Water Gauge, Stiffeners, and Insulation

Gary Bases

November 1, 2001

You may be wondering what water gauge, stiffeners and insulation have in common? Well the answer is a lot. The design and installation of a lagging and insulation system on a selective catalytic reduction (SCR) system for reducing nitrogen oxide emissions will depend heavily upon the stiffener arrangement. The stiffener arrangement will depend on many factors, including the water gauge that the casing of the SCR, or flue plate design is to be based upon. Unfortunately, that explanation tells you very little of what you want to know.

The environment is an important issue in our lives today. The Ozone Transport Assessment Group (OTAG) identified power plants as the most significant source of nitrogen oxides (NOx) emissions in the country. Based on OTAG’s recommendations, the United States Environmental Protection Agency (EPA) proposed that nearly two-dozen states and Washington, D.C. reduce their emissions of air pollution by the year 2003 (This has since changed, with the date moved back). One way to reduce NOx emissions is by installing SCR systems. An SCR system is basically a large box placed in the gas flow of exit gas flue that sprays ammonia into the gas and thereby reduces the nitrogen oxides in the flue gas that exits the stack. An SCR system will only work correctly if the flue gas temperature remains at or above its operating design temperature requirements.

These SCR systems must be insulated and lagged correctly to prevent excessive heat loss and for personnel protection. If you read "Lagging 101" in the April 2001 issue of Insulation Outlook, then you would know that the first thing you do when designing a lagging and insulation system is to review the area to be insulated and look at the stiffener size and pattern.

If the stiffener pattern and size is so important that it’s the first thing you look at when designing a lagging and insulation system, it would behoove us to understand how the size, shape and pattern of the external stiffeners is developed.

Casing Design

There’s a lot of work involved in an SCR system, including the design of the casing for the SCR box that the gas will pass through. This casing design will vary from plant to plant depending upon where the SCR system can be placed. Most often these systems will be needed at existing power plants, so space will be at a premium. Along with the physical location and restrictions, there will be other factors that will effect the design such as the gas temperature, nitrogen oxide levels, and the water gauge to which the casing is to be designed. Each of these factors (physical restrictions, gas temperature, and water gauge) will effect the insulation and lagging design. Of all the factors, in my opinion, water gauge is the most important and least understood.

The term "water gauge" refers to an instrument that indicates the level of water, as in a boiler, tank, reservoir or stream. The measure of the amount of water shown on the water gauge instrument is measured in "inches." That in itself still doesn’t tell us very much but it’s as good a place to start as any.

Stiffener Sizing

Stiffener size is based primarily on three essential items, span, pressure, and temperature and is limited by stress or deflection. So the question arises as to how or where to begin for sizing stiffeners?

After a layout has been completed for a flue or duct system, the individual components must be designed from a structural standpoint to withstand forces due to the following:

1. internal static pressure of the contained fluid (in this case spent gas or air)

2. transient pressure designated for the system (where we find reference to water gauge)

3. dead weight of plate, stiffeners, insulation, lagging, etc.

4. wind and seismic loading

5. expansion joint actuation forces

6. coal ash accumulation (if applicable)

7. load transfer from other equipment

All of the previously mentioned factors must be taken into account for the design of the SCR casing. You will notice the absence of any reference to insulation thickness requirements. However, to simplify what we’re trying to discern (how water gauge affects stiffener design), lets look at one particular way (there are many ways to do this) for calculating stiffener sizing.

Side stiffener calibration example for a horizontal flue:

Specific Design Information

  • Duct 8 feet high x 16 feet wide x 3/16" plate
  • 24" stiffener spacing
  • Temperature of 800 degrees fahrenheit
  • Pressure at steady state+25" H20
  • Pressure at transient state
  • Flue wall weight
  • F9 = 12000 PSI
  • S MT=19200 PSI
  • Span = 8 feet =96"

Using the side stiffener calibration information, we will be able to determine the size of the stiffener by using the following formula:

You then would go to a book like the AISC Manual of the American Institute of Steel Construction to find a stiffener size that’s close to the above calculation. In this case it would be a 3 inch x 3 inch x ¼ inch angle that has a Z equal to 2.5 cubic inches.

Transient Bending (Positive Pressure)

Now we will take our chosen angle size and recalculate to see if our chosen angle will work by checking to see if is less than value.

We now compare this value to the value of 19200 PSI and we see that our choice is acceptable because it’s less than 19000 PSI. As you can see, to calculate stiffener sizing is quite complicated. There’s still another step before you can officially say that the 3 inch stiffener is okay. You must calculate the transient bending or negative pressure, but I think you’re getting the picture now.

There are two key elements in the previously mentioned formulas that have a direct affect upon insulation and lagging systems. They are both found in the formula for "M," stiffener spacing and water gauge pressure.

Now I must apologize to all those who do this for a living. It’s not my intent to oversimplify but only to develop a point to show how a stiffener size calculation works. Today, all such calibrations are being done on a computer, and that’s exactly my point. Years ago those people doing the calibrations considered the insulation and lagging systems when determining their stiffener sizing and spacing. That’s not the case today. It wasn’t more than 15 years ago that the norm for the water gauge pressure for a flue, duct, or casing system was around 17 inches. Today it’s 35 inches or more. That’s why today it’s common to have very large stiffeners (12 inches or greater) on very wide spacing (greater than 48 inches).

I also must point out that it wasn’t a coincidence that the early stiffener spacing matched the average width of a mineral wool board of 24 inches, 36 inches or 48 inches wide. The early designers of flue and duct considered the insulation application because their water gauge pressure was half what’s required today. The higher the water gauge, the bigger the stiffeners.

Insulation System of Yesterday’s Water Gauge

The stiffener sizing of yesterday was based on a much lower water gauge pressure (around 17 inches) and were spaced normally at 24 inch, 36 inch or 48 inch spacing. This spacing allowed the insulation to be placed between the stiffeners without having to cut to fit. Some companies actually made standards about stiffeners and insulation. Some examples:

1. When the external stiffeners are spaced 6 feet or more apart, the insulation should be humped over these stiffeners.

2. When the specified insulation thickness is within 2 inches of equaling the total stiffener height, than the insulation thickness is to be increased to bury the stiffener with at least ½ inch thick or more.

3. External stiffeners should all be of the same height on any one rectangular surface as required to meet stiffener span designs.

Taking the same flue as described earlier, the minimum insulation thickness based on 800 degrees fahrenheit and having to meet a surface temperature of 130 degrees fahrenheit, with an ambient air of 80 degrees fahrenheit and an external wind velocity of 50 fpm with aluminum lagging would require 4 inch thick of a mineral wool 8 pound density board insulation. Per the design specifics the thickness of insulation should be enough to bury all stiffeners. This one layer of 4 inch thick insulation would be notched to allow for the stiffener flange and then placed between the 3 inch stiffeners. After the insulation has been applied, than an outer lagging would be installed over the now flat insulated surface.

Insulation Systems Based on Today’s Water Gauge

The insulation systems of today would never be able to meet the previously listed design specifics of humping or burying of the stiffeners. The stiffeners, being designed today on hot flue, duct, or SCR casing (700 degrees and greater), are quite large and much farther apart. It’s very common to find 6 inch, 8 inch, 12 inch or even greater stiffeners. This is due in part to the water gauge number being used in their design calculations and in part because they haven’t considered the required insulation thickness and its application.

I’m not sure why the water gauge number was increased, but I’m sure that there was a very good reason for it. Be that as it may, the point is that we must work within the design parameters to find an appropriate and economical system to insulate and lag these SCR systems.

The square foot area of additional flue work, including an SCR system, is staggering (100,000 square feet per SCR system is common). The stiffeners being designed on these SCR systems, and on the associated flues, are very large. It would be impossible to hump them or to bury them. Let’s take a look at some of insulating and lagging options.

Insulation and Lagging Systems for a hot SCR System

The design parameters for the insulation and lagging systems will be the same as before. The insulation type will be a mineral fiberboard. The outside surface temperature will be 130 degrees fahrenheit, with an ambient air of 80 degrees fahrenheit and an external wind velocity of 50 fpm. The fasteners shall be spaced to withstand a 30 pound per square foot suction or pressure wind loading and all systems are to be considered outdoors.

H Bar System

This system uses a pre-fabricated support system, much like the manufacturers of continuous gutters (that is, installed over the outside of the stiffeners). These H-looking steel channels are attached to the external surface of the stiffeners and form a picture frame, in which the insulation sits. The lagging will then be attached directly to the H-bar frame. Unfortunately, this system isn’t recommended for hot flues of more than 450 degrees fahrenheit because of the potential heat transfer between the stiffener and the H bar system. Adding an insulation system directly to the flue plate prior to installing the H-bar system may minimize the potential heat loss and would then make this a viable and realistic option. This is a pre-engineered system that’s designed and fabricated off site and then installed at the job site or at the flue fabricator’s shop.

Insulation Pins and 22 Gage Sub-girt

This system utilizes a 22 gage sub-girt plug welded to the external stiffeners, with the insulation and lagging then installed. The insulation and lagging will be attached to the sub-girt by a separate support system (pins, clips, Z clips, and or sub-girt). This is a field fabricated and designed system either at the job site or at the flue fabricator’s shop.

Pre-insulated Lagging Panels

This system consists of a shop or field fabrication of an insulated lagging panel. This insulated lagging panel will then be attached to the outside of the stiffeners directly or to a sub system made from angle iron. This is a pre-engineered system that can be fabricated at the job site or at an off site facility and can be installed at either the job site or at the flue fabricator’s shop.

Final Thoughts

To insulate an SCR system, a combination of one or more of the previously mentioned insulation and lagging systems will be required. This will be very expensive, especially when you compare them to the insulation system of years gone by (one layer of insulation and lagging).

An average SCR system being installed at an existing power plant may cost as much as $50 million (including insulation and lagging). An improperly designed and/or installed insulation and lagging system will have an adverse effect on the SCR’s ability to operate correctly.

No longer is it economically feasible to bury or hump stiffeners. So it will pay to pay attention to the insulation and lagging design. Stiffeners more over 7 inches high are very difficult to insulate and lag economically. So look at the water gauge requirements of your SCR project and before a stiffener design is set, consider what type of insulation and lagging system that will be utilized on the SCR system. Planning ahead will save money. A well designed and installed insulation and lagging system will save money and energy at a rate that’s essential for an efficient plant operation.