Rich Caustic Control Valve Sizing

[chemdesign]

Does anyone have experiece with sizing the rich caustic level control valve on a mercaptan extractor in butane service? If so, can this valve be sized with the assumption that the vapor pressure of the rich caustic is very low which allows the actual valve pressure drop be used in the liquid sizing equation as opposed to the allowable (i.e. choked) pressure drop? The downstream pressure is approximately 1/2 the upstream pressure and the valve recovery coefficient Km is 0.85. I need to resize a control valve in this service, and for some reason the exisitng valve is operating at a much greater % open than would be expected. Thanks.

 

[TD2K]

My first question is are you sure of the dP available for the valve and the flow rate? While the inlet pressure is usually pretty each to check, the downstream pressure could be affected by unknown line losses if you are calculating that.

I would think the solubility of butane in caustic is pretty small and thus the vaporization of the butane should also be minor. Don't know if you have any data on that, my data/experience on these systems is pretty limited.

Have you tried talking to any of the vendors for these systems for general information? They won't help you on proprietary areas but I wouldn't think this would fall in that area. If it is, they might be willing to say 'it's not much of a factor' or 'it is a factor you need to consider'.

 

[chemdesign]

TD2K,

Thanks. i have tried talking to the licensors of caustic processes but have not had a warm response since this is not their process. i did verify the dowstream pressure via a pressure survey and am using that in the sizing equiation. i can't believe there is enough vaporization in this system to cause choked flow.

 

[TD2K]

What are your pressures? What's the inlet pressure?

The dP/P1 ratio of 0.5 is of significance in vapor service since that is about the point where you can get choked flow but it's really of no significance in liquid service.

 

[chemdesign]

Inlet pressure is 135 psig, outlet pressure is 80 psig, with 17 gpm flow. Valve Cv calculates to be 2.5, but actual Cv from % open in the field is more like 5.

 

[roker]

hello,

did you check if flashing may occure, what is the temperature? why the dp on the valve is so high? may be it is undersized/

regards,
roker

 

[TD2K]

The dP the control valve takes is set by the system and hydraulics, it has nothing to do with the size of the valve (unless the valve was wide open which isn't the case).

A simple example is letting down 150 psig steam to 50 psig. Assuming the piping is short enough that the pressure drop in the piping is insignificant, the dP will be 100 psi whether the valve is 1" or 20".

 

[roker]

TT2K,

I agree with your example of steam etc., but this is not the case, I thought to get more details from "chemdesign" so the system can be analysed and solved.

regards,
roker

 

[chemdesign]

My original question was to see if anyone had experiece with valve sizing in the specific application of rich caustic being letdown from a mercaptan extraction column. The existing valve is nearly wide open when sizing calcs indicate that it should not be. Note the valve is located right at the oulet of the column, prior to the rich/lean exchagner.

The four possibilites that i see are:

1. the rich caustic has alot of butane in it (much more than is predicted by process simulation). I don't believe this is the case since the sister valve on the propane mercaptan extractor is not further open than it should be.

2. Most of the mercaptans flash out of the caustic as the pressure is dropped across the valve. I don't think this is likely since it takes a 20 tray tower with lots of stripping steam to regenerate the rich caustic, so it is not easy to get the mercaptans back out of the caustic, especially without heating it.

3. The valve is plugged somewhere between upstream and downstream bleeders at the control station.

4. At some point someone changed the valve trim and never changed the tag on the valve or the datasheet in the plant's records.

I would appreciate feedback if someone else has ever experieced difficulty in sizing a level control valve in this application.