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.
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.
Expanded and tensilized PTFE, or ePTFE, is manufactured by a proprietary process. ePTFE provides toughness and abrasion resistance, while a rubber backing lends flexibility.
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.
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.
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).
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.
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.
Abrasives in the media drastically reduced the diaphragm life. Initially, the company used two-piece PTFE diaphragms with rubber backings, which developed concentrated areas of wear that cracked and flaked off into the process media and pump motors. The PTFE was forced into the air side of the pumps, which damaged their air distribution sections and cost the company more than $5,000 per pump in downtime, repairs and other costs.
The company’s maintenance, repair and operations (MRO) distributor suggested using an ePTFE composite diaphragm with molded-in ribs to reduce concentrated flex points and compare its performance to the diaphragms they had been using.
After five weeks of operation, the two types of diaphragms were removed from the pumps and examined (see Images 8-9).
The ePTFE composite diaphragm showed no crease marks or signs of significant wear or chemical attack. The two-piece diaphragm was torn apart by the metal flakes, and the elastomer backing had indications of wear and chemical attack, eventually leading to rupture and pieces of the diaphragm mixing with the media. Because the composite diaphragm is bonded together, the ePTFE does not disintegrate easily and enter the media.
The ePTFE composite diaphragms lasted more than seven months, while the original two-piece conventional PTFE and elastomeric diaphragms lasted an average of five weeks.
3. Mining Company
A precious metals mining company wanted to get better life out of the diaphragms in its vertical AOD pumps. These pumps operated outdoors at temperatures ranging from 15 to 100 F and inlet pressure of 40 pounds per square inch (psi) for more than 20 hours a week. The media typically consisted of 70 percent hydrofluoric acid concentration.
The company used chlorosulfonated polyethylene synthetic rubber until it was discontinued by the primary manufacturer before trying fluoroelastomer and EPDM as replacements. These diaphragms had to be replaced every four weeks. Each replacement required two employees working an average of seven hours. This added up to 22.75 work days annually per pump.
The company’s head of maintenance selected an ePTFE composite diaphragm. It was tested for six months and the company retrofitted its pumps with ePTFE composite diaphragms. Now on a routine maintenance schedule where the diaphragms are replaced every six months, the company is experiencing greater efficiency resulting from decreased wear on their pumps and systems, less downtime and reduced maintenance. Employees are also less likely to be exposed to highly corrosive acids while repairing the pumps.
4. Automobile Manufacturer
A major U.S. car manufacturer was using primarily AODD pumps to move phosphoric, sulfuric and hydrochloric acids and an abrasive slurry of dirt and metal flakes. The temperature of the media ranged from 100 to 120 F.
The company was using AODD pumps ranging from one-half to 3 inches with varied diaphragm life. They were using two-piece conventional skived PTFE and elastomer diaphragms with an average life of three months. Eventually, however, the company selected ePTFE composite diaphragms as a more durable alternative.
Testing began and the difference was noticed within months. The PTFE was no longer fragmenting and breaking off into the media; wear from the metal flakes was greatly reduced; and the rubber was not being attacked by the phosphoric acid. Based on these test results, the company began changing out the failed diaphragms for ePTFE composite diaphragms, extending diaphragm life from three months to 18 to 24 months and reducing downtime by more than 80 percent.