A Light in the Dark
The monroe County Department of Environmental Services (MCDES) in New York is responsible for solid waste policy management, fleet services, engineering/facilities management and wastewater management. The biggest component of the department’s budget and overall responsibility is wastewater management, known as the Pure Waters program.
Treatment plants and pump stations
The county’s VanLare and Northwest Quadrant wastewater treatment plants are located on the south shore of Lake Ontario. Both plants use state-of-the-art centrifuges and biosolid offload facilities to process biosolids for delivery to landfills. These biosolids are then mixed with garbage to enhance methane gas production, which is collected and used to provide fuel for a landfill gas power plant where electricity can be sold back to the grid.
The VanLare facility is supplied by a gravity feed siphon from a deep tunnel storage system and the Cross Irondequoit Pump Station. A recent upgrade to this pump station provided four new pumps and controls for the entire station. Energy monitoring was included to ensure compliance with peak demand electrical costs and vibration protection on all associated pumps and motors in three axes on each piece of equipment. In the past, only one axis was monitored and some problems went undetected if the pump wasn’t vibrating in that direction. Now, if the problem occurs in the vertical, horizontal or axial plane, the vibration sensors will display a problem and shut down the pump if a limit is achieved.
There are three large pump stations that serve the Northwest Quadrant facility. When these pump stations first went online, the mechanical health of the machine was determined by a route-based vibration data collector and trending. Readings were taken every three months to look for any problems.
Unfortunately, anything out of the ordinary would be missed unless someone was standing in the station when the incident occurred. A pump would be reported as shaking, only to find the problem mysteriously gone when later investigated. Problems continued to occur with high pump vibration, and determining the source was difficult because the events were inconsistent. Therefore, IMI 640B01 4-20 mA sensors were installed in all three pump stations. These two-wire loop powered sensors offered continuous monitoring of the overall vibration levels. The data was sent through the SCADA system, which consists of hundreds of miles of fiber-optic cables and approximately 40 microwave radios. The current output from the sensors allowed for simple interaction with the SCADA system.
In the early 1990s, the MCDES decided to shift the communication focus away from telemetry and toward newer technologies for added reliability. The critical downtime suffered on lease lines was becoming too costly in the form of communication failures that resulted in personnel being dispatched to investigate. Waiting for repair through another agency was becoming all too common. Speed was also an issue. More data can be transferred faster through fiber optics and radio. This allowed the MCDES to identify key process upsets as they happened and display the conditions of the station in real time.
The control room at the VanLare facility is a communication center that is the hub of the MCDES’s maintenance operation. It has four 8- by 8-ft projection screens for displaying weather radar, a deep storage tunnel system, and a wide array of varying size pump stations and security cameras. All maintenance activities for the sewer collection system and plant operations are coordinated through dispatchers that monitor the alarms from these pump stations and tunnel sites, as well as the fire and security systems for Monroe County facilities. The data provided instant results.
“It was like turning on a light bulb in a dark closet,” said one worker. The MCDES began to see a pattern developing at all three pump stations. Because variable frequency drives were used, at certain speeds the vibration would go off the scale and then return to normal as the pump changed to a different speed. Impeller build-up was usually suspected, and the pump was back-flushed or pulled, only to find no debris in the pump. With a bump test and run-up and coast-down test, the results showed a case of resonance. As with most resonance issues, the problem was to stiffen or not to stiffen. The MCDES wanted to avoid arbitrarily placing braces anywhere because of the possibility of shifting the problem to a different area or exciting another natural frequency.
An operational deflection shape (ODS) analysis was done to determine why these pumps were shaking. They were all of similar design and base construction, so solving the problem on one meant solving all of the resonance issues with the pumps. The ODS and a finite element analysis are now being used to construct a model to properly support and stiffen the structure to move any unwanted natural frequencies away from the operating range of the pumps. This would not have been as easy to find had the MCDES not installed the on-line sensors and trended them versus the pump RPM.
In the now
The MCDES now uses the 4-20 mA sensors as a first line of defense against problems that can arise during start-up or acceptance testing of new or rebuilt pump stations. Because they offer a current output, the sensors can be easily incorporated into existing systems without the burden of sophisticated data analysis procedures and equipment. These sensors are now standard in all new upgrades and installations of critical equipment. A vibration limit is also incorporated into all of the pump specifications and critical machines in the treatment plants and collection system. Reliability has become a major focus of the maintenance department and using these on-line sensors has helped to achieve that goal.