|From Ocean Navigator #58 |
The most common kind of bilge pump is a submersible centrifugal pump. These will sometimes flush surprisingly large objects through the pump, but should anything become jammed between the impeller and its housing, the pump will be stopped dead. If the pump is not properly fused it will probably burn up. More insidiously, hairs and other fibrous matter wrapped around the center of the impeller will slow the pump down and seriously impair performance. On the plus side, if water flow to the pump is either blocked off, or the pump remains on once the bilge is dry, the pump will probably be undamaged.
Less common in bilge pumping applications are various kinds of electric diaphragm pumps and flexible impeller pumps. A diaphragm pump has inlet and exhaust valves which have a limited ability to pass solids, but are generally very little affected by hair. As a general rule of thumb, the larger the pump, the larger its valves, and the larger the solids it will pass. If the pump is confronted with something it can’t handle, the valves will jam in the open position at which point the pump will stop moving fluid but the pump motor will not be damaged. Diaphragm pumps can run dry indefinitely.
A flexible impeller pump also has a limited ability to pass solids; once again, the larger pumps can handle larger solids. If the impeller is confronted with something it can’t pass, the vanes are generally ripped off the impeller hub. If the pump runs dry, the vanes will fail after a relatively short period of time. Flexible impeller pumps are most useful when connected to an electromagnetic, engine-driven clutch. This clutch can be energized either by a manual switch—it should be the spring-loaded type that has to be held on so that the pump is not accidentally left to run dry—or an automatic level switch. Mounted like this these pumps will move high volumes of fluids; larger pumps are excellent in damage-control applications.
Clearly, all three types of pump require some kind of a strainer (also called a strum box) on the inlet, but should the strainer plug up, it will render the pump inoperative, sometimes when it is most needed. In the case of a rubber impeller pump, a plugged strainer may cause the impeller to self-destruct. Consequently, in choosing a strainer, two factors are important: the size of the holes and the overall size of the strainer. Both need to be as large as possible, with the hole size limited by the maximum particle size the pump can be expected to pass, and the overall strainer size limited by what is practical in a given application—the greater the overall surface area, the less likely the strainer is to get plugged.
Submersible pumps almost always have the strainer built into the base, so the user has no decisions to make about adding one. All other pumps, however, will require an add-on filter. A relatively fine-mesh strainer is generally recommended for the smaller diaphragm pumps (this makes this type of pump suspect in a bilge pump applications). Larger diaphragm pumps and flexible impeller pumps rarely seem to have trouble when used with mesh sizes from an eighth of an inch to a quarter of an inch. The best approach, however, is to check with the pump manufacturer.
If the bilge is to be sucked dry, the strum box will have to be designed so that the strainer surface is all on the lower side, close to the bottom of the bilge. In this case, there needs to be some provision for rapid removal of the strum box so that it can be cleaned quickly when necessary. If a few inches of water in the bilge are tolerable, the strum box can take the form of a perforated pipe, which will maximize the strainer area for a given size of filter, reducing the chances of clogging, and will also make cleaning easier.The switching maze
Float switches: A mechanism is required to turn a pump on and off. There is much to be said for making this a manual switch, since it forces one to regularly check the state of the bilge, but, in practice, the boating public demands an automatic switch that is generally backed-up with a manual override (this override is, in any case, a requirement of the American Boat and Yacht Council standards for small craft).
By far the most common switch is a float switch, but, in addition, there are switches operated by air pressure, and at least three forms of electronic switch. The variety of switches is testimony to the fact that no one type has been found to be universally satisfactory in bilge-pumping applications. It is instructive to see why this is the case.
The concept of a float switch is simple. A rising water level lifts a hinged float which is used to turn on a switch. A falling level drops the float, turning off the switch. Many of these switches have a sealed vial in the float that contains a drop of mercury (a highly conductive liquid metal). When the float rises, the mercury runs to one end of the vial, closing a circuit between two terminals and switching on the pump. When the float drops, the mercury runs the other way, opening the circuit.
Mercury switches can develop problems if the drop of mercury breaks up, reducing its current-carrying capability. The switch can then heat up, melting the insulation on nearby wiring, which, in turn, may cause shorts through the bilge water. In any case, this heating will cause the insulation to go hard, reducing its flexibility. So instead of mercury, some switches use a rolling ball which mechanically triggers the switch (the ball itself is not part of the electrical circuit).
The problem with any switch in which the wires are carried into the float is that the wires constantly flex or twist as the float moves up and down, with an obvious risk of eventual failure, especially if the insulation has become hard. A way around this is to mount the switch in the fixed part of the unit with a sealed diaphragm over the switch. As the hinged float moves up and down with changing fluid levels, a lever on its base operates the switch. The wires themselves remain static.
The location of a switch in relation to its pump is important. Should a switch be mounted in a position where it is offset from its pump, when the boat is heeled on one tack, the switch will not respond until considerably more water is in the bilge than is needed for a level-state response. When the boat is on the other tack, there is a very real danger that the switch will stay energized after the pump has sucked itself dry, causing the pump to run continuously, creating a heavy drain on the batteries and a risk of pump failure. A float switch should always be mounted as close to its pump as possible and in the same fore and aft plane. In practice, many are simply clipped into the base of the pump, but in this case it is essential to ensure that the switch is aligned fore-and-aft: if it is set to one side of the pump, the pump will definitely run dry on one or other tack.
The very nature of a float switch is such that the change in water level between switching on and turning off is only an inch or two. Even when a switch is mounted on the same fore-and-aft line as its pump, with a boat pounding into a head sea, or rolling from side to side, the action of a small amount of water surging backwards and forwards in the bilge can flick the switch on and off until eventually something fails (the wiring; the switch points or motor; or the hinge on the switch). This situation also creates an unnecessary drain on the batteries.
From a mechanical point of view, the hinge on these switches is vulnerable. As we, and many others, have found, a relatively small piece of trash caught in the hinge can jam the switch in either the off or on position. Some switches include a cover to keep debris away from the switch, but since the cover must have holes to allow the free-flow of water, this can only be a partial solution to the problem.
Air and electronic switches: These sorts of difficulties led to the development of other types of switches. An air switch has a bell housing which is installed with the open end facing down. A length of tubing connects the top of the housing to a diaphragm within a separate switch housing. Rising water in the bilge first closes off the bottom of the bell housing, trapping the air within it, and then raises the air pressure within the housing. This in turn moves the diaphragm to operate the switch (this is the same mechanism that is used for controlling the level on a washing machine).
The clear advantages of such a system are that there is no electrical wiring in the bilge, and there are no moving parts in the bilge to jam or break. But there are drawbacks. Once again, the range between on and off is quite narrow (less than two inches) and the unit must be installed on the same fore-and-aft axis as the pump. In turbulent conditions, water sloshing around the bilge can displace air in the bell-housing with the result that the switch is not activated until the water level is higher than normal. If the tubing from the bell housing to the switch has any dips at any angle of heel, water can gather at the low spot, effectively blocking the action of the switch. And if the tubing gets kinked or crushed or holed the switch will fail to operate.
An electronic switch avoids many of these problems but introduces new ones. The traditional type has two stainless steel sensors mounted a small distance apart. If the water level rises to the level of the sensors, it forms a circuit between them which activates the bilge pump switch. When the water level falls below the sensors, the circuit is broken, opening the switch. This seems straightforward enough, but in real life the conductivity of the liquids the sensors are likely to encounter varies widely, which causes difficulties. Fresh water, particularly clean rain water, has a very low conductivity; whereas salt water, especially dirty salt water, is highly conductive; oil is non-conductive; while some bilge cleaners are not only quite conductive but will also form a slimy film across the sensors.
If the sensor circuitry is set to react to fresh water, the switch may stay permanently on when coated with bilge cleaner. On the other hand, if the sensitivity is reduced, a coating of oil may add enough resistance to cause the switch to stay permanently off. A Teflon-coating on some sensors helps to reduce the extent to which various contaminants can adhere to the sensors, improving reliability. Even so, a balancing act still needs to be struck between sensitivity settings. If the sensors are not kept reasonably clean, the switch may malfunction.
Instead of measuring conductivity, some newer electronic sensors use a beam of light to detect the presence of fluids. In a typical unit the sensor emits a short pulse of light at timed intervals (for example, every 30 seconds). In the absence of fluid, the light is reflected back by a special lens and the system remains quiescent. When the lens is covered by fluid, the light is refracted through the fluid, which triggers the control unit into activating the pump. These sensors will work in just about any fluid, including heavily contaminated oil. Depending on the complexity of a system, multiple sensors can be used to operate more than one pump, or to sound a high-bilge level alarm, and so on. Stop useless cycling
Assuming functioning electronic sensors, it is no good having the pump come on the second the sensors detect fluid. For one thing, the level would immediately drop, turning the pump off, causing the pump to cycle frequently at short intervals. And, any surging of the water in the bilge would cause the switch to repeatedly flick on and off. Some sort of a built-in delay is needed.
This delay can be achieved with sensors at different heights—the higher sensor activating the circuit and the lower one breaking it. The other way is to program the unit such that a single sensor must be continuously immersed for a certain length of time (e.g., 12 seconds) before the switch is activated, with the switch remaining activated for a certain length of time after the sensor is no longer immersed.
These kinds of switches track fluid levels through conductivity or refraction. In recent months, we have seen the introduction of another kind of electronic switch that tracks fluid levels through the current draw of the bilge pump motor. The sensing unit and switch are built into the pump itself. There is a timer which turns the pump on at pre-set intervals (every two and a half minutes in the case of one popular unit). If there is water in the bilge the pump will have to work harder than if there is no water, in which case its current draw will be higher than in the no-load situation. The sensing unit measures the current draw to determine whether to keep the pump running or not. Once it senses a no-load draw, it shuts the pump down.
These automatic bilge pumps have certain obvious advantages. Since the sensing unit and switch are built into the pump itself, there is no need for any external switching device which makes installation simpler and immediately solves all problems with offset switches and fluctuating (surging) water levels. The principal disadvantage is that the pump is cycling on and off repeatedly whether there is a leak into the bilge or not (every two-and-a-half minutes is 24 times an hour, 576 times a day, 4,032 times a week, and 17,472 times a month). This is bound to accelerate pump and switch wear, and will impose an unnecessary drain on the batteries. Granted, the draw is a small one: A 1,100 gallons per hour (GPH), 12-volt pump typically consumes 0.25 amp-hours per day. A less obvious problem, but perhaps more serious, is that if the pump impeller develops an increased resistance to movement (such as might happen when hair becomes wrapped around the impeller or some other obstruction) the sensor will "interpret" this as water in the bilge and the pump will run constantly. That won’t take long to flatten a battery. Plumbing and electrical matters
Other than pumps and switches, there are a few more key aspects that need to be considered in any bilge pump installation. The first of these is plumbing.
A bilge pump is, by definition, installed below the waterline. If its discharge line should also be submerged at any time, there is the potential for water to siphon into a boat. The discharge side of a bilge pump must exit the hull above the waterline at all angles of heel. If this is not possible (for example, on low-freeboard, tender vessels), the discharge line from the pump must be looped high enough to put it above the waterline at all angles of heel and fitted with a vented loop (siphon break) at the high point.
Any measures taken to prevent siphoning will unfortunately create "head pressure" or “back pressure” which the pump must overcome. This head pressure has several components, including the total height the bilge water must be lifted, resistance created by the overall length of the discharge line, its internal diameter, restrictions to flow caused by bends, and the surface texture of the hose, particularly corrugated hose (see accompanying article for a thorough discussion of head pressure). Since most pumps are rated in an open flow environment, which is to say there is no restriction on the suction side and no hoses attached to the discharge side, the minute the pump is placed in a real life situation it will suffer a loss of capacity.
Centrifugal pumps are particularly sensitive to head pressure. The Rule company has provided a dramatic illustration of this at recent boat shows by placing one of their pumps and a comparably-rated Attwood pump in a tank of water, with the pumps discharging into an eight-foot-high pipe. The Rule pump, which is rated at 360 GPH, moves less than 200 gallons, while the Attwood pump is only capable of lifting the water around four feet and so is stalled out entirely. For bilge pumping applications on small boats, a pump should be rated at a head pressure of at least three feet, and for larger boats, eight feet.
The discharge hose from a centrifugal pump must maintain a steady rise, at all angles of heel, up to its thru-hull or vented loop. Otherwise, when the pump stops running after pumping the bilge, although most of the line to the pump will drain down, the low spots will trap water. When the pump restarts, the trapped water can act as a plug, air-locking the pump, effectively stalling it out.
Suction hoses (needed with diaphragm and flexible impeller pumps) must be carefully routed to avoid kinks and damage. It has to be assumed that at some point the filter will become completely blocked, throwing the full suction capability of the pump onto the hose, which must be able to withstand the vacuum created without collapsing. Hose connections must be airtight and should be made with all-stainless-steel hose clamps (this includes the screws—look for the designation 300 SS, 316 SS, or AWAB, or else test with a magnet, checking for non-magnetism).
Irrespective of how clean a bilge is kept, it is advantageous to mount a bilge pump or its strum box on a small pedestal. Heavier sediments will settle out below the level of the pump or filter, and then can be periodically cleaned out manually. In addition, whenever a centrifugal pump is shut down, the water column in the discharge hose returns to the bilge, back-flushing the system. With the pedestal, contaminants will be washed clear of the pump. (Note that in situations with external level switches, long discharge hoses, and small volume bilges, the residual water in the hoses may raise the bilge level enough to kick the pump back on, causing it to cycle repeatedly. In this case, some form of a check valve will be needed in the discharge line, but this is not particularly desirable since these valves are prone to sticking in the open position and to getting plugged. Any such valve will need to be accessible for maintenance.)
On the electrical side, the single most important thing is to use the correct size of wiring. Very often, especially on larger boats, there are long cable runs to and from a bilge pump. For a given current flow, the longer a cable run, the larger the cable needs to be. Undersized cables cause voltage drop, which impairs pump performance, shortens pump life, and, in extreme cases, becomes a fire hazard. All connections must be made with marine-grade terminals set well above any potential water level and preferably sealed with glue-type heat-shrink tubing.
If an external switch is used, it will be in series with the pump, which is to say the full pump current will be flowing through the switch and its wiring. Many of the cheaper switches have a current rating which is barely adequate for normal pump operation and will not handle continuous overloads such as those that occur when an impeller is partially obstructed. The switch points or wiring are likely to burn out. There is no substitute on such a critical piece of equipment as a bilge pump for a properly-sized (and preferably an over-sized) switch.
Finally, the bilge pump must be properly fused in accordance with the manufacturers recommendation. Aside from the usual kinds of problems against which protection is needed (accidental shorts, etc.), there is always the possibility of a bilge pump becoming jammed with debris which, in the case of a centrifugal pump, will produce a locked rotor state in the electric motor. In such circumstances the current draw of the motor increases sharply. Without a fuse, the pump will burn up or, more seriously, the pump housing or wiring (maybe the switch wiring) will melt down. Keeping dry below decks
At this point it would be nice to tie things together by laying down some prescriptions concerning what pumps and what switches to use in what applications. But the truth is, properly installed, they all work in most situations. The keys to remember are to size the wiring and switches for the maximum current rating of the pump, to properly fuse the circuit, to keep all electrical connections above the highest water level, to position external switches so that they work properly at all conceivable angles of heel, to make sure the pump discharge is siphon-proof, to keep all hoses as short and straight as possible, to use the largest diameter hose feasible on the discharge side, and to supply a filter on the suction side.
Beyond this, it is important to understand the potential weaknesses of a given installation (so that one does not become over-reliant on it) and to stress fundamental aspects of housekeeping and seamanship. The bilge must be kept clean, objects properly stowed, and, above all, it should be a part of the ship’s routine to check the bilge on every watch.
Contributing editor Nigel Calder is the author of several books, including Boatowner’s Mechanical and Electrical Manual, published by International Marine.