Pipe flange connections have been used in chemical plants, petro-chemical industry and so on. They are tightened with a lot of bolts and nuts by torque wrench, impact wrench and so on. When the impact wrench is used for tightening a lot of bolts and nuts in pipe flange connection with gaskets, it is well known that a large scatter is assumed to occur in tightening the lots of bolts in the connection using the impact wrench. In the present paper, the scatter in bolt preloads in a pipe flange connection with 20”nominal diameter using air impact wrenches and air torque wrench (JGUN) was measured. Firstly, the best operation time for the air impact wrench was examined and it was determined as 3 seconds. The tightening coefficient was calculated taking into account the interference of the adjacent bolt loads and it was obtained as αA=1. 38∼1. 48 while the conventional value was around αA=2. 5∼4. 0. In addition, it was found that all axial bolt forces tightened by the air impact wrench were larger than the target bolt force. Then, the tightening was carried out using an air torque wrench. The tightening coefficient by the air torque wrench was obtained as =1. 321. 41 while all axial bolt forces were smaller than the target bolt force. The leakage tests were carried out for the connection tightened by air impact wrench and air torque wrench. The tightness parameter of the connection tightened by the air impact wrench was obtained as 8081070 while it was as 661 for the connection tightened by the air torque wrench. As the result, the impact wrenches such as the air impact wrench and the air torque wrench were applicable to the actual bolt tightening because the tightening coefficient was smaller than the conventional value.
