valves

[brodieg]

how does a dubble block and bleed valve work

[druiaghtagh]

27 2 03 in this category

[jenxxie]

It works in basic terms, by being essentially two valves in the one body.  between these two independent valves there is a 'vent' or 'bleed' connection, normally itself "valved" too.

The principal is that the line is 'doubly protected' or isolated, thus protecting equipment or personnel downstream of the valve,  two-fold.

With regards to certain valves, such as the typical API6D Trunnion-Mounted ballvalve,  it is sometimes described as being of a 'double block and bleed design', and in this particular instance, the valve itself is a 'single valve'  where the valve seats are 'pressed' onto the ball, usually in what is known as the 'double piston effect', thus ensuring a 'shut-off' at both sides of the ball, or in effect a "double block".  There is typically then a vent or sometimes vent AND drain connection(s) from the cavity to relieve the pressure and/or drain the media which is subsequently 'caught' between the two seating surfaces of the valve.  ( It is somewhat debateable in true engineering terms if this really constitutes true double block and bleed ).

The reality is though, that true double block and bleed is two valves in the one body casting or forging, with a vent port between the two, which is independently valved itsef.    The concept / requirement is relatively new, and has been borne largely out of recommendations by the HSE  etc as a result of  accidents / near misses  in highly critical applications or situations where recommendations have been put forward to counteract any possibility of repetition of such incidents.

Many Manufacturers can offer either pipeline, or instrumentation DBB Valves in a variety of materials of construction and varying class ratings end end connections to suit almost any application.