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ContinuousWave: Small Boat Electrical
Designing Automatic Pump Control Systems
|Author||Topic: Designing Automatic Pump Control Systems|
posted 06-28-2008 10:19 AM ET (US)
In small boats a very common installation is a simple system of a pump to clear water from a sump which is controlled by a sensor of some kind that detects the presence of water. In many cases the sensor will be a float which located in the sump compartment and actuated by the water level in the compartment. While this may seem at first glance like a very simple system, there is a subtlety in its design which must be given careful consideration.
The intent of the control system is to actuate the pump when water is present in the sump, and to deactivate the pump when water is removed from the sump. That sounds simple, but there is a catch. Most pumps cannot remove every last drop of water in a sump, and no matter how long the pump runs, some water will remain in the sump. To minimize the amount of water, the pump should be mounted as deeply in the sump as possible. It should be at the lowest point of the sump. The physical constraints of the sump area and the pump will dictate how successfully the pump can be located.
After the pump has been installed in the deepest location possible, add some water to the sump. Then run the pump and expel as much water as possible. It is very likely some water will remain. The amount of water remaining is very important in determining the next step: location of the float switch.
The float switch has to be located so that it is above the water level that will remain in the sump even with the pump running. And the float switch has to be located so that it will return to the OFF condition even with this water present. It is extremely important that the float switch be properly position. It must be able to cut off the pump even though there is water remaining in the compartment.
If the float switch is not properly located, there are two possibilities: it is too high or too low. If the float switch is located too high, then it will cut off the pump before the sump as been cleared of as much water as possible. This leaves a little more water in the sump than the absolute minimum if the pump ran until it was dry. If the float switch is located too low, the pump will never be able to lower the water level enough to cut off the float switch. You will have a system in which the float switch triggers the pump to run, and once running the pump stays on forever (or until the battery dies). Of the two possible outcomes, leaving a bit of extra water in the sump is much preferred over having a system that only turns ON and never turns OFF.
To insure that the float switch position is proper, establish the water level in the sump which will remain when the pump has run dry. Position the float switch above this level so that its float reaches the OFF position at this level of water in the sump. Also, take into account possible variations in side-to-side trim on the boat. If the float switch is located farther off keel centerline than the pump, a shift in boat trim could change the water level in the compartment in such a way that the pump runs dry before the float switch reaches OFF.
Once you have selected a float switch location, test your system to check that it is working as intended and that the pump stops after it runs dry. You will have contructed a successful automatic pump control system.
posted 06-28-2008 10:36 AM ET (US)
It is often seen in a sump pump that while running it maintains water in the exhaust side of the pump that has not been expelled from the system, and, when the pump stops, the water trapped above the pump flows by gravity back into the sump. This action creates a situation where the water level in the sump will be slightly higher after the pump cuts off than when the pump is running dry. It is important that the float switch not be positioned so that it is activated by this change in water level.
If the float switch is placed too close to the minimum water level, it can be cut off by the water level when the pump is running dry, but, when the water rises slightly after the pump starts, the float switch can be triggered ON again. If this occurs the system will become unstable and will oscillate ON and OFF forever.
In a control system where the feedback is too tightly coupled to the active element, you often get this type of oscillation. The cure is to create some hysteresis or dead band in the control loop.
Most mechanical float switches inherently have hysteresis or dead band in their action. This usually comes about due to the mechanical design of the switch itself. Generally there is a movement of weight in the float itself which changes the buoyant characteristic of the float. When the float is OFF, it usually becomes less buoyant than when it is ON. In this way, the switch creates hysteresis in the control loop, avoiding a tendency for the system to oscillate.
posted 06-28-2008 10:45 AM ET (US)
There are lift check valves available to eliminate the problem of back flow from the discharge line. These are most useful when long runs are used in deeper draft boats but may also be useful for some Boston Whaler boats.
posted 06-28-2008 10:57 AM ET (US)
In a small boat the distance from the pump to the overboard water outlet ("the head on the pump") is usually not very great. Typically the head on the pump will be only about 18-inches at the most. The volume of water which will be trapped above the pump exhaust is determined by this distance and by the diameter of the pump exhaust hose. When the pump stops running, most of this water runs back into the sump.
The design of the sump, its shape and area, determine how much of a change in water level occurs when a particular volume of water is added. If the sump is large in volume and relatively flat, little change in level occurs, but if the sump is small and has a tapered shape, a relatively significant change in water level can occur from a relatively small volume of water added.
In order to keep the control system stable, the hysteresis or dead band in the control loop has to be greater than the variation in water level which will occur when the pump transitions to OFF from active pumping. If a system is installed where there is a lot of head on the pump, and hence a larger volume of water, it could be possible for the change in water level to be greater than the hysteresis in the float switch. A check valve in the system may be necessary.
Some pumps control themselves by various mechanisms. In some systems the pump operates continually at a fixed interval, and runs until it clears water from its intake. In these systems, a dead band in the control system is introduced by forcing a minimum ON-Delay. Typically once the pump has run and shut off it enforces on itself a certain minimum delay before it will run again from automatic command. Since the pump is unlikely to have cleared every last drop of water from the sump, and since there is likely to be some water trapped above the pump which returns to the sump, there is a good chance that when the OFF-delay expires, the pump will find water present. This triggers the pump to run briefly, until it clears its intake. A system such as this appears to be in constant oscillation, but with a time delay interval forced onto it.
It seems like a good idea to actually check the operation of any self-controlled pump to make sure that it does not fall into such an oscillation in its control and operation. It is inevitable that water will get into a sump, and having a completely dry sump should not be relied upon as the only condition which can break the oscillation.
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