Written By TechStar Solutions Specialist, Don Wyatt
Do you have Fired Heaters, High Performance Furnaces, Incinerators, Selective Catalytic Reduction Units (SCRs), Flare Lines, Tanks to blanket, Reactors to monitor, Marine Terminals for loading or unloading ships and barges? If so, then you probably either have online gas analyzers or need them. Many people that own these assets really don’t like online process analyzers or don’t like what it take to keep them running. Traditionally these analyzers have been more complex then most of the instrumentation that is used in our industry. This leads to performance less than the user would expect.
Maintaining analyzers often requires a different skill set than traditional instrumentation. Many analyzers require the gas sample be removed from the process and transported to the analyzer. These extractive analyzers use complex sample conditioning systems. These systems use a combination of tubing, pressure regulators, flow meters, valves, filters, etc. to condition the sample to meet the analyzers specific requirements.
The Yokogawa TDLS8000 was designed to eliminate most of the issues with traditional gas analyzers. The impact of the issues that could not be eliminated completely are reduces significantly . The primary method of installation is insitu. Most applications are installed directly on the process. Yes, in most installations we eliminate the sample system which is typically the trouble spot with analyzer installations. I say most because some processes still exceed the temperature or pressure specifications of the analyzers. Also, in some cased the line sized may not be large enough for insitu installation. We also must consider accessibility to the analyzers. An analyzer that you cannot get to will most likely not be maintained adequately.
Yokogawa has a large installed base on fired assets. These installations will be a combination of percent Oxygen (O2), ppm Carbon Monoxide (CO) and percent Methane (CH4) To accomplish this the installations use an analyzer to measure O2 and an analyzer the measure CO and CH4. So, you can see it take two analyzers the measure the three gases. Each of the analyzers are SIL 2 Hardware and SIL 3 software. This allows the analyzer to be easily considered in your safety calculations. We have seen these analyzers use for closed loop control and safety applications.
Another interesting insitu application is Ammonia (NH3) Slip for SCRs and Selective Non-Catalytic Reduction Systems (SNCR). Measuring NH3 is very difficult in extractive systems as the process gas tends to continue reacting as it travels down the sample line. This leads to lower readings at the analyzer than is actually in the process. The TDLS8000 can measure the NH3 gas directly in the process. This produces a representative NH3 measurement and eliminates all of the issues with gas sampling.
The last set of applications listed above are what we refer to as Limiting Oxygen Concentration applications. They all have the potential to have a flammable gas and oxygen present. All you need is a spark to have a really bad situation. The goal for these applications is to monitor O2 and the signal is feed back to a system the keeps the O2 level less than 5%. This value will vary depending on the application and the operating company. These applications are typically extractive even when using the TDL8000. The sample systems are very simple as we can typically maintain process temperature and pressure. We typically use flow control and minimal filtering.
What make the TDLS8000 so different than traditional analyzers? I is the second generation Yokogawa TDLS that was purpose designed to measure insitu on Refining and Petrochemical process. This includes the electronic housings, area classifications, ambient temperature ratings, process temperature and pressure ratings, process connections, communication protocols, etc.
The analyzer is made up of the Sensor Control Unit (SCU)(Receiver), Launch Unit (LU) (Laser – Source), Optional Human Machine Interface (HMI) and a Utility Panel for insitu applications or Simple sample system for extractive systems. A laser emits a specific wave length of light in the near infrared where the gas to be measure will absorb the light. Different gases require different lasers. The absorption features are very narrow and specific. The helps us pick wave lengths where there is no interference from other gases in the measured stream. The telecommunication laser that are used have life expectancies greater than 10 years. The signal processing used gives the analyzer a drift spec that is measured in years not weeks. It is very possible that the analyzer will not require calibration during its operational life span. It is possible to calibrate, but it is not typically required. Yokogawa does understand these are process analyzers and the will need to be challenged and validated. There are two (2) ways to do this. The analyzer can be removed from the process and placed and a validation cell and validated with zero and span gas. This can be time consuming and only practical during a shut down or turn around. The most common validation we see in practice is what is called an online validation or bump check. The analyze has valve drivers and the software to introduce a known span gas into the integral validation cell, which is part of the analyzer enclosure, and preform a standard addition test. Since there is no way to remove the process gas from the measurement path we have to do an additive measurement.
Here is how it works: First the analyzer measures the process, this will be the baseline, and stores the value. Second a known concentration of the measured gas is introduced into the integral validation cell. After stabilization the measurement of the validation gas plus the process is captured. Third, the validation gas is removed and the process only is measured. The process before and after are measured to check the baseline value. Then the baseline values compared to the validation plus process value. Since we know the pressure, temperature, path length and concentration of the validation gas we know how much should have been added to the process gas. These valued can be displayed and output along with a pass/fail flag. This is all possible because the analyzer electronically zeros itself about 1000 times a second.
In conclusion, when we consider the long life of the optical components, the long term stability of measurement, the ability to validate in place, the noncontracting measurement technique, the elimination or simplification of sample systems, we see a compelling long term cost of ownership. This along with reliable meaningful measurements lead to a safer work environment.