Lecture8-AdditionalNotes

8/9/2019 Lecture8-AdditionalNotes

1/5

Page 1of 5Lecture 8 Additional Notes

Lecture 8: Steam Generators Additional Notes

Boiler Fittings and Mountings

An overview of the necessary ttings, accessories and controls for a boiler

from nameplates and safety valves to gauge glasses and level controls.

A number of items must be tted to steam boilers, all with the obective of

improving operation, e!ciency and safety.

Boiler name-plate

"n the latter half of the #$th century e%plosions of

steam boilers were commonplace. As a conse&uence of

this, a company was formed in 'anchester with the

obective of reducing the number of e%plosions by

subecting steam boilers to independent e%amination.

After a comparatively short period, only eight out of the

## ((( boilers e%amined e%ploded. )his compared to

*+( steam boiler e%plosions in boilers not e%amined by the scheme. )his

success led to the oiler -%plosions Act #88*/ which included a re&uirement

for a boiler name0plate.

Safet !al!es

An important boiler tting is the safety valve. "ts function is to

protect the boiler shell from over pressure and subse&uent

e%plosion. 'any di1erent types of safety valves are tted to

steam boiler plant, but generally they must all meet the following

criteria2

)he ma%imum set pressure of the safety valve shall be the design or

ma%imum permissible wor3ing pressure/ of the

boiler.

)here must be an ade&uate margin between the normal operating

pressure of the boiler and the set pressure of the safety valve.

)he discharge pipewor3 from the safety valve must be unobstructed and

drained at the base to prevent the accumulation of condensate. "t is good

practice to ensure that the discharge pipewor3 is 3ept as short as possible

with the minimum number of bends, so that the allowable bac3pressure

Fig" 8"# Boiler

Fig" 8"$ Boiler


2/5

Page %of 5Lecture 8 Additional Notes

indicated by the valve manufacturer is not e%ceeded. "t will be &uite normal

for the internal diameter of the discharge pipewor3 to be more than the

internal diameter of the safety valve outlet connection, but under no

circumstances should it be less.

Boiler stop !al!es

A steam boiler must be tted with a stop valve also 3nown

as a crown valve/ which isolates the steam boiler and its

pressure from the process or plant. "t is generally an angle

pattern globe valve of the screw0down variety. 4igure 5.6.5

shows a typical stop valve of this type. 'any boilerma3ers

use 7 spheroidal graphite/ iron valves as standard.

)he stop valve is not designed as a throttling valve, and should be fully open

or closed. "t should always be opened slowly to prevent any sudden rise in

downstream pressure and associated waterhammer, and to help restrict thefall in boiler pressure and any possible associated priming.

Feed&ater c'ec( !al!es

)he feedwater chec3 valve as shown in 4igures 5.6.9

and 5.6.:/ is installed in the boiler feedwater line

between the feedpump and boiler. A boiler feed stop

valve is tted at the boiler shell.

)he chec3 valve includes a spring e&uivalent to thehead of water in the elevated feedtan3 when there is no

pressure in the boiler. )his prevents the boiler being

;ooded by the static head from the boiler feedtan3.


3/5

Page #of 5Lecture 8 Additional Notes

,S control

)his controls the amount of )otal >issolved 7olids

)>7/ in the boiler water, and is sometimes also

referred to as ?continuous blowdown?. )he )>7 in a

sample of boiler water is compared with a set point@ if the )>7 level is too high, a &uantity of boiler water is

released to be replaced by feedwater with a much

lower )>7 level. )his has the e1ect of diluting the

water in the boiler, and reducing the )>7 level. n a

manually controlled )>7 system, the boiler water

would be sampled every shift.

A typical automatic )>7 control system is shown in

4igure 8.8

.ressure gauge

All boilers must be tted with at least one pressure

indicator. "t should be mar3ed to indicate the normal

wor3ing pressure and the ma%imum permissible wor3ing pressure B design

pressure. Pressure gauges are connected to the steam space of the boiler and

usually have a ring type siphon tube which lls with

condensed steam and protects the dial mechanism

from high temperatures.

Pressure gauges may be tted to other pressure containers such as

blowdown vessels, and will usually have smaller dials as shown in 4igure5.6.$.

Gauge glasses and /ttings

All steam boilers are tted with at least one water

level indicator, but those with a rating of #(( 3= or

more should be tted with two indicators. A gauge

glass shows the current level of water in the boiler,

regardless of the boiler?s operating conditions.

auge glasses should be installed so that their

lowest reading will show the water level at :( mm above the point whereoverheating will occur. )hey should also be tted with a protector around

them, but this should not hinder visibility of the water level. 4igure 8.#(

shows a typical gauge glass.

auge glasses are prone to damage from a number of sources, such as

Fig" 8"8 ,pical

automatic ,S

Fig" 8"0 ,pical

pressure gauge


4/5

Page $of 5Lecture 8 Additional Notes

corrosion from the chemicals in boiler water, and erosion during blowdown,

particularly at the steam end.

ater le!el controls

)he maintenance of the correct water level in a steam boiler is essential to its

safe and e!cient operation. )he methods of

sensing the water level, and the subse&uent

control of water level is a comple% topic that is covered by a number of

regulations.

23ternal le!el control c'am4er

)he function of the level controls or alarms is chec3ed daily using the

se&uencing purge valves.

Fig" #"+"1#

nternall mounted le!el controls

Air vents and vacuum brea3ers

=hen a boiler is started from cold, the steam space is full of air. )his air has

no heat value, and will adversely a1ect steam plant performance due to its

e1ect of blan3eting heat e%change surfaces. )he air can also give rise to

corrosion in the condensate system, if not removed ade&uately.

)he air may be purged from the steam space using a simple coc3@ normally

this would be left open until a pressure of about (.: bar is showing on the

pressure gauge. An alternative to the coc3 is a balanced pressure air vent

which not only relieves the boiler operator of the tas3 of manually purging air

Fig" 8"16 Gauge glass


5/5

Page 5of 5Lecture 8 Additional Notes

and hence ensures that it is actually done/, it is also much more accurate

and will vent gases which may accumulate in the boiler. )ypical air vents are

shown in 4igure 5.6.#9.

=hen a boiler is ta3en o10line, the steam in the steam space condenses and

leaves a vacuum. )his vacuum causes pressure to be e%erted on the boilerfrom the outside, and can result in boiler inspection doors lea3ing, damage to

the boiler ;at plates and the danger of overlling a shutdown boiler. )o avoid

this, a vacuum brea3er see 4igure 5.6.#9/ is re&uired on the boiler shell.

Fig" #"+"1$

,pical air !ents and !acuum 4rea(ers

Documents

Lecture8-AdditionalNotes