API 6D Slab Gate Seats: RTFE vs Devlon vs Metal to Metal

[Gorman3]

A customer of mine wants some large bore API 6D slab gates, media is corrosive brackish water. The customer absolutely insists on RTFE seat inserts. Usually a 900# valve would come with Devlon seats, with PEEK being a more expensive alternative for the highest temperatures.

For whatever reason they wont accept Devlon or any nylon type product. Is there any good reason for this other than ignorance of the product? Is there some problem with Devlon in corrosive service? I offered PEEK but they don't want that either.

I am thinking a metal to metal seat with ENP & TC is the next option. I have little knowledge of metal seated slab gates. From what I know there must be a higher precision during machining to assure the metal seats do not gall during operation. Otherwise can they hold water as well as a soft seat? Is the design much different that a soft seat slab minus the groove for the seat ring?


Ultimately I think this is a case of an end user's engineer who approved the wrong item and doesnt want to acknowledge his mistake and go through the trouble of presenting another design change.

thanks for your help.

 

[gerhardl]

You state: 'Usually a 900# valve would come with Devlon seats, with PEEK being a more expensive alternative for the highest temperatures.'

This might be the norm for your factory, but around the world constructional details and material variations will vary from one factory to another, and from use for one fluid range to another. On top of this the 'equal' resilent materials will vary more or less in properties from one manufacturor to another.

If a customer requires a specific resilent material combination, the reason might be 'non technical':
a) The customer has good experience or good references from one brand of valves using the materials.
b) The customer has good experience from a brand, and wish to favour this brand, disqualify or up the price for competitors where this is not a standard.
c) The customer has negative experience with other materials
d) The customer is open for alternatives, but is using 'old internal standards'

If your valve has a superior construction, good references and long standtime for equal purposes, you should let the customer know. Maybe you can offer both type sealings, or offer a demonstration valve in 'your' materials. Free trial periode if not performing to expectations?

Most important here is to check if you can supply to a competitive price (price/lifetime?) and acceptable deliverytime.

Good luck!

 

[Gorman3]

Thanks.

In oil and gas Devlon is standard as it is the most cost effective material to handle a wide temperature range in pressures engountered over 900. I think the customer doesn't really know what he wants. So aside from explaining to him what he needs I'd like to know if there is a possible interaction between Nylon the the brackish media. I'm fairly confident there is not just looking for some confirmation.
thanks

 

[subchrissea]

A lot will depend on the bore size of the valve but under normal circumstances Devlon would be perfectly acceptable and would not deteriorate in the brackish water. The non metallic seating such as Devlon is used for two purposes in a slab gate valve. Firstly, to act as a wiper on the gate to clean away debris. Secondly, as a softer seat to offer sealing at a low pressure differential.

The sealing mechanism on a slab gate valve is self energizing I.e. Using the pressure of the line to press the gate onto the opposing seat. If there is a low pressure differential on large gate valves there isn't enough energy to form a seal hence the need for a softer seat. If the client is insisting on reinforced TFE this could just be a personal preference or an existing spec that no one wants to change. If your client is so insistent and not prepared to reconsider, then it may be easier to give him RTFE on the understanding that it will cost him extra.

 

[sircrashahlot72]

I do know that Nylon products are reported to absorb water and can swell, and this is why there is some resistance to using them in water service. I've not had any experience of issues with this, but it is stated in test materials that they will adsorb water... For a large valve in high pressure, the PTFE may not hold up. PEEK would definitely be the better option.