The Purge
The purge refers to the process that the air purge or the steam purge are often used to clean the pipe based on the service condition of the working medium and how dirty the inner surface of the pipe is after the process pipeline system is installed.

The general requirements of the pipe purging are as the following.
Regulating valves, the main valves, throttle valves, safety valves and instruments shouldn't be equipped with orifice flanges before pipes are purged, and protective measures like removing sealing elements should be taken for the above valves and instruments with welding connections.

The order of the purge should be from the main pipe, the branch pipe to the drain pipe, and the dirt which is removed from the pipe shouldn't get into the pipe that is purged.

The dirt which is removed by the purge must not pollute the environment, and it's forbidden to arbitrarily discharge the dirt.

Restricted zones should be set up before we purge pipes.
　　
After we finish purging the pipe, no other operation should be carried out so as to avoid the cleaning being affected. 

A. The air purge
1. Usually, air purges are adopted for pipes which transport gases in the process pipelines.

2. Large compressors of production equipment or large container storage of equipment should be used for intermittent purges; the pressure of the air purge should not exceed the design pressure of the container and the pipe, and the flow rate should be lower than 20m/s.
　
3. Compressed air without oil should be used when we purge the pipe in which the oil should not exist.

4. The inspection method of the air purge is to set up the target board covered with the white cloth or coated with the white paint at the air vent, and then inspect the target board. If there is no rust, dirt or other stuff on the target board, the air purge will be qualified.

B. The steam purge　　
1. The steam purge is suitable for the pipe which transports the motive steam or the heat. Thermal expansions are not considered in the designs of non heat distribution pipelines; therefore, steam purges are not applicable to non heat distribution pipelines.

2. A great deal of steam should be used for steam purges, and its flow rate should not be lower than 30m/s. 

3. Pipes should be warmed and drained timely, and heat displacement of pipes should be checked before steam purges.
 
4. The process of steam purges are as the following: making the steam move through the pipe, warming up the pipe gradually, keeping the temperature for one hour, steam purging the pipe, stopping the steam and cooling the temperature, warming up the pipe gradually, keeping the temperature and steam purging the pipe, and repeat this process for not less than three times. 

5. Aluminum target boards with smooth surfaces should be used for checking medium and high-pressure steam pipes, and wooden target boards should be used for low-pressure steam pipes. The target board should be placed at the vent where the steam goes out. If there is not dirt, the air purge will be qualified.