What are the 3 primary contaminants in raw water?
- Dissolved solids
- Suspended solids
- Dissolved gases
What are the 2 sources of water used in power plants?
Surface water and ground water.
What are the 2 forms of water treatment used in power plants?
External and internal treatment
Why do we treat boiler feed water?
- Prevent sludge from depositing on boiler surfaces
- prevent scale from forming on boiler surfaces
- prevent corrosion of boiler metal
- Prevent carryover with the steam leaving the boiler
What are Cations? What are Anions?
- A Cation are positively charged ions
- A Anion are negatively charged ions
How are suspended solids removed from raw water?
By coagulation or by filtration.
How are minerals removed from raw water?
With the use of water softeners, demineralizers, and distillation.
How are dissolved gases removed from make-up water/feed water?
By deareation or degassification.
A measure of the water's acidity or alkalinity.
Is 7.0 on the PH scale acidic or alkaline?
The scale reads 0 to 14. (7 is neutral.)
How are chemicals added to the boiler?
Thru the DA, chemical pumps, injectors, or by slug feeders.
What is meant by the term "cycles of concentration"?
The ratio of impurities in the boiler water divided by the amount impurities in make up water.
What PH level do we try to maintain in boilers?
Between 10.0 and 11.0
What is foaming? What is priming?
- Foaming is the existence of a layer of foam on the surface of the water in the boiler drum. (caused by excessive alkalinity).
- Priming is the carry over of slugs of water that are suddenly discharged from the boiler with the steam.
What is carry-over?
The continual discharge of inpurities and water with the steam.
Name some conditions is a boiler that could cause carryover.
Excessive alkalinity which causes foaming, high water level, and the failure of mechanical separators in the boiler.
What chemicals are used to adjust boiler PH?
- Sodium hydroxide (caustic soda)
How do you blow down a boiler?
- Find true water level, 1/2 glass or better
- Open cooling water to blow down tank
- Open quick opening valve (inside valve)
- Open slow opening valve (outside valve) fully and close
- Close quick opening valve
- Check water level and close cooling water valve
What causes condensate return line corrosion?
Dissolved gases (O2 and CO2)
Explain the terms "dry lay-up" and "wet lay-up"
- Using the dry method the boiler would be drained cleaned and isolated from water and steam. Desicants would be placed in the boiler to prevent moisture. The dry method is used for long term shut down.
- Using the wet method the boiler would be drained cleaned and filled with warm treated water at a higher level than normal. Pressureised slightly above atmos. and isolated from steam and water lines.
- The wet method is used when the boiler may be needed on short notice.
Name 3 classidications of pumps.
What is the purpose of a feedwater regulator?
To control the exact amount of water returning to the boiler.
What are the 3 types of feedwater regulators?
- Single Element (senses drum level)
- Double Element (senses drum level and steam flow)
- Triple Element (senses drum level, steam flow, and feedwater flow)
What does a safety valve do?
A safety valve is designed to relieve pressure in a boiler that has exceeded what the valve was set for.
What is gauge pressure? What is absolute pressure?
- Gauge presssure is the pressure which does not include ambiant pressure.
- Absolute pressure is gauge pressure plus ambiant pressure.
What is radiation, conduction, and convection?
The three ways heat can be transfered.
which safety valve will lift first, drum or superheater? Why?
The superheater safety valve is the lowest set valve, therefor it will lift first to allow steam flow to prevent over heating of the superheater.
What are the controlling factors of a single, double, and triple element feedwater regulators?
- Single (drum water level)
- Double (drum water and steam flow)
- Triple (drum water level, steam flow, and feedwater flow)
How often must refrigeration equipment be inspected?
At least once a year.
What is a ton of refrigeration?
The melting of one ton of ice in 24 hrs.
Name at least five safety devices on a boiler.
- Safety relieve valve
- Non-return valve
- Check valve
- Low water cut-out sensor
- Fire sensor
What is Tertiary air?
- Air that is supplied to the combustion process that supports both primary and secondary air.
- Often introduced into the combustion process in a different location.
Why do we purge a boiler?
To remove explosive gases before lighting the fire.
How many btu's are required to change water at 212F to steam at 212F?
970.3 btu's per pound.
What is air made up of? By volume and weight?
- Volume: oxygen= 20.91% nitrogen=79.09%
- Weight: oxygen= 23.15% nitrogen=76.85%
Why is sodium sulfite used in some boilers?
It is used as an oxygen scavenger.
What are the two major categories of pumps?
Positive displacement and non-positive displacement.
Name 4 general classification pumps?
Reciprocating, rotary, centrifugal, and jet pumps.
Explain the operation of a feedwater injector?
High pressure stream of fluid is directed through a nozzle designed to produce high velocity. The high velocity fluid creates a low pressure area in the mixing chamber causing the suction fluid to flow into the mixing chamber.
What is the maximum feedwater temperature when using injectors?
130 to 150F
What are the operating mediums for duplex pumps?
Steam, compressed air and water.
How do you set the valves on a duplex pump?
Close steam supply, exhaust valves and drain pump. Lock out tag out pump. Then center the pistons, plumb the rocker arms, square the valves over the ports, equalize the lost motion and move one piston off center.
What is meant by the term "lost motion"?
- Lost motion is play in the connection of the valve rod to the steam valves in the steam chest.
- Lost motion causes a pause at the end of each stroke to allow the water valves to close tightly.
What is the effect of too much lost motion? Not enough lost motion?
- Too much lost motion will cause the piston to strike the cylinder head.
- Too little will cause the piston to short stroke.
What is a cushioned valve?
A cushioned valve prevents the steam piston from striking the cylinder head by trapping steam in the cylinder to cushion the piston.
What is the area of a piston 6" in diameter?
Area = d2 x .7854 6x6x.7854=28.27 in2
Does a duplex pump displace an equal amount of water on the forward and reverse stroke? Why?
Yes. Because the net effect of the piston rod on the displacement is equal on both sides of a duplex pump.
What type of pump is a metering pump? What type of pump is a vacuum pump?
Both are positive displacement pumps.
What type of service would you expect to see a rotary and gear pump used for?
Both are typically used to pump heavy fuel oils.
Cavitation is the sudden formation and collapse of low pressure bubbles in water by mechanical forces.
What would you do if your pump began to cavitate?
Parcially closing the discharge valve may be done as a temporary fix while you further investigate the cause.
What is the best way to avoid pump cavitation?
Provide correct net positive suction head (NPSH) to the pump.
Explain the operation of a centrifugal pump?
As the impeller rotates water enters the center and is thrown outward by centrifugal force. The water leaves the impeller at high velocity and is converted to pressure in the volute section of the pump.
Name 3 advantages and disadvantages of centrifugal pumps?
- Advantages: inexpensive, few moving parts, low vibration
- Disadvantages: alignment is critical, inefficient at some loads, cavitation
Define head presssure?
The pressure exerted by the height of a given fluid, in feet of head, or psi
Define (NPSH) net positive suction head?
The minimum suction conditions necessary to prevent cavitation in a pump.
Define velocity head?
The kinetic energy per pound due to the vertical distance traveled by fluid.
What do the mumbers 8 x 6 x 12 refer to in a duplex pump.
- First number: diameter of steam piston in inches
- Second number: diameter of fluid piston in inches
- Third number: length of the stroke in inches
The pressure gauge reads 75 psi and the safety valve that is set to blow at 150 psi is blowing. What should you do?
Confirm that the pressure gauge is correct by attaching another gauge.
As the pressure increases, what happens to the latent heat of vaporization?
The latent heat of vaporization increases.
How much pressure will a column of water 10 feet high exert at it's base?
What do the following numbers represent:
- 970.3 BTU's is the latent heat of evaporation for one pound of water.
- 2.31 is the vertical height in feet of one pound of water.
- .491 is the pressure of 1 inch of mercury.
- 14.7 psia is atmospheric pressure.
What is specific gravity?
What is specific heat?
Specific gravity is the weight of any substance when compared to the weight of water.
Specific heat is the amount of heat required to raise the temperature of one pound of any substance one degree F.
What is an open feedwater heater?
What is a closed feedwater heater?
An open feedwater heater or (D.A.) is a heat exchanger in which the steam and water come in direct contact
A closed feedwater heater is a shell and tube heat exchanger in which the steam and water do not come in direct contact.
How many btu's is a boiler horsepower?
What is an H.R.T. boiler?
A fire tube horizotal return tubular boiler.
What is a reinforcing pad?
Where would you find it?
The reinforcing pad strenthings the connection around the blow-down tube at the bottom of the boiler.
Explain the operation of a thermoexpansion feedwater regulator?
As the water level drops within the drum, the steam space inside the tube (thermostat) increases causing the tube to expand. This expansion causes the tube to move a mechanical linkage which will open the regulator valve. The opposite will happen as the level in the drum begins to rise.
Explain the operation of a thermohydraulic feedwater regulator?
As the level in the boiler drops. so does the level in the inner tube creating a larger steam space within the inner tube. The latent heat from the steam in that space is given up to the condensate in the outer tube causing the condensate to expand. The expansion of the condensate pushes on the bellows moving the regulator valve towards the open position allowing more water to enter the drum. If the water level rises, the opposite will occur and the valve will start to close.
What is balanced draft?
What is forced draft?
What is induced draft?
- Balanced draft is produced by using an FD fan and an ID fan together.
- Forced draft is produced when an FD fan pushes air into the boiler.
- Induced Draft is produced when an ID fan draws air through the boiler.
How would you calculate boiler efficiency?
OUTPUT DIVIDED BY INPUT
How many gas passes in a Scotch Marine boiler?
Name 2 types of air preheaters?
Economizer and Regenerative air preheater.