Are pipe unions directional

No. Some old-school plumbers and pipefitters believe that unions should be installed in one specific direction, and some apprentice training books even say so, but this is only a bit of folklore that has no basis in fact. Anvil International, the maker of most steel pipe fittings, has an illustration http://www.anvilintl.com/ProductSearch/ProductInfo.aspx?ProductId=9960 of an assembled union which shows no internal impediment to flow. Nowhere in any of Anvil's product literature is there any installation instruction regarding flow direction. Control valves and check valves, which do have directional requirements, have flow arrows stamped onto them, but unions do not. Additionally, the circuit setter valves (such as those made by Bell & Gossett) with union ends are installed with the union end facing opposite directions relative to flow (i.e., male end upstream on the return valve and male end downstream on the supply valve).