Restriction orifice under special conditions

[garfio]

I need to size a restriction orifice to be installed in a depressurization line. I expect two-phase flow upstream the restriction (liquid/gas) and some additional flashing downstream.
The initial upstream pressure is 1100 psi and the downstream pressure is about 5 psi.

For this particular conditions (two-phase, big pressure drop, flashing), I cannot find a method for sizing the orifice. Could somebody help me with formulas or methodologies to size this orifice plate?

Thank you

 

[JLSeagull]

Consider a choke for high pressure blowdown application instead of a restriction orifice. The choke vendor sizing program may handle two-phase flow. I have used fixed chokes with "bean" sizes in 64th inch increments.

 

[Montemayor]

You can't find a method or equation for 2-phase choked flow because, in my opinion, there isn't any. At least I haven't found or heard of one (that would work) in 47 years. But just because my efforts haven't had success doesn't mean others won't. However, I seriously doubt it, based on the principles and theory of choked flow.

A "choke" with a "bean" is mentioned here. The term "choke" is often battered about in the oil patch, and it can have at least two meanings. One of them is the choked flow of a a gaseous stream to yield a constant mass flow rate at sonic velocity. The other is a simple throttling of a 2-phase stream or of a liquid stream - with a throttling valve or with a fixed orifice. I believe you are looking to apply the former and not the latter. Am I correct? If so, then an oil field "choke" may not serve your purpose.

If you are trying to depressurize a vessel or system, then a standard control valve with instrumentation can achieve your goal - depending on your constraints and application (which you haven't mentioned). I hope this helps.

 

[sailoday28]

Is the flow single or two component?
For one component, particular H20, there is plenty of literature with analytical solutions and comparison to experimental results. I have mentioned this on previous posts.
Fauske and F. Moody and Levi are a few of the well known names in that field.

I may be contacted at sailoday_28@yahoo.com

Regards

 

[monaco8774]

Lavalos

I am not sure of your aplication and may be way off the mark as my experience is oil and gas offshore, but generally if you are talking depressurisation having to happen within a certain time period from a vessel usually the sizing basis is gaseous phase only, if its from a flowline or manifold you could size for worst pressure drop which is liquid phase flashing and then look at what the rate for gaseous phase would be to ensure there are no problems for either case.

Dropping the pressure that far with the temperature drop on the gas from joule thompson could give you issues with icing up and I would avoid a design that requires uncontrolled multiphase depressurisation simply as you may get some pretty fierce reaction forces on pipework at the same time as you have low temperatures in the steelwork.

 

[garfio]

Thanks for all your answers. I think I should give some more detail to clarify my requirement.

I am working in the depressurization of a reactor in a biodiesel project. The reactor will normally work at 900 psi and 370 F, with a mixture of methanol and oils (50%-50% by weight). At normal operating conditions, the mixture is liquid, and methanol is close to critical conditions. Depressurization shall reach 100 psi in 15 minutes.

A software simulator gave me some interesting information: After a fast pressure drop during the first seconds, pressure will hold for minutes (around 400 psi),while the methanol boils in the vessel. When there is no more liquid left, the pressure will drop to my target level.

The depressurization will be done with a line connected to the bottom of the vessel (by process design), so I will get liquid and some vapor in my restriction device. In order to do the simulation and get a grasp of the flows, I modeled my restriction with a control valve (only option in the software), that calculated the flow based on the upstream pressure only (choked). Now I need to do a better sizing of the orifice, to attain the mass flow required.

The model gave me temperatures as low as 170 F downstream the valve, what is good. It also showed that there will be additional flashing (obvious, right?)

I need to be sure that whatever restriction I use, is going to allow the mass flow I need to comply with the time, but a the same time should not be too conservative to avoid having real flows so high that will cause problems with the flare.

Whatever additional information you can provide me will be truly appreciated. For instance, with the lack of specific formulas for conditions like this, what formulas and methodology would you use to size the orifice? would you think of restriction different from an orifice?

Thank you again

 

[Montemayor]

Thanks for giving us what I consider to be some of the necessary basic data. This should be done at the very outset. Is this an academic project or is it a theoretical exercise? Please be frank and tell us all the facts so that we can accurately identify the concerns.

For example, you state that “methanol is close to critical conditions”. I don’t know if you are an experienced process engineer, but methanol’s critical temperature and pressure are 1,174 psia and 463 ^oF respectively. You are not close to what you state. Perhaps you are referring to other “critical” conditions.

You cannot “depressurize” a process vessel or reactor with efficient accuracy by draining the liquid – which is what you are proposing. If you want to depressurize the reactor you should vent the upper, vapor region. Once the vapor/gas is vented you have, in effect, depressurized your vessel. The pressure in a reactor (as in a boiler) is primarily generated by the generation of vapor or gas. This is the major source of pressure in a heated vessel. There is always an expansion of the contained liquid in any heated vessel, but the normal vapor space allotted makes the “swelling” of the liquid a minor contribution to the overall pressure rise. Therefore, the indicated manner of quickly and efficiently “de-pressuring” a vessel is to vent the top gas/vapor phase – not the bottom liquid phase. Do you (or your process colleagues) have a special reason for draining the liquid in attempting to lower the pressure of the reactor?

You further state “The depressurization will be done with a line connected to the bottom of the vessel (by process design), so I will get liquid and some vapor in my restriction device.” You will get 100% liquid INTO your throttling device and probably a 2-phase mixture OUT of the device as this is probably an adiabatic flashing operation.

What kind or type of reactor do you have and what are the contents and their levels within the reactor?

 

[sailoday28]

Montemayor (Chemical)You state"You will get 100% liquid INTO your throttling device and probably a 2-phase mixture OUT of the device as this is probably an adiabatic flashing operation."
For the moment, assume the line out of the vessel as horizontal. If there is little subcoolng of the liquid in the vessel and the flow in the line is adiabatic, friction will help the flow become 2 phase upstream of the control valve.
Please correct me if I am misunderstanding your statement.

Regards

 

[JimCasey]

sailoday,
While preflash is possible, he has stated he is taking the flow off the bottom of the vessel. This implies that saturation conditions exist at the surface of the liquid in the vessel. THe hydraulic head from the surface to the inlet of the valve will likely be greater than the line loss from the vessel nozzle to the valve. So, preflash will be unlikely.

Normally flashing can be predicted by modeling the throttling process as adiabatic and isenthalpic. Set the enthalpy of methanol at the saturation point of its liquid phase coming into the valve and solve for the percent flash by setting the inlet enthalpy equal to the percent vapor x the vapor enthlpy at the discharge pressure, and 1-x times the liquid enthlpy at the discharge pressure. THen add the bean oil, as it won't be flashing or cavitating

 

[sailoday28]

JimCasey (Mechanical)I agree and the elevation will give some subcooling, however, I don't know where the control valve or orifice is located. Friction upstream of the control valve can add to flashing. If you and Montemayor (Chemical)know that the frictional losses are negligible-then you can count on no flashing upstream of the control device.

With regard to an adiabatic process through the valve, if KE change is negligible, then isenthalpic is a good approximation.

Regards

 

[garfio]

I appreciate all the input and discussion. Let me give you some additional info based on the questions and discusion
so far.

It is a REAL project, so your help is really appreciated.

My reference to been "close to the critical conditions" refers to the methanol, indeed. The safety valves are set at 1100 psi, which will put the product at 1210 psi when relieving. One operating case will takes the reactor to more than 400 F. I think it is close enough to critical conditions. The depressurization could be initiated any time so the initial pressure could be as high as 1210 psi.

The reaction is not exothermic, the only heat input would be from fire. The reactor works 100% full of the liquid mixture. The depressurization is done from the bottom of the reactor, and this corresponds to a proprietary design.
I don't have details into this but my feeling is that removing the liquid will help to remove the content faster.

I used the Hysys depressurization utility to predict the process. As I mentioned before, it predicted an abrupt reduction in pressure during the first seconds, and then the pressure stabilizes at around 400 psi. That level corresponds to the boiling point of methanol at the operating temperature, so It had sense to me.

After the pressure stabilized, the model gave me presence of vapor at the inlet of the discharge valve (bottom of reactor), somewhat unexpected because the reactor was modeled as a separator, so I should expect only liquid as you said. Could that have to do with the heat input (fire) that was included in the model? Any of you with experience using Hysys could confirm that? or could there be other reason for the model to give this result?

To add to the previous point, I should say that the reactor has internals (think porous media) that will produce a pressure drop so, even if the model was oversimplified, this pressure drop could cause some flashing before the restriction (or not?). Obviously the model did not account for it.

Based on the previous discussion, please confirm me that my real expectation should be to get liquid only, because that will make a difference in the sizing of the orifice. If that was the case, what formula could I use? I will have flashing downstream anyway.


Hysys offers several flow equations to model the "liquid" exit valve. I chose the general equation (other options
are supersonic, subsonic, fisher, massoneilan). The general calculates the flow based on the inlet pressure (so it uses a choked condition). Don't sure that this is applicable to liquid, but if not, why does Hysys offers that choice for the liquid valve?

I used this equation to determine the flow rate that allows me to depressurize in the given time. Now I have to size an orifice that will give me the same flow rate.

Any additional input in how to calculate the orifice?

 

[jeap]

I understand you are looking for a time to discharge the reactor (not specifically depressuring). Let me tell you that an accurate calculation of the orifice diameter under those conditions is not possible, due to flashing conditions at vena contracta.

From my point of view, you should use the CV calculated through hysys and use it to find an equivalent orifice diameter for the service you described.

This way, you at least have initially a closer number. The only way to know if it allows depressuring the reactor in 12 o 18 minutes is, unfortunatelly, during the event.

 

[dcasto]

Once you select an orifice that gives you the calculated results, just be prepared to cahnge it out to the size you want. I've done all the calculations, installed the calculated 3/8" orifice to get the desired depressureing of ethylene and it was just to slow so we put in a 1/2" and engineering and ops were satisified.

You stated you needed 100 psi in 15 minutes as your measure, not 50,000 lbs/hr, not 30 ft/sec. You will have a variable rate as you depressurre, because of your "it may be 1200 psig and maybe 400 F". Just make the installation such that you cane safely change the RO.

 

[garfio]

The only problem is that this is an emergency depressurization so I won't have the opportunity to test it.

Regarding the variability of the flow, take a look above about the results of the simulation. The model gives a nearly constant pressure during the time the methanol is boiling, meaning the the flow will be uniform enough during this time (10 minutes). That is the condition that I want to calculate the orifice for. After that, the flow will gradually reduce to 10 psi at 15 minutes.