By Joanna Robinson
.....a.k.a. PTFE bellows or expansion joints (of course!). Their unique properties make them particularly suitable for certain specialist applications.
Selecting the right approach
PTFE is very slippery and corrosion resistant. It's both high and low temperature tolerant, has minimum porosity and - most importantly - is non-reactive. Think Teflon, the best known brand name of PTFE-based formulas ... FlexEJ regularly sells PTFE products into the chemical and pharmaceutical industries for example. We’ve also sold them to plants producing white plastic granules where it’s imperative to maintain their colour throughout production – keeping their “whites right” you might say.
We offer a choice of design : either the whole bellows section is manufactured from PTFE or alternatively a standard rubber bellow fitted with a high quality PTFE liner. Both types are FDA approved.
But which to select? As well its minimum porosity and non-reactive qualities, PTFE is good for media flowing at moderate industrial temperatures. A bellow made from all PTFE can withstand temperatures up to 235 degC. It’s very supple too – with high lateral and axial movements depending on size and number of convolutions.
But this ‘bendiness’ can be a problem; these bellows tend not to be inherently very strong although to some extent this can be overcome by adding reinforcing rings which help to aid stability. Another factor to consider with a pure PTFE joint is that at higher temperatures, the bellow can’t tolerate higher pressures. At 235 deg C, the pressure rating drops to just 3 Barg for instance.
So what if the media demands corrosion resistance and a high temperature but there is significant pressure involved? In this instance, a high quality standard rubber bellow fitted with a PTFE liner may well be a suitable solution. At FlexEJ we recommend the Elaflex brand fitted with their own PTFE liner – it’s made of pure virgin PTFE, is seamless with a thickness of approximately 1mm. Elaflex advise, however, that the movement range becomes restricted by around 50% when a liner is fitted. For example a PTFE lined Elaflex RedBand expansion joint is rated up to 16 Barg at 100 deg C but the lateral movement reduces from 40mm to 20mm (depending on size) when the liner is added.
Making the right choice requires knowledge and experience – as we’ve tried to show here, there are a number of factors to weigh up. And then there are price implications and longevity statistics to consider. At FlexEJ our engineers have been successfully advising and selling these products for well over ten years so make good use of our experience and get in touch, we’d be pleased to help.
For more information on how to select the best expansion joint for your installation call Gareth Hardy or Danielle Jobe on +44 (0)1384 881188 or use LIVE HELP or our GET A QUOTE form.