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.

 

[zdas04]

Ball valves do cost more than gate valves, and they're darn sure worth it.

Gate valves take a lot of effort to cycle (an 8" gate valve requires up to 100 turns). I started in steam plants where they were common and my first Oil & Gas project I specified them--some of the operators are still cussing my name. Also, in natural gas service, gate valves are not advertised as bubble-tight, while ball valves (especially trunnion) are.

Globe valves are a good choice in throttling service in most fluids. Any throttle valve will develop wear patterns on the seating surface fairly quickly and will not provide a postitive shut off. Also the changes in direction within the globe valve create a non-zero pressure drop. If you are using a globe valve in block-valve service then you're wasting compression (or pump) hp. It may be small, but over time it adds up to significant wasted energy.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[rsv]

I agree with the effort used by ops. but, I forgot to mention that the site is mostly automated so gates can use smaller actuators and take up less room.

Good gate valve manufacturers are offering resiliant seats which I assume will be as leak tight as ball?

 

[zdas04]

If it don't say "bubble tight" it aint.

David

 

[BSQUAREDBUCKEYE]

Ball Valves versus Gates and Globes?
1) Rotating in the packing instead of reciprocating the process from within or the environment from the outside through the packing gland along with the stem.
2) Elimination of the bonnet gasket and potential leak path.
3) Sealing member rotates in it's own volume instead of having to displace a volume.
4) A full ball is a balanced trim. A half ball, gate or globe valve plug would be unbalanced. This would be important with actuators if you lost your gas or air supply.
5) No collection zone in the bottom of the valve. There has to be room for the gate to move to and may have difficulty displacing what ever has collected there.
6) Smaller overall envelop dimension.
7) Ease of automation - ninty degrees to go full open to full closed. Not multiples of turns.
8) Ball valves for control must be sized by Cv not line size. They can have the same capacity as a globe that is twice their size. In other words, they can be half line size. Ball valves have equal percentage inherent characteristics. So when applied to systems where 30% or less of the system drop is applied to the valve, will provide a "linear" installed characteristic.

 

[JimCasey]

It appears the general trend of this thread is going in the right direction, but I will add my 2-cent's worth (after adjusting for inflation)

Gate valves were invented many years ago. They could be manufactured with the machinery and materials available in the early industrial revolution.They had a head-start on the market. Ball valves were invented in 1949. This was made possible by the availability of some of the polymers created in the 20th century. Although PTFE was invented in 1928, its development and availability as an industrial material enabled the development of the ball valve.

Ball-Valves shut off tight. "Bubble-tight" means nothing, but API6D does.

Ball valves are more compact. Generally manufactured to the same lay-length as gate valves (ANSI B16.10), but look at the dimansions of the assembly: Gate valve has to have a "chest" or bonnet, equal to the valve diameter for the gate to occupy when the valve is open. Then there's a packing box, and a yoke that has to provide for full diameter travel of the actuator stem. Then there's the actuator which has to be beefy enough to provide the "Hammer blow" feature necessary to seat and unseat the gate. Then above the actuator it is necessary to provide room for one diameter of stem to stick out when the valve is open (frequently inside a tube to protect the grease on the threads).So a motorized gate valve is typically around 5 diameters tall, and almost always has to be installed with the stem vertical to reduce bending loads on the "tower". Compared to a ball valve which has a short packing box, a short bracket, and an actuator.

Environmental concerns. Cycling a gate valve drags a diameter's worth of valve stem through the packing box, and drags a diameter's worth of environmental contamination ofn the valve stem back into the packing box. All the towering mechanism above the packing box puts a hopefully small but finite side load on the packing due to imbalanced mass. line vibration, etc. Packing in a gate valve therefore wears very quickly. Compare to a ball valve where the microscopic roughness on the valve stem moves packing particles through a rotational path that remains inside the stuffing box. Net migration of mass from/through the packing assembly in a ball valve is orders of magnitude lower than the gate valve. With a less expensive, simpler packing design a ball valve can have zero leakage to the environment for much longer than a gate valve.

Speed: A large gate valve requires minutes to close with a big electric actuator grinding away. Ball valves need speed controls on pneumatic actuators to slow them down to prevent water hammer. Which would you want to isolate your piping in the event of a fire?

Throttling:
Gate valves are guided very loosely except when the gate is wedged into the seat. If the operator tries to throttle with a gate valve all these loosely-affiliated pieces will jingle together due to flow eddies. Control will never be precise because the relationship of the trim pieces is not precise. Wear will be rapid, and catastrophic failure is likely. Ball valve: Guided positively. Small ball valves have the ball secured in an interference fit between two seats. "Floating" balls don't float very much, and careful fitting of the stem tang into the ball slot ensures accurate positioning of the ball. Larger trunnion valves are guided positively by top and bottom bearings (trunnions) so the ball is not deflected bt process effects, and the ball-to-stem joint is usually a positive connection.


Conclusion: if you want a cheap valve that is seldom if ever cycled and can live with a small trickle when it is closed or if you have large maintenance apes with long cheater bars to try to get tight shutoff, then gate valves are OK. If you cycle the valve with any regularity, or ever throttle with it, and want a high probablility of zero leakage either from upstream to downstream or from inside to outside, than a ball valve is a better choice.

 

[Ashereng]

Hi rsv,

From your question "Why use ball valves on Gas pipelines instead of gate & or globe valves, am I missing something?", I would like to ask you,

"What do you want this valve to do?"

Depending on what you need the valve to do, you need to select the valve appropriately. The valve is selected "fit for purpose".

Having said that, there are usually multiple options for valves for each application. There are also many factors in valve selection (valve sizing, cost, etc. are each a component to be considered in determining the selected valve).

As with anything, go to the expert/professional for advise. If one needs information on pumps, consult a mechanical rotating equipment resource. If one needs information on valves, I strongly suggest consulting your I&C resource (that's what they do) and ask them for further info and advice.

All of the above posting provide valuable information, insight, descriptions and value. However, they are not sufficient to select a valve as is, without further data and information.

One other note, the term "Bubble Tight" is sort of misleading.

ANSI B16.104-1976/FCI 70-2-1976, defines 6 seat leakage classs (note that they actually say "seat leakage" classes). All classes have an allowable "leakage rate".

Class VI is intended for resilient-seating valves (or soft seat), and provides that the leakage limits depends on valve size and ranges from 0.15 to 11.5 ml per minute for valves sizes 1" to 12", and 1 to 81 bubbles per minute.

The difference between the class include for different valve type, the allowed leakage, and testing conditions.


If you do not have an I&C resource available to you, this forum may be of help to you. I have seen other postings where people have provided sufficient information that someone was able to provide a solution.

Cheers!

 

[stanier]

Ball valves can be fire safe if they have a secondary seat and that is a requirement for gas service. Some gases contain componenets that cause seats to stick and you would be better off with a ball valve as there you wipe the seat. If it is real sticky then a valve like the Orbit valve is used. This has a complex mechanism where the ball moves axialy away from the seat and then rotate 90 degrees.

Careful of the quick opening if your gas has condensate. A slug of condensate accelerated through a ball valve by opening too quickly can cause damage.

There a re few books around on the subject of valve selection. I havent had the opportunity to review any. Have other forum members found any of real value? Too many books around are just cribs of "old wives tales" and stuck together as an authorative work. Best use for them is a door stop. These are generally titled "Handbook of... or Guide to..".

 

[jsummerfield]

ANSI or ASME B16.104 is withdrawn. FCI 70-2 was just recently updated to coordinate with the IEC. Zero bubbles is a term within the API standards such as 598. FCI is a control valve leakage rate. Control valves leak. Zero bubbles is bubble tight and clearly defined in the standards. Typically API 6D ball valves are suitable for isolation unlike control valves.


John

 

[ChokeEng]

Better yet, because of the sheer mass of the valve, many 6D pipeline ball valves (>12" NPS) can pass API fire test requirements without having to rely on additional "firesafe" seals because it's extremely difficult to introduce enough heat into the test environment to compromise the soft seals during the test interval. 6D ball valves excel in isolation applications and can usually be supplied in double block & bleed should the application require it.


don f.

 

[jsummerfield]

A 30-inch Class 2500 ball valve is large. So is everything else in the 30-inch system. An automated shutdown valve offshore needs to close within 45 seconds. As the valves become larger than 30-inch the actuators require quick exhaust and other tricks to meet 45 seconds. A 30-inch rising stem valve could be very hard to automate and meet 45 second closure.

Ball valves don't suit all applications. API 6D ball valves are typically limited to services below 400 degree F.

John

 

[Ashereng]

Hi rsv,

jsummerfield is correct. API 6D does have a "bubble tight", "zero bubble" classification.

I would like to highlight that "bubble tight" is a factory test/verification. Once the valve is installed and put in service, the bubble tight may no longer be there as the valve will start to deteriorate. If a true bubble tight is required, then there would also need to be a maintenance program to verify that the valve is still indeed bubble tight, and to replace the valve if required.

In my above posting, I was thinking throttling. I work more with throttle type valves - characterised ball, globe, butterfly/disc - I guess the mention of cheap, in conjunction with globe and ball got me.

Sorry about that rsv.

 

[jsummerfield]

I too agree about factory verses the field installation. Run that rusty hydrostatic testing sludge through those soft seats and they may begin passing sufficiently to be a problem.

John

 

[MikeHalloran]

I'm no pipeline expert, but I conjecture one reason for using ball valves instead of gate or globe valves is that you can pass a pig through them.



Mike Halloran
Pembroke Pines, FL, USA

 

[JimCasey]

Mike: "FULL PORT" Ball valves are piggable. MOST ball valves are "regular port" which coincides with having the port one line size smaller than the valve size. E.g.: 3" port in a 4" valve. A regular port ball valve probably still has less pressure loss than a gate valve because the gate valve has a substantial discontinuity around the seating surfaces where there is a pocket for the edges of the gate, vestigial guiding surfaces, the bottom lip of the gate, etc. Compare to a regular port ball valve that has gently tapered transitions from line size to port size. That becomes pretty close to a large-beta venturi in service and causes almost no restriction. Or, a Full-port valve, for which the custoemr pays a premium, has the same pressure loss as an equivalent length of straight pipe. Maybe even less because the valve's internal surface finish is probably better than he pipe's.

 

[MikeHalloran]

Thanks, Jim.

Does that regular/full port distinction apply to larger valves too? I.e., if you ordered a 30" ball valve, and didn't explicitly specify it, how big would you expect the ball bore to be?





Mike Halloran
Pembroke Pines, FL, USA

 

[Ashereng]

Hi Mike,

When I spec my valves, I specify bore size. I feel that this is a good habbit, and it eliminates misunderstandings.

 

[jsummerfield]

Expect that the bore of a reduced port ball valve to be at least one line size smaller than the connecting flange size.

John

 

[Sircrashalot]

I would expect manufacturer's of 30" 2500# valve to specify the bore size very clearly on their quote and drawings. Also for a serious valve such as this, I would expect that the specification would clearly indicate what the bore should be.

Andy

 

[zdas04]

In large-bore valves, the "regular" trim is finally starting to not be the norm. It is still a good idea to check, but when I've asked for availability of 16-inch and larger trunnion ball-valves for the last couple of years the on-the-shelf valves have been full port and the reduced-port valves were special order. I've also noticed that the last quote I got had a lower price for full-port than reduced-port. I can't think of any reason I'd install a reduced port ball valve in a new service.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem