INSTALLATION OF RADAR LEVEL INSTRUMENTS IN ANAEROBIC …€¦ · Radar, level measurement, foaming,...

INSTALLATION OF RADAR LEVEL INSTRUMENTS IN ANAEROBIC DIGESTERS

Phil Ackman and Thao Le County Sanitation Districts of Los Angeles County

1955 Workman Mill Road Whittier, CA 90607

Phone: 562-699-7411 E-mail: [email protected]

ABSTRACT There are numerous well-proven options available for level determination instrumentation. Instrumenting treatment processes present some unique challenges. Some of these include the ability for wastewater or residuals to clog and blind instruments. Level determination in an anaerobic digester presents these problems as well as others including foaming, surface agitation, and operation in a methane environment. The radar level instrument provides a new technology that has the potential of overcoming these problems to provide reliable and accurate level readings in Digesters. KEYWORDS Radar, level measurement, foaming, digesters INTRODUCTION The Los Angeles County Sanitation Districts operate 11 wastewater treatment plants. The largest plant is the Joint Water Pollution Control Plant (JWPCP) located in Carson, California. This plant has an average flow of 320 MGD with a design capacity of 400 MGD. The major unit processes are Primary Sedimentation, Pure Oxygen Activated Sludge, Anaerobic Digestion, and Biosolids Dewatering. When this work was initiated in 2001 JWPCP was operating 17 circular anaerobic digesters, which are 125 feet in diameter with a capacity of 3.75 million gallons. There were also 23 rectangular digesters which had a capacity of 0.75 million gallons each. These rectangular digesters were at the end of their useful service life and a project to construct 7 additional circular digesters of similar design to the existing digesters was due to begin shortly. The circular digesters have a flat fixed roof and are mixed with five draft tubes. These tubes mix the primary and thickened waste activated sludge using recirculated digester gas. These digesters are operated in the mesophilic temperature range (93ºF). The physical digester characteristics and sludge characteristics lead to the formation of a layer of foam on top of the digester. The depth of this layer of foam can vary from several inches to several feet. The depth of the foam layer can vary in the same digester from day to day. On rare occasions there can be hydraulic or other problems such as partially blocked runoff lines or leaking feed valves that would change

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Copyright 2006 Water Environment Foundation. All Rights Reserved©

the balance of foam in the digester over short periods of time. The normal static sludge level in JWPCP’s digesters is about 4 feet from the roof. This typically provides ample headspace to prevent foam or sludge from being pushed out of the digester seals. Never the less the previously mentioned hydraulic problems have on rare occasion caused a digester to overflow. It is always the intent of the Districts and JWPCP Operators to prevent such overflows from occurring. Not only are overflows low frequency events but the time between the event trigger and occurrence can be quite small. Plant operators for routinely check the digesters for proper functioning and level. JWPCP is manned 24 hours a day with 3-operator work shifts per day. Operators use a glass porthole type device to visually examine the level of sludge and approximate foam depth in the digester, see Figure 1. The treatment plant operators have developed considerable expertise in estimating the levels visually. However, since there is a considerable time interval between operation checks they are not always sufficient to detect precursors to an event. Instrumentation that detected the level of foam or liquid in the digester reliably would be of great benefit. An instrument, which would only measure the level of liquid in the digester, would miss events in which the foam layer had increased in depth so that it was close to the digester roof but the liquid level was normal. Typical instruments such as ultrasonic level, pressure measurement, floats, level switches, etc. either respond only to liquid level or would potentially be compromised by solids build up on the instrument. Ultrasonic type level instruments were installed on several of these digesters many years ago but had problems with absorption of the signal by the methane in the headspace and could only detect liquid levels. For these reasons the instruments fell into disfavor with the operators and were abandoned. Radar level instruments using microwave frequencies have only been on the market for about 10 years. These instruments claim accuracies of 0.1 inch and are thought to be highly resistant to failure. In addition, they have the ability to operate in a carbon dioxide and methane atmosphere and are capable of detecting foam if the density is greater than 0.1 g/cm3. Although no data on digester foam density was available it was thought that the solids content of the foam would render it visible by the instruments.

Figure 1 -- Digester Portholes without and with Internal Cleaning Wipers TEST METHODOLOGY A radar level instrument would be purchased and installed on one of the operating digesters for long term testing and evaluation. A Miltronics IQ160 instrument was selected and purchased.

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The Districts have used Miltronics instruments in numerous applications and have been pleased with the reliability and support for these instruments. There are numerous other manufacturers producing radar instruments. It was not the intent of the testing to examine differences that might exist between vendor’s products rather to determine if this technology could perform as needed in this application. The IQ160 was designed to be placed in tanks such as digesters. The instrument had a sliding waveguide with a clamp seal, which allowed the unit to be flange mounted to a gate valve and lifted up to, allow removal without taking the digester tank out of service. The unit was mounted on an existing gate valve and flange. The unit was placed on a digester that was being returned to service from cleaning. This would allow better access to the instrument during setup since the digester is first filled with water before being returned to service with sludge. The placement on an isolation valve meant that this instrument could be removed from the digester by loosening the collar and sliding the unit up until the horn cleared the gate valve, then closing the valve. This prevents escape of digester headspace gasses to the atmosphere, which might initiate odor problems, safety issues, and regulatory notifications. The installation of the radar unit in the valve and spool could create some potential performance issues. If a standard radar unit were used, reflections of the radar signal off of the valve and spool would lead to excessive signal noise and unreliable readings. The extended waveguide and horn directs the radar signals into the unit and compensates for the reflections off of the valve and spool. Installation of the test unit in Digester No. 8 that was being returned to service from cleaning, had some setup benefits but also a disadvantage. This digester had no previous level instrumentation and therefore no existing conduit or cabling for that purpose. JWPCP uses a Bailey Distributed Control System for plant process and equipment monitoring and control. The closest termination cabinet for the DCS was about 510 feet away. It would be somewhat time consuming and expensive to place this instrument on the DCS for testing. Continuous readings were desired to fully evaluate the performance. If the instrument failed to perform this expense would be wasted. To solve this problem a dedicated data logger was used to collect data. Real time data capabilities would be lost, however continuous data would still be collected without the cost of wiring. The data logger used was a HOBO unit with a shuttle downloader. This unit is small and battery operated (see Figure 2). The data logger was mounted on a backboard near the instrument. The shuttle downloader was also battery powered and plugged into the data logger. This unit was about the size of a book of matches and is used to transport data from the data logger to a personal computer. The advantage of this was to eliminate the need to have or take a laptop to the data logger to obtain data. This set was inexpensive and allowed several weeks of data to be stored on the data logger between downloading. The radar level instrument testing was to occur in two phases. Phase 1 would involve determining whether the instrument could achieve the required accuracy of water and foam detection in an operating digester. Phase 2 would be contingent upon success in Phase 1 and would focus on long-term performance of the unit. Resistance to fouling by grease, sludge, or other material would be examined. How stable would the reading be over time; would the instrument drift or require excessive maintenance and oversight? The depth of foam is quite

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variable, only long-term testing would allow the instrument to experience the full range of foam depths.

Figure 2 – Test Unit Installed on Digester Number 8 with Data Logger and Views of a Standard (left) and Waveguide (right) Type Radar Instrument

TEST RESULTS The radar unit was setup and installed by District personnel in the digester. Miltronics instruments have somewhat similar setup parameters across their instrument lines. This makes it somewhat easier to setup a new instrument if you have some familiarity with other Miltronics products. There are potentially 75 parameters, which can be used to define the operation of the radar unit. As with many instruments default settings were used on almost all of the parameters, leaving tank dimensions and readout options as the setup parameters that were modified. The units initial operation was successful; it tracked the filling of the digester with water and maintained the level measurement all the way to the digester roof. The digester was then returned to service and filled with sludge. The instrument was calibrated to readout level with respect to distance from the roof. This was consistent with the operator’s manual reading reference point. Validation of results from a test instrument usually requires an accurate reference reading. In the case of this digester, no other level instrument was available to compare results against. The only level readings were provided by the Treatment Plant Operators and were based upon visual estimations of the top of the foam level. The operators used the known geometry of an internal overflow pipe flange to determine levels, see Figure # 3. Verification of these readings would require opening the digester, which was not a viable option.

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Since no direct accuracy check would be possible, the evaluation of the instrument would be based mostly upon readings stability and comparisons with operator estimates. The instrument as installed appeared to work well with one problem. Occasionally there would be a spike of short duration in which the reading was off by several feet. The recurrence interval of the spikes was about one every 4 hours, but not regular. Since the level in the digesters change slowly an averaging function could have been used to eliminate the spikes. However, before implementing such an averaging of readings work needed to be done to determine if the spikes could be eliminated. The radar unit has the ability to output the echo profile for troubleshooting, this allows viewing of the echo details. A knowledgeable technician can review this data to determine why the software in the radar unit is selecting the wrong echo for level determination. The District did not initially have the required software to allow viewing and storage of these echo profiles on a laptop. A Miltronics representative was called out to perform this task. It was determined that reflections of signals off of the manway and roof edge were being misinterpreted as level signals. Modifying the “near blanking” parameter solved the problem with level spikes. The “near blanking” parameter tells the instrument to ignore any level that measures less this number. In our case, the near blanking could be set to the level of the concrete roof, since the level cannot go above this. After this modification the levels readings were stable. In order to attempt to verify the accuracy of the instrument, manual readings, which are taken by the operators once per 8-hour shifts, were collected for several weeks. In addition, the digester had experienced very little foam production during the time in which the radar unit was installed in the digester. The ability of the unit to detect the top of the foam layer was important for successful instrument functioning. To enhance the production of foam in the digester the loading of waste activated sludge to the digester was increased. This can result in additional foam production. The graph labeled Figure 4 displays the results of this testing. During the initial portion of the waste activated sludge enhancement test, the operator and instrument agreed fairly well. The increased feeding of the digester did indeed produce more foam on the surface. Levels of foam had built up to about 1 foot, which is still a modest level in comparison with historical high values. Deeper foam levels caused the operator and instrument agreement to diverge somewhat. The difference was still only 0.3 feet, which is still quite good considering the operator readings are visual estimations. Several things are evident from review of this data. The radar unit detected foam quite well. The unit appears to accurately measure the level and the Operators are quite adept at estimating levels although there is more variation. Probably different operators having a somewhat different estimate of a similar level cause this. The radar unit in digester was monitored for a total of 6 months. During this time period the accuracy of readings for both foam and water were verified. The unit suffered no degradation of performance due to any build up of material on the instrument. The instrument performed as it was anticipated, giving accurate level determinations in the digester. A decision was made to purchase and install these units in the existing 17 circular digesters. These would be purchased and installed by Districts’ staff. Installation of the radar level instruments on each digester required physical mounting, power connection, and signal line. These digesters all had flanges and spools that would accommodate the Instruments.

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Figure 3 – Cross Section of JWPCP Anaerobic Digester

Radar Level Instrument Test Results

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Figure 4 – Results of the Comparison Between Operator and Radar Data

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Providing power required running new conduit from local junction boxes. As discussed previously the signal line would require long distances to get to the closest DCS termination cabinet. Because the digesters are far apart there is limited opportunity to combine the level signal wires into one conduit. An analysis of the costs of running the level signal wires determined that wireless radio transmission of the signal would be more cost effective. TESTING OF WIRELESS RADIO SIGNAL TRANSMISSION Wireless technologies have recently emerged to provide low cost alternatives to expensive signal wire installations over long distances. With the intention of using the digester level signals for monitoring purposes only, it would be appropriate to experiment with the wireless signal transmission. Elpro Technologies offered a free trial test of their industrial wireless I/O modules that use hopping spread spectrum frequencies. These modules operate in the license-free 900 MHz radio band for a typical distance in plant and factory environments of 3000 feet. The longest distance between digester tanks to the control center is approximately around 1300 feet. There are several building structures and plant equipment blocking the line-of-sight path between the wireless transmitter and receiver units. The ability of the radio signal to penetrate obstacles or reflect from surfaces over such short distances would be examined. The trial wireless radio I/O units were installed by District personnel to link the level indication of the Digester No. 8 into the DCS cabinet in the treatment plant control center. The Elpro Technologies representative came on-site to provide training and to assist in commissioning the new installation. One radio transmitter unit was installed in a field junction box to convert the 4-20mA analog signals from the digester radar level instrument to radio telemetry signal. At the other end the radio receiver unit was installed in the control center building with the antenna mounted on top of the building to receive multiple wireless signals from all digester radar level indications and to produce 4-20mA signal outputs to the DCS. The wireless I/O modules were easily configured using a Windows-based program and via the RS-232 communication serial port.

Figure 5 – Wireless Transmitter Installed on Digester Number 8

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After activation, the wireless radio signal worked accurately and reliably. The digester level indication signal was present on the plant HMI software. The operators could seek the levels and alarming was activated. After the successful installation and performance test additional wireless modules were purchased and installed to bring other digester level indications into the plant DCS. All units are currently working well and so far require little or no maintenance. Startup of Radar Level Instruments on New JWPCP Digesters The time lag between Design and completion of constructions for a large treatment plant expansion is anywhere from 3 to 6 years. Because of this the radar level instruments were specified to be installed on JWPCP’s seven new circular digesters before testing on the instruments was complete. If the units performed poorly they would have been removed from the contract. It was preferable to receive a credit if they failed rather then design them after construction had begun and pay the contractor an extra work claim to install them. When these digesters were started up the sludge level indications were accurate when the level readings were low. As the levels increased to normal operating range the level readings became inaccurate and erratic. The installation specifications stipulated involvement of the manufacturers representative during setup and the units were purportedly field-tested. Records indicated that the Contractor had followed the specifications and did the required testing prior to startup; therefore troubleshooting the units would become the responsibility of the plant staff. Since similar units had been performing well on digesters set up by plant staff the problem was thought to be setup of installation related. The plant Engineering staff and Electrical and Instrumentation Technicians examined each units setup parameters and found numerous units with incorrect setup parameters. After these adjustments were complete readings improved but some units still were experiencing stability and accuracy problems. One of the problematic units was removed from the digester to determine it’s exact placement geometry inside the tank. It was discovered that the waveguide (straight rod section) was too long and was sticking too far into the tank. The waveguide acts to reduce the interference produced from the spool. The geometry of the waveguide also means that the instrument cannot read levels above the end of the horn end (conical section). Therefore installation of the unit involves some tradeoffs. The signal quality improves as the horn extends further into the tank from the spool or roof. However, the unit can’t read levels above the horn according to the manufacturer. The Districts circular digesters normal operating levels leave about two to three feet of space between the roof and the top of the foam. For this reason it was desired to track the readings as close to the roof as possible. The installation of the units on the new digesters created a one to two foot dead zone where readings were not possible. The Districts conducted a test to verify the manufacturers claim that accurate readings are not possible above the location of the horn. Sludge levels were lowered then raised slowly. The results clearly showed a cutoff of good signal level when the sludge level reached the horn tip. The solution to the problem should have been simple, loosen the collar and raise the unit until the proper position is reached. As can be seen on figure 6 the power and signal conduit are rigid and

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Figure 6 – Installation on Radar and Conduit on Unit for New Digesters Will not allow for any movement of the meter. The units could have to be rewired with flexible conduit or shorter waveguides installed. Shorter waveguides were determined to be the most cost effective solution to the problem. During the replacement of the waveguides several additional problems were noted. Closing the isolation valve without removing the meter crushed one of the waveguides. Several of the radar horns were noticed to have fallen off into the digester, causing the unit to fail. The horns were mounted onto the waveguide by a clamp. It was unclear whether these fell off during installation or subsequent position adjustment. In response to this problem the manufacturer redesigned the mounting configuration to use solid connections. The desire to measure and track levels as close to the digester roof as possible creates a greater quantity of signal noise and false echoes. After the problems noted above had been corrected there were still several digesters that fluctuated intermittently between correct and incorrect level readings. A Miltronics representative evaluated the performances of the instruments and their echo profiles. There was evidence of false echoes and excessive signal noise in the echo profiles. It was recommended to utilize the false echo suppression feature on the instrument to de-sensitize the receiver from any base noise caused by internal antenna reflections, nozzle echoes, or surrounding equipment. The baseline echo profile was recorded when the tank was either empty or at lower levels. Then the auto false echo suppression feature was activated to adjust the TVT (time varying threshold) curve to ignore false echoes in the profile. Since the background noises may shift over time, the false echo suppression profiles can be re-learned and re-adjusted as necessary to improve the instrument performance. The District decided to purchase the software and received training to enable staff to perform the troubleshooting and corrections on units as needed.

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Figure 7 – Echo Profile Before False Echo Suppression

Figure 8 – Trend of Digester Level Showing the Affect of False Echoes

TVT baseline curve

Echo profile

The radar instrument will read the first or largest echo above the baseline curve as the digester level

After false echo suspension feature is activated

Before the false echo suspension feature is activated

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Figure 7 depicts the echo profile of the instrument for a digester that was still reading incorrectly. The profile is obtained by using the manufacturers software on a laptop computer and connecting to the instrument. It can be seen that the false echo overwhelms the true level signal somewhere in the range of 3 meters on the graph. Figure 8 shows a trend of the level for this digester that an operator would see on the plant DCS system. It is not physically possible for the levels in the digester to be varying this quickly; therefore this is indicative of the levels oscillating between correct and false echoes. Operators seeing this type of output from the instrument will quickly lose confidence in the instrument and ignore its readings. Figure 9 shows what the echo profile looks like after implementation of the false echo suppression feature. The subtraction of the signal from the false echo leaves a strong signal from the liquid/foam level in the digester. A typical trend of digester levels that are reading correctly can be found in exhibit 10. The level readings show no oscillation and level changes occur over a much longer time frame.

Figure 9 -- Echo Profile of Digester after Implementation of False Echo Suppression

False echo from tank nozzle & isolation valve

Echo from the tank level

TVT curve filter false echoes

Radar instrument echo profile

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Figure 10 – Trend of Digester Levels Depicting Correct Operation the Radar Units Conclusions The radar level instrument was tested extensively in the Sanitation Districts JWPCP digesters and they were able to detect the level of foam and sludge reliably. The instrument was unaffected by solids accumulation, foam, or the atmosphere. The geometry of the District digesters and the desire to track the level up to the digester roof did provide some challenges. The instrument and the software were capable of performing under these circumstances but setup and maintenance is more time consuming and complex. The startup of new digesters constructed by a contractor demonstrated the difficulty of installation and setup of a relatively new technology that they had limited familiarity with. Installation details and setup instructions were insufficiently detailed and resulted in numerous errors in setup and installation. These problems could have been avoided if knowledgeable plant staff had more interaction with the contractor during startup. However, staffing and division of responsibilities make this difficult to achieve. At this point JWPCP has radar level instruments working in 15 of 24 circular digesters and plant staff is continuing to install units in the remaining digesters.

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INSTALLATION OF RADAR LEVEL INSTRUMENTS IN ANAEROBIC …€¦ · Radar, level measurement, foaming,...

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