Insulation can also be used as a protective measure against high temperatures causing burns, where plant operators that may come into contact with the pipe. You almost always have insulation if there is heat tracing because heat tracing becomes a safety concern if someone could walk up and touch the hot surface of the tracing and/or pipe.
Where gas is the fluid in the pipes, completely dry gas often won't require insulation or tracing, or low point drains (other than for hydro testing purposes), but this is entirely dependant on the composition of the gas (fluid) and the conditions of the piping. If there are no liquids present in the dry gas, there shouldn't be a concern for hydrate formations from temperature fluctuations.
This brings the next question: why do we use heat tracing? Heat tracing (electrical, steam, etc), is used to heat the pipe, which in turn will heat the fluid in the pipe. This is common in systems where liquids are present and the risk of freezing or separation is a concern, whether it be due to pressure or temperature fluctuations. Controlling the temperature with heat tracing will allow the fluid to handle pressure fluctuations without the risk of freezing under most normal operating conditions.
Without going to far into it, on P&ID's (Piping and Instrumentation Diagrams) you will see symbology such as this to identify insulation and tracing:
The "38H" identifies the insulation as being 38mm thick and for hot service. The "ET" is for electrical tracing. Consult the lead sheets for your project for more information.
You will most often find insulation and tracing as part of the line number. For example a line number may look like 88.9-P-AB-1000-38H-ET. Just from reading the line number, you know the piping should be insulated and electrically traced. This line number should be the same on the P&ID and in the model, and on all drawings relating to that pipe.
With regards to your piping layout, insulated pipes must be on pipe shoes, and spaced further from adjacent pipes in order to accommodate the thickness of the insulation. This is one of the biggest mistakes that designers make! Forgetting to put pipe on shoes and not leaving enough space between other pipes! As a general rule of thumb, make sure you have at least 50mm of free space between the outside edge of the insulation and the pipe next to it. If the adjacent pipe is insulated, you need 50mm minimum from the insulation on that pipe too. Basically, from any surface. However check with your design lead and/or client specifications to see if they have minimums. 50mm's to 75mm's is common, and a good way to start, but always check to ensure it meets the project needs.
I hope some of these basics are of help, and I encourage you to do further reading and studying to expand your knowledge. This should at least be a good starting point for learning.
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