Extended stem valves 1

[bck2ftr]

Where can I find advantages and disadvantages of extended stem valves?

Is it a Good Engineering Practice to use extended stem valves?

 

[Danlap]

Extended stem normally applied whenever the valves are somehow not in a close proximity with the Operator location when he's about to operate the valve (i.e. valve located under water / under ground, too hot of pipeline/valve temperature, potential toxic release / considered as confined space, etc.). I believe disadvantages of this are only Plant ergonomic layout (shall effected due to some extended bonnet may obstruct gangway, etc.) and also if not calculated prior, the extended bonnet may suffer bent due to moment by actuator or additional heights --> thus might get stuck.

Extended Bonnet has a different meaning. Its function is to "isolate" the gland from the high or low temperature zone so as to maintain integrity of the stem seal and the operation of the valve.
Best practice definition of low & high temperature should extended bonnet applied is normally depends on Plant's best practice. below -50 deg C and Or above 200 deg C may requires bonnet extension.
Yes, Graphite (standard) could withstand lower or higher then that, however shall it requires to operate the valve during emergency (before gland packing could establish itself towards ambient temperature of -20 up to let say 100 deg Celcius) experiences shows that this repetitive actions will disrupt gland integrity. Then of course leaks or burst through "stem".

 

[bck2ftr]

OK.
Thanks Muktiadi

 

[Danlap]

Your welcome,
to add into perspective. Shall valve and or its medium temperature is higher than 400 deg Celcius, thus requires a very long extended bonnet in order to maintain Gland Packing temperature. Then Heat Dissipation Bonnet could also be used.
Answering your first question, it is generally stipulated in ASTM C1696.
However yes, generally it is best practice. The combination of Gland material, heat dissipation / extended bonnet material and Operation behavior (whether they operate the valve at the lowest or highest design temperature) are driven by Manufacturers' R&D and also End user.

 

[zdas04]

To paraphrase Muktiadi's excellent post, the use of any particular valve configuration should be dictated by the application. I've used a lot of extended stem valves in valve cans. I've buried a few extended bonnet valves. The cost difference was substantial.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat