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PEEK vs Carbon Filled PTFE for ball valve seats 3
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MikeHalloran
Mechanical
PEEK has come down a lot in price since the last time I looked at it. A 2-1/2" dia rod, probably about the right size to make a 2" ball seat, is only a little over $400 a foot, per professionalplastics.com . They don't provide an online price for carbon/PTFE. If performance is otherwise equal (I have no opinion about that), then price may become a valid selector.
What will you do for a stem seal?
Mike Halloran
Pembroke Pines, FL, USA
What will you do for a stem seal?
Mike Halloran
Pembroke Pines, FL, USA
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moltenmetal
Chemical
PEEK will survive these conditions long term- it has an upper service temperature limit in seats and seals somewhere nearer to 600F (315 C). It does however result in a much larger stem torque than does reinforced Teflon, requiring larger actuators.
Carbon-filled PTFE will probably perform acceptably long term, at 400 F and low pressure. Exceed 450 F though and the carbon-filled Teflon is likely to creep away on you.
There are some valve manufacturers who have been creeping up the well-established upper temperature limit of PTFE from 450 F (232 C) where it has been so for the entire previous generation, to 500 f (260 C), by claiming that their reinforcement/filler material makes that possible- metal powder, graphite, carbon fibre etc. Aside from vendor datasheets making this claim, I've seen no evidence from testing or service history that you can extend that limit to 500 F continuous duty in valve seats and seals with any of these fillers and still expect satisfactory long-term performance. I've asked for that evidence here and received crickets in response, so if anybody has it they've been silent about it.
Where you may be able to get away with this is in services where the service temperature is no more than 450 F but you need a "design" temperature of 500 F to handle brief upsets- then, perhaps, you'd be OK.
Carbon-filled PTFE will probably perform acceptably long term, at 400 F and low pressure. Exceed 450 F though and the carbon-filled Teflon is likely to creep away on you.
There are some valve manufacturers who have been creeping up the well-established upper temperature limit of PTFE from 450 F (232 C) where it has been so for the entire previous generation, to 500 f (260 C), by claiming that their reinforcement/filler material makes that possible- metal powder, graphite, carbon fibre etc. Aside from vendor datasheets making this claim, I've seen no evidence from testing or service history that you can extend that limit to 500 F continuous duty in valve seats and seals with any of these fillers and still expect satisfactory long-term performance. I've asked for that evidence here and received crickets in response, so if anybody has it they've been silent about it.
Where you may be able to get away with this is in services where the service temperature is no more than 450 F but you need a "design" temperature of 500 F to handle brief upsets- then, perhaps, you'd be OK.
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Our steam is about 400°F, and a few years ago we switched to PEEK-seated ball valves for vents and drains. Been working well; one caveat - PEEK seats really increase the required actuating force on a ball valve. Check the force required to open or close the valves before you standardize on anything.
Also agree with moltenmetal, the 'new' uprated valve seating material is not backed with testing or history. If your valves will be hard to maintain or replace, go with PEEK and a longer handle.
Also agree with moltenmetal, the 'new' uprated valve seating material is not backed with testing or history. If your valves will be hard to maintain or replace, go with PEEK and a longer handle.
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