Maintaining Wastewater Treatment Systems

Continuous vibration monitoring of pump stations at a major wastewater treatment plant pays off for the City of Tampa.

The Howard F. Curren Advanced Wastewater Treatment Plant (HFCAWTP) is a state-of-the-art facility that treats all wastewater discharged from the City of Tampa, fl, system from approximately 100,000 accounts. The plant has a license capacity of 96 million gallons per day (MGD), with an average daily flow of 60 MGD. The final product, or effluent water, is discharged to Hillsborough Bay or used as reclaimed water for cooling and irrigation. This high-quality water meets all state and federal requirements.

The plant has developed and is currently executing an optimization program that includes automation of processes and procedures when possible, and reducing scheduled vs. unscheduled downtime and maintenance, transitioning from a reactive to proactive organization ready to address issues and problems. Because the Howard F. Curren facility is the City of Tampa’s (COT) only wastewater treatment facility, it is imperative to minimize flow interruptions, unscheduled downtime and overflows.

The use of reliable pumps to transport wastewater from various locations in the city is critical for maximizing flows and maintaining biological efficiencies by producing a constant flow. When the pumps fail, backup pumps are used to keep the flow going. Failures often can be very damaging to the pumps and auxiliary equipment. Installing a protection system that monitors the vibration levels and can be integrated to a shutdown circuit can minimize flow interruptions and the amount of costly damage to that equipment. The price of a new pump motor can be as high as $450,000; the cost to repair an existing unit can approach $175,000 after a catastrophic failure. In an effort to help prevent these types of failures, the HFCAWTP and Connection Technology Center, Inc., a vibration analysis hardware and process equipment manufacturer, investigated different equipment and system options for monitoring this crucial application.

The application
There are eight major pump stations that collect the wastewater and deliver it to the treatment plant. Each major pump station has many smaller stations that will feed it—either through pump systems or gravity feed. There are approximately 224 pump stations within this system.

Three types of pumps setups are typical of these stations: Direct coupled, submersible and vertical shaft. The direct coupled stations will have the motor and the pump on the same floor, with the motor in an overhung position and supported over the pump. The vertical shaft stations will have the motor and clutch or VFD-controlled motor typically two stories above the pump, with the shaft coupled in one or two places.

Each major lift station has three or more motor-pump systems, with one pump typically running at a time to ensure system redundancy. Major failures can cause overflow issues, not to mention extensive damage or complete failure with auxiliary equipment such as valves, VFDs and wiring.

For the purposes of determining where to install a protective system, the major stations were identified as the areas to have the critical equipment monitored.

Vibration considerations
The general vibration considerations that are periodically monitored in the pumping systems at this plant include cavitation, mechanical failure and mis-alignment. Cavitations often will accelerate the mechanical failures of the pump, such as discharge valve failures and impeller wear. Faults due to mechanical issues also are accelerated due to increased flow. Possible mechanical failures include breaking or dropping the impeller or impeller shaft and/or bearing failures.

Other unique vibration considerations at this plant are associated with the alignment of the vertical shafts to pumps, requiring coupling shafts up to 20 feet in length, and accessibility of the equipment, which is often very difficult.

The application is made even more challenging by the fact that these remote pump stations are not manned, and the periodic monitoring may not be sufficient to capture any transient type of faults that could lead to failures.

Process/protection considerations
Periodic monitoring may be sufficient to identify general, long-term machinery conditions, but to capture transient conditions that can cause catastrophic failures, continual monitoring is required. Because the pump stations are unmanned, a system is in place to alert a technician at the plant that there is an issue with the pump station equipment. If there is an issue, corrective actions may be necessary in order to prevent the premature failure of the equipment and overflows.

Ensuring this capability required integration of the vibration system with the plant SCADA system. The output parameters of the vibration system, in this case 4-20mA output proportional to the overall vibration levels of the equipment, will feed into the SCADA system and allow the technician to observe a “status” of the equipment at the stations. This is an ideal situation, as many issues can be identified quickly before the effects of a catastrophic failure occur. However, this integration is often difficult based on the available resources of both the SCADA system and the plant personnel to integrate this.

Another solution that can be implemented as a stand-alone or integrated with the SCADA system is to provide a local relay or shutdown system that can be tied into the motor control circuit to shut down the pump system in the event of a catastrophic failure. Such a solution can limit the extent of the damage to the pump and limit/prevent the damage to auxiliary equipment, as well as minimize interruptions of the flow to the plant.

Equipment & system selection considerations
For the initial unit, a system of low-cost accelerometers mounted to mounting targets connected to a remotely mounted process controller enclosure was specified, with integration to the main plant SCADA system. The equipment was selected based on the following considerations.

Accelerometer selection…
To select the proper accelerometer for the monitoring of components, the following vibration frequency criteria was taken into consideration:

• Pump vane frequencies
• Pump cavitations frequencies
• Motor fault frequencies
• No clearance issues that would require low-profile sensors
• Historical vibration data and experience with the equipment

Frequencies for detecting vibration faults should be within the frequency response of the selected accelerometer. For accelerometer specification, the motor and pump vane frequencies did not require a special frequency response, and a standard, 100 mV/g accelerometer, with a frequency response between 0.5 – 15000 Hz, was selected for this application.

Mounting hardware selection…
To provide the optimum vibration transfer between the machine surface and the accelerometer, a mounting system that utilizes the full frequency span of the accelerometer needed to be considered. A mounting target attached to the prepared machine surface (prepared with an installation tool kit [MH117-1B] that can be resharpened for multiple installations) with an adhesive was selected. The adhesive-mounted target facilitates excellent vibration transfer, and the full frequency range of the sensor can be utilized. Another advantage to the adhesive-mounted target is that the machine surface does not need to be drilled and tapped. A flat mounting target with a ¼-28 threaded hole was selected for this function.

Cable selection…
In light of the environment, the cable connecting the accelerometer to the enclosure needed to be robust, chemical resistant, water resistant and reliable in caustic conditions. A Teflon-jacketed cable with molded connector and stainless steel locking ring was chosen.

Signal conditioner selection…
Because of the required inputs into the process controller, a field-configurable signal conditioner with a display that can be easily seen in a variety of lighting conditions was chosen, as each pump that is monitored can have unique vibration levels. The signal conditioner also needed to be able to re-transmit the 4-20mA outputs in order to eventually integrate with another process control system and SCADA. Power for the signal conditioner(s) and the sensors are provided by the internal process controller.

Process controller selection…
The selected process controller allowed for field configuration, incorporated a display that permitted visual identifi- cation of the vibration level and included a power supply for the signal conditioners.

The ability to set up two different alarm levels, as well as a time delay to prevent “nuisance alarms” that might occur if a spike in vibration levels due to a transient event also was determined to be important for this system. The controllers are powered from 120 VAC input into the enclosure, which was provided by the facility.

Enclosure selection…
The selected enclosure allowed for easy wiring into and out of it. This enclosure also has proven to be unaffected in a highly corrosive atmosphere. The process controllers and the signal conditioners were factory-wired. The wiring of the sensors into the enclosure, any re-transmitted signals out of the enclosure and 120 VAC power into the enclosure were done through pre-defined cable entry and exit cord grips/conduit. The wiring was attached at a termination block that was clearly identified for the type of connection required. (See Fig. 1 for an example of the termination identification.)

The easy wiring minimized the time required to install sensor cables and integrate the components of the system into the enclosure, and ensured that the system was completely integrated prior to delivery.

Financial analysis
Justification for the Howard F. Curren Advanced Wastewater Treatment Plant project was determined based on a review of the approximate cost of a pump station motor repair versus the price of a typical two-channel monitoring system. The repair cost for an 800 hp motor could go as high as $175,000. The price of the monitoring system was approximately $2500—or roughly $1500 per measurement point.

The initial approval to outfit one major lift station was decided in 2006, and a unit has been in service since that time. The project justification was further underscored by a subsequent motor failure at another pump station. The estimated cost of that motor repair was close to $160,000—a fact that renewed interest in the relatively low-cost 24 hour protection device.

Approved monitoring setup
The approved system was to be used as a monitor to notify the plant of problems with the pump or motor, especially during off-hour operation. As shown in Fig. 2, this system consists of two permanently mounted sensors, with cable from the sensor wired to the enclosure. Mounted inside are: two process controllers, two signal conditioners, and two transmitters (for the 4-20 mA output process signals). The box also has a window to permit viewing of the process controller displays for overall vibration level readings.

• The signal conditioner was scaled to less than 0.51.0 IPS, with a frequency range between 5 and 50 Hz.
• Two relay outputs were configured based on experience in required alarm settings. The baseline vibration on the machine was observed to be 0.2 IPS, peak. From there, relay/alarm settings were set at 0.35 IPS, peak for the first level, and 0.65 IPS, peak for the second alarm level, with time delays of approximately 30 seconds for each level. If the vibration does not maintain that amplitude (or greater) for that length of time continuously, the relay does not activate. The levels, time delays and relay action (latching, latching with clear, manual reset) can be adjusted on the process controllers.
• The system was mounted at a lift station with a flow capacity of approximately 35 MGD and connected to the main plant SCADA system. Relays are in place to shut down the pump/motor if there is an event that could cause serious damage to the equipment. Sensor location selection The sensor mounting locations were selected based on historical data and accessibility of the measurement location point. In order to monitor the pump and motor, for the direct driven system, a sensor was placed on both pump and motor. Enclosure mounting location selection The cable was routed from the pump and motor to the enclosure, which was mounted on a fixed wall. This is located near the shut-off switch, which was installed to protect the pump and motor equipment. Major benefits of the system can be seen in the following features and capabilities:
• A turn-key system solution
• Easy wiring terminations
• Field-configurable signal conditioners and process controllers
• Allows for re-transmission of the process signal
• Allows for integration into a SCADA system
• Allows for settings to shut down the equipment
• Two relays with independent input levels with latching options
• User-friendly components
• Permits access to “live” data to hard to inaccessible points
• Offers multi-functions vibration and temperature

Results
The installed system has identified possible pump cavitations occurring in the early morning hours during low-flow periods. These types of cavitations can escalate rapidly, putting a pump and motor in danger. For example, another station at this plant that did not have the approved system in place subsequently failed—possibly due to cavitation—requiring repairs to the equipment and costly unscheduled downtime.

Conclusion
The following factors were critical in convincing management that vibration monitoring has benefits to the Predictive Maintenance Program and City of Tampa (COT) and could be considered for expansion into other pump stations:

1. Cost of the equipment is much less than the cost of repair or replacement of pump and motor
2. The system protects critical equipment with relays to trigger alarms or shutdown
3. 4-20mA outputs feed into SCADA system for continuous, online monitoring.
4. Continuous monitoring can identify possible issues that would not have been observed otherwise.
5. Protecting pump and motor systems during increasedflow events can reduce unscheduled maintenance or repair by alerting the plant of issues before they become catastrophic.
6. The system permits easy access of dynamic data for route collection and/or detailed analysis.
7. Required maintenance will be identified more precisely and accurately, thus reducing unscheduled downtime, repair cost and overflow issues.

Tom LaRocque is the engineering manager for Connection Technology Center, Inc., in Victor, NY. A Certified Vibration Analyst: Category III, he holds a B.S. in Engineering from Clarkson University. LaRocque is a member of the Central New York Chapter of the Vibration Institute. Telephone: (585) 924-5900 ext. 817; e-mail: tlarocque@ctconline.com

Gary Kaiser is a senior application engineer for Connection Technology Center, Inc. A Certified Vibration Analyst: Category III, he previously worked for Eastman Kodak for 23 years. While at Kodak, Kaiser spent 9 years in the vibration analysis group. He also is a member of the Central New York Chapter of the Vibration Institute. E-mail: kaiserg@ctconline.com

Joe Spencer is a mechanical specialist with the City of Tampa, fl. A Certified Vibration Analyst, he has 30 years of field maintenance experience.