Dumping Condensate from High Pressure Mains into adjacent Pumped Condensate Returns to to the Steam Plant Should be Avoided

Transcription

1 Dumping Condensate from High Pressure Mains into adjacent Pumped Condensate Returns to to the Steam Plant Should be Avoided

2 HighTemp Condensate 1. When 338 o F condensate passes through trap to 15 psig, it can t exist as 338oF water at 15 psig. The hottest water can be at 15 psig is 250 o F, so 88 Btu/# is liberated as flash steam. (because 88 Btu s= ( )oF *1.0 Btu/oF per pound for water) Btu Happens to be ~10% of the heat of vaporization of 15 psig steam. ( 88/ 945 = 9.3% actually) Thus, for every 1 pound of condensate passed by the trap, ~1/10th pound of steam will be gener-ated while 9/10 pound will remain condensate. 3. BUT, at 15 psig, steam occupies 800 x the volume of condensate. ( Steam Table ). So even though, by mass there s only 1 part steam to 9 parts condensate, by volume, the ratio is 800 parts to 9 parts water, or ~ 89 to 1. So by volume, 89 times as much steam as condensate enters the pipe. 4. The Point: a lot of 250oF steam is being injected into water that is subcooled down to below 212oF. 100 psig, 338oF Steam 338 o F 100 psi 15 psig 90oF Pumped Condensate 15 psig into Hot condensate flashes to 250oF condensate & 15 psi steam. 250 o F DP accelerating water into collapsing void = 30 psia 9 psia (vapor pressure of 190oF water) = 21 psid IMPERIAL Low Press Pumped Condensate Return 15 psig flash steam (@ 250 o F) bubbling into 190 o F condensate What will Happen?????? The Steam Bubble may collapse with a C.I.W.! So why not avoid them with a dedicated High Pressure CR line?? (next slide) If condensate-flow R T > 1.0, steam continuously condenses as it enters pipe and no bubble can grow, so NO HAMMER. If condensate R T < 1.0, steam bubbles will form and grow, but MAY NOT HAMMER if they re left alone to flow out the system. But the situation is UNSTABLE: Rise To Plant IF cool condensate dumps in downstream so Rt >>1.0,BANG. IF a Pump restarts increasing flow, so Rt >>1.0, BANG. Steam cuts off (e.g. trap closes) so Rt >>0, BANG.

3 The Steam Bubble May Collapse You d like YES,! It depends on the Condensing Ratio Q cond.cap. > Q stm.in IMPERIAL Can We Tell If It Will? m c (T sat T cond ) * 1.0 Btu/# > m stm h fg m c (DT subcooling ) > m stm h fg at Pressure in condensate line at Pressure in condensate line m s #/hr of 250oF Steam 90oF Pumped Condensate 15 psig m c I. If Q cond.cap. > Q stm.in in steady flow, then 250 o F All flash steam entering the pipe will be condensed by the subcooled condensate flow. No steam bubble can grow, so there is no steam bubble to collapse. Things are copasetic. m c (60 of ) > m stm (945 Btu/#) (60 Btu/#)

4 II. Now suppose Q cond.cap. > Q stm.in in steady flow, All flash steam entering the pipe will NOT be condensed. Steam bubbles will grow and travel downstream < IMPERIAL So What? No Problem if steam bubbles make it to the Condensate Receiver BUT, Suppose another branch of Condensate Return tee s into CR Main before the Receiver and brings more subcooled condensate If m s #/hr of 250oF Steam m c (T sat - T c ) > m stm Bubble ( h fg Btu/#) (60oF) (946 m c #/hr of 190oF Cond. mc 15 psig m c 250 o F 200 of 250 o F

5 But What about Unsteady Flow?

6 Suppose Q cond.cap. > Q stm.in initially so all flash steam is condensed IMPERIAL m s #/hr of 250oF Steam Bang! The trap starts blowing steam bubbles again because m c DT s.c. < m s h fg 190oF m c Then Condensate Flow decreases, say, due to a large condensate pump shutting off So flash steam bubbles again survive in the PCR When the Condensate Pump cycles back on, the condensing Ratio flips back Steam Bubbles Collapse and Watercannon may occur against the check valve Lesson: Better not let Q cond.cap. go below Q stm.in, to allow steam bubbles to persist

7 IMPERIAL Suppose Q cond.cap. < Q stm.in Initially due to a large incoming steam input so steam is not all condensing m s #/hr of Flash Steam Bang! 190oF m c Then Steam input is abruptly cut off, say, due to a large trap cycling off so the condensing Ratio flips to Condensing, then the Steam Bubbles will Collapse, water may run up and and hammer against the check valve

8 IMPERIAL If there were a high point in the system for steam to collect, Q cond.cap. > 1.0 Q stm.in is not enough to prevent steam from persisting at the high point. m s #/hr of Flash Steam Bang! 190oF When Steam input is abruptly cut off The Steam Bubble will Collapse causing Water to run up and and hammer against the check valve The Condensing Ratio was found to have to be as high as 140% to guarantee no waterhammer where high points were built into the PCR.

9 IMPERIAL Affect of Blowing Traps Steam will warm condensate flow so entering steam bubbles will tend to persist Increased Flow Volume due to the space taken up by the flash steam will increase velocity in the PCR line. thereby increasing the DP loss in the line. That will bump up the Line pressure, and most significantly, the force of any waterhammer Incoming Condensate from Building Receivers will still be < 212oF, say about 190oF When 190oF condensate dumps into CR Main, it will have more subcooling relative to the steam, and be more likely to cause the steam bubbles to collapse. I.E. ther will be more Waterhammer. < m s #/hr of Flash Steam From Blowing Trap m c #/hr from Condensate Receiver at 190oF 215 of 25 psig o F m c (60oF) (77oF) (946) m c (T sat - T c ) > m stm Bubble ( h fg Btu/#) 220oF (933) 267 o F

10 Work Arounds to Avoid Separate HP CR Line

11 Sparger in CR to Break Up Flashing Steam Manhole which housed 60 psi Steam Main and CR was at End of Steam Line, Last Building When Building CR Pump was On, there was 60 times the cooling capacity needed to condense all flash steam, so all flash steam exiting sparger was condensed. When CR pump cycled off, local forward flow in CR Main went to zero. If trap cycled on into the 15 psi line, at ( )oF/ 946 Btu/# *800/1, = its condensate entered the CR as 48 parts steam by Volume, compared to 1 part water. When Bulding s CR Tank refilled, Pump re-started oF Bang

12 Separate, Dedicated High Pressure Condensate Return IMPERIAL All the steam traps on the High Pressure Steam Mains Discharge to it. It will be 2-Phase flow. Mostly steam, some small volume of condensate Its Pressure is still dictated by that needed to get the 2-Phase mix back to the Plant To be simple, the Pipe should run downhill all the way to the Plant because It s easy for the steam to go up hill, not so condensate. it will pool before a rise until it almost blocks the pipe, then Blurp up in slug flow when the pressure behind it builds enough to lift it over the rise. This CAN (but might not) result in Severe Slug Flow So if the 2- Phase fluid needs to go uphill, it needs special treatment to not be sluggy 100 psig Steam 50 psig Steam 190oF Cond 338 o F 100 psi 15 psig 15 psig 89 x vol of 15 psi steam to 250oF condensate 250 o F steam 300 o F 50 psi 15 psig 40 x vol of 15 psi steam to 250 of condensate 250 o F steam 250 o F Condensate Which Brings up the Question: How do you get the liquid condensate up to the Plant? How about a pumping it from a Separator Tank, Route the steam to LP Steam, the Condensate to a Receiver, flashing it, then pumping it psig + How Do Have Trouble Run line uphill Dump Subcooled Water into it (i.e. water at less than 15 psi saturation temp of 250oF) Say some Building s Condensate receiver at 190oF. slope Enough water head so Pump doesn t cvitate

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