I am going to be brewing nb Belgian tripple, using 1.5 L yeast starter. Do I need to use a blow off on the primary? Thanks in advance for the assistance.
Can’t really answer that question. There are many factors that will determine fermentation activity. If you’re worried about a blow-off, then take the safe route and install one.
There is never a downside to using a blow off hose, but this is often a downside to not using one.
That is what made my decision easy
Do your self a favor. Use YeastCalc to help you build your starter. I wish I would have known about it before I made my Belgian Triple. A member on this forum brought YeastCalc to my attention.
After the fact I found that I under pitched the yeast for the triple. If I remember, I made a 1700ml starter using a cup of DME.
I used a blowoff tube. Everything was fine until the activity seemed to quit after about 4 days. I then put in an airlock and moved the carboy to a warmer spot. Man what a mess I had. I had gunk all over the place! Get that blowoff tube in and don’t mess with it for at least 2 weeks. 
Thanks for the advice, I will set up a blow-off tube. This forum is a big help. 
If you are using 3787, unless you have a lot of head space, you will almost definitely need a blow off tube.
[quote=“560sdl”]There is never a downside to using a blow off hose, but this is often a downside to not using one.
That is what made my decision easy[/quote]
EXACTLY!
Just a little advice. I still had active fermentation going on after 3.5 weeks. It wasn’t going crazy or anything, but it still was producing some C02. I think I kept it in the primary for 4 weeks, took a few hydrometer readings then racked into the secondary. I let it in there about 3 weeks to clear then racked to the keg. In June it will be 8 months since brew day. I took a sample about a month ago to be sure there is no infection due to the fact of screwing around between the blow off tube and the airlock. I switch between the two 4 times. That carboy blew it’s spout 3 times. I had pasty stuff all over. SWMBO wasn’t real thrilled with the show it was putting on. A yes the yeast was 3787.
WY3787/WL530 will have a large krausen. I don’t use a blow off tube, but I do use 2 carboys and split the 5-6 gallon batch between them. If you blow all of the yeast out of the blow off tube, you won’t have enough yeast to finish the beer. Tripels are not very tasty when they are sweet. I ferment in the mid 60s for a week, and then let it rise to the mid 70s to make sure it finishes dry. My last tripel took about 3 weeks and went from 1.084 to 1.000 without any hot alcohol flavors. It is very tasty.
