A Two Phase Steam/Condensate Question
A Two Phase Steam/Condensate Question
(OP)
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.
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.





RE: A Two Phase Steam/Condensate Question
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.
RE: A Two Phase Steam/Condensate Question
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
RE: A Two Phase Steam/Condensate Question
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.
RE: A Two Phase Steam/Condensate Question
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.
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
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.
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
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.
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
I think you have assumed the pipe is filled with condensate...
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
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?
RE: A Two Phase Steam/Condensate Question
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
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
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.
RE: A Two Phase Steam/Condensate Question
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.
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
I think mass and energy conservation laws are helpfu but ignor two phase dynamics.
Nature abhors a vacuum - steam most of all.
I think the following two phase flow regime occurs inside the non-vented, dead-ended 6"nps pipe with a static steam pressure at 6 psig.
- In horizontal sections, 0.2 kg/s condensate at a depth of 0.3" flows down the 8% gradient at 2 fps (based on the Manning equations).
- Overhead, high quality steam roars along at 50 fps in response to the vacuum created by the imploding steam layer against the cool pipe wall, however only 0.3 psid is required to create this flow (which is confirmed by single phase analysis and applying the Martinelli-Nelson Friction Multiplier).
Static pressure reduction from steam friction losses would require a very large vent, which would generate unacceptable steam release during intermittent blow-off (50 kg/s for 15 minutes every 12 hours and very often during warmup), so its size will be restricted.
Assuming some positive pressure at the vent, the 2 fps condensate stream will accelerate down the 30' drop and arrive at the distribution header doing 44 fps, throughly mixed with high quality steam and (I think) with some static pressure.
Perhaps a 45 degree inclined descent would help mitigate mixing and pressurizing?
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
I tried a 1/2"dia vent size and detected no significant reduction in saturation temperature/pressure at the vent for the reasons stated above.
I think 3 additional 1/2"dia vents installed directly above the 3"nps pipes and an inclined 30' drop would help relieve pressure, reduce mixing and reduce the amplitude of water cannon loads.
I am severely limited in what I can do because of the dual blow-off function of this line (0.2 kg/s Continuous and 50 kg/s Intermittent).
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
The flow of 0.2 kg/s maintains a steady, saturated pressure of 6 psig at 230 degF. The condensate temperature is the same at the beginning as the end of the 250' line because the steam pressure drop is not significant at this flow.
Reducing the pipe gradient would increase condensate depth and invite steam void collapse transients.
Insulation would need to be removed in order to reduce condensate temperature, however the line needs to be insulated and heat traced for freeze protection.
The 50 kg/s intermittent blow-off is quite unremarkable except for the low howling noise and has never caused support damage in the distribution headers.
The direct contact condensation mode map (attached to my previous post) provides more information on this issue, if you are interested in it.
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
RE: A Two Phase Steam/Condensate Question
Which is why you've got this "pressure equalisation line", which would be better described as a vacuum break line.
For the new arrangment, would suggest one minor upgrade: To prevent steam from running out through this vacuum break line, add a check valve on the vacuum break line air intake, and also include a bug screen to prevent critters from getting in. Use a dual plate check valve with low torque springs and place this check valve on a horizontal section. Else you could also use a tilting disk type check valve.
RE: A Two Phase Steam/Condensate Question
Environmental restrictions on morpholine and hydrazine release to the air limit vent size.
RE: A Two Phase Steam/Condensate Question
If that is unacceptable, how about incorporating some form of radiator or fan coil to reject some heat before the blow down heads into the discharge pipe. Or better yet Heat your boiler feed water with this rejected Heat.
RE: A Two Phase Steam/Condensate Question
I am saying that the vent size is restricted by the steam that is ejected during the 50 kg/s (396,832 pounds per hour) intermittent blow-off, which generates approximately 100 psig backpressure at the vent location.
RE: A Two Phase Steam/Condensate Question
I wish you were correct, and I apologize for not showing the vacuum breakers installed on each 3"nps distribution line just upstream of each elbow.
These vacuum breakers work very well following intermittent blow-off (when continuous blow-off is not in service), when condensation attempts to pull the lake up into the boilers.
This water hammer scenario is not a problem with the buried pipe configuration and should not be with the elevated pipe configuration.
I actually want steam to 'run out' of this vent to prevent pressurizing the header and loading the 14' water cannons.
RE: A Two Phase Steam/Condensate Question
Alternatively, if you estimate the backpressure at the end of the 250ft line is 100psig during a 50kg/sec intermittent blowdown, a better solution would be to (a) increase the line size on these 3inch tails to decrease the backpressure and (b) immerse these discharge tails deeper down into the lake to prevent a breach of the liquid seal.
100psig backpressure does seem too much for a liquid seal - So, for example, if you have 6 new tails each 4inch dia would bring the max backpressure down to 50psig, then ask for these tails to dip down 3.5x33 = 115ft below the water surface. Am presuming the bulk of this backpressure is due to the hydraulic limitation of these tails.
RE: A Two Phase Steam/Condensate Question
I will assume that you are being serious, (although I wonder about the 115 foot extension) so if you open the direct contact condensation mode map that I attached on my Aug 8 entry, you will see the conditions under which water cannons operate.
Briefly, it is when the rate of steam flow down a submerged vertical pipe slows until it is less than the rate of condensation at the interface with the much cooler water inside the pipe.
The vacuum created allows atmospheric pressure to push this captive column of water up the pipe at an unbelievable rate and deliver a significant momentum force to the first elbow.
RE: A Two Phase Steam/Condensate Question
From first principles, the total surface area presented at the current water - steam interface in these 3inch tails at the lake isnt much; there is some local cooling effect, and we could account for this approximately in the compressible flow calcs that you'd be using to compute max built up backpressure. The cooling effect from this pipe being exposed to the atmosphere and the condensation it causes should also be taken into account in the compressible flow calcs. Suggest using isothermal compressible flow behaviour for each of say 10segments in the 250ft pipe and another 2-3segments in the tails at the lake. Use average conditions for each segment with trial values for segment temperature. But I suspect the max built up backpressure will be in summer at zero cross windspeed and little or no cooling effect.
This 50psig example described earlier is only to illustrate how you can set up these liquid seals. If your lake depth isnt much, you can adjust the no of tails and the dia of each tail to derive a new backpressure to suit the liquid seal depth you may be limited to at this lake. You can then scrap this open vent line you've currently got on this sketch. If it all seems a little too much, reduce the intermittent flow of 50kg/sec to derive a new backpressure that is manegeable.
RE: A Two Phase Steam/Condensate Question
That being said I really think you need a blowdown separator somewhere in the building. Do you have any low pressure steam requirements where you could Harvest flash steam and send it into a low pressure system?
RE: A Two Phase Steam/Condensate Question
Changes to continuous and intermittent blow-off flow rates, pipe diameters and process are not on the table.
The elevated re-route is proposed to avoid digging and replacing buried pipe because it will interrupt production less during installation.
I am convinced that the elevated re-route will present an elevated threat to production following installation due to distribution header pressurization, which will increase the incidence of water canon degradation.
Unless someone out there has analytical or real world experience with two phase flow dynamics in vertical lines, I will snip this thread.
RE: A Two Phase Steam/Condensate Question
That leaves us with the following options I can see so far, some of which are previously suggested by others:
a) Blowdown sep with excess pressure steam bled back into steam plant
b) 2phase blowdown cooler, either fin fan type or ambient cooled, to cool this stream down to less than 100degC
c) Quench this blowdown 2phase stream to less that 100degC by instrumentated automated injection of raw or inhibited water
d) Reduce the blowdown rate to suit the current cooling - condensation capacity of this line - ie automate the blowdown rate at the blowdown valve to permit a stream temperature of no more than 100degC at the tails.
e) Cool this blowdown stream before the blowdown valve to less than 100degC - this single phase cooler should be less of a challenge to design and operate that the alternate 2phase cooler option (a). You could cool this with fin fan aerial cooler, or a heat exchanger with cold BFW on the other side of the HX.
At the moment, without any of these mods, I can only see an increase in ambient cooling / condensation capacity with this elevated pipe, when compared to the previous buried line, so with blowdown rates kept constant at 0.2 kg/sec, you'd now have less backpressure at the tails. But at higher blowdown rates, high backpressure / breach of the liquid seals in the tails is to be expected without some means of dealing with excess flash steam.
RE: A Two Phase Steam/Condensate Question
Thanks to all who participated in this discussion.
My report will read something like this:
My calculations indicate that the following two phase flow regime occurs inside the non-vented, dead-ended 6"nps blow-off pipe at a static steam pressure at 6 psig.
In horizontal sections, 0.2 kg/s condensate at a depth of 0.3" flows down the 8% gradient at 2 fps (based on the Manning equations).
Overhead, high quality steam roars along at 50 fps in response to the vacuum created by the condensing steam layer against the cool pipe wall, however only 0.3 psid is required to create this flow (which is confirmed by single phase analysis and applying the Martinelli-Nelson Friction Multiplier).
The vent size required to reduce static pressure from steam friction losses would have to be very large, which would generate unacceptable steam release during intermittent blow-off (50 kg/s at 100 psig).
I tried a 1/2"dia vent similar to U1/4 and detected no significant reduction in saturation temperature/pressure for the reasons stated above.
The 2 fps condensate stream will accelerate down the 30' drop and arrive at the distribution header doing 44 fps, thoroughly mixed with high quality steam which should increase its static pressure.
Possible water cannon mitigating solutions might include:
- Two large vents (for redundancy) with excess flow valves at the 30 ft elevation to prevent intermittent blow-off releases
- Vents on each 3” pipe instead of vacuum breakers with pipes or hoses discharging outside the screenhouse
- An inclined 30 ft drop into the south pit to reduce steam/condensate mixing.
- Perforations in the walls of the 3” pipes (below the 240 ft ‘low water level’) to reduce the static pressure in the distribution header and the potential momentum load (length of water column in the cannon).
RE: A Two Phase Steam/Condensate Question