What Ball Valve for DB&B?

[meidam]

I'm currently looking to install ball valves to operate as a double block and bleed (DB&B). The specifications are as follows:

4" full bore, 900# rated, Stainless Steel, RF ends, metal seated, with a manual gear acutator, Temp: 400°F

However, I am considering several options and would appreciate some input. First, what's the advantages/disadvantages of the traditional DB&B compared to a single, trunnion mounted ball valve with a DB&B feature. Also, I'm considering using floating ball valves instead of the trunnion mounted design. Though trunnion mounted ball valves are better for higher pressures, the valves in my DB&B are on the downstream side of a pressure let down valve and only see about 60# of pressure. However, would the seat design still be comprised if I used floating ball valves?

Thanks

 

As the specification is for 4" 900#, you can not use floating ball. At this rating only trunnion mounted ball can be used.
As for the DBB function, it depends why DBB was specified.
If DBB is selected for positive isolation of process for maintainance purposes, then in a high press system you need two valves plus a bleed. There may be manufacturers who could offer an integral valve of 2+ bleed.
Normally trunnion mounted ball valves have block and bleed facility by design. In your case you may be able to use a single valve as your downstream pressure is only 60 psi

 

[zdas04]

People talk about trunion ball valves for high pressure applications, but I also specify them for very low pressure. A floating ball valve relies on dP to shift the ball into the downstream seat - the more the dP the tighter the seal. In low differential pressure (say 5-10 psid) there is not enough energy to seat a large (say above 6-inch) floating ball valve and you can get inconsistent seals.

The DB&B specifications vary by company (OSHA just says you need it for hot work, not how to get it). Many companies allow that two independent seating surfaces with a bleed between satisfies the DB&B maintenance requirements - in those cases a trunion ball valve is adequate. I really don't like to rely on the small ports in most manufacturer's bleed mechinism so I specify that once the body cavity has been blown down, the bleed nut should be fully removed which gives you a 1/2 inch vent that will not tend to plug with normal line debris.

Some companies require that two independent valve mechinisms be used (such as two block valves located some distance apart with a vent/drain between them, or a specially designed DB&B valve as mentioned above). In that case either a floater or a trunion will give similar results.

With a 60 psig system why do you install 900 ANSI valves? Seems like gnat-hunting with a howitzer to me.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com