Table of Contents
1 What is LPG?
Before jumping into the specific about LPG storage, the article will discuss the basic about LPG. LPG is a gas obtained from the refining of crude oil and natural gas processing. LPG is mainly used for cooking, heating and auto fuels. LPG or LP gas is an abbreviation which refers to liquefied petroleum gas. The term LPG is most commonly used to refer to the common grouping/mixture of two of the “light hydrocarbon” family, Propane (C3H8) and Butane (C4H10). Both of these hydrocarbons are gasses under normal atmospheric conditions (1 atm. pressure and ±15 °C). LPG furthermore actually refers to hydrocarbons that can be liquefied under moderate pressure at normal temperatures and include the aforementioned gasses as well as propylene, butylene and isobutane (Green, 2008). The liquefaction process of these gasses generally involves the compression of the gaseous state mixture to condense the gases (Hahn, 2018).
The gaseous, natural state of LPG takes up 250 times the volume of the liquefied product. This allows for efficient and economical transport of LPG via various types of transport methods including:
- Railways;
- Gas cylinders or
- Barges and ships.
This, in turn, allows for LPG to easily reach rural areas with inadequate electrical infrastructure (Ezzel, 2016).
1.1 LPG Basic Properties
LPG as previously mentioned consists mainly of mixtures of propane and butane. The following table indicates the chemical properties of propane, butane and a common mixture of 60% propane and 40% butane.
Table 1: Properties of Propane, Butane and a 60:40 Mixture (AFROX)
| Property | Propane | Butane | 60:40 Mixture |
| Molecular weight | 44.09 | 58.12 | 49.7 (Avg.) |
| Carbon content (wt%) | 81.72 | 82.66 | 82.15 |
| Hydrogen content (wt%) | 18.28 | 17.34 | 17.85 |
| Carbon: hydrogen ratio by weight | 4.47 | 4.77 | 4.6 |
| The density of the liquid at 15 °C (kg/l) | 0.510 | 0.575 | 0.536 |
| The boiling point of the liquid at atm. Pres. (°C) | -42.1 | -0.5 | -42.1/-0.5 |
| The density of gas at 15 °C and atm. Pres. (kg/m3) | 1.86 | 2.46 | 2.10 |
| The volume ratio of gas: liquid at STP* | 274:1 | 233:1 | 258:1 |
| The volume of gas from 1 kg of liquid at STP (l) | 537 | 405 | 484 |
| The mass ratio of gas: air at 15 °C & atm. Pres. | 1.52:1 | 2.01:1 | 1.716:1 |
| Latent heat of vapourisation at 15 °C (kJ/kg) | 20.43 | 21.27 | 20.77 |
| Vapour pressure at 20 °C (kPa abs.) | 710 | 110 | 500 |
| Sp. The heat of vapour at atm. Pres. (cal/g⋅°C) | 0.388 | 0.397 | 0.392 |
| Net calorific value at 25 °C (MJ/kg) | 46.0 | 45.6 | 45.8 |
| Gross calorific value at 25 °C (MJ/kg) | 49.8 | 49.4 | 49.6 |
| Wobbe number (kcal/Nm3) | 19 000 | 21 600 | – |
| Limits of flammability in the air (vol% gas) | 2.2 – 10 | 1.8 – 9 | 1.8 – 10 |
| Limits of flammability in oxygen (vol% gas) | 2 – 50 | 2 – 50 | 2 – 50 |
| Max. the flame temperature in air (°C) | 1 930 | 1 900 | 1 900 |
| Max. the flame temperature in oxygen (°C) | 2 740 | 2 700 | 2 700 |
| Max. flame speed in 25 mm tube (cm/sec) | 82 | 82 | 82 |
| The air required for combustion at STP (m3/kg LPG) | 12.1 | 11.93 | 12.03 |
| Air: gas vol. the ratio for combustion at STP | 22.5 | 29.5 | 24.9 |
| O2 vol. for combustion at STP (m3/kg fuel) | 2.56 | 2.51 | 2.54 |
*STP refers to Standard Temperature & Pressure (0 °C and 100 kPa absolute)
2 LPG Storage: Key influences
The construction of a bulk storage facility for LPG has a few key influences which need to be considered to safely and effectively store LPG in a bulk installation.
2.1 Location
The location of the facility can in some cases greatly influence the properties of the LPG being stored due to the ambient conditions on-site. The vapour pressure of LPG varies according to the temperature of the surroundings and in areas of higher ambient temperatures, a higher pressure will be required to liquefy the LPG. A graphical presentation of the changes in vapour pressure for propane due to temperature is indicated in Figure 1. The graph is derived from the available vapour pressure data (Green, 2008).
Figure 1: Vapour Pressure of Propane at various Temperatures
As can be seen in the figure above the difference in vapour pressure between 300K (26.85°C) and 315K (41.85°C) is 10 and 14.3 bar. Close consideration should therefore be taken regarding ambient temperatures as this can be hazardous with regards to chosen design pressures of the vessels. In other words, higher ambient temperatures will require higher design pressures to ensure a safe design.
2.2 Logistics
Logistics regarding site access and transportability of materials can be another hurdle that has to be overcome when designing bulk facilities. Larger vessels might be challenging to be transported as pre-fabricated; since the road, rail or port facilities might not be able to accommodate the large sizes.
These vessels will have to be transported in either pre-fabricated section or as material sheets that will have to be assembled or bent on site. Both options, however, require on-site construction and therefore certain allowances with regards to plot space allocation will have to be considered for construction of the vessels on site.
Horton spheres are usually constructed on-site from the ground up due to their complex design, once they reach a certain size, whilst bullets can be transported in units or sections and installed or assembled on site.
2.3 Constructability and practicalities
In conjunction with the aforementioned logistical constraints, should the decision be made for on-site construction of the vessels whether it be in sections or from the ground up, constructability should be taken into account.
For on-site construction adequate room for the necessary construction, facilities should be allowed for and considered before the decision is made to complete on-site construction. Smaller vessels can be used for example; like in the case of multiple smaller bullets, but this will, in turn, lead to much more piping and connection materials, whilst the total storage allowable will also decrease to more sections with additional safety distances to adhere to.
2.3.1 Safety distances
When constructing an LPG facility strong emphasis should be placed on designing the facility according to the correct and applicable codes and standards in the area/country of construction. These standards will provide an engineer with the correct information and guidance to correctly and more importantly – safely design an LPG bulk storage facility.
One of the key concerns when designing an LPG bulk storage facility is the correct implementation of safety distances. These govern the required safety distances between storage (pressurized) vessels with other storage vessels, whether it is another LPG storage vessel or another chemical storage vessel. These safety distances also govern the allowable minimum distance between an LPG pressure vessel and the plot borders, roads etc.
The safety distances limit the overall storage that can be stored safely at a certain facility. The use of an internationally accepted standard is therefore very important during the feasibility stages and estimations of the possible storage on site. In South Africa codes like the SANS 10087, and NFPA 58 codes are adequate and proper sound engineering judgement should be practised when implementing the codes.
3 LPG Storage: Gas Composition
When designing the facility, proper investigations should be made regarding the typical composition/ratio of propane to butane should be done to establish the correct design solution for the facility. Table 1 indicates the large difference between the vapour pressures of propane and butane. Should the actual LPG being stored contain more propane than anticipated during the design stage, the vessels could fail due to insufficient design pressures and cause major safety hazards, whilst overdesign could increase the CAPEX of the facility when the propane content of the LPG gas stored is much lower than designed for.
During operation, it shall, however, be the responsibility of the operators to ensure that the incoming LPG conforms to the propane/butane ratio according to which the facility has been designed.
4 LPG Storage solutions
4.1 LPG Spheres
The first spherical LPG storage vessels (‘Hortonspheres’) were constructed in 1923 by Chicago Bridge & Iron Company (CBI) and allow for effective large volume storage of LPG. (Ezzel, 2016)
The largest benefit with regards to the sphere is their ability to store very large amounts of LPG in proportionally small areas. This ability arises because an LPG sphere has a very large volume to surface area ratio. Furthermore, the required wall thickness of an LPG sphere of the same diameter as that of a bullet is much less. LPG bullets can, however, reach extreme lengths (up to 70m), allowing for higher storage volumes.
Large spheres, however, have a large concentrated load (point load) on a small section of earth, leading to higher groundwork design constraints. Spheres can also not be moved once constructed as they lose integrity during deconstruction them (due to a high amount of welding points) compared to a bullet that can be split into fewer sections. (BNH Gas Tanks, n.d.).
Spheres’ complex designing procedures usually also extend the construction period for the spheres but in turn, spheres allow for less piping and connections when compared to multiple bullets. (BNH Gas Tanks, n.d.).
4.2 LPG Bullets
Another form of storage (and probably the most common) is the LPG bullet.
4.3 Above-ground
Aboveground storage in bullet-form is similar to that of spheres. Bullets, however, are usually installed pre-fabricated in smaller units with cases of large (50+m) in length have been done in various locations across the world with some reaching lengths of 70m as in the case of Sunrise Energy LPG Import Terminal (Engineering News, 2014).
Bullets have the advantage of being able to be moved in sections as well as having a more uniformly distributed load across the ground surface due to multiple supports and often multiple (smaller) bullets. Settling occurs less readily and bullets are therefore a safer option in locations with more severe weather conditions (Ezzel, 2016) or challenging geotechnical areas. Bullets can also be transported should it be necessary via minimal sections after vessel deconstruction leading to minimal welded seams and possible weak points after reconstruction.
Regular maintenance can also much more readily be completed in the case of bullets compared to a large sphere, with the possibility of some of the bullets staying in operation during scheduled maintenance, whilst the use of a large sphere will shut down all operations until maintenance is complete (BNH Gas Tanks, n.d.).
4.4 Mounded/Buried
Mounded bullets are bullets that are buried beneath a mound, typically trapezoidal, consisting of sand/ground and a cover of a binding material like asphalt. The mounding is usually also fitted with walls at two opposing ends with some cases being surrounded with a wall. Mounded or buried bullets are a trade-off between available storage and the necessity to allocated extra civil and mechanical work to accommodate the actual mounding. The use of mounded bullets allows for narrower spacing between adjacent LPG bullets as well as less strict safety distances with regards to other properties in close proximity.
This is turn allows for more available plot space for the actual storage of LPG. When constructing mounded bullets, it is, however, important to consider the gravitational force that the mound material exerts on the shell of the bullet and also on the supports for the vessel. Connections on underground tanks should be located in positions that are easily accessible to operate and maintain.
5 LPG Storage: Conclusion
Plot space constraints (due to safety distances), ambient conditions, site access, tie-in points as well as client requirements are important considerations before commencing the design. The available plot space will determine the maximum storage volume of LPG, due to the safety distances and applicable standards as mentioned earlier.
Further considerations will include the other decisions mentioned earlier in this article. These will include practicalities with regards to construction as well as typical incoming gas composition and location.
Once all of the aforementioned and other project-specific decisions have been made and sound engineering practice and judgement has been exercised by experienced professionals; the decision can be made on the type of storage vessel and the amount of LPG that the facility can accommodate. Each design will therefore be unique, based on specific site requirements and not be based on previous projects and/or designs.
6 References
Ezzel, G. (2016, November 14). LPG Storage Bullet Tanks vs. LPG Storage Spheres / ‘Hortonspheres’. Retrieved from TRANSTECH ENERGY: https://www.transtechenergy.com/lpg-ngl-storage-news/lpg-storage-bullet-tanks-vs.-lpg-storage-spheres-/-hortonspheres
Hahn, E. (2018, July 05). What is LPG? Liquefied Petroleum Gas – Propane. Retrieved from ELGAS: https://www.elgas.com.au/blog/492-what-is-lpg-lpg-gas-lp-gas
AFROX. (n.d.). Liquefied Petroleum Gas. Retrieved from AWS Group: http://www.awsgroup.co.za/data/L.P.G.pdf
Green, D. &. (2008). Perry’s Chemical Engineer’s Handbook, 8th Edition. Kansas: McGraw Hill.
BNH Gas Tanks. (n.d.). LPG Horton Spheres. Retrieved from BNH Gas Tanks: file:///Volumes/GoogleDrive/My%20Drive/EPCM%20-%20Gerhardo/Articles/LPG%20Comparison%20Research/LPG%20Horton%20Sphere.webarchive
Ezzell, G. (2016, November 14). NGL & LPG Storage Infrastructure News. Retrieved from Transtech Energy: https://www.transtechenergy.com/lpg-ngl-storage-news/lpg-storage-bullet-tanks-vs.-lpg-storage-spheres-/-hortonspheres#
