Drainage by Pumping Balloon to Trap Insects

New Zealand Journal of Agriculture, Volume 95, Issue 1, 15 July 1957, Page 73

Drainage by Pumping

Balloon to Trap Insects

By

D. F. SCOTT,

Machinery Instructor, Department of Agriculture, Auckland MOST land drainage systems in New Zealand depend on gravity flow for their outlet, but sometimes any of the following can make it necessary to use pumping: Drainage into a tidal estuary; drainage into a stream or river subject to flooding; situations where outlet is through a drain or drains not well maintained; drainage of low-lying, flat areas where the required depth of outlet drain is lower than that available. IN most schemes where pumping is used it is possible to get gravity drainage for a limited period. In tidal areas gravity drainage is available between tides, and in flood areas between floods, so that it is usual to provide a gravity outlet in the form of a flood-gate, which is set as low as conditions permit. Sometimes during the initial stages of developing land, such as reclaimed tidal flats or low-lying swamp, when open drains alone are used for infield

drainage, it is satisfactory to use what gravity drainage can be obtained by the use of flood-gates. This is because open drains can tolerate high water levels without any ill effects. However, if it is desired to install tile drains or combined tile-mole systems, high water level in outlet drains cannot be allowed and often at this stage pumping must be resorted to. Types of Pumps Pumps used for land drainage in this country are usually axial-flow pumps and consist briefly of a vertical tube containing an impeller which looks something like a ship’s propeller. Pumps of this type are manufactured in New Zealand by well-known firms; a simple type of farm-built flood pump with a limited lift is made by a Waikato farmer-engineer, and ordinary centrifugal pumps are also used to a limited extent. Some of the smaller installations use centrifugal, sump-type pumps with capacities of up to 12,000 gallons per hour, and one of the biggest flood pumping installations in the country, at Kerepehi on the Hauraki Plains, uses a 33in. centrifugal pump. Most farm installations, however, will use the axial-flow type of pump mentioned above. These axial-flow pumps have high efficiencies when used for the purposes for which they were designed.

Vertical axial-flow pumps used in this country are of two types. One is fitted with a direct-drive motor so that the impeller of the pump travels at motor speed. To deal with variations in lifts and quantities of water pumped the makers supply different impellers. The other type is driven by V belts. This means that the same impeller can be used over a range of operating conditions by changing pulley ratios to give different impeller speeds. Both types of pump have advantages. The direct-drive machine has no V belts, so that transmission losses are eliminated, but some users prefer the indirect drive because it allows easy variation of pump speed and means that an ordinary electric motor fitted with a thrust bearing for vertical drive can be used. The direct-drive pump requires a special flange mounting motor. Many farmers using flood pumps have had trouble with eels. The usual difficulty was that an eel would get into the pump and might be partly cut up by the impeller, but ten. quite a large piece of eel would become draped over the leading edge of the impeller and in that position would put the impeller out of balance and cause vibration and overloading of the motor. Often eels collected in this way would gradually work inward toward the centre of the impeller and become wound round the impeller shaft, causing further overloading. One make of pump has eliminated this trouble by sharpening the impeller and so shaping its leading edge that eels or small pieces of root are flung to the outside and there cut between the sharpened impeller and the pump casing. Both with flood-gates and flood pumps it is desirable to have some

means of straining floating debris before the flow enters the pump bay. Roots and other pieces of timber can jam the flood-gate or damage or overload the pump. It is preferable to fit some form of screen that can be easily cleaned, as in some installations the amount of debris coming down is considerable, particularly during flood time. Screens can be made from either metal or wood. They are often sloped so that the farmer can clean them easily with a drag. Some pump users have found that metal screens supported on a wooden beam and electrified with a charge of about 12 volts are very effective in preventing eels approaching the pumps. For safety reasons installations of this type must be made by a registered electrician and passed by the local supply authority before they are used. Generally the efficiency of axial-flow pumps falls off when they are used on heads greater than 10ft., and for lifts greater than this either an ordinary centrifugal pump or a combina-

tion of the two known as a mixed-flow pump is used. The mixed-flow pump is rather similar to the axial-flow pump, but its impeller gives a centrifugal action as well as a screw action, with the result that it can be used at a somewhat higher head than the plain screw or axial-flow pump. Mixed-flow pumps can be used on heads of up to 24ft. with reasonable efficiency. Centrifugal pumps are generally used only on very large schemes. Horizontal-shaft centrifugal pumps have the disadvantage that they require priming equipment to fill the pump with water before it is started. Another possible fault of this type is that the engine or electric motor is usually mounted at the same level as the pump and could easily be submerged in a major flood or a collapse of a flood-bank. With vertical drive pumps the motor can be mounted well above the pump to obviate this risk. The most satisfactory method of driving flood pumps is by electric motor. These motors are very simple

to control automatically by a float switch, and automatic control is almost always a necessity in these installations. Electric motors require practically no maintenance and can be easily coupled to pumps using either a direct drive or a system of belts. Unfortunately the site of a flood pump is usually dictated by factors beyond the control of the designer of the drainage system, and often a new installation may require a long extension to electric power lines if the plant is to be driven by electric motor. For this reason quite a number of flood pumps are driven by internalcombustion engines, usually diesel, and recently a well-known make of diesel engine has been satisfactorily fitted with an automatic starting and stopping device which enables it to be operated from a float switch. Even though automatic control can be achieved, diesel engines still require much more attention than electric motors. Such things as cleaning of

fuel and oil filters and checking and changing of oil must get regular attention during the operating season and fuel has to be carried to the pump site. Electric motors, however, are not trouble free by any means, especially when large motors are used under automatic control in fairly remote rural areas. Unfortunately voltage regulation on rural power systems is not good and motors must often run on low voltage. Though protective devices are fitted, burnt-out motors are not uncommon and often the first indication the farmer has that anything is wrong is the water level rising in the drainage system. Where a pump house can be seen from the homestead it is a good idea to fit outside pilot lights which are connected in parallel with the motor. This means that when the motor is running the lights will be on, and if the lights go out when the pump should be running,, the farmer can investigate. Farmers should consult the suppliers of their motor or their power supply authority for advice on the purchase of adequate protective gear that will save their motors in the event of single phasing, low voltage, or overloading caused by debris jamming the pump. Size of installation The size of the pumping installation will depend mainly on the area being drained. It is good practice to keep

to a minimum the area that must be handled by the pump. Where possible, drainage water from higher country should be diverted to by-pass the pump system. Every gallon of water the pump handles will cost money, so it is in the farmer’s interest, because of both initial cost and running cost, to keep the size of his installation as small as possible. In choosing a pump one must consider the area, the heaviest likely rate of rainfall, and the percentage of runoff. This run-off coefficient, as it is called, will vary greatly according to soil type, the topography, and the vegetation covering the area. To design a pumping system where the calculations are based on run-off a considerable amount of investigation work would be necessary over several years before the installation was made. For this reason it is usual to use as an approximate guide, at least in Auckland Province, the figure of lin. per 24 hours. This means that to drain 10 acres the pump will have a capacity of 10,000 gallons per hour and to drain 100 acres 100,000 gallons per hour. This figure is probably on the generous side and the writer is aware of many pumps which handle greater areas than this. English pub-

lications mention the figure of |in. per 24 hours with a maximum of fin. At this rate of run-off a 10-acre block would require a pump capacity of between 2500 and 4000 gallons per hour. Where the pump is providing an outlet for mole or tile 1 drains the figure of lin. per 24 hours should be adhered to. It is usual to mount the pumps in a concrete pumping bay. This same concrete work is used to mount the flood-gates when they are used. Because of the large volumes of water handled by the pumps precautions must be taken to prevent scouring, particularly on the outlet side of the pumps, where the water may often fall several feet on to the bottom of the outlet drain. The drain and the land beside it should be protected either by a concrete apron or by metal or wooden fluming. On the intake side of the pump it is also usual to provide a concrete floor for the same reason. The pump should be set up so that its intake is submerged by at least Ift. of water at its lowest pumping level. If this is not done, the pump may suck air. In large installations it is good practice to have two small pumps rather than one big one. In this way a breakdown of one pump or motor does not put the plant completely out of action and also in most systems there will be considerable variation in the level of water on the intake side during pumping. For most efficient operation one pump would deal with the maximum lift required and the second pump would be chosen to work at greatest efficiency for a lower lift. As both pumps would be controlled by float switches, each would operate only at the appropriate times. To ensure that the pumps run for a reasonable time instead of starting and stopping frequently, adequate capacity should be provided in the pumping bay or the drain close to the pumps. The range of movement of the float switch can be adjusted to set the starting and stopping levels of water at the pumps. If the pump runs for short periods only, the distance between the on and off stops on the switch can be increased. Power Requirements The power necessary to drive any pump can be calculated from the quantity of water handled per minute and the height of lift involved. The lift is measured from the level of the impeller to the centre of the pump discharge pipe (see the top-left sketch on page 74). If the discharge is submerged, the lift is measured from the level of the impeller to the surface on the discharge side. This will of course

be the maximum head. The actual head will vary in operation according to the height of water in the drains and possibly the flood level outside the bank. The water horsepower can be calculated as follows: — H.p. = gallons/minute X head in feet

3300 Usually the pump will be only about 50 per cent, efficient, so the motor horsepower will be at least twice the water horsepower, and in addition a further safety factor of about 50 per cent, is often used to allow for low voltage and other overloading that may occur. When the pump is likely

to work under submerged-outlet conditions it will be necessary to fit a flap gate to the pump’s outlet. Under some conditions, particularly in peaty soils, there may be considerable subsidence after drainage. This will mean that in time the lifts of the pumps will have to be increased. In practice what usually happens is that initially one pump suitable for the present conditions is installed and after subsidence has taken place a second pump with a greater lift is added. Where subsidence is considerable the cost of pumping will increase as the subsidence takes place, and at the same time the assistance of gravity drainage will be less, so

more of the water falling on the catchment must be pumped. For this reason it is important to have some idea of the likely , extent of subsidence before installing pumps. With many flood pumps driven by electric motors no special arrangements are made for cheap power. This is because power boards usually require time switching or ripple control switching, which will permit them to stop the pumps during peak load periods. Under these conditions cheap power is usually . available. With small installations that do not run for long periods during the year it is probably preferable to pay for power at a higher rate to have 24-hour pumping. Where large installations are planned and particularly where pumps must run for long periods, it may be preferable to provide sufficient storage capacity in the main drains and pumping bays to permit pumps to be switched off during peak load periods. Under these conditions the power board will probably supply power cheaper. In big installations the possibility of two-rate metering allow-, ing cheap pumping at night should also be discussed with the power board. Pump Efficiency A factor that has considerable effect on both the initial cost of pumping equipment and also its running cost is the efficiency of the pump itself. Well-designed pumps used on jobs for

which they . were designed will run much more cheaply than . inferior units. This is because the pump will take less horsepower to drive it and yet will do .the same job as an inferior machine with a much larger motor. Though the efficient pump may cost more initially, it should be remembered that the inferior machine will require a larger motor which will cost more and will use more power over its entire life. A further point that should be remembered is that many power boards impose on electric motors a standing charge of so much per horsepower per month so that the inferior pump with the larger horsepower motor will be further penalised by higher running costs. For running economy it is important to use a pump designed for the particular job. If water is to be lifted only 4ft., a pump with a 6ft. lift should not be bought. Such a pump will do unnecessary work and will cost more initially and more to run. Stop-banks Nearly all flood pumping installations will require stop-banks, as usually the land to be drained is endangered by either flood water or high tides. Adequate banks are important; otherwise pumps may be handling water that is either leaking through or running over unsatisfactory flood-banks. Flood-banks should be built at least Ift. to 18in. above the maximum flood or tide level.

In tidal areas it is usually not economical for farmers to build and maintain banks that are subject to wave action, so banks should be placed so that they are protected, and if there is a likelihood of occasional waves, considerably more freeboard should be allowed. It should be remembered that one flooding with salt water can do considerable damage to pastures and crops and may waste years of work.

If stop-banks are built from material dug from a drain running round the inside of the bank, a reasonably wide berm should be left between the bank and the drain. This will allow the passage of machinery for cleaning the drain and topping up the flood-bank, but what is more important is that it will reduce the possibility of seepage from the outside of the bank to the drain during high tides or floods. If porous material such as pumice ,or sand is used in the bank, no drains should be dug anywhere near the inside of the bank. The width of the bank both at its base and at the top will depend on the height, but as a general rule it is good practice to have a bank with a top width equal to its height and a batter of 1| to 1. No vegetable material or timber should be incorporated in the bank as it is being built and the site of the bank should be cleaned of all surface soil and vegetation before building begins. Small banks can be conveniently built with a bulldozer or an angledozer; large ones are better done with a carry-all, or where a drain is being dug round the inside of the . bank a dragline excavator can conveniently dig the drain and swing round to deposit the spoil on the bank as it goes. Banks should always be protected from stock by complete fencing and should be grassed as soon as possible after completion. The species of grass used will depend on the locality, but grasses such as kikuyu and buffalo grass are excellent because of the binding nature of their roots. Buffalo grass is useful near the sea because of its resistance to salt wind, ' and where farmers are reluctant to use kikuyu owing to the risk of its spreading, pasture grasses can be used. In trials Italian ryegrass has shown , its value in establishing a quick cover, which is one of the main requirements, especially where the bank is likely to suffer from erosion from wind or water.

Banks will require topping up from time to time, so this should be borne in mind when nearby fences are being planned, and room should be left for the necessary machinery. There will be considerable subsidence in all new banks owing to consolidation of the material. Depending on what the

bank is made of, subsidences from 30 to 50 per cent, can be expected on a new bank. Banks should be built high enough to allow for this. Banks consolidated by machine have from 8 to 10 per cent, less volume than banks with soil deposited without compaction. Flood-gates As mentioned earlier, flood-gates will very likely be used to give gravity drainage when possible. On tidal works it is usual to site the flood-gate about level with low-water spring tides, but this may not be possible if the outlet channel between the flood-gate and deep water is likely to silt up. The flood-gates should be placed as near as possible to a deep channel. Often a tidal creek will provide this, and only a very short length of channel will be ' required between the flood-gate and the deep water. Flood-gates will also require protection against erosion, particularly on their outlet sides. Gates can be made of a variety of materials, either concrete or cast iron being the more durable. However, wooden gates if properly made are quite satisfactory and are less liable

to damage due to slamming caused by wave action or the wake from boats. Quite frequently flood-gates collapse owing to unsatisfactory foundations. Where possible firm land should be chosen as the site for the gate, but where this is not possible piles may

be necessary or else the structure of the flood-gate supports and pipes leading through the bank should be braced sufficiently and reinforced to prevent collapse. A rough guide to the size of floodgates to be used is to allow 1 sq. ft. of flood-gate area for every 30 acres of flat land, and for flat land with some hill water 1 sq. ft. per 20 acres is suggested. Obviously these figures can be taken only as a rough guide, as such factors as the time the floodgate is open and the rate of run-off from the catchment are most important. As flood pumps are often placed in positions that, may be submerged in major floods and as flood-banks may be damaged, it is usual to provide a means of hauling the electric motor above likely flood level in an emergency. In some installations the motor is permanently mounted quite a distance above the normal position and a specially long shaft used to couple to the pump. Though the initial outlay on drainage pumping schemes is usually fairly high, a properly designed system using efficient pumps is well worth while and the outlay appears quite reasonable when considered on a per acre basis. Naturally, pumping will be used only when gravity drainage is not possible, but there are considerable areas in Auckland Province that are already partly drained and could be still further improved by the use of pumps. References J. G. Sutton, United States Department of Agriculture Technical Bulletin 1008, “Design and Operation of Drainage Pumping Plants’’, 1950. E. A. G. Johnson, "Land Drainage in England and Wales”. R. C. Lough, Proceedings of New Zealand Institute of Engineers, “Notes on Preparation of Drainage and flood Control Schemes”, 1946.