Amount of heat required to produce 50 psig in pipe with water in it
Amount of heat required to produce 50 psig in pipe with water in it
(OP)
I am wondering how much steam would have to be provided to a pipe (3 ft long, 2 in diameter, 316 SS) containing water to produce a vapor pressure of 50 psig. I'm assuming the pipe is half full of water (or 0.0325 cu. ft.) at 140F -- and that the water is stagnant and isolated (i.e. no flow and the pipe is then essentially a constant volume container). The pipe is heated via a steam jacket with steam at 327F and 85psig. The pipe is insulated so I've assumed heat loss to be negligable here. The problem I'm having is that I don't know how to take into account the fact that as the water inside the pipe vaporizes, the pressure builds. This increased pressure then increases the boiling point of the water which means additional heat would be required to vaporize the water.
The temperature of saturated vapor at 50 psig is 297 F if this helps.
Thanks so much in advance and please let me know if other information is needed!
The temperature of saturated vapor at 50 psig is 297 F if this helps.
Thanks so much in advance and please let me know if other information is needed!





RE: Amount of heat required to produce 50 psig in pipe with water in it
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Amount of heat required to produce 50 psig in pipe with water in it
RE: Amount of heat required to produce 50 psig in pipe with water in it
Are you going to regulate the pressure with a vent valve or a pressure relief valve?
How are you going to refill the water?
You no longer have a "pipe half full of water" ....
You now are building a "boiler" that is more easily capable of blowing up and killing nearby people than a terrorist's pipe bomb.
RE: Amount of heat required to produce 50 psig in pipe with water in it
Your issue is more that the steam jacket will require a flow so there will be some temperature loss in the steam, but maybe not that much.
for such a small system I suspect there will be lots of other issues which mean your energy transfer is not ideal
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Amount of heat required to produce 50 psig in pipe with water in it
RE: Amount of heat required to produce 50 psig in pipe with water in it
So I'm not actually sure now what your question is and how you are going to measure it
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Amount of heat required to produce 50 psig in pipe with water in it
Here's my thought process. Since at 50 psig, the boiling point of water is 125 C, I'm imaging the remaining liquid in the pipe to be at this temperature. At the point of reaching 50 psig, we have a small amount of steam at 147 C and the rest of the water at 125 C. I can then use the sum of these ethalpies to do as LittleInch suggested. Does anyone see flaws in this logic?
Thanks
RE: Amount of heat required to produce 50 psig in pipe with water in it
RE: Amount of heat required to produce 50 psig in pipe with water in it
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Amount of heat required to produce 50 psig in pipe with water in it
Aeboarder, The boiling point of water is completely irrelevant to the problem. The total pressure in the pipe will be the partial pressure of air plus the partial pressure of water, which is equal to the vapor pressure of water.
There will be more heat used to heat up the pipe and water than to vaporize water. Remember to use absolute pressures and not gage pressures for your calculations.
RE: Amount of heat required to produce 50 psig in pipe with water in it
LittleInch, the steam is saturated.
RE: Amount of heat required to produce 50 psig in pipe with water in it
RE: Amount of heat required to produce 50 psig in pipe with water in it
So what real world interest is there in knowing how much heat (calories) is required? If you open a valve you will vent a small amount of steam or hot water. What is the concern?
RE: Amount of heat required to produce 50 psig in pipe with water in it
Composite, I found the boiling point relevant because this is the temperature I'm assuming liquid water to be when at this pressure. If I know this temperature, I can begin to calculate the amount of energy needed to bring it up to that temperature.
The real world interest is investigating if a small steam leak could provide enough heat to cause this to happen. If so, this would justify building in a pressure relief valve.
RE: Amount of heat required to produce 50 psig in pipe with water in it
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Amount of heat required to produce 50 psig in pipe with water in it
It is not a question of heat. It is a question of temperature, and my first post explained why there is no safety issue. So please take a day or two and think a little before posting again. These are very elementary chemE concepts, which are admittedly somewhat complicated for non-chemE's who haven't thought about it before.
The expansion of liquid water is insignificant if there is a vapor space. It is very significant if there is no vapor space.
This could require a pressure relief.
"I found the boiling point relevant because this is the temperature I'm assuming liquid water to be when at this pressure. If I know this temperature, I can begin to calculate the amount of energy needed to bring it up to that temperature."
This statement is wrong and make no sense. If there were no air in the half-filled pipe, then any water in the pipe will always be at its boiling point, whatever the temperature. With air in the pipe the water can never reach a boiling point, whatever the temperature.
RE: Amount of heat required to produce 50 psig in pipe with water in it
Thermal expansion is a concern only when a system is full of liquid, isolated and heated. You assumed that the pipe is half full so it should not be an issue except that thermal expansion should have a slight affect on the pressure.
If you pipe is full of liquid, it may be a big problem because liquid is incompressible.
http://abiquim.org.br/congresso/cong_cd/fullpapers...
RE: Amount of heat required to produce 50 psig in pipe with water in it
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Amount of heat required to produce 50 psig in pipe with water in it
RE: Amount of heat required to produce 50 psig in pipe with water in it
I have prepared a simulation in Aspen HYSYS to calculate the Temperature and Pressure of Water in a Externally Heated Tank with closed outlets.
This is just an example calculation. However, it can be "tuned" for this case :)
I have also prepared a small report that includes HYSYS Simulation Snapshots. You may find the report in the attachment.
Kindly, share your views on the report :)
Here are the first two paragraphs for you to read while report is being downloaded :)
Synopsis
It is desired to calculate the temperature and pressure of water / steam in a tank with closed outlets after addition of a certain quantity of heat to the water.
In this example calculation, a very small quantity of water (0.203 liters) has been considered. The total volume of tank has been considered to be 0.0654 ft3. The internal diameter of tank is considered to be 2 inches and length of the tank is considered to be 3 feet.
The initial temperature and pressure of water in the tank is 76.71 Deg. F and 51 psig.
The initial percent level of water in the tank is 10.02 %
Nitrogen blanketing has been provided to the water. The pressure of nitrogen gas above water surface is 51 psig.
Results of Calculations
It has been observed that after addition of 166.66 BTU of heat to the water, the temperature of water is increased to 394.8 Deg. F from 76.71 Deg. F and pressure of water is increased to 323.8 psig from 51 psig.
The percent level of water in the tank is also increased from 10.02 % to 11.01 %.
RE: Amount of heat required to produce 50 psig in pipe with water in it
The pipe volume is 1.8 litres. Assume half filled with water, so vapour volume is 0.9litres. Heat system to a temperature T. The desired pressure in the system is 435kPAa. (app. 50 psig) The air volume, Va, is now 0.9*(273+T)/273*100/435. The steam then occupies the remaining volume (0.9-Va) assuming the liquid volume increase is negligible. The water partial pressure is then (0.9-Va)/0.9 * 435kPa. Play with T until the water partial pressure equals the steam pressure (steam tables) at that temperature. That equates to about 131C. I calc the steam required for this to be about 0.42kg. Bulk of it going into heating up the water from 20 to 131C. So very dependent on the initial assumption of how full the pipe is. Doesn't take lot of water to boil to increase the pressure significantly, less than 3g. So if have couple teaspoons of water in the pipe, pressure/temperature about the same but steam required to heat it up loads less as no need to heat lots of liquid up to 131C.....less than 0.09kg steam required if pipe about 10% full to start.
Assuming the system is allowed to come to equilibrium, i.e. at the steam tracing temperature of 164C then the pressure will have increased to 830kPaa or 110psig. I calc the steam required for this to be about 0.54kg, again being based on half full to start.
Check my calcs but I think the methodology is correct - maybe not for the purists as am using ideal gas law.
RE: Amount of heat required to produce 50 psig in pipe with water in it