Use API 609, the industry standard for butterfly valves. The demands of a pump shaft sealing system are much higher than one for a butterfly valve shaft.

   We already use API 609, but it deals with packings only in paragraph 5.4 (5th edition, May 1997, page 4) and says very few words about this topic.
   We are trying to improve our valve packings and looking for a test system to qualify our packing Suppliers...
   In addition, I found out (from API website) that exist some User's Specifications similar (but not equivalent) to API 682, such as:
 - Shell 11 GS-682;
 - Exxon IP 10-1-3;
 - Aramco 31-SAMSS-012;
 - Sadaf 11 GS-982-SE.
   Can anybody help with this ones too?

What are you trying to improve the packing for, fugitive emissions?
Stipulate your required maximum stem leakage, and get the manufacturer to comply.
Take a look at ANSI/ISA 93.00.01 - Standard Method for the Evaluation of External Leakage of Manual and Automated On-Off Valves.
Most Manufacturers wshould be able to offer you test results from tests they have conducted.
To get the supplier to carry out this just for you is expensive and probably un-neccessary.
You could devise a test based on API 6A PR2 Performance Verification Test (Appendix F2.3) to be more specific to your requirements. We are doing something similar.
Hope this helps

Terry,
   I took a look at the standards you mentioned, thank you.
   We are looking for a way to qualify packings not only for fugitive emissions, but for any relevant aspect.
   Regarding fugitive emissions, we already carry out helium tests according to User's Specifications, the most common of which is Shell MESC SPE 77/312 (and 77/307 for Vacuum service).

Bit late

From a UK perspective, we have completed the ISO and 77/312 type testing, in essence you have to be carefull with the mechanical testing, whilst we succeded in meeting the fugitive emission standard such as 1000 cycles, max 50 ppm
for a gate valve the limiting factor is the cycles at or around 1000 cycles the valve top works tend to fail this is very relevent when looking at small forged steel valves as about 100 is the maximum cycles.

One other point, the most crucial factor in overall performance from a emission standpoint is temp cycling

Cheers