TECHNICAL CORNER

Mr. Beer will be remembered for his outstanding service to the Association and its predecessor. His Authority was an original member of the West Midland Group and he was Chairman of the Group in 1960-1961, remaining on the Council of the Association until reorganisation.

In addition to his activities on behalf. of this Association, Mr. Beer served on the Council of the Institute of Trading Standards Administration since 1961, becoming Chairman of this Institute in 1974. Award

B.W.C. Thacker, Past President of the Association, has been awarded the Queen's Fire Service Medal. We offer him our heartiest congratulations

Items for Publication

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Presented by PHOENIX

What's the Hold-up? When the tail pipe of a tank wagon compartment is first connected to the filling pipe of an underground storage tank, there is a considerable amount of air trapped in the connecting hose as well as in the vehicle tail pipe and the tank filler pipe. Air interposed between liquid surfaces in this way tends to produce the classic condition known to all plumbers as "air lock". The weight of air in the pipe is negligible and so the effective "head" of liquid to establish flow depends largely only on the depth of liquid in the carrying tank. -Not infrequently this is insufficient to overcome the head of liquid above the bottom of the fill pipe in the storage tank. In such cases, equilibrium is established and no flow takes place.

Before flow can start, additonal pressure is necessary. The simplest way of getting this is to replace the trapped air in the tank wagon tail pipe and hose by liquid, ihereby achieving a sufficiently greater head of liquid. So far as a plumber is concerned, the classic case he encounters is of an air lock in a pipe connecting to a bathroom tap, fed from the water tank in the roof. To overc.ome the problem, he has available mains water pressure. US1·ng a hose ' he will connect the mains water tap in the kitchen to the affected bathroom tap and open them both. Th en . ma1ns ater w pressure will push water and trapped air out of the pipe and up into the tank in the roof and, hey presto, the household system is working again!

The tank wagon driver has no such simple cure open to him, but with the ingenuity one has grown accustomed to expect, he has devised another remedy. He leaves the hose connection to the tank fill pipe loose, in theory only until sufficient air has escaped and been replaced by liquid to give the additional head he requires to establish flow. In practice, however, it not infrequently occurs that the signal for tightening the connection is the appearance of liquid running down the outside of the fill pipe.

There are tank and pump fitters who have been known to anticipate the tank wagon driver's need. They drill a onesixteenth inch hole in the wall of the fill pipe just below its point of penetration through the tank skin. Without anyone being the wiser, air can then be displaced without hindrance into the tank vapour space and never again will the words "air lock" pass a tank wagon driver's lips as he tightens his connections once and for all.

The question which petroleum officers must answer is whether they should object to the one-sixteenth inch hole in the wall of the fill pipe. It is apparent that this hole will connect the tank vapour space to atmosphere at low level, when the filling pipe cap is removed. It is unlikely that the one-sixteenth inch hole would prevent flame penetration to the tank vapour space under the right conditions. However, four 1/32 inch holes or, at most, sixteen 1/64 inch hOles would probably be satisfactory on all counts provided that the holes were not more than 23 inches from the fill pipe cap. This

would meet the case of most directly filled tanks but would still leave a problem when a tank is filled through an off-set filling pipe.

When off-sets are provided the only truly safe way of preventing air locks appears to be to fit the off-sets with vent pipes separate from but treated in the same way as the vent pipes of the storage tanks. It could well be argued that this will add unreasonably to the cost of installation. If this is so, the more fruitful approach might be to consider the provision of a suitable permanent device to remove air on tank wagon hose connections.

rlho's Spitting?

. When a tank wagon fills a storage tank 1t sometimes happens that people standing beneath the tank vent pipe are showered with small droplets of petrol. An event like this not only generates a heated response but frequently sparks off long discussion as to the cause. Ignoring the theories, two facts usually emerge, firstly, that the tank concerned has a long horizontal run of vent pipe and secondly that the spitting starts as the tank nears its full condition.

Before we attempt to theorize on our own account, we should carry out a simple experiment - using water because it is safer. The first step is to fill a tube some 780 mm. long with mercury. A suitable bath is then half-filled with mercury and the mercury filled tube inverted in it so that the open end of the tube is submerged. As this is done mercury flows out of the tube until the height of the mercury column in the tube corresponds wi th barometric pressure. If barometric pressure is 760 mm Hg then the column of mercury in the tube extends 760 mm. above the level of mercury in the bath. In the tube over the mercury a vacuum is formed; this is often called the Torricellian vacuum.

If a small droplet of water is now introduced by means of a pipette into the bottom of the tube, the droplet will float to the top of the mercury column. Here it will disappear as it immediately "flashes off" to fill the Torricellian vacuum with water vapOur. The process can then be repeated finally the droplet of water does not d1sappear but forms a thin film of water on top of the mercury. The Torricellian vacuum has now become fully saturated with water vapour and water can no longer evaporate It '1 W1 1 be noted that at this time the . mercury column has been pushed down the tube. (the temperature being 20 C.) so that the height of the column is only 742.5 mm. The difference between this figure and true atmospheric pressure, as noted known as the vapour pressure. traction shows that the vapour water at 20 C. is 17.5 mm.Hg. above·, is Simple subpressure of

If atmospheric pressure falls below 760 mm. the size of the Torricellian vacuum expands and in consequence the space is no longer fully saturated with water vapour. The immediate effect is that the thin film of water left on top of the mercury column "flashes off". When atmospheric pressure rises the volume of the Torricellian vacuum is reduced and water is wrung out of it. This water condenses on top of the mercury column.

It is now possible to relate the experience gained from the experiment to the behaviour of petrol vapour in a tank. If the atmosphere in a tank is fully saturated with petrol vapour then any increase of pressure will canse some of the vapour to condense.

Because petrol vapour is heavier than air and takes a long time to diffuse the usual condition inside a petrol tank is that fully saturated atmosphere exists only in a comparatively thin layer immediately above the liquid. When a tank is filled, the atmosphere at the top of the tank is first displaced.

Thus the fully saturated layer is pushed into the vent pipe only when the tank starts to become full. The additional pressure to which this layer is subjected in the vent pipe causes petrol to condense. The of petrol are entrained in the fast flow1ng vapour/air stream and are ejected finally from the open end of the vent. The additional pressure in the vent pipe arises from resistance to the flow of . . turn laden air through the pipe. Th1S 1n 1S a function of the rate of flow and the physical characteristics of the pipe. The most important physical characteristics to be taken into consideration are the length and cross-sectional area of the pipe and. the number of restrictions, elboWS, etc., Wh1Ch Occur along its length.

Oil companies are not likely to be very happy about reducing rate of flow to prevent spitting from vent pipes. Th · 1S being so the alternative is to ensure that vent pipes are as short and straight as ·ble pOSSl with the largest convenient cross-sectional area.

The fact that change of pressure can cause condensation of petrol vapour in vent pipes does underline the need for making certain that all vent lines fall back to the storage tank and that all U-bends in vent runs are avoided.