Inertial Filter Probe

Principle of Operation

The basic principle of operation of the inertial filter is to accelerate the particulate material contained in the process gas in a vector direction with sufficient velocity to prevent the particles from sticking to the walls of the sampling tube and extract a gas sample to be transported to a gas analyzer.

An eductor is used to generate a high velocity (70-100 feet per second) axial gas flow through the inertial filter. The flow rate is dependent upon the gas density, temperature, diameter of the sampling tube, absolute pressure, and particulate loading. Particles in the high velocity stream continue to travel in the straight vector direction, and the sample stream is withdrawn at a very low filter face velocity (0.006 feet per second) separating the sample stream from the initial particulate material.

Particles entrained in the high velocity axial gas flow are prevented from depositing on or penetrating into the porous filter wall by the ballistic effect of particle inertia. The low radial velocity also inhibits particles from penetrating the porous wall. Compressed air is used as motive force air to the eductor and controlled with a needle valve.

A venturi flowmeter, which will measure 100-400 liters per minute, is located between the inertial filter and the eductor. This flowmeter is used to measure the high velocity axial gas flow and to monitor flowrate. The pressure differential on the flowmeter is fed to a transducer board that converts the pressure to a 4-20mA signal

While the filter is on-stream, sample flow continues without interruption so that representative samples are furnished on a continuous basis. The housing annulus has very low volume to minimize sample dwell time and assure fresh samples. Analyzer response lag time is minimized yielding timely analysis of process conditions.

Mercury Sampling Problems in Stack Gas
It is difficult to sample mercury (Hg) from stack gas without altering the ratios of oxidized to elemental Hg and without changing the concentrations of either species by losses in the separation of particulates through filtration. The original ratio of elemental mercury (Hg°) to oxidized mercury (Hg⁺²) should be preserved during the transport across the filter media. Current state of the art sample conditioning systems rely on heated filters to extract stack gas from the flowing process, remove the particulate material, and transport the clean sample to the sample conditioning system. These filters can become coated with particulate material, attenuating the Hg concentrations transported across the filter. Blowback is employed to periodically remove the particulate coating on the filter by back purging dry, compressed air under high pressure (100 psig). Hg° is oxidized to Hg⁺² across the filter media, caused in part by reaction with artifacts in the particulate matter coating on the filter media as well as reaction with the filter media itself. To achieve good analytical results, the filtering process at the sample point should not change the species of Hg existing in the sample, and should not attenuate (absorb) the Hg.

The Solution: Durinert®
A standard inertial filter will not yield accurate results when sampling Hg species in a process gas. Due to its chemical nature, Hg° and Hg⁺² are difficult to transport across a filter media. The filter media must be isolated from the Hg species to prevent oxidation of elemental Hg and to avoid reduction of oxidized Hg to elemental Hg.

We use Durinert® coating on all sampling components that come in contact with the gas sample stream which provides a protective inert coating on the stainless steel components to prevent Hg reduction.