Calculate approx water temperature spraying to atmosphere via HP steam leak

[dwf_90]

I am looking for help calculating the condensate temperature being sprayed to outside atmosphere via a leak. The source comes from a tube and shell heat exchanger using HP steam to heat natural gas. We have a leak on the steam side and management is asking to attempt to tight the flanges of the heat exchanger heads, while it is under pressure. Thankfully for safety reason this is no longer being considered I am curious of the temperature that the condensate will be if some one had attempted to work on this steam leak while it it spraying to the atmosphere. This isnt the first time this issue has came up with this heat exchanger, previously the gasket has been cut aprox 1" in width sometimes in a few different spots. Again not attempting to work on this equipment while under any amount of pressure or temperature im just curious so I can present some good data to our safety department and managers. Management wanted to reduce pressure from 1858 psi, 530 F to 500 psi, 452 F and 500 psi,385 F. i would like to know the calculation or how to calculate the condensate temperature when the exchanger is at 500 psi , 452F and 500 psi 385F? Thank in advance.

 

[IRstuff]

When you say "condensate" are you referring to the water that's inside of the exchanger?

Is the exchanger ambient natural convection or forced, i.e., is there air flow around the exchanger?

Can you quantify the mass flow rate of steam leak? At its crudest, one would assume some sort of mass flow rate at some temperature that mixes with the air on the outside, which is what will cool it down. The caveat is that if this is a safety question, then one must ignore the thermal mixing, since someone could have their hand or face right next to the leak, and only consider the mass flow rate impinging on body skin. For a small, or slow, leak, that might be tolerable, since the heat rejection rate of the skin might be sufficient that the worker can register the heat increase and move away. A fast leak potentially can delivering scald-producing amounts of steam, which would be really bad.

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[dwf_90]

Thanks for your reply, the condensate that I am referring to is after the steam has leaked to atmosphere and then condensed. I am attaching the exchanger specs as well as a video of a similar leak that has occurred on this piece of equipment. The side (tube side) of the exchanger utilizes natural convection.I cannot quantify the mass flow rate of the leak.

 

[georgeverghese]

When steam escapes from the leaking gasket, this is an isenthalpic expansion. So lookup steam tables and match the enthalpies at P1 and at atmospheric pressure. Most likely the steam will all be superheated at 1bar abs, but may within a few seconds, cool and condense to 100degC.

 

[IRstuff]

If it's condensed, by definition, it's no more than 100C.

If you have condensate, then you can capture it and measure it.

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[goutam_freelance]

It is very difficult to calculate theoretically. The following is an approximate sequence of events.
1. At leakage point, the pressure is critical and velocity is sonic. The enthalpy reduces in proportion to increase in kinetic energy
2. Just after the leakage point the pressure becomes atmospheric through a series of shock waves and at each shock front enthalpy and velocity is lost to turbulence and friction. It is at this stage steam condenses and becomes drops and plume.
3. Little beyond the shock zone, water droplets evaporate to attain a temperature corresponding to partial pressure of water vapour. It may be noted that due to injection of large amount of steam the partial pressure of water vapour increases locally and so temperature of condensate increases too.
But overall the temperature of condensate beyond the shock zone will be much less than 100 deg C as the steam is not exclusively bordering a body of water.

It is difficult to estimate the condensate temperature. It depends on
a) Existing humidity in air before leak, i.e partial pressure of moisture.
b) Quantity and rapidity with which the jet of steam mixes with surrounding air. This will determine the partial pressure of moisture in surrounding space.
Safety Concern
1. Presonnel within the rapid shock wave zone are in high risk of burning as the steam may still be superheated.
2. At a distance from shock zone the risk of burning is much lower.


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[georgeverghese]

That is some horrendous leak you've got there, looks almost suicidal to attempt to tighten the head flange bolts from this view. What is the rational for the lower pressure of 500psig?

 

[MortenA]

From a thermodynamic point of view your water outside of the HX must be 100ºC - but cooling as it sprays in the air. Water can MAX be 100ºC at atm pressure! I believe goutam_freelance is overcomplicating the problem. You have to consider that you have the water and the steam in the HX - depending on where the leak is either only steam escapes or water is pushed out by the pressure. If you consider the steam phase and look at a water P-H diagram you should look at the saturation curve (because there is both steam and water in the HX) and your pressure (think it was 6 barg) then as the steam leaks this will be an iso-enthalpatic expansion (all work is converted to heat - so enthalphy is constant. While you may find some areas where is this not true that is only of academic interest because get far enough away from the leak and this will be true - and in the thermodynmics the "route" does not matter (not very Kirkegaard om afraid ;-) ). In this case the steam does not throw any condensate - only when it cools further in the air this will happen. If the water is pushed out you can look at the other side of the diagram. This will tell you that a certain fration of the water will flash to steam - the steam will still be 100ºC - and the amount of steam that flashes off will cool the water to 100ºC.

 

[dwf_90]

georgeverghese I agree about the leak being suicidal. Noone is going to attempt to fix this while under pressure. The leak that I posted in the GIF was at normal operating pressure at 1900 psia. Our management team recklessly tried suggesting to reduce pressure to 500 F. Their justification was the pressure is safer in their minds and the temperature would be ok. Neither of these response were excepted and data is being collected for our safety department. I would never allow maintenance to be preformed under these conditions.

 

[goutam_freelance]

then as the steam leaks this will be an iso-enthalpatic expansion (all work is converted to heat - so enthalphy is constant.

This is true only when we can neglect the kinetic energy. Here since the velocity is sonic at leak exit, the kinetic energy is considerable.
h_vessel=h_exit+(1/2)(V_exit)^2
Potential energy is neglected.

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[MortenA]

@goutam_freelance

The ehthalpy remains constant at the boundary conditions... As i said, there will be some volume where you could experience higher than 100 deg C - but its academic. You would be hurt anyway!

BUT if the process would be more constant entrophy (work being "taken out of the system") then that would only be lowering the temperature. And you CANT add work in this system!

 

[goutam_freelance]

The ehthalpy remains constant at the boundary conditions.

It is not clear which boundary you are talking about.
In a natural process you need to consider Laws of Nature. Here the applicable law is 'First Law of Thermodynamics' or 'Law of conservation of Energy' by which energy remains constant (not enthalpy). Here we exclude relativistic processes.

The 'Energy' is sum total of potential, kinetic and enthalpy for a steady state adiabatic process . So in a natural steady state adiabatic process you should not consider enthalpy is constant unless kinetic and potential energy changes are negligible.

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[davefitz]

Regrding safety of the defined configuration, there are at least 2 comments to be made related to maintenance of a fuel gas heater, where such leaks between 2 different systems can occur. Both comments underscore the need to apply a double block and bleed valve whenever 2 different systems can interact due to leakage across a heat exchanger.

first, a double block and bleed on the steam side can protect the workers that are working on the heater while the steam side is pressurized.

second, I have been involved in a fatal accident investigation of the explosion of an air cooled condenser ACC in Monterrey Mexico in 2012. The unit was a combined cycle plant , normally supplying LP steam in a cogen configuration to a salt factory . During a rare outage, the steam system was depressurized , but the gas side was apparently pressurized due to leakage across the single block valves. Fuel gas leaked into the steam system ( across the tube to tubesheet rolled connections) , eventually finding its way into the air cooled condenser. A welder who was repairing a drain valve on the ACC sparked the explosion, killing one worker. This accident could have been prevented, if a double block and bleed valve configuration was provided on either the fuel gas or steam connections to the fuel gas heater, but neither were provided, as the design engineer made the fatal assumption the plant would never, ever shut down ( as defined in the power purchase agreement).

Incidentally, the same investigation revealed an unusual chemical reaction occurs in a HRSG when organic feedwater chemicals are used to prevent tube corrosion; high tube metal temperatures combined with entrained iron oxide particles ( which behave as a catalyst) will break down some feedwater chemicals and result in the formation of flammable chemicals which also accumulate in the ACC.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[MortenA]

@goutam_freelance: Its the first law of thermodynamics....

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing two kinds of transfer of energy, as heat and as thermodynamic work, and relating them to a function of a body's state, called Internal energy.
The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.

So when steam leaks out of a hole no energy is converted to work (or very little) and no energy is added. So the energy must remain constant for the system.

I knicked this from Wikipedia (because im lazy), but basically its the same for an orifice or a leak.

The throttling process is a good example of an isoenthalpic process in which significant changes in pressure and temperature can occur to the fluid, and yet the net sum the associated terms in the energy balance is null, thus rendering the transformation isoenthalpic. The lifting of a relief (or safety) valve on a pressure vessel is an example of throttling process. The specific enthalpy of the fluid inside the pressure vessel is the same as the specific enthalpy of the fluid as it escapes through the valve.[3] With a knowledge of the specific enthalpy of the fluid and the pressure outside the pressure vessel, it is possible to determine the temperature and speed of the escaping fluid.

And with constant enthalphy you can use diagrams that has been made many years ago to find you final split between steam and water and the temperature.

 

[Bibalan]

Theoretically, considering leak of condensate in 2553psia and 276F to atmosphere, outlet temperature will be 100C.
93% of water will be in vapor phase. (calculated by passing water through a valve from operating condition to atmospheric pressure in HYSYS)
Other pressure and temperature conditions which you have mentioned, will effect on % of vapor and liquid, but the temperature will remain 100C.
According to my experience, if operators are under direct shower of 100C water or steam, working on equipment for more than a few seconds would be very hard, even using adequate cloths. Also, as you had repetitive problem on same exchanger, accidents like fail of not and bolts while tightening would be probable. So pressurized maintenance is not recommended.