7 replies to this topic

[Raj.R]

Dear Sir,

Please clarify me that how the Storage tank Inlet Nozzle Size determined? Is there any standard for calculating the size? What else to be taken into consideration Velocity/Momentum? What is the use of spare inlet nozzle?

[fallah]

deepakrraj,

The inlet line of the storage tanks is usually sized based on maximum fluid velocity of 3m/s. The inlet nozzle size could be equal or greater than the inlet line size.

Fallah

[kkala]

Probably mentioned 3 m/s is 3 ft/s (typo)? I have seen a local instruction on drawoff nozzles ( what they are is explained in http://beta.globalspec.com/reference/81435/203279/chapter-14-draw-off-nozzle-hydraulics-nozzle-cavitation-due-to-lack-of-hydrostatic-head) for refineries, saying that in general velocity shall be 3 ft/s, or lower.
Concerning local refinery practice, we size ingoing / outgoing tank nozzles same as the relevant pipe lines, despite above instruction; I do not know whether this is quite good practice. Nevertheless velocity is (locally) limited to 1 m/s or less, in case of flammable liquid that could contact air (e.g. during LLLL status).
See also http://www.cheresources.com/invision/topic/9843-vessel-nozzle-sizing.

Edited by kkala, 10 October 2012 - 06:14 AM.

[Raj.R]

Dear Sirs,

Thank you for your clarification. Is there is any specific standard avaliable for this? In our case the inlet velocity is 7 m/s, is it allowed to provide the another inlet through spare nozzle (i.e 2 inlet nozzles to the tank) ? Is there any limitations for floating & fixed roof storage tank?

[kushal.raval]

Dear kkala Sir,
I believe 3ft/s velocity is mandatory for first filling of tank to avoid flashing and/or roof imbalance in case of floating roof tank. For first start up this may be achieved by throttling of upstream valves. However, for regular operation about 2-3 m/s velocity is optimized design for nozzle sizing. Regarding 2 inlet nozzles, there should not be any technical problem for this.

[Raj.R]

Dear Mr. Kushal.raval,

Thank you for your clarification.

[kkala]

Dear kushal.raval, your post gives explanations on mentioned 3 ft/s and reason of not generally applying this limitation on nozzle sizing. Specifically for drawoff nozzles (post No 3), they can be directed to a pump / drain, so lower velocities can be justified for their sizing. Yes Indeed, this is not valid for all nozzles. Static electricity has to be also taken into account for flammable liquids, though it may not increase nozzle size (see below).
Concerning flammable liquids, it is pointed out that velocity into the tank has to be below 1 m/s (close to 3 ft/s) to avoid risk of spark (*) due to static electricity and subsequent explosion or fire, if the moving liquid can contact atmospheric O2. This does not necessarily mean nozzle sizing to 1 m/s, a "diffuser" can be connected downstream the tank nozzle in order to reduce liquid outgoing velocity.
We locally apply this always if LLLL is not clearly (a few cm) above the upper part of the nozzle, in case of cone roof tanks.
If nozzle is clearly below LLLL (and nozzle velocity > 1 m/s), special precautions have to be taken during complete tank emptying (e.g throttling outlet liquid, as in post No 4, etc) and later during tank filling. Floating roof tanks keep the roof (through legs) at about 0.8-2.0 m height from bottom during complete emptying, so during it (and later filling) these are like cone roof tanks.

(*) static electricity measures are not always limited to 1 m/s, see http://www.cheresources.com/invision/topic/13488-top-loading-arm-size-and-static-electricity-generation/ and related forum threads.
Syntax correction from "yes" to "indeed" on 20 Oct 2012.

Edited by kkala, 20 October 2012 - 11:58 AM.

[kushal.raval]

thanks kkala sir for this useful info.