What is a backseat of Gate Valve
What is a backseat?A backseat is a seating arrangement inside the bonnet. It provides a seal between the stem and bonnet and prevents system pressure from building against the valve packing when the valve is fully open. Back seats are often applied in gate and globe valves (source: wermac.org).
How to use the backseat?
The backseat should only be used to isolate the gland packing from the line medium for example when gland packing leakage is evident and until the valve can be de-pressurised and repaired.
How to not use the backseat?
Under no circumstances should the backseat be used to allow gland packing replacement or repair while the valve and system are pressurized.Having clarified these 3 questions, the issue seems quite simple.
But it is not!One of the most common errors done on site is to believe that if the Gatevalve is back-seated (fully open position – and backseat sealing made), this will help to prevent wear of the packing rings.
Unfortunately using the back seat in this way over a longer period of time, could result in some serious issues:a) Since the body, bonnet, stem, and disc heat up and expand at different rates, experience shows that valve can get jammed/blocked in back-seated positon.b) The gland packing will dry out since it’s not exposed to the steam, and could immediately blow upon closing of the valve.The simple but important rule for valve users, preventing misuse of the backseat!“Once the valve is brought into fully open position (back-seated), turn the hand wheel back one full turn”(this should be clearly stated in every Installation, Maintenance and Operating Manual of Valve Manufacturers),The other issue valve users experience is the problem with actuated valves. It does not matter if electric-, pneumatic- or hydraulic actuators are used. If the valve manufacturer does not correctly adjust the actuator in the open position, we will get back to the same problems (as described earlier). Field experience on high pressure/temperature drain valves (globe valve), pneumatically actuated and usually in fail safe open position, showing clearly the premature use of the gland packing rings.
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