What is the condition described as "not allowed" in relation to compression fittings upstream of service regulators?

The condition described as "not allowed" in relation to compression fittings upstream of service regulators refers to safety standards and best practices in gas piping systems. Compression fittings are designed to create a secure connection in gas piping, but using them upstream of service regulators can pose a risk. This is because compression fittings can potentially loosen over time due to vibration, thermal expansion, or other environmental factors, leading to gas leaks.

In gas installation practices, it is crucial to ensure that all fittings in high-pressure areas, particularly those located upstream of safety devices like service regulators, are capable of withstanding the necessary pressures without failure. Instead, more reliable types of connections, such as welded or flare fittings, are recommended in these critical locations to enhance safety and integrity of the gas supply system. Therefore, using compression fittings in this specific context is explicitly marked as "not allowed" to ensure the safety and reliability of the gas piping system.