On a recent visit to evaluate several level transmitters on chemical storage applications at a new combined cycle power plant, we had the opportunity to work with the EPC firm, plant personnel and the chemical supplier. This provided an interesting look at the level instrumentation package for the chemical storage side of things from an engineering perspective, as well as from a daily operations point of view.
Ammonia, Acid and Caustic Chemical Storage
Although important measurements, the ammonia, acid and caustic storage tanks are not difficult level applications for Magnetrol® and Orion Instruments® by any stretch. However, we found that small nuances on how the applications are monitored relative to technology can have a dramatic effect on the day-to-day practicality and reliability of the type of instrument(s) used. Additionally, there are safety considerations when replenishing these chemicals, which can be addressed simultaneously with inventory monitoring by implementing a few simple, cost-effective modifications to the instrumentation package.
These chemical storage tanks can be horizontal or vertical vessels six to ten feet in diameter/height, with the ammonia storage tank usually the largest of the three. It is not uncommon to see some type of non-contact level transmitter (Ultrasonic being the most prevalent) installed to provide level indication to the control room with a local display at the base of the tank, either in series with the 4-20 mA transmitter output or repeated from the control room. The signal to the control room tracks inventory, acts as a high alarm for overfill protection and establishes the resupply interval. The local display facilitates monitoring the offload of chemicals from the supplier’s truck.
There are a number of level technologies you could throw at these applications. Our preference is Through-Air (Non-Contact) Radar or Guided Wave (Contact) Radar on the acid and caustic tanks and the latter for the ammonia storage. This doesn’t imply non-contact Ultrasonic Level transmitters or other technologies are not up to the task. Simply put, radar is indifferent to the changes in the contents of the vapor space of these vessels occurring throughout the course of the day. Oftentimes, these changes affect the Ultrasonic burst causing what we refer to as a nuisance alarm, e.g., it loses the signal intermittently or the level indication becomes erratic only to recover about the time a technician arrives on the scene. These types of issues are difficult to isolate since they are intermittent and not linked to an installation or configuration anomaly or hard fault in the instrument. If a competitively priced technology that does not require calibration and is unaffected by changing process conditions is available, we would take that path. Radar meets these three criteria.
Performance Perspectives from the I&C Team
Discussions with the I&C technician and chemical supplier during the evaluation process were confined to reliability, remote indication and performance verification of the level transmitter. Both preferred an independent visual indication along the lines of a Magnetic Level Indicator (MLI) on all three tanks. From the technician’s perspective it allows for easy verification of the level transmitter’s performance and adds a layer of redundancy in case the transmitter was out of commission for whatever reason. We have a lot of confidence in our instrumentation hitting the mark right out of the gate, but we have to admit it is nice to have a second opinion on the reading following initial commissioning. Sticking these tanks is usually not an option during normal operation.
Level Gauge Readability During Product Transfer
The chemical supplier’s insight focused on readability during the transfer of product from the truck to the tank. Even though he normally offloads a fixed quantity of material which the vessel should accommodate based on level indications prior to dispatch, knowing the level during the transfer process is a safety measure to prevent overfilling. His comment was that MLIs can be read easily from a distance with the occasional glance while managing other tasks, whereas, he had to be on top of an LCD-type display to monitor progress. This was particularly important when working with the ammonia storage tank.
Considerations When Top Filling
One item worth pointing out that would simplify the commissioning of the instrument is the close proximity of the top-fill piping to the instrument mounting nozzle on the smaller acid and caustic tanks. Since non-contact technologies are ubiquitous on these applications and rely on projecting a circular or elliptical signal footprint perpendicular to the surface of the material being measured, locating the instrument nozzle as far away from the turbulence generated while top filling is the ideal. Furthermore, such close proximity allows the spray pattern created as the chemical enters the vessel to interact with the transmitted beam, which could cause a loss of signal during the fill process leaving the supplier blind as to the remaining space in the vessel. Tweaking the instrument’s configuration to overcome such obstacles is possible with time and patience. On the flipside, separating these two entry points during the design phase of the vessels would eliminate any potential problems without adding cost or extending the commissioning time. The present nozzle/fill line configuration is another point to argue the case for an MLI or opting for Guided Wave Radar technology to eliminate any potential interference and excessive turbulence near the transmitter.
After surveying each application and visiting with the I&C department and chemical supplier, we reviewed the various options with the engineering team. Our collaboration yielded three options to improve the reliability and enhance the day-to-day functionality of the instrumentation package.
Three Options to Improve Reliability and Functionality
The easiest modification was to replace the originally specified Ultrasonic technology with an entry level Through-Air Radar (Magnetrol Model R82), an easy fix to improve reliability by eliminating the vapor space issues noted above without adding to the overall cost. The balance of the inventory management scheme remained the same, i.e., remote LCD indication and so forth.
Taking things a step further, we considered incorporating a Guided Wave Radar (Magnetrol Eclipse® Model 706) in place of the non-contact technologies. The Guided Wave Radar would add cost to the instrument itself as compared to an Ultrasonic or entry level Through-Air Radar. However, if we leverage its remote transmitter option in lieu of purchasing an independent remote indicator, we can offset most of the additional cost by eliminating the secondary display and its associated costs: wiring, mounting, configuring, etc
Another benefit to this approach is more flexibility in the instrument nozzle mounting location relative to the vessel top fill piping arrangement. The Guided Wave Radar is a contact measurement whose sensing element can ignore turbulence and the chemical spray pattern previously mentioned. This added benefit not only simplifies commissioning of the instrument but separates the measurement from the tank dynamics for improved reliability.
Lastly, we looked at including some of the “wish list” items the I&C technician and chemical supplier noted and added a visual indicator (Orion Instruments Atlas™ or Aurora® models) for level verification, redundant and diverse measurement technologies and improved readability during normal plant operation. This approach does add cost to the instrumentation package even when taking into account the elimination of peripheral items included in a minimal installation. Aside from the ammonia storage tanks, which traditionally have the process connections in place to accommodate an externally mounted device, adding similar process connections to the acid and caustic tanks would bump up the cost of the vessel as well. In the grand scheme of things, the additional costs are minor compared to the long-term benefit, but it is something that needs to be taken into consideration for chemical storage monitoring.