A 17,000 Kg valve, supported by a +Y at the center will not pivot. The only pivoting would be due to adjacent pipe elements where the weight distribution on either side of the valve is not symmetrical. You can always put an additional node in your valve so you have 3 consecutive rigids, with one short rigid equal to the finite length of your real-world Y support in the middle. Then place your 2 Y supports on those two center nodes, each representing one end of the Y support. I think this is over-kill, but it can be done quite easily.