A Two Phase Steam/Condensate Question 1

[Bambie]

The continuous boiler blow-off system at our plant discharges a constant mass flow of 0.2 kg/s at 6 psig into a lake via 250' of buried 6"nps pipe with a 12:1 drainage slope.
At the lake surface the 6"nps pipe branches into three 3"nps pipes that drop 14' vertically below the lake surface where they are anchored.
Direct contact condensation (water cannon) plagued this design and damaged anchors until a vent was installed in the 6"nps line which depressurized the 3"nps pipes and maintained the steam/water interface at lake surface elevation.
MIC degradation of the buried pipe has resulted in leaks, so rather than replace it, there is a proposal to re-route the 250' of 6"nps pipe 30' above ground and then drop it down into the existing 3"nps distribution lines.
A vent is proposed at the 30' elevation.
My concern is that a 0.2 kg/s steam/condensate mixture descending 30' under gravity and atmospheric pressure could pressurize the 3"nps distribution lines and 'load the water cannons' so to speak.
My question is whether there is any way to predict the static pressure in this downcomer and whether the 3"nps lines could be effectively vented to prevent water cannoning.
Please see the isometric sketch attached.

 

[mk3223]

to flow up 30' up of the 6" pipe aboveground, the condensate line pressure may be 13 psig at least. Is your system can run at higher pressure?
Regarding the maximum static pressure of the 3" distribution under the water, it can be calculated by adding the 30' high os the liquid head to the system.

 

[MJCronin]

Why do you have a discharge of this hot pure condensate into a lake ?

Most boiler plants take this stream and direct it into a deaerator or into a steam surface condenser.

A flash tank discharging into atmosphere is another option

Do you have a deaerator as part of your plant ?

MJCronin
Sr. Process Engineer

 

[Bambie]

MJCronin,

This is a power plant and maintaining boiler chemistry is paramount. Blow-off heat recovery and inventory recycling was deemed un-economic when the plant was built.

I was hoping the discussion could be about the thermal hydraulics happening in a descending column of steam and condensate.

mk3223,

The flow is always down hill. The boilers are actually 100' above the buried section of pipe.

 

[chicopee]

If you are in the USA, I would think that the environmental people that deal with live organisms, fish, crustacean etc... would be very disturbed until an evaluation was made.
By flowing the steam up 30' you risk the chance of forming condensate before the steam ascends the 30' level, particularly during cold weather. Nonetheless, I would not recommend it. The line should always be pitched downward in the direction of flow which was done with the initial buried installation per your statement.

 

[lilliput1]

Blowdown is required to be cooled to maximum 140F before discharge. Use blowdown cooler.

 

[Bambie]

Chicopee,

The plant is 45 years old - all evaluations have been made and environmentally approved.
The proposed re-route maintains a down hill gradient...the only difference between the two geometries is where the 30 foot drop is located.

lilliput1,

The blow-off header is located in the CCW intake channel where it is thermally tempered before entering the lake as discharge, guaranteeing environmental compliance.

 

[KoachCSR]

You can predict the hydraulic behavior of such two-phase flow using a specialized software package such as RELAP. It won't be cheap, however.

 

[Bambie]

Attaching the sketch again...I don't know where the previous one went.

 

[BronYrAur]

It would seem to me that once the pressure is vented off, the remaining liquid will drop in temperature as is physically drops down the vertical pipes. The water will be below the boiling point and thus should not hammer. I am not familiar with the term "cannon" used in this context, but I assume you mean when flash steam hits water. Any idea what temperature the condensate is just before it gets to the lake?

 

[Bambie]

BronYrAur,

The temperature really depends on how well the vent works (saturation at 0 to 6 psig) is (212 degF to 228 degF).

The 3"nps water cannons can fire slugs when steam flow enters zone 1 on the mode map attached.

0.2 kg/s is .067 kg/s per 3" pipe, which is 15 kg/s/m^2 if pure steam, so water hammer is assured at 6 psig - less amplitude (I suspect) at lower pressures.

 

[chicopee]

At least the sketch explains a few things. If my math is correct, it would take the steam 1.7 hours to travel the length of 250' when it reaches the vent. It seems that during that period of time the steam would have cooled off well below boiling point at ambient temperature unless the plant is located along the equator. If I were you I would do a heat transfer evaluation so that you may have a better idea as to the temperature expected at the vent. When the pipe was buried, heat loss in the ground was probably minimal at steady state which would explain the "canon" effect.

 

[Bambie]

chicopee,

I think you have assumed the pipe is filled with condensate...

 

[georgeverghese]

Doesnt seem like you'll have the concern you've described with this reroute - the water level inside the 3inch lines can be no more than the water level in the lake (give or take a few inches to account for flowing frictional loss), assuming the vent enables pressure equalisation with the outside atmosphere.

 

[lilliput1]

Put blowdown separator with vent to release steam at the bottom of the 5" drop then introduce the cooling water with self acting valve & remote temperature sensor then discharge to the (3) 3" lines. Extend the vent up 30 ft.

 

[Bambie]

lilliput1,

I presume your suggestions result from careful assessment of the two phase mixture leading you to believe there is a significant probability of water hammer?

 

[BronYrAur]

I can't speak for anyone else but what Lillipu1 describe is very common. Most blowdown gets discharged into a city sewer system. There are restrictions on the temperature allowed to be dumped into the sewer as well as possible temperature restrictions on me sewer pipe material itself. Most blowdown systems that I have seen go to a separator which allows flash steam to vent off and then a cold City water pipe is connected to it to temper the water before it hits the sewer. The city water is regulated by a valve and temperature probe.

I think in your case once you blow off the flash steam, you no longer have danger of water hammer. That is just my experience with nothing to back it up in terms of calculations

 

[chicopee]

Before investing into solutions mentioned above, I would do a time/incremental length step heat transfer analysis to determine the temperature at the vent. I suspect that the temperature would drop significantly from the saturation temperature at 6 psig; the time for the blowdown fluid to travel to the vent may be of the order of not less than 16 minutes which was figured for saturated steam with a quality of 1/2%. As the saturated steam travel along the 250' pathway it will cool down and the mass of the fluid will increase thereby slowing down.

 

[LittleInch]

That's a new one for me - had to go and look it up and still struggling a bit to see where the cannon bit comes from and what orientation the 3" pipes are, but clearly this is a researched phenomena and if the outflow isn't big enough then clearly some undesirable things start happening with 6psi wet steam coming into contact with water at a much lower temperature.

However, once you stuck the vent in there, to me this now looks like a very long slim separator with condensate simply draining out the bottom and a free vent on the top. Only if your system starts to fill up and start slugging in the 250' long pipe and blocking up the vent will you start to have problems, but so long as its at the same angle and no pockets I can't see why this would be any different to what you have now.

~So whether you have a 30' drop into the header or a gentle slope into the header makes little difference as far as your scheme goes. IMO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Bambie]

Chicopee,

I think you are underestimating the flow rate down an inclined pipe.
Please see my calculation attached for the Manning estimate.
This methodology predicts a traverse time of 2.3 minutes.
I have also attached a HDS-3 Design Chart for cross-checking the equations.