Purging hydrocarbon vessels of flammable vapour

Emptying hydrocarbon storage tanks, reactors, pipelines or other vessels is dangerous because there is a high risk of creating an explosive mixture if the purging operation is not managed correctly

Emptying hydrocarbon storage tanks, reactors, pipelines or other vessels is dangerous because there is a high risk of creating an explosive mixture if the purging operation is not managed correctly.

Vessels have to be purged with nitrogen to reduce the hydrocarbon content to below the lower explosive limit (LEL) before air can be allowed into the enclosure, but many gas detectors are not well equipped to measure hydrocarbons at this low level and are prone to burn-out if exposed to higher levels.

The infrared technology in the Crowcon Triple Plus+IR has been specially designed for this application.

With Triple Plus+IR, the purge vent gas is monitored to determine when there is less than 100% LEL equivalent flammable gas concentration.

The advantage of using a monitor (rather than an estimated time-lapse) is that the costly nitrogen purging can be stopped as soon as it is safe, with no need to guess the time or to run the purge longer than necessary.

The infrared detector in the Triple Plus+IR is a unique dual range infrared sensor for flammable hydrocarbons scaled with both 0-100% volume and 0-100% LEL ranges, with an automatic range switch between these.

Conventional meters are not sensitive enough for this job: if using only a 0-100% volume range, the required critical readings are in the bottom 5% of the scale and cannot be read with a high degree of confidence.

Purging is therefore usually continued until an effective zero reading is obtained, wasting nitrogen.

When using the Triple Plus+IR, however, as the hydrocarbon concentration falls below the equivalent 100% LEL concentration, the range switches from 0-100% volume to 0-100% LEL, so a clear and immediate visibility of a safe hydrocarbon level is given.

This allows the earliest possible termination of nitrogen flow and a switch to air.

Purge monitoring is also necessary in the reverse mode.

Before hydrocarbon can be re-introduced, a low oxygen level is essential (usually <2% by volume).

Nitrogen is therefore once again used to flush out the air, and the oxygen sensor fitted to the Triple Plus+IR is used to monitor the falling oxygen content of the exhaust line.

Once the oxygen content is low enough, the nitrogen flow can be stopped and the hydrocarbon re-introduced.

The monitor is then used to follow the refill process determining when full 100% hydrocarbon concentration is reached allowing venting to stop.

Conventional detectors frequently burn-out or become poisoned when exposed to these high concentration levels.

Back to top