1. Nomograph

The nomograph is illustrated in the first two figures of this report entitled Nomograph 1 and Nomograph 2. It has been devised to establish the protection requirements for storage tanks to prevent excess boil-off of flammable vapour which could lead to ignition from a fire on an adjacent tank.

Both versions of the nomograph define the cooling water requirements as a function of the tank size, the boiling point of the stock contained in the tank and the separation distance between the tank being protected and the tank on fire. Nomograph 1 contains the version for un-insulated tanks and Nomograph 2 the version for insulated tanks.

This section of the report is divided into three parts.The first part describes how the nomograph is used, the second part lists the limitations of the nomograph and the third part outlines the basis on which the nomograph has been developed.

1.1 Use of Nomograph.

Both nomograph versions are used in the same way.

The boiling point of the stock and the size and shape of the tank are used to establish a Pivot point within the area contained by the two main vertical axes representing the spacing distance and the cooling water rate. A straight line through the pivot point then connects corresponding points on these two main axes.

1. The stock boiling point line (red line) is identified.
    
2. A blue line associated with the relevant tank height is chosen.
    
3. The appropriate tank capacity is identified (a horizontal green line) and this is used in conjunction with the blue line identified in (b) above to give a vertical green line.
    
4. The intersection of this vertical green line and the red boiling point line determines the pivot point.

The spacing distance between tanks is calculated as a fraction of the tank diameter and the point representing this value on the left-hand axis is joined to the pivot point and then projected to the right-hand axis, where the necessary cooling water rate is read off the scale.

Conversely, if the water rate is known the method can be used in reverse to determine the minimum spacing distance to provide the necessary protection.

The results calculated by this approach are based on the cooling requirements of the tank which is exposed to radiant heat from the fire. However, there is another consideration which defines a minimum water rate and this is governed by the temperature which the cooling water might reach to provide the necessary protection. If the water rate is very low, this temperature could exceed the
boiling point of the stock or the boiling point of water. To ensure that this limitation is not violated guidelines are given below for both versions of the nomograph on the minimum water rates which can be specified by this method.

MINIMUM COOLING WATER RATES - UN-INSULATED TANKS

MINIMUM COOLING WATER RATES - INSULATED TANKS

(To be used with NOMOGRAPH 2)
This nomograph should not be used for values of the cooling water rate less than 2.5 x 10^-3.

1.2 Limitations of the Nomograph.

1. Both nomographs have been produced using identical techniques.The first version is designed for tanks having no insulation and the second for tanks having thermal protection by way of insulation corresponding to a thermal resistance of 259 m2K /kw.
    
2. Only tank sizes up to and including those with a capacity of 300,000 Bbls are included in these two versions of the nomograph.
    
3. Tank aspect ratios (the ratio of tank diameter to tank height) are limited to values less than 3.
    
4. Only two tank heights of 15m and 20m are considered.
    
5. The two adjoining tanks must be of the same size and height.
    
6. It is assumed that cooling water is applied to only those vertical sides of the tank which receive radiative heat from the tank on fire.
    
7. It is assumed that the only ignition source is the tank fire itself and hence that the vapour diluted with air to a composition lying within the flammability limits must drift from the protected tank into the fire for an ignition to occur.

1.3 The basis for the nomograph.

The nomograph is based on a theoretical calculation which contains no safety factors.The procedure is to select the worst case conditions for the ignition of a plume of vapor produced from the tank as the result of radiation from a neighbouring tank which is on fire.The conditions from which the selections are made are:

1. Atmospheric stability categories
    
2. Wind direction relative to the centre line common to both tanks
    
3. Wind speed.

For each combination of these conditions, the calculation is performed using a range of values for the variables which are accounted for in the nomograph, viz:

1. Tank height
    
2. Tank capacity
    
3. Boiling point of the stock
    
4. Spacing distance between the tanks.

The procedure for the calculation is described in detail in the next section but a brief account is provided below as an overview.

For each tank spacing, radiation incident on the tank from the flame on the adjacent tank is determined using calculated values of the flame shape, orientation and temperature and by employing
rigorous integration procedures to evaluate the proportion of the radiation leaving the flame which is absorbed by the tank. Flame emissivities and tank absorptivities are assigned constant values. The heat requirement to produce a vapour plume from the tank which just intersects with the flame at its lower explosive limit (lel) boundary is then evaluated using the appropriate Pasquil dispersion model for a neutrally buoyant plume from a finite sized source. Finally, the cooling water rate needed to ensure that the radiant energy absorbed by the tank and its contents does not support these heat requirements is obtained using the usual heat transfer equations.

The relationship between the tank spacing and the cooling water rate so obtained is then curve fitted to an exponential equation which yields two constants.These constants are used to calculate the pivot point on the nomograph.The whole procedure is repeated for each tank size (height and capacity) and each stock boiling point for the two tank cases of no insulation and thermal insulation to some fixed resistance.

Other variables were tested for sensitivity and it has been shown
that:

1. The effect of tank height can be expressed in linear form over the entire range of variables used
    
2. The effect of let value is of second order and hence for these calculations the stock boiling point is a more sensitive variable than its flash point
    
3. Liquid level in the tank need not be considered as a separate variable since the worst case corresponds to a full tank.

All the calculations are incorporated in the overall procedure without the use of any additional factors. However, a number of elements of the procedure do imply a certain margin of safety.

1. 'Worst case' combinations are always selected in the calculation
    
2. Where a choice of calculation procedure is available and there is no sound technical basis for selecting one of them, the choice is then made on 'worst case' considerations.

Continue to section 2, Theoretical Basis >>