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.

 

[georgeverghese]

Realised (to my horror) last night that my previous suggestion of deleting this vent line and dipping these tails deeper into the lake is flawed - it assumes that all flash steam is fully condensed by the time it reaches the tails, and this is most likely not true. Albeit, condensation rates would be higher now that you are going for this elevated 250ft line, but perhaps you've done the heat transfer calcs that show that total condensation of flash steam is not possible, which is why you've got this excess pressure - steam vent line.
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.

 

[Bambie]

SNIP.

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).

 

[lilliput1]

You should use a blowdown separator and blowdown cooler. The blowdown separator would have side tangential entry and vent at the top sized equal to or larger than the inlet pipe. The tangential entry will help separate the liquid condensate from the flash steam. Cooling the condensate will prevent reflashing to steam. top of drops should also be vented.