double block and bleed

[chrisrich]

what is the view on using a single block valve with a DPE (double piston effect) instead of using conventional DB&B valve arrangements. How does the double piston effect work. does it provide 3 genuine levels of safety similar to conventional DB&B valves [/b]

 

[TerryOsser]

Double Piston effect seated Ball valves work by having a seat design such that both line pressure and body cavity pressure (if the first seat should leak), when the ball is in the closed position, will force the seat seal onto the ball. This is apposed to Self Relieving Seats that lift off the ball allowing the bocy cavity pressure to be relieved either upstream or downstream, or both depending on the configuration of the seats. The body cavity drain (or vent) is used as the bleed, on which you must place an isolation valve. Normally an isolatable pressure relief valve is required off the vent or drain to prevent cavity overpressurisation. When in the closed position the bleed is opened to monitor any leakage into the cavity. If there is no leakage the primary seal is good. If there is leakage, depending on the amount, the bleed can be piped away (preferential route) and work can continue. Same as a traditional DBB.

Benefits are that you can use one ball valve with DPE seats rather than the traditional arrangement of three valves. A DPE seated valve costs around 10% more than a traditional seated valve. There is the obvious associated weight and space saving.
The downside is that if the valve needs replacing for whatever reason, the line needs to be shut down. So Operations need to buy into this. With the traditional DBB, (assuming the downstream valve is still sealing), the valvbe to be replaced can be done so whilst the line is still pressurised. The single bodied DBB cannot be blinded.

The DPE DBB valve satisfies API 6D (pipeline)requirement, and API RP14E. Some Operators dont like it as they prefer to have 2 independantly bodied valves. Ask the client.
We have just replaced the 'traditional' DBB arrangment with DPE ball valves, after satisfying A Hazop study.