How to Design Inverted Siphons? | Waste Management

While designing the inverted siphon the following points should be considered:

(1) Construction of inverted siphon should be simple.

(2) Changes of direction in inverted siphons should be easy and gradual.

(3) For trouble free working self-cleansing velocities should be provided in the siphon. The velocity in siphon should not be generally less than 1 m/s.

(4) To ensure self-cleansing velocities for the wide variations in flows the inverted siphon is generally provided with two or more pipes not less than 200 mm diameter laid in parallel so that up to certain flow one pipe comes into service and when the flow exceeds this flow the excess flow is taken by the second and subsequent pipes.

On most separate or partially separate sewerage systems, two pipe siphons will usually suffice, the first taking up to 1.5 times the average dry weather flow and the second pipe taking the balance of the flow.

On larger, particularly on combined sewers three or more pipes are provided to obtain better results.

In this case the flow is divided between the different pipes as follows:

(a) First pipe to carp up to 1.5 times the average dry weather flow;

(b) Second pipe to carry from 1.5 times to 3 times dry weather flow; and

(c) Remaining pipes to carry the remainder of the flow which will usually be the storm water flow in excess of that carried by first and second pipes.

(5) For the selection of proper sizes of pipes the minimum, average and maximum flows of sewage in the sewer should be considered.

(6) The total length of the siphon consists of straight length and the lengths of falls, bends and rise.

(7) The design of siphon is made on the basis of pipe running full under pressure. It is, therefore, necessary to know the maximum available head. Also, the losses of head due to friction, bends, etc., should be properly worked out and allowance should be made for these losses of head in the design of siphon. Further in the design of siphon allowance should also be made for the loss of head due to increased friction on account of roughness in the siphon caused by silting.

(8) The possibility of silting of siphon should be avoided or minimised.

This can be achieved by:

(i) Providing screens and detritus pits above the siphon inlet,

(ii) Duplication of the pipeline for diversion of flow (provision for standby),

(iii) Laying the pipes in such a way that they could be drained to some lower point.

(9) The design of inlet and outlet chambers should allow sufficient room for entry by service personnel for cleaning and maintenance of siphon.

The inlet chamber should be large enough to include the channels with side channel weirs (or lateral weirs) which ensure the correct distribution of sewage into the different pipes.

The outlet chamber should be so designed as to prevent the backflow of sewage into pipes which are not being used at the time of minimum flow.

(10) If the length of siphon is more, hatch boxes at intervals of about 100 m should be provided to clean out the pipes if they become silted up. There should be a vent pipe in the hatch box to avoid the formation of air-locks in the siphon.

(11) It is advisable to provide a diversion or by pass arrangement for the siphon so that when the siphon is choked up due to any reason or during any other emergency the flow of sewage can be diverted from the inlet chamber to a nearby stream if possible or sewage may be pumped from the inlet chamber to the lower reach of the sewer line.

Fig. 6.16 shows the details of an inverted siphon. It consists of an inlet chamber, outlet chamber and three pipes laid in parallel to each other. The pipes have two sloping lengths which are connected by flat lengths. The main sewer M enters the inlet chamber which has three channels as shown in the section on CD in Fig. 6.16.

When the sewage flow is small it flows in the central channel and enters the siphon pipe 1. When the quantity of sewage flow increases beyond the capacity of the central channel the excess quantity overflows into the side channel on the left and enters the siphon pipe 2.

Further when the flow of sewage is in excess of the capacities of the central channel and the side channel on the left the excess quantity overflows into the side channel on the right and enters the siphon pipe 3. At the outlet end the siphon pipes discharge sewage into the corresponding channels provided in the outlet chamber which in turn discharge sewage into the main sewer N.

The siphon pipe 1 which carries the minimum flow is usually of smaller size, and requires more drop in order to achieve self-cleansing velocity. Thus the inverts of all the siphon pipes 1, 2 and 3 may be at the same level at the inlet end, but at the outlet end the invert of siphon pipe 1 is lower than that of the siphon pipes 2 and 3 which come into action during the periods of higher flows. However, the top of the siphon pipe 3, which comes into action only during the period of heavy flow, should not be above the top of the outlet sewer N.