Boiler Blow Down: American Style

My lecturer was telling us about what to do if the boiler level is found to be too high. The list goes like this:

Close feed v/v, secure fires, stop fans, close stop v/v, call the chief and bridge, blow through the gauge glasses, and use the blow down v/v to bring the level down to normal. Then he says, but if you’re writing the USCG exam, you must choose the surface blow down v/v, not the bottom one.

Why? No one seems to know. Any one here have the skinny on why the surface v/v is a better choice than the bottom v/v?

[QUOTE=Emrobu;196280]My lecturer was telling us about what to do if the boiler level is found to be too high. The list goes like this:

Close feed v/v, secure fires, stop fans, close stop v/v, call the chief and bridge, blow through the gauge glasses, and use the blow down v/v to bring the level down to normal. Then he says, but if you’re writing the USCG exam, you must choose the surface blow down v/v, not the bottom one.

Why? No one seems to know. Any one here have the skinny on why the surface v/v is a better choice than the bottom v/v?[/QUOTE]

Well, other than being called for in the boiler guide, I’d think it might be safer to use the surface blow (worst case - drops to a certain level and no more) vice bottom blow (worst case - can drop all the way)? Emergency or remedial actions should be preferentially effective and safe, I imagine one holds more risk, thus less safe and possibly incorrect for exam? Just my two cents.

Correct, if the valves fail you lose all water and boom goes the dynamite. Surface blow should get you down to the steam drum baffle plate level.

[QUOTE=Emrobu;196280] Any one here have the skinny on why the surface v/v is a better choice than the bottom v/v?[/QUOTE]
If a bottom blow valve seat is cut and does not seal when shut the amount of feed loss can be extremely high and continuous. If a surface blow valve fails to seat the results are not quite so potentially disastrous. Remember, there are two valves in series to provide some protection against the failure of one of them, neither are meant to be throttling valves because the seats can be easily cut.

I would rather leak a bit of steam than water. Besides, losing a bit of steam doesn’t screw up the chemistry so badly and doesn’t require draining the boiler to repair.

PS. There is no dynamite in a boiler and if the fires were out at the high high point of this exercise the only explosion likely to happen that day would be the result of some dildo trying light a burner off the brickwork after they got the level under control.

These textbook answers are wrong. First off, to get carry over, one has to REALLY fuck up and totally fill the drum. Even when the high-high or top of sight glass is reached in most boilers, there is still a bit of height to go the get through the separators. Second, it’s likely more harmful to the super heaters due to rapid temp changes and the water will probably not make it all the way to the turbine.

And lastly, the MAIN REASON this is wrong is because blowing down decreases the pressure in the drum and causes drum swell!

In the real world with modern controls (assuming feed valve isn’t stuck open), it is very likely the reason you have a high level on the drum is because the boiler pressure is decaying. More fire is the solution to that. Or, reduce steam demand. Opening a blow down with the fire out will just exasperate the problem and make the drum level go higher (and waste lots of expensive condensate).

http://controlguru.com/dynamic-shrinkswell-and-boiler-level-control/

[QUOTE=johnny.dollar;196373]These textbook answers are wrong. First off, to get carry over, one has to REALLY fuck up and totally fill the drum. Even when the high-high or top of sight glass is reached in most boilers, there is still a bit of height to go the get through the separators. Second, it’s likely more harmful to the super heaters due to rapid temp changes and the water will probably not make it all the way to the turbine.

And lastly, the MAIN REASON this is wrong is because blowing down decreases the pressure in the drum and causes drum swell!

In the real world with modern controls (assuming feed valve isn’t stuck open), it is very likely the reason you have a high level on the drum is because the boiler pressure is decaying. More fire is the solution to that. Or, reduce steam demand. Opening a blow down with the fire out will just exasperate the problem and make the drum level go higher (and waste lots of expensive condensate).

http://controlguru.com/dynamic-shrinkswell-and-boiler-level-control/[/QUOTE]

I have a story about “press” in a steam drum, too. . . .happened while maneuvering. . . entering Rotterdam. Fun was had by all. . . .

[QUOTE=Emrobu;196280]My lecturer was telling us about what to do if the boiler level is found to be too high. The list goes like this:

Close feed v/v, secure fires, stop fans, close stop v/v, call the chief and bridge, blow through the gauge glasses, and use the blow down v/v to bring the level down to normal. Then he says, but if you’re writing the USCG exam, you must choose the surface blow down v/v, not the bottom one.

Why? No one seems to know. Any one here have the skinny on why the surface v/v is a better choice than the bottom v/v?[/QUOTE]

The question posed has to do with what to do, not why the situation occurred.

With regards using the Bottom Blow (when the boiler is operating); Its [B]only[/B] purpose is to remove sludge, scale, and sediment that collects in the bottom of the mud drum. Opening the bottom blow valve can disrupt the natural circulation of water within the boiler resulting in tube or component failure. It should be done strictly in accordance to the the manufacturer’s operating instructions. Generally the bottom blow (and water wall drain) valves are opened and closed in short bursts.

If you have a high water situation opening the surface blow piping will bring the water level down to a safe level but not low enough to be unsafe.

The only time the gauge glasses are blown down is to remove debris that may have accumulated to obscure the reading of the water level, to ensure the piping to the gauge glass is clear, or confirm the gauge glass is full/out of sight. When done care must be taken not to washout/damage the mica protecting the glass. It is done (again) in very short bursts. With that said it is not done to drain down the steam drum.

Let’s go back to the original question and set of conditions.

[I]“Close feed v/v, secure fires, stop fans, close stop v/v, call the chief and bridge, blow through the gauge glasses, and use the blow down v/v to bring the level down to normal.”
[/I]
Then the statement being questioned is presented:

[I]"… if you’re writing the USCG exam, you must choose the surface blow down v/v, not the bottom one." [/I]

With regards using the Bottom Blow (when the boiler is operating); Its [B]only[/B] purpose is to remove sludge, scale, and sediment that collects in the bottom of the mud drum. Opening the bottom blow valve can disrupt the natural circulation of water within the boiler resulting in tube or component failure. It should be done strictly in accordance to the the manufacturer’s operating instructions. Generally the bottom blow (and water wall drain) valves are opened and closed in short bursts.

There is no circulation to speak of, the feed is closed, the steam stop is closed, and the fires are out. Bottom blows are handy for removing enough water to correct chemistry problems or a high level condition. Of course they should not be opened for long periods at high loads.

The only time the gauge glasses are blown down is to remove debris that may have accumulated to obscure the reading of the water level, to ensure the piping to the gauge glass is clear, or confirm the gauge glass is full/out of sight. When done care must be taken not to washout/damage the mica protecting the glass. It is done (again) in very short bursts. With that said it is not done to drain down the steam drum.

Blowing down the glass was probably added in order to prove that a high level existed in the drum and not just in the glass. I don’t think anyone would attempt to blow down a boiler through the glass.

Since I was not in the classroom when the statement parameters were made I made certain assumptions as you did Steamer. I took the feed v/v to reference the feedwater regulator and the stop v/v to reference the feed stop and not the steam stops. If the steam stops were closed, opening the superheater vent should have been included.

The best way to correct chemistry problems is to secure and completely drain the boiler but sans that the surface blow piping is what is recommended if the boiler is online. Bottom blows are more effective in removing sludges and other heavy materials that settle in the lower drum and headers and best done after the boiler has been secured for awhile. See Osbournes Vol I, MEBA’s Modern Marine Engineering Vol 2, Combustion Engineering’s V2M8 Boiler Operating Manual for reference.

If you have a high water situation opening the surface blow piping will bring the water level down to a safe level but not low enough to be unsafe. The reasons why have been stated.

Agree with your last statement which is in line with my original post.

Appreciate all the thoughtful replies.

By feed valve I meant water inlet, by stop valve I meant steam outlets, and the gauge glass blow was only to prove the boiler’s level. The real-ship system that I know best doesn’t have a superheater, just an economiser. Reckon that valve should be closed, though too, good point. I think these instructions were meant to be as general as possible.

I was sitting here practising my boiler and feed system sketches, and thinking. Do you reckon that the top blowdown valve is less prone to damage because it doesn’t get as much pressure going through it? The bottom blow down valves have the head from the whole boiler plus the steam pressure on them. Point well taken about it being a lot safer to have a damaged top blowdown valve than a bottom.

[QUOTE=cmakin;196379]I have a story about “press” in a steam drum, too. . . .happened while maneuvering. . . entering Rotterdam. Fun was had by all. . . .[/QUOTE]

Well? Tell us a story. :slight_smile:

      • Updated - - -

[QUOTE=johnny.dollar;196373]These textbook answers are wrong. First off, to get carry over, one has to REALLY fuck up and totally fill the drum. Even when the high-high or top of sight glass is reached in most boilers, there is still a bit of height to go the get through the separators. Second, it’s likely more harmful to the super heaters due to rapid temp changes and the water will probably not make it all the way to the turbine.

And lastly, the MAIN REASON this is wrong is because blowing down decreases the pressure in the drum and causes drum swell!

In the real world with modern controls (assuming feed valve isn’t stuck open), it is very likely the reason you have a high level on the drum is because the boiler pressure is decaying. More fire is the solution to that. Or, reduce steam demand. Opening a blow down with the fire out will just exasperate the problem and make the drum level go higher (and waste lots of expensive condensate).

http://controlguru.com/dynamic-shrinkswell-and-boiler-level-control/[/QUOTE]

That’s an interesting thought. I’ll bring it up with my instructor. He’s got a steam ticket, so I’ll get his take on this. Maybe “what caused this high level?” is something we need to know before we can fix it.

If we are having water problems can we do a continuous blow using vacuum drag for make up?

[QUOTE=Fraqrat;196413]If we are having water problems can we do a continuous blow using vacuum drag for make up?[/QUOTE]

I wasn’t sure what you meant, so I typed it in to google. So glad I did.

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[QUOTE=Emrobu;196422]I wasn’t sure what you meant, so I typed it in to google. So glad I did.[/QUOTE]

You have just ruined one of most favorite steam plant terms, how do I clear short term memory?

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Okay. “Press” story. We were inbound to Rotterdam, maneuvering. I was one of two Day Third Engineers on an SL7 for Sealand. It was supper time and both of us (Day Thirds) were on supper relief. I was taking bells on the Starboard Wheel and my partner on the Port. . . . Pretty routine, you know. . . a few bells. . . all of a sudden I get a low steam drum level alarm. . .what? Look at the glass and it is still going down. Feed pump pressure okay. . . wait. . . losing turns. . . crap. . hit the throttle valve flipper and NOTHING. . . oh, shit. . .EOT is ringing. . . .At least they want fewer turns. . . I tell my partner that I have a situation so he makes some calls. . . I see the hand pump for the throttle, line it up and start pumping away. . . crap. Still nothing. . . Pop of the cover of the console over the emergency pump and see that the little reservoir of hydraulic oil is cracked and all has leaked out. By this time, other engineers are down. . refill the reservoir and have control with the hand pump. . . Investigation found that the spring in the spring grid coupling had completely rusted away. Went up, ate supper and came down and replaced the coupling. Made a bit of OT. . .

On a side note, if you have seen that insipid movie, Contraband with Mark Wahlberg, you have seen the emergency throttle pumps in action. In the scene where they are trying to control the CP propeller and the engineers are using hand pumps, in reality, they are on one of the SL7s in New Orleans, pumping on the emergency throttle valve pump. . . .

From the time I got the low drum level alarm to when we had control back was probably no longer than 5-7 minutes, but it sure felt like an hour. . . . .

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Circa 1992, any of that look familiar?

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Some. Don’t recall the throttle wheels, but they must have been there. The hand pumps for the emergency throttles are covered by the ash trays. . . I was on the MACLEAN in 81.

That is the USNS Denebola and Wikipedia says that was at one time the Resource. The ash trays were clutch if you had to stand there and answer bells on one side and keep up with both bell books.

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Blow down -UK style.

Usually both scum and blow down valve are single (although not for all boiler types) valves therefore there is always the possibility of wire drawing the valve seats. To avoid this:
+crack the O/B discharge 1/2 turn
+open scum/ blow down slowly until fully open
+watch G/G
+shut scum/blow down at the appropriate level
+shut O/B discharge once pipe work has cooled to avoid vacuum in the O/B line

Also JonnyDollar is not quite right above, Babcock Marine Radiants were notorious for filling the steam drum with water due latency in the feed system controls as well as swell and shrinkage

Under steady state if the M/E, T/A or Cargo pump trips pressure in the boiler rapidly increases causing the water level to plummet and the feed controller to ramp to max, cooling the steam drum but filling the drum with water. Due to the small volume of the steam drum compared to say and ESD III, Babcock fitted a ‘High High’ solenoid to act almost instantly once the water level was above the gauge glasses to avoid carry over. So yes it is possible to fill the steam drum with water without cocking up.

D
(Steam 2Eng, EOOW)

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I don’t know about spring grid couplings. Is this the thing?