What size should takeoff connections for instrument and control piping be if the pressure is 900 psi or less, or temperature is 800 degrees F or less?

The recommended size for takeoff connections for instrument and control piping under the conditions specified—pressure of 900 psi or less or temperature of 800 degrees F or less—is indeed 1/2 inch. This size is typically preferred because it strikes a balance between being large enough to allow for adequate flow and pressure integrity without incurring unnecessary costs or complications associated with larger fittings and piping.

Using a 1/2 inch size ensures that instruments can function effectively, providing accurate readings and reliable control without creating excessive pressure drops or flow resistance. It is also compliant with industry standards and practices for such specifications, ensuring compatibility with most typical instruments and control devices used in high-pressure situations. This standardization simplifies installation and maintenance while maintaining safety and performance.

In contrast, larger sizes, such as 3/4 inch or 1 inch, are generally used for higher flow requirements or when larger volumes of fluid are necessary, which isn't typically the case for instrument takeoff connections in these pressure and temperature ranges. A 1/4 inch takeoff might be too small to handle the desired instrumentation needs effectively in this application.