Hello Jaffaotc,
I have a bit of past experience with sealant injection on gas and hydrocarbons service valves.
First of all, I must point out that I haven't used Valtek or Furmanite products for at least 5 years so improvements may have been made to these Sealants that I am unaware of.
The majority of my experience is with Sealweld greases and sealants.
The reason we chose to use Sealweld products is that furmanite's product was for emergency use only and would harden like a resin over time. Valteks products were also very viscous and did not flow very well.
The system with Sealweld, if followed correctly, is very effiecient although it is no where near the cheapest option of these 3.
Firstly, you are supposed to inject valve cleaner into the injection point at pressures upto 10k psi, this clears a passageway throught the grease channels. I believe that it even breaks down old sealants such as the Furmanite resin one....but this takes time. If time permitted, we would leave the injection system pressurised for up to 12 hours if the cleaner did not penetrate immediately.It never took longer than this and this was worst case scenario.
After this, there are a couple of options of sealant depending on how bad the leakage is. There is a product called totalube 911 which is marketed as a valve grease but even this can cure leakage if the leak path is small enough. or if it is a case of removing foreign matter away from the seat contact area.
Contrary to Valvits advice, the Sealweld stuff is also recommended for use as preventative maintenance, it never hardens so it doesn't cause blockage problems at all. If finances permit.....although they rarely do....you could do a lot worse than to grease and lubricate ball vales on natural gas lines as a routine operation.
Safety is a major factor too, the Sealweld injection equipment operates at up to 10,000psi so you must ensure that grease fittings are rated to whatever pressure you are injecting at and that they are in good condition. I could go on for ever here about the safety implications, I recommend contacting one of these companies and get them to advise you for your specific situation.