Hybrid Electrical Section 5.doc

SECTION V: ELECTRICAL SYSTEMS

5.1

GENERAL

The electrical system for the hybrid cooling tower consists of the raceway, conductors and enclosures for the power, control and measurement systems. These are subdivided into Lighting & Receptacles, Fan Circuits, Roller Doors Circuits, Motor Operated Valves and Heat Tracing.

ALWAYS USE PROPER SAFETY PROCEDURES!

ALWAYS TAG OUT CIRCUITS BEFORE PERFORMING MAINTENANCE!

ALWAYS MEASURE BEFORE TOUCHING CONDUCTORS!

ALWAYS USE ONE HAND!

5.2. LIGHTING & RECEPTACLES

5.2.1 Lighting - The lighting consists of 6 metal halide lights mounted around the tower illuminating equipment areas, walk ways and ladders. The stairtowers are illuminated as well.

Around the perimeter of the tower the luminaries are mounted to the handrails with U Bolts. Replacement of the lamps, requires that the luminary be opened the lamp removed and the new lamp installed. Changing the ballast is more complex requiring that the casing be opened, the wiring be removed and the ballast unscrewed.

Lights for the tower are controlled by switching breakers supplied by the owner in the electrical area of the chemical shack.

5.2.2 Receptacles - Near each electrical motor there is a 15 amp GFI duplex receptacle for small tools or potable lighting.

5.3 Fans

For each of the three fans the owner has provided a Two (2) speed motor starter. Each motor starter has 2 separate sets of contactors, one for high speed (1800rpm) and one for low speed (1200rpm). The starters have individual interlocks for fan vibration, and oil level as well as emergency stop switches and fire protection system interlock. Control of the motor starters is from the reactor plant console and is by others.

In addition to the 2 sets of 480vac 3 phase to power the motors, each has protective circuits for excess vibration, and low gear box oil level. Each is also supplied with a locking emergency stop switch which prevents the fan from running.

5.3.1 Fan Motor - The fans are driven by 75hp 2 speed electrical motors thru gear reducers. The motors are powered by 480vac 3 phase electricity. Six (6) wires feed each motor. Only one set of 3 wires is energized at a time, one for high speed one for low speed. The owners supplied motor starters provide the electrical energy and are located in the chemical shack. When the motor is not energized auxiliary contacts on the motor starter provide 120v to the motor space heater preventing condensation from accumulating in turn preventing rust from forming on machined surfaces.

5.3.2 Space Heater - Each motor has a 120vac space heater to prevent condensation of water during times when the motor is cold. Since the motor is warm during operation the heaters must be activated only when the motor is off. As such the motor starter provides 120vac 120w to the motor heater when the motor is de-energized. This heater is integral to the motor.

5.3.3 Oil Sump Heater with Thermostat – Each gearbox is equipped with a Kim HotStart Oil heater and thermostat. If either the oil heater or thermostat fails they may be replaced.

When oil is replaced re-energize the oil sump heater circuits with proper safety tag out procedure!

5.3.3 Protective Interlocks

5.3.3.1 Gear Box - Each fan is coupled thru a reducing gear thru the drive shaft to the electric motor. A vibration switch is mounted on the gear reducer. The level of the oil is monitored with a level switch. Sump heaters maintain the oils viscosity in cold weather.

5.3.3.2 Vibration Switch – Each gear box has a Robert Shaw Vibration Switch with 2 pole output, and a remote reset. When the fan motor is started, 120vac is provided to the switch to hold the switch in a closed state during the normally high vibration starts. After 10 seconds the Robert Shaw times out and is subject to vibration tripping.

When high vibrations occur the vibration switch opens, opening the motor interlock circuit, de-energizing the motor starters. The vibration switch can be manually reset at the switch by pressing a button or remotely at the Emergency Stop Control Box near the motors.

5.3.3.3 Oil Level Switch - The Oil Level Switch manufactured by Murphy and consists of a single pole float switch within each gear box. Should the oil leak out or be deplenished the float switch will open and the motor starter will be de-energized preventing damage to the gear box and possibly to the fan blades, drive shaft and personnel. The level of the oil switch is not adjustable nor “resetable” without replenishing the oil.

5.3.3.4 E-Stop - The Emergency Stop Switch is a manual switch located next to each motor allowing maintenance personnel to insure the motor will not be energized remotely. The switch opens the same circuit as the vibration switch to perform this task. It is “lock”able providing the function of local disconnect.

5.3.3.5 Fire Protection – The owner has pending a Fire Protection System Interlock consists of a 4 pole relay within an enclosure in the electrical area of the chemical shack. The relay is powered by the local 120vac panel thru normally closed contacts in the fire protection system dry pipe. The dry pipe opens the switch should pressure drop within the dry pipe. The same drop in pressure opens the dry pipe causing water to flow into the fire protection system putting out any fire.

When the switch opens the relay is de-energized and 3 of the contacts open, opening the interlock circuit from the Estop/Vibration Switch to the motor starter, causing the motor starter to de-energize.

5.4 Roller Doors

The roller doors modulate the flow of air thru the tower causing the effluent air (plume) to be dry heating before leaving the tower thru the fan stacks. When high efficiency is needed the dry doors (upper level) are closed forcing air thru the wet doors (lower level). When more air is drawn thru the wet section, the more effective evaporative cooling takes place and the tower is more efficient.

During periods of cold ambient temperatures, the towers efficiency is not critical but the size and density of the plume is at a maximum. To reduce the size and density of the plume, air is redirected thru the dry heat exchangers at the top of the tower by opening the dry doors.

5.4.1 Motors - The roller door motors are ¾” hp 460vac 3 phase reversible motors with integral thermal overload protection thermostats. The thermostats are in line with the roller door control logic and prevent operation if the motors are overloaded.

5.4.2 Manual drive - Each roller door has a manual chain drive with electrical release to disable the motors.

5.4.3 Door Edge Switch -Each roller door has a door edge switch which can be used to prevent the door from closing in the event something interferes with the door. A bypass switch located on the motor controller allows operation of the door if the door edge switch fails due to water shorting the contacts. This is a 24v circuit.

5.4.4 Open/Close Limit Switch - Each roller door has a pair of limit switches located in the worm drive section. A nut rides along the worm gear and hits the limit switch at one end when the door is open and at the other end when the door is closed. The limit switches are adjusted to set the 100% open and 100% closed position of the door. These are 120v circuits

5.4.5 Position Sensing – It is only critical to control the position of the wet doors. Dry doors are either fully open or fully closed. The position of the wet east doors is visible using the site security cameras, but the position of the wet west doors is determined by using string pots mounted on the top of the door. The string is actually a stainless steel cable wound around a spool which is connected to a variable resistor thru a worm drive within the string pot. As string is pulled off the spool the variable resistor output changes controlling a 4-20ma transmitter changing its output from 4 to 20 ma over the entire position of the roll up doors. Owner provided 24VDC powers the string pots.

The string pot is manufactured by Unimeasure and is part # HX-P420-100*E3*L3MR.

5.4.6 Motor Starters - The roller doors are driven by CTS Engineering fabricated motor starters. The motors starters have 480v/120v transformers to derive the 120v control signals and 120/24v transformer with rectifiers for 24vdc safety edge circuit. As the roller door is reversing two motor contractors (Automation Direct Part # GHBN-3-01A) within the motor starters are used to provide forward/reverse connections. The two motor contactors are mechanically and electrically interlocked (Automation Direct Part # BM0H) to prevent both from closing at the same time.

5.4.6.1 Fusing - The 480v/120v control transformer (McMaster-Carr Part # 7962K54) is fused with 2 type CCMR ½ Amp fuses. This style fuse is required as the fuse holders are rejection type. The output of the 480/120 transformer is fused with a single type CC BAF-2 fuse.

The 120v/24v control transformer is fused with a ¼” 2 ½ amp fuse. The fuse holder is used as a disconnect for the roller door. Simply pull out the top of the fuse holder handle to de energize the motor starters, the fuse holder handle should be tagged out making the motor safe. Note that all voltage at the motor - 480vac and 24vdc is removed with fuse holder for F4 pulled out and tagged. Also note that no indication of the condition is given at the console and continued operation and indication of the opposite roller door at the reactor plant console is possible.

5.4.6.2 Local Control - Individual Local control of the roller doors is possible by using the manual control stations near each roller door motor. The controller has two controls OPEN and CLOSE, the buttons must be pressed for the doors to move. When completely open or closed the motor will be de-energized and the doors will stop moving. No indication is given at the local control station.

5.4.6.3 Remote Control – By owner the roller doors are controlled in tandem with the indicating lights showing the state of both doors. If both doors in the set are closed the light will indicate closed, if only one door is closed the light will not indicate closed. The console has a T handle return to center switch. The switch has separate contacts for each side which keeps the control circuits independent.

5.5 Motor Operated Valves

There are 2 motor operated valves on the cooling tower. The first is a bypass valve which dumps water directly into the basin bypassing the heat exchanger. The second is a start up valve which bypasses the dry section of the tower dumping the water into cell 3 of the tower. Each valve can be controlled either locally or remotely from the control room.

5.6 Heat Tracing & De-Icing

5.6.1 Breather Valve Heat tracing - Located at the top of each tube bundle is a vent and a manual breather Valve. When filling the tube bundles the vent (acting as a float valve) permits air to escape. When water reaches the vent the float closes the valve. Shortly thereafter the water drains down thru the tube bundles and a vacuum builds at the top of the bundle.

During shutdown the tube bundles must be drained. In order to drain the tube bundles the vacuum must be released. The manual breather valve performs this function.

In order to assure that the manual breather valve and vent are not damaged and are operational they must be protected from freezing. The heat tracing performs this function by keeping the piping above freezing temperatures. The heat tracing is self regulating and consists of 9 feet of Nelson CLT5-CB. Each side of the tower has it’s own circuit for heat tracing. The heat tracing begins operation when the temperature drops below 40(F.

5.6.2 Roller Door Rail Heaters – Located along the wet door rails at the bottom of both sides are silicon rubber covered heaters. The heater is a Chromalox Part # SL-N-3-15-0-10-S-120V-225W connected in series with a thermostat also Chromalox part # T-N-40P-KIT. To insure the heaters make good contact with the rails and to protect the heaters from mechanical damage they are covered with anodized aluminum L which is screwed into the channels. A terminal box is mounted on the lower end of the L. The Rail Heaters share the same circuits with the breather tube heat tracing.

5.6.3 De-Icing System – To prevent extensive condensation on the doors and ice during extreme freezing conditions the drain from the heat exchanger is routed to a horizontal header mounted behind each wet door. The header has 20 adjustable nozzles aimed to spray a curtain of hot water onto the back side of the doors during freezing conditions. At each wet rolling door a 120v solenoid valve allows the flow from the heat exchanger drain when the temperature drops below the set point of a thermostat mounted on the south wall near the corners of the tower. The thermostat set point is adjustable from below freezing to 90(F. It is recommended that the thermostat be left at 40(F.

Besides the PVC piping, the parts for the de-icing system are available from McMaster-Carr. The nozzles part # 32925K77, the solenoid valve is part # 4738K176, the thermostat is 1846K26.