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Force cooling the HRSG after shutdown may be on everyone’s mind at midnight on Friday, so your team can start HRSG maintenance work ASAP. But how much consideration has been given to the long-term effects of force cooling? Could quick access this outage mean more maintenance next outage? That’s no fun.Force cooling should be performed with an awareness of the impacts on the HRSG so the risks and rewards can be judged.Spin CoolingForce cooling the HRSG with air by spinning the combustion turbine cools the gas-side rapidly, and effectively turns the superheater and reheater panels into condensers. This creates the risk of excessive component stress in drum shells and nozzles, headers of superheaters and reheaters, and even in gas baffles and flow distribution elements.Accelerated Drum Pressure DecayReducing HP drum pressure rapidly causes the inside surface of the drum to cool much more quickly than the outside of the drum. This creates stress, and drum nozzle cracks are often the result. Shell-to-drum head welds may also be at risk. Components in the drum attached to the shell, such as baffle plates and steam separation housings, are subject to stresses as well during these rapid transients.

Action to Minimize Risk:• Keep drum temperature ramp rate within the acceptable limits

for both increasing and decreasing pressure. These limits should be determined specifically for each HRSG design. HRST can help assess and determine the limits for your HRSG.

• Consider alternatives to spin cooling to speed up maintenance.◊ Increase natural draft through the HRSG during shutdowns

as an alternative to spin cooling. Opening HRSG access doors on the roof and lower sidewall areas can boost natural draft.

◊ Add extra roof doors to increase cooling and improve access, both of which can quicken HRSG internal access for maintenance work. Roof doors can also help locate tube leaks quicker, so repair plans can be developed sooner.

◊ Evaluate and optimize the location of roof cable openings. This can allow skyclimber cable set up and positioning

while the upper HRSG header areas are cooling. Skyclimbers can decrease maintenance time by eliminating the need for scaffold set-up and tear down. Continued on page 3

HRSG Rapid Cooling for Maintenance

HRSG Gas Side Corrosion - Page 2Beware the combination of sulfur and humidity!

Welcome our Page 4New Employees!

Tube Leak Pain Relief Page 4In A Box

Superheater or Reheater Page 5Failures got you Feeling Drained?

Recent Project Success Page 5 Attemperator Liner Page 6Inspection: Be diligent!

Water Hammer Page 7

HRSG Academy Page 8

TM

In This Issue of Boiler Biz:

YOUR HRSG SOLUTIONS NE WSLE T TER VOLUME 15 | ISSUE 1 | 2014

BIZ®

BOILER

NEWS


HRST, Inc. strives to be the world’s most trusted supplier of HRSG and boiler related technical

services and products.

HRST, Inc.6557 City West Parkway

Eden Prairie, MN 55344 USAwww.hrstinc.com1-952-833-1427

Downcomer pass-through nozzle stress during cooling

HP Drum head weld crack examples

Copyr ight © 2014 by HRST, I nc. Al l r ights reser ved. Volume 15 - I ssue 1 - 2014

Sulfur-related corrosion damage in HRSGs is becoming more common. Primarily these are oil fired combined-cycle plants and some natural gas (NG) fired plants with higher sulfur content in their gas supply. Sulfuric acid can form during operation and condenses before leaving the stack. Sulfuric acid also forms from sulfur-containing deposits that absorb moisture when off-line. This acid results in HRSG damage to tubes, drain piping, inner liners, outer casing, pipe penetration seals, and stack inner surfaces. During OperationTurbine exhaust gas contains sulfur dioxide (SO2) which reacts with excess oxygen and forms sulfur trioxide (SO3), which in turn, reacts with a water vapor in the gas and forms gaseous sulfuric acid: SO₃ + H₂O -> H₂SO₄In a properly designed HRSG operating at base load and with the design fuels, the sulfuric acid normally does not reach its acid dew point temperature and leaves the stack in a gaseous form. This is not the case for low-load operation and transient stages such as start-up. A misconception is that as long as exhaust temperature is higher than acid dew point, the condensation will not occur. Yet what about tube metal temperatures (TMT)? The TMTs are often lower than the design temperatures and subsequently below acid dew point temperatures during start-ups and low load operations. When gas comes in contact with cooler tubes, sulfuric acid can condense on the tubes. For base loaded plants, this issue is not significant, as the start-up and low load times are short compared to the overall run time. However, for plants that frequently run at low loads or cycle, corrosion can become a problem. • Other plants that may have issues with acid dew point are plants that have

frequent sulfur spikes in their fuel supply. Such spikes increase the acid dew point temperature and condensation occurs at higher exhaust temperatures than expected. That is why it is important to monitor fuel composition and calculate the acid dew point temperature if sulfur content changes. The acid dew point temperature is a function of oxygen and moisture in the exhaust gas, as well as the amount of sulfur in the fuel. Figure 1 shows an estimation graph for the acid dew point as a function of sulfur in a fuel for exhaust from a GE 7FA turbine. It is important to note that oxygen and fuel flow are set to be constant in this graph, yet will vary from one plant to another, depending on turbine water injection level and other fuel constituents. Another very important factor in calculating the acid dew point, and one which will vary between plants, is the presence of CO and SCR catalysts. With the SCR catalysts, the amount of SO3 can triple. With CO catalysts the amount can double. With both SCR and CO catalysts, the amount of SO3 is four times. This makes a big difference in the sulfur dew point!

OfflineSulfur-containing deposits that accumulate on tubes near the stack can become even more corrosive when off-line humidity and lower temperatures allow acid to form. (Figure 2). After HRSG shutdown, sulfur that has deposited on the tubes, headers and walls throughout the HRSG can become very corrosive if water ingress occurs. The moisture reacts with the sulfur, creating acid that can be recognized for the wet and thick appearance, and can be found in lower crawl spaces and at the bottom of a stack (Figure 3).The main sources of moisture inside of an off-line HRSG are: 1) humid outside air that found its way inside (either via stack or access doors that were left open) and 2) rain water ingress through roof casing penetrations, like pipe seals and roof doors. Therefore, for sulfur-challenged HRSGs, it is important to maintain a dry off-line environment and to implement a rigorous corrosion inspection program. Authored by: Souren Chakirov

HRSG Gas Side CorrosionBeware the combination of sulfur and humidity!

Figure 1. Graph shows influence of turbine exhaust moisture, temperature and fuel sulfur on turbine

exhaust sulfur dew point temperature.

Figure 2. Example of heavy tube fouling containing sulfur.

Figure 3. Example of heavy sulfur deposits in a Lower Crawl Space of an oil fired HRSG.


continued from page 1

Thermal Stress Damage to Superheaters and ReheatersSuperheaters and Reheaters still under pressure will form condensate as the relatively cool GT spin cooled air is blown across the hot tube coils. This has the potential to cause damage from cool condensate “raining down” and accumulating in hot lower headers. Additional damage can result if accumulated condensate is not removed from the system before the next start-up. Action to Minimize Risk:• If possible, keep pressure as high as possible and steam temperatures

as low as practical on the steam side just prior to the CT trip. This will help minimize the temperature difference between the condensate and the lower header when they contact. When using this approach, carefully monitor attemperator steam outlet conditions for at least 50°F of residual superheat.

• Evaluate the HPSH and Reheater for proper drain sizing, location and automated valves with a mechanism or logic for condensate detection and removal under all conditions. HRSGs designed after about 2007 should have them since they are now required by ASME Section I Boiler & Pressure Vessel Code. But if your HRSG was designed before then, get them checked! HRST can help!

Damage to Non-Pressure Part Gas-Side ComponentsHRST has observed a correlation between spin cooling and damage to gas-side components like tube coil gas baffles, flow distribution perforated plates, and turning vanes. Spin cooling will cause a reverse in the “normal” start-up expansion and flexing of gas-side components. This can drive or accelerate non-pressure part gas-side component failures.

Action to Minimize Risk:• Inspect gas-side components more frequently if spin cooling frequency

increases. Put extra inspection focus in the Inlet Duct and Module 1. Module 1 inspection should include front and rear tube faces, plus the upper and lower crawl spaces.

• If non-pressure part damage is found, evaluate the design and criticality of the components. HRST can help! Failure of gas baffles can result in tube failures and outage safety hazards, so their importance should not be underestimated.

Flushing Water from the Economizer InletA method of force cooling the HRSG is flushing water through the economizer and then draining from the evaporator. Damaging HRSG economizer stress can occur if the flow of water is stopped and then restarted. If flushing is used to cool the evaporator, it will result in more rapid cooling of the steam drum and that could create stress.Furthermore, when spin cooling, the tube coils will cool faster than the drums. If water flow is stopped and resumed during this period, the water exiting economizer coils will be cooler, and the steam drum will still be hot, creating another potential source of damage to the drum. Action to Minimize Risk:• Care should be used to maintain continuous water flow and avoid

starting and stopping, using a simultaneous draining and filling process.

• The flushing process should always be done within the acceptable drum pressure depressurization ramp rate.


Reheater lower header bowed by condensate accumulation

Gas baffle failure in Module 1 of an HRSG that had undergone

multiple spin cooling events in the previous year.Authored by: Jonathan Aurand, Bryan Craig, P.E., Lester Stanley, P.E.,

Dan Baldwin

Steam drum baffle with attachment weld crack


Tube Leak Pain Relief In A Box - Take one out and tell your boss the pain is now under control! Have you ever found a tube leak on a Friday afternoon and needed a piece of spare tube? I know some of you have, because I’ve gotten some of those calls! What a headache! The size & material of the tube determines how easy it will be to find. HRSG cold end? Not too hard. HRSG hot end? Can be very hard. In the last Boiler Biz issue, we described the value of preplanning repair scenarios to save outage time and reduce the drama. Now we want to show you how to avoid a Friday afternoon panic to source tubes for the weekend repair!Box of Pain Relief!Let us help you with a box of spare tube stock! We’ll do the review and analysis to select the best set of tubes to put in the box so everywhere in the HRSG is covered. Some engineering is required for this.Stocking some slightly thicker tubes may help them be useable in several locations. Some material substitutions may also help. Care is required, and we will provide code calcs to prove the material change is proper. Did you know that at lower temperatures a SA-178A (carbon steel) tube has a higher allowable stress than a SA-213 T22 (alloy steel) tube? In this case, substitution may need to be a thicker T22 tube than the original carbon steel. Result: One very effective box of pain relief! We’ll package the box of tubes with MTRs that are clearly organized and labeled. Authored by: Darryl Nagel

Randy OjedaRandy Ojeda joined the HRST engineering team in March 2014. He is a graduate of the University of Wyoming with degrees in Civil and Architectural Engineering. His work with an HRSG OEM for over 17 years, has provided comprehensive experience in design and manufacturing in both the power generation and petrochemical industries. Randy’s experience includes a wide range of HRSG applications in both domestic and international markets. He also possesses over 10 years of engineering experience in a wide range of non-HRSG industrial applications. Randy resides in Minnesota and is looking forward to when the last two of his five children (four boys and one girl) are off to college.

WELCOME OUR NEW EMPLOYEES!Jerry CovingtonJerry Covington joined the HRST Field Technical Advisor team in December 2013. Jerry brings a wealth of industrial piping experience, having worked extensively over the past ten years as an

NCCER Certified Pipefitter for several major construction contractors in the Gulf Coast Region. He also has a strong background in industrial safety, and possesses extensive experience in

rigging applications. A member of our Gulf Coast Regional office, Jerry currently resides in Texas with his wife and their 2 children. In his spare time Jerry likes camping, fishing, and doing other outdoor activities with his family. Jerry enjoys hunting and sport shooting, and is an avid

saltwater angler and kayaker.

HRST selected tubes, labeled with MTRs inside. Box enclosure sized to fit in plant store room. Door and tags for easy inventory checks.


Drum Door Alignment Blocks Manway Gaskets Liner Studs Sidewall Access Doors

Floor Pipe Penetration Seals Roof Pipe Penetration Seals Sidewall Pipe Penetration Seals Roof Access Doors

Drum Door Alignment Kits Liner Retainer Clips Coil Gas Baffles Liner Washers

Tube Supply Internal Stack Expansion Joint Debris Cover Internal Thermal Liner for Desuperheater Steam Drum Manway Door Assembly Upgrades

Forged Nozzle Assembly Fabric Pipe Penetration Seals Thermometer Assembly Steam Drum Agglomerator Mesh Pads

LP Evaporator Feeder Pipe Assemblies Vent Silencers Steam Drum Chevrons P91 Pipe & Fittings

Inlet Duct Floor Liner SH/RH Drain Modifications Downcomer Feeder Pipe Upgrade Tube Tie Repair Sleeves

Below is a partial list of recent product and design projects shipped to our clients.

Superheater or Reheater Failures Got you Feeling Drained?From our HRST inspection experience, we often find evidence of problems related to superheater or reheater drains. These problems can be bowed SH/RH tubes, or perhaps bent or cracked drain pipes. Often, repeated tube failures in SH/RH can be traced to problems with the drain system.Of the drain problems we encounter, most can be traced to one or more of the following categories:

1. Cool or saturated steam by-pass through the drain system.2. Low point in pipe system that cannot be drained.3. Low point drains that are not operated.4. SH/RH drain expansion interference during lateral or vertical thermal displacement.5. Drain pipe flexibility (expansion overstress).6. Drains undersized and/or not automated.Why so many categories? Unfortunately, there are many factors that contribute to these problems. Sometimes the problems are fairly subtle. We recently re-designed one customer’s drain system that was correctly designed to most common criteria, and was being operated properly. Unfortunately, there was a slight low-point at the end of a long pipe run that joined two sections of reheaters. This low point allowed some water to accumulate because the available drain was slightly up-hill from the low point. Undrained water was being swept up into the reheater and causing bowed tubes. The solution was to reposition the drain to the low point (see illustrations).What should the proactive plant staff do to avoid drain related issues? Careful outage inspection of the drain arrangement is a great start! We can help! Depending on the HRSG OEM and the vintage of their design, we probably have insight on where to place extra inspection focus. HRST has addressed all six categories over the past few years, and has recently formed an engineering/design team to better tackle design problems and issues related specifically to drains. The team can provide proven solutions to all six problems above.

One final note: ASME has added requirements in recent code revisions of Section I to address HRSG drains. These requirements are good changes that help provide better drain function to HRSGs. However, these changes are the bare minimum, and in many cases, do not cover all of the six problem categories. Generally it requires an experienced approach to solve existing problems and to specify the correct design for new construction.

Authored by: Guy Thompson, P.E.

Recent Project Success

BEFORE

AFTER

Reheater drain line crack found by HRST before failure. Caused by lack of expansion allowance.


Attemperator Liner Inspection:Be diligent!Be diligent by having a borescope inspection of your HRSG attemperator piping section! Each year we borescope many attemperators as part of our inspection service. Cracked liner sleeves and broken sleeve supports are becoming a more and more common finding as HRSGs gain operating years and cycles. Sometimes we find cracks in the piping itself. Borescope inspection should be timed for when the attemperator probes are pulled for inspection.Did you perform this inspection a couple of years ago? Maybe no problems were found? Excellent! But don’t scratch this item off your planning list. We see a trend with HRSGs that are about 10 years old….no problems two years ago, then big problems this year. Once an attemperator internal thermal sleeve reaches the end of its fatigue life, degradation happens quick! Our service is typically purchased with our Standard HRSG inspection, so a very cost-effective “adder” to the project. We can also design and supply upgraded components for this area, if problems are found. Authored by: Lester Stanley, P.E.

HRSTPRODUCTS

HRST Retrofit Tube Coil Gas Baffles

Designed for your exact arrangement and includes on-site

Technical Advisor assistance.

HRST engineer performing attemperator borescope inspection.

Installation of new attemperator liner. Designed and supplied by HRST.

Internal liner cracks.Found before failure, barely.

Typical arrangement of probe style attemperator with internal liner.

Robust hardware protects tubes

HRST Gas Baffles minimize gas bypass


We addressed water hammer in Boiler Biz way back in 1999. I related a story about experiencing water hammer up close and how it scared me to death.How do you hammer things with water anyway? Water seems kind of soft and squishy. Well, here in Minnesota, liquid water is scarce for six months out of the year. You could pound nails with Minnesota water. Seriously though, water hammer occurs when an abrupt change in momentum is imposed on a volume of water. Water, being rather heavy and essentially incompressible, resists changing direction instantaneously. Think of the time you did a bellyflop off the diving board. It smacked you pretty good, didn’t it?Four types of water hammer are encountered.Type #1 Thermal Shock – Visualize a steam bubble in sub-cooled water. The outer surface of the bubble is touching the sub-cooled water and rapidly condenses only to expose the next layer of steam to the cool water. The bubble implodes. Wham! This kind of water hammer is characterized by sharp high-pitched banging noises and a shaking of the affected components. Water hammer of this type has the potential to damage. Gauges are particularly vulnerable to the short intense pressure spikes.Type #2 Flow Shock – Hammer occurs when slugs of water are flowing in a pipe with high velocity steam. All is well until the slug of water comes to a turn. Bam! The slug of water wants to go straight and the pipe resists. The pipe shakes violently. Pipe supports are stressed to the breaking point. And then it’s over. What just happened??Type #3 Hydraulic Shock – This form happens when a rapidly moving column of water is suddenly stopped. Say we have high flow in a pipe and a valve slams shut. The water slams against the valve with a single bang. Typically this is not as violent as Type #2 because the velocity of pure water in a pipe is much slower than the velocity of a slug of water being driven by steam flow.Type #4 Pressure Driven Thermal Shock – This form is similar to Type #1. Rather than sub-cooled water and steam coming together, the water and steam start out as a two-phase saturated mixture. With a sudden rise in pressure, the saturation temperature of the steam suddenly rises. The water temperature is unaffected but at the new pressure the water is now sub-cooled. The steam bubbles in the sub-cooled water begin to implode. This kind of water hammer is characterized by a rumbling that lasts for a few seconds.What you should knowType #1 water hammer occurs when steam and sub-cooled water come together:• Steam enters a tank that contains sub-cooled water.• A bottled up economizer has been steaming. Flow begins

and sub-cooled water enters, collapsing the steam.• Sparging steam is injected into a cool HRSG.When the combination of steam and sub-cooled water is a

given, the resultant hammering can be reduced to a growl with a design that makes the bubbles small. Implosions are still occurring, but they are small and far less violent. Often, Type #1 water hammer occurs when the vapor phase wasn’t supposed to be there in the first place, say a steaming economizer. Most economizers can be vented. Venting of the steam and water hammer cannot occur.Type #2 often occurs in reheater piping on start-up. The water wasn’t supposed to be there. The solution is simple. Make sure the piping is drained before steam flow begins. Well, okay, not so simple if the drains don’t exist. When you see reheater piping off the supports and mechanically damaged insulation you ought to suspect a water hammer event. Chances are, you’re

right.Type #3 water hammer is not as common in our industry. Obviously, if water hammer is the result of a valve that closes suddenly, then arranging for the valve to close more slowly solves the problem. Failing that, a water hammer arrestor (like a shock absorber or accumulator) can be installed. Their use is rare in boiler piping, but they are quite common on hydraulic systems.Type #4 water hammer is less common in HRSG applications and requires a sudden pressure increase on a steam/water mixture. If it happens, perhaps either the steam or the water didn’t belong in the system.An HRSG example we investigated was water hammer after overnight shutdowns. The client had an LP Economizer with a drum level control valve at the outlet and a leaking check-

valve at the inlet. Each morning, when the condensate pump was started, hammering was heard. The sound resembled a freight train when it starts moving, due to the slack between cars. The noise period was brief, since the series of vapor pockets collapsed quickly with the increase in pressure (and saturation temperature).Another solution is good ear protection so you barely know it is happening. Just stand back in case something lets go.

Water Hammer

Authored by: Robert Krowech, P.E. Contributions by: Ned Congdon, P.E.

Cold reheat pipe with insulation damage from Water Hammer

Type #2 Flow Shock Water Hammer

SAINT PAUL, MNHRSG ACADEMY!

JUNE 16-18, 2014

This 3-day course focuses on the key points that HRSG operations and maintenance personnel need to understand to minimize forced outages, avoid unnecessary repairs and improve performance. Each presentation is full of photos and illustrations showing actual problems. We share our lessons-learned from hundreds of HRSG inspections and problem investigations. We also do our best to make it an adventure!

G.H., TransCanada Energy, “Excellent common sense approach to inspection, maintenance and repairs. Good preventable maintenance approach to avoid extended shutdowns..”

W.H., Magnolia Energy, “A wealth of useful information professionally done!”

The Learning Adventure Continues!

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BIZ®

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NEWSTHE BOILER BIZ NEWSLE T TER IS AVAIL ABLE FREE TO OPER ATORS AND OWNERS OF POWER PL ANTS. VISIT W W W.HRSTINC.COM TO SIGN-UP.

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