Can solids accumulate? Special corrosion mechanisms may occur under a solids' deposit.
Depending upon the answers to these questions and other particular circumstances, piping systems with intermittent or rare use may require special materials.
Let's look at two rules-of-thumb: one for freezing and another for corrosion.
To address freezing, anything longer than a tee connection to a blind flange or valve is too long unless a plant provides protection measures such as heat tracing, insulation, circulation systems, additives, etc. Flanges on the unused branch of a tee fitting haven't suffered an excessive number of problems due to freezing. However, the amount of incidents increases when line is attached to the tee.
One way to ensure that rarely used pipe segments, including lines to pressure relief valves and connections between piping systems, don't suffer water problems is to make them come out the top of a line and not have pockets.
For corrosion protection, any branch from the flowing line that has a length/diameter ratio exceeding three likely will cause a problem. Avoid branches with elbows or secondary tee connections.
For welded-cap (Figure 1) or fused-cap connections, consider any length of pipe without flow a dead-leg. The real question for a capped pipe is why it's there. If you can cut off a cap to get into a pipe, you usually can cut into the pipe itself.
Sometimes a dead-leg is necessary — for example, you might put pipe in during a shutdown that won't be used for a long time. In such situations, consider special arrangements that isolate the pipe from the process, use special materials to reduce corrosion, or arrange required preventive maintenance.
These rules-of-thumb and pointers provide some guidance about analyzing and addressing dead-legs. However, you must use engineering knowledge for your specific system to decide what must be done.
ANDREW SLOLEY is a contributing editor to Chemical Processing. You can e-mail him at Asloley@putman.net.