Depending on the type of material being handled, especially the amount of abrasives and solids, ePTFE could be the best diaphragm material.
by Michael Brooks
November 21, 2017

Air operated diaphragm pumps are used in many applications requiring abrasive and chemical resistance. When both are needed, an elastomer with some chemical resistance, or a polytetrafluoroethylene (PTFE) diaphragm with an elastomer backing, is typically used.

abrasivesImage 1. Abrasives in the test slurry, which consisted of one part abrasives and 70 parts water. (Images courtesy of Garlock Sealing Technologies)

The problem with most elastomer diaphragms in abrasive chemical applications is they eventually weaken from chemical attack and erosion from abrasives. This results in loss of tensile strength and durometer, making them vulnerable to rupture and disintegration into the media.

PTFE diaphragms can provide superior chemical resistance, but less abrasion resistance and flexibility than elastomer diaphragms. Several factors influence the abrasion resistance and flexibility of PTFE.

Most diaphragm manufacturers use a skived PTFE made from a conventional or modified material, both of which offer strong chemical resistance. Conventional PTFE is highly crystalline and has less flexibility than modified PTFE. Modified PTFE tends to have less abrasion resistance because of its amorphous structure.

ePTFE composite diaphragmImage 2. The ePTFE composite diaphragm after 24 continuous hours of testing (155,000 cycles and 3,456 gallons pumped)

Expanded and tensilized PTFE, or ePTFE, is manufactured by a proprietary process. ePTFE provides toughness and abrasion resistance, while a rubber backing lends flexibility.

Test Results

Internal testing was conducted to compare the performance of ePTFE composite diaphragms with two-piece, rubber-backed PTFE diaphragms (see Images 1-5). It involved two identical 1.5-inch inlet/outlet air operated double diaphragm (AODD) pumps at the same elevation and air inlet, media inlet and outlet pressures. One was fitted with an ePTFE composite diaphragm, and the other with a two-piece diaphragm with PTFE facing the media and a thermoplastic elastomer facing the air side.

diaphragm after testingImage 3. The ePTFE/fabric/rubber diaphragm after 194 continuous hours of testing (1,250,000 cycles and 27,000 gallons pumped)

The media was a slurry of one part abrasives and 70 parts water. The abrasive media consisted of 41 to 53 percent silicon dioxide, 7 to 31 percent iron oxide, 17 to 25 percent aluminum oxide and 3 to 15 percent calcium oxide with trace amounts of various inorganic materials.

Many users of air operated diaphragm pumps encounter problems with abrasives and chemicals in the media. These problems result in expensive downtime, equipment damage, maintenance and repair costs, and lost production.

An applications engineer or pump manufacturer should be contacted when selecting a diaphragm. One-piece ePTFE composite diaphragms resist chemical attack and physical disintegration from abrasives to deliver long service life, improved productivity and greater safety. Four examples show companies resolving problems by using ePTFE composite diaphragms.

ePTFE after testingImage 4. The ePTFE composite diaphragm after 372 continuous hours of testing (2,400,000 cycles and 53,000; gallons pumped). The ePTFE began to tear, but did not break apart from the diaphragm.

1. Chemical Cleaning Company

Delta Tech Service Inc. in Benicia, California, specializes in decontaminating oil refinery equipment. The company designs and fabricates skids that utilize two 3-inch AODD pumps joined at the manifolds, allowing users to simultaneously operate one or both pumps.

One of the challenges in the field was the short life of the fluoroelastomer diaphragms that came with the pumps. Ethylene propylene diene monomer (EPDM) and conventional skived PTFE diaphragms with thermoplastic elastomer backings were used, but none lasted longer than a 30-day rental period (see Images 6-7).

PTFE overlaysImage 5. Conventional skived PTFE overlays after 24 continuous hours of testing (155,000 cycles and 3,456 gallons pumped)

Delta Tech then tried an initial set of ePTFE composite diaphragms that lasted for more than 90 days—more than four times the life of the EPDM and two-piece PTFE/elastomer diaphragms. Most of Delta Tech’s pump applications involved coke dust, condensate and various chemical cleaning agents, which can soften the thermoplastic elastomer and tear through the PTFE.

By converting to the one-piece ePTFE composite diaphragms, the company realized significant savings in maintenance costs and system downtime.

PTFE and elastomer diaphragmImage 6. The two-piece PTFE and thermoplastic elastomer diaphragm is shown. The conventional PTFE separated from the assembly, and pieces of the diaphragm entered the media after abrasives and flexural stresses tore it apart.

2. Zinc Refining Company

Extracting metals from a refining process requires pumps that can move various chemicals, metal fragments and dust particles. A U.S.-based company that specializes in the extraction of zinc uses 10 AODD pumps to move slurries of sulfuric acid (up to 90 percent concentration), hydrochloric acid (up to 36 percent concentration), Di-(2-ethylhexyl) phosphoric acid (D2EHPA), kerosene, metal powder and metal flakes. AODD pumps were used because of their ability to handle solids and move to different areas of the process.