Bottom Blowdown - Required Flow

[Jason5000]

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.

 

[steamdog]

The point of bottom blowdown is not to remove water from the boiler to control water chemistry, but just to remove the sludge that may collect there. Years ago, when I worked for the manufacturer of these bottom blowdown valves, we recommended to blowdown only quickly, maybe ten seconds for each blow. This will suck the most sludge out without wasting water. Number of blows varied depending on exact boiler conditions. From twice a day to once a month!!

It is this quick short blowdown that is most effective.

 

[Jason5000]

What I'm really looking for is how much flow should the bottom blowdown be sized for to be effective at removing sediment from the boiler. I've never seen a sizing basis for one of these. I'm wondering if there are any parameters like a ratio of incoming boiler feed water, or a flow relative to the size of the boiler, something that provides the basis for how much flow is necessary for a good bottom blowdown...

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.

 

[chicopee]

Blowdown frequency and duration are based on your water chemistry.

 

[steamdog]

You cannot put a restriction orifice on the bottom blowdown valve because it would get clogged with exactly the stuff you are trying to remove from the boiler. The code says you are not even allowed dams or pockets in the valve for stuff to collect.
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.

 

[insult2injury]

Most likely, the blowdown tank/vent was incorrectly from the start. Using a smaller valve may be an option, provided you are still able to clear out the drum effectively. It may require you to blowdown more frequently.

I2I

 

[rmw]

Unless you don't have a top (continuous) blowdown.

rmw

 

[MisterDonut]

If the relief valve is opening, I would suspect the vent are not large enough, as insult2injury stated above.

 

[davefitz]

As misterdonut stated, an undersized blowdown tank vent line is common.

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.

 

[Jason5000]

Thanks davefitz, and to everyone who has responded, a code reference was just what I was looking for. For the record, we're not using the bottom blow for water chemistry; we do have a continuous blowdown for that.

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.

 

[unclesyd]

Just make sure you have the right line.

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.

 

[bonair]

ASME BPV code has strict requirements for bottom blow off piping, valves, etc. as steamdog stated in his comment. Last year I had a client who wanted to capture top blow down and bottom blow off from a fire-tube in a common pressure-rated tank and then send it for use in a process rather than discharging to sewer. I put together a "concept" design, but would not make an engineering design for construction due to unclear code and NB limits for the non traditional approach without more research. I agree with davefitz. Something doesn't sound right with the flash steam vent.

Hope this helps. My experience is with fire-tubes, but they need to get rid of mud and crud as with water-tubes.

 

[grantak]

My experience with the boilers I service is that most of them discharge steam from the blowdown tank during bottom blowdown.
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.