>>inhibited graphite packing for valve stems in seawater service<<
OK, Why?
TFE-based packings seal more tightly, have vastly less friction, are easier to adjust properly, are chemically inert. Don't know if they are less expensive but valve vendors sure charge more for graphite.
Graphite packings are superior for high temperatures over 400F at the packing box, (not likely for a seawater application) or fire-safe services (A seawater leak will not contribute fuel to a conflagration). Graphite packings stand up better to radiation greater than 10e6 rads (again not a likely consideration).
So I continue to wonder: Why would you want to use graphite packing on a seawater application?
BTW, as I understand it the inhibitor is a percentage of zinc chips in the graphite, so it would effectively inhibit corrosion of the stuffing box bore and valve stem from galvanic corrosion until the zinc is gone. Once the zinc is gone the galvanic corrosion between the valve stem, graphite, and corrosive electrolyte is gonna make the packing box look like an 89-cent Wal-Mart barbecue grill used once and left out in the rain.
I don't mean to jump on you, but I have had painful experiences where somebody reads a magazine article about the marvel of graphite packing and does a plant-wide changeout to graphite. Then a bunch of problems popped up the I, the OEM valve supplier, was supposed to fix even though my company was not consulted before our equipment was modified.
Thanks Jim,
No worries about jumping - I'm dealing with someone else's spec. Valves have been purchased for a large floating offshore platform, and many have inhibited graphite packing. I think the issue of galvanic corrosion is going to haunt us, and was looking for corroboration of that as there is an arguement that "inhibited" means we will be ok. I Could find no specs or internet material on the life expectancy of inhibited graphite to back me up however.
As a valve supplier, which packing would be more leakproof/longer lasting: aramid fiber or a combination of ptfe/aramid (to specify in place of the inhibited graphite) and are both readily available?
I appreciate your answer, thanks.
I thought I knew about packing until I called im my local packing (Garlock) salesman once. I learned more in an hour meeting with him than I had learned during 25 years in the valve bidness. It's always good to speak to a specialist, and free, too.
I did have many years of non-diappointment using aramid-fiber yarn packing with a TFE overbraid; Crane 285K. One advantage to a braid over the solid packings is that it can absorb grit and still seal.
TFE-Chevron packing is even more popular. Your packing guy probably has this in preassembled sets. You can even drop in a coil spring to live-load it which compensates for wear or thermal transients.
I googled "inhibited graphite packing" and got a lot of hits. Here's the link to Crane's:
Grampi1,
Thanks for your response. I'm trying to find out - and it's not readily available in a spec - how much the inhibitor increases the corrosion resistance. I've been told by a vendor that I can only expect 18 months increase in life by using inhibited graphite. On an offshore platform, that's not enough. The aramid/PTFE or Chevron TFE seems less problematic.
