Abstract: Welded modular blocks, provided in a variety of standard selectable configurations, permit fabrication of gas panels and other fluid flow control systems for high purity, leakproof applications. Each module is pre-welded and provided with mating ports to receive gas control components such as valves, controllers, pressure regulators and the like. Each module is also provided with one or more connector tube stubs which may be welded to a tube stub of another module to form an array of modular blocks and associated components. The components are bolted to the modular blocks and may be readily removed for servicing. The welding of the standardized blocks obviates costly seals otherwise needed to prevent leakage.