Monitoring Vibration on Tunnel FansSafety FirstThe worlds developing infrastructure createsmany technical challenges for engineering andconstruction. Quite often names like “Chunnel”(English Channel), “Big Dig” (Boston), and“Nam Wan” (Hong Kong) are feats to marvel atas they provide passages underground,underwater, or straight through a mountain fortoday’s highways and rapid transit systems.

Commuter Train Exiting Underground TunnelAlthough the tunnels provide a very unique formof infrastructure, they also come with their ownset of environmental problems. It is only naturalin today’s world that the tunnels will becomepolluted with COx from gasoline based engines,and NOx and smoke from diesel based engines.These toxic gas emissions need to be clearedfrom the tunnel and replaced with fresh air. Thisventilation process keeps those of us that use thetunnels healthy, and in the case of an emergency,safe.

VentilationPurging of toxic gases with a supply of clean airrequires Tunnel Jet Fans. These fans providelongitudinal ventilation of the tunnel and are theprimary sources of clean air. Jet fans provide avery high impulse or thrust of clean air into thetunnel and through the tunnel. They are oftenfound in multiple sets mounted on the roof orside wall of the tunnel at selected intervals. Thenumber of fans required is based on tunnellength and design.

A Pair of Tunnel Jet FansAxial flow fans are also used to vertically clearsmoke from the tunnel in case of a fire. Thesevery large axial flow fans are mounted in verticalshafts, and act as forced air chimneys duringemergencies. Often a series of dampers is alsoplaced in the roof of the tunnel to allow the

longitudinal jet fans to assist air flow to the axial flow ventilation fans. This combination quickly clears the tunnel of smoke and allows emergency crews to take the necessary actions.Process MonitoringThe critical nature of these fans to provideenvironmentally safe air requires 100%performance 24 hours/day. Monitoring thevibration of these fans is one of the requirements to make sure that 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.

Typical 4-20 mA Loop Power CircuitUtilizing such a common industrial circuit designmeans that the fans can be monitored by thetunnels “in house” PLC or DCS control system.This makes it easy for the operators to trend andtrack fan vibration during normal day – dayactivities.

The vibration monitoring of the fan can beaccomplished in two different manners.

1. General purpose accelerometers, like theAC102 Series, can be placed on the fan,and connected to a transmitter, like theVX Series, that filters and converts thevibration signals to 4-20 mA. Thiscombination will provide process controland allow for dynamic vibration analysis.

AC102 Series Sensors with Transmitters

2. Dual output loop power sensors, like theAC400 Series, could be installed on thefans. This dual output sensor filters andconverts the vibration signals to 4-20 mAto provide process control and alsoallows for dynamic vibration analysis.

Sensor MountingThe number of sensors to mount on each fan willbe determined by the design of the fan and theextent of monitoring required. A securemounting method is required, and the timeshould be taken to properly locate and mount thesensors. Several options are available forpermanent mounting.1. Stud mounting with spot face2. Mounting pad with adhesive3. Zerk fitting adaptor4. Fin mount

Horizontal Vibration Sensor Mounted onTunnel Fan Motor

CablesCable management is also a concern with tunnelfans. Quite often there is a significant amount ofair flow and long distances involved from thesensors to the transmitters, or directly to thecontrol room in the case of dual output sensors.Care must be taken and plans put in place toassure that the cables are run in a manner thatprotects them. Access holes and conduit pipesmay be required to protect the cables.

Completed Conduit ConnectionJunction Boxes

Sensor outputs will be collected by junctionboxes, and then processed by the PLC or DCS.If you are using the LP400 series sensor, theSB142 or SB242 series junction boxes willmange the dual outputs.

Drilling Access Hole for ConduitGood organization of the wiring will providerobust and understandable wire runs. Each wireshould be labeled, and matched to thecorresponding sensor.

If an alarm occurs, the vibration analyst canmeasure the dynamic vibration and analyze thefrequencies to determine root cause.

SummaryMonitoring the vibration on tunnel fans is critical to maintain fresh air for health and safety providing reliable sources of infrastructure.