PTFE Diaphragm Guide
A diaphragm is asked to do two incompatible things: flex millions of times, and resist chemical attack. PTFE is specified because it does the second better than anything else — the design work is all about surviving the first.
Why PTFE, and why it is difficult
Elastomer diaphragms flex readily but are attacked by aggressive media. PTFE is chemically almost inert, but it is stiff, it does not recover elastically the way rubber does, and it cold-flows. A PTFE diaphragm therefore fails by fatigue at the flex line rather than by chemical degradation.
That is why most PTFE diaphragms are not plain discs. They are either two-piece — a PTFE face bonded or clamped to an elastomer backing that provides the elasticity — or profiled with a convolution that gives the movement somewhere to go.
The flex line is the design problem
Stroke length relative to diameter governs life. A diaphragm asked to move too far for its diameter concentrates strain in a narrow band, and that band is where it will crack. If a diaphragm is failing repeatedly at the same radius, the answer is usually a larger diameter or a shorter stroke, not a thicker diaphragm — thicker material concentrates the strain further.
Grade selection
Unfilled PTFE is normal for the process face, because chemical resistance is the reason PTFE was chosen. Filled grades are stiffer and generally reduce flex life, so they are specified only where the diaphragm also has to resist pressure-driven cold flow into a port or support ring.
Clamping
The clamped edge must not be a stress raiser. A sharp clamping edge cuts the diaphragm at the point of highest bending, so clamp faces are radiused and the clamping load is set to seal without crushing. Overtightening a PTFE diaphragm is a common installation error that shortens life dramatically.
Temperature
Flex life falls as temperature rises, because creep accelerates. A diaphragm rated for a given cycle count at ambient will not achieve it hot. Where the process is hot and cycling, expect a shorter replacement interval and plan for it rather than treating it as premature failure.
When to replace
On cycle count, not on visible condition. A PTFE diaphragm typically shows no warning before it cracks through, so scheduled replacement based on pump hours is the correct maintenance strategy for anything where a leak matters.
Related pages
See PTFE diaphragm for the product, and PTFE ball for AODD pump for the check balls in the same pump. Material behaviour is covered on virgin PTFE, and terms in the PTFE glossary. Send pump model, stroke and media to HEMSUN.
