11 replies to this topic

[go-fish]

Is there any criteria for sizing the outlet and inlet nozzles of atmospheric storage tanks?

The vessels group in my company uses a spreadsheet for outlet nozzle sizing and the systems group maintains 1 psi/100 ft criteria for sizing the connecting pipeline (in case the outlet goes to a pump suction). There is usually a difference between the nozzle and connecting pipe with nozzle one size higher than pipe. Therefore, I have to use a reducer in most of the cases.

For inlet nozzles, same size is maintained as line size. Do not know the reason though.

[ankur2061]

go-fish,

There is a very recent post on nozzle sizing. Read the same to get an answer to your query.

http://www.cheresour...-nozzle-sizing/

Regards,
Ankur.

[go-fish]

I read that post. However, it seems to be the other way round in my case. In that post, the outlet nozzle was kept as line size and inlet nozzle was checked for momentum losses. And in my case, its opposite. I am wondering if there is a specific criteria for tanks like restricting maximum outflow velocity.

If there is a mismatch between pipe and nozzle size, do we match the nozzle to pipe or other way? We are talking about large sizes like 36" and 30" so even one size change could have considerable effect on material cost.

Has anyone who has worked in tank farm area experienced this anamoly?

[ankur2061]

go-fish,

For large atmospheric storage tanks the following are the most common practices followed for the critical nozzles:

1. Inlet nozzle generally kept same as line. No issues if inlet nozzle one size smaller or larger than inlet line size. Momentum criteria not critical since less or insignificant concern related to separation of phases, liquid entrainment & mechanical vibration due to large open area of discharge inside the tank.

2. Outlet nozzle preferred to be same as pump suction line size to prevent extra head loss due to enlargement (expander). Lower size than the pump suction line generally avoided. It is important to note that expander gives greater head loss than reducer.

3. Normal Vent nozzle size equal to or greater than the bigger of the inlet or outlet nozzles. Proper sizing is required to be done based on inbreathing / outbreathing calculations for optimum nozzle size.

4. Overflow nozzle size greater than or equal to the inlet nozzle.

Hope this helps.

Regards,
Ankur.

[go-fish]

I am consistantly getting tank outlet nozzle sizes one size higher than pump suction connecting pipe and I have to use a reducer to match up. Do you recommend that I reduce the tank nozzle size to match the connecting pipe?

[Art Montemayor]

Fish:

If what you are stating is that the Tank's pump-out suction nozzle is one size bigger than the Tank's pump-out pump, than that is the normal, conventional practice employed in the field. This is done in order to ensure that the NPSH available to the pump-out pump is well within the NPSH required by the same pump. It is always normal practice to employ an eccentric reducer at the pump's suction flange (with the straight side at the top).

This has been my experience and is illustrated in just about every Unit Operations text book of centrifugal pumps connected to their source tanks.

[go-fish]

Fish:

If what you are stating is that the Tank's pump-out suction nozzle is one size bigger than the Tank's pump-out pump, than that is the normal, conventional practice employed in the field. This is done in order to ensure that the NPSH available to the pump-out pump is well within the NPSH required by the same pump. It is always normal practice to employ an eccentric reducer at the pump's suction flange (with the straight side at the top).

This has been my experience and is illustrated in just about every Unit Operations text book of centrifugal pumps connected to their source tanks.

No, I meant my tank outlet nozzle is coming out to be one size higher than the connecting piping to the pump. I already maintained 1psi/ 100 feet criteria for line sizing.

[fallah]

No, I meant my tank outlet nozzle is coming out to be one size higher than the connecting piping to the pump. I already maintained 1psi/ 100 feet criteria for line sizing.

May i know about what you will gain due to above action.

Economical issue: Bigger nozzle size

Technical issue: Probably higher inlet loss

[Art Montemayor]

fish:

You are not supposed to apply a general pressure drop allowance to pump suctions. You should determine what your SPECIFIC pump NPSH requirement is and allow a SPECIFIC pressure drop for the specific line length - not a pressure drop per 100 feet of length.

Otherwise, you could have problems.

[imran.idris]

Dear Go-fish,

What I understand is that you specifically want to know about the reason why outlet nozzle size is kept one step bigger than the pump suction line or the tank outlet line.
See my comments below:

• It is usually done in order to reduce the effect of eddy currents / vortex formation at the exit of tank and into the outlet nozzle.
• In atmospheric tanks, where available pressures are very small, by adding a bigger nozzle you actually save the cost in piping. As the sudden change in diameter can cause an unacceptable pressure drop which may result in selecting a higher outlet piping size.
• Sudden increase just acts like an orifice plate and reducer will provide the same effect as venturi provides in piping.
• So, bigger outlet nozzle with reducer is the way forward.

Regards,
Imran Idris
http://forums.thepetrostreet.com
imranidris@thepetrostreet.com

[go-fish]

Thank you for your detailed explanation. So is it a general practice to use reducers after nozzles?

[kkala]

... So is it a general practice to use reducers after nozzles?

No it is not, seeing that in my present job there is a practice to have same tank nozzle diameter as the inlet or outlet piping. Nevertheless I believe a bigger nozzle size is indicated in most cases in order to have greater future flexibility. I remember the case of a 8000 m3 refinery tank, fed with a 3" line. This tank may change use in the future, requiring a much higher filling rate. I assume it not easy to place an additional nozzle on an operating tank (have certainty on this concerning a pressure vessel).
So having a bigger nozzle available could avoid future trouble, and this is evidently true for suction lines too. If mentioned tank was connected to (say) 16" line, I would not think of bigger nozzle unless specifically instructed.
Generally it is a good idea to ask for Owner's policy in such case, or think of the size potentially needed in the future.

Edited by kkala, 24 April 2010 - 06:15 AM.