Is LPG considered incompressible?

API 521 states that a relief valve is normally used with incompressible fluids. Can LPG be considered as incompressible? and a relief valve be used for LPG bullets?

Is it like..When petroleum gas becomes LPG, it is already compressed to the maximum and is not compressible anymore...hence LPG is considered incompressible and a relief valve is used for protecting LPG bullets???

Your bullets are not full till the top, so that logic does not work, I think :)
But relief devices for that type of service definitely exist.

Yes, you consider it a liquid if you are sizing a thermal relief valve. However, a regular PSV sized for fire case will be sized on vapor or supercritical service, as that is what will be relieved as the LPG heats up.