Desuperheater 2

[garfio]

While looking for a desuperheater that provides a very low superheat degree (per process design), I came across the Komax desuperheater that offers a simple equipment and to be within 5°F of the saturated temperature. For the specific applications, they offered to be within 1°F of saturation. Anybody has experience/comments about this equipment?

(

http://www.komax.com/products/desuperheater.html)

 

[davefitz]

There are many desuperheaters that are provided within the scope of supply of some large control valves which allow desuperheating into the wet region, if required. The control logic is sometimes called " enthalpy control", and you will need to measure the inlet water temp, water flowrate, inlet steam T + P, and estimate the steamflow rate. The PLC will calculate steam properties in real time and permform a 1st law heat balance and control the reate of water flow to provide an outlet steam enthalpy of your choice.

In the case of steam dumps into a surface condenser, the outlet steam volumentric flowrate is directly proportional to the outlet steam pressure. For some toher control valves ( such as Lesley valves), the valve stem position and the dp across the inlet to the valve vena contracta can be combined to yield the apparent inlet steam flowrate. The water flowrate is monitored by an orifice plate flow element.

 

[garfio]

Thanks for the information. My question was more in the direction of confirming that this equipment (with only a temperature feedback) could get at less than 1°F superheat. There are several types of desuperheaters, but most of them offer only between 7 and 10° of seperheat. Any experience?

 

[davefitz]

The accuracy of the thermocouple is usually in the range of 5-8 F due to corrosion of the bimetal connection. Most conservative guidelines on a boiler is to maintain a 15 F superheat, if temperature controlled.

If you are going to use a 1 F superheat, better make sure the ratio of water to steam flow may not exceed 0.35 :1 ; this is a backup safety measure to avoid water hammer and/or other damage due to excess water flow entering teh steam devices. I ahve seen temperature controlled desuperheaters overpsray to the level of very high water in the steam line.

 

[davefitz]

I forgot to add: To even hope for a 1 F pinch point with temp controlled spray , the outlet temperature thermocouple must be located sufficiently far downstream that a steam transit time of greater than 0.7 seconds from spray nozzle to thermocouple is ensured. This distance may need to be increased based on the spray water droplet surface tension ( function of steam pressure) or spray water subcooling . Also, to ensure no water is separated from the steam prior to evaporation, the elbow downstreamof teh spray must be sufficiently far downstream to ensure a 0.35 sec transit time from spray nozzle to elbow.

Above guidelines are from an old Fisher spray attemporator design book, and confimred by techpapers published by Deutsche Babcock design engineers in the 1980's. Based on expereience with damage caused by not meeting these guidelines, I concur with their accuracy.

 

[davefitz]

error:

reviewing old notes, the transit time from spray nozzle to thermocouple to be GT 0.35 sec, and to downstream elbow GT 0.15 sec.

brain cells are retiring at a fast rate now.

 

[JimCasey]

.35 seconds is still a long way:
Every time I have used the ideal gas laws to calculate Mach, steam came out with its speed of sound somewhere around 1500 ft/sec. (I know that's variable according to temp, but it's still a rule-of-thumb number)
If the discharge from the valve expands to 1/3 Mach velocity, then that's 1500/3=500 ft/sec, so the 0.35 sec transit time to the T/c occurs 175 feet downstream!..

 

[insult2injury]

500 ft/s is a fairly high velocity for a steam line especially so close to saturation temperature and in the presence of erosive spray droplets. Typical steam velocities for this application would be closer to 150-200 ft/s. Most thermocouples would not be designed for a velocity of 500 ft/s.

I2I

 

[davefitz]

Yes, its a long way. In commercial practice, it might only be met on long runs of transfer piping ( reheater inlet , bypass system discharges). We rarely see boiler mfrs meet this design guideline- and the result of a short spray header outlet pipe straight section is water separating from the steam at the next elbow, and liquid water hitting some downstream feeders and tubes while some other feeders and tubes get dry steam.

The large difference in inlet enthalpy to the individual tubes of the next heat exchanger results in large heat exchanger outlet tube -to- tube temperature differences and resulting loss in hottest tube creep life and high thermal stress at tube to header weld interfaces.

One way the boiler mfrs try to skirt this issue is to limit the range of boiler loads for which you can use the spray, and mandate a minimum 20 F superheat measured at the downstream thermowell.

On HRSG's we have seen faiulures of large 12" F22 forged nozzles on P91 intermediate reheater inlet headers ; the combination of the 2:1 step change in pressure part wall thickness combined with a rapidly alternating steam temperature hitting the nozzle to header weld ( due to alternating impaction of liquid water from the spray) had lead to a prompt fatigue failure in about 18 months.

On LP bypass /dumps from the HRSG reheater to condenser we have seen gross failures of the spray outlet pipe due to waterhammer when there was inadequate transit time from spray nozzle to monitoring thermowell and outlet temperature control was used to control spray water flow. Relocating the thermowell downstream onto the condenser nozzle , upgrading the spray nozzle type, converting to enthalpy control with overide to prevent the ratio of water flow to steam flow from exceeding 0.4:1 ended the waterhammer issue.

 

[CJKruger]

My cynical view on desuperheaters are:
- Buy the most expensive one you can afford.
- Buy the one with the largest pressure drop you can accommodate.
- Install the TI way, way, way down the pipe.
- Clearly specify your turndown requirements.
- Design the rest of the plant assuming the desuperheater does not work.

 

[rmw]

Now that is what I call a positive approach-seriously said. I like it.

rmw