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This is to get your attention. This is what 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 basics of

attemporation/desuperheating and give you some things to look at so this doesnt

happen at your plants.

Root Cause: The primary failure mechanism has been preliminarily identified as ID initiated

thermal fatigue cracking resulting from thermal downshock from an upstream

attemperator.

Another common mode of failure with desuperheating problems.

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Desuperheating (or attemporating) is the process of cooling steam. There are no easy rules

or formulas regulating the process. A number of different rough guidelines must be

examined. The amount of straight pipe with minimal turbulence, location of the measuring

element and style of spray nozzle are critical. There is very little margin for error. Even in a

well engineered system there is limited capability to operate outside the design conditions.

The reality is engineers frequently fail to understand the implications of failure to follow

recommended practice. This results in long term operating issues for systems that are

either poor designed or being operated outside of their design condition. This paper will

examine desuperheating in detail.

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Efficiency,

Why desuperheat?

Piping, metallurgy

Condenser limitations

Heat transfer/HX

By far the most common method of desuperheating is by mixing water in a stream of

steam. At typical layout is shown in figure 1. Water passes through control valve and is

sprayed into stream of steam. The temperature is measured at a point downstream and

the control system regulates the water flow/control valve position based on the measured

temperature.

Slide shows a sphere of liquid water surrounded by an atmosphere of superheated steam.

Assuming it is in a perfectly insulated box (no heat loss) the vapor is cooling and the liquid

is heating (Note 1). Heat is always in motion from warmer bodies to colder (Note 2). The

rate of the heat transfer is driven by the temperature difference between the liquid and

vapor and the surface area of the liquid sphere. It takes fixed amount of energy (measured

in British Thermal Units, BTUs) to increase the temperature of the water sphere. This

energy come from the steam vapor thus cooling it.

Note 1: The perfectly insulated box is an example of the First Law of Thermodynamics or

the Conservation of Energy. The energy of a closed system is constant. In slides 5-9 the

perfectly insulated box (closed system) has a constant energy of 13,842 BTUs.

Note 2: The motion of heat from colder bodies to warmer is an example of the Second Law

of Thermodynamics. Isolated systems spontaneously evolve towards thermal equilibrium.

This is somewhat of a simplification and the second law is beyond the scope of this paper.

The liquid sphere is a temperature below the saturation point. This is the condition water

normally enters a desuperheater. It is very difficult to deliver liquid water at the saturation

temperature. For a period of time dependent on the surface area of the sphere and

difference in temperature between the superheated steam and liquid water the liquid will

heat (and steam cool) with no change in state of any liquid. During this period the actual

cooling amount is relatively small since it takes very little energy to heat the liquid (Note 3).

Once the surface of the liquid reaches the saturation temperature for the steam pressure

the steam cooling progresses more rapidly.

Note 3: The amount of energy to heat a body of liquid water is small relative to the

amount of energy required to change the state from liquid to vapor. Saturated water at

400 PSIA has an energy level of 424.2 BTU/lb. Saturated steam at 400 PSIA has an energy

level of 1204.6 Btu/lb. This difference is called the latent heat of evaporation, in this case

780.4 Btus are required to change a pound of saturated water to steam. Note that this is a

constant temperature process. The latent heat of evaporation changes with pressure. It

varies from over a 1000 BTUs per pound under vacuum conditions to zero at the liquid

vapor critical point of 3208 PSIA. For exact values consult a steam table.

This shows the liquid sphere heated to the saturation temperature. The liquid now begins

changing state rather than simply heating. Assume the insulated container maintains the

liquid and vapor mix at constant pressure; the container will need to expand as the liquid

evaporates (Note 4). As the liquid evaporates the sphere gets smaller reducing the surface.

At the completion of the liquid evaporation the steam is cooler by the amount of energy

required to heat the liquid to the saturation point and evaporate it. The time required is

the sum of the two processes.

Note 4: The specific volume of water increases with the change in state. Liquid water at

400 PSIA saturation has a specific volume of 0.01934 cubic feet per pound. Steam vapor at

400 PSIA saturation has a specific volume of 1.1610 cubic feet per pound. The pound of

water will increase in volume by a factor of 60.

The time required to evaporate the liquid is a function of surface area and temperature

difference between the steam/vapor and water/liquid (Note 5). The process conditions

largely dictate the temperature difference. Therefore the time required to cool the steam

is largely driven by the droplet size. Smaller drops mean shorter times as shown in figure 5.

The available time is a function of the steam velocity and straight length of pipe.

Note 5: The heat transfer equations are reasonably complex and outside the scope of this

paper (http://en.wikipedia.org/wiki/Heat_equation).

You do not need to know this equation, 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. We 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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Lower the final steam temperature, a more water is required, doesnt really change the

time to evaporate much.

Might increase a little, more collisions, might not change at all.

Might get even decrease with better atomization of the water, smaller drops.

Water flow increases to ~ 10kpph

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Doubling the steam flow will also increase the water demand to ~ 10kpph, and the time

to evaporate stays about the same (small delta T change), but the flow velocity in the

pipe doubles.

Increased steam flow requires increased water flow. This is more water droplets with an

increased steam velocity. Increased steam flow requires increased water flow.

Well designed system can handle ~ 3:1 turndown, meaning if it is designed for 300,000

PPH, could cool 100,000

Paying attention to the details, might get 5:1.

Typically guidelines may specify the number of straight lengths down stream as a function

of pipe diameter.

There are few no one size fits all rules of thumb, the math has to be done.

What do I want you to take away .

Surface area counts, small drops, broken spray nozzles dont work.

Just like pouring water in the pipe

What kind of flow conditions to combined cycle plants put on bypasses, HP and HRH

desuperheaters?

Is it more than 5:1?

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Bypass valves, a great idea right.

Combine two functions into one device, pressure reducing station and desuperheater all in

one.

It does reduce cost, but there are some compromises.

Need a diffuser to get short length and quiet it down.

D DMAA simplemechanically atomized desuperheater with single ormultiple, fixed-geometry spray nozzles is

intended for applications with nearly constant load. The DMA is installed through a flanged connection on the

side

of a DN 150 (NPS 6) or larger pipeline.Maximum unit CV is 3.8.

D DMA/AFA variable-geometry,mechanically atomized, back-pressure-activated desuperheater with one,

two, or three spray nozzles is designed for applications requiring control over moderate load fluctuations. The

DMA/AF

desuperheater (figure 2) is installed through a flanged connection on the side of a DN 200 (NPS 8) or larger

pipeline. Maximum unit CV is 15.0.

D DMA/AF-HTC The DMA/AF-HTC is functionally equivalent to the DMA/AF, however it is structurally suited

for more severe applications. The most common applications include boiler interstage attemperation, where

the

desuperheater is exposed to high thermal cycling and stress, high steam velocities and flow induced

vibration. In addition to this specific application, the DMA/AF-HTC is suitable for other severe desuperheating

application

environments. The DMA/AF-HTC uses a construction optimized to move weld joints away from high stress

regions.

Not all nozzles are the same, they can have sizes and Cv ranges.

This is good news in that it can help if there are basic design/ sizing issues

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And my personal favorite, a pipe with holes, no nozzles.

Los of issues with these.

Can work, but only under a very narrow flow condition and near perfect control valve

sizing/pressure drop allocation.

From the moment the liquid enters the steam stream the process starts to unravel. Steam has turbulence, turbulence creates collisions between the droplets and they collide to form larger droplets (and reduced surface area). Gravity begins bringing the droplets to the bottom of the pipe. And eventually an elbow may be encountered and centrifugal force drives the liquid to the wall of the pipe. Overspray 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 representative of the actual steam temperature. Other 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 liquid and steam. This makes it extremely difficult to get the vapor to the 100% steam quality saturated condition (which is a desired condition in the process industries for heat exchangers).

Poor droplet formation (atomization); large drops are harder to accelerate to the steam velocity, take longer to evaporate and the large mass makes them more subject to turbulence (changes in direction).

Change in process conditions; increasing the flow or lowering the pressure increases the steam velocity lowering the available time to evaporate.

Change in process conditions; decreasing the flow or increasing the pressure results in the control valve throttling too low (sitting close to the seat)

so there is insufficient pressure left to get adequate atomization. Low velocity and big liquid drops result in water simply pouring into the bottom of the pipe.

Leaking control and block valves; water collects on the bottom of the pipe creating stress on the piping system. Once the water drops to the bottom of the pipe it essentially becomes unavailable for cooling the steam. Very little of it ends up evaporating; it is removed by drains or exits into the process.

How many people do all of the basic maintenance?

At what interval?

How about the thermocouples and infrared?

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Typical find, usually hard to see on the borescope.

Note the crack near the base of the flange.

Where do you think that water went?

Spray pattern?

Nozzle stuck open

Once again, if there is anything you take away, inspect sprays, small drops

Instrument the pipe

HRST is working on a portable package to record pipe temperatures.

Bryan Craig, Craig Dube, Jacob Bartol or myself can get you started.

This is an elbow downstream of a HP desuperheater.

Red line is the difference between the bottom of the pipe and the outside of the elbow.

Early in the startup the bottom is cooler, likely meaning water is rolling down the bottom of

the pipe.

Then the difference goes to zero.

Suddenly the outside gets very cold relative to the bottom and finally after startup they

equalize.

Well look at this condition closely later on.

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Leaking HRH desuperheater, compounded by drain issues

Another view of previous

Was like this for years when NAES took what we call care, custody and control

About a year after CCC a crack developed in the HRH inlet to the turbine

Took a boat sample.

Results were less than clear, OEM lab showed creep, other metallurgists were not quite in

agreement

Due in no small part to the lack of history on the balance of the HRH piping system the

plant went into a extended outage to assess the piping system and take boat samples at

numerous locations.

Waterhammer downstream of a bypass

More bypass damage

Example of how it pipes get quenched

Id 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.

Can someone tell me what would happen?

Root Cause: 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 spider web or crazed pattern and

subsequent 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 Reheat Spray Water Regulating Valve,

designated 2HR-TV-2040.

Red Line delta, notice delta return to zero in the middle, but only because water is at the

bottom of the pipe too!

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Note 1: Pipe is hogged, see page 3

Note 2: Pipe is not round, possibly over 0.5 , see page 8 for wall thickness

Note 3: Location of failures, see pages 4 through 11

Note 4: Wall Thickness 0.625 as measured on outlet November 2013

Note 5: Transition back to 0.500?

Note 6: Moves ~ 18 inches upstream in new design

Note 1: Pipe is not round and may be hogged (installation contractor to specify maximum

allowable hogging)

Note 2: Extend Length of body to total length of ~ 10 feet ~ 0.625 inch wall thickness

Note 3: Reverse water supply piping and move spray nozzles downstream, ~ 12 inches

(distance to be confirmed)

Note 4: Add liner, 0.5 inches off pipe wall, starting 6 inches downstream of sprays, 48

inches total length

Note 5: Machine back to this point, existing 0.780 wall, replace with 0.625 wall

Note 6: Drain location to be determined depends on slope hot/cold (slope 1/8" per foot

towards condenser)

Note 7: Hanger relocation possible up to 12 inches downstream (to be confirmed)

Note 8: Installation contractor to have available ~ 16 length P22, 0.5" wall thickness, 30

inch OD for contingency of hogging

No easy answers, more straight run is the answer 9 times out of 10.

This hairpin was added to increase straight length from ~ 8 feet to 18 feet.

Think the cost was something like 750k per unit.

Tube failures were essentially eliminated.

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