Amount of heat required to produce 50 psig in pipe with water in it 4

[aeboarder]

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!

 

[waqasmanzoor]

Dear All,

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 %.

 

[Mutt]

On simple calc basis using steam tables and ideal gas law....and metric

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. .

 

[reena1957]

Using simple partial pressure rules, one can calculate the vapor pressure of water vapor to be added to the initial air(and a small water pressure). I got 137.7 Deg C as the temperature to which water has to be heated to produce the 50psig total pressure. By calculating the number of moles of water intially present (say at 40Deg C) and finally at 137.7 Deg C, one can find the amount of water vaporized, which comes out to be 1.11 gms. Now using the latent heat of steam in the jacket, one can calculate the amount of steam required to heat the water to 137.7 deg C and vaporize 1.1 gm of water - this works out to be 185.2gms or 0.408lbs.This neglects the heat loss and the heat required to warm the pipe, etc. which at most may be equal to the same amount of steam used to heat the water and produce staem. So In alll , you may need only one pound of steam. If you are worried about the maximum pressure that will be generated inside your pipe, it will not be more than the staem pressure in the jacket(assuming no air inside the pipe). So your safety requirement for the pipe will be satisfied if you design it for 85psig.