Isolation of HP Steam Systems

[TMOC]

I would people's feedback on isolation systems for HP Steam.

The Company I work for rates Positive Isolation (blank or removable spool) higher than Double Block & Bleed. I do not have an issue with this.

On our HP Steam system - Class 600 - I am getting mixed feedback. Although Positive Isolation is more secure and DB&B, some are concerned about swinging a blind against HP steam isolated behind a single isolation valve. The push is therefore for DB&B which can pose other safety issues along with economic consideration.

Places I have worked in have positive isolation using a single (high-performance) isolation valve and a spec blind (or blank). A bleed is fitted between. I have received advice that all isolations in steam power generation plants are DB&B utilising welded valves.

Another option is to have DB&B and Positive Isolation, however if fully welded valves are used, there is nowhere to swing the blinds.

Can people give me some advice on which is the "best" system for HP Steam Service based on their experiences and possible knowledge of safety requirements / legislation.

Thanks

TMOC

 

[25362]

Trevor A. Kletz on his What Went Wrong - Case histories of process plant disasters (Gulf) proposes (fig. 1.1) for steam above 600 psi, all-welded double block and double bleeders to isolate plant up to pressure or pressure lines from equipment under maintenance.

 

[DSB123]

TMOC,
On our site the norm for high pressure lines (including HP Steam) is to use DB&B with all valves being fully welded.