IS MOUNDED BULLET FOR STORING LPG IS BETTER THAN USING SPHERE? AND WHY?
The choice between a bullet and a sphere is a matter of economics and project logistics. So one is better than the other only in the matter of which has a cheaper overall installed cost.
The sphere of course uses less material than a bullet for the same amount of liquid storage but is somewhat more difficult to fabricate. The diameter for shipping of a storage tank is in the order of 10 to 14 feet depending on the method of transport. This would limit a sphere's storage capacity if it is shop fabricated. For this reason, you usually only see spheres used for very large volumes (maybe in the order of 300,000 gallons or more) because they must be field fabricated to compete with the bullet in cost.
For smaller storage tanks, the bullets are preferred because they can be shop fabricated and more easily transported.
COMPARISION OF LPG SPHERES WITH LPG STORAGE TANKS:
1) Least initial investment and financial risk in case of LPG multiple storage tanks of as compared to sphere of similar or larger volume.
2) LPG sphere of same volume needs more time for construction as compared to LPG multiple storage tanks.
3) LPG Multiple LPG storage tanks makes the project faster, economical, easier and will in turn help the company to start the terminal as soon as possible which in turn will generate good immediate returns.
4) LPG Storage tanks are much safer since it weight is distributed evenly on a larger area thus reducing chances of collapse as compared to sphere, especially in small islands prone to earth quakes, cyclones and Tsunami as can be verified from the recent incident in Japan where a row of LPG sphere went under severe fire condition.
5) Multiple LPG storage tanks have an added advantage that the load is distributed over a larger area thus the problem of uneven settlement / collapse is largely reduced.
6) In case of installations with Multiple LPG storage tanks if there is any problem / maintenance / repair / breakdown of any one tank the outlet and inlet valves of the same can be closed and the tank can be isolated, also in case of emergency the LPG product in the tank under repair / maintenance can be easily transferred to another tank located in the same plant thus increasing safety of the plant. Whereas in case of sphere if there is any problem with the sphere or its valves or pipeline the entire plant comes to a stand still (This is a loss to the company in terms of business loss, product loss, as well as extra demur-rage that the company may have to pay to the shipping company if unfortunately the LPG ship also arrives the same time when the sphere has generated a possible breakdown). But in case of multiple storage tanks even if one of the tank generate a possible breakdown and the LPG ship also arrives the same time still the LPG from the ship can be unloaded to the remaining tanks and the company will have to incur least loss due to ship demur-rage.
7) Though there is a common notion that for the same volume a sphere has the least surface area and least thickness and hence lesser weight and less cost but our past experience suggests that weight saved in making a sphere as compared to storage tank is lost as wastage in the development of various sections (petals and crown) thus leaving little or no benefit of the weight saved.
8) The fabrication cost increases considerably in case of sphere as compared to multiple LPG storage tanks , since the sections have to be pre fabricated at shop, shipped to site, adjusted, assembled and welded at site piece by piece in proper sequence, heat treated, radiographed and tested at site. Fabrication of sphere requires lot of time since the petals / sections are required to be assembled piece by piece in proper sequence and completion of one stage is entirely dependent on completion of the earlier stage as so on and so forth. More over there is a continous requirement of close monitoring, blockage of all construction equipments and manpower till the entire completion of work sequence by sequence, this makes the fabrication of sphere still more costlier and time consuming as regards to blockage of manpower, machinery rent etc.
9) Sphere once fabricated cannot be moved or relocated from one place to another and if in case any relocation (due to sale of plant or plant relocation) is required the sphere has to be cut into several small pieces and transported to site and then reassembled and re-fabricated at new site this is as good as manufacturing a brand new sphere at site and hence the after sale value of a sphere is much less and as compared to LPG storage tanks which can be sold out for good returns.
