7 replies to this topic

[Teknas]

Dear All,

I am working on a project where we have rerouted the column overhead vapors to a new reboiler and thereby generating steam. In the original scheme, our client had a air cooler. The existing vapor line had a pressure control valve for controlling column pressure, without a bypass connection.

In the new scheme, we also went ahead with same concept of pressure control, but now the client insists on a bypass to the control valve.

The vapor line is 24". The contract apart, please tell me is the bypass to control valve necessary, because if the vapor line control valve is taken down for maintenance how can a 24" manual bypass valve control the column pressure ? Is it physically possible ? In my experience I have never seen such an arrangement.

I would also like to clarify that the steam generated by the new reboiler is just to reduce load of the boiler and save on fuel. The original connection to the air cooler is maintained, but blocked.

[Zauberberg]

Hello Sanket,

Can you upload a sketch of existing and the new (proposed) system? In that way, it would be much easier for us to get the full picture.
I think I know what is your concern, but I want to be sure if I got it right.

Regards,

[djack77494]

Hello Sanket,

Can you upload a sketch of existing and the new (proposed) system? In that way, it would be much easier for us to get the full picture.
I think I know what is your concern, but I want to be sure if I got it right.

Zauberberg is right to request a sketch, but venturing out a bit into the unknown, I know I would not want to be the person expected to achieve column pressure control by manually adjusting a 24" valve. I'd like a crew of 5 additional operators to assist, and would still hope that valve movements would be slow and minimal. In short, manual adjustments to replace an actuator for a 24" control valve really are not practical.

If the existing air cooler will still be in place, why not use it as a backup. Then if you ever need to take the reboiler off-line, just route your overhead vapors through the system that is currently in use and in place. Flexible, known to work, and even cheaper than a bypass valve.

Doug

[JoeWong]

I really doubt if you can find a large (may be not 24", but i guess at less 12") bypass valve if you stick the principle that isolation valve shall not be used throttling.

What's the purpose of bypass valve ? Check with your client.

Is the system demand for a bypass valve ?
If it is to control the column pressure manually by changing the opening of manual bypass valve, the operator would have a "good time" with the company. Personally i really doubt if this procedure is workable at site and safe to do so. That's i always have problem with control valve bypass valve. I would rather go for 2 x 50% control valves or 1 x 30% and 1 x 70% control valve or 2 x 100% control valve if process demand for it.

Flexbility & operability and safety
But generally i am having hard time with the operator and start-up team as they are looking for start-up bypass. They want flexibility. Well...i have started-up some plants. Flexibility is required. Sometime it is useful when we have "problem" and infact this encourage a lot of illegal operation.
Anyway, there is a conflict between flexbility & operability and safety.

In many event, it end up a extreme large PSV downstream of the control valve and bypass valve, increase flare capacity, line size, acoustic induced vibration problem, etc.

Cost saving using manual bypass for very infrequent event
Sometime we have also though of a manual bypass can be used for intermittent peak which is very infrequent. Say you need a large flow during ""XX" event but normal flow is just 50% of the peak flow. Then you provide a 50% control valve and another bypass valve to cater for the 2nd half of flow. You may just open up the bypass valve and let the control valve maintain a good flow via both control valve and bypass valve.

I don't really know your system, plant operating philosophy (manned, unmanned, fully automated, etc), it is very hard to judge if a bypass is required.

I am hoping that you can consider to review the main purpose of bypass valve, check with your client, request for the clarification if they insist.

[Teknas]

Hello Sanket,

Can you upload a sketch of existing and the new (proposed) system? In that way, it would be much easier for us to get the full picture.
I think I know what is your concern, but I want to be sure if I got it right.

Regards,

Hi Zauberberg,

I have uploaded the scheme for your reference. Please have a look.

Attached Files

•  Overhead_Scheme.xls   26KB   116 downloads

[Technocrat]

Hi Sanket,

I understand that you need a bypass valve to the new control valve (say B1) so that in the event you take B1 for maintenance you can maintain the column pressure by adjusting the bypass valve. But it is very difficult to operate this big manual globe valve. The bypass valve need not be always a manual globe valve. As per me there are two options to control the pressure in case you take the control valve for maintenance:
1. There will be two control valves in parallel (B1 & B2), both are exactly same to each other, so that when one control valve is under maintenance the second one will be in line.
2. If you don't want two control valves in parallel due to cost/space constraint then the original control valve (A) on the Air Cooler will remain as it is, when B1 or B2 or both valves are under maintenance then take A in line temporarily. When B1/B2 is back in line then take A out of loop.

Regards.

[Padmakar Katre]

Dear,
Could we think to have a MOV (Motorized Opertaed Valve) in the bypass with the provision of the local as well as remote operation. Here one thing I want to know from you is that what was the practice if there was problem with the Pressure Control Valve in the original scheme other than shut down of the column. I think to run your column on the manual pressure control is a big deal. Either to have a spare control valve with same CV as main control valve or to have the bypass valve as MOV. Just waiting for your comments.

[Zauberberg]

Hello Sanket,

Thanks for uploading the sketch, things are more clear now.
Here is what I would do if I were in your position:

1. Basically, there are no differences in two schemes submited to your analysis: first system doesn't have a bypass at all, but repairing/replacement of a control valve would require tower shutdown. Your concerns in the second scheme are reasonable - I hope no one is thinking about replacing the control valve while the tower is operating, and maintaining column pressure by manipulating bypass (usually globe) valve? It cannot be serious approach.

2. In the second (revamp) scheme, what I would think about is flexibility of condenser heat removal demand VS reboiler heat input demand. Any significant difference between these two values will lead you to operational and safety concerns. Do you plan to remove air-condenser or it will serve as supplementary heat sink in case reboiler heat demand is not sufficient to maintain your tower heat balance? This is the most important question.

3. Check the composition of overhead vapors: if there are no non-condensables present in the system, throttling of overhead vapor flow or having hot vapor bypass control scheme is not the optimum solution, but it will work. Instead, I would rather go for flooded condenser pressure control. This scheme, however, involves installation of additional exchanger (or reusing existing air-condenser), because I haven't seen reboiler being applied in such services - lack of flexibility, I guess.

I am attaching one good article written by Andrew Sloley, famous distillation expert. It is related to column pressure control.
I am also looking forward for more information from your side.

Best of luck,

http://www.clarkson....gn/distnprs.pdf