Selection of Instrument for Oxygen Content Detection during Nitrogen Purging of Storage Tanks

For the nitrogen gas displacement and inert gas protection conditions of the storage tank, the oxygen content detection is divided into two types: online continuous monitoring and portable on-site detection. The instruments must be adapted to the explosion-proof area conditions to ensure the accuracy and reliability of the monitoring data during the displacement process, safety interlock, and completion acceptance. The tank area is a flammable and explosive hazardous area. All on-site instruments must meet the Ex d IIC T6 explosion-proof grade, which is compatible with the safety regulations and safety assessment requirements of the station.

The preferred choice for online monitoring is the extraction-type electrochemical fuel cell oxygen analyzer, which is the most suitable instrument for nitrogen gas displacement conditions. The instrument operates at room temperature, has no high-temperature components, and the range can be selected from 0 to 10% VOL, meeting the entire process monitoring from the initial oxygen content of air at 21% VOL to the qualified indicators. The accuracy of the trace level can meet the conventional displacement acceptance standards of ≤1% VOL. The equipment is equipped with 4-20mA, RS485 signal output and relay alarm contacts, which can be connected to the PLC/DCS system to achieve automatic nitrogen supplementation when oxygen content exceeds the limit and safety interlock protection, ensuring the inert safety of the storage tank. The instrument must be equipped with a 316L stainless steel sampling pre-treatment system to complete water removal, filtration, pressure stabilization, bypass displacement, and avoid sensor drift and damage caused by water vapor and impurities.

For on-site displacement acceptance, multi-point verification, and online instrument comparison, an intrinsically safe pump suction portable oxygen analyzer is used. The pump suction sample can stably extract the gas phase gas at the top of the tank, solving the problem of inaccurate sampling at low pressure inside the tank, and is suitable for operations in explosion-proof areas. It can store the detection data and meet the retention requirements for completion acceptance materials.

This condition does not recommend oxygen analyzers based on the zirconium oxide principle, as they are prone to interference from water vapor and combustible gases at high temperatures, have large zero-point drift, and poor stability. Laser oxygen analyzers are expensive, and the cost of ordinary storage tank displacement projects is low, and conventional conditions do not require their use. The overall selection is stable and reliable, compatible with biogas and hazardous storage tank nitrogen displacement processes, and meets the requirements for long-term operation and safety acceptance.

Common for displacement acceptance: 0-10% VOL, resolution 0.01% VOL; used for blowing down from 21% to below the qualified indicators during the displacement process, controlled to ≤0.5% or ≤1%

High-purity trace oxygen requirement: 0-1000 ppm, used for conditions requiring several ppm levels after displacement

Signals: 4-20mA isolated output + RS485 Modbus RTU, dual-channel relay alarm, can be linked to the nitrogen inlet valve interlock

The tank interior has condensation, oil and gas, and trace impurities. Pre-treatment is a must, protecting the sensor and preventing drift damage

Gas path material: 316L, can be uniformly matched with the pre-treatment system of your previous biogas desulfurization dew point analyzer, reducing the number of spare parts

Installation location: sampling port in the gas phase space of the storage tank top, avoiding the nitrogen inlet port and liquid level liquid surface, taking the real gas phase inside the tank; do not measure at the nitrogen inlet, it will be inaccurate.