A to Z about boilers

Boiler Mountings

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. 

Boiler Water level Control

A regulated supply of water must exist for a boiler generating steam. With the system in balance without leaks the flow of water to the boiler will be equal to the flow of steam. When there is a change in steam demand then the flow of water will have to be changed.

One Element

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The water level is measured using a suitable device. This level is sent to the controller as a measured value. The controller compares this measured value against a set point. Any deviation from this set point caused a demand signal to be generated.

Two Element

A single element controller is considered suitable for only the smallest of boilers. The reason for this is the effect of stepped load changes

Should there be a sudden demand for increase steam flow there will be a requirement to increase the firing rate. The increased firing rate increases the ebullition or bubble formation in the water and the volume of water increases or swells. This will be seen as an immediate increase in the water level.

For a single term controller the tendency will be to close the feed supply valve. However, the increased firing rate will generate more steam flow from the boiler and in reality an increase in water flow would be required

For a stepped load increase there will be tendency for the water level to rise due to swell as described. The Water level controller will give a reduced demand signal. The steam flow transmitter will detect an increase in steam flow and send an increase demand signal via a conditioner which adjusts this signal to suit the system. These two signals pass through a Summator. The resultant effect will prevent any immediate sharp change in the demand signal to the feed flow control valve.

Three Element

Variations in feed pressure will vary the true feed flow rate to valve demand signal. For improved control an additional feed water flow element may be added. This is the case only when the cost and complexity can be justified due to improved plant efficiency and critical water level control.

Uptake Fires

On break out of an uptake fire the priority is to boundary cool to contain the fire and give cooling effect.

Modern ship

An uptake fire generally starts when the load on the boiler is reduced. This is due to the quantity of excess air being very low at high loads.

Should a fire break out then the possibility of speeding up and reducing the excess air should be considered.

The amount of feed heating should be reduced to lower the inlet feed temperature and aid with cooling parts.

Where the possibility exists of damage to the superheater, then after first relieving pressure, it should be flooded.

Older ship

Where the excess air on older boilers is high even at high loads a different plan of attack must be used.

The flames should be extinguished and the air shut off. The amount of feed heating should be reduced.

The safeties should be lifted to keep a high steam flow and hence high feed flow requirements. ( the boiler is now being fired by the uptake fire )

Lifting the safeties give the added advantage of reducing the boiler pressure and hence corresponding saturation temperature of the water aiding the cooling effect

Tackling the fire

If a direct attack should be made on hot non-pressurised parts then the nozzle should be set to solid jet and aimed at the seat of the fire.

This should not be carried out on hot pressurised parts due to the risk of a steam explosion.

Dry powder is a suitable extinguishing medium.

Disassociation

Under certain conditions an extremely destructive fire, commonly known as a hydrogen or 'rusting' fire, may occur Under high temperatures water will tend to disassociate to hydrogen and oxygen. The percentage amount increases with increased temperature These will recombust again liberating heat In a fire there is a danger that the use of superheated steam as an extinguishing agent (say sootblowers on an air heater fire) could in fact feed the fire and accelerate the growth. For example the displacement which occurs about 707^oC

Heat + Hot 3Fe + 12H₂O Ћ 3FeO₃ + 12 H₂

see Theory section for a more complete explanation

Tackling this type of fire is very hazardous and consists mainly of boundary cooling and shutting off water and air supplies as effectively as possible.Under no circumstances should steam smothering be considered.

A typical scenario for this fire is a badly cleaned uptake igniting leading to tube failure