Introduction

At present a number of large consumers are using regular 15kg LPG cylinders. The problem faced by the consumers is that the last 1 to 3 kg of gas in the cylinders does not come out. With the result they have to return cylinder with this quantity of gas, for which they have already paid. To overcome or reduce the problem many solutions like hot water circulation, steam injection etc. in the cylinder room have been tried out and the magnitude of the problem has been reduced, but not completely eliminated. The solution lies in replacing these cylinders, with bullets which will not only ensure proper flow of gas but will also eliminate the expenditure required for hot water circulation or steam injection, etc.

Principle of Evaporation

To understand the problem let us first understand the priciple of evaporation of LPG. Liquefied Petroleum Gases, (LPG) as the name indicates, are the gases derived from petroleum crude or associated gases. These gases under moderate pressure are converted into liquid and filled in the cylinders. The withdrawal of gases from the cylinder lowers the container pressure. This causes the liquid to “boil”, in a effort to restore the pressure by generating vapours to replace that which has been with dran. The required “Latent heat of Vaporisation,“ is surrendered by the liquid and causes the liquid temperature to drop. This heat loss in turn is made up by heat transfer only from surrounding ambient air. As the heat transfer from air to vapour is negligible only the portion of cylinder having liquid will be able to take part in the transfer of heat. When the withdrawal rate from cylinder is faster, the fall in liquid temperature is also fast, which causes the atmospheric moisture to condense around the cylinder and the evaporation of liquid to form gas gets affected.

Higher the quantity of liquid in the cylinder higher will be the rate of evaporation for same ambient conditions. When the liquid level falls below a certain level the evaporation rate is very slow and not capable to cope-up with the higher withdrawal rates. So this quantity of liquid goes as a waste in the cylinders.

In case of a bullet, the level of the liquid is always above a minimum specified level so this problem of lower evaporation rate does not come up.

Bullet size

Bullet is a cylinder tank which is capable of holding the LPG under pressure. Normally the size of the bullet should be equal or more than the estimated monthly consumption of LPG. Due consideration has also to be given to the sizes of tank lorries carrying LPG from refinery of storage point. They are normally 3 MT, 5 MT, 7 MT, and 10 MT capacity. 5MT and 7 MT tankers are very common. For a consumer having 600 cylinders per month consumption the storage capacity should be about 10 MT (approx. one month consumption)

As per the explosive rules these storage vessels (bullet) have to be tested and certified at least once in five years. So it is suggested that at least 2 bullets each of 5 MT capacity or 3 bullets each of 4 MT capacity should be installed. The total space required for first arrangement will be about 29mx26 meters while that for second will be 17.5 mx 16.5 m. It is therefore advisable that 3 bullets of 4 MT capacity each should be installed.

The size of a 4 MT bullet be approx. 1.5m diax6.5 overall length. The bullets have to be installed parallel to each other and the distance between the bullets should be minimum 1.5 metres. All around the bullets a clears space of minimum 5 metres should be provided A 2 metre high industrial fencing has also to be provided around this space.

Construction of Bullet

The bullet should be designed to LPG Vapour pressure at 130°F i.e. about 200 Lbs psi and a test pressure of one and a half times this pressure. After fabrication the bullet should be stress relieved and all the welded joints should be of X-ray quality. They should also have the following.

Man Hole : For inspection and cleaning of bullet etc.

Liquid valve for filling : Normally a spray header or a down pipe with perforations should be used so that the pressure inside the bullet does not go up while filling the bullet.

Pressure relief valves: Two pressure relief valves should be provided to take care of any excessive pressure build up in the bullet.

Pressure gauge: A pressure gauge should be installed to monitor the pressure inside the bullet.

Vapour outlet connection: For taking the vapour from bullet for use

Liquid Level Gauge: To Know the level of liquid in the bullet, one marking should also be provided on this for minimum ullage required which is minimum 5% of the capacity of bullet.

Vapour equalizing point To be used at the time of decanting the tank lorry for connecting to tank lorry with a compressor.

Drain: To drain water after the hydraulic testing of bullet.

Vapourising Capacity of Bullet

The vaporizing capacity of a bullet largely depends on the surrounding ambient conditions and the area of bullet in contact with the liquid. The Vaporising capacity ‘V can be calculated as following:

K= Coefficient of heat transfer which can be taken as lOk.cal/where m²h/oc difference in temp.

A= Area of bullet through which heat transfer will take place i.e. area in contact of liquid which is approx. 5m² for 4 MT bullet when it is is ¼ full. (Dimension of bullets as given earlier) ta=ambient temperature.

tm= The minimum permissible liquid temperature (either the temperature at which vapur pressure is just higher than the first stage regulator pressure or the temperature at which frosting on the exterior of the container occurs, whichever is the higher value.)

L= Latent heat of vaporization for LPG which is approx. 86 k.cal/kg. Taking the ambient conditions as 5°C. (The temperature at which frost will start). The vapour pressure of gas at 5°C will be approx. 30 psi absolute. (Taking LPG mixture as 90% commercial butane +10% commercial propane).

Therefore V= 10x5x[5-(5)] 86 = 5.8 kg/hr

Evaporation from three bullets=l7.4kg/hr. Normally the ambient temperature is higher and all the bullets will not have minimum level of ¼ full, so the evaporation rate will be higher.

Pressure Regulations

The using points in the installation are likely to be at various distances from the storage point and they can also be at various height levels. It is therefore advisable to have two stage pressure regulators. The first stage pressure regulator should reduce the Pressure regulator should reduce the pressure to about 10 PSI and should be installed near the storage tanks and the second stage pressure regulator should be near usage point giving a of 11 inches water guage. The capacity of regulators will depend on the maximum load at any point of time and individual load at any point of time and individual appliances consumption figures.

Pipe Size Selection

The selection of pipe sizes etc. will mainly depend on the consumption pattern of the installation, as well as on the individual appliance consumption. However after knowing the required flow quantity, length of pipe line and allowable pressure drop the following empirical formula may be used to find inside diameter of pipe

Q = Flow (reduced to 1 atm absolute) in M³/hr at 15°C or ft³/hr at 60°F which can be taken as l.75 (50% propane 120 RVP

D = Actual inside diameter of pipe in cm. Or in inches.

H = Allowable pressure drop in cm water guage or inch water gurage.

S = Specific gravity of vapour relative to air at 1 atm. Absolute at l5°C or 60°F which can be taken as 1.75 (50% Butane = 50% Propane 120 RVP)

L = Length of pipe in metres or feet. Formula and Units to be chosen depending on Metric or British system.

After finding out this diameter equivalent length of pipe should be found for various fittings and this should be added to the straight run length. The flow should again be cross checked with this new equivalent length.

Safety and Protection

As LPG is a highly inflammable product, adequate safety and fire fighting facilities to be provided. Following points may be kept in view while putting up the installation:

Storage of LPG for large consumers