by Tom LaRocque, Gary Kaiser, & Joe Spencer
August 8, 2008

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.