5 replies to this topic

[abhi_agrawa]

Hi!
In a plant that we are designing, the heat to the reboilers in the distillation column is supplied by LP Steam. The heat flow to the reboiler is controlled by controlling the flow of condensate. The arrangement that we use is shown in the attached drawing.
On the condensate line, the control valve is between a reducer and expander. Would somebody please tell me why do we need the reducer and the expander.
Thanks,
-abhishek

Attached Files

•  Drawing1_Model__1_.pdf   3.75KB   135 downloads

[Art Montemayor]

Abhi:

It is not unusual to see a control valve that is at least 1 size less than the pipe that it is located on. However, since, as you state, you are designing the plant you are in a better position to tell us why you have need for a system where the control valve size is 5 times less than the pipe it is located on. We are given no basic data on the design, so I'm not in a position to be able to find out what problem - if any - there might be in making the piping 4" or even 6". Here, I'm assuming the calculations for the correct control valve size are correct. I don't have any problems with an oversized LP condensate line, other than the expected economic argument against needless capacity. I have never seen (or even dreamt) of a reboiler condensate line that was 8" in diameter. That is a huge quanity of condensate and I can't imagine the heat duty on the reboiler.

In defence of the designer(s), perhaps they require an extremely low condensate velocity. Or perhaps there is a need to expect future expansion in the condensate flowrate and the investment in the piping is made now. If I were to see this installation in a plant I would normally suspect that the piping size has been made bigger than required for future capacity expansion. I believe we are all aware that there will be flashing through the condensate control valve and the subsequent flow will be 2-phase. However, in all fairness, I believe we are only talking about a 25 or 30 psi drop (this is what I call LP condensate) so the amount of steam vapor created will not be very large. In the worst engineering case, this might be a plain blunder or a case of "belt and suspenders" - i.e., a design engineer making it super sure that his design has sufficient capacity.

[abhi_agrawa]

Art,
Thanks for a quick reply. I'll clarify the things. The plant is in the designing stage and I am a part of the team. I do not have much experience with design, and that is what made me curious.

In this paticular valve, the line size for condensate comming to the valve is 4", the control valve is 3" and then it expands to 8". Since downstream of the control valve the flow is 2 phase, we have a 8" line.

All the control valves in the plant have a reducer/expander around them and are 1 size less than the line size. I wish to know why the reducer and expanders are put? Why not have the line and the contol valve of the same size? So, in this case, why is the line size of the condensate to control valve not 3" ?

Thanks,
-abhishek

[djack77494]

abhishek,
As a matter of practicality, control valves are very often sized to be smaller than the line they are attached to. This is because the criteria used for sizing the lines are quite different than that used to size the valve. Especially in a potentially flashing service, such as yours, you would want to sharply limit the velocity and pressure losses upstream of the valve, dictating a relatively large line. For the control valve, you're "burning" pressure. You've got a flowrate and a pressure drop to "use up". No sense in sizing the valve larger than necessary. It could easily be 2 or 3 (very rarely 4) sizes reduced from line size. I'd offer another comment concerning the other valves in this system. Make the bypass (globe) valve no larger than the control valve. Make the isolation valves one size larger than the control valve, but, of course, no more than line size. Do your best to ensure there won't be any flashing upstream of the control valve, and also to locate the control valve as close as possible to your destination.
Happy designing,
Doug

[Ritesh Suratwala]

Hi,

Let me give you my view on same.

Lets take your example only. Heat exchanger outlet pressure is fixed based on source pressure and exchanger pressure drop. Similarly, control valve outlet pressure is fixed based on destination pressure. That means control valve pressure drop is almost fixed for given flowrate. Now with given pressure drop, if you want to maintain certain flow, it will ask for fix absolute opening in valve irrespective of control valve size. So, even if you are using higher size valve, you will not be able to utilize 100% openeing of that valve due to plant capacity constrain. That means you will end up only with adding un necessory cost.

Line size is defined based on some correlations and control valve size are based on some other correlations. So, for given flow rate, if you calculate line size and control valve size, most of the time you will end up control valve size 1 size lower than line size. Note that, formulas give minimum required line size and control valve size. So, technically nothing wrong if you go for same valve size as line size, but as explained above, that is nothing but adding un necessory cost.

[Adriaan]

Make the bypass (globe) valve no larger than the control valve.

NU UH!

Why do you need a bypass valve?

The very obvious answer is "so you can work on the control valve while the system is still in use".

In practice a bypass is often used under .... special conditions (such as during startup) so you want a bigger bypass valve (unless you are certain that twice the control valve capacity will always suffice) to quickly drain the system - which would be full of cold condensate.

A secondary consideration is that usually the bypass is located under the control valve; any fouling tends to collect down there and when you have a small bypass the accumulated stuff will very probably clog your bypass up (see the picture). Even in the cleanest systems some fouling occurs!

 deposits.bmp   8.5KB   110 downloads

The obvious addition I'd make to the design then would be to add a drain just before the bypass valve and to use that drain from time to time to flush out such deposits.