This page contains a Flash digital edition of a book.
Even Dryer Bilges We wanted to follow up last month’s “Keeping the


Bilge Dry” article with more bilge pumps. That article was about submersible bilge pumps and this one is about diaphragm and impeller pumps, known as positive displacement pumps, PDPs. (PDPs have an expanding chamber on the intake side and a decreasing chamber on the discharge side so they force the water through).


By Jack and Alex Wilken First thing though: Why not just


use a submersible bilge pump? Answer*: Price, output, quietness,


and ease of installation seem to favor the submersible. However, there are two clear advantages for PDPs 1. They are able to pull the last bit


of water out of your bilge (Driest bilge!) This is possible because the pump is mounted up out of the bilge and the pickup is cup-shaped with screen covering the open/down side.


16


14 12 10 8 6 4 2 0


Head in feet


Impeller pump Debris: Some of these pumps


handle debris better than others. In order of size from big pieces to small: we have the single diaphragm, impeller and multi-chamber diaphragm pumps.


Installation: It is recommended


that with a multi-chamber diaphragm pump you install a filter in addition to the screen on the pick up. As the smaller chambers and valves clog easily, the filter goes between the pick


Multi Chamber Diaphragm must have protection from debris


the impeller pump. Because the PDPs are to be mounted above the maximum bilge water level – this is the level above which electrical or mechanical systems would be adversely affected by bilge water – you should have a shorter wire run to the terminal block. In all cases heat-shrink crimp-on electrical fittings should be used. Mounting the pick-up: This is


usually simple because there are both straight and right angle models. These


is recommended. The vacuum switch deals with switch malfunctions or a blocked pickup. This is important as at least one manufacturer states that their impeller pumps should not run dry for more than 30 seconds. (See drawing ‘Impeller Pump Vacuum Switch.) The single diaphragm is okay with just the screen and will not be damaged by running dry. The flow rates of all PD pumps are not effected by head (back pressure) in the same high percentages as with submersible pumps. (See chart ‘Effect of pressure/head on bilge pumps’.)


Comparison of Systems: The


water level or bilge pump switch and wiring system is the same for PDP and submersible pumps with the exception of the addition of the vacuum switch for


Multi Chamber Diaphragm Pump


Inline Filter


Submersible pump


0 2 4 6 8 10 12 14 16 18 20 USGPM The effect head or back pressure on bilge pumps 2. They are able to pump against


pressure. This allows the use of a petroleum products filter. (Best for a ‘Green’ solution, see chart of Effect of Head Pressure on Bilge Pumps.)


48° NORTH, FEBRUARY 2011 PAGE 46


up and the pump intake. (See drawing ‘Multi Chamber Diaphragm with inline filter.) For the impeller pump the screen should suffice, but a vacuum switch on the intake side of the pump


Vacuum Switch


Impeller Pump


Impeller Pumps are damaged if they run dry


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82