Hot Water. Gravity Flow
Hot Water. Gravity Flow
(OP)
Dear all,
May could you help me?
We transfer hot water at saturation conditions (pressure 4barg, temp.152 deg.C)through the 8" piping (piping lenght is about 700 m) from point 1 to point 2, when point 1 is 30 meter higher than point 2. Water goes from the high point to the low point by gravity.
The water flow rate through the piping varies from 100 tonnes/hr to 350 tonnes/hr.
I want to know what is the right way to control the pressure at the upper part of the piping (point 1)in order to prevent water "flash"(vacuum???) and thereby to prevent creating two-phase flow?.
Any help would be appreciated.
Thanks in advance,
May could you help me?
We transfer hot water at saturation conditions (pressure 4barg, temp.152 deg.C)through the 8" piping (piping lenght is about 700 m) from point 1 to point 2, when point 1 is 30 meter higher than point 2. Water goes from the high point to the low point by gravity.
The water flow rate through the piping varies from 100 tonnes/hr to 350 tonnes/hr.
I want to know what is the right way to control the pressure at the upper part of the piping (point 1)in order to prevent water "flash"(vacuum???) and thereby to prevent creating two-phase flow?.
Any help would be appreciated.
Thanks in advance,





RE: Hot Water. Gravity Flow
Your basic data is confusing. It would help if you were specific in your query with regards to the basic data and your scope of work. When you give only generalities all you can hope for are wild guesses and more generalities. For example, is this a real-life application or an academic discussion?
If you have saturated water at 4 barG and you are flowing from a “point” (tank, pipe?) that is located 30 meters above the tank (?) where it is drained to, it is totally erroneous to label this as “gravity flow”. Water goes from the high point to the low point not by gravity, but by the pressure difference. Granted, the effects of gravity help, but it isn’t a requirement in this application as I see it.
At 480 gallons per min (100 tonnes/hr) you are really trying to push the water through a pipe only 8” in nominal diameter. And you state that you are going to push 3.5x that flow rate as well (1,680 gpm!) through the same pipe? You don’t state the number of elbows, tees, etc. and other fittings and resistance in the 700 meters of pipe run, but I hope you don’t have much. Have you done any hydraulic calculations at all on the system?
Without your detailed explanation and basic data, we can’t tell how you have the system configured to ensure 100% liquid-filled – which is a pre-requisite to flowing the fluid as pure liquid flow and without self-venting characteristics. In order to apply the Darcy equation (which I believe anyone would) you must establish 100% liquid-full piping. To ensure this, you must maintain a “seal” at the bottom of your piping run. And this will undoubtedly produce a flashing result as the saturated water exits the “seal”. The seal in this case can be a control valve that is maintaining a positive liquid level in the 700 m of piping.
This is an easy Darcy equation + Bernoulli equation application. But I’m assuming that I can visualize what you are proposing (or have) with accuracy. That means that I’m not sure, since I don’t have all the basic data and a complete explanation of the application.
RE: Hot Water. Gravity Flow
Keep the pressure at point 1 above the vapor pressure of water at 152ºC, whatever that may be (I'm too lazy to look that up right now.)
Perhaps a backpressure controlling valve at point 2 would do the trick, but ideally it should be at the end of that pipe segment in order to prevent vaporization and column separation and surges at any point within that segment.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Hot Water. Gravity Flow
Here is one possible answer. Assuming that the water is to discharge to atmosphere at Point 2 and you want no flashing along the line, you must flash the condensate to atmospheric pressure before it enters the pipe at Point 1. Taking your higher flow of 350 tonne/h and assuming the pipe is just 700 m of straight pipe (i.e. no bends, valves etc) then the pressure drop in an 8" Sched 40 pipe over this length is 26 m of water at its density at 100C. This means the elevation difference of 30 m is sufficient to achieve your desired flow and it is not necessary to keep any pressure at Point 1. Of course, this assumes Point 2 is at atmospheric pressure and the pipe is evenly sloped.
Under these conditions it definitely becomes a two phase flow problem, but not because of any flashing. The two phases would arise because the flow you want is less than the siphoning capacity of the line and you will draw air into the pipe. The safe way to do this would be to have a much larger diameter pipe at Point 1 that drops vertically 30 m. The larger diameter is necessary to achieve self venting flow. Then the 700m of horizontal flow in the 8" pipe would be normal flooded single phase flow.
But as I said, this is just one answer based on a particular set of assumptions. Give us the correct information and we will give you the correct answer. GIGO.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Hot Water. Gravity Flow
Thanks for your usefull answers. I would like to continue our discussion.I will try to be more specific in my problem.
First of all, this is a real-life aplication. As I wrote,
we transfer hot water at saturation conditions (pressure 4barg, temp.152 deg.C)through the 8" gradually sloping piping (piping equivalent lenght is about 800 m) from an horizontal steam/water separator to an heat-exchanger(water is used as a heating media) , when a steam/water separator is located about 30 meter higher than heat-exchanger water inlet nozzle.
Since water/steam supply to the separator does not constant,therefore water flow rate from the separator through the piping varies from about 100 tonnes/hr to 350 tonnes/hr.
There are two control valve in the line. 1st control valve (level control valve) is installed adjacent to the separator (at water outlet) to control water level in the separator. 2nd control valve (pressure control valve) is installed at water outlet line from heat-exchanger,which control the water pressure above its saturation pressure 4 barg at heat-exchanger inlet (to avoid flashing into the tubes).
As far as I understand ,the pressure at the upper part of the piping will be changed (due to water flow rate changes),therefore, I would like to know what is the right way to control the pressure at the upper part of the piping (outlet from the separator or downstream of the level control valve)in order to prevent water "flash" and thereby to prevent creating two-phase flow?.
Thanks in advance,
Mike
RE: Hot Water. Gravity Flow
I am surprised your system works the way you describe it. I would expect the two control valves in the same line to fight each other. But anyway, I believe your problem is easily solved.
The fact that your second valve is able to control the pressure at the inlet to the heat exchanger at 4 bar, even though the supply pressure 700 m away is also 4 bar, confirms my previous calculation that the pressure gained by the decrease in elevation is greater than the friction drop through the line. Your level control valve at the separator is achieving nothing. In fact, all it is doing is adding unnecessary pressure drop into the line.
You should remove the level control valve (or lock it fully open) and change the function of the pressure control valve at the heat exchanger outlet to control the level in the separator. The hydraulics of your system are such that you will always have at least 4 bar at the inlet to the heat exchanger, but for safety's sake you could retain the pressure sensor at the heat exchanger and use it to give an alarm if the pressure goes under 4 bar.
Also, if you get any heat loss over the line, and the temperature at the inlet to the heat exchanger is less than 152 deg C you could safely allow a lower inlet pressure than 4 bar, i.e. you could go down to the saturation pressure at the new temperature.
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Hot Water. Gravity Flow
Thanks for your reply.
Regarding the level control valve your analysis is correct,but I forgotten to note that we have two separators which connected in parallel and are fed from different sources.Each separator has independent level control system with a level control valve. Two short 6" outlet lines are joined into one 8" line. Therefore,I think we can'not remove the level control valve(s)as you suggested.
The question is that in spite of we control at least 4 bar at the inlet to the heat exchanger (by pressure control valve),if we can get hot water "flash" at upper part of the piping(downstream of the level control valves) at low flow conditions. For example at 120 tonnes/hr, when friction losses through the 8" piping are about 0.5 bar and elevation difference is 30m (2.8 barg).
I did a quick calculation (based on Bernoulli equation):
P1=2.8 BARG+4 BARG- 0.5 BAR-4BARG
P1=2.3 BARG (pressure at upper part of the line)
At this pressure 2.3 barg and 152 deg.C we will get a "flash".
What is your opinion?
Thanks in advance,
Best Regards.
Mike
RE: Hot Water. Gravity Flow
I am not surprised that you get flashing (and probably vibration and/or hammer) under low load conditions. As I said before, the level control valve just introduces an unnecessary restriction, and the LC valves will be more closed under low loads.
Even with two tanks you can control them the way I suggested before. If the levels in the two tanks are the same (or nearly the same) then just treat them as one tank and control the level in both of them with the valve after the heat exchanger.
If one tank has a level higher than the other then leave the existing LC system on the higher tank, and control the heat exchanger valve according to the level in the lower tank. If the higher tank level goes up then its LC valve will open, effectively draining its contents into the lower tank. This will raise the level in the lower tank which will result in the valve at the heat exchanger opening and taking the excess water away.
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Hot Water. Gravity Flow
I don't understand it. If the fluid pressure is 4barg at the top and 0.5 bar is the pressure drop and 2.8 barg is pressure added due to elevation then just before the entry into the HX, the pressure should be morethan 4 barg. If at all, there is flashing, it should be during higher flowrates when the frictional losses exceed gravitational head and your water pressure drops below saturation point. But, as I roughly calculate, even there shouldn't be any problem with 350ton/hr flowrate.
Actually 2.3 barg is the total pressure difference between the HX entry and separator outlet.
RE: Hot Water. Gravity Flow
The result is that straight after the first valve there is an area where the pressure suddenly drops (Delta P over the valve) but the temperature is still close to 152 and the condensate flashes. It has to flash to fill the line with low density vapor to reduce the pressure at the PC valve from 6.8 bar to 4 bar. At higher flow the loss in pressure at the heat exchanger is due to friction, but at low flow the pressure still has to be lost to meet the controller's requirements and the only way to do this is to lower the density and therefore the static head.
The 2.8 bar that Mike uses will not exist at low flow rates because the line is not full of liquid. Although I disagree with Mike's actual numbers, I believe he is right to conclude that there is an area of low pressure at the beginning of theline, just after the LC valve.
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Hot Water. Gravity Flow
I got your point but I am still confused as to why the OP is maintaining 4 barg at the HX inlet. Ideally, I would take it as [saturation pressure at 1520C+elevation-frictional loss at maximum flowrate-loss across the first valve]. Say it as PcontThis will always ensure saturation pressure at the outlet of the separator valve. The losses at low flowrate will further reduce during low flowrate and as you rightly said, the pressure before the HX increases and the HX outlet valve opens up till the pressure reduces to Pcont. As Pcontis higher than Psat at 1520C, there shouldn't be flashing.
...or I am terribly missing something.
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