Boiler Water Test

Posted on Monday, June 04, 2012 by Unknown

Corrosion and tube failure caused by water chemistry

pH	- 9.6 to 10.3
PO₄	- 4 to 20 ppm
N₂H₄	- 0.01 to 0.03 ppm
TDS	- < 150 ppm
Cond pH	- 8.6 to 9.0
Cl	- 20 ppm
O₂	- 10 ppb
Si	- 10 ppb

Chlorides

TDS

Phosphates

Hydrazine

Alkalinity Phenolpthalein

Alkalinity Methyl orange

Sulphite reserve

Ammonia in feed

Boiler water treatment

Alkalinity

Treatment

For pressures below 20 bar dissolved O₂ in the feed does not cause any serious problems so long as the water is kept alkaline
However cold feed should be avoided as this introduces large amounts of dissolved O₂ are present, for pressures greater than 18.5 bar a dearator is recommended

Feed Treatment Chemicals

Sodium	Hydroxide
Calcium	Bicarbonate (CaCO₃ + Na₂CO₃)
Magnesium	Bicarbonate
Magnesium	Chloride.
.	.
Sodium	Phosphate
Calcium	Carbonate
Calcium	Sulphate
Magnesium	Sulphate
All in this column precipitated as hydroxide or phosphate based sludges	All in this column form sodium salts which remain in solution

Sodium Hydroxide

Sodium Carbonate Na₂CO₃ ( soda ash )

Sodium Hexa Meta Phosphate NaPO₃ (calgon)

DiSodium Phosphate Na₂HPO₄ (Cophos II)

TriSodium Phosphate Na₃ PO₄ (Cophos III)

Chemicals are normally added as a dilute solution fed by a proportioning pump or by injection from pressure pot.
Use of chemicals should be kept to a minimum.

Injection over a long period is preferable as this prevents foaming.

Excessive use of phosphates without blowdown can produce deposits of phosphides on a par with scale formations.

Therefore it is necessary to add sludge conditioners particularly in the forms of polyelectrolytes, particularly in LP blrs

Neutralising amines

Hydrazine N₂H₄

see above

Bramine ( cyclohexalamine )

(Bull & Roberts amine treatment)

Neutralising amine as with hydrazine. Used with hydrazine to maintain feed water alkalinity within parameters. As a knock on effect will slightly increase boiler water alkalinity.

Stable at high temperatures so is used more than hydrazine to control the steam line alkalinity as there is less chance of copper corrosion which occurs with the prescience of ammonia

Proper boiler water treatment eliminates sludge and scale deposits within the boiler. However, over along period of time a film of copper and iron oxides build up on the tube surface. Most of these oxides are transported from oxides of corrosion within the feed system to the boiler with the condensate.

Bramine reduces this corrosion and eliminates the build up of these oxide deposits.

Mechanism of function
Condensate from the condenser is very pure and slightly acidic, often referred to as 'hungry water'. It can dissolve metals in trace amounts to satisfy this hunger.
Distilled make up water aggravates this situation containing much dissolved CO2 and hence being acidic carries its own corrosion products.
Trace amounts of bramine are introduced into the system to establish an alkalinity level greatly reducing the effects of the hungry water.

Some of the bramine is used almost immediately, most however, passes on to the boiler where it is then transported through boiler water, boiler stm drum, stm lines back to the condenser. It has no effect anywhere except the condensate system.

Bramine also has a cleaning effect and may assist in the cleaning the film off the tube over a period of time.
Bramine is safer to handle than Bramine and will protect all metals.

Hydrazine however readily breaks down to form ammonia which whilst protecting ferrous metals will attack those containing copper

Filming amines

Shows neutralising tendencies, main function however is to coat piping with a molecular water repellent protective film

Injection of amines
May be injected between HP and LP turbines in the X-over pipe or after the dearator.
Adding in X-over pipe-reduces corrosion of copper alloys
Dearator only effective as a feed heater

Adding after dearator -Dearator correctly performing as a dearator and feed heater. If possible the best system is to have a changeover to allow norm inj into the X-over at sea and injection after the dearator when the turbine shut down

Limits of density/pressure

Sludge conditioning agents

Coagulants-

Antifoams

Dispersing agents
- Sludge conditioners such as starch or tannin.
- Prevent solid precipitates uniting to form sizeable crystals e.g. MgSO4

Treatment in boilers (non congruent)

LP tank blrs (<14 bar)

Na₂CO₃- precipitates salts, provides alkalinity
MgSO₄- Sludge conditioners

Na₂CO₃ can break down to form NaOH in higher rated boilers hence initial dose with Na₂SO₄

Medium pressure tank blrs (<17.5 bar)

Na₂CO₃(3) + sodium phosphate(4) + sludge conditioners(1)

Medium to High pressure water tube <60 bar

Na₂CO₃ + Na₂HPO4 +sludge conditioners

Oxygen scavengers also used to allow magnetite (Fe₃O₄) layer to form in the boiler

Boiler operating above 42 bar require a dearator.

HP to UHP blrs (42 to 80 bar)

Due to level of decomposition of Na₂CO₃ . NaOH preferred for better controllability Na₂HPO₄

NaOH attacks the magnetite layer. Congruent treatment used.

Permissible limits

		Shell	WT	WT	WT	WT	WT
TEST>	PPM	<17.5b	<17,5b	<32b	<42b	<60b	<85b
Hardness	CaCO₃	<=5	<=5	<=5	<=1	<=1	<=1
P.alk	CaCO₃	150-300	150-300	150-300	100-150	50-100	50-80
T.D.S.	CaCO₃	<=7000	<=1000	<=1000	<=500	<=500	<=300
Cl	CaCO₃	<=1000	<=300	<=150	<=100	<=50	<=30
PO₄	PO₄	30-70	30-70	30-70	30-50	30-50	20-30
N₂H₄	N₂H₄	--	--	--	0.1 -1.0	0.1 - 1.0	0.1 - 1.0
SO₃	SO₃	50-100	50-100	50-100	20-50	--	--
SiO₂	SiO₂	--	--	--	--	--	<=6.0
Fe	Fe	--	--	--	--	--	<=0.05
Cu	Cu	--	--	--	--	--	<=0.02
pH	pH	10.5-11	10.5-11	10.5-11	10.5-11	10.5-11	10.3-11
Limits for feed water
Cl	CL	<=10	<=5	<=1.0	<=1.0	<=1.0	<=1.0
O₂	O₂	----	----	<=0.006	<=0.003	<=0.015	<=0.01
NH₃	NH₃	----	----	----	-----	-----	<=0.5
Fe	Fe	----	----	----	-----	<=0.01	<=0.01
Cu	Cu	----	----	----	-----	<=0.01	<=0.01
pH	pH	----	----	----	8.5-9.2	8.5-9.2	8.5-9.2

Boiler Mounting

Posted on Monday, June 04, 2012 by Unknown

Definition - various valves and fittings are required for the safe and proper working of a boiler . Those attached directly to the pressure parts of the boiler are referred to as the boiler mountings.

Minimum requirements for boiler mountings

- two safety v/v's
- one stm stop
- two independent feed check
- two water gauge or equivalent
- one pressure gauge
- one salinometer v/v or cock
- one blowdown/scum v/v
- one low level fuel shut off device and alarm

Functions

SAFETY V/V-protect the boiler from over pressurisation. DTI require at least two safety v/v's but normally three are fitted ,two to the drum and one to the superheater. The superheater must be set to lift first to ensure a flow of steam through the superheater.
These must be set to a maximum of 3% above approved boiler working pressure.MAIN STM STOP-mounted on supherheater outlet header to enable boiler to be isolated from the steam line if more than one boiler is connected. V/v must be screw down non return type to prevent back flow of steam from other boiler into one of the boilers which has sustained damage (burst tube etc) v/v may be fitted with an emergency closing device.
AUXILLIARY STOP V/V- similar to main stops but connected to the auxiliary steam line
FEED CHECK V/V'S- a SDNR v/v so that if feed p/p stops the boiler water will be prevented from blowing out the boiler. The main check is often fitted to the inlet flange of the economiser if no economiser fitted then directly connected to the boiler. The Auxiliary feed check is generally fitted directly to an inlet flange to the drum with crossovers to the main feed line. Usually fitted with extended spindles to allow remote operation which must have an indicator fitted.

WATER GUAGES- usual practice is to fit two direct reading and at least one remote for convenient reading.
PRESSURE GUAGES-fitted as required to steam drum and superheater header
SALINOMETER COCKS OR V/V'S-fitted to the water drum to allow samples to be taken.Cooling coil fitted for high pressure boilers.
BLOWDOWN COCK- used to purge the boiler of contaminants.Usually two v/v's fitted to ensure tightness . These v/v's lead to an overboard v/v.
SCUM V/V-These are fitted where possibility of oil contamination exists. They are designed to remove water and/or contaminants at or close to normal working level.

SAFETY VALVES

At least two safety valves have to be fitted to the boiler. They may be both mounted on a common manifold with a single connection to the boiler. The safety valve size must not be less than 38mm in diameter and the area of the valve can be calculated from the following formulaC x A x P = 9.81 x H x E
where
H= Total heating surface in m³
E = Evaporative rate in Kg steam per m² of heating surface per hour
P = Working pressure of safety valves in MN/m² absolute
A = Aggregate area through the seating of the valves in mm²
C = the discharge coefficient whose value depends upon the type of valve.
C=4.8 for ordinary spring loaded valves
C=7.2 for high lift spring loaded valves
C= 9.6 for improved high lift spring loaded valves
C= 19.2 for full lift safety valves
C= 30 for full bore relay operated safety valves

LIFT PRESSURE

The safety v/v must be set at a pressure not exceeding 3% of the approved boiler working pressure. It is normal to set the suphtr safety below that of the drum to ensure an adequate flow of stm for cooling purposes under fault conditions. Similarly the superheater should be set to close last.

10% ACCUMULATION OF PRESSURE RULE.

With all the flames in full firing the stm stop is closed, the boiler pressure must not increase by more than 10% in 7 minutes for water tube of 15 mins for tank boilers with the safety lifted. this is normally waivered for superheater boilers. Instead calculations and previous experience used.

BLOWDOWN

The pressure drop below the lifting pressure for a safety v/v is set at 5% by regulation although it is more normal to set v/v's at 3% to prevent excessive loss of stm. For boilers with a superheater it is important that the superheater v/v not only lifts first but closes last.Adjustement of the blowdown may be necessary following adjustment of the popping setpoint (Increaseing set point lengthens blowdown). Adjustment is achieved by altering the height of the 'adjusting guide ring' on the full lift safety valve design shown below. Over raise adjustment of this ring can lead to mal-operation with the valve not fully opening

SETTING

Must be set with the surveyor present except when on the waste heat unit. A chief engineer with three years experience may then set the safety valve but must submit information to surveyor for issue of certificate.
Superheated steam safety valves should be set as close to operating temperature as possible as expansion can alter the relationships between valve trim and guide/nozzle rings which can effect the correct operation of the valve.

Easing gear to be checked free before setting valves. Steam should not be released as this can damage seat.

Improved high lift safety valve

Differences in the ordinary and high lift designs

Ordinary	High Lift	Improved high lift
Winged valve	Winged valve	Wingless valve
No waste piston	Waste piston	Waste piston
	No floating ring	Floating ring

For superheated steam the aggregate area through the seating of the valves is increased, the formula is
As = A(1 + Ts/555)
where
As = Aggregate area through the seating of the valves in mm² for superheated steam
A = Aggregate area through the seating of the valves in mm² for sat steam
Ts = degrees of superheat in ^oC
As is greater than A due to the higher specific volume of superheated steam requiring more escape area.
The manifold pipe must have an area equal to at least Н of A, the exhaust must have a diameter dependent on the type of valve but up to 3 x A for a full bore relay operated valve.
A drain pipe must be fitted to the lowest part of the valve, it should have no valve or cock and should be checked clear on regular occasions.

Materials

Materials for all parts must be non corrodible. Common materials are Bronze, Stainless steel or Monel metal, depending on the conditions of service. The valve chest is normally made of cast steel.

Full lift safety valve

This is a modern version of the high lift safety valve incorporating the piston and reaction force effects to improve valve lift. In addition the inlet pipe is tapered to give a nozzle effect increasing the reaction on the lid.
The initial lift is produced when the steam pressure under the disc exceeds the spring pressure. As the valve begins to open a thin jet of steam escapes and is deflected by a small angle on the nozzle ring. As the lift increase the steam begins to react against upper guide ring increasing to 'full bore'lift. Full Borelift is defined as that point where the area of the nozzle, rather than the lift, limits the discharge capacity of the valve. The form of the valve offers an increased area to the steam jet stream and the design allows for a piston effect of the valve trim assembly as it enters in the guide ring cylinder, both these effects increase lift and improve action of the valve
The guide sleeve is adjustable allowing alteration of the blowdown.
With boiler pressure dropping the valve begins to close. When the lid just exits the guide sleeve there is a loss of the reaction and piston effect and the valve tends to snap shut cleanly.
Blowdown adjustment is achieved by altering the height of the adjusting Guide Ring. On some designs a second adjustable ring is mounted on the nozzle, this allows adjustment of the 'warn' or 'simmering'period and increases the popping power. Adjustment of this ring is critical to operation, after factory setting it is generally unnecessary and no attempt should be made to remove slight 'warn'

Full lift safety valve

Seen fitted to large high pressure boilers.

This design offers sveral advantages over simple high lift valves

Easing gear

This is fitted to safety valves to allow manual operation of the valve in an emergency.

Gauge glasses

General

The requirements are that there must be two independent means of reading the boiler water level. Normal practice for propulsion plant boilers are the fitting of two direct reading level gauges and a remote display readout.

Low pressure boilers( up to 17.5 bar)

Small vertical boilers may be fitted wit a series of test cocks to ascertain the level, this is deemed unsuitable for boilers above 8.2 bar and/or 1.8m in diameter.

Tubular gauge glass

Medium pressure boilers

Reflex glass is used due to the fact that light falling on the glass is reflected by the steam but not by water, and so the glass appears bright where there is steam and dark where there is water.

High pressure boilers

A ball is located in the water side to prevent large quantities of water entering the engineroom in the event of the glass failing and the subsequent large expansion of the water as it flashes off to steam. An orifice restrictor is fitted to the steam valve.
Mica is placed on the water side of the glass to protect against erosion and chemical attack of the high temperature water.

General gauge glass faults

Due to the evaporation of water leaking through the cock joints a build up of deposits can occur. This leads to restriction and eventual blockage of the passage. If this occurs on the steam side then the level tends to read high as the steam condenses.
Another reason for blockage is the cock twisting, hence the cocks are all arranged so that in their normal working positions, i.e. steam/water open , drain shut, the handles are all pointing downwards. Possibility of the sleeving rotating on the cock has led to the use of ribbed asbestos sleeves which must be carefully aligned when fitting.
For tubular gauge glasses the length of the tube is critical and should be checked before fitting

Bi-color gauges

$refraction through water of different colors$

Igema remote reading indicator

Fitted in addition to gauges required by statute and not in place of them. The red indicator fluid does not mix with the water

Igema remote indicating water level indicator

Equilibrium condition is when H= h + rx where r is the specific density of the indicator fluid.
If the water level rises h increases and x reduces. Therefore H will be reduced and water will flow over the weir of the condenser to maintain the level constant. If the water level were to fall h would be reduced, x increase and H would be increased. Water therefore must be made from condensing steam in order to keep the condenser level constant.

Burners & Refractory

Posted on Monday, June 04, 2012 by Unknown

Combustion of fuel in furnace and burner design

Process

The heat producing constituents of the fuel are hydrogen, carbon and sulphur.

- Carbon to carbon dioxide - 34
- Hydrogen to water - 120.5 ( assuming the water vapor is not allowed to condense)
- Sulphur to sulphur dioxide - 9.3

The main cause of heat loss with the process is that taken away by nitrogen. Therefore, to achieve maximum efficiency the excess air should be kept to a minimum. However there is a limit to the reduction in the excess air in that the combustion process must be fully completed within the furnace and within a finite time.
The main type of combustion process is called the suspended flame. The flame front remains in the same position relative to the burner and quarl.. The fuel particles pass through the flame completeing their combustion process and exiting at the same rate as the fuel entering.

Primary Flame-To burn oil the temperature must be raised to vaporisation temperature, this can not be done in heaters due to gassing but is done by radiant heat in the flame. The lighter hydrocarbons in the atomised spray are rapidly heated and burnt in the primary flame. The heavier fractions pass through this achieving their vaporisation temperature. The primary flame is essential to good combustion. By design the primary flame exists where it receives maximum reflected heat from the shape of the quarl. The size of the primary flame ( shown smaller than actual in drawing) just fills the quarl space. Too large and impingement leads to carbon deposits building up. Too small unheated secondary air reduces combustion efficiency. The tip plate creates vortices reducing the mixing time for the air/fuel and reduces the forward speed of the flame

Secondary Flame-Here the heavier fractions are burnt. The velocity of the air and fuel must be matched to the required flame propogation rate.

Combustion in furnace space

For proper combustion of fuel in the furnace and adequate supply of air must be supplied and intimately mixed with a supply of combustible material which has been presented in the correct condition.

Air- it is the purpose of the register, swirler vanes and (vortice) plates, and quarl to supply the correct quantity of air for efficient combustion suitably agitated to allow proper mixing.

- prevent thermal shocking
- improve the combustion process
- improve plant efficiency (bled steam and regenerative)

Fuel It is the purpose of the burner to present the fuel in suitable condition for proper combustion. Generally this means atomising the fuel and giving it some axial (for penetration) and angular (for mixing) velocity. For effective atomisation the viscosity of the fuel is critical, for fuels heavier than gas or diesel oils some degree of heating is required. It should be noted that the temperature of the fuel should not be allowed to raise too high as this can not only cause problem with fuel booster pumps but also can cause flame instability due to premature excessive gassification (is that a real word-answers to the normal address)
The smaller the droplet size the greater the surface areas/volume ratio is, this increases evaporation, heating and combustion rate.

Combustion zones

Register- supplies the correct quantity of excess air. Too little allows incomplete combustion, smoking, soot deposits and flame instability. Too much excess air reduces combustion efficiency by removing heat from the furnace space, may cause 'white' smoking and promote sulphurous deposits. In addition too much excess air increases the proportion of sulphur trioxide to dioxide promoting increase acid corrosion attack in the upper regions.
The register and to some extent the quarl determine the shape of the flame, short and fat for side fired boilers, long and thin for roof fired.

Flame burning off the tip- may occur after initial ignition or after a period of high excess air. The effect of this is to move the primary flame away from the quarl thereby effecting the combustion process leading to black smoke and flame instability. Two methods of bringing the flame back are to reduce excess air and introduce a hand ignitor to ignite the fuel correctly, or to rapidly close then open the register damper

Types

- Pressure jet
- Spill type pressure jet
- Variable orifice pressure jet
- Spinning cup
- Steam assisted
- Ultrasonic

Turndown ratio ratio of minimum to maximum flow ( roughly the square root of the ratio of maximum to minimum pressure)

Pressure jet

This is the simplest and oldest design of burner. Atomisation of the fuel is achieved by forcing the fuel under pressure through an orifice at the end of the burner, the pressure energy in the fuel is converted to velocity. Spin is given to the fuel prior to the orifice imparting centrigual force on the spray of fuel causing it to atomise.The disadvantage of this burner is its low 'Turn-Down' ratio (in the region of 3.5). The advantage is that it does not require any assistance other than supplying the fuel at the correct pressure. Due to this it is still seen even on larger plant were it is used as a first start or emergency burner.
Anouther disadvantage over assisted atomisation burners is the lack of cooling from stam or air means the burner must be removed when not in use from lit boilers to prevent carbonising in the tube