ON/OFF service valve, Through-conduit, forged body and bolted bonnet constructions as a parallel expanding gate with simultaneous tight sealing on both seats. Upstream and downstream seal is merely provided by mechanical action caused by gate expansion and is not affected by fluctuations or vibration of the flow. Gate expansion against the seats in either open or close position, also allows a very long integrity of sealing surface as well as valve stem by isolating the valve body from flow.