Open Bypass for Thermal Relief

[RJB32482]

Right now we have a styrene line that has about 400 foot of piping that is 4" Sch. 40. It pumps styrene into a reactor at around 2000 lb/min. There is a blocked in section that is blocked in by 2 valves. In the past, they installed a bypass that went around one of the valves and instaleed a valve in it. The bypass is about 9 feet long and reenters into the main header behind the first block valve that blocks in the section. The valve in the bypass is closed when pumping and open when not. Problem is the valve in the bypass gets plugged up a lot.

Is there a good reason not to just remove the bypass valve except loss of flow rate while pumping to the reactor? What would be the best way to calculate the loss of flow while having the bypass open?

Thanks. A 1989 article in CHE Mag. is where I got this idea.

 

[Ashereng]

Are you thinking of replacing the bypass valve with a PRV?

 

[RJB32482]

No not a PSV. Just an open bypass that will relieve the fluid whenever the blocked in section valves are closed.

 

[Ashereng]

If you remove the bypass valve, how are you planning to relieve the fluid between the 2 block valves?

 

[RJB32482]

When the valves in the main header is closed, the open bypass might also be full but where it bypasses to (from in between the 2 valves to before the first valve)is not blocked in. This will relieve the fluid blocked in.

Thanks.

 

[StoneCold]

If the bypass valve line runs parallel to the main line downstream of the pump. Which is what I think you are saying, then you won't loose any flow when the pump is running.

If the bypass outlet is in front of the pump and the bypass inlet is down stream of the pump then the pumping loss will be the amount of material that can flow through the bypass line at a differential pressure equal to the operating pressures.

You might want to put an orfice plate in the line to slow it down a little but it sounds like a good solution to the problem.

Regards
StoneCold

 

[RJB32482]

Thanks for the info. The bypass inlet and outlet is both on the discharge side of the pump. The bypass is pretty much a loop around with 2 elbows going around the block valve. So no losses in pumping capacity should occur.

Thanks.

 

[EGT01]

RJB32482,

Just to point out the obvious, one reason for not removing the bypass line's block valve is you may not be able to easily isolate the line that you are trying to protect. Are there times when you may need to have the line completely isolated and how do you plan on accomplishing complete isolation if there is a bypass line without a block valve.

Also, do you know why the bypass line gets plugged up? I don't have experience with styrene but isn't it prone to polymerization even at room temperature? I can imagine a dead leg of stryene could be a problem over time but in a small diameter line that essentially has no flow through it, I can imagine even without a block valve, the bypass line may still have a plugging problem.

You may want to go back to the root cause of your concern, why is it necessary to block in your 4" line in the first place? Are there other means of avoiding overpressure of the 4" line from thermal expansion of the liquid? If you presently have a procedure that says open the bypass valve when you block in the 4" line, why not just make your procedure be that you never close one of the valves in the 4" line?

 

[RJB32482]

One of the valves in the 4" line is for the whole styrene header system and the other valves are for each of the reactors. So you can't just open the 4" valve in the main header line.

Thanks

 

[StoneCold]

EGT01 Makes a good point. Even if you decide to leave the bypass open most of the time, you should at lease have a block valve there.

Regards
StoneCold

 

[RJB32482]

Increasing the line to 2" might help in the plugging factor while still not effecting the flow rate. Any thoughts if this would help or not?

Thanks.