Boilers Flashcards
State the reasons for boiler water treatment.
The reason we carryout boiler treatment is to ensure that the boiler can produce a high quality of steam. Treatment is also used to make sure there is a continuous heat exchange while also providing corrosion protection.
State the reasons for conducting each of the following boiler water tests and describe how the tests are conducted :
Alkalinity to Phenolphthalein;
This can cause high residual stress which will lead to the steel to crack. To test do the following;
Take 100ml of filtered sample boiler water
Add 10 drops of phenolphthalein (will turn pink if alkaline)
Add drops of N/50 standard solution of sulphuric acid until sample turns clear.
Ml of sulphuric acid used x 10= p.p.m of alkalinity
State the reasons for conducting each of the following boiler water tests and describe how the tests are conducted :
Chlorides Tests
This test is to check how much chloride salt is within the boiler and feed water system. Using the same sample remaining from the P Alkalinity test, carryout the follow to test for chlorides;
Add 2ml of N/50 sulphuric acid
Add 20 drops of potassium chromate indicator (turn a pale yellow)
Add drops of N/35.5 standard solution of silver nitrate (whole sample becomes reddish-brown)
Ml of silver nitrate used x 10= p.p.m chlorides
State the reasons for conducting each of the following boiler water tests and describe how the tests are conducted :
Phosphate reserve.
This is used to react with scale forming salts to produce non-adhering sludge. It also reacts with sodium hydroxide to increase alkalinity. Take two samples of boiler water
20ml sample of boiler water
Add equivalent amount of vanadomolybdate reagent and fill one tube with the sample
Fill the other tube with an equivalent mixture of pure distilled water and the reagent
If phosphate is present, then sample will turn yellow
colour is measured against a shade disk = p.p.m phosphate
Sketch a section through an ESD III roof fired boiler, clearly indicate on the diagram:
i) the gas flow from furnace to above economiser;
ii) the gas temperatures to be expected at various point from the furnace to above the economiser.
163
440
1100
1316
Describe the function and purpose of each of the following:
i) Downcomer tubes;
The downcomers are external to the furnace and take boiler water from the steam drum back down to the water drum within the boiler allowing the circulation of the system.
Describe the function and purpose of each of the following:
Steam drum;
The steam drum holds both water and steam. The water enters the steam drum from the feed water for the economizer and circulates the system. Steam is the generated and leaves at the top to be passed via the superheater and leave the boiler
Describe the function and purpose of each of the following:
iii) Riser tubes.
The riser tubes allow the water and steam to go from the water drum into the steam drum to allow the water and steam to circulate the system.
Describe with the aid of a sketch a water level indicator that utilises a “differential pressure” cell to provide a remote reading.
This is known as Igema remote water level indicator. The lower section of the U tube contains a red fluid which has a greater density than water.
As the water level falls ‘h’ will be reduced and ‘x’ and ‘H’ will increase. Water in the condenser reservoir being maintained by condensing steam.
As the water level rises ‘h’ will increase and ‘x’ and ‘H’ will reduce. Allowing water to flow over the weir in the condenser reservoir to maintain a constant level.
State the precautions that should be taken to protect the device when first putting it into service.
Make sure the device can withstand the boiler pressure it will be fitted onto.
Explain what is meant by the term “swell and shrinkage” as applied to boiler water level control.
Increasing the load will cause an increase in steam bubbles. This will displace the water, causing the level within the drum to rises. This will also happen when demand decreases, causing the water level to drop.
Explain where a “baffle plate” may be fitted and it’s purpose;
Its located at the bottom of the steam drum and can be fitted either to direct the steam/water mix into the cyclones or to improve circulation.
Its located at the bottom of the steam drum and can be fitted either to direct the steam/water mix into the cyclones or to improve circulation.
Describe with the aid of a sketch the purpose and functionality of a “cyclone separator”;
Cyclone Separators are used to separator the steam and water mixture returning to the drum. The water swirls around creating a vortex, pushing the water to the outer edge and to the bottom and the steam outlet at the top.
state the purpose of a “dry pipe”.
Removes any water droplets from the steam leaving the drum.
With reference to boiler water test results, in EACH of the following cases state the reasons for the change in the reading:
i) Total dissolved solids and chemical reserve reducing;
Leakage below water boiler level
With reference to boiler water test results, in EACH of the following cases state the reasons for the change in the reading:
Phosphate reserve reducing, chlorides and total dissolved solids increasing;
There is a sea water leak within the system
Explain what is meant by the following terms:
i) Alkalinity.
Alkalinity is the higher end of the pH scale and can be used to neutralise the acid.
Explain what is meant by the following terms:
ii) Hardness.
The sum of of the temporary and permanents hardness salts and is an indication of the total scale forming potential of the water.
Explain what is meant by the following terms:
iii) Chlorides.
Chlorides are referred to as permanent hardness salts which include Sodium Chlorides (Common Salts) and Magnesium Chlorides.
Explain the principle of operation of the air ejector sketched at question (a).
Extracts the air from the condenser to allow vacuum to be maintained. The steam is condensed from the air by feed water passing through cooling coils. The air is released through a vacuum retaining valve. The condensate drain from the first stage passes through a sealing loop back to the condenser. The condensate drain from the second stage passes through a sealing loop to an atmospheric drain tank.
Explain the rules regarding the fitting of these devices to a boiler, identifying the difference in requirements between a boiler fitted with a superheater and a boiler without a superheater and the minimum permitted bore of a safety valve.
All boilers without a superheat should have two safety valves fitted to the main steam drum or shell. Group 2 boilers and upwards should be fitted with a third safety valve on the main steam outlet on the superheater. The bore must never be less than 38mm in diameter.
Explain what is meant by the term “blow down” when applied to safety valves.
The difference between the set pressure and the pressure at the valve closes after releasing the excess pressure.
Explain why the following are used:
i) flat seats;
Flats seats provide an equal lift, therefore the seat has a greater escape edges
Explain why the following are used:
easing gear;
Easing gear allows us to manually opening the valve to the maximum effective lift from a remote position.
Explain why the following are used:
drain pipe.
Drain pipe is fitted to the lowest part of the valve chest on the discharge side to vent off and remove any dirt or water.
Describe how the kinetic energy of the escaping steam may be utilised to increase the lift of the safety valve.
The valve is lid and seat is designed to direct the steam downwards and upwards against the valve lid and seat allowing extra lift against the spring force.
With reference to a large marine boiler plant;
State why superheaters are used.
Super heaters are used to turn saturated steam into dry steam while also acting as a heat exchanger with the exhaust gases heading up the funnel and then steam flowing down the funnel.
With the aid of a sketch explain where superheaters are located in the boiler.
The superheater is fitted at the lower part of the furnace, below up the uptake. This allows the exhaust gases to pass over as it leaves towards the economizer.
With reference to a large marine boiler plant;
Explain why they are located in the position stated.
As this allows the exhaust gases to flow over the superheater as it leaves the boiler and up the funnel. Therefore, providing, maximum temperature allows us to get the best superheated dry steam.
Explain how the superheater is protected when steam pressure is being raised from zero to working pressure.
By leaving the vents and rains open until enough steam can be circulated to protect the superheater.
Explain how the lift of a marine boiler safety valve is calculated with respect to the diameter of the valve seat.
HE = CAP
A=HE
CP
H = total heating surface of the boiler E = Evaporative rate in kg of steam C = Discharge co-efficent P = Working pressure A = Area
State the number of safety valves that must be fitted to a high pressure marine boiler that incorporates a superheater and where the safety valves are fitted.
Two safety valves must be fitted to the main steam drum and a third must be fitted to the main steam outlet from the superheater.
State at what pressure a boiler safety valve is set to lift and the minimum allowable bore of a safety valve seat.
Boiler safety valve will lift depending on the valve design and boiler design. The bore must never be less than 38mm in diameter.
With aid of a sketch describe how, in an ‘Improved High Lift’ boiler safety valve, the design of the safety valve lid and the shape of the seat help overcome the resistive forces of the valve spring.
The valve is lid and seat is designed to direct the steam downwards and upwards against the valve lid and seat allowing extra lift against the spring force.
State the materials used in the construction of the principle components of an Improved High Lift Safety Valve.
Valve chest – Cast steel Valve body – Cast steel Valve seat – Stainless Steel, monel Valve lid – Stainless Steel, monel Spindle – Stainless Steel Guides/ Bushes – Bronze Spring – Tempered Steel
With the aid of a detailed sketch, describe a water level gauge glass suitable for use on a high pressure water tube boiler operating at greater than 40 bar pressure. Clearly indicate on the sketch any safety devices that are fitted.
Micra sheets are fitted between the glass surface facing the water and steam.
Explain why it may be necessary to blown through a gauge glasses.
To unsure that the cocks are not chocked or and all passage ways are clear. Otherwise if the steam is chocked then this will create a vacuum and water will rise till it fills the glass.
List the steps for blowing through a boiler gauge glass.
.
Shut the steam and water chocks
Open the drain cock, watch for condensate and the flow to stop.
Open water cock and check flow then shut
Open steam cock and check flow then shut
Shut drain cock
Open water cock and fill to the top
Open steam cock and watch water level return to working level in boiler
Describe in full the manufacturing processes involved in the construction of a boiler drum.
The steam drum is constructed in two halves from two different thicknesses of steel. The greater thickness is on the bottom half to increase the strength around the tube drilling. This will be cold rolled then checked for its ovality. Then a submerged arc welding process will join the two half together using burden.
The drum ends are then welded on while the body rotates on the jig. The drum is pre heated then a submerged arc welding process joins the ends on. The holes are the marked and drilled for the tubes. After completion a series of test are taken from the excess metal. The test the manufacture will carryout are;
- Tensile test
- Across weld test
- Charpy impact test
Identify the materials from which boiler drums are constructed and the plate thicknesses used.
The drum is constructed by cold rolled 0.16 carbon or carbon-manganese steel.
Steam drum plate thickness is 75mm
Water drum plate thickness is 35-38mm
The drum end is made of 0.16 carbon or carbon manganese steel at a thickness of 25-30mm
State the tests to which the boiler drum material is subjected to.
The test the manufacture will carryout are;
- Tensile test
- Across weld test
- Charpy impact test
Describe how the material to be tested is obtained.
The welding run off material can be removed and used to carryout the tests
- Tensile test
- Across weld test
- Charpy impact test
With the aid of a sketches describe a design of a water wall used in the construction of a modern water tube boiler, identifying how the tubes attached to the drums and headers.
Water walls are used in water tube boilers to contain the heat of the furnace, therefore reducing the refractory material required. They also contribute to the evaporation rate by receiving radiant heat. They are fitted to the drums and headers by being tapered then when heated they expand into place or welded into the drums and headers
Identify and describe the stages of the combustion process that take place in the burner unit.
The rotary cup burner uses centrifugal forces to create atomisation by the fuel being injected into the side of a spinning cup. Fuel is then thrown off the rim of the cup and atomised with the air.
Explain the meaning of the term turn down ratio as applied to boiler burners, clearly stating how the ratio for a burner is derived.
Turn down ratio is the maximum and minimum fuel throughputs while maintaining a stable flame condition.
Explain what is meant by EACH of the following terms and why the levels of EACH term is monitored and controlled: -
i) Alkalinity;
Alkalinity is the higher end of the pH scale and can be used to neutralise the acid.
Explain what is meant by EACH of the following terms and why the levels of EACH term is monitored and controlled: -
Hardness.
The sum of of the temporary and permanents hardness salts and is an indication of the total scale forming potential of the water.
For EACH of the following cases state the reasons for the change in the reading:
i) TDS and hardness increasing, alkalinity and chlorides increase slightly;
Fresh Water Contamination
For EACH of the following cases state the reasons for the change in the reading:
Hardness, TDS and phosphate reserve all decrease.
Leakage below boiler water level
State the reasons for conducting a boiler water test for Alkalinity to Phenolphthalein and describe how the test is conducted.
This can cause high residual stress which will lead to the steel to crack. To test do the following;
Take 100ml of filtered sample boiler water
Add 10 drops of phenolphthalein (will turn pink if alkaline)
Add drops of N/50 standard solution of sulphuric acid until sample turns clear.
Ml of sulphuric acid used x 10= p.p.m of alkalinity
Explain why the testing of boiler water is critical to the performance of the boiler.
All waters cause problems or damage the boilers due the chemicals that are contained within. By testing we are able to monitor these chemicals and keep them under control.
Identify what would cause a change in the water test results reading in EACH of the following cases:
i) Total dissolved solids and chemical reserve reducing;
Leakage below boiler water level
Identify what would cause a change in the water test results reading in EACH of the following cases:
Phosphate reserve reducing, chlorides and total dissolved solids increasing;
Sea water contamination
Explain what is meant by the following terms:-
i) Alkalinity;
Alkalinity is the higher end of the pH scale and can be used to neutralise the acid
Explain what is meant by the following terms:-
Hardness;
The sum of of the temporary and permanents hardness salts and is an indication of the total scale forming potential of the water.
Explain what is meant by the following terms:-
ii) Chlorides.
Chlorides are referred to as permanent hardness salts which include Sodium Chlorides (Common Salts) and Magnesium Chlorides.
State three functions of the deaerator;
- Removes any gases from the water by scrubbing
- Provides sufficient head pressure above the feed pump.
- Heats feed water to about 105 degrees c
Identify where in the machinery space the deaerator is located and why it is located there.
It is located in the highest part of the system to provide a constant head for the feed pump suction.
State two functions of the deaerator.
- Provides sufficient head pressure above the feed pump.
- Heats feed water to about 105 degrees c
State the function of and explain the principle of operation of an air ejector.
An air ejector removes any incondensable and remaining steam vapour from the condenser and thereby maintains a vacuum.
