Boiler Blow-off Blues

[yamoffathoo]

Engineering is proposing to install 4"nps tell-tale drains immediately downstream of 4"nps boiler blow-off valves on horizontal lines. The idea is to give operators 'handier' drains to monitor. This arrangement is probably going to produce flow disturbances from drain water entrainment. Blow-off flows are 100lb/sec at 450 psig and 300 degF. I'd appreciate references to help convince Engineering of this folly.

 

[Duwe6]

Get your Safety Guy involved. ". 100lb/sec at 450 psig and 300 degF." is a lethal steam hazard. It will 'flash' back to steam and nobody will be able to see anything.

If Safety cannot visualize that, take him/her up to the boiler PSV's. Each has a "Try Handle". Attach a 6-ft or longer lanyard to the handle on your biggest PSV and pull it - hard. That never fails to put "the fear of God" into operators, engineers, and especially Safety folks.

p.s. put your earplugs in extra tight and extra deep for this

 

[BigInch]

You might find some references here.

http://www.kirsner.org/pages/condInduceWatHamText.html

http://www.kirsner.org/index.html

From "BigInch's Extremely simple theory of everything."

 

[rmw]

I hope Duwe6's suggestion was rhetorical. Don't pull that string unless you are ready for the ramifications of what is going to happen if the safety valve doesn't re-seat. Sometimes they don't and if the emergency that lifted the safety valve in the first place justifies the unintended aftermath, folks generally accept that as part of the incident.

On the other hand, if it doesn't reseat after this "demonstration" tripping a plant or process for a frivoulous purpose is usually heavily frowned upon.

His/her point about the boiler blow off (and when you mentioned water I assumed that this is blowdown) flashing to steam is well taken, however.

All in all, however, something doesn't make sense about the scenario you presented. A 4" drain on a 4" blowoff (blowdown?) line??? How far does the horizontal line you mention go after the blow off valves? Can you elaborate more?

rmw

 

[yamoffathoo]

Duwe6, I think we got off on the wrong track, the handy drain valves would only be opened during an outage, never during operation.

BigInch, thanks for the links.

rmw, Using B31.1 definitions, the boiler tubesheet blow-off is flashing saturated liquid. There is 20' of horizontal 4"nps line downstream of the boiler blow-off motorized valves then a lateral into a 6"nps pipe which runs approximately 300' down to the lake. The proposed 4"nps branches would be positioned 5' downstream of the MVs.

 

[Duwe6]

Unless those 4" drains, and/or your MOV's are interlocked to make it essentially impossible to open them while there is pressure in the system, the flashing condensate enveloping the person opening the drain is a valid scenario. A 3/4"NPS drain valve, or even a 1/2-inch tubing valve would be enormously safer. Just route the opening down to just above the floordrain. Will still provide visual indication of draining with a greatly reduced risk. 4-inches of steam/flashing condensate is a scary amount of energy to let loose.

Was [sort of] serious about pulling the Try Handle on the PSV. The latest edition of NBIC still requires boiler operators to "try" every boiler PSV every 6 months. Nobody I know does it, but the requirement is still in the [mandatory] NBIC book.

 

[yamoffathoo]

Duwe6, Not only safer, a 1/2" or 3/4" weldolet and drain valve would be far less of a flow disturbance during blow-off. I can only think that a full sized branch connection is intended to intercept all drainage flow - a smaller connection might miss some? I was hoping to find experience with vortex shedding and flow transients from such a large connection. Between blow-offs, the MOV's could leak and fill up this bucket and when they open, high velocity steam could pick up the water and throw it at the first elbow.

 

[BigInch]

Have your ops/safety guys give Kirsner a call.

From "BigInch's Extremely simple theory of everything."

 

[MJCronin]

"Boiler blow-off... runs 300 ft into a lake" ? Why ?

Why is there not more of a conventional blow-off system (i.e. tanks, drins and a vent) installed ?

Why am I the only one here that is uncomfortable with this extended and certainly unique blow-off system ?

 

[yamoffathoo]

These are boilers for turbo generators at a thermal station. Recovering, polishing and re-injecting demineralized water is too costly.

 

[yamoffathoo]

Here is a sketch of the proposed drain connection.

 

[Duwe6]

Proposed sketch looks very good to me. Make sure that the horiz run is well supported - somebody will probably want to climb on it. Also restrain the drain outlet pipe well enough that if it happened to see a 450 psi 'blow', it won't whip around.

Plan for the best, insure against the worst.

 

[yamoffathoo]

Duwe6,
Please explain your advice...how could a 4"nps bucket of water downstream of the blow-off line look 'very good' to you? Is this advice supported by analysis or experience?

 

[yamoffathoo]

I think the following might happen:

1) The leaky blow-off valve fills the branch with cool condensate

2) Initial blow-off is 100% saturated liquid due to the slave/master opening sequence 3"nps slaves on each boiler, 4"nps master on a group of boilers
3) As hotter blow-off enters the master MV, a flashing mixture with increasing velocity acts like a steam ejector and drain water is entrained
4) This very wet mixture of steam and water separates when the 4"nps blow-off pipe empties into the 6"nps pipe and liquid velocity drops
5) A growing tsunami wave of drain water with high velocity steam on top is then accelerated along the 6"nps pipe and momentum forces are transferred to the 90 degree thermal loop elbows
6) Axial and transverse pipe motion fatigues anchoring and guiding supports
7) Even after the drain water inventory is depleted, the 4"nps branch connection would continueto cause flow disturbances from vortex shedding in the high velocity steam flows

Unless I am out-to-lunch, this doesn't sound 'very good' at all.

 

[Duwe6]

I am seeing purpose of this drain is to allow visualization of the end of the dumping of cold condensate from a shut-down boiler. For that it will work well.

If an operator 'jumps the gun' and opens this tell-tale valve early and the condensate is still hot enough to flash, he closes the valve. The valve being a hand-operated gate valve, it cannot be rapidly. A quarter-turn ball valve WOULD be dangerous, as the operator can get surprised by what is in the line, just like your scenario yamoffathoo.

The above presupposes competent boiler operators. If the boiler system is being run/operated by less-than-competent operators, there will be dozens/hundreds of other spots in the system to get similar problems at this tell-tale can present to an idiot. Boilers are inherently dangerous and rather unforgiving if you are incompetent.

 

[yamoffathoo]

For the love of Pete!

Is there anyone who will comment on the propensity for a size-on-size branch connection to cause flow transients from condensate entrainment and vortex shedding?

Lets leave the bloody operators out of this !

 

[yamoffathoo]

Another sketch to aid with visualization.

 

[BigInch]

How can you leave operators out of it?
If you can do that, you don't need this thing.

A 4 on 4 isn't a telltale tap, it's a diversion. Any downstream piping will reverse flow as well.

Transients on a blowdown are guaranteed with any fluid and much worse with phase changes.

From "BigInch's Extremely simple theory of everything."

 

[vpl]

After watching this thread for a while it's becoming apparent that the issue isn't whether or not the proposed drain line is properly engineered. It's about yamoffathoo being able to tell engineering at his place of work that they're wrong.

I'd appreciate references to help convince Engineering of this folly.

He's already decided that the design is no good. He wants anonymous Internet input supporting his position so he can shove it under someone's nose.

The comment about leaving the bloody operators out of it, when combined with the comment of convincing Engineering of its folly, makes me wonder if yamoffathoo isn't one of the bloody Operators.

Patricia Lougheed

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[rmw]

I too had begun to suspect that Yamoffathoo is one of the operators too. I believe he/she is overthinking this while not being burdened by a lot of engineering facts.

First a steam ejector works by accelerating steam in a nozzle to supersonic speeds creating a low pressure zone and then decelerating the stream in a diverging section difusser whereupon a shock wave, the equivalent of a "hydraulic jump" occurs and the pressure increases. I can't picture this process being possible to be created in some blowoff piping.

Second, there is always two phase flow in blow off lines as they take a high pressure saturated fluid to atmospheric pressure. It is the name of the game, and blow off piping survives this abuse for years

Third, blow off piping unless immaculately designed and installed can and usually does contain pockets of cold fluid left over from previous blows or valve leakage, fluid which is disposed of by the fresh blow stream and replaced by new fluid and or new leakage after each blow.

Fourth, there have been a lot of dedicated people trying to assist you with your problem and your 'attitude' towards our colleague VPL is annoying if not downright off-putting. I'm speaking for me at least.

If you are an operator trying to get us Engineers to help you gang up on one of our own, then you are on your own here bub.

rmw