#2 Splitter- Condensate Flow Valve Utilization in Gas Plant 1

[DavidStCyr]

I have a condensate question regarding our #2 splitter reboiler a stab-in reboiler, which has two identical exchangers. Each exchanger’s condensate output will flow through its own flow valve. This condensate re-route includes use of 2 FCV’s that were not previously being used. We currently flow through LCV 755 (a 2” valve), as we dump directly to sewer from the #2 splitter reboiler. This (LCV 755) level regulator controls the product bottoms level in the tower by controlling the steam regulators on each reboiler, as well as the condensate level in each. The proposed re-route of condensate to a collection tank would then differ the total condensate flow through two FV’s to a collection tank, instead of direct discharge from the regulator to the sewer.
Basically, right now we are running through a 3” line, through this 2” LCV755 and then to the sewer. We are now looking to run the same condensate through the two FV’s in 8” parallel lines, one per exchanger. My question is would the change to these larger control valves pose any issues controlling the level in the reboilers. The spec’s for the proposed valves are listed below. Any help would be greatly appreciated!

FV-800 & FV801

Size: 6”
Total Flow: ~40 Mlbs/hr
P1 (min, norm, max):
30, 40, 45 psig
P2(min, norm, max):
9 psig, 15-16 psig, 21 psig
Condensate is about 267°F

 

[SeanB]

It depends on the Cv of the valve. If you have the valve make and model you can contact the vendor and have them check rate the valve for the new operating conditions. If it is a Fisher valve you can download software to do the check rate yourself.

 

[DavidStCyr]

Thank you SeanB!
The valves each have a max Cv of 271. Based on the new operating conditions the max Cv required is around 35.
Based on the manufacturer data, the valves can pass the require Cv at about 45% valve travel. Would you say these valves are oversized? Also, can oversizing a valve adversely affect controlling the level in the reboiler? Thanks!

 

[TD2K]

Do you have a sketch of the system? I've having some problems following the exact control scheme.

Sounds like you have equal percentage trim if they are going to be about 45% open given the required and installed Cv.

Oversizing a control valve means it's going to run towards the closed position. If you look at the Cv curve, a small movement in valve position will result in a large change in Cv and thus flow. 45% wouldn't give me a concern, 10% might.

It's interesting that over a reasonably wide range of reboiler operating pressures (30 through 45 psig), the temperature is given as a constant at 267F (saturation temperature for 25 psig steam). From your writeup, I would expect the condensate to be leaving the reboilers saturated, that's going to increase the required Cv for your level control valves and increase the amount of flashing through those control valves.

 

[SeanB]

You need to look at the required Cv at your normal and minimum flow operating conditions as well. At the maximum the valve appears to be suitable. Check the required Cv and valve opening at the minimum and normal condition. That is where you might run into problems.

 

[DavidStCyr]

TD2K,
We checked with the manufacture and they confirmed the valve would indeed flash so they recommend hardened trim. Thanks!

SeanB- I checked the required Cv and valve opening at the min and normal condition and they fall between 22-30%. The manufacture recommends a valve to operate between 20%- 80%. So I think we are ok here. Thank you all for your help!