Question: Boiler manholes
Why are boiler manholes elliptical in shape?
Answer:
The boiler manhole-lid is mounted from the inside of the boiler so that the
boiler pressure will help to keep it tight. Hence the manhole has to be
elliptical in shape to make it possible to take the lid out from the boiler.
Questions and Answers
Question: Boiler pressure
In tanker vessels we use boilers for cargo heating.
Lets say that steam pressure to deck line adjusted to 5 bar.
What will be most economical? Boiler pressure adjusted to 7 bar or to 16 bar ?
Answer:
The higher pressure would be advantageous if the boiler is oil-fired,
but the difference is hardly measurable.
If you have a diesel engine exhaust gas economizer then 7 bar boiler pressure
would gain more heat from the exhaust gases.
Questions and Answers
Question: Cavitation
in a two flow line
We have a steam genereting facilty using process heat. The condenser is a
vertical condenser with process vapours on tube side and boiler feed water on
shell side. The outlet of the shell side is a two phase flow. Pipe thickness
reduction of the shell side outlet pipe is observed for last couple of years.
We replace the piece of pipe every one year. I want to know the probable
reasons for this to happen.
Answer:
I guess it might be a cavitation problem. Calculate the flow rate. Decreasing
the speed of the flow might be a solution i.e., increase the pipe diameter
Questions and Answers
Question:
Boiler drum level control goes haywire
We are having 100 tones FBC boilers. Frequent problem observed is that whenever
there is sudden load change the boiler drum level control goes haywire leading
to tripping of boiler and turbine on drum level low or high. Our drum level
control is 3-element control in auto mode.
Answer:
The most common fault with a three-point level controller is the steam flow
transmitter.
Loosen the impulse pipes and cleanse the holes into the measuring orifice.
Questions and Answers
Question: Firing light crude
oil
Can a Main Boiler built to fire 380 cst HFO, be fired with light crude oil
directly from the cargo-tanks?
Answer:
A completely new fuel system is required, from deck to the burner rails of the
boiler. To prevent any possibility of gasses leaking from flanges, there have
to be ducting enclosing the entire fuel-system with forced draft fans that vent
30 times the volume of the trunking to the outside. Also there have to be a
burner hood to be constructed all over the burner roof, equally vented.
Naturally there have to be new burners and so is the burner management.
There are two main contractors who are capable and willing to carry this out:
HAMWORTHY-Combustion Engineering in UK and SAACKE in Germany.
Questions and Answers
Question: Seawater in the
boiler
If the boiler had to be operated with seawater what would be the result.
Answer:
The salinity will rise rapidly since the salt remains in the boiler while the
water boils off. Salt will son precipitate and accumulate on the bottom and
also on the heating surface where it, just as boiler-scales, inhibit the heat
transmission to the water and causes the metal to overheat and in worst case
burst. You may also get foam in the boiler that will cause difficulties to
maintain the water level and water droplets might follow with the steam,
causing problems with turbines and engines.
It is very dangerous to operate a boiler with salt in it, and you have to
control the salt concentration by frequently blowing off from the bottom of the
boiler and form the water surface to keep the salinity below 9.5% (boilermakers
and classification societies may recommend other values).
It would also be a good measure to reduce the capacity of the boiler.
After this emergency operation it would be wise to open up the boiler for
inspection since seawater further accumulation of scales.
In the old days some ships sailing on lakes used the lakes water as make up
water for their boilers, but even that water caused problem with salt in the
boilers although it is supposed to be fresh-water.
Questions and Answers
Question: Combustion air
preheating
Please tell me how air inlet temperature affects boiler efficiency. What are
the benefits of air preheating?
Answer:
The combustion air will be heated to the flame temperature. This heating cost
money. If you have some waste heat to be used for preheating the combustion air
it will pay.
Questions and Answers
Question: Water in the
heavy fuel oil
Is it possible to overheat heavy fuel oil thus causing any water in it to turn
to steam and cause problems at the pump and burner?
Answer:
Yes it is. The temperature of the heavy fuel oil is very often 130°C to
150°C and water introduced to that temperature would immediately evaporate
into steam; well it depends on the pressure too. When water is boiling it
expands about 1600 times. The situation might be dangerous since the safety
valves are not designed for steam.
This kind of problem is very likely to occur when you change fuel oil tank and
some water from a poorly drained pipe mixes with the heavy fuel oil.
Questions and Answers
Question:
Oil showing in the water level gauge glass
Whilst on your engine room rounds, you discover oil showing in the water level
gauge glass of an auxiliary boiler. Describe the remedial actions you would
take, explaining why such actions must not be delayed.
Answer:
Stop the burner immediately.
Oil present, even small quantities, in boiler water will cause foaming and
moisture carry-over. It also forms a heat insulating film, sometimes a
carbonized layer, over tubes or shell surfaces. Even a very thin layer may
result in tube or plate material failure due to overheating.
The oil manifests itself by forming an oily ring inside the water gauge
glasses, at the water level.
Questions and Answers
Question: Emergency
low boiler water level
You are an officer on watch, & finds the boiler water-level gauge glass to
be empty & the burner firing...What is your course of action? (Assuming the
gauge glass to be clear & good working order)
Answer:
Normally a boiler is provided with two independent sensors for emergency low
water level burner cut-outs. So this would never happen. However, if it does,
don’t take any chances! Shut off the burners immediately!
Before you start raising the level in the boiler you have to find out if any
part of the furnace walls has been overheated. If you raise the level over a
glowing steel-wall then the boiler might produce more steam than the safety
valves can handle and a nasty explosion would be the result.
Questions and Answers
Question:
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?
Answer:
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.
Questions and Answers
Question: Fluctuating
boiler water level
The feed water control valve is fully open and the water levels fluctuate at
normal boiler load.
Answer:
Check if:
- the control valve really is fully open by means of the hand-manoeuvre
device.
- 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
>=30 psi)
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
control loops.
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.
Questions and Answers
Question: 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
electric motor.
Answer:
- 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
- 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.
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.
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
considerably. Questions and Answers
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