20 °C.

Step 1 — mass, supports and hanger load

At 4.2 mm wall the duct is about 3.78 kg/m, so the 24 m run is roughly 91 kg of duct. At the specified 2.5 m spacing that is ten spans and nine intermediate supports, with each hanger carrying about 14.2 kg once the dynamic and condensate factor is applied. Nine hangers on a 24 m run is not an onerous support count, which is the point: PP duct is light, and the support problem on this run is not strength, it is giving the duct room to move.

Step 2 — thermal movement

The temperature change is 40 K, and movement per metre is α · ΔT = 0.15 × 40 = 6.0 mm/m. Over the full 24 m the run wants to grow by 0.15 × 24 × 40 = 144 mm. That is the number that has to be absorbed, and it is centimetres, not millimetres. It is also the reason the run cannot be anchored at one end and left free somewhere the layout does not allow — the free end has to be somewhere real, and the drawing has to show it.

Step 3 — fixed point and expansion joints

The clean solution is a single fixed point at the centre of the run, with each 12 m half moving outward into an expansion joint at its far end. Each half moves 0.15 × 12 × 40 = 72 mm, so each end joint is selected with at least that much travel — an 80 mm travel joint covers a 13.3 m span at 6.0 mm/m, which clears the 12 m half length with margin. Ends that are fixed to a fan or a scrubber nozzle are not free ends, so the joint sits close to that connection and the fixed point moves accordingly; the design rule is fixed point first, joints around it, not the reverse.

Step 4 — what the restraint would cost if it were wrong

Suppose the run were clamped rigidly at every support instead, with both ends fixed to equipment. The section area is π × 0.315 × 0.0042 = 4,156 mm², and the fully restrained axial force at 40 K is E · A · α · ΔT, which at the short-term modulus of about 1,500 MPa comes to roughly 37 kN — close to 3.8 tonnes along the duct. Creep relaxation reduces the sustained value to around 12 kN over time, but the first heat-up applies the higher figure, and it is the one that cracks a flange bolted to a steel mating ring or bulges a wall panel. The welded joints on this run also total 24 m ÷ 3 m of standard length, so eight field welds at roughly 990 mm of weld circumference each, every one of which is a leak path that the pressure test has to clear.

Step 5 — the handover numbers

The figures this run generates are the ones that belong in the installation record: nine hangers at 2.5 m and 14.2 kg each, a central fixed point with two 80 mm expansion joints, a datum of 20 °C, and a design movement of 144 mm to be confirmed on first heat-up by watching the joints actually move. If the joints do not move when the system is commissioned, the run is being restrained somewhere the drawing does not show, and that is the fault to find before the duct is concealed. The fittings and support components for a run like this are drawn from the same PP duct fittings range as the straight sections, so the wall thickness and joint geometry stay consistent end to end.

Installation and Commissioning Checklist

The list below is the sequence we work to on a PP duct installation, condensed to the checkpoints that decide whether the run performs. It is written to be walked on site, not read in an office, and the items in it are the ones that are expensive to revisit.

Before installation

1. Shop drawing walked against the as-built route; ceiling heights, columns, nozzle positions and clearances all measured.
2. Support positions marked on existing structure, or brackets designed and anchored.
3. Datum temperature recorded in the method statement, with worst-case range agreed.
4. Welding procedure and welder qualifications documented for the joint type and position.
5. Materials stored under cover, off the ground, ends open; welding rod sealed and dry.

During installation

1. Spools handled with slings, not dragged; no load carried on the duct wall.
2. Supports spaced to the deflection-based schedule, with saddles wide and smooth.
3. One fixed point per segment; all other supports sliding or guiding, free to move axially.
4. Expansion joints set to the gap that matches the datum, with travel available in both directions.
5. Edges cut square and deburred; weld zone scraped fresh before welding.
6. Welding shielded from draughts; filler rod matched to parent material in grade and colour.
7. Flanges gasketed full-face and bolted evenly in a crossing pattern, not over-torqued.
8. Wall and floor penetrations sleeved, duct free of the structure at the penetration.

Testing and handover

1. Leak test at the specified pressure before any insulation, cladding or ceiling is closed.
2. Every weld, flange and joint sprayed and watched; no follow-up items left open.
3. System balanced to design airflow, dampers set and locked.
4. Fan checked against its curve, current draw recorded, treating device commissioned jointly.
5. Run taken through its temperature range and expansion joints seen to move.
6. Installation record issued: datum temperature, test result, balanced flows, fan current.

The components and straight duct for a run are ordered together so the wall thickness, diameter and joint details match throughout — the industrial ductwork range covers the sections, fittings and connections a specification normally calls for.

Frequently Asked Questions

How far apart should PP duct supports be?

Between about 1.5 m for small diameters and 3.0 m for large ones, decided by deflection rather than by a steel-duct table. Our schedule runs 1.5 m at 110–160 mm, 2.0 m at 200–250 mm, 2.5 m at 315–400 mm and 3.0 m at 500 mm, and it shortens where the duct runs hot, carries a fitting, or takes a point load. The full table, with the hanger load at each spacing, is given above.

How much does polypropylene duct expand?

About 6 mm per metre over a 40 K rise, using a design coefficient of 0.15 mm per metre per kelvin. A 24 m run moving through 40 K grows by 144 mm. Steel over the same rise moves about 0.5 mm/m, so the difference between the two materials is roughly twelve to one, which is why PP duct needs expansion joints and steel duct of the same length usually does not.

What temperature is PP duct welded at?

Three figures are in circulation and they describe different things. The material melts near 165 °C, the welding gun air temperature is set around 250–320 °C with roughly 300 °C as the common working value, and the interface the two materials reach during welding sits between them. A specification quoting 300 °C is normally the air temperature; a note capping the material at 165 °C is guarding against distortion. The bead, not the dial, is the acceptance check.

Can PP duct be glued or solvent-welded?

No. Polypropylene has no room-temperature adhesive that holds under chemical duty and is not solvent-cementable, unlike PVC. Permanent joints are hot gas or extrusion welds per DVS 2207-3 and 2207-4, and joints that need to come apart are flanged with a gasket. Tape and mastic have no place in the specification of a chemical exhaust run.

Does every run need an expansion joint?

Every straight run long enough to generate more movement than its restraints can accommodate does. In practice that means placing a fixed point and sizing joints so each straight segment between anchors moves into a joint, with the joint travel divided by the movement per metre giving the maximum length it can serve. Short runs between equipment connections may need none; long straight runs between buildings usually need several.

How is a PP duct leak tested?

Blank the ends, pressurise to a low positive pressure of a few hundred pascals, and spray every weld, flange and joint with soap solution, watching for bubbles. Larger systems are tested to a leakage class quoted as a percentage of design flow at a stated pressure, agreed before the test. The test happens before insulation, cladding or ceiling work, because a joint that is covered cannot be sprayed.

Can PP duct be installed outdoors?

Yes, with two provisions. Polypropylene degrades under ultraviolet light, so an outdoor run is either pigmented for UV resistance or shielded, and it is not left bare for years. Thermal movement outdoors is larger than indoors because the surface can reach well above air temperature in sun, so the design range has to account for solar gain rather than ambient alone on a shaded run.

How do I balance the system after installation?

With dampers set at each branch to give the design airflow, then locked so the balance cannot drift. A PP duct damper provides the adjustment point without breaking the chemical resistance of the run, and the setting is recorded on the commissioning sheet so a later change can be referred back to the number the system was balanced to.

Getting the Installation Right From the Drawing

A PP duct installation is decided by a handful of numbers set before the first spool is lifted: the support spacing at operating temperature, the movement the run has to absorb, the position of the fixed point, and the welding procedure that makes the joints tight. Get those four right and the rest is execution. Get them wrong and the duct will look correct on the day it is installed and fail in the first year, and the failure will be blamed on the material rather than on the detail.

Xicheng manufactures polypropylene duct, fittings and the components a run needs, and we work with the installation parameters on this page as our own fabrication and site data. If you are planning a run, replacing one that has sagged or cracked, or writing an installation specification and want the support and expansion figures by diameter, send us the medium, the operating temperature, the airflow and the layout, and we will come back with the spacing, the movement and the restraint pattern for your system.

Useful starting points: the polypropylene duct guide for the material picture, PP duct versus PVC duct if the material choice is still open, and what a PP duct is for how the components fit together before you specify them.

Contact our engineering team with the medium, the temperature and the layout, and we will give you the support and expansion figures for the run you are planning.