Jabsco Pump Catalog

The raw water flow capacities indicated earlier are for diesel engine cooling systems and include a safety margin of some 30% to ensure adequate engine cooling under adverse operating conditions. As gasoline engines, unlike diesel engines, have a high heat rejection rate at idling speed, an increase in pump flow capacity of some 10% at maximum rpm is recommended. While flow capacities can be affected by salt deposits or scale in pipes due to operation in tropical regions, seawater pollution or age and wear of pumps and systems in general, this is usually a gradual process with ample warning that preventive maintenance can counteract. Far more serious is a reduction in cooling water flow due to adverse conditions caused by an inadequate system. When marine vegetation such as seaweed gets stuck in seawater inlet strainers of insufficient mesh or hole size, this can cause a significant reduction of the cooling water supply to the pump. On the other hand, no amount of pump flow capacity will safeguard against a sudden but total blockage — for example, a plastic bag covering up the hull inlet. System safety margin can be substantially increased by providing two separate hull inlets usually on either side of the keel, joining the pump inlet pipe below the water line. In actual practice, the likelihood of both inlets being covered simultaneously has been found to be insignificant. As heat exchanger and cooler manufacturers’ recommendations for a given engine performance and corresponding pump flow will include appropriate safety margins for minimum and maximum flow, it should be kept in mind that excessive water velocities through the heat exchanger pipes due to “overkill,” i.e., fitting oversize cooling pumps, may cause pipe erosion and accelerated pump wear through excessive pump operating pressures. Low water velocities through heat exchanger tubes, on the other hand, could result in the formation of sludge. Therefore, too small a pump (insufficient safety margins) may cause inefficient operation of heat exchangers and coolers for much of the time. Safety Margins To provide reliable performance under seagoing conditions, a raw water cooling pump must operate in a correctly designed cooling system. It is, therefore, essential that cooling water should reach the pump without having to overcome undesirable resistance or restrictions. The following general rules are given for guidance: • Suction pipe bore must not be smaller than the pump inlet connection, but if total suction pipe exceeds 10' (3m) in length, the bore should be one size larger, particularly if the pump is operated at high speeds. Use the same size bore pipe throughout, i.e., avoid sudden enlargements or contractions. Use long tapered sections of pipe at any change in pipe bore. • The suction pipe run should be as straight as possible, i.e., avoid unnecessary bends. Do not use square or standard elbows, instead use long sweep bends. • Do not fit gearbox or engine oil coolers in the pump suction system. Always install after the pump. • Seacocks of the same nominal size as the suction pipe work should be of the ball or plug type, giving full through bore in the open position. The handle position should clearly indicate whether seacocks are open or closed. • Seawater inlet strainers should have a hole or mesh size of at least 1/8" (3mm) diameter and up to 3/16" (5mm) for larger pumps, but smaller than the heat exchanger tube bore. • Check frequently that inlet strainer is not clogged. If in doubt clean thoroughly. • Fast boats (over 12-15 knots) and planing craft must be fitted with inlet scoops positioned in a permanently wetted area of the ship’s hull to create sufficient inlet pressure at higher boat speeds. Flush inlet fittings are not suitable for fast boats. Suction System 97 11 Flexible Impeller Pumps Introduction