Desuperheater question

[trb7578]

A local WtE facility that I am observing had a recent thrust bearing failure in the turbine. They stated the reason that th turbine shifted was the desuperheater is injecting too much water pre-high temp superheater, and is therefor making it through the superheater as droplets and not being fully vaporized and is then impacting the first stage blades, which I am finding hard to beleive. The steam line is a 750°, 600 psig line, and the desuperheater is injecting water 250° as needed. The mass flow rate of the steam is an average of 468.2 klbs/hr. If my math servs me correctly they would need to inject about 488.4 klbs/hr of 250° water bring the steam from superheated to saturation levels. The limits of the nozzle is no where near that high. Are they trying to cover them selves from another issue in the boiler, and basically lie to me, who works for the observational engineering consultent hired by the county/operators, or is my math completely skewed because I am scrathing my brain and trying to remember things I have not touched for about 5 years. Or has anyone else actually heard of an issue simular and can point me in a direction to better understand how this is possible. Thanks

Thom Bartczak
HDR ONE COMPANY | Many Solutions

 

[TheBlacksmith]

You're doing a it as a bulk mass problem. Teh condiitons may not be homogeneous. If the water is injected in large droplets (i.e. poor or missing spray nozzles) it may not all evaporate before reaching the turbine. Also the pipe is probably cooler than the center; I have seen water collect in low flow dips and bends, then get entrained by a sudden up power and damage a turbine.

 

[davefitz]

first , your math is off. If the spray water has an enthalpy of 200 btu/lb, the steam inlet is at 1300 btu/lb, and the outlet steam is 1200 btu/lb, the the ratio of spray water flow to inlet steam flow is 0.15:1 , so an inlet steam flow of 468 k lb/hr only needs 70. kpph of water to spray it down close to sat steam conditions.

second, there are many ways to end up with excess water going to the turbine, due to an incorrectly designed or operated spray. I am sure the design or operation is violating most of the following tules of design and operation:

a)do not operate spray at loads lower than permitted by boiler mfr

b)steam outlet leaving the attemp should be monitored by a thermocuople placed a distance downstream to ensure at leasst a 0.3 sec steam resident time between spray nozzle and thermocoupl.e

c) steam shuld not be sprayed below 20 F superheat unless advanced logic reliably ensures that overspray will not exceed 8% SBW

d) 1st elbow downstream of spray nozzle should be at least a distance greater than needed for a 0.15 sec steam transit time from nozzle to elbow

e)superheater header drain and main steam line drain to be opened during startup ops per STG op instructions and ASME turbine water damage guidelines

f)the flowarte of spray water to be measured by flow element , and ratio of spray water flow to steam flow not to exceed 0.25 :1 for this low pressure low temp boiler case ( this protects against overspray in the event you did not follow items a thru e).

g) get the spray nozzle vendor data sheet that defines the actual spary nmozzle atomization turndown limits, and do not exceed these. For a simple inhjection quil desing , this is only 2.5 :1. For spring loaded desings, this may be 40:1.

 

[davefitz]

error- item c to read " 8% liquid by wt".

 

[MikeHalloran]

The typical marine Diesel wet exhaust system injects 130F water at a velocity of <20mph (pressure limited) into a 1000F gas stream moving at >100mph. If the water were injected radially, the gas would move ~8 ft in the time it takes the water to reach the center of the pipe. The water is rarely injected radially, and even if it were, some rocket scientists still put a rubber elbow immediately downstream of the injection point and wonder why it gets burned. (the numbers are inexact but representative)

Yeah, it's not a desuperheater, but it's trying to do pretty much the same thing. My point is, as Blacksmith says, the mass balance doesn't tell you enough. You have to work out the geometry and the time for interaction of a unit mass of water with the steam.



Mike Halloran
Pembroke Pines, FL, USA