Multi Hole Probe System

Multi Point
General Description:
The Universal Analyzers Multi-Hole Probe Tube and Sampling System are designed to obtain a representative sample when there is cross-stack stratification in a Continuous Emissions Monitoring System. Both the US EPA (per 40CFR60 Appendix A-4) and the South Coast AQMD permit the use of a Multi-Hole probe system as an alternative solution to gaseous stratification. A typical system will include two Multi-Hole Probe Tubes (each with a dedicated heated probe filter), two short heated transition bundles, and a two-point mixing/averaging module.

Principle of Operation:
One major source for bias, even in a well-designed single point CEMS sample system, is that the reference method and CEMS system measure from different points. In the presence of a stratified stack, this bias is amplified and cannot be accounted for with a simple correction factor. An example stratification profile is show below. What this profile does not show is that it could be varying with time and/or load as well. By employing a Multi-Hole Probe System, it will automatically correct for these spatial and temporal variations by taking a continuous, weighted average.

RECLAIM (REgional CLean Air Incentives Market) is another powerful reason to consider a Multi-Hole Probe System. Ever-reducing NOx emissions are making it difficult to meet incentive with traditional sampling methods. By meeting incentive (i.e. being at or below 7.5% relative accuracy):

  • RATA testing may be reduced to once annually, as opposed to biannually
  • There is potential to gain emission credits for exchange via RECLAIM
Multi-Hole Probe Tube Design:
The Universal Analyzers Multi-Hole probe uses equally sized holes and an oversized sample pump. Under significant vacuum the holes act in the same manner as a critical orifice establishing equal, critical flow across each. The hole locations are defined by EPA Method 1 for Sampling and Velocity Traverses. Because the number of sample ports and their locations are dependent on stack geometry, each Multi-Hole probe system design is unique.

Multi-Hole Probe Testing:
Each Multi-Hole Probe System is bench tested to rigorous standards. The tests are performed in accordance with AQMD Source Test 511 in conjunction with EPA GD-031. The system is tested using reliable, NIST Traceable equipment. Universal analyzers also generates a report that details the system design, system test, and bench test results which stand as evidence that the system meets the two major requirements for a Multi-Hole probe system: • The flow rate for each of the individual sample ports falls within 10% of the average of all ports. • The sum of the individual sample port flows is within 10% of the total flow through the sample probe.

Multi-Hole probe systems excel as a remedial solution for a cross-stratified stack in scenarios where the user is attempting to measure low pollutant levels. Because of the limitations of the design, the system is geared toward low particulate processes. Sampling conditions involving high particulate loading and/or entrained moisture can be detrimental to long-term multi-point probe performance, and its use in these environments is not practical, due to the uncertainty regarding reliable performance and comprehensive maintenance requirements.

Multi-Hole Probe versus Other Remedial Methods:
Current remedial methods to handle cross-stack stratification are flow mixing or straightening devices and sample site relocation. Flow mixing and/or straightening vanes demand extensive engineering and labor resources, they can be very costly, and could potentially delay startup or production. Sample site relocation, though a simpler solution than mixing or straightening, may not always be practical or even possible. A Multi-Hole Probe System can be designed and built in a matter of weeks, it is an economical solution, and it is accepted by the US EPA and SCAQMD.