Are You Using the Correct Bends for Your Pneumatic Conveying System?

To get the best lifespan and product quality out of your pneumatic conveying system, you need to be sure that you’re using the correct bends within your configuration. Whether it’s for a dense phase or dilute phase system – with a pressure or vacuum process – it’s vital that your system operates with the most appropriate equipment.

Conveying bends in pneumatic conveying systems generally fall into three separate categories:

  • Standard Long Radius Bends
  • Short Radius Elbows
  • Specialty Bends

Standard Long Radius Bends

Standard long radius bends are perfect for most dilute phase pressure and vacuum conveying systems using bulky materials that aren’t prone to any issues relating to abrasion or degradation, whether thermally or physically.

The bends are formed from a straight length of pipe / tube and rolled to a long radius (typically a rolling radius that is  10 x the pipe diameter), sometimes filled with resin when being rolled to prevent the pipe being pushed oval as it goes through the rolls.
The main appeal of the standardised long radius bend is that they don’t suffer the large pressure drop of a short radius elbow and they are readily available, easy to utilise in most sizes and formats and also considerably cheaper than their specialty counterparts.

Conversely, every set-up will have its faults, and unfortunately, the standard long radius bend is no exception. They are far more prone to wear and abrasion and product degradation and damage.

Typically fragile and friable products like granulated sugar or grits and flakes can damage as they travel around the outer face of the bend at high velocity and break down into smaller particle sizes and even dust.

Also, low melt temperature products such as certain plastics and chemicals heat up due to friction as they pass around the bend at high velocity and the elevation in temperature causes the particles to partially melt and deposit on the bend. Typically Polypropylene pellets can generate heat causing them to partially melt and leave deposits on the inside face of the bend called “snakeskins” (when they fall off the bend they look like a snakeskin) and as the pellet skids around the bend and heats up it generates a tail sometimes called “angel hairs”. When many of these angel hairs have been formed and they break off the pellet they can gather together in cyclones, hoppers and receivers and they can knit together into a mat and are sometimes called “birds nests”.

Also, the internal integrity of the bend itself is more prone to wear and abrasion after substance exposure from abrasive products like sand and products with high silica / mineral content. As the particles move at high velocities around the outer internal face of the bend, scouring occurs causing wear and eventual puncturing of the bend.

Short Radius Elbows

Elbows are mainly used in Dense Phase conveying systems where the material is conveyed at high pressure and low velocity. Typical products suited to this type of conveying are products that fluidise easily and ones that have a tendency for air retention. The product behaves similar to a fluid and can be transferred at very low velocity and in a dense condition (high ratio of solids to air). Due to the fluid conditions and low velocity, short radius elbows are perfectly suited to these type of systems.

Short Elbows on Dilute Phase Systems:

In dilute phase systems the conveying velocity is higher as the product needs to be kept in suspension in the air stream. Short radius bends are generally not suited for dilute phase systems, the particles being conveyed are subjected to an abrupt change in direction which causes loss of momentum and velocity, as the particles slow down they can fall below the suspension velocity (saltation velocity) and clump together. Increased pressure and energy is required to accelerate the particles back into the air stream and to their normal conveying velocity. The effect of this is that short radius elbows increase the pressure on dilute phase systems and can lead to blockages. 

SPECIALITY BENDS

Blind-Tee Elbows

By introducing a secondary impact zone through a pocket of material, there is less abrasion involved with a blind-tee elbow. This allows for the safer passage of product, as it lessens the amount of high impact contact to the walls of the elbow.

Perfect for: short, dense or dilute phase systems with dedicated product types to avoid the cross-contamination issue.However, due to the second impact zone, the pressure is heavily dulled in this system, increasing the possibility of cross-contamination.


Spherical-Chamber Elbows

Creation of a spherical chambers or pot that produces a gathering of product and rotating movement offers lower abrasion and friction as the product abrades against itself as it changes direction instead of abrading against the pipe wall. Vortice elbows have a bulbous extension which create a circulating flow pattern or a pocket of material that cushions the impact of the incoming stream


Perfect for:
abrasive fluidisable powder products. 

Long Radius Speciality Bends

Abrasion Resistant Lined Bends

With the inclusion of a liners or wear-plates in the main impact area, this type of bend is used when abrasive products are transferred in conveying lines. They are available in a multitude of sizes and designs. Typically lined with Basalt inserts for maximum abrasion resistance or with hardened steel backplates or channel backed to allow material to fill the void and act as the wear zone.

 

Long radius bends with hardened steel back plates. These are bolted on and can be easily replaced. Pneumatic conveying bends lined with ceramic / Basalt inserts / tiles for maximum abrasion resistance Long radius bend with a channel back plate. Back plate filled with resin / concrete.

Perfect for: dilute-phase and dense phase systems using pellets and powders. Good for strong, moderately-abrasive products.

Not sure on the correct set-up for your pneumatic conveying system set-up? Contact us here at STB Engineering, and one of our specialist advisors will be able to discuss and answer your queries and questions and help resolve any conveying issues you may have.