Question: Boiler manholes
Why are boiler manholes elliptical in shape?
The boiler manhole-lids are mounted from the inside of the boiler so that the
boiler pressure will help to keep them tight. Hence the manholes have to be
elliptical in shape to make it possible to take the lid out from the boiler.
Differential pressure transmitters for the steam drum level
Way is the high pressure leg of the transmitter connected to the water side and
the low-pressure leg connected to the steam side?
The signal from the transmitter ought to increase when the water level raises
and decreases when the level falls. Furthermore the signal shall be zero, and
give impulse to stop the burner, in case of transmitter malfunction, power
failure or cable breakdown.
Both requirements will be fulfilled if the transmitter is mounted with the
high pressure measuring point connected below the lowest water level and the
low pressure measuring point connected above the highest water level. The
output will increase when the level is raised. To compensate for the water
column in the reference leg the output signal's zero-point has to be elevated.
This is the common method.
If the transmitter is swapped, with the low pressure side to lower end and
the high pressure side to upper end, then the signal will decrease when the
level is raised.
This signal can be used to control the level as well, but the signal can not be
used to stop the burner for emergency low level in case of power failure or
cable breakdown. This system requires an extra sensor to trip burner at
emergency low water level.
One can of course use the emergency high water level alarm to stop the burner,
but this is not correct. The emergency high water level shall stop the feed
water pump and whenever applicable stop the steam turbine, but not the burner.
Fluctuating boiler water level
The feed water control valve is fully open and the water levels fluctuate at
normal boiler load.
- the control valve really is fully open by means of the hand-manoeuvre
- all stop valves in the line are fully open.
- the suction filter to the feed water pump is satisfactory clean.
- the feed water pump discharge pressure is sufficient.
- the feed water control valve pressure drop is normal. (>=2 bar or
If all these are fund to be in order, then you should recalculate the
control valves KV-value (CV-value). Under-sized control valves do exist, even
though over-sized control valves more often cause malfunction of automatic
The tuning of the controller(s) is dependent on the system you have, but do not
even try to tune the control loops until the above criteria are fulfilled.
The burner starts and stops very often
The burner starts and stops very often, sometimes every second minute. An
alarming temperature-raise has been observed in the combustion air fans
- Increasing the burners turn down ratio would be a nice solution, but it's
not always possibly.
- Run the burner in minimum load, i.e., prevent the burner from increasing
the load just after the burner start.
- Install a five to ten minutes' time-delay in the fan-motor stop function.
Then the fan will continue to run during the shortest burner stops and the
combustion air fan motor will get a little rest from the start current.
Answer: by Mr. Axel Friese
The effect will be that the burner stops all the time only in minimum load
(load is reduced when it comes above the control set point). When the burner is
required to start again, the increased starting pressure set point works out
very nicely because it compensates the drop in boiler pressure during purging
and lighting up time.
- Increase the "burner stop" set point as much as possible in
relation to other switchpoints or setpoints in the controls
- Raise the "burner start" set point abt. 0.2-0.3 bar above(!) the
control set point.
When the burner finally ignites, the actual boiler pressure is only very
slightly below the set point. Modern PID controllers notice this fact and keep
the burners in low load until the "stop" setpoint is reached again.
If the "burner start" set point is kept in the traditional way (some
0.5 bar below control set point), the PID controller notices a high deviation
between set point and actual value and thus increases the burner load
unnecessarily. The burner will fire up and will not be able to adjust itself to
the actual (low) steam demand. This results in overfiring and very soon the
"burner stop" set point will be reached.
Resetting the parameters to the increased "burner start" and
"burner stop" values will reduce the number of starts and stops
likely source of errors
In which part of a boiler control system is it most likely to get a failure.
When you have problem with a boiler control system you should keep in mind that
most faults occur outside the control cubicle, but on the other hand, your
problem might not be among the most common.
| Statistically calculated faults in control
|Transmitters and sensors
|Loss of electric power
Question: Open steam
Why has a steam valve, or at least a big steam valve, to be opened slowly?
If you have a one liter of water standing in the pipe just after the valve and
open the valve too fast, then you will get a projectile of one kg rushing down
the pipe. At next valve, bend or other obstacle the speed of the water mass
will be converted into pressure. You can hardly imagine the damage this energy
Thermal stress is an other reason to be very cautious and drain out water
carefully when you open a steam valve.
A large steam valve ought to have a small by-pass valve to simplify preheating
of the pipe.
Low viscosity fuel for high viscosity burners
Our boiler plant is designed to be fired with heavy fuel oil of 600 cSt @
Would it be safe to run it on Gas Oil of 7 cSt @ 40°C?
Well, this is a question for the burner maker. Don't forget to ask them
It is outmost important that the fuel oil heaters are by-passed when firing Gas
Oil or Diesel Oil. These fuels must never be heated above their flash-point.
- For a steam atomized burner you have to switch over to compressed air
atomizing since light fuels must not be atomized by means of steam. This burner
will probably fire the fuel without any problem since they normally are
designed for fuel viscosity between 10 and 20 cSt.
- A rotary cup burner, on the other hand, might give some problem with the
flame since this atomizing method normally require a fuel viscosity of about 25
to 60 cSt.
In the event of failed ignition, gas-oil is more likely to cause an
explosive mixture than heavy oil. However, a dangerous situation should never
occur if the procedure for such failed ignition is followed. Upon two
consecutive failed ignition attempts, a thorough investigation into the cause
should be sought and the furnace correctly purged inline with the manufacturers
and ruling classification societys requirements.
N.B. 7 cSt @ 40°C = 5.5 cSt @ 50°C.
The viscositys of lighter fuels are normally given at 40°C and the
viscosity of heavier fuels at 50°C, 80°C, 100°C or even higher
For conversion of viscosity and density at one temperature to viscosity and
density at an other temperature I recommend The FUEL OIL Calculation Program.
Oil Ignition Quality, CCAI
What is the CCAI of a fuel oil?
The CCAI, the Calculated Carbon Aromaticity Index, is a measure of the Fuel
Oils Ignition Quality.
The value can be calculated using the following, ISO 8217 Ann.C, formula:
CCAI = D-81-141 Log10 Log10( VK+0. 85)-483 Log10((T+273)/323)
D = density (kg/m³) at 15°C
VK = kinematic viscosity (mm²/s) at temperature T (°C)
The CCAI-calculation can easily be carried out by means of
The FUEL OIL Calculation
Steam valves open causing a sudden large load
I have two 82,000 lb/hr natural gas fired boiler that is designed for 300 psig
and operating at 205 psig (saturated). The boilers serve a large campus with
numerous buildings, each with an integrated building management control system.
Due to an unresolvable characteristic of the building control system,
occasionally all of the building steam valves open causing a sudden large load
on the boilers that lasts for 20 to 30 minutes. The demand for steam is not
real in that no heat is actually required by the buildings. When this condition
occurs we have a serious water carryover problem. My question is how can we
maintain boiler pressure and water level while either ignoring or controlling
the sudden false load. Our combustion control system is a PLC based system,
metered/cross limited air-fuel ratio , three element drum level and oxygen
Shall we try to correct thorugh the control algorithm or add backpressure
control valves. I look forward to any advise.
The steam capacity doesnt seem to be sufficient to supply all the fully
open control valves. First of all recalculate the control valves. Over-sized
control valves are very common cause of problems.
Level moves up and down in the sight glass
I have a 500hp cleaver brooks steam boiler when the boiler is on either med or
high fire the water level moves up and down in the sight glass and causes low
water trips. We have 2 of these connected to the same system and 1 boiler does
this and the other does not. Both are hooked up to an automatic skimming
system. Any thoughts?
Foam in the boiler? Check the boiler water and the skimming system.
Obstruction in the feed water sypply? In that case the feedwater control valve
would be fully open.
Question: Composite Boilers
What is the inherent problem in Composite Boiler?
There are different types of composite boilers. Normally one part of the boiler
is heated by means of a fuel oil burner and the other part is heated by the
exhaust gases from a diesel engine.
- Heating of one part of a boiler at the time often causes thermal stress
that may lead to leakage.
- One single composite boiler does not fulfill normal requirement of
redundancy when the steam is used for essential service purpose.
Running out of boiler water chemicals
How to operate auxiliary boiler onboard ship for at least 10 days period when
all boiler chemicals are finished.
Pay attention to the measured values of the boiler water quality, when/if the
salt concentration increases then blow off from the bottom of the boiler more
If you use make-up water from an evaporator it would not be too alarming. Using
tap-water as make-up water would require more attention.
Also read about seawater in the
Is it possible to transport superheated steam of the order of 45 t/h at 30 bar
pressure and temp of 300 deg from a aux boiler to a distance of 1.5 Km?
Calculate with a velocity of 15 m/s (49 ft/s). To avoid water hammering the
pipe-line should slop slitely downwards in the steam flowing direction. To
start up the line you will need a drain valve on every 30 m (100 ft).
Question: Light oil
Is it possible to replace atomizing media ( Compressed air in present
condition) to steam for light diesel oil burner?
No it is NOT possible. The light oil must not be heated higher than its