7 replies to this topic

[raviteja goriparthi]

hiiiiiiii all

I want to calculate the boil off from an LPG double wall storage tank. The condition are

Import of LPG from Jetty:

One tank at a time shall be receiving cryogenic LPG from the cryogenic ship tanker.
LPG import flow rate from Jetty: 3,000 m3/hr.
LPG composition: C3 - 40%; C4 - 60%
Operating temperature: (Estimated) - 32 deg C @ atm pressure
Pressure at Ship pump: 10 - 15 kg/cm²g (assumed)
Density of LPG: 500 – 580 kg/m³ @ 15 deg C
LPG import line size from Jetty: 20”. Import pipeline shall be above ground, insulated line and total pipe length is 3.0 km.

LPG Storage
LPG storage tank having capacity of 50,000 m³ shall be provided. Tanks shall be double containment type, atmospheric with suspended roof. A steel-in-steel design is envisaged. The outer tank dimensions are 57 m diameter and 25 m height. There will be vacuum with perlite insulation in the annulus. The tanks are to be provide with boil off recovery system with compressor and condenser.And the insulation thickness is 1,000 mm . The inner tank dimensions are 55m dia and 24 m height.

Awating your valuable suggestions.

Thank you all.
g.raviteja

[kkala]

Below are some notes for want of something better, since no response has been received. It is supposed that a rather realistic BOG rate has to be assumed for the preliminary design to proceed, while precise value shall be determined (under the conditions specified) during detailed engineering in communication with vendor.
A. Let us first limit the matter to the LPG refrigerated storage. Below are references probably useful:
1.http://www.hydrocarb...facilities.html
2. http://www.eng-tips.....cfm?qid=263281 (reported figure for LPG BOG rate: 0.1%).
3. http://www.tokyo-gas.../j_txt/42e.html (information only)
4. http://webwormcpt.blogspot.gr/2010/08/how-boil-off-gas-bog-is-generated.html (showing complexity of BOG rate estimate).
5. Attached "BOG-LNG.doc", explaining design policy for refrigerated tanks (even though referring to LNG).
B. Reference 2 indicates max BOG rate 0.05% for LNG and 0.1% for LPG refrigerated atmospheric tanks, percentages assumed to be w/w per day, based on quite full tanks. So BOG ~ 50000*500*0.1%=25000 kg/day (approx ρ=500 kg/m3, it may be higher).
Nevertheless only one reference has been found for that 0.1 % w/w concerning LPG storage (temp=-32 oC, versus -162 oC for LNG). Furter investigation is needed, advice would be welcomed.
Γ. Concerning the ship unloading line of 3 km, additional boil off during LPG transfer may correspond to heat load (per unit mass) of approx 15/500 bar*m3/kg = 3 kJ/kg, plus heat transferred from ambient air through the (insulated) line. The 3 kJ/kg is energy ΔP/ρ supplied by the pump and transferred to heat by friction (roughly, ΔP=15 bar, ~1250 kW in total, static elevation neglected), while LPG temperature / pressure are same in ship and storage. However comments on this inexperienced opinion (or a rule of thumb as in Β) are again welcomed, since actual situation looks more complex. See attached "P4-20_Yacine Zellouf-1.pdf", even if referring to offshore LNG tanks.

Attached Files

•  BOG-LNG.doc   41.95KB   1723 downloads
•  P4-20_Yacine ZELLOUF-1.pdf   1.06MB   5097 downloads

Edited by kkala, 12 October 2012 - 07:52 AM.

[Art Montemayor]

g.raviteja:

Is this a student assignment problem? This does not seem like a real-life, industrial problem:

• At -32 ^oC you are far removed and much warmer than the cryogenic zone (-100 ^oC). Your boil-off may be analogous to an LNG tank, but the rates will be different because of the different driving force;
• A transfer pump pressure of 10 kg/cm²g seems much too low for a 3-km long pipeline. For a project of this size no one is going to waste time on calculations based on an "assumed" pressure drop. Please submit your calculations to arrive at a 20" OD pipeline. Surely you must have done some work on this.
• What is your basis and reason for doing this calculation? For a project of this magnitude, an in-experienced engineer would not be assigned to carry out the calculations.

Await your reply and calculations.

[kkala]

I think, Art, the technical details in post No 1 do not indicate a student project. Per my guess (post No 2) it may be a preliminary basic design, to be elaborated e.g. for rough budget estimate. This should have been clarified in the beginning.
LNG and LPG storage under (almost) atmospheric pressure seem to have similar methodology to estimate BOG rate (ref 1). I was surprised at reported (max daily) 0.1% for LPG, versus 0.05% for LNG and 0.04% for NH3 (ref 2), which probably means inferior isulation for LPG storage, or some other reason.
Estimation precise enough of LPG BOG rate seems to me almost impossible now (under assumed design) . Detailed Engineering Contractor (or Supplier of turn key package) would specify it later (ref 5), if the project is realized. At present a rather realistic BOG rate is needed (based on data of similar projects) to proceed in compressor specification (e.g. http://www.ihi.co.jp/compressor/en/products/process-gas/gas-recipro/lng_bog/index.html, for LNG).
Above concerns LPG atmospheric storage. Ship unloading line contribution to BOG rate is another cup of tea, requiring methodology from an expert (both BOG rates have to be considered).
raviterja goriparthi will hopefully clarify above assumptions. If the matter has been settled already (post 1 dated 11 Sep 12), he is kindly requested to advise applicable BOG rate, especially for LPG atmospheric storage alone. It is hard to find it in common litterature.
References in parenthesis (e.g. ref 2) concern post No 2 by kkala.

Edited by kkala, 13 October 2012 - 05:52 AM.

[Robert Montoya]

Dears the EN 1473 described which flows are described that you should consider. Annex an extract of standard, to calculate it should use the API 2000 and process data.

Attached Files

•  EN 1473_07.pdf   32.05KB   2369 downloads

[Anjaney Shukla]

Does anybody has the deign procedure for cryogenic/ LNG tank

[raviteja goriparthi]

First of all thank u very much for u r feedbacks and i'm " very sorry" for the too late replay...

[kkala]

Have you come to any conclusion about the BOG rate in the mentioned LPG storage tank? It is not easy to find such information in literature (post no 4 by kkala).