Why use ball valves. 4

[rsv]

I don't know whether this is a simple Q or not.
Why use ball valves on Gas pipelines instead of gate & or globe valves, am I missing something?

I am new to Gas pipelines having mostly worked in petrochem & powerstations where gate & globe valves are the standard for isolation because they are cheap & reliable.

I have used small 150# ball valves in Pharmaceutical plants but these are 30" 2500# and must cost way more than gate valves.

 

[rsv]

Thanks for all the replies there have been some interesting points.

Apart from the pigging aspect why not use a butterfly valve although they were not mentioned in my original post I would imagine they are a lot cheaper & initially as leak tight as a ball & there would be no need for cavity relief?

My initial post was because of thoughts of long term seat reliability which has been mentioned above.
Metal seated valves especially with HF are allowed very marginal leakage across the seats but I would not expect it to deteriate as much as a soft seated valve.
I know trunnion mounted ball valves have cavity plugs & seat / stem sealant injection ports on larger valves to compensate for the above but, that makes me think the manufacturers are expecting these valves to leak.

Good point about emissions but I guess you could use bellows sealed.

As far as automation goes I can see the point as far as speed goes but the size of a pneumatic actuator for a large ball valve is incredible compared to an electric actuator for a gate valve especially for parallel slide.

As far as getting rid of the bonnet gasket - it is replaced with a body gasket on most ball valves & to pass the fire proof tests most ball valves require graphite packing to the stem & seat assemblies.

So ball valves appear to be the new kid on the block because our machining has gotten better but, I still think they are expensive (not quite on target but 1.5" metal seated 1500# ball cost £200 comparable globe £40) or maybe I am just a little old fashioned.

 

[JimCasey]

When you compare to buterfly valves, it's important to make a distinction: Rubber lined/High-performance (Double-offset) or Triple offset.

In general, butterfly valves are limited because the vane bisects the flow path. So pigs, slurries, etc. meet an obstacle, and the sealing edge of the vane is directly exposed to erosion. Also, the vane generates a significant dynamic torque which peaks at about 70 degrees open. It makes butterfly valves a little challenging to throttle with. Since the vane is on a shaft, which can be considered a beam supported outside the pipe cross-sectional area, they are limited in the shutoff they can take. ASME 150 and 300 are universally available in HP valves, ASME600 is seen less frequently but still not impossible to find, but by the time you get above class 600 the shaft diameter has to be so big it's almost hard to tell when the valve is open.

Rubber-lined butterfly valves are typically rated pretty low in shutoff pressure...225 psi or less. DOuble-offset valves must have resilient seats so once the application temperature exceeds the capabilities of available polymers, metal seats are used and leakage drops usually to FCI70.2 class IV(altho there are some class VI metal seats.) Metal seats available to around 1100F, but they ARE a bit fragile since they must be made of thin stock to be conformal. Rubber-lined and Double offset valves are position-seated. They are as closed as they get when the major axis of the vane is at 90 degrees to the pipe axis. If you go past this point, they begin to open again.

Triple offset valves can use metal seats for zero leakage. They are torque-seated. Usually there is a conforming insert, but the amount of distortion it must accomodate is small and the sealing member can be quite robust. To close the valve, run the disc into the seating surface, and torque like mad to cause the surfaces to conform to each other.

Butterflies are less expensive than ball valves of the same size, shut off better at low pressures, and are very compact. Butterfly valves usually are available in 2" and larger, while ball valves from 1/4 inch are the norm.

Please have the following etched on a granite slab and displayed in a place of prominence:

THERE IS A BEST VALVE FOR EVERY APPLICATION

Type X valves are not universally better than type Y valves. One valve type does some things better than other valve types. Knowing what you want the valve to do, and selecting the right valve to do it is the key to succesful application.

 

[Ashereng]

Hi rsv,

I would like to add that in a butterfly/disc valve, the sealing elements tend to deteriorate "faster" (a relative term) than those in a ball valve. This results in leakage.

Your original post pertained to shutoff service, but did not give specifics to what level of shutoff is required, it is difficult to say whether a butterfly is suitable in your application.

My above posts have indicated that I feel once a valve is installed, bubble tight does/may exist - this is still valid. Having said this, a butterfly may be appropriate, it all depends on the condition, requirements and service.

My suggestion is to have your I&C (instrumentation and control) resource person size/select the valve fit-for-purpose for your application. That is what we do.

 

[rsv]

Thanks for all the comments.
From what I have gleaned from the above there is nothing cut & dried, it is still a question of personal choice / experience and how long you think resiliant seats will last compared to conventional HF seats.

The reason the question was asked in the first place was because I joined a project 1/2 way thru' after the valve types had been decided, I would have probably gone for gate valves but I wasn't sure if that was because of my background.

I have heard a lot of stories over the years about very early failures of soft seated ball valves are these just stories or is it still true?

 

[Ashereng]

rsv,

As with most stories, at one point, it was probably true, and now, probably less true.

Ball valves have come a long way "over the years", along with everything else (e.g. my car, laptop, TV).

At one point, ball valves were not really used for throttling. Now, there are many characterised ball valves designed specifically for throttling (and the characteristics can be engineered to give exactly the response you want).

With regards to the early failure of valve seats, companies like Orbit, have specific designs to minimise seat failure (they ball moves off the seat befor rotation). Other companies have improved the characteristics of the resilient seat with better materials. The use of trunnion ball valves with springs has also increased.

Also, there are many types of ball valves, and, the wrong valve in the wrong service may also give rise to these horror stories.

My suggestion, as stated above, is to have your I&C (instrumentation and control) resource person size/select the valve fit-for-purpose for your application. That is what we I&C people do.

You stated above "From what I have gleaned from the above there is nothing cut & dried, it is still a question of personal choice / experience and how long you think resiliant seats will last compared to conventional HF seats."

Your I&C resource will choose the right valve solution based on sound engineering reasoning and knowledge, and not so much on "personal preferences" (sometimes, there is a right answer) and "how long you think resiliant seas will last" (because they have actual data/information).

 

[rsv]

Thanks Ashereng.

I don't think it would be a good idea for me to ask my I&C resource, as my job is to specify the valves - I don't have a problem with doing this.

The question was put to get other peoples opinions on why they thought ball valves were better for gas pipelines & I have found all the responses interesting.

As long as the valves are either actuated or not used very often it appears to boil down to the (perceived) holy grail of Tight shut off & zero emmissions to the enviroment & a smaller footprint.

I personally believe that all valves will leak probably sooner rather than later but, that the actual leakage rate although it sounds a lot compared to zero is not that bad.
If you are really really concerned about leakage then use 2 valves in series regardless of the valve type.

I would very much have to agree that the stem sealing arrangement on ball valves is very much better than standard gate / globe valves but, if it is a real concern or the process is toxic then a bellows seal gate will beat a ball valve on emmissions.

If you are using automated valves then the pros and cons are horivontal real estate for ball valves & vertical real estate for gate valves and this would probably depend on the type of plant.

I have known ball valves that could not be moved after long periods with no use because scale formed around the seals & ball and the actuator did not have the torque required to move the valve whereas a gate valve would move but may leak slightly across the seat?

 

[Ashereng]

rsv,

My apologies. No offence meant.

I too am in I&C, and I often ask my peers for help in areas where I deal less in, and/or am less experienced in.

You mention a good point though with valve failure (it won't close when you want it to, or vice versa). For almost all ESD applications, using the SIL analysis/method, there is a requirement to do regular checks/maintenance on the SIL valves. The reason is to avoid an on-demand failure. Any valve that is neglected will eventually fail.

In line with the ESD application - for SIL3 application, one of my client's standard is 2 parallel trains of 2 valves in series. This is very expensive. It does however have a probability of failure that is very very small, and allows for testing without taking the process down.

 

[rsv]

No offence taken.

When it comes to SIL rating 2 in series does mean you can reduce the SIL level required of the individual valve/actuator so I think it is a good idea.

I believe that people get really hung up with the SIL rating of the actual valves (It is after all an instrument safety system 2oo3 poling etc.)- I know of only two manufacturers in Europe that have had independent sil rating of there valves & 1 is not a ball valve but an Axial valve.
Even though most manufacturers have not provided fault tree analysis etc. to achieve sil rating, I believe that ball valves from reputable manu. would be OK.

 

[Ashereng]

rsv,

I have not come across any TUV SIL rated valves. I have on the detection side (pressure xmtrs, etc.), and of course, on the control system side.

For our SIL application, because the client was very large, they had maintenence data from which the valve availability/failure rates were mined. We were actually able to use actual data! Haven't happend since, but it was nice.

Another common practice is to use a non-SIL valve (perhaps a globe throttling control valve) up stream in series with a SIL rated ESD valve. This way, the shut-off requirement would be lower, since you are taking credit for the upstream control valve shutting down the flow somewhat.

Ooops - I think we should probably start a new thread for this.