Question About Escape of Pressurized Hot Water

[crshears]

Hey all,

I was recently told something that doesn't square with what I've come to believe about steam flashing; hopefully you can tell me if I'm way off base . . .

My brother in law related how back in the day he worked at a very large greenhouse and flower growing facility that used pressurized hot water to warm the greenhouses during the colder months.

He was told that if he ever heard an increasingly louder roaring sound while moving about on site, he was to return from the area where the sound was along the same path he had been taking and report the situation to the pipe fitters and engineers for resolution. He was further advised that escaping steam from any leak would be invisible, and liable to cut off the hand or arm of anyone unlucky enough to walk through the escaping jet.

I have heard of this being the case for 600# saturated steam, or superheated steam at almost any pressure above, say, 100# or so, but it did not make sense to me that this would occur with pressurized hot water at 260° Fahrenheit.

I therefore went to my trusty steam tables and observed that the vapour pressure of water at 261°F is 36 psia, or, say, 21 psig. These values suggest to me that a pressure of, say, 30 pounds per square inch above atmospheric would be more than adequate to preclude any flashing of said hot water into steam, even right at the suction inlets of the hot water circulating pumps.

I further noted that the heat content of water at 261°F is listed as 229.6 BTUs, whereas water at 212°F has a heat content of 180 BTUs. The heat gain from 212°F to 261°F being ~29.6 % of the heat value at 180°F suggests to me that if said hot water system developed a leak, the escaping fluid would be a two-phase [ meaning consisting of mixed steam and hot water ] mixture of around 73% hot water and 27% steam - - the latter of which would, of course, upon escape to atmosphere, expand to steam somewhere in the neighbourhood of 1,700 times of its volume as water.

My reasoning is that since the heat content of the flashed steam is less than that of the water escaping with it, water would also exit from the leak in more than sufficient quantity to be readily visible.

Am I missing something?

 

[goutam_freelance]

You are generally right. There will be about 5.2% steam and 94.8% hot water.

 

[EdStainless]

I had a customer tell me about their system.
Slightly hotter, 285F but in the same range.
The only leaks that they really worried about as a danger were the ones that were small enough that water wouldn't leak, just steam.
They didn't see it as a serious personal injury issue, just as system security one.
At 60psi they weren't concerned about 'cutting off a hand' but rather the risk of burns.

 

[LittleInch]

Hey all,

I was recently told something that doesn't square with what I've come to believe about steam flashing; hopefully you can tell me if I'm way off base . . .

My brother in law related how back in the day he worked at a very large greenhouse and flower growing facility that used pressurized hot water to warm the greenhouses during the colder months.

He was told that if he ever heard an increasingly louder roaring sound while moving about on site, he was to return from the area where the sound was along the same path he had been taking and report the situation to the pipe fitters and engineers for resolution. He was further advised that escaping steam from any leak would be invisible, and liable to cut off the hand or arm of anyone unlucky enough to walk through the escaping jet.

I have heard of this being the case for 600# saturated steam, or superheated steam at almost any pressure above, say, 100# or so, but it did not make sense to me that this would occur with pressurized hot water at 260° Fahrenheit.

I therefore went to my trusty steam tables and observed that the vapour pressure of water at 261°F is 36 psia, or, say, 21 psig. These values suggest to me that a pressure of, say, 30 pounds per square inch above atmospheric would be more than adequate to preclude any flashing of said hot water into steam, even right at the suction inlets of the hot water circulating pumps.

I further noted that the heat content of water at 261°F is listed as 229.6 BTUs, whereas water at 212°F has a heat content of 180 BTUs. The heat gain from 212°F to 261°F being ~29.6 % of the heat value at 180°F suggests to me that if said hot water system developed a leak, the escaping fluid would be a two-phase [ meaning consisting of mixed steam and hot water ] mixture of around 73% hot water and 27% steam - - the latter of which would, of course, upon escape to atmosphere, expand to steam somewhere in the neighbourhood of 1,700 times of its volume as water.

My reasoning is that since the heat content of the flashed steam is less than that of the water escaping with it, water would also exit from the leak in more than sufficient quantity to be readily visible.

Am I missing something?

I think you're missing where on the pipe the leak is. Right on the top you're more likely to get steam, but the third hand tale you're telling is not the same conditions.

There is a lot of steam missing here and some tale from superheated steam which can be nearly invisible has been superimposed onto a lower pressure and temperature system.

But don't mess around with a leak on a superheated liquid system.

 

[Snickster]

I believe the pressure in the hot water pipe would be about 50 psig max if it was just a hot water recirculation system? Any fluid escaping will be liquid water on the inside of the pipe and flash to part water/part steam as it passes through the pipe opening. At the smallest cross-sectional area of the flow is called the vena-contracta where the pressure will reach atmospheric and maximum velocity will occur. The maximum velocity of the water is based on the available head of the water in accordance with the equation:

Head = V^2/2g where head is in feet, V velocity is in feet per second and g is gravitational constant 32.2 ft/sec^2

So 50 psig inside pipe would equal 50(144)/62.4 = 115.5 feet of head available.

The maximum ideal velocity of water would then be 115.5 = V^2/2(32.2)

or V = 86 ft/sec

I don't think this could cut off anyone's finger or arm and would be clearly visible.