Key Takeaways
- Welding is the primary joint; flanges are the removable interface. Installing PP air duct means joining extruded PP sections into a leak-tight run, supporting it against long-term deflection, and compensating thermal expansion before the system is concealed: weld the permanent straight runs, and keep flanges for the equipment ties that must come apart.
- Follow the parameter window, then test-weld. Published welding parameters are a starting window, not a guarantee: same-material rod, a test weld on scrap, and a visual bead check set the real numbers for your crew on the day.
- Flanges need a sequence, not just a torque value. PP flange joints relax over time, so bolting runs in staged rounds toward the manufacturer’s target torque with a re-torque scheduled later, and gaps are never closed by pulling bolts harder.
- Support spacing is deflection- and temperature-limited. A PP run sags under its own weight over years, and hot service makes it sag faster, so spacing comes from a deflection-limited chart for your diameter at your operating temperature — not from a metal duct table.
- Test every joint before it disappears. Weld and flange checks belong to the pre-concealment stage, when a leak still costs minutes to fix rather than a wall, a liner or a shutdown.
An installation crew standing over a pallet of extruded PP sections reaches for the habits that served it on galvanized sheet — mechanical seams to close joints, metal hangers at familiar spacing, long runs welded rigid end to end. None of those reflexes transfers cleanly to plastic, and what it takes to install PP duct that stays airtight starts with a different picture of the joint, the support and the temperature. Installing PP air duct means joining extruded PP sections into a leak-tight run, supporting it against long-term deflection, and compensating thermal expansion before the system is concealed. This section names the differences that break those habits and the decisions they force.
How to Install PP Duct: What Changes When the Duct Is Plastic
Why steel habits fail on PP ducts
The primary joint in a PP duct run is a field weld, made by hot-gas welding in the DVS 2207-3 practice — not a mechanical seam. Where galvanized duct closes with mechanical seams and lets the flange carry the load, a PP run fuses into one homogeneous piece, and flanges appear only where the system must come apart — at a fan, a scrubber, a damper or an access section. Plastic also answers heat differently: steel ductwork shrugs off service temperatures that push PP grades toward their continuous-service envelope of about 90–100 °C depending on the grade. If the material itself is new to you, what a PP air duct is covers its composition and joint types first.
The two structural differences that break copied habits sit beneath the joint question. PP’s linear thermal expansion runs about six to eight times that of steel — coefficient 72–90 ×10⁻⁶ m/(m·°C) against steel’s 10.8–12.5 — so a long run anchored rigid at every point pushes against its own supports as the line warms. PP is far less stiff than steel, so a run creeps and sags under its own weight for years; a narrow point clamp concentrates the load, where a wide cradle spreads it. Supports are engineered for the plastic, not inherited from a metal table — PP duct vs alternatives carries the system-level comparison.
The three decisions before the first cut
Before the first cut, settle three decisions, because every failure this material is known for traces back to one of them. Joint method comes first — weld the permanent runs, flange the equipment ties, keep a coupling only where a section must come out later. Support system second — set spacing from a deflection-limited basis for your diameter at service temperature, and spread the load through wide cradles rather than point clamps. Expansion layout third — choose the fixed and sliding points and the compensation hardware before long runs are closed, so leaks at joints, sag at supports and cracked flanges stay off the job.
The cost of deciding late lands on the order form before it lands on the roof: a crew that locks its choices after cutting buys rod, flanges, cradles and expansion pieces twice, then still fails acceptance when an unmapped joint leaks or a sagging span misses its check. Decide the three first and the install becomes a sequence of checks — every weld, flange and hanger either confirms a decision or flags one you skipped.
After this section you can name the three decisions PP duct forces on your crew — joint method, support system and expansion layout — and decide them before the first cut. You can also say why locking them in early separates a run that passes acceptance from one that buys its parts twice.
Choosing Joints and Fittings: A Connection Map for PP Duct Systems
The fittings map: type, joint and where it belongs
Every interface on a duct drawing takes a different connection, so the job before fabrication is to assign a joint to every interface on the sheet. The map below walks the seven interfaces a PP run typically shows — extension, turn, branch, equipment tie, termination, diameter change and access — and picks for each from the three joint families: weld, flange or coupling. Read your own drawing against it and the choice is made per interface, on purpose, instead of by habit.
| Interface | Connection | Why it belongs there |
|---|---|---|
| Straight-run extension | Field butt weld (hot-gas welding in the DVS 2207-3 practice) | Permanent and homogeneous — the run fuses into one continuous piece |
| Direction change | Welded elbow, or molded-socket elbow | Turns the run inside the same joint family as the straight sections |
| Branch takeoff | Welded tee or wye | Homogeneous junction with no extra leak path under service load |
| Equipment tie-in | Flange with gasket | The removable interface — fans and scrubbers must come apart for service |
| Run end | End cap; rain cap where a stack terminates outdoors | Closes the bore and sheds weather at external terminations |
| Diameter change | Concentric or eccentric reducer | Steps the bore smoothly, with no ledge for condensate or debris |
| Access or temporary section | Flanged spool or mechanical coupling | Reopens later without cutting a weld out of the live run |
Welding owns every permanent, load-bearing interface on the run. A butt weld fuses the same material into one homogeneous piece, so no gasket, clamp or fastener is left behind to creep, relax or age with service. If a connection never has to open, weld it, because a welded joint is also the cheapest one on the job: it adds no parts to buy, no gasket to stock and no torque to schedule.
Flanges mark the other boundary — every interface that must open, at a fan, a scrubber, a damper or an access section, becomes a flanged joint closed against a gasket, so it parts with a wrench instead of a saw. Fittings meet the run the same way the straight sections do: molded socket fittings are closed into place with back-welding, the standard manufacturer practice, which keeps the whole line inside one weld family across the φ20–600 mm outer-diameter range. Socket and flange faces are cut from the same wall section as the duct itself, so duct wall thickness selection is the companion reference that keeps every mating face consistent with the run you are welding.
When a coupling or mechanical joint makes sense
A coupling earns its place where an interface must reopen without cutting a weld. Maintenance access is the first candidate — a cleaning hatch, a filter section or a spool that comes out on schedule — and so is a fan connection that should not pass vibration into a rigid welded line, where a short mechanical link isolates the equipment instead. Staged installs and retrofit tie-ins justify one too: when duct arrives in phases, or a new branch must join a line already running, a mechanical joint closes the connection without hot work beside an operating system.
A coupling is a deliberate interface, not a shortcut for permanent joints, and it does not belong on straight sections a weld would close with fewer parts. Every mechanical joint you add brings a gasket, a seal to inspect and a line in the maintenance schedule, and each one is a future check the welded run does not carry — so the test before you specify one is whether that section will genuinely come out again. When it will, the PP air duct coupling range is the category to order from; when it only might, weld it and cut later if you must.
After this section you can choose the joint for every interface on your drawing — weld the permanent runs, flange the equipment ties, and keep a coupling only where a section must come out later. That same map doubles as the start of your order list — every row names the fitting that ends up in the crate.
Hot Gas Welding PP Duct Sections: Parameters and Step-by-Step
The previous section’s connection map assigned a weld to every permanent interface on the run; this section is how that weld gets made so it holds. Below is the parameter window your crew works inside — air temperature, air flow, welding speed, rod and feed — and the visual check that makes those welds hold. Treat every value below as a window to start from, and fix the day’s settings with a test weld before any production joint.
The parameter window (and why test welds set it)
Hand welds on PP duct follow the DVS 2207-3 practice, and the table below is the starting window a crew reads before it lights the gun.
| Process | Parameter | Window |
|---|---|---|
| Hot-gas hand welding | Air temperature | 305–315 °C, measured 5 mm from the nozzle centre |
| Hot-gas hand welding | Air flow | 40–50 l/min |
| Hot-gas hand welding | Welding speed | 60–85 mm/min |
| Hot-gas hand welding | Rod and feed | 3 mm rod fed at 8–10 N (rods of 3–4 mm are typical) |
| Extrusion welding | Extrudate temperature | 210–240 °C (for heavy-wall or large-diameter joints) |
The window is a starting point, not a guarantee, because ambient temperature, joint geometry and material grade each shift the setting that melts the rod cleanly. Weld too cool or too fast and the rod fuses cold, closing the joint with a bead that carries little load; run hotter than the window and the surface degrades instead of fusing. Both failures cost the weld its strength, which is why crews test-weld on scrap from the same batch and read the bead before any production joint.
The rod itself carries half the job. It must be the same material as the duct so rod and parent fuse as one, and it stays dry and clean from storage to the gun. A contaminated or moist rod shows up as porosity in the finished bead — the defect the visual acceptance check below is looking for.
Joint prep to visual acceptance
Joint prep stays compact: clean and bevel the joint faces so the rod fuses into fresh material, tack the sections to hold alignment, then run the root and cover passes at the window settings. The same rod-and-parameter rules govern every pass whether your own crew lays the weld or a field welding service does — and they are the rules the manufacturers’ own welding method reference documents, where the factory joints your duct arrives with were made the same way.
Visual acceptance gates each joint before the run moves on: a good bead is glossy and uniform, free of porosity, cracks or unmelted rod. A dull, porous or cracked bead sends the joint back to prep — clean, re-bevel and weld again — because the bead is the visible record of a fusion you cannot inspect from inside the bore.
After this section you can set up a field weld — choose the day’s settings from the window, with the nozzle temperature measured 5 mm from the nozzle centre, air flow and rod feed included — run a test weld on scrap from the same batch, and read the bead before any production joint goes into the run.
Flanged Connections to Fans, Scrubbers and Dampers: The Assembly Protocol
The connection map in the previous section assigned flanges to the equipment ties that must open — the fan, the scrubber, the damper — and a PP flange seals only when it is aligned, staged and re-torqued. The bolting protocol below comes from PP flanging practice (PPI TN-71), read with one boundary: it was written for PP pressure piping, so a duct crew takes the sequence and the discipline, and takes target torque values from the flange manufacturer’s table for low-pressure duct service.
Flange anatomy and gasket choice
A PP duct flange is typically a welded stub flange on the duct end working against a loose backing ring, lap-joint style, so the bolt load squeezes the joint through the backing ring rather than through the pipe wall itself. The ring spreads the clamp force around the full circumference while the welded stub stays with the duct, so an equipment tie opens and closes without stressing the plastic wall. The seal lives on the flange faces, and they must be clean, flat and undamaged before anything is bolted; a gouged or chipped face is a leak path no torque sequence recovers. The PP air duct flanges range is the category to order these parts from.
Gasket choice and condition decide the seal before the torque wrench does. Match the gasket material to the media and the temperature, with the supplier stating the compatible grades, and inspect the gasket before every assembly for cuts, hardening or embedded debris. Never reuse a gasket that has already been compressed; a half-seated or hardened gasket leaks no matter how good the bolt torque is, because a joint seals when bolt load crushes fresh gasket material into the face irregularities.
Bolting: the three-round sequence and PP re-torque
Bolt the joint in staged rounds toward the manufacturer’s target torque, then re-torque it after the PP has crept — the table below is the sequence with the check that belongs to every step.
| Step | Action | Check |
|---|---|---|
| 1 | Align the flanges and tighten all bolts by hand | Faces parallel, bolt ends out for inspection |
| 2 | Tighten in at least three staged rounds — about 30% of the target torque, then 50–70%, then 100% (target torque = the flange manufacturer’s value for the size) | Torque wrench each round |
| 3 | Run a final circular re-torque pass at 100% | No bolt turns at target |
| 4 | Measure the gap between flange faces at eight equally spaced points | Gaps even — flanges pulling parallel |
| 5 | Re-torque all bolts 24–48 hours after initial tightening | PP creep compensation |
PP flange joints relax far more than metal ones because the plastic creeps under sustained bolt load, so a joint that felt tight on installation day can leak a week later unless it is re-torqued — the 24–48 hour pass is part of the sequence, not a follow-up. One piece of background keeps the rounds honest: the nut factor, meaning lightly greased or coated bolts tighten differently from dry ones (K≈0.16 vs 0.20). Lubricate non-coated threads and nut faces — dry assembly is itself a defect — and take the target torque from the flange manufacturer’s table, not from habit.
Two prohibitions govern the whole sequence. Never tighten or loosen flange bolts while the system is pressurized; a joint that must be worked is isolated and bled down first. Never use bolt tension to close a gap between the flange faces — cinching means misalignment or a foreign object in the joint, and pulling the gap shut hides the cause while the backing ring takes a load it was never meant to carry.
After this section you can run a PP flange bolting sequence — staged rounds toward the manufacturer’s target torque, a circular re-torque pass, eight-point gap checks and a 24–48 hour re-torque — and decide the schedule for it before the first bolt, instead of pulling bolts to a guess. The flanged equipment ties at the fan, scrubber and damper then keep their gaskets seated for the life of the run.
Duct Hangers and Support Spacing for PP Ductwork
The run the previous sections built is welded and flanged; now it gets carried. Carrying is where plastic punishes metal habits hardest, because a PP duct sags under its own weight for years and sags faster hot. Support spacing is a deflection decision at your service temperature, not a table lookup.
Why plastic duct spacing is not a metal table lookup
A PP run takes its spacing from a deflection judgment at its operating temperature, and the three bases below replace the metal lookup.
| Basis | What it says | How to use |
|---|---|---|
| Manufacturer deflection-limited chart | The PP duct manufacturer publishes a support-spacing chart per diameter, covering about φ50–630 mm, with curves for service temperatures around 20/40/60/80/100 °C and built on limiting long-term deflection — one well-known chart limits sag to 1 cm after 10 years. Spacing shrinks as temperature rises. | Read the spacing for your diameter at your operating temperature from the manufacturer’s chart, never from a metal table; confirm the value with the supplier for your project. |
| Metal baseline, contrast only | Sheet-metal duct hangs at up to 8 ft (2.5 m) for sizes under 125 mm, 6 ft (2 m) for 125 to 1000 mm, and 4 ft (1.2 m) above that — vertical round duct at up to 12 ft (3.6 m) and rectangular at up to 10 ft (3 m) (published support-spacing table). | Contrast only — the metal row is an upper bound for a stiffer material, not a spacing for PP. |
| PP pipe temperature derating, pattern only | Support tables for PP pipe shrink roughly 25% between 60 and 180 °F — 2 in Sch 40 pipe drops from 3 ft to 2 ft, and 4 in from 4 ft to 3 ft. | Take the pattern — hot service tightens spacing — not the table values, which are written for small-bore pressure pipe. |
Read the three rows as one argument: the manufacturer’s chart is the decision, the metal row is the contrast that shows why the copying habit fails, and the pipe row shows which way temperature moves the answer. No universal PP spacing value exists in published industry tables, so the rule stays two-step — read the chart for your diameter at your service temperature, and confirm the value with the supplier for your project.
That discipline matters because the failure is delayed, not absent. A span copied from a metal table reads fine on installation day, then sags over years as the PP keeps creeping under its own weight, fastest where the run runs hot. A low point that looks minor becomes a condensate trap once the line carries acid fume, and what the maintenance log later records as material failure was a support error all along. Keep the decision paired with the design-side partner that sized the run — the ventilation duct sizing and design guide.
Support hardware: wide cradles, sliding vs fixed points
Duct hangers for a PP run need wide cradles or bands that spread the load across the full bearing surface, not narrow point clamps that press a line into the wall. A sheet-metal hanger bites on a stiff section that shrugs off point loads; the same clamp on PP concentrates the hanging force on a thin band of plastic, and that line is where indentation and sag begin. Specify the cradle width and the bearing surface with the hanger, and order the support hardware with the duct package instead of from the metal rack.
The cradle is only half the layout, because the support system also has to let the run move. Fixed points stay few and sit where movement must stop — beside heavy equipment such as a fan or scrubber, where the flanged tie-in from the last section must not be dragged as the line warms. Sliding supports carry most of the run and let the duct slide axially, while guides at direction changes keep that movement on its intended axis. Mark the split between fixed and sliding points now — the expansion section next builds on it.
After this section you can set hanger spacing for a PP run the right way — from a deflection-limited chart for your diameter at your service temperature, with the metal baseline held only as contrast — and choose wide cradles inside a deliberate fixed-and-sliding layout instead of copying a metal table.
Thermal Expansion: Compensating Long PP Runs
The last section split each run’s supports into fixed points and sliding points so the duct has somewhere to move; this section sizes that movement and picks the hardware that absorbs it. Growth is a predictable quantity — it follows from the material’s coefficient, the run’s length and the temperature rise — so it is designed for, not discovered after the line is closed and hot.
How much does PP duct grow?
PP’s linear expansion coefficient runs about 72–90 ×10⁻⁶ m/(m·°C) against steel’s 10.8–12.5 — roughly six to eight times (published expansion-coefficient table) — and that single gap explains why a PP run grows where a steel run barely registers. The growth is linear in the run’s length and in its temperature rise, so the movement is entirely predictable: ΔL = α · L · ΔT.
A few tens of degrees of warming across a straight industrial run pushes that formula into the tens of millimeters — enough to drag a flanged tie-in off its gasket or bow a span between its supports. The full worked arithmetic belongs to the installation example later in this guide, so here the formula stays one you can run on your own length and temperature rise. Note that the coefficient is for unfilled PP, and grades vary slightly, so confirm the value on your material’s datasheet before you size anything.
Compensation hardware: expansion joints and layout
Axial expansion joints, sliding sleeve joints and flexible connectors are the three compensation families, and manufacturers list all three as catalog families for PP duct systems, socket, clip-band and flanged ends included. The axial joint is a molded PP bellows section welded or flanged into the line that compresses and extends with the run; the sliding sleeve joint is a larger-diameter sleeve the duct end moves inside, the usual choice for bigger bores; and the flexible connector is a short fabric-reinforced sleeve clamped at a fan tie, where vibration isolation and thermal growth arrive together.
The layout rule pairs with the fixed-and-sliding split marked in the last section: fix an anchor near the heavy equipment end of each straight run so growth is directed toward the compensation hardware, slide the supports between, and fit a compensation piece wherever the accumulated growth exceeds what a stub or a branch can flex to absorb. A run left rigid from equipment to equipment spends its growth as stress on the flange bolts you tightened in the assembly protocol earlier. Fixed points and runs belong on the design drawing before installation day — the discipline air duct design principles sets out — so compensation lands as a drawn decision, not a field improvisation.
After this section you can estimate how much a run grows between installation and operating temperature, and decide where fixed points and expansion hardware sit so the movement goes somewhere harmless.
Common PP Duct Installation Mistakes (and the Fix)
Six mistakes cause most of the rework on a PP duct installation, and each one is the shadow of a discipline the previous four sections already covered. None of the six announces itself on installation day; the sagging low point, the cracked flange and the leak show up weeks later, usually after the joint has stopped being reachable. So use the table below as a pre-sign-off audit: walk your finished run against all six rows while every weld, flange and hanger is still exposed, and settle each row from the evidence on the run itself before the line is insulated or closed in.
| Mistake | Why it fails | The fix |
|---|---|---|
| Metal hanger spacing and narrow clamps carried over from sheet-metal work | A narrow clamp presses a line into a low-stiffness wall, and metal spacing lets a PP span sag for years before the low point shows | Wide cradles, spacing from the manufacturer’s deflection chart for the diameter at service temperature |
| Rigid clamping at every support | The run has nowhere to expand, so thermal growth turns into stress at the nearest flange or a bow between supports | A few fixed points where movement must stop; sliding supports between; guides at direction changes |
| Welding with a moist or dirty rod, or skipping the test weld | Contamination shows up as porosity, and an untested setting welds the whole run cold | Same-material rod kept dry from storage to gun, and a test weld on scrap before any production joint |
| Welding outside the 305–315 °C window (too cool or too hot) | Cold welds and overheated welds both lose strength — one never fused, the other degraded | Set the gun at the window measured 5 mm from the nozzle centre, then judge the bead: glossy and uniform, no porosity or cracks |
| Pulling flange bolts to a guess in one pass | Uneven bolt load creeps loose in PP, and the joint that felt tight leaks a week later | Three staged rounds toward the manufacturer’s target torque, an eight-point gap check, and a 24–48 hour re-torque |
| Sealing seams with tape and leak-testing after the ceiling closes | Tape is not a seal on a fume line, and a leak found behind a closed ceiling costs a teardown instead of minutes | Weld beads and flanged gaskets as the seals, every joint leak-checked before insulation goes on |
Run through the six rows once more over the completed run: each row either confirms a decision the earlier sections made or catches one that slipped while the run was still open. Do the walk before the concealment stage starts, because it is the last moment a fault costs minutes instead of a strip-out. A run that clears every row is ready to be handed over, and the duct insulation and sealing guide is where the work goes next.
After this section you can audit an installed run against the six mistakes that cause leaks, sag and cracks — and decide, before you sign the section off, whether each one is cleared.
Worked Example: Installing an Acidic Exhaust Branch with a Fan Tie-In
This section is a worked example, not a new rule set: it runs one installation through the decisions the earlier sections already set and ends in a method statement a foreman can hand over. The line is a φ250 mm PP branch for mildly acidic fume — ≈18 m of horizontal run, installed at 20 °C ambient, running near 60 °C — with straight sections welded, welded elbows for the two direction changes, and one flanged fan tie-in. Every value below is the example’s own arithmetic inside windows this guide has already covered, and each one is confirmed against the supplier’s data before fabrication.
The step-through
1. Assign the joints. Straight sections are field butt welds in the DVS 2207-3 practice, the two direction changes are welded elbows, and the fan tie-in is a flange with gasket — every interface takes the connection the map assigned before the first cut. φ250 mm sits inside the standard φ20–600 mm outer-diameter range, so this branch is a standard line through that map.
2. Set the weld window. The crew welds inside the hot-gas window: air at 305–315 °C measured 5 mm from the nozzle centre, 40–50 l/min of flow, 60–85 mm/min of travel, and a 3 mm same-material rod fed at 8–10 N. A test weld on scrap fixes the day’s settings before the first production joint.
3. Bolt the fan tie-in. Hand-tighten, run three staged rounds toward the flange manufacturer’s target torque, finish with a circular re-torque pass, and check the gap at eight equally spaced points. Schedule the 24–48 hour re-torque while the fan is locked out.
4. Set the supports. The run serves near 60 °C, so read the spacing for φ250 mm at that temperature from the manufacturer’s deflection-limited chart — the basis that limits sag to 1 cm after 10 years. The metal table’s 6 ft (2 m) band is contrast only: the plastic runs tighter per the chart, on wide cradles, with sliding supports between the fixed points.
5. Size the growth. Growth is ΔL = α · L · ΔT, with the unfilled-PP typical value α ≈ 0.08 mm/(m·°C) for this example: 0.08 × 18 m × (60 − 20) °C ≈ 58 mm. That ≈58 mm is the example’s arithmetic, not a rating: one welded elbow absorbs part of it, and the rest goes to an axial expansion joint placed so the fixed anchor at the fan end directs the growth toward it.
6. Check before concealment. Read every weld bead while exposed — glossy, no porosity — leak-test each joint, and re-measure the flange gaps after the 24–48 hour re-torque. A fault found here still costs minutes at an open joint; behind a closed ceiling, the same fault costs a strip-out.
7. Write the method statement. The single sentence a foreman hands over: “φ250 mm PP duct branch, ≈18 m horizontal run, welded straight joints per the hot-gas window (305–315 °C air at 5 mm, 40–50 l/min, 60–85 mm/min, 3 mm rod), one flanged fan tie-in tightened in three rounds to the manufacturer’s target torque with an eight-point gap check and a 24–48 hour re-torque, wide-cradle hangers at the spacing read from the manufacturer’s deflection chart for 60 °C service, and about 58 mm of thermal growth (typical α 0.08 mm/(m·°C) over 40 °C) absorbed at the expansion point — leak-checked before concealment, all values confirmed against the supplier’s data.”
What changes the answer
Change one input and the answer moves in a direction the earlier sections already drew:
– Hotter service, toward the ≈90–100 °C grade envelope: growth and support spacing both move against you. – A longer run: 30 m at the same 40 °C rise reaches ≈96 mm by the same arithmetic, so the compensation hardware grows with the line. – A warmer installation day: the rise from a 20 °C start shrinks, and so does the allowance. – A larger bore, φ400+: chart spacing tightens again, and heavy-wall sections may justify extrusion welding. – Flanged tie-ins at both ends: the fixed-point layout carries more of the design, because both equipment connections must hold position while the middle of the run grows. – Stronger media: gasket material and chemical compatibility move to the supplier’s datasheet.
After this example you can run the same step-through on your own line and decide the entries of a complete method statement — joints, weld parameters, flange rounds, support basis and expansion allowance.
FAQ: How to Install PP Duct (Joints, Supports, Leaks)
Can I use regular duct tape to seal PP duct joints?
No — cloth duct tape dries out and loses adhesion, so it cannot hold an airtight joint on a PP exhaust line carrying fume at temperature. The seal on a PP system is the weld bead on a permanent joint or the gasket in a flanged one, and acceptance is a visual bead check plus a per-joint leak test while the run is still exposed — not tape.
Why is the hot-air welding temperature (305–315 °C) higher than PP’s melting point?
Because the 305–315 °C figure is the air temperature measured 5 mm from the nozzle centre, not the temperature of the rod or the joint faces. PP itself melts around 165 °C, and the air cools as it crosses from nozzle to seam, so the gun must run hot for the interface to reach melting; the test weld on scrap is what confirms the day’s setting.
How far apart should PP duct hangers be?
There is no universal PP lookup table like the metal one. Read the spacing for your diameter at your operating temperature from the manufacturer’s deflection-limited chart (the basis that limits long-term sag), keep the sheet-metal rows as contrast only, and remember hot service tightens spacing — pipe support tables lose roughly a quarter of their span from 60 to 180 °F. Confirm the final spacing with the supplier.
| Question | One-line answer |
|---|---|
| Can I use regular duct tape to seal PP duct joints? | No — cloth duct tape dries out, so the seal stays the weld bead or the flanged gasket, judged by a visual bead check and a per-joint leak test while the run is exposed. |
| Why is the hot-air welding temperature (305–315 °C) higher than PP’s melting point? | 305–315 °C is the air temperature measured 5 mm from the nozzle, not the material temperature — the air cools before the seam melts (PP melts around 165 °C), and the test weld fixes the day’s setting. |
| How far apart should PP duct hangers be? | No universal PP spacing exists — read your diameter at your operating temperature from the manufacturer’s deflection-limited chart, hold the metal rows as contrast only, and remember hot service tightens the span. |
After these answers you can settle the three doubts that surface on every PP duct job — and decide when a value needs your supplier’s confirmation instead of a chart.
Pre-Sign-Off Checklist and What to Order
A PP duct installation is signed off layer by layer, in the order the layers can still be fixed cheaply. The two lists below close the guide: walk the acceptance layers while every weld, flange and hanger is still exposed, then turn those decisions into an order list that names real parts. Each row references a discipline the sections above already covered — by now the run either confirms a decision you made or flags one you skipped.
The acceptance checklist
Run the checks below in this order, before any liner, blanket or ceiling goes over the run.
| Layer | What to check | When |
|---|---|---|
| Weld beads | Every bead glossy and uniform — no porosity, cracks or unmelted rod | Before any concealment |
| Joints under leak test | Soap-solution or pressure-decay check per joint, per the acceptance level the engineer sets | Before any concealment |
| Supports and cradles | Spacing read from the deflection chart for the service temperature; wide cradles seated, sliding points free to move | Before concealment |
| Expansion allowance | Fixed and sliding points placed as drawn; compensation hardware free to compress and extend | Before first heat-up |
| Flange condition | Faces parallel — gaps even at the eight check points; re-torque done 24–48 hours after initial tightening | After the re-torque window, before handover |
The order is the economics of a missed check: each layer is sequenced by how expensive a miss becomes, not by how visible the layer is. A bead re-welded today costs minutes, because the joint is still open and the gun is still warm; a leak found behind a liner costs the liner and the strip-out needed to reach the joint. Work down the rows and every fault class is caught while it is still cheap to fix.
The checklist is written for the crew foreman and the commissioning engineer to walk together over the exposed run. The foreman re-opens what fails on the spot, and the engineer holds the acceptance level each layer is judged against. Acceptance is a shared walk with the torch and the wrench still in reach, not a paperwork event signed after the line is covered.
What to order and confirm
Each line below pairs the item with the specification basis that sets its size or quantity, and the person who confirms it.
| Item | Spec or quantity basis | Confirm with |
|---|---|---|
| Welding rod | Same material as the duct, 3–4 mm typical; quantity from the joint count and passes | Welding method reference / supplier |
| Flanges and gaskets | Welded stub flanges with backing rings per diameter; gasket material matched to the media | Flange supplier’s table — the PP air duct flange range is the hardware category |
| Target torque table | The flange manufacturer’s torque values for the sizes ordered — not a generic figure | Flange supplier |
| Support hardware | Wide cradles and threaded-rod drops, spacing read from the deflection chart per diameter and temperature | Manufacturer’s chart / supplier |
| Expansion pieces | Axial joints sized to the growth the ΔL arithmetic gives; elbows and rain caps close the routing and the stack end | Supplier — see the 90° elbow and rain cap categories |
Order against the interface map from the connection-map section, not against a catalog: assign every crate item to an interface you drew — rod to the welded runs, flanges to the equipment ties, cradles to the drawn support points, expansion pieces to the spans you compensated. When each item matches an interface you drew, nothing gets bought twice and nothing ships once on a guess.
The difference between a PP line that passes and one that gets reworked is decided before the first cut: the joint map, the support chart and the expansion allowance carry the outcome, and the sections above are how you settle those three while the run is still on paper. The values those disciplines leave open — rod size, gasket material, the torque table for your sizes — are exactly what the supplier conversation closes, so the order list above is the script for it. When you are ready to install PP duct on your site, browse the PP air duct range and the complete PP duct guide, then decide the remaining values with your supplier and line up the rest of the system behind the run. And once the run is in service, the PP duct maintenance & troubleshooting guide carries the leak, noise and airflow checks that keep it there.
