G2 Gas Technician Module 12 Revised Already Passed

G2 Gas Technician Module 12 Revised Already Passed What does an Aquastat do? Its an automatic operating switch (normally open). Sense water temperature of a boiler and closes when temperature drops. What does a low water cut off do? Automatic safety control (normally closed). Senses water level in the boiler. If level drops the switch opens and shuts off the gas valve. What does a water flow switch do? Automatic safety control (normally open). Senses water flow and switch closes, in series with gas valve. What does a Pressure Control Switch (pressuretrol) do in a steam boiler? Automatic control that senses high and low steam pressure in a boiler. It closes to start burner on low pressure and opens at high temperature to stop the burner. Controls steam pressure in the boiler. What does a high pressure control do in a steam boiler and at what psi does it open? Automatic safety control that is normally closed. It senses high pressure in the steam boiler and opens the switch. This is at 15 psi when it opens. How does a Thermocouple work? Two dissimilar metals that are joined at two junctions. Heating one of the junctions produces a DC voltage. The temp difference between the hot and cold metals determines the quantity of the DC voltage. How much voltage will a thermocouple produce? It produces 25 - 30 millivolts, sufficient to hold a solenoid like a pilot stat energized. Where do you screw in a thermocouple and how tight should it be? The thermocouple is screwed into the pilostat coil, it should be tightened a 1/4 turn past finger tight. What are Thermopiles? Thermopiles are a collection of thermocouples connected in series. How many milivolts can a Thermopile generate? What is unique about this system? It can generate 750 milivolts and it is sufficient to operate a milivolt gas valve system. This system does not require power. Thermocouple milivolts in a open circuit what is a good range? 20-30 milivolts Thermocouple milivolts in a closed circuit what is a good range? 10-15 milivolts What milivolts indicates a good thermopile in an open circuit? 700-800 milivolts What is the purpose of the pilot drop out test? To find the minimum voltage produced by the thermocouple that will still energize the pilot stat. Usually between 2-4 milivolts What is the flame failure response test time for a thermocouple/thermopile residential system. 90 seconds Closed circuit test(thermostat OPEN) on a thermopile system. 600 milivolts Pilot flame installation rules. Must no impinge on main burner or heat exchanger, away from drafts and within the pilot flame about 3/8" to 1/2" What is flame ionization? The process of heating air molecules to allow for separation of electrons allowing a flame to conduct an electrical current. When an AC signal is directed through the flame it will conduct an electrical charge across the flame to the ground. What is flame rectification? When you over size your ground to prevent a flame detector from getting a false positive and thinking there is still flame present due to carbon build up. Closed circuit test(thermostat CLOSED) on a thermopile system. 400-450 milivolts What does a flame rod do? Senses a pulsating dc current that signals flame is present. How should you size a flame rod? Why? Must have 4 times less surface area then the ground. Normal practice is 10 times smaller. This causes the signal to flow easier from the flame rod to ground. Pilot flame rectification- What is flame failure response time? Trial for ignition safety lockout range? Distance for spark gap? Potential pre purge time? Flame failure response 0.8 seconds Total for ignition lockout between 15 & 90s Spark gap 1/8 to 3/16" Pre purge up to 45 seconds Direct spark ignition- Trial for ignition time? Flame failure response time? Trial for ignition 4 seconds Flame failure response 0.8 seconds Types of ignition control modules? intermittent pilot, direct spark ignition and hot surface ignition Sequence of operation of an ignition control module? 1. Safe start check 2. Ignite flame 3. Energize the main gas valve 4. Prove pilot or main gas flame 5. Monitor flame during cycle How does a solenoid valve work? When energized it creates an electromagnet that provides opening and closing for the valve. How does a pilotstat energize? By the thermocouple milivoltage What does a combination gas valve include? Pilotstat, automatic gas valve, main gas and pilot regulators What speed does a diaphragm open? Close? Opens slow and closes fast What type of valve is fast opening and fast closing? Solenoid valve Most ignition control modules flame failure time? 0.8s Combination gas valves have what in line with each other? Pilot gas valve in line with the main gas valve How far should a flame rod project into the flame? 1/2" What causes a noisy lifting flame? High pressure Excessive draft causes what at the pilot location? Wavy blue flame The process of conducting electricity through a flame is? Flame ionization Two causes for induced draft switch failure? Failure of motor and blockage Who can change a low water cut off? Plumbers and steam fitters. Maximum operating pressure of a steam boiler? 15 psi What determines the strength of the magnetic field? The strength of the current Ina purely resistive circuit what happens to the current and voltage? They are in phase with each other In inductance which one is the leader current or voltage? Voltage leads the current In capacitance which one is the leader current or voltage? Current leads voltage Formula for inductive reactance? 2 x π x F x L = inductance reactance F = frequency of ac voltage L = inductance of cool in Henries Formula for capacitive reactance? 1/(2 x π x F x C) = capacitive reactance F = frequency C = capacitance What part of the motor is the stator? Stationary part of an electric motor. Another name for stator? Field winding What causes the rotor to rotate? A stator induces a counter magnetic field, it is in opposition of the opposing fields causing it to rotate. What are the two types of bearings? Sleeve or ball bearing Three causes of bearing failure. Dirty, old or incorrect oil, Over-tightened belts and improper alignment of the drive Motor speed formla. RPM = (Hertz x 120)/# of poles When changing a motor what should you do? Check voltage supply and amp draw while under full load. Where do you use a splash proof motor? Outside in moist conditions Where do you use explosion proof motors? How are they made? Used in explosive atmospheres and are totally enclosed What does the start winding do? Has a higher resistance then run winding causing it to be out of phase with it. This provides more torque on start up. Why must the start winding be removed from the circuit after initial start up? It will over heat and damage the motor. How does the start winding get removed? A centrifugal switch mounted on the motor shaft will open at 85% of rated speed taking the start winding out of circuit. When does the starting relay remove the start capacitor from the circuit? At 85% of rated speed. What is back emf? Counter voltage generated by the motor How can you increase back emf? Greater number of windings across the stator How high can back emf be compared to line voltage? 2 1/2 times When would back emf reach maximum value? When the motor reaches maximum speed. How are the contact switches different on a potential relay vs a current relay? Potential relays are normally closed and current relays are normally open. How are capactiors rated? Capaciity (micro farads) and voltage What material is the run capacitor and the start capacitor made of? Start capacitor is made of plastic and the run capacitor is made of metal. If you need to replace a capacitor what would you do? what else can you do if the best option is unavailable? Replace the capacitor with same capacity and voltage. If that is not an option Run capacitors +/- 10% and start capacitors withing 10-35 micro farads Formula for capcitance in parallel? Ct = C1 + C2 + C3 Safety precautions when handling a capacitor. They can hold an electrical charge for a long period. To prevent injury and equipment damage bleed the charge across the terminals with an appropriately sized resistor.