Selection of seat material based on Operating or design temperature.

[ET1983]

Dear all,

I know that, many threads explain about the selection of seat material and its pressure and temperature ranges for specific fluid services. Also many vendor charts show the Seat material specifications and its limitations with respect to the pressure and temperature.

However, my question is whether the Seat material is selected based on the Operating conditions of the fluid service (or) on the design conditions of the fluid service.

 

[TD2K]

Unless I had a very, very good reason I use the design temperature. Why? Because that is the temperature that the system can operate to so my materials in turn should be good for those materials.

 

[pdp123]

Agreed with TD2K
Also look for Upset conditions.....

Regards
PDP123

Hard Work with Smartness is the key to success

 

[SJones]

How much do you believe in the accuracy of a quoted maximum operating temperature as opposed to the all encompassing envelope of the design temperature?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[ET1983]

Thanks all for the inputs.

However, i have seen cases where the design temperature is around 200+ deg celcius and the operating is somewhat around 120-130 deg celcius for sour service environment, where they look for tight seal.
As we all know that, Soft seals give better sealing than the Metal seat. In these cases, selecting a Valve seat for the operating temperature will'nt be a right choice.

Also metal to metal seats are costlier and also..the damage on the seats wud be more wen used with sour service application.

Please comment.

 

[moltenmetal]

Mechanical engineers dealing with metals and alloys know that unless you're near the top end, 50 F doesn't mean much and a 50 F design margin can be added without really influencing cost significantly. With seal materials however, 50 F difference in continuous exposure temperature can be the difference between adequate service life and quite rapid failure. A 50 or 70 C difference in temperature is a totally different world! You need to use care to avoid tripping across a seal temperature limit arbitrarily.

If the process is truly out of control in temperature by a significant amount such that significant excursions long enough to bring mean seal temperatures in valves higher than their limits for extended periods are a real possibility, then you must be conservative. In that case, chances are your system as a whole will suffer from other problems too and could use some additional work. But if, as in many systems, temperature and pressure are tied to one another (by the vapour pressure of a liquid system etc.), or protected by controls, then tighter limits can be meaningfully set for the system's P/T envelope and better sealing material selections are possible.

Too many designers are sloppy or arbitrary with safety factor selection, and in many cases safety factors are "chained" or arbitrarily added to by subsequent members of the project team, each in an effort to cover their own backsides. This can easily result in bad selections, where under-performing or expensive high temperature materials are selected needlessly.

If you eliminate a class VI shut-off valve in favour of a class IV metal-seated valve merely in an effort to be "conservative" on operating or upset temperature limits, you're not really doing the project a favour!

This kind of sloppy safety factor selection has, in my opinion, resulted in some "grade inflation" of some seal materials' stated temperature service limits over time. Whereas Teflon-containing mateirals were once universally agreed to be of little use when continuously above 450 F (232 C), many are now making claims about extending reinforced Teflon valve seat limits to 500 F (260 C). I've asked for but have never been given the testing data which shows that the up-rating is based on anything. Rather, I think what has happened is that people are using Teflon-seated valves in 450 F services with 500 F design temperatures and getting away with it because they never actually operate above 450 F.

 

[Danlap]

Hi,

Unless you have a very good reason to claim warrant, then most of the case the manufacturer is off the hook after trouble occurs related to the installed valves. This include leakage rate, material vs operation (P/T) compatibility, etc.
Talking about soft seat, I assume the discussion is over Ball valve, (some) Plug valves, (Double / Single offset) Butterfly valves and limited number of Gate valves.

Sour service = NACE MR 0175. Or other words wetted (metal) trim recommended to meet certain hardness level (among other requirements). Common method is Hard Facing
What is tight seal (lets use ISO 5208 as reference):
Is Class A tightness (zero leakage) throughout its lifetime? Or Class B is enough for all of its lifetime?
PS: Let's say life time is 5 years conservatively.

Ball valve:
By using soft seat, one cannot guaranteed that harder particles (let say one grain of sand/salt) may not pass the pipeline and scratch the Seat. Thus, you lost the leakage Class A within several stroke after service.
Metal seat. As single ball valve, one can expect Leakage Class B (this is small by the way) for longer period of time. As double Ball valves (sometime with bleed) installed in series, its considered as Tight Shut Off.

Plug valve:
Many OEM suggest Viton/O-Ring/Rubber Sleeve to be replaced every 3 years or xx cycle (whenever criteria meet first)

Double offset Butterfly valve:
I don't have to elaborate this. Many disadvantages compare to advantages.

Gate valve soft seat:
Operator must carefully close the valve with delicate force, in order not introduce excessive force towards the vulnerable seat.

Therefore, unless the manufacturer can confirm that their Soft Seat is suitable for 200+ degree (sour service), provide performance warranty and idiot proof (for operator). suggest to use Metal seat whenever in doubts.
Costlier? It would be costlier to find a very reliable soft seat at 150-200 degree Celcius and shall the soft seat valves leaking sooner than it was supposed to.

Regards,



All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

 

[ET1983]

Thanks to moltenmetal and Danlap to share their knowledge and experience.