Wednesday, June 22, 2011

Boiler and Control System


Economizers are heat exchangers which are fitted in a boiler to increase the efficiency of the boiler. This is done by extracting the heat from the exiting gas and using it to heat the feed water entering the boiler.

To obtain an acceptable degree of efficiency and reduce fuel consumption as much as possible by introducing further heat recovery surface so that the gas temperature at the funnel may be as low as practicable, the gas temperature leaving a boiler cannot be reduced much below 30oC above the saturation temperature. In radiant types a much higher exit gas temperature is usually found. To carry out this further heat exchange, surfaces such as economizers and air heaters are commonly used.
In many radiant boiler types, economizers are also found arranged integrally within the boiler unit. In this location they consist of a number of multi-loop elements of plain tubes connected at their ends to inlet and outlet headers.
Since are situated in a hot gas temperature zone and are required to perform a considerable heat exchange duty, a portion of the water pumped through them may be converted into steam. These steaming economizers are arranged so that water enters the lower header and the steam and water mixture leaves from the top header to the steam drum where the steam and water separate.
Economizers are used externally to boilers for further heat recovery. Economizers are found in the cooler gas zone and are fed with water temperatures around 116oC or 185oC depending upon whether the feed cycle includes high pressure feed heaters after the de-aerator.
Before going into economizer inspection, first inspect the gas side of the boiler. It gives you a clear picture of boiler working condition and the efficiency of heat transfer surfaces.
1.      Check exterior of drums for sign of tube roll, leakage, corrosion, soot erosion and overheating.
2.      Condition of outside drum insulation.
3.      Drum seals for signs of air leakage.
4.      Inspect drum support for cracks and expansion clearance.
5.      Check all the blow-down connection for expansion and flexibility of support.
6.      Inspect all piping and valves for leaks.
7.      Visually check water wall tubes and fins for cracks.
8.      Check exterior of all tubes for corrosion, carbon-build up, erosion, blisters and sagging.
9.      Inspect tubes at soot blower for sign of steam impingement.
10.  Check header seals for signs of air leakage.
11.  Examine exterior of headers for corrosion, erosion, thermal cracking and condition of insulation.
12.  Condition of refractory.
13.  Around the burner assembly check refractory, tube condition and accumulation of soot or carbon.
14.  Check soot blowers for distortion, worn bearings, rubbing of tubes, condition of nozzle cracks, freedom of movement and effective lubrication.
1.      The major problem at the economizer section is low temperature corrosion and problems from gas side deposits.
2.      Sliding and leaky expansion joints at the casing may allow accumulation of soot with severe acid attack.
3.      Inspection of tubes bends by opening the inspection covers needs to be carried out to check these.
4.      Uptake area may show cracked expansion bellows sign of acid corrosion.
General cleanliness of these areas indicates the combustion performance in boiler.
Design and Setting of the Marine Boiler Safety Valve
CATEGORY:Safety valves are fitted to protect the boiler from the effect of over pressure. At least two safety valves are fitted to each boiler steam drum, but if there is a super heater, another safety valve should be fitted on it.
Introduction: Marine Boiler Safety Valves
The pressure setting of the superheater safety valve should be less that the designed pressure of the boiler, i.e. less than that of the steam drum safety valve, to ensure flow of steam through the superheater under blow off conditions. The pressure setting of one steam drum safety valve should be same as the design pressure of the boiler. The pressure setting of another safety valve should be 2-3 % more than the designed pressure of the boiler.
Classification of Boiler Safety Valves
There are three types of safety valves used in marine boilers:
1.      Improved high lift safety valve
2.      Full lift safety valve
3.      Full bore safety valve
Boiler Safety Valve
Improved High Lift Safety Valve:
1.      Wingless valve improves steam flow and reduces risk of seizure.
2.      Waste steam pressure acting on the piston gives increasing valve lift.
3.      Special shaped seat deflects steam towards lip on valve and increases valve lift.
4.      The valve lifts, the force to compress the spring increases, so the higher valve lifts the greater the increasing in boiler pressure.
5.      Waste steam pressure keeps cylinder in place while piston moves, also by having a floating cylinder, seizure risk is reduced.
6.      A lip is placed around the valve seat so that when the valve lid lifts, escaping steam is trapped in the annular space around the valve face, the resultant build–up of pressure acting upon the greater valve lid area causes the valve to lift sharply. This arrangement gives another advantage to close the valve cleanly and sharply with very little blow down effect.
7.      The improved high lift safety valve makes use of waste steam pressure to increase the valve lift; this is done by allowing the pressure to act upon the lower spring carrier which fits within a floating ring so forming in effect a piston. The pressure acts upon this piston causing it to move up, helping to compress the spring and so increasing the valve lift.
8.      Loose fitting key or pad lock is provided to ensure proper closing of valve.
9.      Loose pin is provided to secure valve lid and allow thermal expansion.
10.  Adjustment of the valve is carried out by means of a compression nut screwing down on to the top spring plate.
11.  A compression ring is fitted after the final adjustment to ensure no further movement takes place.
12.  A cap is then fitted over the compression nut and the top of the valve spindle, a cotter is passed through and padlocked to prevent tampering by unauthorized person.
13.  Clearance between this cap, the valve spindle and cotter are such as to prevent the valve being held down externally.
14.  Easing gear is fitted so that in the event of an emergency the valve can be opened by hand to a full lift ¼ D to release the boiler pressure.
Valve Area: As = A × (1 + Ts / 555)
·         As- Aggregate area through the seating of valve (mm2) for superheated steam.
·         A-Aggregate area through the seating of valve (mm2) for saturated steam.
·         Ts- Degree of superheated steam in oC.
15. Valve Area (As) greater than (A) due to specific volume of steam increases with increases of temperature at constant pressure and more escape area is required to avoid accumulation of pressure.
16. The area of valve chest must be at least (1/2) A.
17. The waste steam pipe and steam passage must be at least 1.1× A.
Manual Hand Trying of Boiler Safety Relief Valve:
To check the proper working condition of the boiler safety valve we carry out the “Hand trying out the Boiler Safety valve” at regular intervals. The safety valve is provided with the easing gear which manually lifts the safety valve and releases the excess pressure in the boiler. When the easing gear is pulled, the valve will be opened by hand to a full lift of ¼ D to release the boiler pressure. Before carrying out the process the boiler safety valve has to be drained.

Boiler Safety Valve Drain:
Draining of the boiler safety valve is necessary as to prevent any build-up of water in the pipe line causing head of water to form over the valve lid so increasing the blow off pressure. So at regular intervals the boiler safety valve should be drained.
1.      Drain pipe must be fitted to the lowest part of the valve chest on the discharge side of the valve.
2.      The pipe should be led clear of the boiler.
3.      The pipe must have no valve or cock fitted through its length.
4.      The open drain of the pipe should be regularly checked.
5.      If the pipe becomes chocked, there is possibility of overloading the valve due to hydraulic head, or damage due to water hammer.
6.      The waste steam pipe of the boiler safety valve should be well secured so that no load of the pipe is on the safety valve, which can be the cause of additional stress on the valve.
Pressure Setting of the Boiler Safety Valve:
If it is found that the boiler safety relief valve is not lifting at the designed lifting pressure, manual pressure setting of the boiler safety valve has to be done for the proper and safe operation of the boiler. The adjustment can be carried out on this type of valve to give the desired discharge and blow down characteristic.
1.      Safety valve pressure setting can be done from high to low pressure or vice versa.
2.      Take necessary personal safety precaution and arrange tools i.e. gagging tool and master gauges.
3.      Slowly raise the boiler pressure and blow off the safety valves manually few times for thermal expansion and to reduce the thermal stress on the valves.
4.      Then screw down all the safety valves higher than the setting pressure at which you are going to set.
5.      Raise the boiler steam pressure 2-3 % more than the designed pressure of the boiler, then stop firing and unscrew the first valve slowly, when it blows off at 2-3 % more than the designed pressure then note this opening and closing pressure of the valve and finally gag it.
6.      Raise the boiler pressure at the designed pressure of the boiler and unscrew the 2nd valve, when it blows off at designed pressure then note this opening pressure and check the closing pressure also. Recheck the setting pressure and gag the valve.
7.      Then set the superheater safety valve lower than the designed pressure of the boiler in same procedure.
8.      Finally take out the gagging tools. Pressure setting should be done in presence of surveyor
Boiler Inspection or Survey Carried Out at Regular Intervals

The boiler is vital equipment on ships. It is used as main propulsion (in steam ships) and for auxiliary heating in other ships. It is very sensitive and dangerous equipment, where there should be regular inspections and surveys carried out to avoid accidents and outages.
Normally boiler inspection will be carried out onboard the ship by a port state control and during the dry dock. They are used to carry out the inspection and see the working condition of the boiler. During the inspection they will conduct an in-depth analysis of the boiler condition considering various factors to find the working condition of the boiler. If necessary they will replace damaged parts of the boiler needed for continued safe operation.
1.      Boilers are inspected to maintain the Class requirement.
2.      Regular internal inspection and external examination during such survey constitute the preventive maintenance schedule the boiler goes through to have a safe working condition.
1.      Water tube high pressure boilers are surveyed at two year intervals.
2.      All other boilers, including exhaust gas boilers, are surveyed at two yearly intervals until they are eight years old and then surveyed annually.
1.      Confirm time available, manpower, and time required.
2.      Check before shutting down boiler.
3.      Check for spares e.g. manhole door joints, gauge glass, packing and steam joints.
4.      Check the tools required e.g. gagging tool, torque spanner, rope, chain block etc.
5.      Check manual for special instruction and past records.
6.      Steam requirement for the next port should be considered e.g. Tankers require steam in discharged Port.
7.      Briefing to other engineers of work involved.
Before inspection is to be carried out, the boiler which is firing should be shut down. These are the steps to be followed before shutting down the boiler for inspection.
1.      Inform the chief engineer and inform the duty officer in the bridge.
2.      Change over M/E, A/E, and Boiler to diesel oil.
3.      Top up diesel oil service tank, stop heavy oil and lube oil purifiers.
4.      Stop all tank and tracing steam heating and carry out soot blowing.
5.      Change over from automation to manual firing of boiler.
6.      Stop the firing of the boiler and purge boiler for three to five minutes.
7.      Switch off power and off the circuit breaker for forced draught fan, FO pump, feed pump, and combustion control panel. Hang necessary notices.
8.      Shut main steam-stop valve and shut all fuel valves to boiler.
9.      Let the boiler cool down, do not blow down now.
10.  When the boiler pressure is about 4 bars, carry out blow down.
11.  When boiler pressure is slightly higher than atmospheric pressure, open the vent cock to prevent formation of vacuum.
12.  Let the boiler cool down.
13.  Once sufficient cooled, open top manhole door first with all safety precaution.
14.  Mark the nut on the top manhole, slacken the dog-nut, and secure it with a rope.
15.  Knock the manhole door gently, but do not open it as it may contain steam or hot water.
16.  Conform nothing coming out; open the door fully with the help of securing rope.
17.  Do not open immediately open the bottom door, since the boiler is still hot and if opened relatively cool current of air will pass through the boiler causing a thermal shock.
18.  Allow further cool down before opening bottom manhole door.
19.  Open the bottom manhole door with the same precautions and open the furnace side door also.
20.  Ventilate foe period of 12 to 24 hours.
21.  Then check for oxygen, flammable vapour, and toxic gasses.
22.  If it is safe, prepare for entry.
These are the steps to be carried out before entering the boiler for inspection.
1.      Prepare a long rope, wooden plank oxygen analyzer, safety hand lamp, and safety torch attached with rope.
2.      Get a pouch to carry tools and keep track of the number of tools to be brought into boiler.
3.      Personnel safety protection wear, e.g. helmet, safety shoes, hand gloves, etc.
4.      No extra instruments to be brought in and clear pocket contents as it may fall into boiler.
5.      Keep an emergency breathing apparatus ready.
6.      Remain in communication and ensure proper lighting.
7.      Check boiler internals before making an entry, e.g. foothold and handhold.
Inspection Carried Out In Boiler Superheater and In Steam Drum

The superheater is a device which converts saturated steam or wet steam to dry steam, and it is used in driving the lager turbines in the marine propulsion system. In the superheating process the temperature of the steam is only raised, keeping the pressure at a constant level.
Superheating process can be done by three methods:
1.      Radiant superheating: In this type, the superheating tubes are placed directly in the combustion chamber.
2.      Convention superheating: In this type of super heaters the superheating tubes are placed outside the combustion chamber on the path of the hot gases.
3.      Separately fired: In this type the superheater tubes are placed in the separate combustion chamber outside the boiler. This is separately fired to maintain the required temperature of the superheated steam outlet.
In the superheater zone the products of combustion were still at a high temperature and deposits from impurities in the fuel condensed out on the tubes, reducing heat transfer and steam temperature. Eventually gas passages between the tubes would become so badly blocked that the forced draught fans would be unable to supply sufficient air to the burners, combustion become impaired and the fouling condition accelerated. Sodium and vanadium compounds present in the deposits proved very corrosive to superheater tube causing frequent repeated failure. Due to the fouled conditions there was a loss of efficiency and expensive time consuming cleaning routines were required.
Inspection on Superheater
1.      Internal and external examination of heaters.
2.      Thermal crack at the headers due to high stresses set up across the thick welded section is possible.
3.      Super heater safety valve and stop valve.
4.      Super heater drains and vents valves and manhole openings to check.
5.      Efficiency of the “screen” plates to ascertain –these protect headers from direct heat of furnace.
6.      Superheater tubes are also prone to high temperature creep failures and thermal fatigue cracking sudden quenching can cause fatigue failure.
7.      Check for deposit accumulation in header.
8.      Drain valve from headers to examine.
Super Heater Walk-In Spaces:
1.      Supports of horizontal super heater tubes to check for burning away and leave the unit unsupported and cause drainage problems.
2.      Super heater support tubes may also crack due to effect of bending fatigue stresses due to misalignment of tubes in the tube holes.
3.      Build-up of deposit is most troublesome defect in super heater. These may result in high furnace pressure, loss of super heater and poor combustion.
4.      Special attention and suspicion to be reserved for tubes through which there still exist gas paths as they operate under excessive metal temperature.
5.      Oxide scaling inside or outside may cause tube failure and worst case hydrogen fire when iron burns in steam at above 700*C in exothermic reaction, and destroys all boiler, economizer and air heater.
Now you have a clear picture on the various inspections carried out on the marine boiler parts for the safe and efficient working of the boiler.
·         STEAM DRUM
·     Inspection carried out in Boiler Superheater and in Steam Dr
Learn how inspection is carried out in boiler steam drum, headers and super-heater tubes.
The steam drum is one of the important parts of the boiler which acts as the reservoir for the steam generated and for water required for the boiler. Mainly all the boiler mountings are mounted on the steam drum and it should possess sufficient strength to withstand the high temperature and pressure of the steam generated.
As before, in inspecting the generating tubes, headers, and superheater tubes of the boiler, the inspection has to be carried out in the boiler steam drum.
Check the steam drum for corrosion, scaling, and pitting:
1.      Manhole seats and surface condition.
2.      Condition of all feed, chemical feed, blow down lines and inside pressure parts or chocking, security, and leaks.
3.      Check for freedom of expansion of drums and headers.
4.      Inspect tubes for corrosion, excessive deposits, flare-cracking, and pitting.
5.      Inspect hand-hole plates and stud threads.
6.      Make a complete waterside examination and check for scale build up as necessary.
7.      Measure thickness of scales by using commercially available gauges.
1.      All internal (removed from drum) checked and tested.
2.      Feed regulator, feed check valve, water gauge fittings, and drum safety valves examined. Attention to securing arrangement of seats in valves covers to valve chest to drum nozzles.
3.      Welded connection of drum to casing to check for any possible damage creating gas leakages.
4.      Areas of drum not protected by tubes from heat radiation and shielded refractory. Thermal cracking of the refractory material to be checked.
Boiler headers are the water feeders to the generating tubes in boiler. The headers are connected in between the steam drum and the water drum. Normally the water from the water drum enters the main headers from there and many generating tubes are connected where the steam is generated.
Rear and Side Wall Headers:
1.      Sufficient doors or handhole plugs to remove for assessment of internal condition of headers and tubes.
2.      Check for pitting and corrosion of headers, rear walls, floors, roofs, and side wall tubes.
3.      Check for casing defects for possible gas or air leakage.
Bottom Header:
This contains the furnace tubes and the down comer tubes. A number of handhole doors is provided for internal inspection and repair to the tubes.
·         Inspection for deposits of sludge must be carried out during the survey.
·         Regular blowing down from this header will be necessary to keep it clear of sludge deposits.
Repairs in Marine Boilers

This article discusses the general repairs needed in the marine boiler and how to repair them directly on board. Some common repairs are leaking of tubes in both smoke tube and water tube boilers, busting of tubes, and leakages in the manhole joints.
Some of the common repair work carried out on the marine boiler while on board the ship is plugging of the tubes and replacing the leaky manhole joints. Other major repairs like the renewal of the damaged tubes and furnace rebuilding must be carried out in the dry dock. The plugging of the boiler leaky tube is a temporary repair which must be carried out in order to fire the boiler. Whatever the situation, and in any condition the boiler must run to supply the working steam.
When the gasket becomes damaged or gets old, smoke starts to come out of the boiler in the case of the water tube boiler. In the smoke tube boiler, the water starts to leak outside the boiler. This must be repaired on board by replacing the leaky manhole joints.
Replacement of a Leaky Manhole Joint
1.      Maintain proper spigot clearance- 1.5 mm to position the door centrally for evenly loading the gasket.
2.      Never use an old gasket.
3.      Do not over strain the door studs, which may stretch.
4.      Pull-up studs by re-tightening the nut after steam rising or warming up.
5.      Avoid causing damage to door by holding it by a rope and gently lowering it inside or taking it out.
6.      Mark the dogs and nuts to fit back correctly in the same door.
7.      Check for wear and tear on the studs and nuts.
8.      Carefully check the matting/ landing surface for corrosion and erosion on the door and boiler before reassembling.
Repairs in Smoke Tube Boilers
Procedure for Plugging of a Damaged / Busted Smoke Tube:
1.      Hydrostatic testing to mark the leaky tubes.
2.      Cut the tubes on one end and clear of the tube plate. At the other end the tube is collapsed inside the tube plate.
3.      Pull out the tube from the collapsed end.
4.      Insert a short tube into the tube plate and weld it in place.
5.      Lap the spare tapered plugs on both stud ends in the tube plates.
6.      Insert the tube plugs and tack weld it.
7.      Alternatively, the plugs can be held in place by a long steel bar threaded and bolted at both ends.
8.      Hydrostatic pressure test to confirm no leaks.
9.      Flush up the boiler and re-inspect the plugs for leaks under full steam pressure.
Temporary Repairing Procedure to Rectify the Leakage in Smoke Tube:
1.      Stop the burner, allow the boiler to cool and remove the soot.
2.      Allow boiler to depressurize, and open the blow down valve to drain the boiler.
3.      Enter the boiler flue box and cut a hole in the side of the relevant smoke tube.
4.      Clean the rim of the smoke tube with a wire brush.
5.      Cut a circular plate (15 mm thick) of the same diameter as the smoke tube and chamfer the top edge to 30 degrees by grinding.
6.      Fit the plate into the top of the smoke tube and weld it in position as shown.
7.      Enter the boiler furnace and cut a similar hole in this end of the relevant smoke tube.
8.      Repeat steps 4 to 6 for lower plate.
9.      Refill boiler and check for leaks before start-up.
10.  Start-up boiler and check for leaks when pressurized.
Note: Any temporary repair to smoke tubes or boiler tubes should receive more permanent attention as soon as conveniently possible.
Repairs in Water Tube Boiler
Instruction for Plugging / Repair of Water Tube Boiler & Economizer:
1.      In case of tube failure, steam pressure has to be removed and the oil burner dismantled.
2.      If the leakage is readily visible from the burner hole, the boiler can be emptied and repairs commence.
3.      Otherwise, the boiler is given pressure by means of the feed pump. The position of the leakage will be indicated by the water flow.
4.      This flow may not be visible from the burner hole. If it is not visible, remove the inspection door and enter the furnace. If the tube failure is still not found, then enter the generating tube section. From here the bottom of the membrane walls and generating tubes can be inspected for leakage.
5.      If the leakage has resulted from the membrane walls or generating tube, the inspection door at the smoke connection pipe must be removed, and the generating tube/ membrane tube in which the failure has occurred is pointed out.
6.      The leakage may also result from economizer.
7.      By removing the inspection door at the bottom of the economizer, it can be determined which uptake has caused the leakage?
8.      If necessary other inspection doors should be removed to point out the damage register.
9.      When a damaged tube or convection register has been removed, and the remaining tube studs have been repaired/ plugged a new tube or register should be mounted as soon as possible.
10.  Operation for longer periods with one or more registers missing involves the risk of further damage to the boiler due to increasing heat leads on the parts next to the ones removed.
Scope of Inspection of a Ship’s Boiler
The boiler is one of the items of equipment on a ship which continuously keeps on running during sailing and in port. As it is running continuously, it has to be cleaned and inspected to check the condition of all internal working parts at regular intervals.
The scope of inspection is to clean the boiler’s internal surfaces and to check for corrosion and scale formation in the boiler. As the boiler normally runs continuously, there are few chances to open the boiler. Thus, during the inspection all the important checks will be carried out and it will be made sure that the boiler will safely work without any problems until the next inspection. Routine inspection is important because salt formation and scaling inside the boiler tubes will reduce the heat transfer rate and ultimately damage the tubes due to overheating.
1.      The inspection should include finding reasons for any abnormality found and should also ensure that any repair carried out does not affect that safe working order of the boiler.
2.      A complete inspection means full internal and external examination of all parts of the boiler and accessories such as super-heaters, air heaters, and all mountings.
3.      The examination may lead the inspector to require hydraulic testing of pressure parts or thickness gauging of plate or tubs that appear to be checked for good working condition.
The Inspection is not completed until the boiler has been examined under steam and the following items dealt with:
a) Pressure gauge checking against a test gauge.
b) Testing of water level indicators and protective devices.
c) Safety valves adjusted under steam to blow off at the required pressures.
d) The oil fuel burning system examined.
e) Testing of remote control gear for fuel shut off valves.
For a gas fired boiler, the chief engineer floats the safety valve at sea at the first opportunity. Survey record is not assigned until a statement is received from chief engineer about the pressure at which the safety valves were set.
Inspection Consists Of:
a) Examination of the items.
b) Statement whether a problem / defect exist.
c) Determining the cause of problem.
d) Define the repair and whether temporary / permanent.
The Main Benefits of Doing Inspection:
By doing the inspection, we are manually cleaning the boiler scales and chemical cleaning of the salt formation in the boiler parts and making the boiler safe for operation. It also helps in checking the redundancy of the stand-by boiler. During the inspection the newly signed in crew members and the ship’s engineer will also have a chance to see the internal parts of boiler.
1.      Boiler must be sufficiently cleaned and dried to make a thorough examination possible.
2.      Boiler should be manually wire-brushed to clean the internal surfaces.
3.      In case of difficulty in manual cleaning, chemical cleaning with hydrochloric acid plus inhibitor to prevent acid attacking the metal without affecting removal of deposits is the best procedure.
4.      For oil contamination, alkali boil-out using tri-sodium phosphate solution is essential prior to acid cleaning. Through water flushing must be carried out after acid cleaning to avoid acid concentration in crevices and captive spaces.
5.      All internals that may interfere with the inspection have to be removed.
6.      Wherever adequate visual examination is not possible, surveyor may have to resort to drilling, ultrasonic, or hydraulic testing.
7.      All manhole doors and other doors must be opened for reasonable time previous to survey for ventilation.
8.      If another boiler is under steam arrangement of locking bar and other security devices must be in position preventing the admission of steam or hot water to the boiler under survey. The smoke trunking, exhaust gas shut-off etc., must be in position and in proper working condition.
9.      Plant’s staff or repairer’s staff should stand by the manhole in case of emergency and to take note for defects/ repairs required.
Before survey, the surveyor should acquaint himself with the boiler type in question (drawings carried on board) and during the survey it is advisable to follow a planned routine in order not to miss parts of the boiler or important items.