Final answer:
Street elbows and closed nipples can be used in a gas piping system, provided they meet safety standards and local building codes.
Step-by-step explanation:
True, street elbows and closed nipples can indeed be used in a gas piping system. These fittings are designed to make connections between pipes in different configurations easier to achieve, especially in tight or restricted spaces. Gas piping systems often require specific materials and configurations according to local building codes, so it is essential that these fittings are made from materials and designs suitable for gas applications and that their usage complies with the relevant safety standards.