Chapter XIX: Appendix: H

GIBB’S MODULE.[65]

(See p. 164.)

[65] This description has been supplied by Glenfield & Kennedy,
Kilmarnock. The modules can, it is understood, be obtained from them.

The attributes of a perfect module are many and varied, but in Gibb’s module they have all been successfully embodied in what is probably the simplest piece of apparatus of its kind ever devised. The following summary of the characteristics of Gibb’s module is, therefore, equivalent to an enumeration of the attributes of a perfect module:--

Gibb’s module

Cannot be tampered with, }
Cannot get out of order, } since it has no moving
Silt or other solid matter in the water } parts, and because of its
cannot affect its action, } extreme simplicity.
Requires no attention, }

It is accurate, } being designed on scientific
Works with very small loss of head, } hydraulic principles.

It is portable, and can be erected at any desired site very simply and
easily.

It is strong and durable.

The range of variation of both up- and downstream water-levels through
which the discharge remains constant is more than sufficient to meet
all the requirements of irrigation canals.

The sufficiency of the delivery can be ascertained at a glance.

The water can be drawn from any desired depth in the parent channel.

When desired, means are provided whereby the supply can be closed or
opened at will.

Means are provided, if desired, for a sudden increase of discharge
when the upstream water-level exceeds a certain limit, so that surplus
water, which might endanger the safety of the canal, is allowed to
escape into the branch whenever the danger limit is reached. The
upstream water-level at which escapement begins can be fixed in
accordance with the requirements of each site, and the action of the
escape notch is independent of the opening and closing of the module.

No designing or calculations are required. These have already been
worked out. Known the discharge required, the module is supplied
complete and ready for setting in position in the canal bank.

HYDRAULIC PRINCIPLE.

The entire absence of moving parts is the chief feature of Gibb’s module; the water simply regulates itself by using up all the excess of energy over and above that required to discharge the correct supply of water. The way in which this takes place will be understood from the following analogy:--

We all know that when we stir tea in a cup so as to make it spin, the liquid rises at the rim of the cup and curves down into a depression in the middle, and the greater the spin the more marked this effect is. It is, we know, the centrifugal force produced by the spin that makes the tea remain high at the rim of the cup. If, while the tea is thus spinning, a teaspoon is held so that it dips slightly below the surface of the liquid near the rim, it will obstruct the flow of the outer portion of the liquid, which will fall in towards the depression in the middle. The reason for this, of course, is that the centrifugal force is absorbed when we interrupt any part of the spin with the teaspoon; hence the liquid must fall, and we know that when liquid falls it uses up “head” or energy.

In Gibb’s module a similar action is made to take place in a steel chamber, semicircular or spiral in plan, through which the water flows in a semicircular path instead of circulating round and round as in the teacup. The surface of the stream, however, assumes the same form as it does in a cup, because it flows under the same conditions. Across the chamber are fixed a number of vertical steel diaphragm plates which take the place of the teaspoon in the above analogy. The lower edges of these plates are of such a shape, and they are fixed at such a height from the bottom of the chamber, as to allow a stream of just the correct required discharge of water to flow under them without interference. But if, owing to an increase of head caused by a rise in the upstream water-level, the water tends to rise higher at the circumference of the chamber, then the water at the surface of the stream strikes against the diaphragm plates, and its centrifugal force being absorbed, it will fall in towards the centre just as happened in the teacup when the spoon was used in place of these plates. In this way the excess head that caused the additional rise of water at the circumference is used up by the fall back towards the centre. The full capacity of the semicircle or spiral for using up excess head or energy in this way is made available by the use of a sufficient number of diaphragm plates fixed at suitable intervals. When the range of head to be dealt with is not large, then a semicircular chamber is sufficient; but for large ranges of head the chamber is made of spiral form so as to lead the water round a complete revolution or more, as may be necessary.

STRUCTURAL DETAILS.

Fig. 29 shows the general form and structure of the type of module suitable for irrigation. Fig. 30 is from a photograph.

The working chamber or shell A is constructed of mild steel plating securely riveted to a framework of angle steel, and the semicircular form of the shell with the rigid diaphragm plates B B riveted to the walls makes a very strong structure, and ensures durability.

The “leading-in” bend C is of cast iron strongly bolted to the steel shell, and is so designed as to deliver the water into the module chamber in a completely established vortex condition.

The socket D on this “leading-in” bend is made so as to allow of considerable latitude in the vertical alignment of the straight leading-in pipe, so that the water can be drawn from any desired depth in the parent channel, and the proportion of silt drawn off is thus brought under control.

-- PLAN. --

-- FRONT ELEVATION. --

FIG. 29.--Details of Gibb’s Patent Module.]

Grooves E E and a shutter F, as illustrated, for closing off the flow through the module, are provided, if required, but all modules are not fitted in this way, because many irrigation authorities consider it undesirable to provide the consumers with unrestricted facilities for closing off their supplies without previously giving notice of such an action.

An escape notch H is provided in the position indicated when desired. It may, however, be found difficult to determine beforehand the upstream water-level at which it is necessary to allow this escape of surplus supply, so that it is generally more satisfactory to cut the escape notch after the modules have been installed and actual experience has indicated a suitable level for the notch crest.

In the standard type of module for irrigation purposes the top of the module chamber is completely open, as shown, and this is the type generally recommended, as it is found that consumers have greater confidence in an apparatus which hides nothing from them. To meet the needs of special cases, however, a second type is also made in which the chamber is completely closed and considerably reduced in height, being thus specially suitable for sites where space is confined.

Pipes I, of diameter suitable for all sizes of modules, are also supplied. These may either be welded steel or cast iron, as desired. An 18-feet length of pipe is usually found sufficient to bring the supply through the canal bank to the module.

All modules supplied are treated with anti-corrosive paint, which ensures the protection of the metal.