Sampling System

Overview Hot and Wet Sampling System HW3 Probe Assembly Heat Traced Sample Line Heated Pump Heated Sample Cell

Sampling System Overview

In order for a Continuous Emission Monitoring System (CEMS) to function as desired it must be configured to deal with the environment and source type it will operate in. The rude fact is that the application engineering aspect of CEMS may be the most important part and it is in sampling systems where that is most critical. But it is not easy, there literally are hundreds of important variables that must be taken into account for every installation, any one of which ignored…will result in serious problems. That means that each system should be custom engineered for the job it is intended for, and not just “Cookie Cutter” applied to all applications—and those doing the engineering must have lots of experience. Indeed, even the best technology is almost guaranteed to fail in the wrong application…our industry has seen far too much of that. 

Over the years we at EcoChem Analytics have gained the application engineering experience that allows us to determine, ahead of time, the best configuration to accomplish the project goal and produce the lowest long-term costs. Those very same engineers who pioneered the first effective CEMS in the seventies, who introduced major technological innovations in the late eighties, and who have successfully engineered thousands of CEMS installations are the key to the success of EcoChem Analytics. The following are important CEMS concepts that they use to insure a successful project.

The sampling system may be the least understood part of a CEMS. In some minds no big deal--just a pump and tube and you’ve done the job. Not so simple, and so many have done such a poor job of dealing with sample system design that many users have rushed away toward hybrid systems—creating new problems instead of doing it right in the first place. The fact is that it is very difficult to extract a sample of gas from the exhaust stream of a combustion source and transport it for analysis without degrading or affecting the gases to be measured. There is the problem of particulate buildup, moisture, corrosive acid condensates--and the gases to be measured may themselves be highly reactive, perhaps even with the sample system itself. However, through proper consideration of the source conditions, required measurements and site geography it is possible to produce very reliable, low maintenance CEMS sampling systems. 

The primary key is to keep the system as simple as possible while dealing with four primary concerns: Particulate build up; Corrosion; Condensates; Reactive Gases. This is accomplished in the EcoChem Analytics sampling systems using specially designed and now refined components. The basic system that the EcoChem Analytics CEMS uses is the “HW” (Hot and Wet) method. The HW sampling system maintains high temperature throughout the sampling and analysis process. This approach provides the unique capability to directly measure HCl and Ammonia. In the HW system only four parts are in contact with the flue gas. The following discussion will describe the HW approach and its underlying principles.

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Hot and Wet (HW) Sampling System 

For the measurement of reactive gases such as ammonia or HCl, or in the case of a sample gas with a very high acid dew point temperature it may be necessary to maintain elevated temperatures all the way through the gas analyzer. The HW sampling system can maintain such conditions at temperatures up to 480 ºF. The HW system also includes the unique capability to directly measure stack water vapor content.

Hot and Wet Sampling System Flow Diagram

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1. HW3 Probe Assembly 

The HW3 Probe Assembly provides specific functions needed for reliable sampling of flue gases while closely maintaining temperatures at elevated levels. Those functions are probe-back purge with instrument air, calibration gas injection, and failsafe inerting to protect the system from corrosion if loss of temperature control should occur.

HW3 Probe Assembly

The HW3 Probe Assembly consists of a coarse (20micron) sintered metal filter mounted on the probe tip, a probe tube, and a fine filter body, which is mounted on a 2" 150PSI flange. A probe tube heater for stacks with condensing conditions can be provided. Standard materials of construction include 316L, Hastelloy, Inconel or even titanium.

Filtration is provided in two stages using sintered metal filters. The coarse filter on the probe tip is rated at 20 microns and is protected by an impact shield. External to the flange is the fine filter assembly which houses the second sintered metal filter that is normally selected for 1 micron. Generally this filter does not need servicing but it can be easily accessed without removing the probe from the flange.

Instrument air purging of the probe tip coarse filter is accomplished through the Fine Filter Assembly by use of a one-way valve for injection and operation of a special bellows valve for isolation. How often the probe is back purged is determined by sampling conditions but even with severe particulate conditions no more than four times a day is required.

Like back purging, calibration gas is injected into the probe by a check valve, using the same bellows valve for isolation. The isolation function is of critical importance while calibrating and the HW3 Probe uses a unique air operated bellows valve and seat to provide positive sealing at temperatures as high as 480 F. The back purge, calibration, and isolation valves are all built into the HW3 Probe Assembly and are of course heated and temperature controlled to the same level. 

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2. Heat Traced Sample Umbilical

The heat traced sample umbilical will maintain the gases at an appropriate temperature all the way from the Probe Assembly to the analyzer enclosure. Good engineering practice dictates that this be short as possible but it is of more importance to place the analyzer in a convenient location. Lengths up to 500 feet are possible. The sample umbilical bundle will contain one or more tubes for the sample gas and may contain additional tubes for instrument air and calibration gas. It may also have conductors and control wires for other functions needed at the  Probe Assembly. The materials of the sample tube may be either Teflon or stainless steel depending on the temperature requirement. Normally the measurement of HCl or ammonia will require the highest sample line temperatures, at about 450 ºF. 

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3. Heated Sample Pump

The motive force for sample system flow is a special high capacity heated pump. A key in reactive gas sampling is that it is important to minimize residence time in the sampling system and that means the pump must have high capacity. The heated sample pump has been specially designed for this service and has proven to be very reliable.

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4. Analyzer Sample Cell

The analyzer sample cell may be of straight or folded path construction depending on the gases to be measured and range of analysis and may include an integral ZrO2 oxygen sensor. The sample cell is temperature controlled and the sample exhaust is always vented to the atmosphere.

It should be noted that in the  Sample System only four parts-- Probe, Sample Line, Pump, and Sample Cell -- are in contact with the flue gas. All parts are highly specialized for this application and in over 400 installations they have performed reliable for extended periods of time.

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