Boiler Blowoff Vessels Frequently Asked Questions

October 17, 2022 by Aaron Rhoade

Madden Engineered Products has provided design assistance and the supply of traditional boiler blowoff vessels since 1953. We find this service becoming more and more of a specialization, and as a result, assisting customers with the design and understanding of boiler blowdown tanks has become a pertinent service for our company.

This article is intended to help end users and engineers understand the more common questions and solutions the Madden teams sees when designing and building boiler blowdown tanks.

Questions in this article (14 total, listed in order):

• Why do I need a Blowdown tank?
• How does a boiler blowdown tank work?
• How long does it take for the boiler blowdown water to cool naturally without additional quenching water?
• What information is needed to properly size a Blowdown Tank?
• What are the other common processes in the boiler room that get sent to a Blowdown Tank?
• What resources are available to support the designing of Boiler Blowdown Tanks?
• What is the difference between a Blowdown “Tank” and a Blowdown “Separator”?
• For how long should a boiler operator blowdown?
• What happens if an operator blows down too much from the boiler?
• Can we blowdown multiple boilers in succession to the same tank?
• What is else is required if we send multiple processes and/or continuous blowdown processes to our blowdown tank on top of intermittent bottom blowdown?
• Why do vent sizing formulas in the NB-27 guide seem to yield such large vents?
• Do I need an exhaust silencer or true vent silencer on a blowdown tank vent?
• Do we need to put an insulation jacket or some type of protective barrier around a boiler blowdown tank?

Now let’s expand…

Boiler Blowdown Tank Frequently Asked Questions – The Basics

1. Why do I need a Blowdown Tank?
   1. Blowdown tanks provide a safe area to blow out extremely hot, high pressure, dirty boiler water. This process is needed for two main reasons: efficient boiling, and equipment longevity. As water boils, sediment and other impurities begin to form. These particles can act as an insulator inside the boiler, reducing efficiency of heat transfer and thereby requiring the burner to fire harder and longer to achieve desired steam production. These impurities can also begin deteriorating the boiler internals if left untreated. One of the easiest ways to help prevent both situations is to blow out the water once a day and replace with fresh water.
2. How does a boiler blowdown tank work?
   1. The final question in this list will expand on this topic, but to put it simply, a standard blowdown tank is designed to receive one 4″ level drop from the boiler steam drum and then hold that volume. This is accomplished with an overflow drain. The water fills the bottom of the blowdown tank up to, but not over this drain, and then begins cooling via natural convection through the tank. The bottom blowdown processes normally occurs once a day, so by the time an operator performs the next blowdown the water should be at room temperature. This room temperature water will then quench the following blowdown to a safe temperature as the mixture rises above the overflow drain and out to the city sewer drain. The process then repeats, over and over.
3. How long does it take for the boiler blowdown water to cool naturally without additional quenching water?
   1. If no other hot blowdown processes are being sent to the tank, typically it takes 4 to 6 hours to cool the blowdown to room temperature, assuming you do not install an insulation jacket around the tank. Madden can estimate this time for you if the average room temperature is known and an insulation jacket is NOT being used. If insulation is used, outside resources would be required for an accurate calculation rather than estimate.
4. What information is needed to properly size a Blowdown Tank?
   1. To size a tank, you need to know the max volume of blowdown water, the operating pressure the boiler will push that water out at, and the line/connection size this water will pass through. If the volume is unknown, it is recommended to account for a 4″ drop in the boiler’s steam drum. In this case, the steam drum dimensions would need to be known.
   Additional Note: this is for standard, intermittent bottom blowdown. If other process will be blown down to the tank, those operating pressures and line sizes would also need to be known.
5. What are the other common processes in the boiler room that get sent to a Blowdown Tank?
   1. Blowing out the bottom boiler connection(s) once a day is a given. However, it is not uncommon to also send condensate return lines to the tank, deaerator overflow, and/or continuous surface blowdown as well.
   Additional Note: If your boiler room is doing significant continuous surface blowdown, (ie., 500 PPH or more), ask Madden about a heat recovery system to save on boiler fuel costs.
6. What resources are available to support the designing of Boiler Blowdown Tanks?
   1. The National Board provides a guide for designing blowoff vessels. The latest edition is NB-27. These formulas are better safe than sorry equations for designing a blowoff vessel. If you prefer to follow this guideline when designing your tank, be sure to reach out to an experience boiler tank manufacturer for assistance. The NB-27 guide often yields much larger tanks than is truly necessary for the application. An experienced company could save you on initial equipment costs and floor space in your boiler room by designing a smaller, sufficient blowoff vessel.

Boiler Blowdown Tank Frequently Asked Questions – More Advanced, Engineering Related

1. What is the difference between a Blowdown “Tank” and a Blowdown “Separator”?
   1. The short answer is “tanks” use overflow drains and little quenching water and are larger; whereas “separators” drain immediately and rely completely on cooling water, but are usually much smaller and less costly up front. We’ve written a full article on this which you can read HERE.
2. For how long should a boiler operator blowdown?
   1. The standard blowdown amount to account for is a 4″ level drop in the steam drum. Madden blowoff vessel calculation sheets estimate the time it would take to achieve a 4″ level drop per your boiler steam drum dimensions, operating pressure, and blowdown line size. On average this is around 60 seconds. However, during normal conditions blowing down 1″ to 2″ is a more common volume. Therefore, most boiler operators will end up blowing down for 15 to 30 seconds, once a day.
   Additional Note: Saying 15-30 seconds is the short answer, but to know exactly what your boiler room needs, it is best to work with a water engineering or chemical supply company for their recommendation. They will base it on the water quality during normal conditions over a period of time. Further ensuring boiling efficiency and equipment longevity.
3. What happens if an operator blows down too much from the boiler?
   1. It is important to note that intermittent bottom boiler blowdown is a manual process requiring the boiler operator to remain at the blowdown valve (usually a slow opening valve and a quick opening, deadman lever valve). The normal operation is to first open the slow opening valve, and then turn open the quick acting blowdown valve. This will last 15 to 30 seconds, and then the operator will shut off the valves in reverse order. Again, never leaving the boiler during this process. The boiler should have level alarms which should ensure the boiler would never be blown down for “too long”.
      As far as the blowdown tank is concerned, no amount of boiler blowdown would “hurt” the vessel, and it is virtually impossible to overfill the vessel. In a situation where the boiler blowdown valves were left open and forgotten, the boiler alarms would soon go off and at worst (concerning the blowdown tank) it would drain too hot to the sewer.
   Additional Note: to further ensure safety, ask Madden about sizing an aftercooler with a cold-water quenching valve on the blowdown tank drain as an added backup cooling measure.
4. Can we blowdown multiple boilers in succession to the same tank?
   1. Absolutely. Madden’s recommendation for this is to simply move the overflow drain further up the blowdown tank in order to provide a larger reservoir for holding and cooling multiple blowdowns. If necessary, the tank height would be increased as well. To further clarify our recommendation here, the easiest way to think of this is, instead of accounting for a 4″ level drop, we essentially account for a 6″, 8″, 10″, 12″, etc., level drop.
   Additional Note: as noted on the previous question, for an added safety measure, Madden can provide an aftercooler with a self-operating temperature quenching valve on the drain line to ensure boiler water does not leave the tank and enter the city sewer at too high of a temperature (typically 140 deg F.).
5. What is else is required if we send multiple processes and/or continuous blowdown processes to our blowdown tank on top of intermittent bottom blowdown?
   1. This has been touched on in both of the previous questions, the short answer is quenching water. The longer answer is expanding on why Madden recommends an aftercooler system with a self-operating cold-water valve on the drain line specifically. Some other vendors will install this sensor and valve directly in the tank’s overflow drain reservoir and inject water into the reservoir as well. Nothing is wrong with this, but we believe it takes away from a traditional blowdown tank’s best feature, reducing the need for cooling water via the reservoir provided by an overflow type drain. Installing an aftercooler package on the drain line instead helps ensure you will use the bare minimum cold water required. Allowing the overflow drain’s reservoir every chance to cool the incoming processes before cold water “reluctantly” turns on to quench to your desired temperature.
   Additional Note: Madden can provide temperature quenching calculations to ensure a regulating valve and surrounding pipe and valves are sized adequately. To do so, we will need to know all temperatures and flow rates entering the tank to assist with providing this calculation.
6. Why do vent sizing formulas in the NB-27 guide seem to yield such large vents?
   1. The National Board’s NB-27 guide for designing atmospheric pressure vessels suggests a vent size that reduces the steam velocity passing out of the vent close to 50 FPS. There is some gray area as the guide states, “approximately 50 FPS” and does not expand further. Reducing this velocity helps keep noise levels lower and helps ensure water won’t carryout of the vent with the flash steam.
      But as alluded to, this particular parameter can yield very large vents. Madden and several other Engineering Firm contacts agree 300 FPS is still a safe maximum velocity through an atmospheric tank vent. However, please note, this statement assumes the vent line will be terminated outside of the building, well away from any personnel. The vent line termination point is the main concern, as this is where personnel would experience the majority of the noise, or the risk of water coming out with the steam and falling around this location.
7. Do I need an exhaust silencer or true vent silencer on a blowdown tank vent?
   1. It is uncommon to need any kind of silencing equipment on the vent line. This assumes an adequately sized vent (under 300 FPS) and the vent line terminates out of the building, away from personnel. Most of the noise will not radiate out of the blowdown tank walls in the boiler room. This is especially true if an insulation jacket is being used for personnel protection. The majority of the noise will travel up and out of the vent.
      As this is an intermittent process lasting less than ¼ hour, OSHA noise level requirements are 115 dBA, rather than the typical 85 dBA limit most are used to seeing. Even if you have a client who wants their blowdown tank to operate at 85 dBA anyways, most tanks will not reach these noise levels within 3 to 5 feet next to the tank, only out the vent on the roof.
   Additional Note: Madden has a good understanding of normal noise levels in a bottom blowdown tank, but Madde does not provide acoustic calculations/estimates. For accurate decibel estimates, you would need to reach out to an acoustic engineering company and/or an acoustic insulation company. Madden can assist in providing flow rates, pressure drops, and velocity estimates for these calculations if needed.
8. Do we need to put an insulation jacket or some type of protective barrier around a boiler blowdown tank?
   1. Because these vessels are designed to cool the blowdown via natural convection (heat dissipating through the vessel into the room’s ambient atmosphere), Madden recommends a protective barrier rather than insulation. Typically this is a metal guard rail/fence built around the tank or a “jacket” made of mesh fence like steel.
      However, we understand it is often easier and more cost effective to get high temperature insulation jackets to accomplish this. This is an appropriate solution, but keep in mind it will take longer to cool the blowdown water naturally.

Conclusion

For further assistance designing boiler blowdown equipment be sure to check out our website. And for more design assistance on a boiler blowdown tank project, please don’t hesitate to reach out to our factory to set up a phone call or Teams meeting.

We are a smaller, family-owned manufacturer who prioritizes responsive customer service and quality products that are easy to work with and very dependable.

If you or your client have an upcoming boiler room project needing blowdown equipment, please reach out to our factory to discuss a possible solution!

Phone: 574-295-4292
Toll Free: 800-369-6233
Email: info@maddenep.com
Website: www.maddenep.com