Closed Pressure Vessel - Hot Water Charge

[chemE72]

For the sake of simplifying as much as possible, assume there is a closed pressure vessel (650 gals) that takes water from a water distribution header. This header is at 100 psig and regulated down to 40 psig and blocked off by a block valve right before the pressure vessel. If we assume the water entering the pressure vessel after opening block valve is 40 psig and I would like to add 360 gals to vessel, could we quantify or assume what the pressure of the tank would be at this volume? Would it approach 40 psig, would it be well under?

 

[Latexman]

Sounds like a homework problem. Is it?

You left out the initial pressure.

You said hot water. You didn't mention temperatures. Is it relevant?

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[chemE72]

Nope. Water properties are constant at 180 F and 40 psig. Flow comes in at 35 gpm once block valve opens. Initial pressure of vessel is 0 psig.

 

[Latexman]

Initial air (I assume it's air) temperature in tank?


Good Luck,
Latexman
Pats' Pub's Proprietor

 

[chemE72]

Just air and I would assume an initial temp of around 20 C (68 F).

 

[Latexman]

Okay, I have an answer. What did you get?

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[chemE72]

Well my question was is there a way of quantifying implying that I am not sure of how to quantify ending pressure

 

[Latexman]

I got 25.2 psig. Do you want the methodology?

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[Latexman]

Initial - 650 gallons of air at 68 F (528 R) and 14.7 psia.

Add 360 gallons of 180 F (640 R) water.

Final - This leaves 290 gallons of air (650 - 360 = 290) at about 180 F.

14.7 x (650/290) x (640/528) = 39.9 psia = 25.2 psig.

Google "Gas Laws".

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[chemE72]

Looks like ideal gas law. This does not take into account the 40 psig coming from the water due to being pumped in. Not sure if that contributes to pressure increase in closed system.

 

[Latexman]

It doesn't. That pressure gets eat up by the pipe, fittings, and throttling valve to get the target flow rate. Once the liquid is in the tank, all you got for back-pressure is the air space compressed by the liquid displacement.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[LittleInch]

I agree with latexman. The 40 psig is just the max pressure you can get. You must have some other restriction to lower the pressure if your flow is 35 gpm.

Flow may not be constant. A system sketch would help.

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