maddocks,
Dual diameter pigs don't work that way. A 6-8 launcher typically will have a 10-inch barrel with a 6-inch straight section. You stuff an 8-inch foam pig into the 6-inch section (it takes some force, but will go). Then at the transition to 8-inch the pig relaxes to the line size. They have been installed all over the world with many successes.
Dual diameter launchers work OK for many things. I just heard about a line with an 18-20 dual diameter launcher being smart pigged. Before a couple of years ago this was not an option in any size. I'm not sure what sizes they can accommodate today. My advice is still, if you think you will ever want to smart pig the line, design long launchers and receivers and put a receiver on the 6-inch line.
There was mention above about running a 6-inch pig and dropping it into the 8-inch line to be pushed home by the 8-inch pig. I've done this, and it has never worked as well as I'd hoped. The big pig will tend to run up on the small pig and the two can get irretrievably wedged in the line. My problem was with "turbo pigs" which are made of plastic and have vanes like the old mandrel pigs. If the big pig can ride up high enough to interlock the vanes, the pigs are never moving again without a cutting torch.
Another problem with dumping the small pig in the large line is that it is a tough build. If you just use a non-barred tee you have to use pigs that are either shorter than 8-inches or very flexible (like a foam pig). A turbo pig or a mandrel pig won't make the corner. A not-barred tee also has the ability for the 8-inch pig to try to go walkabout up the 6-inch line in certain field maintenance scenarios.
I have a picture of a dual-line piggable drip that has worked to allow multiple launchers to share a receiver (one system has 5 launchers and one receiver, been working fine for 15 years) on my web page under samples (it is in the "Rules of Thumb for Gathering System Equipment" file). They work, but they're expensive and while I have built a couple with different diameter lines (using a reducing lateral), I haven't had much luck with the actual pigging in that mode. The photo above the
Gathering System Samples heading is a dual-line piggable drip. It works well, the two streams interfere with each other and drop out a lot of liquid in a gas gathering system.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
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