# Pressure Vessel

1. Jun 23, 2004

### NASNAS

I have a question for you all, say you have a pressurized cylinder with water in it, it is at 200*F and at a pressure of 300psig. Now you open a drain valve on the bottom to the atomosphere.

Will it flash to stream or just come out as hot water or a combo of the two?

What equations and info do tyou use to figure this out?

Thanks!

2. Jun 23, 2004

### Artman

Here is a calculation for flash steam:

% flash steam = (100 * (hf1 - hf2)) / hfg2

where:
hf1 = enthalpy of liquid at pressure p1
hf2 = enthalpy pf liquid at pressure p2
hfg2 = latent heat of vaporization at pressure p2

This calculation is from ASHRAE Systems 2004.

Now if I'm interpretting this correctly, that equals

% flash steam = (100 *(393.84 - 180.07)) / 970.3 =
% flash steam = (100 *(213.7)) / 970.3 =
% flash steam = 21377.00 / 970.3 = 22.03%

So approximately 22% should flash to steam.

Do not try this experiment at home!

3. Jun 24, 2004

### NASNAS

what units are you using? I used 80*F as the ambient temp.

I got hf1=169 BTU/lb, hf2=49 BTU/lb and hfg2 970 BTU/lb.

so I came up with 12% steam.

4. Jun 24, 2004

### Artman

Sat. Liquid Enthalpy hf of 393.84 at 300 psi Abs Pressure and 180.07 at atmospheric pressure of 14.696 psi Abs Pressure.

I'm not positive that I am right, but the formula calls for the hf at the given pressure, not the given temperature. Maybe someone else can verify this.