We are manufacuturers of valve stem, flange and all sorts of other industrial sealing products. During 2003, I looked into the idea of taking discreet emission measurements at the valve utilizing sensors but we never pursued it. At that time I found that end users recognized the potentional value but it was not one of their top concerns. I also found that some end users veiwed the idea of automatically reading the emissions at a stem seal, say once a day or once a week, was good since leak detection personnel would not have to be hired and deployed once a quarter, but bad in that if the valve was out of compliance they would be more immediately responsible to document the event and repair the leak within the EPA guidelines. I suppose that if cheap reliable sensors are available EPA could push for their use as BACT items.
Jim Drago, PE, Engineering Manager, Garlock Sealing Technologies
ANSWERS:
MY EXPERIENCE is limited to using bellows seal or twin seal designs on toxic, radioactive or high vacuum applications to prevent out or in-leakage round the stem. These designs were helium tested to 10-10 cc/sec.
I found that bellows seals are not only expensive, but also tend to provide a sense of false security, while their life is unpredictable.
In case of the twin or dual packing arrangement, the bonnet is tapped between the two packings and this connection is piped to a vent disposal header or waste container. The tap can also be used as a purge connection, with the purge media being at a higher pressure than the process. In this case, if leakage develops, it is the purge material that leaks into the process.
The idea of using discreet emission measurements at the valve utilizing analytical sensors seems to be a good one, and I will ask some of my more experienced colleagues about it.
Béla Lipták, PE, CONTROL Columnist
I NEVER cease to be amazed by the lack of knowledge and expertise on the subject of valve stem sealing fugitive emissions. There is little point in worrying about sensors to detect same, compliance issues, etc. The technology of a stem sealing system is available and has been for a number of years.
What is this technology? Simply the use of the KVSP stem packaing technology developed and marketed by Dupont. While Dupont doesn't necessarily sell this product directly, it is readkily available from most control valve manufacturers, except those who do not keep up with technology advances, are too cheap to provide it, or prefer to downplay it in favor of their own sealing technology.
The KVSP packing system is available in several variations, including fire safet per API 607 Qualification. While a bit more expensive than standard Teflon-based packing and graphite, it pays for itself in a relatively short ime in several ways. It is probably the nearest thing available today to a universal control valve step packing. So much so that Dupont has standardized it for all of their control valve needs, with few exceptions.
I wrote a section in Liptak's IEH4 Handbook that covers this in more detail. I invite anyone with interest in KVSP for control valve stem sealing to contact me.
