8 replies to this topic

[shan]

I sized vessel inlet/outlet nozzle based on momentum limit (density*veocilty^2) or velocity limit. I never understand what the theories behind the limits are. Why should we just use connection piping size as nozzle size?

[fallah]

I sized vessel inlet/outlet nozzle based on momentum limit (density*veocilty^2) or velocity limit. I never understand what the theories behind the limits are. Why should we just use connection piping size as nozzle size?

Outlet nozzle sizing to be done using line sizing criteria.
Inlet nozzle sizing in vessel,as you mentioned,to be done based on momentum limits regarding existence/type of inlet device.

If there is no inlet device inside vessel momentum severely limited.
If there is simple inlet device such as half pipe or baffle moderate momentum be allowed.
If there is complex inlet device such as schoepentoeter or other vane pack type higher momentum be allowed.

All above limitations would be considered in order to control the force of inlet fluid momentum (especially lquid) may lead to vessel damage due to erosion,vibration,...

Finally,it is worth to mention that connection piping size may be different with inlet nozzle size due to different sizing criteria.

Hope above help you out

[ankur2061]

Shan,

An old thread on momentum criteria for nozzles should be able to give you some more insight on the subject. Below is the link:

http://www.cheresour...ov2-for-nozzle/

Regards,
Ankur.

[shan]

Hi Fallah/Ankur2601,

Based on your replies, it seems to me the reason for the inlet nozzle momentum limits is to protect vessel internal devices. Therefore, there are only maximum momentum limits and no minimum momentum limits. Then, the inlet nozzles shall be always equal to or larger than the connecting line sizes to not increase or reduce the momentum. However, I saw the vessel nozzle sizes are smaller than connecting line sizes (reducer on upstream of nozzle) in many cases. Could you provide an explanation?

Regards

Shan

[fallah]

However, I saw the vessel nozzle sizes are smaller than connecting line sizes (reducer on upstream of nozzle) in many cases. Could you provide an explanation?

The case as you mentioned above rarely happens.It may be done in order to decreasing equipment nozzles size and certainly provided that full inlet device would be exist.Anyway,the reason could be difference between line sizing criteria and nozzle sizing criteria.

Would you please attach a sample case along with relevant data may help for better assistance.

Thanks

Edited by fallah, 16 February 2010 - 02:46 AM.

[Technocrat]

Dear Shan,

Please find attached Excel spreadsheet for calculation of momentum value and selection of inlet device.

Regards.

Attached Files

•  momentum calcs.xls   28KB   1727 downloads

[shan]

Hi Everyone,

I checked a few larger line size/smaller nozzle size cases. I think the smaller nozzle sizes are from the vendor's package. Therefore, I agreed that we should specify the nozzle sizes the same as or larger that the connecting line sizes to reduce momentum.

Regards

Shan

[fatimah]

Hi there

just have something to add; my comment is based on Ludwig book. please find it here

http://books.google....nozzles&f=false

usually for liquid, the velocity limit is always based on viscosity of the liquid. the lower the viscosity, the higher the velocity limit.

for vapor, our consideration is based on molecular weight & inlet pressure.

for liquid/vapor mixture the velocity limit is lower than vapor only fluid as we need to consider the possibility of entrained liquid droplets travelling at the same speed as the vapour as this could cause severe pitting and erosion.

and yes, we use different nozzle size to meet the required velocity

[kkala]

I checked a few larger line size/smaller nozzle size cases. I think the smaller nozzle sizes are from the vendor's package. Therefore, I agreed that we should specify the nozzle sizes the same as or larger that the connecting line sizes to reduce momentum.

I would add that not all design aspects get optimized in a factory. Someone may have thought that existing nozzle is enough, or not worthwhile increasing its size, even though the new line directed to it has bigger diameter. He may not be informed on the latter, or decide to increase nozzle size at first chance (it may never happen). Pending points are often forgotten if installation apparently works smoothly, unless they are important safety matters touching legislation.

As a local practice here for refineries, nozzle size is same as connecting line size. Momentum rule is not taken into account, though it may be useful.
If high velocity is required in atmospheric tanks to "agitate" stored liquid, this is realized by "jet mixer" downstream of the nozzle.
If velocities < 1 m/s are needed in atmospheric tanks to avoid static electricity, "difusers" are placed downstream.
Provision for higher flow rates in the future is not known to be made during tank design / construction.