A Simplified Walk Through of the NB-27 Guide for Blowoff Vessels

September 25, 2020 by Aaron Rhoade

*Note to Reader, Interchangeable Terms:  Blowoff Vessel, Blowoff Tank, Blowdown Tank

If you’re not an experienced engineer involved with boiler room equipment design, The National Board of Boiler and Pressure Vessel Inspectors’ guide for blowoff vessels can be a little daunting. Other than using the complex formulas themselves, part of the unease some individuals may experience is with reading the more open ended statements within the guide. There are so many factors that can effect a blowdown tank’s design it would be nearly impossible for the National Board to write comprehensive guide on each. So naturally they leave some things a bit ambiguous as to the way something can be designed or fabricated (Reference page 2 of NB-27 document).

This article is intended to help break down the NB-27 guide into layman’s terms so to speak. We will address the ins and outs of pertinent formulas as well as discuss frequently asked questions we receive when presenting an NB-27 designed blowoff tank to our own clients.


As mentioned in our introduction, the Guide starts out stating “It does not address details for all possible arrangements of boiler blowoff equipment”.

This is because there are policy differences from state to state when it comes to pressure vessels and acceptable draining procedures to city sewers. There are many different ‘waste’ water processes that can be sent to a blowoff tank, both intermittent and continuous. For the sake of this particular article’s length, we will leave this be for now and focus on ‘standard’ intermittent bottom boiler blowdown applications.

Let’s get started, we’ll note the section’s title and then give our brief overview….


The guide starts with a summarized section explaining why clean, quality water is important to a boiler room’s performance. Treating your water is often one of the more underappreciated aspects to the boiler room’s overall design. Quality water increases boiler efficiency and equipment service life. And one process used to achieve this is blowing down water from the boiler, which helps remove impurities that can cause corrosion, scale, and dampen efficient heat transfer from the fuel being burned to the water needing to be boiled.

Thus the blowoff vessel is one pertinent piece of equipment needed to accomplish this. It provides a safe way to contain this extremely hot, high velocity and high pressure blowdown process. At Madden our core business is focused on exactly this, water conservation and energy efficiency in the boiler room. You can review other equipment we have (here.)

The remainder of this section in the NB-27 Guide summarizes the nuances of sizing a blowoff vessel. Blowoff vessels are sized based on boiler operating pressure and the size and length of its blowdown line. Read through this section of the Guide for more details if you’re interested.

For the sake of helping this article stand alone, here’s a succinct list summarizing the rest of this section in the guide, including physical pieces involved and the vessel design parameters:

  1. Connections: Vent, overflow style drain, manway, cleanout drain, and centrifugal inlet with wear plate
    • Note on inlet and wear plate: this specific design helps with the tank’s longevity and it helps the blowdown process itself. The water blown down from the boiler is very hot, under high pressure, and moving at a high velocity. I.E. it has a lot of energy. When it enters a large atmospheric vessel, this energy “relieves” itself in the form of steam flashing. The centrifugal motion helps steam separate from the remaining condensate as well as suppresses wear and tear on the vessel.
      • The metaphor we use to help visualize this is thinking about your sink at home, steam will rise out of faucet if you turn it all the way to the hot side. Times this by a multiple of 3-10 and you have the industrial boiler version.
  2. Ancillary equipment: Pressure gauge, sight glass, and a cooling water valve if you are also adding a continuous blowdown line to this tank (but we’ll leave the cooling water valve for another article).
  3. Vessel Parameters/Design: Blowoff vessels should be designed and fabricated to meet ASME Sec VIII, Div. 1 standards. They should be designed with a minimum pressure rating of 50 PSI. And when sized appropriately the water exiting the overflow drain should not be above 140 deg F.

On to the next section…

“Boiler Blowoff Vessel Sizing”:

This section is mainly an introduction, for the purpose of this article we’re going to skip over it.

“Volume of Boiler Discharge Water”:

And so we begin. Here the National Board has to make quite a few assumptions for the sake of starting somewhere.

The 3 formulas you see in this section provide us with the expected volume of water the blowoff vessel will see given the boiler(s) operating conditions. Assumptions: 1.) the boiler will be operating at about half the level of the steam drum; 2.) the operator will open the blowdown valve long enough to allow a 4” level drop in the drum.

Understandably most of these numbers they have used are what we would consider ‘better safe than sorry’ values.

So, per the drum diameter and length, we can use the NB-27 formulas here to estimate a cubic foot volume that will discharge into the blowdown tank.

And this brings us to our first commonly asked question to go over in the latter section of our article.

*FAQ #1 – Why a 4” level drop and do we really need to account for this? (Again, see latter section for discussion)

“Blowoff Time”:

In a way, this section goes hand in hand with the prior section, as both relate to the quantity of water removed during the blowdown operation. There are so many considerations for water quality in the boiler room it makes it difficult to provide clients a firm answer for just how much to blowdown, and therefore, how long to blowdown.

  • Are you treating your water with chemicals before entering the drum?
  • Do you have a deaerator tank removing oxygen from the water?
  • Are you in an area with naturally cleaner water, or known to contain a lot of impurities?
  • Are you also blowing down from the surface?

The list goes on and on.

So sticking to layman’s terms, this section is easy, if you want to know an estimated amount of time you should leave the blowdown valve open to achieve a 4” level drop in your boiler, the NB-27 guide has formulas to provide a solid estimate.

But what we do want to point out in our walk through here is this: note that a 4” level drop isn’t always necessary for designing your tank. See the FAQ section of this article for more details on this.

“Sizing Vent Pipe”:

We would argue the most important take away from this section is the National Board’s recommendation to design the blowdown tank’s vent to ensure the exiting steam has a velocity of 50 FPS. And this also is our next frequently asked question.

*FAQ #2 – Why 50 FPS and what does the National Board consider “approximately” 50 FPS? Just how strongly should we heed this parameter?

The reasoning for 50 FPS has to do with water staying entrained in the flash steam when the boiler blowdown water enters the vessel. When the hot, high pressure saturated blowdown comes into our tank, which is vented directly to atmosphere, a relatively large amount of energy is going to dissipate/release. It does this in the form of “flashing” steam.

If the vent is too small the resulting velocity of steam passing through it will be too high. Steam moving through the vent fast enough will result in water carryover. Water leaving the tank through the vent along with the steam is a recipe for someone at the plant receiving an unwanted shower. Dangerous.

Sizing the vent for lower velocity not only aids in condensate and steam separation within the blowoff tank, but it also minimizes noise through the vent.

But all this being said, read the following FAQ section for further discussion on the velocity design options.

“Sizing the Blowoff Vessel”:

In this section of the Guide the National Board goes over, 1.), setting your tank’s minimum diameter based on the velocity and density of flashing steam; 2.), the expected nominal water level in the tank based on your chosen diameter; 3.), the expected high level of the water level in the tank; and 4.), the resulting length of the cylindrical part of the vessel from tangent to tangent.

The 1st formula uses the mass flow rate of the blowdown water divided by the product of the density of steam inside the vessel and the velocity of steam inside the vessel. It’s a little hard to tell in this section (see the example section in the appendix of the NB-27 Guide for further detail) but here the density of steam is an assumed value of 0.0404 lbs/cubic ft.

Breaking this formula down in Layman’s terms is a bit tough, it’s involved. Each of the 3 factors within this formula need aspects derived within themselves as well. For a more comprehensive walk through of this formula please feel free to reach out to our group here at Madden.

Summarizing this in Layman’s terms the best we can however: steam has an expected rate at which it will expand (as well as an expected density) and expansion can cause pressure build up. In blowdown tanks you want minimal pressure, just enough to help push steam out the vent, ~ 2 PSI. And this is what the minimum diameter setting ultimately provides, room for safe expansion.

This above value you’ve come up with is, again, what the National Board says should be the minimum diameter for the blowdown tank. You can make this larger if needed based on your boiler room’s available footprint. But making it any smaller could cause the blowdown tank to operate under pressure which will start to bring us back to the potential steam shower we reference earlier.

Once this value is set, the 2nd and 3rd formulas take this value in conjunction with the value derived in the “Volume of Boiler Discharge Water” section to let us know where the water level would reside after one blowdown (“normal water level”) and where the water level would reside on all the recurring blowdowns that quench with the previously cooled blowdown (“high water level”).

The 4th and final formula is then relatively self-explanatory, based on the diameter you’ve chosen the national board suggests sizing the tangent to tangent height of the blowdown tank by multiplying the diameter by a factor of 1.75 and then adding the value of the “high water level” to this. And this leads us to our 3rd FAQ:

*FAQ #3 – (this isn’t so much a question as it is a statement), a lot of our clients will come back after reviewing a NB-27 based blowdown tank design and ask why it’s so tall.

To put it simply, it’s because the National Board uses a relatively linear formula based on the diameter you chose and it’s designed to ensure expanding steam won’t build up too much pressure in the tank.

There’s other reasoning’s to support this more straight forward formula but we’ll leave that overview for another article. That being said, see the FAQ section for our recommended variations to consider if a strict NB-27 blowdown tank won’t work for your boiler room.

“Time to Cool”:

We do not often exercise or worry about this formula. A typical boiler room will allow the remaining blowdown water in the tank (after steam flashes) to cool to room temperature in about 3-6 hours. Consequently, this isn’t a common concern for clients, as most boiler rooms don’t operate more than 2-3 boilers at a time. This means blowing down boilers individually once per 8 hour shift is relatively easy to do. Should your boiler room have more than 2-3 boilers operating and/or other processes are added to this tank we can certainly help you take a closer look at this formula.

In Layman’s terms, this section takes the assumed air temperature surrounding the blowdown tank (to be safe we use the hottest expected room/outdoor temperature the tank resides) and confirms how long you should wait to blowdown again based on the exit temperature you want draining at your facility via natural convection cooling only.

To add to this (why we are not often concerned with these formulas), when adding other processes to the blowdown tank other than intermittent bottom blowdown from the boiler, the National Board recommends adding a tempering water valve to the system. And assuming this valve will function as it should, draining water above 140 degrees is again not much of a concern as it will kick on cool quenching water whenever it sees drain temperatures above 140 degrees.

If you need help choosing a cooling water tempering valve we can help specify and source this for you as well as design complete cooling water spools if needed.

“Sizing Horizontal Blowoff Vessel”:

The remainder of the NB-27 guide goes over sizing a blowoff vessel with a horizontal orientation. Many of the formulas carryover with a few exceptions.

We will leave this section of the guide for a later date. If you have a need to discuss horizontal designs please feel free to contact us for assistance.


*FAQ #1 – Why a 4” level drop and do we really need to account for this?

The short answer for this is, yes.

If you don’t have either an industrial water treatment expert or the boiler manufacturer specifically telling you otherwise you should stick to the NB-27 guide’s recommendation here. If you were to plan on a lesser level drop from the boiler, but then end up doing more, you would be risking water carrying out of the vent with the steam as well as the remaining condensate draining out the overflow style drain before the next cycle. I.e., steam shower and drain temperatures above 140 deg F.

That said, the longer answer is not always, and ironically it very often will be a no; but can be a bit difficult to arrive at this design variation with confidence.

With proper water treatment before entering the steam drum, a 4” level drop is overkill to keep sediment and other impurities from building up in the drum. A few manufacturers even state in their literature that blowdown tanks will be designed for a 2” level drop unless otherwise requested by the client. However, boiler manufacturers willing to state this are few and far between.

Ultimately a water engineering or treatment firm is the most appropriate entity to inquire about this. But note they will want to thoroughly look through your system and incoming water as a whole before recommending how much/long you should blowdown your boiler(s).

When it is all said and done this could be worth investing in, specifically having a water quality engineering firm oversee your boiler room processes, either continually or at least during the design phase of your boiler room. They could potentially save you thousands of dollars in oversized equipment by specifying the appropriate sized water softener, chemical feed system, etc.

*FAQ #2 – Why 50 FPS and what does the National Board consider “approximately” 50 FPS? Just how strongly should we heed this factor?

As noted earlier in the article, the main reasons for this number is to reduce potential water carryover exiting the system with the steam as well as noise. This is a better safe than sorry value, however, depending on the system size this can result in some relatively huge vents. On larger boilers operating at high pressure, a vent size of 18”-36” might be needed to achieve exit velocities of 50 FPS. And depending on how far you need to carry the steam to leave your building, this piping cost can also be relatively huge.

At Madden, we sometimes guide clients to target 300 fps for blowdown tank vents as a maximum. We believe there are situations where this is a safe and reasonable exit velocity. We’ve yet to hear this resulting in issues from a customer in our 65 years designing tanks. This maximum velocity is also supported by several other engineering firms we work closely with.

If considering a more “relaxed” velocity number for your blowdown tank is pertinent we would encourage looking at the surrounding area where the vent piping exits the building. If there is relatively no potential for personnel to be around this area, 300 fps could be fine for your design. If someone could potentially be nearby this vent, we’d recommend you sticking closer to the 50 fps to be better safe than sorry.

Ultimately, this is up to your facility and safety personnel at the site to decide.

*FAQ #3 – (this isn’t so much a question as it is a statement) A lot of our clients will come back after reviewing an NB-27 based blowdown tank design and ask why it’s so tall.

To put it simply, steam just takes up a ton of space. And this equates to potential pressure increases within a vessel. The NB-27 formula ensures their suggested design will safely handle this significant ‘energy expansion’.

Now, in suggesting our potential design alternatives, we want to be clear that we do not want this section to appear as if we disagree with the NB-27 Guide’s formula for the tank’s height. It is a sound equation that relies heavily on the minimum diameter setting formula. Most of their attention is spent ensuring the tank can adequately handle this burst of steam and water entering the vessel. Once this minimum is set, they then provide a simple and safe formula for a resulting overall height.

What we do want to point out is that we see this formula as significantly “better safe than sorry” in some larger boiler applications. We feel confident our alternative design recommendation here would meet the National Board’s Page 2 note of other appropriate ways to design the room or vessel.

The reason we say “some” is because there’s an infrequently noticed threshold in the tangent to tangent height formula where the results go from seemingly logical to illogical. For example, on larger, higher pressure boiler blowdown applications you’ll start to see relatively large tangent to tangent heights relative to the suggested diameter minimum. Naturally, you’ll want to then increase the diameter to yield a smaller overall tank height. But what you may find is the point where the tangent to tangent height will stop lessening as you increase the diameter and instead start to increase.

When and if you find yourself in this situation, and/or in general the NB-27 design results in a tank size that does not work well with your building’s roof/ceiling height, we feel confident suggesting a few design alternatives. For example:

With our knowledge and experience using the various editions of the National Board’s Rules and Recommendations for the Design and Construction of Boiler Blowoff Systems over the years, we feel it is safe to ‘manually’ apply a formula based on what the implied blowoff vessel’s overall capacity should be.

To explain further, our thought process behind this: Once the volume of discharge water is determined (section 2 of guide), and the minimum diameter is then established (in section 5), we can use the NB-27 recommended tangent to tangent formula/dimension to derive what this tank’s overall capacity needs to be to function as a safe blowoff vessel.

In using this value (the implied overall capacity the tank should have) we can then increase the diameter size but lessen the tangent to tangent dimension accordingly, keeping the overall capacity the same. This will result in shorter tanks than the NB-27 formula would suggest when increasing the diameter size.

And that concludes one design edit we feel is a safe alternative to larger NB-27 tank designs.

There are more design alternatives you could consider here as well if your blowdown tank will be located in a facility or situation where we do not need to be concerned with personnel around the unit and/or a drain system that does not go directly to the city sewer. The other alternative involves an even smaller tank, but realizing it will increase velocity through the vent.

If you want to discuss this more thoroughly feel free to contact us.


That concludes our simple walk through of the NB-27 Guide for designing blowdown tanks in regards to standard intermittent bottom blowdown processes.

There are of course many other factors needing to potentially be considered with these designs:

  • Multiple boilers being blowdown
  • Continuous surface water also being blown down
  • Other processes in the plant under pressure being sent to the tank before exiting
  • Total water quality before entering the boiler
  • How much condensate return the system sees
  • Etc

And if you’re in this situation we would be happy to help further.

We hope this walk through of the National Board’s newer NB-27 guide helps in having you feeling more confident with your blowoff vessel design.

For further design assistance and/or a quote on fabricating a boiler blowdown tank for your application, please reach out to our group at info@maddenep.com or call 574-295-4292.

Thank you for reading our article, have a good one!