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Mike Conley

Combustion – Part 1

By | Combustion

PART 1 OF 3

Greetings and Welcome back to our blog series!

In keeping with world events this past year, we would like to present our version of volatility better known as the process of combustion in conventional boilers. The goal is to take you on a short journey through the basic concept of combustion including its process, application, effects, fuels, along with sequence of operation, checking (testing) & setting combustion as related to commercial & industrial boilers. So let’s fire it up!

We will first review basic process as we begin to better understand safe-reliable burner combustion in a boiler. Chances are you benefit from the combustion process in your daily life, driving an automobile, cooking, heating your home or a variety of other ways. You were likely exposed at an early age to the direct process of combustion watching your folks build a campfire or playing with blue-tip stick matches to ignite your favorite tobacco, experimenting with alcohol lamps or Bunsen burners in a school lab. Combustion is all around us.

There are three basic components that must be present in combustion for this chemical reaction to take place.

  1. Oxygen
  2. Fuel
  3. Heat (spark or other means)

Better illustrated by the common triangle shown below.

To obtain safe, reliable burner combustion, the above 3 elements must be introduced at the right time and quantity.

For sustained effective and efficient combustion, we follow the Three T’s of time, temperature, and turbulence.

The Time of combustion refers to rate of reaction. Fuel (natural gas, propane etc…), is introduced to the combustion zone as described in the steps below. The volumetric rate is metered by burner architecture via orifices, lances, spuds or other similar means. Size and quantity of these delivery methods result in gas velocity and volume required to achieve rated capacities and limit emissions during the resident time in the main furnace.  Air-fuel ratios must be controlled to maintain stable, clean flame geometry within the furnace throughout various firing rates.

The Temperature of combustion is inherent to above attributes. Once we establish the primary or main flame (see sequence below), the heat present will maintain the temperature to continue the chemical reaction.

The Turbulence of combustion comes from the introduction method of fuel with air mixing and is vital to achieving stable and complete combustion. As a result, methods used will help to abate VOCs (volatile organic compounds) including  reducing carbon monoxide and nitrogen oxide (NOx) emission. Boiler furnace and refractory design play an important role in turbulence produced to effect an efficient clean flame pattern.

CFD (Computational Fluid Dynamics) modeling is complex process often used to develop a good match between burner & boiler in commercial & Industrial boilers. This complex process will only be mentioned here due to the extensive analysis required for its development.

Example of an Industrial Low NOx burner (NatCom)

   

These combustion systems are designed and engineered on our burner-control systems, to meet the most stringent emission requirements mandated to date.

The Following steps outline the burner combustion flame sequence:

Step #1. A call for heat is signaled to the boiler’s flame safeguard (sequence) control. This control starts a combustion air fan (oxygen) which in-turn energizes a pressure switch confirming fan air pressure. The boiler’s burner fan is engineered by the manufacture to produce enough combustion air to fire the burner at its variable, min./max. capacity. The sequence controller initiates an internal timer to “pre-purge” combustion chamber to accomplish a minimum of four (4) complete air changes per code, through combustion chamber and flue passages to flue stack outlet prior to start. This code requirement offers inherent safety by removing residual unburned fuel or in the case of a leaking fuel valve, miniscule amount of fuel in the combustion chamber at light-off so as not to cause an explosive environment resulting in unwanted occurrence.

Step #2.  Once timed sequence of pre-purge is complete and all safety parameters met, air dampers are electrically positioned to start.

Note: Some burners often found on condensing boilers include VSD (variable speed drive) fan motors, increasing fan RPM for pre-purge, then decreases to a proper set point for light off to accommodate pilot fuel for light-off.

Step #3. Simultaneously a gas pilot solenoid valve opens, while an ignition transformer energizes spark or glow plug (heat) to ignite pilot (fuel) establishing pilot flame prior to main fuel valve opening.

Step #4. Pilot flame is confirmed via flame rectification (Scanner Cell or Flame Rod) to flame safeguard control where sequence is automatically advanced to allow for main fuel valve(s) to open establishing a low fire in the boiler’s combustion chamber.

Step #5. Once main fuel valve(s) is open and flame signal remains strong, pilot fuel valve and spark or glow plug are de-energized, the flame safeguard control releases burner operation for firing rate /modulation to the respective controller packaged with boiler or from remote signal from a Plant Master or other firing rate type control. Assuming boiler has been properly warmed, modulation control will position firing rate to match boiler load.

Step #6. When steam or hot water load is satisfied and system demand is less than fuel (energy) input by the burner in low fire – minimum firing rate, burner’s flame safeguard control will begin burner shut-down sequence. This process includes a “post-purge” time to eliminate unburned fuel from chamber and flue passages, prior to shut down. At this point the boiler/burner is ready to start the sequence over again upon demand.

Note: The sequence above is very general. There is a lot more going on electronically relative to the combustion process than what is mentioned in this blog.

For more information relative to the variety of products offered as new or retrofit in your existing boiler plant, contact our Sales or Parts department.

Typical Flame Safeguard Sequence Control

Hope to see you next month for Part 2 of “Combustion”, where we will look at boiler stack draft and different fuels as it relates to combustion.

 Information in this blog is being furnished by D. J. Conley Associates Inc. and by those having numerous years of experience in design, installation and application with generation of heating and process steam and hot water products and services. This information, along with supplemental data obtained from a variety of sources, is for the beneficial use of its audience only. We cannot be held liable for the application or misapplication of products or methods associated with this data which may cause unfavorable issues or harmful outcomes. There are many circumstances beyond our control at play in individual systems. You are welcome to contact us in the event questions should arise.

Boiler Waterside Care & Treatment – Part 3

By | Water Treatment | No Comments

In this final installment of Boiler Water Treatment, we will focus on the following three components:

  • Chemical Treatment Application
  • System Design/Treatment Injection Sites & Cautions
  • Signs Your Treatment Program Requires Attention

CHEMICAL TREATMENT APPLICATION

Evaluation and analysis of all facets of chemical treatment must be examined prior to implementation. 

Chemical treatment formulas have changed over the years along with applicability, codes, laws, and rules for safe use and compliance. A variety of material types and manufacturers are available; though many formulations are similar, application and follow up is key to a successful treatment program.

Due to other complexities like injection line type and size, distance to equipment being treated, and method / location of injection and pump types, proper evaluation is required to be sure the best solution is applied. Additional factors such as bulk delivery, storage, safe handling and use of PPE, along with liability and cost, help to determine the right fit for your treatment plan.

By partnering with your boiler supplier, engineer, and certified water professional, you can be confident the best treatment advice is provided.  Misapplication can lead to major loss, severe penalties, ruptures, spills, and poor treatment, including the potential for pollution, energy efficiency loss from mud / scale build-up, and equipment failure. Additionally, health concerns may exist from steam injection in humidification, sterilization, and food processing where toxic steam may come into contact with humans.

Example of boiler operating characteristics that demand different treatment methods.

A steam boiler operating at 10 psig with 10% soft water make-up, 3% blowdown, while receiving 87% condensed steam back in the form of hot treated condensate to the feedwater tank or deaerator with minimal “direct” steam use.
 

VS

A steam boiler operating at 100 psig, having 55% direct steam use such as sterilizers, humidification or other live steam injection, 5% blowdown (60% soft water make-up) and 40% return condensate.

 

Treatment Timeline

After your boiler has been installed and is ready for start-up, the process begins.

  • Steam Boilers: Boil-out should be employed per manufacturer recommendations.
  • Hot Water Boilers: If cleaning & flushing a new boiler with the entire hydronic system is the plan; be aware during the system flush, sediment may drop out in the boiler due to having the lowest delta P of all system components. This sediment can collect on tubes (Firetube) or in tubes (Watertube) and lower drum(s).
  • Proper boiler isolation, drain, and rinse is recommended after a combined boiler & system flush to ensure sediment, welding slag from manufacturing, and other particles are removed
  • Formal treatment injection should begin immediately upon boiler water fill.

A word about Boiler ECONOMIZERS on steam boilers: If you have multiple boilers, each having a flue gas economizer, and one or more boiler is in hot stand-by,  it is recommended that a by-pass line with orifice be installed. This will recirculate water from a common feedwater header at economizer, to outlet piping of economizers, back into the deaerator, allowing treated water to continually flow at a low rate through economizer to protect tubes from corrosion.

SYSTEM DESIGN / (POSSIBLE) TREATMENT INJECTION SITES

The system design below shows what a typical boiler plant can look like. Common treatment injection sites are shown by an ORANGE ARROWS and will vary with each application.

SIGNS YOUR TREATMENT PROGRAM REQUIRES ATTENTION

There are specific markers given that are a red flag that your water treatment care program is in need of review.

  1. Water leaking around the base of your boiler (Watertube) or from under the front or rear head (Firetube Boiler) of your boilers.
    1. If upon further inspection it is determined the water on the floor is not from typical condensation during warm up or other assembly deficiency and the boiler is up to temperature/pressure, we recommend the boiler is cooled and opened for internal visual inspection and possible hydrostatic testing.
    2. If signs of leakage appear to be coming from another source such as the boiler flue gas stack or economizer, further investigation into those areas is warranted.
    3. If boiler headers, shell and/or tubes appear to be the source, a hydrostatic pressure test is required to pinpoint cause.
  2. If boiler stack vapor (plume) is excessive in a non-condensing boiler, the same inspection as above is in order.
  3. If your boiler’s stack temperatures are excessive, replace temperature gauge with a calibrated gauge to ensure it is not defective.
    1. Once you determine gauges are accurate, inspect the waterside for scale and/or mud build-up and proper flows through tubes.
    2. If no indication of waterside problems, inspect fireside to determine structural condition and verify no gasketing failures.
  4. Other signs include, visible sediment in the water, discoloration in the gauge glass and/or blowdown water, water bounce with constant burner cycling or other operational anomalies.

Finally, there is an often-unspoken added benefit to a comprehensive water treatment program which involves a bit of human nature. Great waterside care avoids the blame game along with numerous hours of meetings, emails, texts, and personal anguish that often accompany boiler vessel and/or tube failure for lack of proper treatment or operational abuse. Many intelligent, well-meaning people often attempt to shift blame when their boiler or related equipment falls victim to neglect, abuse or lack of care. The evidence is often not in their favor.

All manufactures of commercial & industrial boiler equipment provide a plethora of resources including periodic training events, custom training designed for a specific site, on-line resources, operator training with video recording along with Operation and Maintenance manuals covering the subject of waterside care. These should be reviewed often with procedures implemented to maintain efficient operation and protect your boiler investment.

Retrofit applications can be particularly troublesome as older buildings, can retain certain older technologies that do not blend well with newer product technology. There can also exist a lack of disclosure with regard to how a building’s older condensate system functions, resulting in a detrimental effect to replacement or retrofit products. All of this can be avoided by meeting with a team of owners, suppliers, engineers and operating personnel to describe and explain system function and best solutions prior to purchase, so all involved understand and are comfortable with selections. The result can be boiler and associated boiler room equipment lasting 30-40 years or more of trouble-free operation as safely and efficiently designed.

Thank you for joining us! Looking forward to meeting you on future blogs!

Blessings!

Mike

Boiler Waterside Care & Treatment – Part 2

By | Water Treatment | No Comments

In the previous blog we reviewed water and boiler feedwater basics.

We continue with emphasis on components suggested for pre-treatment. Ask your boiler manufacturers’ representative and water treatment consultant for recommendations, as there may be additional requirements related to specific boiler technologies and your particular application. As mentioned in Part 1, oxygen corrosion and scale buildup are the two major factors in boiler waterside failure. Rest assured, if you have a boiler made with processed iron or steel, your boiler will corrode over time. This happens from concentrations of caustic or acid in water, with oxygen attacking the boiler’s magnetite layer which, if not protected, can happen at an advanced rate.

CLOSED LOOP HYDRONIC SYSTEMS

Traditional Low Temperature Hot Water Systems generally operate between 150° F to 200° F while condensing boilers expand that range to 130ᵒ F and below, taking advantage of the latent heat from condensation once dew point is reached. Here are a few tips when operating hot water boilers in closed-loop systems:

  • Always include either a digital or analog low volume/flow meter to record and identify make-up water loss.
  • Weekly readings should be taken to ensure system leaks are not present.
  • Losses can occur from leaking drain valves, piping, air vents, expansion tanks, air separators, system valves, pump seals, coils, or other heat exchange equipment.
  • Once discovered, they require immediate correction to avoid damage or costly repairs.
  • Upon repair, system water must be replenished and chemicals added, then verify proper chemistry is restored.
  • Boiler warm up should follow, bringing system water temperature and flow to normal operating throughout the building, driving off oxygen and releasing trapped air via vents.

Some manufacturers and owners prefer softened water for make-up to a hot-water system while others consider it unnecessary if properly treated and monitored. Other water loss can be due to normal preventive maintenance as required by manufactures. With code changes through the years certain jurisdictions only require internal inspection of waterside on ASME Section IV heating boilers every 3 years however manufacturers recommendations must be followed to maintain warranties and adhere to best practice.

Medium and High Temperature Hot Water Closed Loop Systems generally operate at temperatures from 230ᵒ to 420ᵒF. These systems require special consideration for make-up because of higher required loop pressure. Due to elevated operating temperatures, make-up water must first be softened and preheated to de-aerate and avoid thermal shock, then chemically treated, and pumped to overcome system head. Other options may apply and should be considered for your individual system.

STEAM SYSTEM PRE-TREATMENT OPTIONS 

The following approach offers a “general guide” to pre-treatment recommendation. A comprehensive analysis of the following system characteristics is utilized to provide best equipment options for your facility.

Information required to select a quality make-up water solution:

  • Incoming water analysis
  • Incoming water pressure
  • Incoming water temperature
  • Percent make-up water anticipated, Includes:
    • Blowdown loss
    • Steam pressure to deaerator or feedwater tank pre-heater
    • Process steam use in lbs./hr.
    • Other system use or loss in lbs./hr.
  • Operating steam pressure of boiler
  • Steam use detail
    • Comfort heat through coils (% returned)
    • Direct – live steam injection (% make-up)
    • Live steam to food process (% make-up)
    • Live steam to humidification (% make-up)
    • Domestic use via heat exchangers (% returned)
    • Sterilization (% make-up)
    • Exhaust steam (% make-up)
    • Flash steam available in PPH (either % return or make-up)
  • Budgetary constraints
  • Physical space
  • Pre vs. Post treatment or both – Evaluation of offset costs
  • Risk tolerance

Answers to the above are analyzed, with resulting answers used to select best equipment options for your facility.

Most modern steam plants include pre-treatment using at minimum, a water softening system, deaerator or feed tank with steam pre-heater, chemical feed pumps and storage. Additionally, most steam boilers incorporate automatic surface blow-off based on measured conductivity in boiler water. Some include blowdown heat recovery systems and/or condensing economizers to preheat make-up water. Choosing the wrong equipment (or none at all) can result in greater cost over the life cycle of your boiler plant. Assistance in selection from engineers, contractors, or suppliers can reduce long-term expense and limit operational liabilities. It is up the owner to make an informed decision.

WATER SOFTENER

When selecting a water softening system to eliminate water hardness, we suggest a twin automatic (shown below) to ensure softened water to the boiler feedwater tank at all times, eliminating hard water by-pass during regeneration. Also, using the “On-Off” vs. modulating make-up water control is recommended where lower make-up water is expected. This eliminates the possibility of “channeling” whereby a small stream of water creates a “channel” through the resin bed which can occur during periods of low flow vs. normal flow across the entire mineral bed of your softener. Channeling can result in the presence of “hard-water” even though your softener resin may not require regeneration. To help size your make-up valve and avoid channeling, low flow limits can be obtained by your water softener’s O & M manual or manufacturer specifications.

 

REVERSE OSMOSIS (RO)

Higher boiler operating pressures, or owner preference may lead to a pure water solution such as Reverse-Osmosis (RO) as shown below. If RO is chosen, further care into boiler and feedwater products selection should be considered and reviewed by plant engineer or consultant. Pure water tends to be aggressive to metallic pipes and other substances that dissolve and ionize in water – not due to acidity, but because of its high purity and lower levels of dissolved substances. Metals such as pump, tanks, piping and components that come into contact with RO water should include corrosion inhibitors. Distribution piping from the RO system to storage should be made of plastic piping. Certain boiler level controls that rely on conductivity may not be as effective when using RO water. Exchanging these type controls for traditional float-type or level transmitters is recommended.

DEAERATOR VS. FEEDWATER SYSTEM

Feedwater tank selection is an important part of your overall steam plant solution.

When choosing the right feed tank system for your steam plant, using answers to the above 10 questions and applying the data to available equipment in a usable format will likely result in proper selection.

Deaerator and feedwater tank systems both serve the same base purpose of reducing dissolved oxygen and carbon dioxide levels in feedwater to prevent corrosion, deliver needed feedwater upon demand, avoiding potential for thermal shock to the boiler and store enough pre-heated and treated water to allow for at least 10 minutes of run-time before reducing the water level below the Net Positive Suction Head required by your feedwater or transfer pumps, leading to system shut-down.

The following chart illustrates the need to consider a Deaerator vs. Feedwater system with pre-heater.

Deaerators are not always the right solution for owners. Steam plants support a variety of applications and designs which vary in size. Consideration should be given to the amount of make-up vs. hot condensate return, whether pumped or gravity, along with consideration of equipment first cost, size, location and headroom. There are also trade-offs such as increased chemical cost over the life of the tank to treat dissolved gases in a feedwater system vs. deaerator. Evaluate all aspects and payback before selecting the best solution for your steam plant.  Often a combination deaerator and surge tank is the best solution for process applications where higher temperatures are being returned at higher rates or additional water storage is needed.

The comparison spreadsheet below illustrates some major differences between Deaerator and Feedwater tank w/Pre-heater.

Comparison spreadsheet

*Comments are based upon Pressurized Deaerator

**Should not exceed temperature setting above 190ᵒF  in case pre-heater overshoot or condensate returns higher than expected to avoid feedwater pump cavitation.

Additional Resources:

Cleaver Brooks / Do I Need a Deaerator?

American Boilers Mfg. Association / Deaerator White Paper

Be sure to catch up with us on Part 3 of 3 – BOILER WATERSIDE CARE & TREATMENT

 

Boiler Waterside Care & Treatment – Part 1

By | Water Treatment | No Comments

One of the more common challenges facing boiler owners, operators and plant engineers is proper water treatment. For many operating engineers and owners  water treatment is not a conundrum as they are fully engaged in the care and importance of a comprehensive boiler water treatment program. For those who are new to the boiler industry or desire a refresher, we offer this three-part blog dedicated to the subject of basic boiler water treatment and waterside care and its implementation.

It begins with reading and understanding manufacturers recommendations and implementing best practices to avoid negative impact from a deficiency in water treatment and waterside care. When you mitigate the potential for a boiler occurrence from vessel and/or tube failure, you accomplish several things; you keep employees and property safe, you avoid costly repairs, down-time & insurance cost escalation including work stoppage, and you extend boiler & associated equipment longevity with higher efficiencies resulting in energy reduction.  Keep in mind there is much more to learn than what will be posted here, but this is a good place to start.

Having a basic understanding of where water impurities originate and how these impurities can affect your boiler and boiler room equipment helps reinforce the need for action. Let’s look at the hydrological cycle below to gain insight as to where elements such as soluble and insoluble hardness salts in our water supply come from.

As you can see, naturally occurring elements and gasses like sodium, calcium carbonate (bicarbonates), magnesium, heavy metals, silica and other minerals, along with dissolved gasses like oxygen, carbon dioxide, nitrogen are all in water received by municipalities. This includes city, county well, water tower, and other means supplied by various methods into homes, schools, manufacturing plants, utilities, etc. These governed sources, though monitored and treated for harmful pollutants to humans, also include a long list of elements not mentioned, but less harmful in boiler water.

Since our focus centers on boilers operating at common pressures and temperatures, 5 – 150 psig steam, we will target those primary elements that cause most boiler & tube destruction. More on hot water boiler treatment later.

High concentrations of calcium and magnesium, oxygen, and carbon dioxide can cause scale build-up, loss of efficiency, and boiler/vessel or tube failure. Lack of proper pre-treatment via water softeners, deaerators, and other equipment or too much/too little chemical treatment and/or blowdown can also lead to improper TDS (total dissolved solids) levels, increased “cycles of concentration” oxygen corrosion and ultimately constitute the majority of pressure vessel and tube failures. In addition, boiler water imbalance can produce poor steam quality (wet steam), priming/carryover & foaming which compound operational issues such as lowering steam temperatures to processes, boiler “water bounce” and nuisance shut-downs, nucleate boiling on boiler tubes and/or tube sheets leading to overheating, stress cracking and failures. 

Examples of tube scaling and corrosion:

Scale: Acts like an insulator, decreasing boiler efficiency (above) as it builds and if not corrected, will ultimately lead to overheating and damage to tubes and vessels.

Oxygen & Carbon Dioxide Corrosion causes pitting and channeling in boiler tubes; can lead to tube leaks and failure. 

Priming & Carryover

Other elements found in smaller quantities can cause problems in boilers supplying steam as the prime mover to turbine generators and other equipment requiring higher temperatures and pressures. Those will not be addressed here as treatment options vary greatly and become more complex.  For Hot Water Boilers with carbon steel (ferrous metal) the primary concern is oxygen corrosion. Other tube materials like stainless steel and stainless alloys, which also require treatment, do tend to hold up better in hot water applications. Boiler vessels and tubes made from  copper (non-ferrous) used in some hot water heating boilers are often low-volume vessels that can provide efficient heat transfer, but come with caution on temperature and pressure limitations and have a higher delta P. Other boilers offering aluminum heat exchangers do have greater heat transfer characteristics, but also favor adherence to a prescribed pH range and are not as complimentary as carbon, stainless steel and copper fin boilers to system piping and components. Be sure to consult your water treatment specialist for proper and ongoing treatment.

The goal of reducing harmful water impurities, maintaining proper TDS & pH levels and eliminating gasses must be a daily mind-set and include chemistry readings and a boiler room log to monitor and adjust as needed. Consideration must also be given to  testing condensate return temperatures and methods to eliminate sources of rogue, untreated, infiltration or contaminated returns. Following boiler manufacturers recommendations for blowdown along with water chemistry control limits will help achieve these goals. To Learn more about suggested boiler water “Control Limits” consider obtaining document ABMA-Boiler 402, “Boiler Water Quality Requirements & Associated Steam Quality for ICI Boilers” via Techstreet.com.

With all sincerity, take heed the importance and possible implications to the lack of proper water treatment, blowdown and preventive maintenance which can lead to  catastrophic pressure vessel failure, serious danger and even loss of life. This blogger cannot emphasize enough the importance of a comprehensive water treatment program for all steam boiler and ASME pressure vessels.

Be sure to catch up with us on  Part Two – Next Month !

Author: M. Conley / D. J. Conley Associates Inc. 1974 – present.

Resources: Cleaver-Brooks Company and various