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
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.
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 formula
C x A x P = 9.81 x H x E
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
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.
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
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
Improved high lift
No waste piston
No floating ring
For superheated steam the aggregate area through the seating of the valves is increased, the formula is
As = A(1 + Ts/555)
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 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 Bore lift 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
This is fitted to safety valves to allow manual operation of the valve in an emergency.
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
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
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
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.