I am specifying valves for a oil centre.

In this specific case, the fluid is hydrocarbon with a high presence of hydrogen sulfide H2S (up to 160000 ppm).
The Owner of the plant defines "Lethal Service" each H2S concentration greater than 1000 ppm.
The piping class is designed according to Cat.M of ASME B31.3.

What are the precautions you would take for preventing stem/packing leakage to the environment?

If ISO 15848 is the chosen standard for fugitive emission testing, would you specify Tightness Class A?

In case you specify bellow seal valves, Would you in any case require the above mentioned emission test?

In this specific case, the fluid is hydrocarbon with a high presence of hydrogen sulfide H2S (up to 160000 ppm).
The Owner of the plant defines "Lethal Service" each H2S concentration greater than 1000 ppm.
The piping class is designed according to Cat.M of ASME B31.3.

Bellows seals are the traditional answer, but they are VERY expensive and have a limited cycle life.  ALso when they fail it tends to be a catastrophic failure.  

Contemporary solution to the problem is to use a double packing set with a lantern ring between the packings, Leakoff from the Lantern ring is piped to a detector and then to safe disposal.  THe second packing set has no differential pressure so it is assured that nothing escapes to atmosphere.  Additionally such packing is customarily live-loaded to compensate for wear or thermal effects.  

Rotary valves avoid a major packing wear mechanism since the stem does not drag in and out of the packing box.  Any micro-roughness in a rotary valve only moves packing particles circumferentially inside the packing stack.  Packing wear is drastically decreased so the packing stays tighter and lasts longer.  Compared to a rising-stem valve where the same shaft surface finish drags particles out of the packing box, and also drags any contamination that lands on the stem back IN to the packing.  This accelerates packing AND stem wear, so leakage occurs much, much sooner with the rising stem valve.  

You say "High pressure" but that's a relative term.  
Some of the tightest valves are rotary valves designed for ASME class 150 and 300.  Example: The Durco TSG4 has a primary seal of a tapered plug within a sleeve. There is a secondary seal of a PFA diaphragm with a pressure-energized reverse-lip stem seal. Then they stack on a double TFE  Live-Loaded packing set. That valve is frequently used for really nasty chemicals such as phosgene or HF.   Above class 300, a ball valve with double live-loaded packing will still outperform a rising-stem valve for lowest stem leakage.

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