While PTFE (polytetrafluoroethylene) tape is versatile and commonly used for sealing threaded pipe connections in various applications, there are certain situations where it may not be suitable or recommended. Here are some scenarios where you should avoid using PTFE tape:

1. High-temperature applications: PTFE tape has a limited temperature range, typically up to around 260°C (500°F). In high-temperature environments exceeding this range, PTFE tape may degrade, melt, or lose its sealing properties. For such applications, specialized high-temperature sealants or gaskets are more appropriate.
2. Oxygen-rich environments: PTFE tape should not be used in systems containing pure oxygen or oxygen-rich atmospheres, such as in oxygen lines in industrial or medical settings. PTFE can react with pure oxygen under certain conditions, leading to combustion or fire hazards.
3. Certain chemical environments: While PTFE is highly resistant to many chemicals, it may not be compatible with certain substances, particularly strong oxidizing agents, alkali metals, and fluorine gas. In such cases, specialized sealing materials or gaskets may be required.
4. Critical or high-pressure applications without proper reinforcement: While PTFE tape can provide effective sealing in many low to moderate pressure applications, it may not be suitable for critical or high-pressure systems without additional reinforcement or support. In such cases, using PTFE-based thread sealants or other sealing methods may be more appropriate.
5. Electrical applications where insulation is required: PTFE tape is not an electrical insulator and should not be used in applications where electrical insulation is necessary. For electrical insulation purposes, specialized insulating tapes or materials should be used instead.
6. Threaded connections with irregular or damaged threads: PTFE tape relies on the integrity of the threaded connections to create a seal. If the threads are damaged, corroded, or irregular, PTFE tape may not provide an effective seal. In such cases, it’s better to repair or replace the fitting or use alternative sealing methods.

Overall, while PTFE tape is suitable for many sealing applications, it’s essential to consider the specific requirements and conditions of the application to determine whether PTFE tape is the appropriate choice or if alternative sealing methods should be used.