10 replies to this topic

[atiq124]

The half pipe coil around a reactor is full of cooling water and the reactor is heated to 145 degree C using hot oil in the internal coils. So, water in external coils is heated to 145 degree C and I want to know the pressure in the external coil at that temperature. 

 

I have calculated vapour pressure water at 145 degree C using Antoine equation as shown below and the result is 3.06 barg pressure. This pressure is same as that of saturated steam at this temperature. The water in pipe (sealed with inlet and outlet valves shut) is liquid and not the steam. I wonder why is the pressure for water and steam at this temperature same ?

 

Sorry if my question sounds stupid or too simple but I am trying to learn and understand. Thanks

 

 

Log10 (P) = A - (B/(C+T))

A= 8.14019

B=1810.94

C=244.485

T= 145 degree C

 

P=10^A - (B/(C+T))

P= 3094.6 mmHg

P= 4.07 bara

P= 3.06 barg 

[shan]

The difference of water and stem at the same temperature is latent heat.

[atiq124]

Thanks Shan, so the pressure of water and steam will be same in this case ?

Also, as the cooling is off and inlet/outlet valves to the coil are closed, there is no dynamic pressure from water. Total pressure at any point will be the sum of static pressure (head) and vapour pressure ?

[shan]

Yes.  Water and steam temperature and pressure are the same at the saturation conditions. (145 C, 3.1431 BarG)

[Bobby Strain]

No! If the coil is blocked full or cooler water, the water will expand until it ruptures the coil.

 

Bobby

[atiq124]

Hi Bobby, we should be draining water from jacket before heating but unfortunately, this is not being practised at the moment. The stagnant water at high temperature with some chlorine contents has been causing damage to external coils but that is another story. Regarding this thread, do you agree that under given conditions total pressure at any point will be the sum of static pressure (head) and vapour pressure ? And that steam and water pressure at 145 degree C are the same ?

Thank you

(ps. the coils in question are rated for 4 barg pressure)

[Bobby Strain]

You are probably asking the wrong question. Maybe you can explain what problem it is that you are trying to solve. You have thus far only thrown us a red herring.

 

Bobby

[atiq124]

Hi Bobby, let me put the question this way.

A pipe full of stagnant water is likely to get heated up from ambient to 145 degree C. This will expand and exert pressure on pipe wall, weldings, connections and other fittings. What should be the pressure rating of pipe & fittings to make sure that it does not burst or leak out. This brings back the same question, what will be the pressure of water (not steam) at 145 degree C ?

Thank you

[Bobby Strain]

You must install a relief device to protect the system from overpressure.

 

Bobby

[K RAVI VARMA]

First, Mr Bobby is right, a thermal expansion relief valve needs to be sized and installed on your system, as blocked discharge condition is arising.

 

Secondly, as you desired to understand & learn by seeking. In the case of a system filled with stagnant fluid with blocked discharge conditions, there is no vapor space to accomodate increased liquid volume as temperature rises, no phase change occurs. There will be enormous pressure rise than at its saturated condition as liquid is incompressible.

 

regards

[shan]

You need to calculate overall water volume change, dV(H2O) in the temperature variation range and pipe section containing volume change dV(pipe) in the same temperature range.  If dV(H2O)<dV(pipe), there will be space for vapor and pipe section internal pressure will be the steam saturation pressure at 145C.  However, if dV(H2O>dV(pipe), the pipe internal pressure will be balanced at the metal elasticity point.