Valve Seat/Stem Material for Monomer Service 1

[TiCl4]

All,

We have manual ball valves in monomer service - styrene, acrylonitrile, arylic monomers, etc, that sometimes have severely short service life. After 6 months of operation in a blended monomer service (emulsion polymerization plant, so these monomers are blended with water, surfactants, etc), these valves become inoperable. The valves have a 15% RPTFE seat material, and the stem packing is PTFE. I need to have our maintenance team pull these apart to figure out which is the issue, but I know that pure PTFE (and maybe RPTFE?) has voids. These voids allow monomer ingress, which then polymerizes over time, swelling the material.

Does anyone have good experience with stem/seat materials in monomer service? Of the pure monomers, styrene seems to have issues the most.

 

[TiCl4]

An addendum: I’ve seen a recommendation in the styrene storage and handling guide for plug valves instead of ball valves for styrene service. The intent stated is that ball valves trap monomer between the valve and seat, which can cause polymerization/seizing. Does anyone have experience with manual plug valves (2-3”)?

 

[shvet]

Does anyone have good experience with stem/seat materials in monomer service?

I do. I have worked at operation team of a proces facility handling acylonitrile,acrylamide,acrylic acid, methylacrylate. For pure monomer environment we use metal-metal gate&globe valves or PTFE lined diaphragm valves, most in range 3/4-2".

We have a negative experience with cheap PTFE (lining, packing, sealing) as it is sensitive to (a) quality of PTFE-powder it is made of and (b) a manufacturer a final piping element is made by, which both are unable to control actually. Most PTFE-powder manufacturers and PTFE-products manufacturers, we work with, cut expenses what drastically increases permeability and weaks bounding of particles it is made of.

What actually do you mean 'inoperable'?

Low-quality PTFE is prone to swelling. In zones of mechanical tenses and chemical environment PTFE degrades and easily get picked off with a fingernail.

I have never met PTFE O-rings in such service. Based on experience with PTFE I recommend avoiding plastic internals in monomer environment where it is possible.
I have no experience with olefin environment.
Regarding to stagnant voids pipng/valves, one is able to google photos of of blown pipings caused by olefins polymerization. I have spent some time to find some in my archive but found nothing as archive is large and there is no enough time.

Hope this helps

 

[TiCl4]

Shvet,

"Inoperable" means the manual valve is stuck and cannot be actuated by hand. The binding force is large enough that putting a cheater bar on the handle will end up bending the handle rather than turning the valve.

 

[shvet]

You do not the exact cause of this "inoperation", correct? You do not know is it sealing ring fault, polymer plugging, metal corrosion, or deformation of packing or internals?

I recall that we have a practice replacing globe/gate valves with diaphragm ones on lines where polymer plugging is a problem as diaphragm valves are immune to polymer. Those can flatten polymer and have no voids so we have no problems with tightening and opening/closing.

 

[TiCl4]

Update:

We've removed a valve and inspected it, with a picture below. It appears that monomer is collecting in the void between the ball and body and polymerizing in place. I think a plug valve would be the best way to prevent this. Diaphragm valves would also work, I think, but I don't think they have the same level of shutoff that ball/plug valves do.

 

[PeterIgg]

If the cavity is the problem and you only need a unidirectional valve, look at a segment type valve, AKA C-Ball. Just a thought.

 

[shvet]

I have been away from computer.

Diaphragm valves would also work, I think, but I don't think they have the same level of shutoff that ball/plug valves do.
Sure diaphragm valves are not the best choice because of poor tightening but they are still immune to a polymer any form of. While any other type are not. Being a person that had fought against polymer
for many years I recommend you to give a look at diaphragm valves one more time. Sure those are not applicable at all system points, but at some points the do provide incomparable benefits.

monomer is collecting in the void between the ball and body and polymerizing in place
In this case globe valves are better as those have no voids and perfect tightening at reasonable cost. Check if piping hydraulic and layout can tolerate those.
I suppose ball valves have been choosen as those are full bore and therefore guarantee free draining/venting.

Note that an ultimate mean to prevent valves polymerizing is washing the stagnant points with a solvent (constant or prior valve closing). I have met such and I should warn you that this is the most expensive option.