Safe operating conditions for Autorefrigeration potential

[kevlar49]

Hi,

I am drawing up guidelines to avoid brittle fracture in propane spheres. The concern is autorefrigeration can cause brittle fracture.

I think the safe operating condition is between the equilibrium vapor pressure curve and the MDMT curve as determined using the method in ASME UCS 66, if the vapor pressure curve is to the left of the MDMT curve on a temperature vs. pressure plot.

Any thoughts?

 

[sshep]

Kevlar49,

My advice is that if you shoot for staying above some minimum temperature while letting down an LPG then you are asking for trouble. Know your MDMT for sure, but to avoid a screw-up focus on the procedure (below) rather than on the temperature.

The general guideline for avoiding auto-refrigeration is to always pressure up the equipment (i.e. the sphere) with vapor of the same composition before introducing liquid. This positively controls the flashing that will occure when liquid is letdown into the equipment. This pressurization must be done with vapor of the same composition. Typically it is possible to get vapor from equipment already in service, otherwise include a small vaporizer (even a heat traced 1/2" stainless line of sufficient length) to generate start-up vapor.

Note: many people have found out the hard way that pressurizing with N2 (or CH4) will do nothing to supress the flashing temperature when an LPG is letdown.

best wishes,
sshep

 

[dcasto]

With propane, don't worry about it. Propane boils at -43 @ 14.696. Your steel should be fine for that condition, if not you may select to have it tested and certified via Charpy testing.

 

[sshep]

Dcasto,

If you were commissioning a new vessel, you could add yourself to the list of those who fell into the trap I mentioned in my previous post. Your reasoning is only true for a vessel with an existing LPG vapor space.

When the partial pressure of LPG vapor is zero, you can't calculate the minimum liquid temperature from the total pressure. This is how evaporative cooling works, and the temperature is not so easy to calculate.

A mistake like this with propane is probably a minor incident, but pressuring up a carbon steel process vessel with N2 in the mistaken belief it will suppress flashing; and then letting down ethane into it could be catastophic.

best wishes,
sshep

 

[pipehead]

sshep, I agree. When I commission ethane or ethylene, I use a vaporizer and purge with hot vapors at 100 psig. Then slowly add the C2 until the pressure is 400 psig. Then cut the heater off and let it fill up.

 

[sshep]

Yesterday in one of our plants which process material no lighter than methanol (BP=65C) and hexene (BP=63C), we had the problem of ice formation on the outside of a flare line. The mechanism for this cooling is to get liquid into the flare line where it is exposed to "dry" offgases which can create subfreezing temperatures by evaporative cooling.

I cite this as an example for imagining what could happen if LPGs are carelessly handled by those who do not recognize the potential power of evaporative cooling. I am also surprized that Kevlar's post did not generate more interest in this safety subject.

best wishes,
sshep

 

[dcasto]

You ought to work with fluids near there critical point. Our rich oil deethanizer operates near there and when we loose the overhead compressor, the pressure rises about 10 psig and the whole overhead line gets coated with ice. As soon as the compressor comes on, the line drops ice for 10 minutes. Its all carbon steel built in 1961.

 

[barqueiroenmarte]

I would like to calculate the resulting temperature when discharging a liquid LPG from a pressure storage to the atmosphere since it is not the saturation temperature at the discharge pressure, according to above posts. Any idea, documentation, web, book... which could help?

Moreover, I understand that in case the expansion is from a LPG pressure storage vessel to another vessel filled with LPG vapor at a lower pressure the resulting temperature will the saturation temperature at the discharge pressure. Is it right?

Thank you for your help.

 

[tkdhwjd]

barqueiroenmarte,

When a liquid LPG from apressure storage is released to atmosphere (open system I assume), why do you say that it is not the saturation temperature at the atmospheric pressure? The example discussed above deals with a closed system with propane partial pressure being lower than the total pressure due to presence of inerts or methane. Perhaps you misunderstood the discussion....

 

[dcasto]

barque, you state that you want to vent LIQUID propane, therfore it is a either saturated or sub cooled, in either case the liquid propane will be two phase on the outlet of the valve. If the propane is subcooled, there will be more liquid propane than if it were saturated.

Get out a Mollier or PV diagram and follow the enthalpy of what you want to do.

 

[barqueiroenmarte]

Sorry, perharps I misunderstood the discussion.


Sshep said in a previous post that "When the partial pressure of LPG vapor is zero, you can't calculate the minimum liquid temperature from the total pressure". I understood that he wanted to mean that in case of liquid propane discharging to a lower pressure (no matter if it is a closed system or atmosphere) the resulting temperature (liquid+vapor) can't be always calculated as the saturation temperature at the discharge pressure, since it is only true when the discharge pressure is due to propane vapor but it in case of discharging to a system full of any other vapor (air, nitrogen, etc) "the temperature is not so easy to calculate".

 

[dcasto]

no problems here barque. The statement "When the partial pressure of LPG vapor is zero, you can't calculate the minimum liquid temperature from the total pressure" is ONLY correct if the LPG liquid is so small as compared to the new mixture, however, you can get the answer with a simple enthalpy balance.

If you open a valve with LPG liquid behind it, the LPG temperature will drop to that of saturated liquid at the pressure, unless the LPG is a subcooled liquid. Then once the LPG hits the tank with the N2 in it, the temperature will change because of the ethanlpy exchange between the N2 and LPG. This will continue until the pressures equalize. What is not stated is whether there is a constaant pressure on the system (due to a release of N2 + LPG vapor and what is the ratio of mass between the LPG and N2. That's when you can't calculate anything.

 

[sshep]

I didn't mean to create any confusion on a technical issue, only to advise caution in the approach being used to start-up new equipment. dcasto's doing a pretty good explaination-I think we both have some ethylene experience so he can validate.

I was pointing out to the originator (minor C3 case) that the boiling point of the LPG at system pressure is an upper bound on temperature for first time introduction. Think in terms of the diffusion of molecules from liquid to vapor at the interface. The boiling point (flashing) assumption implies that the concentration of vapor at the liquid interface is 100% LPG. This is great thinking for distillation and evaporation in still air (your case?), but in cases of extreme evaporation driving force the temperature can get lower than the boiling point. Good thing for us or our sweat would only cool us to 100C, and our cooling towers give us boiling water as well.

Vaporization requires heat. Flashing and evaporation get heat first from bulk liquid then the surroundings. Your atmospheric release will immediately flash, the bulk liquid will cool to the system boiling point with the vapor fraction determined by the feed enthalpy. The remaining puddle will evaporate at the boiling point at rate determined by heat transfer from the surroundings- very quickly at most latitudes. A big puddle might freeze the surroundings and limit the rate of vaporization. Theoretically, if a wind were to blow across your "puddle" the temperature could drop below the boiling point as the vapor near the interface becomes leaner. Why are you making a vapor cloud? This puddle is dangerous! NOTE: you can simulate evaporation with a process simulator by mixing any liquid stream with N2 vapor at the same temp and pressure. The mix will be colder by the amount of liquid vaporized and the effect is real.

That was the theory, this was the point:

It is common practice to pressure up ethylene vessels with vapor to suppress flashing. Imagine this screw-up: bring carbon steel equipment up to operating pressure with N2 instead. Immediately on introduction of liquid ethylene (which you never expected to vaporize) the pressure shoots up well above operating pressure and the vessel relieves. Ice forms on a line which is supposed to run at ambient conditions, but fortunately for you no brittle fracture. The ice melts and the pressure is vented back to operating conditions. Congratulations, you have just survived a "virtual start-up".

best wishes,
sshep

 

[dcasto]

I'm not hijacking the thread, just elaborating on issues, real or precieved, about commisioning equipment.

There are two main reasons we would fill a vessel or line up with N2 to about 700 psi before adding ethylene, Fist to makesure there was no O2 (or ppm levels at most of O2) and second to keep the temperature of the system from being to cold as pointed out. The low temperatures didn't bother us as much as the O2. Here's an example why. We had a power company drill into our ethylene line. The segment was blocked in and we set up our portable flare to burn the ethylene in a controled fashion on one end. Meanwhile, the rupture point burned ethylene too (and the auger truck too). After about 24 hours the whole pipeline was down to about 20 psig AND -110 F. It took us two days to dig up the line because of the ice. We replace 10 feet of pipe and went back on line. The pipeline was not ruined, nor was there a threat of what most people believe with -110 degrees on API X52 carbon steel.

Just before I left the ethylene business, I created a new way to fill vessels with ethylene and propylene (LPG too). I rented an oil field test separator with a line heater and choke. The unit is rated to 2000 psi and we put high pressure LPG in it and heated the LPG in the water bath heater. The choke was used to lower the pressure to 50 psig. The LPG was 50 psig at about 50 Deg F. We used that to purge the line and vessels, no N2 was used. Then we used it to pressure up the system to operating pressure.

 

[GL431]

Hey guys, thank you for an interesting thread. Dcasto, I would like to know more about that portable flare of yours. Is that a standard utensil to depressure pipelines of the type you have described? Can one use it also for liquid LPG pipelines? Do you have a pointer to literature on it (duty specification, operating instructions etc.)?

Thanks in advance