Piping Design Rules of the Thumbs: 

1. Line velocities and pressure drops, with line diameter D in inches: liquid pump discharge, (5+D/3) ft/sec. 2.0 psi/100 ft; liquid pump suction, (1.3+D/6) ft/sec. 0.4 psi/100 ft; steam or gas, 20D ft/sec, 0.5 psi/100ft.
2. Control valves require at least 10psi drop for good control.
3. Globe valves are used for gases, for control and wherever tight shutoff is required. Gate valves are for most other services.
4. Screwed fittings are used only on sizes 1.5 in. and smaller, flanges or welding otherwise.
5. Flanges and fittings are rated for 150, 300, 600, 900, 1500, or 2500 psig.
6. Pipe schedule number = 100P/S. approximately, where P is the internal pressure psig and S is the allowable working stress (about 10.000 psi for A120 carbon steel at 500˚F). Schedule 40 is most common.