Is a liquid drain trap suitable or not? 2

[DjazAutomation]

We have a storage tank for gas condensates in our Gas central processing facitliy. We are facing some difficulties to drain only water from the tank without draining Gas condensates (Liquid). Hence, we loose significant qunatities of gas condensates when the operator drains the tank manually.

I wonder if a liquid drain trap is suitable for our application since I noticed that liquid traps are usually for removing a liquid from a gas vessel. While in our application, we need to remove a liquid (water) and leave another liquid (condensates) inside the tank.

SG of condensates: 0.7
SG of water: 1
Pressure inside the tank (Upstream) : < 5 Barg
NPS Drain pipe: 4'', Class 150
Downstream pressure: (Atmospheric)

Do you think that the solution is feasable? Is there any manufacturer that proposes a suitable product for this application ?

Any help would be much appreciated.

Thanks a lot

Djaz

 

[ash9144]

You could use a coriolis to detect the density difference and open/close based on that.

You could probably use a conductivity probe to detect the interface as well and control using that.

 

[gerhardl]

I doubt there exists valves or liquid/steam/gas traps that will be suitable for the purpose.

A drain trap is (either) normally built with a float mechanism, balanced to be floating (and hence open or inverted constructed to close) in a certain amount of liquid, usually water.

It would probably be difficult to dimension and construct the valve/trap with a proper closing force and performance to close for the liquified gas even if there is a stable level interface of water (1.0) lowest and liquid gas (0,7) above.

Or, the drain tap could be of the other main construction principle: thermostatic, constructed to open and close at different temperatures. (This is based on the principle that the two phases gas/liquid transforming draws a temperature curve related to pressure. Ref steam tables.) If water and gas condensates have the same temperature (and same pressure) as here, this will not work.

Do others have knowledge of 'contraptions' for this purpose?

If not there seems only to be a limited number of possibillities left: (suggestions)

Mechanical drainage combined with closing signal from fast operating gas detection unit.

Separate drainage tank with checkvalve closing return to tank for liquid, heating element to turn gas from liquid to gaseous, (temperature and pressure controlled) and water left liquid, and overpressure protection to return overpressure gas to tank, combined with suitable mechanical liquid drainage. (Necessary safety devices in addition, explosion protection required ?)

Time controlled drainage (empirical data) combined with water liquid drainage well (pipeline piece straight down from tank in suitable dimension), if a well is not already installed, to give significant height of water liquid, in stead of drainage from the (much) larger complete tank bottom.

...or any combination of above.

Others with suggestions or comments?

 

[DjazAutomation]

Thank you ash9144 and gerhardl for these valuable information.

I think now that a liquid drain trap is definetly not suitable for this application. I even asked Spirax Sarco, I got the same answer. So, I am comfortable with this decision. Thank you for that.

Moreover, I was wondering about a cost-effective solution that relies on one of these options:
- Using a detector (Density or conductivity) as ash9144 proposed and use the signal to tell the operator to stop draining.
- Using magnetic level indicator (Like those of Magnetrol), and choosing the right float to measure the water level only (Heavier liquid). We do have a free nozzel on the tank to connect the lower Tap of the level indicator but on the other hand, we don't have any nozzel to connect the upper nozzel of the level indicator. Is there any way to overcome this problem?

What could you suggest regarding these solutions?

Thank you very much for your help.

Djaz

 

[gerhardl]

Suggestion:

You need to know a bit more and analyze the actual conditions at site to construct and specify the real needed assembled equipment.

At least temperature, pressure,flow amount and liquid details (material compabillity) and what happens when you tap out the liquid from a higher pressure to atmosphere would be necessary.

One idea would be to arrange a vertical tube of suitable size to give a highest possible column to detect the interface between liquids and constructed to keep the outflow stable not to mix and disturbe the interface too much.

In addition: shut down valve between tank and tube,checkvalve in addition ?, tapping/closing valve a bit above bottom of valve, a detector of different density (or viscosity) necessary distance from tapping valve at the column and suitable for column and conditions, and one manual bottom drainage valve at bottom of column to occasionally drain out particles and dirt.

 

[DjazAutomation]

Thanks gerhardl, it seems to be a great Idea (I drew a sketch based on it).
However, I am wondering what would be the best technology to use to detect density. Reliability and maintenability of the instrument is really a concern for me.

Regarding the actual conditions, I still don't have all the required information (Others than those already given).
It's only to have a rough idea of the solution. the detailed engineering will be determined later to say if the solution is feasible or not.

Thanks a lot for your valuable help.

Djaz.

 

[ash9144]

For density measurement, my choice would be coriolis meter. But this is a big expense for this problem.

I think conductivity would be your best option. Need to mount the probe as close to the tank as possible with the valve downstream of the probe.

 

[gerhardl]

Try to contact a reliable good local supplier with detailed product knowledge with details on your problem and sketches, and ask for a preliminar quotation with advice for best/cheapest instrumentation and a price idea, pending your decision. Or ask two competitors, before you go out and ask for firm price.

I do not feel that I have sufficient detailed knowledge on suitable instrumentation, and you will have far more details on necessary accuracy and needed detection sensitivity and speed, and mounting possibillities than anyone else.

With the significant diffeerence in density it should perhaps not be necessary with very precise detection. If any instrumentation is available to be clameed on the outside of the draining well, this could perhaps be a suggestion.

Conductivity for the two liquids could perhaps be different. Any instrumentation available utilizing this?. (Flowmeter on magnetic principle? Just a loose, untested idea.)

 

[DjazAutomation]

Thank you all for your suggestions.

As you proposed, I'll try to get in touch with Density or conductivity detectors supplier in order to get accurate information about the suitability of the solution and the constraints that I need to take into consideration in the design.

Again, many thanks for your valuable help.

Djaz.