Piping Components

Please give us more information.

Such as:

For both Line Classes give the following information:

Base Material ( e.g.: Carbon Steel vs Copper)

The Commodity in each of the two systems

The size range for your area of interest for both

The Design pressure and temperature range for both

With this we might be able to offer some insight

However, it is possible that whomever created these specs just left the nipples out.

Do you mean threaded nipples?

If so, my guess would be that they are not allowed in a high temperature, high pressure, high corrosion, or otherwise dangerous service (cyanide gas, perhaps?) or any combination of those.

What is your spec that they are not allowed in?