The City of Tampa's Howard F. Curren Wastewater Treatment Plant uses vibration analysis hardware and process controller equipment to protect critical machinery against damage from mechanical failures or environmental changes, ensure survivability and prevent unscheduled downtime and costs. This system uses relays to trigger alarms or shutdowns and is integrated to the main plant's Supervisory Control and Data Acquisition (SCADA) system. Here is a look at how this system was chosen and installed.
Introduction and Overview
The Howard F. Curren Wastewater Treatment Plant is a state-of-the-art facility that treats all wastewater discharged from approximately 100,000 accounts in the City of Tampa system. The plant has a permitted capacity of 96-mgd, with an average daily flow of 60-mgd. The treated 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.
Inspired by an optimization program, the plant's current maintenance program includes changing monitoring processes and procedures when possible, reducing scheduled versus unscheduled downtime and maintenance and transitioning from a reactive to proactive organization. Because the plant is the City of Tampa's only wastewater treatment facility, it must minimize flow interruptions, unscheduled downtime and overflows.
The use of pumps to transport wastewater from various city locations is critical for maximizing flows and maintaining biological efficiencies by producing a constant flow. When the pumps fail, backup pumps maintain flow. Pump failures can often be damaging to the pumps and auxiliary equipment. Moreover, the cost of a new pump motor can be as high as $450,000, and the cost to repair an existing unit can approach $175,000 after a catastrophic failure.
A protection system that monitors the vibration levels and can be integrated to a shutdown circuit minimizes flow interruptions and the amount of damage to that equipment.
The plant worked with Connection Technology Center, Inc., a vibration analysis hardware and process equipment manufacturer, to investigate different equipment and system options to monitor this application.
The Wastewater Department installed one unit as a trial on a large (700 hp) pump and motor combination at a major pump station. Vibrations were detected and repairs made for less than $500 that saved damage to the expensive pump and motor. Plans are in place to install monitors at all major pump stations over the next 2 years.
Application at City of Tampa
In their investigation, the team first considered the pump stations. Eight major pump stations collect the wastewater and deliver it to the treatment plant. Each major pump station has many smaller stations that feed them-either through pump systems or gravity feed. There are approximately 224 pump stations within this system.
The three typical pump configurations are direct coupled, submersible or vertical shaft. All are variations of centrifugal pumps. The direct coupled stations have the motor and the pump on the same floor, with the motor in an overhung position and supported over the pump. Submersible stations have the pump and electric motor in a common housing. Pumps are installed in wet wells up to 35 feet deep and stay submerged in water. The vertical shaft stations typically have the motor and clutch or VFD-controlled motor two stories above the pump, with some shafts as long as 20 feet with a center support bearing.
Each major lift station has at least three or more motor-pump systems, with one pump typically running at a time to ensure system redundancy. Major failures can cause overflow issues as well as extensive damage or complete failure with auxiliary equipment such as valve, VFD and wiring.
In determining where to install a protective system, the team identified the major stations and their critical equipment.
Vibration Considerations
Each pump type presented a unique vibration challenge. General vibration considerations applicable to the direct-coupled and submersible pump systems include cavitation, mechanical failures and misalignment, which can be periodically monitored. Cavitation will often accelerate mechanical failures like discharge valve or bearing failures and impeller wear. Mechanical faults are also accelerated due to increased flow.
A unique vibration consideration for the vertical shaft system is the difficulty of aligning the vertical shaft to the pump. The system requires coupling shafts up to 20-ft in length, and accessibility is often difficult.
These remote pump stations are not manned, which complicates the monitoring and vibration considerations; periodic monitoring may not be sufficient to capture any transient type of faults that could lead to failures.
Process/Protection Considerations
Periodic monitoring might be sufficient to identify general, long-term machinery conditions, but to capture transient conditions that can cause catastrophic failures, the team determined that continual monitoring was required. Given the unmanned pump stations, an integrated system could alert a technician at the plant of an issue with the pump station equipment.
In this situation, the team concluded that the vibration system should be integrated with the plant SCADA system. The output parameters of the vibration system, 4-20mA output proportional to the overall vibration levels of the equipment, could feed into the SCADA system and allow the technician to observe equipment status. Ideally, many issues could be identified quickly before catastrophic failure. Unfortunately, this integration was difficult based on the available resources of both the SCADA system and the plant personnel.
Another solution that could be implemented alone or integrated with the SCADA system was a local relay or shutdown system tied into the motor control circuit to shut down the pump system to prevent equipment damage. This system could limit/prevent extensive damage to the pump auxiliary equipment, as well as minimize interruptions of the flow to the plant.
Comparing the cost of repairing a pump station with an 800-hp motor ($175,000) with the price of a typical monitoring system (a two channel system is approximately $2,500, or approximately $1,500 per measurement point) justified the project. The initial approval to outfit one major lift station was decided in 2006, and a unit has been in service since then. Another pump motor failure costing an estimated $160,000 further justified the project and renewed interest in a relatively low cost 24-hour protection device.
Equipment and System Selection Considerations
For the initial unit, the team specified a system of low cost accelerometers mounted to targets connected to a remotely mounted process controller enclosure, with integration to the main plant SCADA system.
The equipment for the system was selected as follows:
Accelerometer Selection
To select the proper accelerometer for the monitoring of components, the following vibration frequency criteria were considered:
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 to 15,000 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 used the full frequency span of the accelerometer needed to be considered. A mounting target attached to the prepared machine surface with an adhesive was selected. The adhesive-mounted target facilitated excellent vibration transfer, and the full frequency range of the sensor could be utilized. Another advantage to the adhesive mounted target is the machine surface did not have to be drilled and tapped. A flat mounting target with a ¼-28 threaded hole was selected for this function.
Cable Selection
Due to the environment, the cable connecting the accelerometer to the enclosure needed to be robust, chemical resistant, water resistant and reliable in a caustic environment. A Teflon jacketed cable with molded connector and stainless steel locking ring was chosen.
Signal Conditioner Selection
Due to the required inputs into the process controller, a field configurable signal conditioner with an easy-to-read display was chosen. The field-configurable signal conditioner also needed to retransmit the 4-20mA outputs to integrate with another process control system and SCADA. Power for the signal conditioners and the sensors was provided by the internal process controller.
Process Controller Selection
The selected process controller was field configurable, had a display that allowed visual identification of the vibration level and included a power supply for the signal conditioners. The controller needed the ability to set up two different alarm levels and a time delay to prevent "nuisance alarms" that might occur if a transient event caused a spike in vibration levels. The controllers were powered from 120 VAC input into the enclosure, which was provided by the facility.
Enclosure Selection
The enclosure selected allowed for easy wiring into and out of the enclosure. The enclosure has also proven to be been unaffected by the 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 performed through predefined cable entry and exit cord grips/conduit. This was attached at a termination block that was clearly identified for the required connections.
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.
Approved Monitoring Setup
The chosen system (see Figure 1 below) 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 allow viewing of the process controller displays for overall vibration level readings.
Figure 1. Approved Protection System
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 did not continuously maintain that amplitude (or greater) for that time, then the relay did not activate. The levels, time delays and relay action (latching, latching with clear, manual reset) can be adjusted on the process controllers.
This system is mounted at a lift station with a flow capacity of approximately 35-mgd, and is connected to the main plant SCADA system for monitoring. Relays are in place to shut down the pump/motor in the event of possibly serious equipment damage. The system can notify the plant of problems with the pump or motor, especially during off hour operation.
Sensor Location Selection
The sensor mounting locations were selected based on historical data and accessibility of the measurement location point. To monitor the pump and motor in the direct driven system, a sensor was placed on both the pump and motor.
Enclosure Mounting Location Selection
The cable was routed from the pump and motor to the enclosure mounted on a fixed wall and located near the shut off switch, which was installed to protect the pump and motor equipment.
Results
The installed system has identified possible pump cavitations occurring in the early morning hours during low flow periods. These cavitations can escalate rapidly, putting the pump and motor in danger of damage.
The following are some key benefits to this system:
- Turnkey system solution
- Easy wiring terminations
- Field configurable signal conditioners and process controllers
- Possible retransmission of the process signal
- Integrated into a SCADA system
- Can be set to shut down the equipment
- Two relays with independent input levels with latching options
- User-friendly components
- Access to "live" data in inaccessible points
- Offers multifunction vibration and temperature
Conclusion
The following factors were critical in convincing management of the benefits of vibration monitoring to the predictive maintenance program and the need to expand the program to other pump stations:
- Cost of the equipment is much less than the cost of repair or replacement of pump and motor
- Protects critical equipment with relays to trigger alarms or shutdown
- 4-20mA outputs feed into SCADA system for continuous, online monitoring
- Continuous monitoring can identify possible issues that would not have been observed otherwise
- Protecting the pump and motor systems during increased flow events can reduce unscheduled maintenance or repair required by alerting the plant of any issues before they become catastrophic
- Easy access of dynamic data for route collection and/or detailed analysis
- Maintenance required will be identified more precisely and accurately to reduce unscheduled downtime, repair cost and overflow issues
Pumps & Systems, August 2008