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Maximum Flow Velocity

velocity pipe

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#1 shajige

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Posted 28 April 2012 - 05:57 AM

Hello All,

I have a confusion with respect to pipe sizing and maximum allowable flow velocity.
In my problem, steam of 80% quality is to be passed through the pipe to a pit. But this is not continous operation,which means that may be 6-7 hours during start-up and similar time during shut down/emergency.
What could be best maximum flow acceptable. The process team of consultant is suggesting 100+ m/s velocity to a sonic velocity at the exit tip by providing smaller size (20" dia).If they have to limit to 50-60m/s the size of pipe will be huge.
My query is whether such high velocities are acceptable for pipe sizing?

Thanks in advance for help.

Best Regards,
SA

#2 ankur2061

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Posted 28 April 2012 - 06:14 AM

SA,

As I understand this is the blowdown line from the HRSG steam outlet line to the blowdown pit and is only to be used for a very short duration (1 or 2 hours) during start-up and shutdown to blow out the high-water content stream from the HRSG to the blowdown pit.

Your consultant is absolutely logical in saying that for this purpose you don't need a very big line size just to keep the velocity in check.

What is more important is to provide proper piping support for the pipe upto the blowdown pit in order to account for the pipe vibrations due to high velocities.

If you are too concerned about the noise during controlled and monitored blowdown for a period of 1 to 2 hours then you can provide an end-of-the-line silencer where the velocity is maximum.

Hope thsi helps.

Regards,
Ankur.

#3 latexman

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Posted 28 April 2012 - 08:15 AM

For short term, non-continuous service (blowdown, relief tailpipes discharging to atmosphere, etc.) having sonic velocity at the exit has been and is still considered good engineering practice to achieve an economic long term cost of ownership.

#4 kkala

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Posted 29 April 2012 - 08:48 AM

Besides noise requirements specified in the Contract could dictate the silencer according to written standards.
Concerning two steam boilers locally ordered, starting valves (discharging steam to ambient in start up) were equipped with silencers to reduce noise to 95 dba from any place where people can be. But if operation lasts 6-7 hours, flow could be rear yet considered as continuous, posing stricter noise limits (85 dba at 1 m distance).
See http://www.cheresou...ea-definition/ , post 7. Country regulations and assessment of the specific case can determine noise limits.

Edited by kkala, 29 April 2012 - 08:55 AM.


#5 shajige

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Posted 01 May 2012 - 10:37 PM

Thanks everyone for the prompt response and I am impressed with the forum where so many experts are volunteering to answer queries.
The stratup may take 4-7 hours and shutodown also may take similar or lesser time to cool down. Hence we can use higher velocity for sizing these pipes as its not a continous operation?
Noise levels also needs to be addressed accordingly.
Anyone has seen/used a silencer for such flow condition (init
Thanks everyone for the prompt response and I am impressed with the forum where so many experts are volunteering to answer queries. The startup may take 4-7 hours and shutdown also may take similar or lesser time to cool down. Hence we can use higher velocity for sizing these pipes as its not a continuous operation.
Noise levels also need to be addressed accordingly.
Anyone has seen/used a silencer for such flow condition (initially water and then 80% quality steam).




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