Monitoring Vibration on Tunnel Fans
Safety First
The world's developing infrastructure creates many technical challenges for engineering and construction. Quite often names like "Chunnel" (English Channel), "Big Dig" (Boston), and "Nam Wan" (Hong Kong) are feats to marvel at as they provide passages underground, underwater, or straight through a mountain for today's highways and rapid transit systems.
Although tunnels provide a unique form of infrastructure, they also come with their own set of environmental problems. It is only natural in today's world that the tunnel will become polluted with COx from gasoline-based engines, and NOx and smoke from diesel-based engines. These toxic gas emissions need to be cleared from the tunnel and replaced with fresh air. This ventilation process keeps those of us who use the tunnels healthy, and in case of emergency, safe.
Ventilation
Purging of toxic gases with a supply of clean air requires tunnel jet fans. These fans provide longitudinal ventilation of the tunnel and are the primary source of clean air. Jet fans provide a very high impulse or thrust of clean air into and through the tunnel. They are often found in multiple sets mounted on the roof or side wall of the tunnel at selected intervals. The number of fans required is based on the tunnel's length and design.
Axial flow fans are also used to vertically clear smoke from the tunnel in case of a fire. These very large axial flow fans are mounted in vertical shafts and act as forced air chimneys during emergencies. Often, a series of dampers are also placed in the roof of the tunnel to allow longitudinal jet fans to assist airflow to the axial flow ventilation fans. This combination quickly clears the tunnel of smoke and allows emergency crews to take the necessary actions.
Process Monitoring
The critical nature of these fans to provide environmentally safe air requires 100% performance, 24 hours a day. Monitoring the vibration of these fans is one of the requirements to make sure they are running smoothly and will not unexpectedly fail. Trending and alarming the vibration in the tunnel control center utilizes 4-20 mA current loops from sensors and transmitters.
Utilizing such a common industrial circuit design means the fans can be monitored by the tunnel's "in-house" PLC or DCS control system. This makes it easy for the operators to trend and track fan vibration during normal day-to-day activities.
The vibration monitoring of the fan can be accomplished using general-purpose accelerometers, like the AC102 Series Accelerometer. The accelerometer can be placed on the fan and connected to a signal conditioner, like our SC Series Signal Conditioners, that filters and converts the vibration signals to 4-20 mA. This combination will provide process control and allow for dynamic vibration analysis.
Sensor Mounting
The number of sensors mounted on each fan is determined by the fan's design and the extent of required monitoring. A secure mounting method is required, and the time should be taken to properly locate and mount the sensors. Several options are available for permanent mounting:
- Stud mounting with spot face
- Adhesive mounting pad
- Zerk fitting adapter
- Motor fin mount
A CTC accelerometer is mounted horizontally on a motor using a motor fin mount pad
Cables
Cable management is also a concern with tunnel fans. Quite often there is no significant amount of airflow and long distances involved from the sensors to the transmitters, or directly to the control room in the case of dual output sensors. Care must be taken and plans put in place to ensure the cables are run in a manner that protects them. Access holes and conduit pipes may be required to protect the cables.
Drilling access hole for conduit
Good organization of the wiring will provide robust and understandable wire runs. Each wire should be labeled and matched to the corresponding sensor.
A completed conduit connection
Enclosures
Sensor outputs will be terminated at an SCE Signal Conditioner Enclosure, which should be mounted in a safe and easy-to-access location. All CTC Signal Conditioners feature a buffered BNC on the front which allows the analyst to access dynamic data from the connected sensor. Output wiring from the SCE enclosure will then be run back to the PLC or DCS.
SCE410 Series Stainless Steel Signal Conditioner Enclosure, for 1 to 8 CTC Signal Conditioners, NEMA 4X (IP66) rated
Summary
Monitoring the vibration of tunnel fans is critical to maintaining fresh air for health, safety, and providing reliable sources of infrastructure.
