HAMBAKER GROUP Dosing Syphons

www.hambakergroup.com

Operation and Maintenance

GeneralThe primary function of a dosing syphon is to act, in effect, as an automatic valve, retaining the liquid in the dosing chamber until there is sufficient to ensure that the rotary distributor, which the syphon feeds, will rotate and continue to rotate until the syphon has discharged all the liquid.

In an article printed 30 years ago, it was stated that defects seldom occur until a dosing syphon has reached a considerable age, by which time maintenance is often left to an attendant who is forced to rely upon experience gained by trial and error, with varying degrees of success.

The biggest change since the article was published has been the reduction in manpower that the English and Welsh Water Authorities and the Scottish Regional Councils have had to introduce. Operational maintenance is now usually in the hands of a team who travel from Works to Works on a predetermined programme with a check list of duties to perform. There is no longer the attendant whose duty it was to see that “his” Works functioned correctly.

This has introduced two particular changes. There is no longer the “make do and mend” approach whereby, as a syphon aged, the attendant used to make it operate, possibly by the simple expedient of raising the dome to enable air to enter it or even to kick the dome to disturb the air inside it. He then usually had the time to watch the syphon operate for several cycles. It is now usually the case, if there is a fault, that the Operational Maintenance Team note and report that a particular syphon is not operating or that the rotary distributor it serves is not rotating.

Neither of these approaches is ideal. The first only overcame a problem temporarily without discovering the cause of the problem, whilst the second approach only reports that a problem exists. In either case, however, the problem can lead to the reduction in the quality of the treated effluent and this can only be rectified by determining the cause and taking the necessary action.

The cause is usually very simple and can be overcome quite cheaply, often by approaching the original equipment manufacturer.

Principles of OperationDosing syphons rely upon small increases and decreases in air pressure within the syphon dome and it is worthwhile contemplating the principles of operation.

Whilst there are a number of different details in syphon design, the basic principle relies upon the rising water level in the dosing chamber causing an air pipe to seal the syphon dome such that a further rise in water level compresses the air trapped inside the dome. When a predetermined pressure differential is reached so the starting “trap” is blown and the syphon will operate automatically.

In a typical syphon, as shown in Figure 1 when the water level is at its lowest point in the dosing chamber (Syphon Bottom Water Level or B.W.L.) within the ain trap pipe of the syphon, the water level is at the standing level of the distributor arms. In other words the syphon has just ceased its operation and the distributor is stationary.

within the chamber. At Top Water Level (T.W.L.) the air pressure within the dome forces water out of the ‘U’ tube ‘D’ at ‘E’ via the second, outlet air pipe. The air inside the dome is then replaced by the water (see Figure 3).

Water enters the dosing chamber and the level rises until, as shown in Figure 2, it reaches the open end of the inlet air pipe at ‘C’, which has allowed air to fill the syphon dome. Upon reaching this level the water seals the air pipe and compresses the air inside the dome as the water level rises

Potential problemsIt can be seen, therefore, that the critical parts of the syphon are the two air pipes. If either or both these air pipes become blocked, or an air leak occurs, the syphon will not function correctly. Similarly if the air pipe lengths are amended, the syphon may operate erratically or not at all. During manufacture, syphons are the subject of very careful design and quality control and each unit is made to suit the distributor(s) which it is to feed, to ensure that the finishing head (B.W.L. to centre-line of arms) is sufficient to ensure rotation of the distributor. The draw down (T.W.L. to B.W.L.) is usually determined by the maximum water level in the Pimary Settling Tank. The final position of the open end of air inlet pipe ‘C’ and the top of the ‘U’ tube ‘E’ are determined after the finishing head and draw down are calculated.If, in operation, the inlet air pipe blocks, it is often the case that sufficient air cannot enter the syphon dome to create the pressure increase that was calculated to be present when T.W.L. is reached.

The result is that either the water level in the Dosing Chamber continues to rise until the pressure is increased sufficiently to blow the syphon trap or that there is not sufficient air within the dome to keep the water level below the syphon stand pipe. In these cases, the symptoms would show as either a rise in the water level, potentially even as far as the weir level from the Primary Settling Tank, or dribbling of water from the distributor arms.Similarly if a blockage affects the outlet air pipe, the trap in the ‘U’ tube may not blow until the water level has reached similar levels to the first example, or the whole syphon may be put out of action.Air leaks in either of the air pipes, or the syphon dome, can have similar effects, usually resulting in the water entering the syphon stand pipe as the water level rises in the Dosing Chamber, causing the distributor to dribble but not rotate.Unfortunately, any or all of these symptoms have led to the all too frequent attempts to alter the air pipe, sometimes for no other reason than for aesthetic purposes.

After the syphon starting “trap” is blown, and the liquid starts to discharge through the syphon, the flow of liquid refills the ‘U’ tube to prime the syphon for the next operation, allowing the syphon to operate automatically.

As the syphon operates, the water level will fall until Bottom Water Level is reached, which in this case is the bottom of the syphon dome. During this time the air inlet pipe at ‘C’ will be uncovered but no air will enter as the other end of this pipe inside the dome is below the depression point and still in the water.

When the syphon takes air in, possibly in a number of gulps, under the bottom of the dome, the inside end of pipe ‘C’ will be uncovered and air will be drawn into the dome giving it a full charge of air ready for the next cycle.

The effects of any alteration of the original air pipes can be as follows:1. The outer leg of the ‘U’ tube controls the

T.W.L. in the dosing chamber. If it is too long, sewage may flood back into the Primary Tank before the syphon starts. If it is too short, the T.W.L. in the dosing chamber may not be high enough to cover the dome completely. A pocket of air is then retained inside the dome and the syphon may function erratically. Alternatively, if the outer leg of the ‘U’ tube is too high the water level inside the dome may rise to such an extent that the free-board below the top of the syphon stand pipe is lost, with consequent tendency for the distributor to dribble.

2. The distance between the lower edge of the air inlet pipe and the bottom of a dome should be quite small, and is carefully determined by the manufacturers. A smaller distance prevents the syphon cutting-out promptly. A larger distance raised the level at which the air inlet is sealed and so reduces the free-board below the top of the syphon stand pipe.

3. If the dome end of the air outlet pipe and ‘U’ tube assembly is allowed to project below the soffit of the dome, an air pocket may be formed within the dome. This may pt the syphon out of action, or it may cause erratic working.

4. If the ‘U’ tube rises above the lower edge of the dome it will not become full of water when the syphon discharges and syphonage will not start its next cycle.

5. The inner end of the air inlet pipe should extend below the top of the syphon stand pipe in order that the speed of filling the dome with air may be assisted by the draw-down effect of the water. On the other hand, there should be a clearance above standing level in order to prevent the inner end of the air inlet pipe becoming sealed before the dome has been filled with air.

Most manufacturers keep detailed records of all their installations and certainly at Adams-Hydraulics Ltd. there are records, going back to the early 1900’s, of all the distributors and syphons that they have made. A number of original manufacturers are now no longer in existence or have ceased making distributors and syphons but Adams can usually supply new syphon domes and air pipes for these installations, even though no original records may exist.After many years of operation, during which time the syphon dome may rise and fall minute amounts, as the air inside it is compressed and depressed, the syphon stand pipe may become worn or notched. This has the effect of very gradually lowering the syphon dome and its associated air pipe. This, in turn, gradually alters the free-board on the stand pipe and may cause the distributor to dribble during periods of low flow.

MaintenanceIt is recommended that the following operations are carried out weekly but experience may dictate different time scales from site to site.

1. Clean the syphon air pipework internally to ensure they are not blocked. Syphons with galvanised steel pipes often have crosses or tees at each bend and have plugs in these fittings, allowing the pipes to be rodded. Plastic airpipes do not have these fittings and, therefore, a hand pump or similar may be necessary to clean the pipes.

2. Ensure the plugs, where fitted to the airpipes, are airtight after cleaning and check the air pipework for air tightness.

3. Clean the chamber and feed pipe between the chamber and the distributor. Where fitted, the washout valve adjacent to the distributor supporting base can be opened to achieve satisfactory cleaning of the feed pipe.

4. Remove any grease or fat that may cause intermittent blockages.

The following should be carried out annually.

5. Raise the syphon dome and air pipe carefully, so that the syphon stand pipe can be inspected for wear or notching. If wear is noticed, order a sleeve pipe to effect repairs. Check and clean all the air pipe at this time. If the air pipe is starting to corrode, order replacement air pipe and dome or dome plate, where fitted.

6. When lowering the dome into position, ensure that it is correctly positioned on the syphon stand pipe.

For their own equipment Adams can usually supply dome plates and air pipes at a lower price than the complete domes. The plates can be fitted onto the existing domes. All that they need to know is the original name of the Works, the name of the original Authority (if supplied pre 1974) and the approximate date of installation.Sleeve pipes can be supplied, where the syphon stand pipe has become worn or notched and, if, for whatever reason, no records can be found, a Representative may visit the site to obtain the necessary dimensions. It is often cheaper, however, if the Authority can provide drawings or other information, detailing the Primary Tank maximum water level, Dosing Chamber T.W.L. and B.W.L. together with the level of the centre line and the distributor arms.For a relatively small outlay, new syphon domes and airpipes can often revitalise old Works and produce improved effluent without major additions to the Works.

Ham Baker Group is a trading name of F. J. Holdings LtdCompany Registration No. 4878424ISO 14064-1

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ADAMSHYDRAULICS

Adams HydraulicsUnit B & ESpitfire HouseAviator CourtYorkNorth YorkshireYO30 4UZ

t: +44 (0) 1782 202300www.adamshydraulics.co.uk

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