Jabsco Marine Pumps and How They Work
Jabsco Boat Pumps fall into three broad categories: variable-volume impeller pumps; centrifugal pumps; and positive-displacement pumps.
Variable-volume impeller pumps.
These work on the principle of a change in the displaced volume of the impeller from one side of a pump chamber to another. The most common variety has a flexible impeller turning in a pump body that has one side flattened by screwing a blanking piece (a cam) into the body. As the vanes on the impeller reach the cam, they are squeezed down, expelling any fluid trapped between them. As the vanes pass the cam, they spring back up, drawing in more fluid.
Less common variations on the same theme are vane and rotary pumps. On vane pumps, the drive shaft is permanently offset in the pump housing such that the impeller (rotor) is always closer to the pump body at some points than at others. Set in slots in the rotor are hard vanes, which are held against the pump body. As the rotor spins, the vanes move in and out of their slots, thus maintaining contact with the pump body. The changing volume between the vanes alternately draws in and expels fluid in much the same manner as the vane action in a flexible impeller pump.
In rotary pumps the impeller does not actually rotate! Instead it is mounted on, but not keyed to, an eccentric shaft. The shaft is centered in the pump housing but with an offset section where it passes through the impeller bearing. The impeller is kept from spinning by an impeller guide held in the pump body and fitting in a slot in the impeller. As the shaft turns, the impeller oscillates in a circular pattern but without actually turning, pulling in and expelling fluid as it goes.
Finally we have gear pumps in which fluid is trapped between the teeth of two gear wheels and then driven out at the point where the gear teeth mesh. Gear pumps are used primarily as oil pumps on engines and hydraulic systems; they are capable of pumping to very high pressures.
Centrifugal Pumps.
Centrifugal Pumps have an impeller with vanes designed in such a way that fluid drawn into the center of the impeller is thrown out by centrifugal force. The momentum generated in the fluid keeps it moving and is what makes the pump work. A less common variation on the same principle is a turbine pump, which has a somewhat different arrangement of the vanes on the impeller but is otherwise the same.
Positive-displacement pumps.
This type moves a diaphragm, bellows, or piston in and out of a pump chamber, alternately increasing and decreasing its volume. These pumps, unlike the others, can work only with a set of valves-one valve lets the fluid into the chamber and traps it; another lets it out and stops it from being sucked back in. Since there is no impeller, there is no shaft entering the pump chamber and therefore no shaft seal and associated leaks.
When choosing which Jabsco pumps in Essex.
Essex is surrounded by water, both salt sea and river water. Shallows and mud flats are present in both. Choice of pump can be critical in challenging environments.
Pumps Pros and Cons...
The most important factors in terms of choosing a pump for a given application are its ability to self-prime, the effect of head pressure on its performance, and its tolerance for solids, chemicals, and running dry. The duty rating of electric pumps must also be taken into consideration.
Self-priming or not. All pumps, excluding the centrifugal ones, are self-priming-in other words, they have the capability to draw fluid up to themselves. Centrifugal pumps have no selfpriming capability whatsoever and must always be installed below the liquid level they are to pump, at all angles of heel. (Note: The addition of a check valve in the suction piping to a centrifugal pump will retain fluid in the pump when it is shut down, thus maintaining its prime. Installed like this, a pump can be placed above the liquid it is pumping, but should the check valve leak back when the pump is shut down, the pump will lose its prime and will have to be hand-primed in some manner-generally by disconnecting the discharge line at some point above the pump and pouring water into the line.)
The effect of head pressure. Head pressure refers to the resistance to flow that a pump must overcome.
Variable-volume impeller pumps.
These work on the principle of a change in the displaced volume of the impeller from one side of a pump chamber to another. The most common variety has a flexible impeller turning in a pump body that has one side flattened by screwing a blanking piece (a cam) into the body. As the vanes on the impeller reach the cam, they are squeezed down, expelling any fluid trapped between them. As the vanes pass the cam, they spring back up, drawing in more fluid.
Less common variations on the same theme are vane and rotary pumps. On vane pumps, the drive shaft is permanently offset in the pump housing such that the impeller (rotor) is always closer to the pump body at some points than at others. Set in slots in the rotor are hard vanes, which are held against the pump body. As the rotor spins, the vanes move in and out of their slots, thus maintaining contact with the pump body. The changing volume between the vanes alternately draws in and expels fluid in much the same manner as the vane action in a flexible impeller pump.
In rotary pumps the impeller does not actually rotate! Instead it is mounted on, but not keyed to, an eccentric shaft. The shaft is centered in the pump housing but with an offset section where it passes through the impeller bearing. The impeller is kept from spinning by an impeller guide held in the pump body and fitting in a slot in the impeller. As the shaft turns, the impeller oscillates in a circular pattern but without actually turning, pulling in and expelling fluid as it goes.
Finally we have gear pumps in which fluid is trapped between the teeth of two gear wheels and then driven out at the point where the gear teeth mesh. Gear pumps are used primarily as oil pumps on engines and hydraulic systems; they are capable of pumping to very high pressures.
Centrifugal Pumps.
Centrifugal Pumps have an impeller with vanes designed in such a way that fluid drawn into the center of the impeller is thrown out by centrifugal force. The momentum generated in the fluid keeps it moving and is what makes the pump work. A less common variation on the same principle is a turbine pump, which has a somewhat different arrangement of the vanes on the impeller but is otherwise the same.
Positive-displacement pumps.
This type moves a diaphragm, bellows, or piston in and out of a pump chamber, alternately increasing and decreasing its volume. These pumps, unlike the others, can work only with a set of valves-one valve lets the fluid into the chamber and traps it; another lets it out and stops it from being sucked back in. Since there is no impeller, there is no shaft entering the pump chamber and therefore no shaft seal and associated leaks.
When choosing which Jabsco pumps in Essex.
Essex is surrounded by water, both salt sea and river water. Shallows and mud flats are present in both. Choice of pump can be critical in challenging environments.
Pumps Pros and Cons...
The most important factors in terms of choosing a pump for a given application are its ability to self-prime, the effect of head pressure on its performance, and its tolerance for solids, chemicals, and running dry. The duty rating of electric pumps must also be taken into consideration.
Self-priming or not. All pumps, excluding the centrifugal ones, are self-priming-in other words, they have the capability to draw fluid up to themselves. Centrifugal pumps have no selfpriming capability whatsoever and must always be installed below the liquid level they are to pump, at all angles of heel. (Note: The addition of a check valve in the suction piping to a centrifugal pump will retain fluid in the pump when it is shut down, thus maintaining its prime. Installed like this, a pump can be placed above the liquid it is pumping, but should the check valve leak back when the pump is shut down, the pump will lose its prime and will have to be hand-primed in some manner-generally by disconnecting the discharge line at some point above the pump and pouring water into the line.)
The effect of head pressure. Head pressure refers to the resistance to flow that a pump must overcome.
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