Continuous Blowdown Tank vs Steam Separator - Is there a difference?

[crazyjpeters]

So, I'm in the midst of evaluating / modifying our CBD tanks, which were really never evaluated when they switched from AVT(R) to AVT(O). In searching for a maximum nozzle velocity, what I've come to notice is that "Blowdown tanks" CBD or IBD, and any design resources/guides seem to show them being around 1:1 diameter : height. However, steam separators (NZ has produced substantial research and design guides) seem to be 1:3 in diameter : height.

The msx steam inlet velocity appears to be fairly pretty accepted as 40 m/s based on research that Bangma did in 1961, and the designs for these TOC (top-of-cyclone) and BOC (bottom-of-cyclone)separators looks to be fairly well established?

But I really feel they're doing the same job, so why the major difference in vessel design? What concerns me is that the tiny inlet nozzle on our existing vessel has much higher inlet velocity, which is too high and blowing through the vessel wall, but if i slow it down to the conventional New-Zealand style TOC velocity, I'm afraid I'll be dropping it too low with respect to centrifugal acceleration, which is what is doing the water removal.

Am I missing something here?

 

[chicopee]

I suspect that the internal pressure within the cyclone separator to be much greater than the internal pressure within the blowdown tank. The principal function of blowdown tank is to control the impurity level of the boiler water whether this control is carried out automatically or manually. Two other reasons for blowdown operation is to control the water level in the steam drum and to keep a lower water temperature before blowdown water is discharged to its final destination. Normally, a large amount of water will be drained during the blowdown period; also there will be a high noise level associated with blowdown. All these factors have a direct relationship to the size and shape of blowdown tanks.

 

[crazyjpeters]

I understand a blowdown tank's function. Note that this is a continuous blowdown tank, not bottom or intermittent blowdown. It is intended to be receiving blowdown constantly, to maintain low silica and to control cation conductivity. I would say in our operation, intermittent blowdown (IBD tank) is used for drum level control, and decreasing water temperature (though the CBD definitely does some of that).

My existing tank is relatively comparable to the Wairakei Separator (I had reviewed McKibbin's paper on this due to the similarities between them). The wairakei separator operates at 11 bar abs, at 184C, and ours at 7.5 bar abs, and 181C I believe. Theirs with a quality of 36%, ours at 50%. Not exact, but very similar, and yet, the tanks look nothing alike. Of course, there's the obvious difference that the Wairakei Separator is a BOC type, with the steam exhaust ported out the bottom of the tank, but that's more of a layout than a significant difference in dimensions.

 

[davefitz]

The BD tank has a manway to allow internal inspection and repair / replacement of the anti erosion liner. Its large dia implies a lower design pressure would be expected, so the allowable pressure loss in its exhaust stack would be low. This imlies a large dia vent pipe if the tank is used at a 250' tall power plant. The low design pressure sometimes allows rationalizing the use of drains from different pressure class sources. It would require spray attemporation to maintain its CS / 304 shell to be held below 800F.

The separator would require boroscope inspection of erosion damage , and likely would be fabricated of a higher alloy that is less likely to erode during operation, It can use a higher designg pressure ( assuming all drains to it are from the same pressure class source) and also may utilize a much smaller vent stack diameter. The higher alloy might be suitable for higher steam temps and might not require attemporation.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick