Below Are Some of the Boiler Questions We’ve Been Asked over the Years –
Question 1: What is a flame safeguard system?
Safeguard system is a set of controls used on a boiler to ensure safe burner operation. Primary functions include:
- A safe way of starting and shutting down the burner. This can be accomplished either automatically or manually.
- A flame safeguard system also starts the burner in the proper sequence. For example it will purge the combustion chamber of gas, light the pilot and then open the main gas valve.
- The flame safeguard system will also continually monitor burner operation when the boiler is on-line.
- The system will protect the boiler from excessive pressure or temperature conditions
- It will also regulate the firing rate according to the demand for heat or steam
- Finally, it will standby during down time, waiting for the signal to start the burner once again.
Question 2: What is meant by Stoichiometric Combustion?
Stoichiometric Combustion is the perfect combination of air and fuel that result in perfect combustion! Sounds good doesn’t it? Unfortunately, it is impossible to achieve in burners that you would commonly find on a boiler.
So why toss this term about? For the simple reason that it gives us a target in which we might compare our combustion conditions against. For example if we supply too little air, the burner will run “rich”. This means that not all the fuel was burned. Not only is this inefficient it also results in sooting that will decrease the heat transfer in the boiler.
Introduce too much air into the process and again, you reduce efficiencies. Not all the fuel is burned. This is why we strive for the perfect balance – Stoichiometric Combustion.
Want to know more about the calculations to determine how much air is required? Just e-mail us.
Question 3: Can you elaborate on just how much air is required to get a good complete combustion?
Thanks for the question. You are looking for that magical “Theoretical” air. The exact amount required to completely burn the fuel. The truth is that there has to be “excess air”. According to the John Zink Combustion Handbook, excess air is defined as “the amount of air needed by a burner which is in excess of the amount required for perfect or stoichiometric combustion. Some amount of excess air, depending on the available fuel/air mixing energy, is required to assure through mixing of the fuel and air for complete combustion.”
To calculate Excess Air, % use the following formula:
= (Air supplied – Theoretical air) x 100
The theoretical air required will vary depending on the fuel that is being fired. I found a table in the ASHRAE 1985 Fundamentals Handbook (pg. 15.8) that lists these values. For example Natural Gas requires 9.6 lb of cu. ft.air/cu. ft of fuel, Propane requires 24 cu.ft. of air/cu. ft. of fuel, No. 2 fuel oil requires 12.7 lb of air/gal of fuel.
As for a general rule of thumb…how about 0.9 cubic feet of air for 100 Btu of fuel.
Question 4. What does foaming mean?
Foaming is a condition that occurs in boilers when there are high concentrations of soluble salts, suspended solids or organic matter. These create foam in the steam space of your boiler that actually look like the foam on a good glass of beer! Now foam on your beer is acceptable, foam in your boiler is not.
When these little foam bubbles pop, they create a liquid that, in turn form slugs of water. Not only does your steam quality suffer, so may your entire system. Remember steam can reach velocities of over 80 miles an hour. Push a slug of water through at that speed and you can seriously damage turbine blades, piping systems and actuators.
Question 5. Please define turndown ratio and tell what the advantages are for a higher turn down. Are there any disadvantages?
A good friend of ours by the name of David A. Scearce addressed this very question in a study he did some years back. Here are some highlights of that report.
First a definition: Turndown ratio is the ratio of maximum fuel input rate to minimum fuel rate of a variable input burner. Traditionally burners on firetube boilers operate in the 5:1 turndown ratio range depending on fuel and size. High turndown burners are considered those with ratios of 10:1 or greater.
Advantages of high turndown burners include:
- Reduction of standby losses
- Limiting thermal cycling
- Saves wear and tear of burner components
- Initial cost and complexity
- Requires more maintenance
- Limitations of Boiler/burner system
The limitations include fuel/air mixing requirements, material temperature issues, flame shape characteristics, flow control limitations and pressure vessel limitations.
Question 6. 1. What is the general construction for furnace refractory and the common defects found.
2. Does the laying/installation of refractory bricks vary much from ” similar to laying pavers”? I’m interested in marine boilers in particular.
1. For boilers, refractory is generally a harden brick type material orcastable cement that is designed to handle extreme heat. At one time this refractory had asbestos as the main component, but this has changed. I have included a link below to the Refractories Institute who will have more detailed information on the components of refractory. Some of the things we look for when servicing refractory in a boiler is for loose or broken tile along with flaking or chipping of refractory surfaces. These we repair immediately.
2. Re-bricking a boiler may be a little more difficult especially in firetube (marine) designs. The reason for this difficulty is found in the circular furnace. Make sure you have the right replacement brick and you follow the manufacturers instructions. Depending upon the manufacturer you may also have an inner door and a rear door that will require pouring new refractory. Remember, if you are doing refractory work, make sure the old refractory does not contain any hazardous material and that you have all the closing gaskets needed in order to close the boiler when you are finished. Let me know if we can answer any other questions for you.
Question 7. I observed that some industrial plants having small package boilers do not practice deaerator system. What should be the boiler rating requirement to have deaerator.
Thank you for your question.
We suggest using a deaerator when:
1. Your boiler plant operates over 75 psig
2. Any boiler plant with limited standby capacity
3. Boiler plants that use 25% or more cold make-up water
4. Any boiler plant that relies on continuous boiler operation
Remember the deaerator is a preventative maintenance tool. It really should be used in every boiler application with the exception of a hot water heating system that uses absolutely no make-up water.
Hope that helps,
Question 8. I have a steam boiler and need to install a Boiler Feed system, how do I make sure that I have the right size tank?
A general rule of thumb is to have at least ten minutes worth of water available at all times. Here is what you need to know to figure this one out. The size of your boiler (bhp) the fact that one bhp = 34.5 lbs./hr. and that one gallon of water weighs about 8.337 lbs. Take all of these facts and plug them into the following calculation:
BHP (Boiler Horsepower) X 34.5 / 8.337 / 60 X 10 = gallons of water needed
So, if you have a 500 bhp boiler your formula will be as follows:
500 x 34.5/ 8.337 / 60 x 10 = 345 gallons But wait, you can’t operate a tank completely flooded so you will have to include a 1.5 safety factor. This will take you to about 517 gallons, so a 500 gallon tank will work.
Maybe an easier way is to plan for 1 gallon storage for every boiler horsepower.
500 bhp = 500 gallon tank
300 bhp = 300 gallon tank
But, be careful! Depending upon your particular process you may need a bigger tank. Especially if you are using your steam for process and large slugs of water are returned in an unpredictable manner! When in doubt call us.
Question 9. Is it legal to weld on a boiler burner? fire tube boiler 150 hp. The air deflectors have cracked and either need welding or replacing.
Boiler pressure vessels require an A.S.M.E. certified welder. The burner does not, but be careful. Changes in the burner cause changes in performance and combustion. Safety of the weld is also a concern. Your safest bet would be to replace the cracked part. Any time work is done on a burner the combustion and safety controls should be checked as soon as possible after completion of the work.
If you provide us with your burner/boiler information we can help you locate what you need. Also, a digital picture is great when trying to figure out what part is actually need. I am betting what you are calling air deflectors is actually the burner’s diffuser.
We provide complete burner/boiler service, parts and training in and (far) around the Pittsburgh area, if you ever need help just give us a call. 412-257-8866
Very Best Regards,
Question 10. My question is, if my boiler has very high conductivity and or Alkalinity, can the boiler be safely blown down while it is under load? (Bottom blowdown).
Continuous surface blow down should be used to control boiler conductivity. The surface of the boiler water will have the highest concentration of dissolved solids along with any oil. The continuous surface blow down will be a very small flow that gets tweaked daily based on your chemical sample results. You did not mention the size of your boiler but a 1/2″ throttle valve is usually sufficient and a 3/8″ throttle valve will probably work also.
The bottom blow down is done at least daily (more often depending on boiler water conditions) to blow out scale and mud from the bottom of the boiler. If possible you should lower the firing rate of your boiler when you do the bottom blow down then return to normal firing rate after the blow down.
Question 11. Our 1,200,000 lb/hr steam boiler normally operates at around 3% excess O2
or 15% excess Air. If it operated at 3% excess O2 what would be the approximate % boiler efficiency loss?
The term Boiler efficiency is a generic term that could mean many things. The best definition of your boiler’s efficiency is the fuel-to-steam efficiency. After all you are paying for the fuel to produce the steam!
There are two pieces of information that you will need. The first is the stack temperature. Simply put, the lower the stack temperature the more heat you are transferring within the boiler. The second bit of information you will need is the amount of CO2 in the flue gas. High CO2 readings with no CO and very little O2 throughout your entire firing range indicate good burner control.
You are going to need some simple equipment to check your fuel-to-steam efficiency. This includes a flue gas analyzer, a stack thermometer, a room thermometer, charts for heat loss in the stack for the fuels you are firing and a correction chart for radiation and convection losses. (ask your boiler supplier for these charts if you do not have them)
Here is an example:
Stack Temperature = 340 degree F.
Room Temperature = 80 degree F.
Gas analyzer reports CO2 = 10% and CO = 0
Subtract room temp from flue gas temp. 340 – 80 = 260 deg. F.
From the Hays Chart, at 260 deg. F. and 10% CO2 you find a stack loss of 15.6% exiting the stack.
Add for radiation and convection losses (available from your boiler manufacturer). We will assume 1% in this example. So 15.6% = 1% = 16.6%
Now subtract that from 100% efficiency
100% – 16.6% = 83.4% Fuel-to-Steam efficiency.
Question 12. How do you calculate boiler turndown. Is there a formula?
Boiler Turndown is a comparison of the Maximum Input of the boiler compared to the Minimum Input that the boiler will properly operate.
To calculate your actual Turndown you need to know what your actual input is at Maximum and Minimum firing rates. The easiest way to do this is meter the fuel usage be it either a flow meter on the fuel oil supply or a gas meter on the natural gas supply line.
Lets do an example:
Assume that at Maximum Input we clocked the Natural Gas meter and found that the meter clocked 1000 cubic feet (pressure corrected) per hour.
Note: You do not need to have the boiler at Maximum Fire for and hour. You can take the reading over a few minutes and correct to one hour.
So, 1000 cubic feet/hour X 1050 btu/ cubic foot = 1,050,000 btu/hr at maximum input
Minimum Input meter clocked is 100 cubic feet/hour
100 cubic feet/hour X 1050 btu/cubic boot = 105000 btu/hr at minimum input
1,050,000 = 10 giving us a 10 to 1 turndown in this case.
My assumptions: Natural Gas but value = 1050 btu/cubic foot
Question 13. Why are manholes elliptical in shape ( advantages ) ?
Manways on boilers are elliptical because the cross section of a man is elliptical; well most men are. Most manways these days are 12″ X 16″. A 12″ X 16″ opening is significantly smaller than a 16″ round hole. A smaller opening means smaller surface area of the backing plate so the smaller plate will have less over all force acting upon it.
Question 14. Can you tell me the proper way to install flexitaulic hand hole and manhole cover gaskets? Thank you ! Brooks Young. I am working on Johnston firetube boilers 165 psi 22k steam per hr.
1. Make sure all seating surfaces are clean, this usually requires wire brushing.
2. Make sure threads on the nuts and bolts/studs are clean and nuts turn freely over the length of the threads.
3. The tricky part is to make sure the gasket is centered on the backing plate and the backing plate is centered in the manhole or handhole. Tighten (evenly where there is more than one bolt/stud involved) making sure the backing plate and gasket stay centered. Once you are sure everything is properly in place and snugged torque to the proper torque spec. for the size of the bolt/stud and materials that you are using or manufacturers recommendations if you have them.
Note: NEVER EVER TRY TO TIGHTEN A MANWAY OR HANDHOLE WHILE THE BOILER IS HOT AND PRESSURIZED! THE MANWAY OR HANDHOLE COULD MOVE CAUSING YOU TO BE SPRAYED WITH HOT WATER AND/OR STEAM!!!!!!!!
Hope this helps!
Question 15: Air-Cured Time for Refractory Replacement.
After major refractory replacement: Castable must be air-cured for a minimum of 24 hours. Some moisture will remain, so firing will have to be done at “low fire” rates on an intermittent basis so as to hold the stack temperature (as shown on the stack thermometer) to not over 150 F – that is to say, below the boiling point of water, since the generation of steam within the refractory material would destroy the strong bond, cause spalling, flaking and cracking.
After initial firing of a repaired read door, the door metal will become exceptionally warm or hot. This is due to water conduction of heat. This condition will exist until such time as all moisture has been driven out of the refractory.
The following heat curing schedule is recommended:
Operate burner for brief intervals at low fire, bringing the reading of the stack thermometer up to 150 F and hold this for six hours. The raise the stack temperature at a rate of 50 F per hour until normal operating temperature is reached and hold this for another 6 hours. The boiler may now be operated in the normal manner or shut down if desired.
The securing bolts around the heads should be tightened periodically as the temperature is equalized and when the boiler is at high fire. The tightness of the sight glass retainer should be checked and the retainer and sight glass should be examined for gas leakage. The air line from the front head of the boiler should be checked for possible blockage.
Question 16. What are the disadvantages of boilers being cycled frequently? I saw a system of 3 boilers which are cycled every 10 minutes. Running a single boiler would meet their heating requirements. Is it advisable to cycle boilers so frequently or is it better to turn down the other 2 boilers and operate only one?
Situations that require an absolute continuation of heat without any interruption even for a short period of time requires the additional boilers to be fired frequently so they remain hot and ready to go. A cold boiler takes time to properly warm-up and be placed in service.
If a loss of heat for 1/2 hour to 1 hours would not cause any problems then running only one boiler is the preferred method. In your case I would run only one boiler at a time. Have one boiler in standby fired every few hours just to keep warm and the last boiler completely off and isolated.
Now, everyone has their own opinion. The above is based on my experiences in the boiler field. Boilers that are turned on and off are heated/cooled/heated/cooled, this can shorten the life of the boiler refractory, cause condensation of flue gas, shorten motor starter contact life among other things. A boiler will last the longest with the least failures if its kept hot and cycled off as few times as possible.
Question 17. I am having problems with chemical carry over and the site glasses are blowing out. This is a laundry operation that is running Monday through Friday in the am only. The site glass assembly is McDonald Miller. Can you explain what can cause this problem ?
The most likely cause for your chemical carryover is that your Total Dissolved Solids are too high (Boiler water Conductivity is another way to express TDS). TDS is controlled by your surface blow down and should be controlled to the recommendations of your chemical supplier (normally 4000 micro-mho plus/minus 400).
Another possibility for chemical carryover is that you are dropping too big of a load too fast onto the boiler which drops boiler pressure suddenly causing the boiler to suddenly boil excessively and carrying over into the steam line. This is a bit harder to fix but can be done.
Now, the gauge glass problem! I am assuming you have the hollow tubular 5/8″ in gauge glass. These never hold up well on a high pressure boiler. They tend to erode from the top and start leaking steam. As long as you stay with the 5/8″ tubular gauge glass you will have this problem. To eliminate this problem you need to change your gauge glass to a prismatic type gauge glass that uses a thick flat glass that is sandwiched into a strong metal housing. It will not fail! Our Parts Department will be happy to quote a prismatic gauge glass assembly to you. We will need the Center to Center dimension and pipe size of your gauge glass connections. We need to know the MAWP of your boiler also.