Lars Josefsson | Boiler Questions and Answers

Boiler Questions and Answers

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Boiler drum level control goes haywire
Boiler manholes
Boiler peak conditions
Boiler pressure

Burner starts and stops very often
Cavitation in a two flow line
Composite Boilers
Combustion air preheating

Differential pressure transmitters for the steam drum level
Emergency low boiler water level
Fluctuating boiler water level
Firing light crude oil

Fuel Oil Ignition Quality, CCAI
Light oil atomizing
Low viscosity fuel for high viscosity burners
Level moves up and down in the sight glass

Most likely source of errors
Oil showing in the water level gauge glass
Open steam valves slowly
Running out of boiler water chemicals

Seawater in the boiler
Steam valves open causing a sudden large load
Transport superheated steam
Water in the heavy fuel oil
 

Question:  Boiler peak conditions
What will be the probable reasons, when the boiler load can not be increased to peak conditions?

Answer:
Soot on the heating surfaces. Even a thin layer of soot will reduce the boiler efficiency. Not the right fuel for the burner. For instance, diesel oil to a rotary cup burner wouldn't do. Too low feed water temperature.


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.


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


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.


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.


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.


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.


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 steamships 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.


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.


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.


Question:   Boiler manholes
Why are boiler manholes elliptical in shape?

Answer:
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.


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.


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.


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

  1. Increase the "burner stop" set point as much as possible in relation to other switchpoints or setpoints in the controls
  2. 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.

Question:   Most likely source of errors
In which part of a boiler control system is it most likely to get a failure.

Answer:
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 systems.
Transmitters and sensors 40 %
Actuators 25 %
Controllers 10 %
Loss of electric power 5 %
Others 20 %

Question:   Open steam valves slowly
Why has a steam valve, or at least a big steam valve, to be opened slowly?

Answer:
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 can cause.
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.


Question:   Low viscosity fuel for high viscosity burners
Our boiler plant is designed to be fired with heavy fuel oil of 600 cSt @ 50°C.
Would it be safe to run it on Gas Oil of 7 cSt @ 40°C?

Answer:
Well, this is a question for the burner maker. Don't forget to ask them too.
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 society's requirements.

N.B.   7 cSt @ 40°C  =  5.5 cSt @ 50°C.
The viscosity's 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 temperature.
For conversion of viscosity and density at one temperature to viscosity and density at an other temperature I recommend  The FUEL OIL Calculation Program.


Question:   Fuel Oil Ignition Quality, CCAI
What is the CCAI of a fuel oil?

Answer:
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)
where
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 Program.


Question:   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 trim.
Shall we try to correct thorugh the control algorithm or add backpressure control valves. I look forward to any advise.

Answer:
The steam capacity doesn't 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.


Question:   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?

Answer:
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?

Answer:
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.

Question:   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.

Answer:
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 frequently.
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 boiler.


Question:   Transport superheated steam
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?

Answer:
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 atimizing
Is it possible to replace atomizing media (Compressed air in present condition) to steam for light diesel oil burner?

Answer:
No it is NOT possible. The light oil must not be heated higher than its flashpoint.