To Run Your pump as Close to optimum efficiency as possible, pay close attention to life-cycle costs and use variable-speed pumps. This can reduce running costs substantially, particularly for larger units. Selecting a pump is a team effort and the user, system designer and suppliers must all understand the requirements. Give pump suppliers flow and head (pressure) and they can select a pump for you, but give them a system design and they may be able to suggest a change in pipe for the whole or part of the system. This can lead to better selection and lower running costs. I had a recent case which illustrates this cooperation, or lack thereof.
It was the usual complaint – the pump didn’t perform. However, the pump had been factory-tested and the results were to specification. On site, I headed for the suction arrangements, confidently expecting a problem with the piping size or the reducer. Nothing wrong there. In fact, it was a nicely laid out system. But, inspecting the discharge system, I heard a fair bit of noise from the first section. Fortunately, a tee piece provided a point on which I could attach a gauge. Due to pressure differences in the system, over half a bar had disappeared. A pump specialist may have anticipated the problem if a layout had been submitted before installation.
The modern centrifugal pump is designed for minimum maintenance. The close-coupled version with greased-for-life bearings and mechanical seals needs no regular maintenance at all. However, the long-coupled industrial pumps differ from agricultural ones. While building services prefer mechanical seals and sealed bearings, farmers traditionally prefer packing and oil-lubricated bearings. However, this system is high-maintenance, and runs the risk of oil contamination due to condensation or gland leakage. The oil level must also be maintained in dusty conditions. nother reason for the close-coupled pump’s popularity is that it doesn’t need careful alignment and regular alignment checks. Today, 80% of field problems I see stem from either poor installation or faulty selection. Most are suction problems.
Good selection means choosing a pump with comfortable suction conditions and as close to the best efficiency point as possible. On one selection I checked recently, the duty point was almost unachievable. This could have been avoided if the client had consulted the pump manufacturers, or understood the problems of running at minimum flow conditions. The pump supplier will always want to know the duty point you require, preferably in metres cubed per hour (flow), and metre head (pressure). They’ll check if this is the total head. Also critical are the static head and pressure losses in the pipeline. If the pressure loss is unknown, the supplier can usually calculate it using the pipe size, class, running length and fittings. The supplier will also ask about the suction condition – the water level either above or below the pump centre line. From this data the supplier can calculate the duty point and select the best pump for the job, avoiding the problems that occur when operating outside or at extreme ends of the performance envelope.
Next in importance is good installation. If your pump is long-coupled (if it has a base plate and a flexible coupling between the pump and motor) then it should be bolted down firmly on to level concrete. The pipes should be supported and shouldn’t hang on the pump flanges. The pump and motor should be accurately aligned radially and axially, and the distance between shaft centres carefully set. This applies equally to old installations and new deliveries. Transport and handling can change the factory alignment. Fit pressure gauges to the suction and delivery flanges. These can establish where on the curve the pump is operating and can be marked to indicate changing performance, whether due to wear or changing conditions. Suction lines must be generously sized to keep suction velocities around 0,75m per second, and certainly below 1m per second.
Fit an eccentric reducer where the pipe diameter reduces to the suction diameter, and a good quality low-loss foot valve with a generously sized strainer where a suction lift occurs. Suction and delivery valves make isolation easy for maintenance or repair. On systems with no foot valve, a good non-return valve in the delivery pipe will prevent reverse rotation when the pump is stopped, and prevent over-pumping when it’s started again. A well-designed suction system will avoid cavitation problems, which can damage the impeller quickly and severely. Remember to fit a voltmeter and ammeter in the start panel. These are useful for problem solving and can be similarly marked to the pressure gauges.
Factors affecting maintenance
A properly installed and selected pump should need very little maintenance. Here are the main check points: Sealing options – soft packing or mechanical seal: Soft packing is cheaper and makes repacking a gland easier, as the pump doesn’t have to be stripped. You can often get away with changing the two outer rows only or, in a crisis, stuffing your old rugby sock in the stuffing box. However, you do have to strip small pumps for a proper change. The downside is the packing must be kept dripping wet, to keep it cool and prevent the pump from sucking air through the gland. To avoid damaging the sleeve and packing, setting must be slowly and carefully done during commissioning and checked regularly. A mechanical seal is more expensive and the pump has to be stripped for replacement. Under optimum conditions it has a very long service life and is generally used where a drip can’t be tolerated, or zero maintenance is expected.
Lubrication: There are several bearing options – greased bearings, sealed bearings and oil-lubricated bearings (generally oil-bath on water pumps). Greased bearings are mostly found on older pumps and require only a small amount of grease, added about every 2 000 hours. In very humid or dusty conditions these intervals can be halved. Use a good-quality grease, typically HLGI No 2, and ensure the grease nipples and gun are clean. Greased-for-life bearings require no maintenance, other than checking for noisy running at regular intervals. Oil-bath builds need frequent checking to ensure the oil level is correct and the oil isn’t contaminated. A white appearance means water in the sump has emulsified the oil. Immediately change the oil and clean out the sump. Use a good-quality mineral oil such as SAE 30, though this can vary with ambient temperature. If possible ask the manufacturer for advice.
Look, listen and feel: The best maintenance is recognising problems early. This is easier if you know the normal running conditions. I recommend a “look, listen, feel” approach whenever the pump is started or stopped. Don’t just press the button and leave. A little more time here can save you money and downtime. Look for leakages. Water may indicate the gland needs attention. This could worsen as the pump develops pressure. Leaking oil could be a sign of overfilling or worn oil seals. Check the oil level. Start the pump and check where the gauges settle. Listen for unusual noises and touch various places around the pump and motor to check if there’s more vibration than usual, which may signal an alignment check is due. Feel the bearing frame to check if heat builds up unusually quickly, pointing to alignment or bearing problems. At least every six months, check the alignment and all the holding-down bolts for tightness. When stopping the pump, check for leaks, vibration and heat. Call Bob Mackie on (011) 824 4810.
A trickle or a flood?
How much water do farmers need to irrigate their crops? Agricultural consultant Johannes Maree looks at one of the most frequently asked questions in farming, and offers advice on the planning stages of crop production.
A plant’s water requirements change from week to week, if not literally from day to day. The best advice is to give your plants water when they need it, rather than irrigating according to a predetermined schedule. Perennial plants need more water during the active growing season than they do in the dormant season. W hen a plant will need water and how much are directly affected by the following factors: type of plant; type of cultivar; stage of growth; water-holding capacity of the soil; climatic conditions such as temperature, light, wind and rain; the irrigation system used; and evaporation. Evaporation isn’t only determined by heat. A soil that contains a lot of organic material will retain more water, while a sandy soil will lose it faster.
Quality and quantity
It’s important to consider both the quantity and quality of available water. High yields and top-quality crops can’t be produced without an adequate supply of good, clean water. It’s advisable to have your water tested at the outset to determine its quantity and quality. hen setting up a crop production enterprise, think in terms of buying water rather than land. availability of enough good, clean water is more important than the quality or position of the land. Generally speaking, good-quality water has an electrical conductivity (EC) of no higher than 0,3mS/cm (milliSiemens per centimetre). An overly high EC means there are too many salts in the water and this can be problematic. Poor-quality water has a negative effect, not only on your plants, but also on the soil and the pumps, nozzles and pipes of the irrigation system. t’s much easier and cheaper to correct or improve poor soils than poor water. Ideally, the primary water source for irrigation, such as a borehole, stream, river or spring, should be able to supply at least two to three times the average amount needed.
Water needs for specific crops Cut flowers: The average water requirement for cut flowers is 4l/m2 to 6l/m2 per day (see Table 1). In winter this can drop to 2l/m2 per day, while in summer it can go up to eight. Therefore, taking 5l/m2 per day as an average: 5l x seven days (1 week) x 10 000m2 (1ha) = 350 000l/ha per week = 35mm/per week. P er cycle (three months: 12 to 13 weeks) = 4 550 000l/ha per cycle = 4 550m3/ha per cycle.
Vegetables: On average vegetables need 4 500 000l/ha of water per cycle. For most vegetables a cycle is normally around three months. This is equal to 35mm per week or 4 500m3/ha per production cycle. Fruit trees: During the growing season, fruit trees need an average of about 250 000l/ ha (25mm) per week. During the dormant season watering must be drastically reduced to about 100 000l/ ha per week (10mm per week) depending on the type of trees.
Irrigating during flowering and fruiting
Sufficient water is particularly important when trees are in flower or fruiting. The above figures are general guidelines for commercial farmers using micro- or sprinkler irrigation systems. Remember to consider hot periods, water availability, better planting methods and greater land usage. According to Charles Crosby of the Water Research Council, 25mm per week is generally adequate for most crops under normal circumstances. However, it’s important to have the capacity to put down 30mm to 35mm a week if necessary, due to crop requirements or excessively hot and dry conditions. Planting densities will also influence water requirements for irrigation. I strongly caution against relying on rainfall figures when calculating irrigation needs and area for potential crop production, except in the case of dryland production. However, rainfall needs to be recorded and necessary adjustments made once production is under way.
To keep in mind
- Be aware of the legalities of getting permission to use water and registering as a water user.
- Generally speaking, borehole water is best because its pH and EC tend to remain more stable over a longer period of time. Furthermore, borehole water generally tends to contain fewer pollutants and diseases.
- Dam or river water is often infected with diseases such as fungal spores and eelworm, and the salt content can fluctuate drastically between the dry and wet seasons.
- A drip system uses approximately 30% less water than an overhead sprinkler system.
- The best time of the day to irrigate is very early in the morning, in the first few hours after sunrise.
- E-mail Johannes Maree at [email protected].
Although 25mm per week is generally adequate for crop production under normal circumstances, it’s important to be able to put down 30mm to 35mm per week if needed, as a crop’s water requirements increase during excessively hot and dry weather. The growth stage will also influence irrigation requirements, as will rain.
An irrigation system must therefore allow for flexible scheduling. This solid set system, using Floppy Sprinklers to irrigate wheat on Louis van Rensburg’s farm Wonderpan near Prieska, is outstanding in this respect.
For calculation purposes
If you had to pour 1l of water into a 1m2 pan, the water would be exactly 1mm deep. Thus, if your rain gauge indicates that 1mm of rain has fallen, it means that 1l of water fell on every 1m2 piece of ground.
There are 10 000m2 in a hectare. Therefore, if you have to irrigate 10mm/ha, you’ll need exactly 100 000l of water per hectare.
1mm = 1l/m2
= 10 000l/ha
Thus 10mm = 10l/m2 = 100 000l/ha.