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This is to get get your attention. attention. This is what can can happen desuperheaters are not maintained, not designed correctly, and/or operator error / controls malfunction. Has anybody here had a line failure downstream of desuperheater sprays? How many people here do routine desuperheater inspections? My objective in this presentation is to enlighten you a little on the b asics of attemporation/desuperheating attemporation/desuperheating and give you some things to look at so this doesnt happen at your plants. !oot "ause# The primary failure mechanism has been preliminarily identified as ID initiated thermal fatigue cracking cracking resulting from thermal downshock from an upstream attemperator.
This is to get get your attention. attention. This is what can can happen desuperheaters are not maintained, not designed correctly, and/or operator error / controls malfunction. Has anybody here had a line failure downstream of desuperheater sprays? How many people here do routine desuperheater inspections? My objective in this presentation is to enlighten you a little on the b asics of attemporation/desuperheating attemporation/desuperheating and give you some things to look at so this doesnt happen at your plants. !oot "ause# The primary failure mechanism has been preliminarily identified as ID initiated thermal fatigue cracking cracking resulting from thermal downshock from an upstream attemperator.
Another common mode of failure with desuperheating problems.
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Desuperheating (or (or attemporating) attemporating) is the process of cooling steam. steam. There are no easy rules or formulas regulating the process. process. A number of different rough rough guidelines must be examined. The amount of straight straight pipe with minimal turbulence, location location of the measuring element and style of spray nole nole are critical. There is !ery little margin for error. error. "!en in a well engineered system there is limited capability to operate outside the design conditions. The reality is engineers fre#uently fail to understand understand the implications of failure to follow recommended practice. practice. This results in long term operating issues for systems that are either poor designed or being operated operated outside of their design condition. This paper will examine desuperheating in detail.
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"fficiency, $hy desuperheat% &iping, metallurgy 'ondenser limitations eat transfer*
+y far the most common method of desuperheating is by mixing water in a stream of steam. At typical layout is shown in figure . $ater passes through control !al!e and is sprayed into stream of steam. The temperature is measured at a point downstream and the control system regulates the water flowcontrol !al!e position based on the measured temperature.
-lide shows a sphere of li#uid water surrounded by an atmosphere of superheated steam. Assuming it is in a perfectly insulated box (no heat loss) the !apor is cooling and the li#uid is heating (ote ). eat is always in motion from warmer bodies to colder (ote /). The rate of the heat transfer is dri!en by the temperature difference between the li#uid and !apor and the surface area of the li#uid sphere. It takes fixed amount of energy (measured in +ritish Thermal 0nits, +T0s) to increase the temperature of the water sphere. This energy come from the steam !apor thus cooling it. $ote %# The perfectly insulated box is an example of the 1irst 2aw of Thermodynamics or the 'onser!ation of "nergy. The energy of a closed system is constant. In slides 345 the perfectly insulated box (closed system) has a constant energy of 6,78/ +T0s. $ote &# The motion of heat from colder bodies to warmer is an example of the -econd 2aw of Thermodynamics. Isolated systems spontaneously e!ol!e towards thermal e#uilibrium. This is somewhat of a simplification and the second law is beyond the scope of this paper.
The li#uid sphere is a temperature below the saturation point. This is the condition water normally enters a desuperheater. It is !ery difficult to deli!er li#uid water at the saturation temperature. 1or a period of time dependent on the surface area of the sphere and difference in temperature between the superheated steam and li#uid water the li#uid will heat (and steam cool) with no change in state of any li#uid. During this period the actual cooling amount is relati!ely small since it takes !ery little energy to heat the li#uid (ote 6). 9nce the surface of the li#uid reaches the saturation temperature for the steam pressure the steam cooling progresses more rapidly. $ote '# The amount of energy to heat a body of li#uid water is small relati!e to the amount of energy re#uired to change the state from li#uid to !apor. -aturated water at 8:: &-IA has an energy le!el of 8/8./ +T0lb. -aturated steam at 8:: &-IA has an energy le!el of /:8.; +tulb. This difference is called the latent heat of e!aporation, in this case <7:.8 +tus are re#uired to change a pound of saturated water to steam. ote that this is a constant temperature process. The latent heat of e!aporation changes with pressure. It !aries from o!er a ::: +T0s per pound under !acuum conditions to ero at the li#uid !apor critical point of 6/:7 &-IA. 1or exact !alues consult a steam table.
This shows the li#uid sphere heated to the saturation temperature. The li#uid now begins changing state rather than simply heating. Assume the insulated container maintains the li#uid and !apor mix at constant pressure= the container will need to expand as the li#uid e!aporates (ote 8). As the li#uid e!aporates the sphere gets smaller reducing the surface. At the completion of the li#uid e!aporation the steam is cooler by the amount of energy re#uired to heat the li#uid to the saturation point and e!aporate it. The time re#uired is the sum of the two processes. $ote (# The specific !olume of water increases with the change in state. 2i#uid water at 8:: &-IA saturation has a specific !olume of :.:568 cubic feet per pound. -team !apor at 8:: &-IA saturation has a specific !olume of .;: cubic feet per pound. The pound of water will increase in !olume by a factor of ;:.
The time re#uired to e!aporate the li#uid is a function of surface area and temperature difference between the steam!apor and waterli#uid (ote 3). The process conditions largely dictate the temperature difference. Therefore the time re#uired to cool the steam is largely dri!en by the droplet sie. -maller drops mean shorter times as shown in figure 3. The a!ailable time is a function of the steam !elocity and straight length of pipe.
$ote )# The heat transfer e#uations are reasonably complex and outside the scope of this paper (http>en.wikipedia.orgwikieat?e#uation). @ou do not need to know this e#uation, the point is to remember that heat transfer is a function of delta temperature, area and of course time.
And since the process normally dictates the pressure, temperature and flow, the only things a desuperheater designer has to work with are surface area and time. $e can increase the surface area by more and smaller drops. Time as well see later on is a function of straight pipe length.
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2ower the final steam temperature, a more water is re#uired, doesnt really change the time to e!aporate much. Bight increase a little, more collisions, might not change at all. Bight get e!en decrease with better atomiation of the water, smaller drops. $ater flow increases to C :kpph
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*oubling the steam flow will also increase the water demand to + %kpph, and the time to evaporate stays about the same -small delta T change, but the flow velocity in the pipe doubles. ncreased steam flow re0uires increased water flow. This is more water droplets with an increased steam velocity. ncreased steam flow re0uires increased water flow . $ell designed system can handle C 6> turndown, meaning if it is designed for 6::,::: &&, could cool ::,::: &aying attention to the details, might get 3>. Typically guidelines may specify the number of straight lengths down stream as a function of pipe diameter. There are few no one sie fits all rules of thumb, the math has to be done. $hat do I want you to take away E. -urface area counts, small drops, broken spray noles dont work. Fust like pouring water in the pipe 1hat kind of flow conditions to combined cycle plants put on bypasses, H2 and H!H
desuperheaters? s it more than )#%?
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+ypass !al!es, a great idea right. 'ombine two functions into one de!ice, pressure reducing station and desuperheater all in one. It does reduce cost, but there are some compromises. eed a diffuser to get short length and #uiet it down.
D DBAGA simplemechanically atomied desuperheater with single ormultiple, fixed4geometry spray noles is intended for applications with nearly constant load. The DBA is installed through a flanged connection on the side of a D 3: (&- ;) or larger pipeline.Baximum unit 'H is 6.7. D DBAA1GA !ariable4geometry,mechanically atomied, back4pressure4acti!ated desuperheater with one, two, or three spray noles is designed for applications re#uiring control o!er moderate load fluctuations. The DBAA1 desuperheater (figure /) is installed through a flanged connection on the side of a D /:: (&- 7) or larger pipeline. Baximum unit 'H is 3.:. D DBAA14T'G The DBAA14T' is functionally e#ui!alent to the DBAA1, howe!er it is structurally suited for more se!ere applications. The most common applications include boiler interstage attemperation, where the desuperheater is exposed to high thermal cycling and stress, high steam !elocities and flow induced !ibration. In addition to this specific application, the DBAA14T' is suitable for other se!ere desuperheating application en!ironments. The DBAA14T' uses a construction optimied to mo!e weld oints away from high stress regions.
ot all noles are the same, they can ha!e sies and '! ranges. This is good news in that it can help if there are basic design siing issues
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And my personal fa!orite, a pipe with holes, no noles. 2os of issues with theseE. 'an work, but only under a !ery narrow flow condition and near perfect control !al!e siingpressure drop allocation.
1rom the moment the li#uid enters the steam stream the process starts to unra!el. -team has turbulence, turbulence creates collisions between the droplets and they collide to form larger droplets (and reduced surface area). Jra!ity begins bringing the droplets to the bottom of the pipe. And e!entually an elbow may be encountered and centrifugal force dri!es the li#uid to the wall of the pipe. 9!erspray can also result in the temperature probe being coated with a thin layer of saturated water resulting in a flatlined temperature reading which is not representati!e of the actual steam temperature. 9ther common issues include> Trying to cool too much= the closer the process steam gets to the saturation temperature the lower the delta T becomes between the li#uid and steam. This makes it extremely difficult to get the !apor to the ::K steam #uality saturated condition (which is a desired condition in the process industries for heat exchangers). &oor droplet formation (atomiation)= large drops are harder to accelerate to the steam !elocity, take longer to e!aporate and the large mass makes them more subect to turbulence (changes in direction). 'hange in process conditions= increasing the flow or lowering the pressure increases the steam !elocity lowering the a!ailable time to e!aporate. 'hange in process conditions= decreasing the flow or increasing the pressure results in the control !al!e throttling too low (sitting close to the seat) so there is insufficient pressure left to get ade#uate atomiation. 2ow !elocity and big li#uid drops result in water simply pouring into the bottom of the pipe. 2eaking control and block !al!es= water collects on the bottom of the pipe creating stress on the piping system. 9nce the water drops to the bottom of the pipe it essentially becomes una!ailable for cooling the steam. Hery little of it ends up e!aporating= it is remo!ed by drains or exits into the process.
ow many people do all of the basic maintenance% At what inter!al% ow about the thermocouples and infrared%
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Typical find, usually hard to see on the borescope. ote the crack near the base of the flange. $here do you think that water went% -pray pattern%
ole stuck open 9nce again, if there is anything you take away, inspect sprays, small drops E
Instrument the pipe L-T is working on a portable package to record pipe temperatures. +ryan 'raig, 'raig Dube, Facob +artol or myself can get you started. This is an elbow downstream of a & desuperheater. Led line is the difference between the bottom of the pipe and the outside of the elbow. "arly in the startup the bottom is cooler, likely meaning water is rolling down the bottom of the pipe. Then the difference goes to ero. -uddenly the outside gets !ery cold relati!e to the bottom and finally after startup they e#ualie. $ell look at this condition closely later on.
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2eaking L desuperheater, compounded by drain issues
Another !iew of pre!ious
$as like this for years when A"- took what we call care, custody and control
About a year after ''' a crack de!eloped in the L inlet to the turbine Took a boat sample. Lesults were less than clear, 9"B lab showed creep, other metallurgists were not #uite in agreement Due in no small part to the lack of history on the balance of the L piping system the plant went into a extended outage to assess the piping system and take boat samples at numerous locations.
$aterhammer downstream of a bypass
Bore bypass damage
"xample of how it pipes get #uenched I’d like to use this slide to point out one thing you should never do … If a bypass valve leaks by the downstream temperature creeps up. Has this happened to any of you? The one thing you should never, ever do is simply open the valve. an someone tell me what would happen?
!oot "ause# The primary failure mechanism has been preliminarily identified as ID initiated thermal fatigue cracking resulting from thermal downshock from an upstream attemperator. The ID cracking which exhibited a Mspider webN or McraedN pattern and subse#uent through wall rupture is concentrated primarily at the extrados of the failed elbow. The apparent root cause of the preliminarily identified primary failure mechanism, ID initiated thermal fatigue cracking and the resultant rupture, currently appears to be attributable to the upstream attemperator Leheat -pray $ater Legulating Hal!e, designated /L4TH4/:8:.
Led 2ine delta, notice delta return to ero in the middle, but only because water is at the bottom of the pipe tooO
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ote > &ipe is hogged, see page 6 ote /> &ipe is not round, possibly o!er :.3 M, see page 7 for wall thickness ote 6> 2ocation of failures, see pages 8 through ote 8> $all Thickness :.;/3 as measured on outlet o!ember /:6 ote 3> Transition back to :.3::% ote ;> Bo!es C 7 inches upstream in new design
ote > &ipe is not round and may be hogged (installation contractor to specify maximum allowable hogging) ote /> "xtend 2ength of body to total length of C : feet C :.;/3 inch wall thickness ote 6> Le!erse water supply piping and mo!e spray noles downstream, C / inches (distance to be confirmed) ote 8> Add liner, :.3 inches off pipe wall, starting ; inches downstream of sprays, 87 inches total length ote 3> Bachine back to this point, existing :.<7: wall, replace with :.;/3 wall ote ;> Drain location to be determined depends on slope hotcold (slope 7P per foot towards condenser) ote <> anger relocation possible up to / inches downstream (to be confirmed) ote 7> Installation contractor to ha!e a!ailable C ; length &//, :.3P wall thickness, 6: inch 9D for contingency of hogging
o easy answers, more straight run is the answer 5 times out of :. This hairpin was added to increase straight length from C 7 feet to 7 feet. Think the cost was something like <3:k per unit. Tube failures were essentially eliminated.
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