How to Specify Wall Thickness and Collapse Strength for Perforated Metal Tubes?

Wall thickness on a perforated metal tube is set by the differential pressure it has to survive, not by the thickness the punch press happens to be running. A perforated tube is a cylinder with a regular field of openings, so it fails in one of two ways: it collapses inward under external pressure, or it splits along the seam under internal pressure. Both scale with the ratio of wall thickness to diameter, which is why two tubes with the same hole pattern can carry entirely different ratings. Guangtong rolls and welds perforated metal tubes from flat punched plate in 304, 304L, 316, 316L and 904L, in walls from 0.3 to 5 mm and diameters from 10 to 800 mm, and the thickness is normally fixed from the duty before the pattern is finalised.

Summary. Collapse pressure rises with the cube of the wall-to-diameter ratio, so doubling wall thickness multiplies it by about eight while doubling diameter divides it by about eight. Open area removes the metal that carries load, and suppliers reduce allowable pressure either by the ligament efficiency or by the square root of the remaining section, a gap of more than two to one. A stiffener ring at mid span is usually cheaper than a thicker wall.

How the Wall Fails Under External and Internal Pressure

When the pressure outside the tube is higher, the failure mode is buckling rather than tearing: the cylinder loses roundness, flattens on one side and collapses, often well below the stress at which the metal itself would yield.

The sizing rule follows from that. Collapse pressure is proportional to the elastic modulus and to the cube of the wall-to-diameter ratio. At a constant diameter, doubling the wall multiplies collapse pressure by about eight. At a constant wall, doubling the diameter divides it by about eight. That is why a large thin tube is the weakest combination available: a 600 mm intake screen and a 50 mm cartridge core cannot share a wall specification at the same pressure.

Roundness carries the same weight, since a tube one percent out of round loses a large share of its theoretical collapse load because the flattened side buckles first. Guangtong measures it after welding rather than before.

When the pressure inside is higher, the wall is in tension around the circumference and the failure is a split rather than a collapse. The hoop stress is the pressure multiplied by the diameter and divided by twice the wall thickness. Doubling the wall doubles the pressure held and doubling the diameter halves it, so geometry that ruins a thin tube in external service survives in internal service at moderate pressure provided the seam is sound.

The seam is the weak line. The longitudinal weld joins the two edges of the rolled plate exactly where hoop stress is highest, so it has to be full penetration with no undercut and no plate mismatch. A partly penetrated seam fails before the base metal does.

Backwash is usually the governing internal case. Reverse flow at 1.5 to 3 times the service rate imposes a far larger differential than normal operation, and the surge when a valve closes can spike two to three times the steady figure, which is why backwash procedures are set from the element rather than the housing.

How Open Area Reduces the Allowable Pressure

Open area does not reduce strength in proportion to the holes, and two conventions appear in supplier quotes.

The conservative one multiplies the allowable pressure by the ligament efficiency, the pitch minus the hole diameter divided by the pitch. The less punitive one multiplies by the square root of the section that remains. On a 2 mm hole at 3 mm staggered pitch, about 40 percent open area, the first gives 0.33 and the second 0.77, more than two to one on the same tube. Neither is wrong, but a quotation is only comparable if the supplier names the convention, which is why Guangtong states it on the drawing.

Hole direction matters as well. A slot along the axis removes less of the metal that stays continuous around the circumference than a round hole of equal area, so it costs less collapse strength, the same logic that makes the continuous slot of a wedge wire element work in fine duty. A slot running around the circumference cuts the hoop path and creates a band that buckles early.

For a support core the target is 30 to 40 percent open area with a solid wall section, since the core only passes what the medium already passes. Sizing it at 1.2 to 1.5 times the media open area of a sintered filter element keeps pressure drop low without paying for strength a higher open area would cost.

How to Size Wall Thickness From Diameter and Pressure

Sizing runs in a fixed order.

• Establish the worst differential, including backwash, surge and cleaning cycles

• Decide whether the governing case is external, internal or both

• Take a starting wall from the band below and check it against the allowable pressure after open area

• Keep hole diameter at least equal to wall thickness, since punching below it distorts the hole and breaks the punch

• Confirm the result with a hydraulic test rather than with arithmetic alone

For core duty at 2 to 5 bar the bands below are starting points rather than code values, and long unsupported spans or pressures above 10 bar move the tube up a band.

Thickness tolerance deserves its own line. Cold rolled plate at 1 mm is commonly supplied within plus or minus 0.05 to 0.10 mm, so a wall ordered at nominal can arrive thin, and if the pattern also sits at the top of its open area range the tube falls below its rated pressure before it sees load. Specify a minimum wall.

Stiffener Rings, Seam Welds and End Fittings

Pressure tries to flatten the cylinder. End fittings stiffen the ends, so buckling on a long tube almost always begins at mid span.

That is the argument for a welded stiffener ring. A ring at mid span halves the unsupported length and raises collapse pressure without touching the wall, and it normally costs less than stepping up thickness across the whole tube, the same trade that governs strength in filter element construction. The order of preference is a ring first, wall second, diameter last, because diameter is fixed by the housing.

End fittings are often the weakest link, since they are frequently taken from stock rather than rated for the tube. The fitting weld carries the full differential the way the shell does, so flanges, threads and their welds belong on the same drawing at the same pressure, and Guangtong quotes them together for that reason.

Where the Requirement Changes by Service Pressure and Medium

The table below sets the wall against the service rather than against preference.

What Buyers Report After Volume Production

The most frequent complaint after several shipments is a collapse during commissioning, and the drawing was usually met. The tube arrived at the nominal wall with the pattern at the top of its open area range, and together they left less section than the differential demanded. Buyers caught by this now specify a minimum wall and ask Guangtong to state which convention reduced the allowable pressure for open area.

The second theme is a leak at the seam. The tube held its test pressure in the shop and weeped at the longitudinal weld after a few thermal cycles, which points to partial penetration rather than to the alloy, so full penetration and a penetrant check are now required.

The third is an out of round tube that will not seat in the housing, caused by ovality from rolling and welding, so roundness is now specified as a percentage of outer diameter and measured after welding.

Buyers who sent the duty and the differential rather than a thickness received the right wall on the first article, and buyers who allowed a stiffener ring got the strength at lower weight than a thicker tube would have cost.

What to Put on the Perforated Metal Tube Order

Each field below removes a specific failure.

A Representative Case: Collapsed Core on a High Flow Water Line

A water treatment contractor ordered 400 mm perforated support cores at 1.5 mm wall and 45 percent open area to keep pressure drop low on a high flow line. The cores collapsed at the first backwash, well below the 6 bar the line was rated for.

Two errors compounded. At 400 mm and 1.5 mm the thickness to diameter ratio puts theoretical collapse pressure far below 6 bar before the pattern is considered, and 45 percent open area had removed much of the section that remained. The second was reading the line pressure as the differential, when reverse flow imposed a far larger external differential across the core.

Guangtong supplied 400 mm cores at 3 mm wall with round holes at 34 percent open area and a stiffener ring at mid span. The assembly held a hydraulic test at 1.5 times the design differential with no permanent deformation and roundness within 0.5 percent of outer diameter. The contractor now sizes cores from the reverse flow differential, and similar duty in wastewater treatment is quoted the same way.

Manufacturer Recommendations From Guangtong

Four points decide whether a perforated tube holds its rating.

First, size from the worst differential, because backwash, cleaning cycles and valve closure surge are the cases that fail tubes.

Second, specify a minimum wall rather than a nominal wall, and state the tolerance that applies.

Third, ask the supplier to name the convention used to reduce allowable pressure for open area.

Fourth, control roundness after welding, because buckling begins on the flat side.

Guangtong perforates and forms tubes in 304, 304L, 316, 316L and 904L, plus Monel, Hastelloy and nickel alloys, from 10 to 800 mm diameter and 0.3 to 5 mm wall, with round, square, hexagonal and slotted patterns and ends open, threaded or flanged. Wall thickness and roundness are checked through quality assurance rather than assumed, and customization service covers thickness and stiffener development against the differential a buyer actually measures. Buyers who want a wall confirmed before tooling can send the duty, the differential and the housing bore through the Guangtong contact desk.

FAQ

Q: How do I choose the wall thickness for a perforated metal tube?

Start from the worst differential and decide whether it loads the tube from outside or inside. Take a starting wall from the diameter band, check it against allowable pressure after open area, and confirm with a hydraulic test. Specify a minimum wall.

Q: Why does wall thickness matter so much more on a large tube?

Collapse pressure scales with the cube of the wall-to-diameter ratio. Doubling the wall at a constant diameter multiplies collapse pressure by about eight, while doubling the diameter at a constant wall divides it by about eight. Large thin tubes are the weakest combination available.

Q: How much strength does open area remove?

It depends on the convention. Ligament efficiency on a 2 mm hole at 3 mm pitch gives about 0.33, while the square root of the remaining section gives 0.77. Ask the supplier which convention the quotation uses.

Q: Which pressure should I use to size a backwashable tube?

The reverse flow differential, not the line pressure. Backwash runs at 1.5 to 3 times the service flow, and the surge when a valve closes can spike two to three times the steady figure, so the governing case is larger and shorter lived than normal operation.

Q: Is a stiffener ring better than a thicker wall?

Usually, on a long tube. A ring at mid span shortens the unsupported length and raises collapse pressure without adding weight along the whole shell, which makes it cheaper than stepping up a thickness. The order is a ring first, wall second and diameter last.