Bottom Blowdown - Required Flow
Bottom Blowdown - Required Flow
(OP)
Can anyone point me in the direction of a Code, best practice, or rule-of-thumb for determining the amount of bottom blowdown required to keep the bottom of a boiler sucked clean and free of silt?
The boiler in question is a 600# process waste heat boiler. The bottom blowdown system currently in place has been proven quite capable of lifting the relief valve on the unit's high pressure blowdown drum. I'm looking for a little guidance to a proper reference to help determine if the bottom blowdown valve is oversized, the blowdown drum and associated piping are undersized/underdesigned, or some combination.
The boiler in question is a 600# process waste heat boiler. The bottom blowdown system currently in place has been proven quite capable of lifting the relief valve on the unit's high pressure blowdown drum. I'm looking for a little guidance to a proper reference to help determine if the bottom blowdown valve is oversized, the blowdown drum and associated piping are undersized/underdesigned, or some combination.





RE: Bottom Blowdown - Required Flow
It is this quick short blowdown that is most effective.
RE: Bottom Blowdown - Required Flow
The Cv of the bottom blowdown valve currently in place on this system is too large for the attached piping with the differential pressure available. What I'm trying to sort through for alternatives is essentially if the valve is just oversized or if my downstream piping is undersized. Could I get by with bottom blowdown valve with a smaller Cv or put a restriction orifice in the line to limit flow? Or is the blowdown valve sized properly, but my high pressure blowdown drum and its associated vent piping are too restrictive to vent all the flash steam from a good bottom blowdown?
I know either solution, further throttling flow or a bigger vent, could eliminate the overpressure scenario, but I don't know which one is really the proper one for long term reliability of the boiler. Since I don't know how much bottom blowdown flow rate this system should be sized for to begin with, I'm stuck on what should be modified to eliminate this overpressure concern.
RE: Bottom Blowdown - Required Flow
RE: Bottom Blowdown - Required Flow
I would say that is would be much less expensive to get a smaller diameter valve than to change all of the piping and the drum. Also, make sure it stays open for only a matter of seconds!! This may also be the problem.
Then, next time you open the mud drum and it is filled with mud, you know the smaller valve did not do a good job!
Finally, not to be argumentative, but bottom blowdown is NOT based on water chemistry, this is controlled by the continuous (top) blowdown.
RE: Bottom Blowdown - Required Flow
I2I
RE: Bottom Blowdown - Required Flow
rmw
RE: Bottom Blowdown - Required Flow
RE: Bottom Blowdown - Required Flow
Often , the scope of supply of the vent line / exhaust stack is not from the same party that is supplying the blowdown tank. As this vent stack may be quite long , the supplier has a strong economic interest in ignoring its process requirement to ensure the blowdown tank does not exceed ( 15 psig) during startup operations.
To confirm the sizing of the vent, obtain from the tank vendor or designer the design steam flowrate expected to be vented during startup operations and confirm the 15 psig tank operating pressure design point. The proceed to calculate what diameter vent is needed to ensure the pressure drop from tank to atmosphere is less than ( 15 psig) .
The max flow thru the bottom blowdown is highly dependent on where the vale is locate ie how much piping is between the valev outlet and the tank inlet. Also ASME sect I and B31.1 have strict desing guidelines for this vale and its piping- it is worth reading.
RE: Bottom Blowdown - Required Flow
The vent is clearly undersized for the flow we're getting, but what's not clear to me is if we have too much blowdown capability or not enough vent. Somebody missed the design on one portion of this system, I'm just now sure which. Just swapping out the valve would definitely be easier, but I'd prefer to make the right recommendation from a process standpoint for one of the potential alternatives.
RE: Bottom Blowdown - Required Flow
I don't recall all the details but on one 650 psig HRSG boiler mud drum we had both a blowdown and drain valve. The blowdown was something like 1" and the drain was 2 1/2". During a turnaround the two lines were swapped and the first time the larger valve was used a blowdown it caused the crash shutdown of the boiler with the resultant quench of the tubes. We were quite lucky that no significant damage was done to the generator tubes, though it took a little rerolling to stop the weepers and new anti-vibration bars for the screen tubes.
RE: Bottom Blowdown - Required Flow
Hope this helps. My experience is with fire-tubes, but they need to get rid of mud and crud as with water-tubes.
RE: Bottom Blowdown - Required Flow
The theory is that the initial rush of water will be subcooled by the cold water in the blowdon tank (Steam must be cooled before discharging into the sewer). This water very quickly turns to steam and along with the blowdown water will discharge out of the top of the vent.
As long as there is no risk to people or property there is nothing wrong with this.
What is critically important is that you do get good flow out of you mud drum blowdown valve to ensure you do draw out all the solids. I would not recommend reducing the size of your blowdown valve or line at all. You just need to make sure the blowdown tank can discharge safely.