Resist Using Check Valves With Bilge Pumps
Bilge pump fundamentals
Water enters inside the hull of every boat at some rate. Eventually all boats will sink without expelling this water, some much more slowly than others. Water runs in through various topside openings when it rains or from sea spray while underway. Water often drips or runs from stuffing boxes where propshafts go through the hull. Wooden boats may leak slightly as their hulls respond to changing stresses. Water gets into the boat when cleaning strainers and replacing seawater hoses. Bilge pumps are designed to move such incidental water back overboard.
A fairly small hole in the bottom of the hull, say 3/4 inch diameter, will let water in much faster than most bilge pumps can get rid of it. The failure of a seawater hose, valve or fitting can sink a boat within minutes or hours. The bilge pumping system is not designed to dewater a boat under these conditions. Water coming in through leaks is not under much pressure, but it is persistent and adds up over time. Water exerts a half-pound of force per square inch for every foot below the surface. Four feet down the water is at 2 psi over atmospheric pressure.
Most modern boats employ automatic electric bilge pumps to keep the bilge dry. The most popular and least expensive pumps are centrifugal pumps with sealed motors. A centrifugal pump will not "suck" water up to it. The pump must be mounted so that it's impeller is well below the water level. When it is not running a centrifugal pump offers no resistance to water flowing down the hose and into the bilge. A discharge through-hull fitting must be mounted well above the exterior waterline so that the boat will not begin to flood at any state of pitch or roll. As soon as the pump starts, the water level will begin to drop in the bilge as the outlet hose fills rapidly with water. If the pump is not installed far enough below the level of the bilge water, it will begin to suck air as the water level drops.
An automatic pump must be mounted so it is lower than the lowest level of the water.
Positive displacement pumps can be mounted outside the bilge and will self-prime, but they are generally 5 to 10 times more expensive than centrifugal pumps.
As the pump discharge reaches the discharge fitting, the pumping rate will slow because it is opposed by the weight of water on the hose, also known as the 'head."
The rate also slows due to the resistance of the water flowing through plumbing fittings and friction against the walls of the hose.
Pumps are typically rated with no hose friction or head pressure losses and at full battery voltage. A 500 gallon per hour pump may actually pump less than half that rate as installed.
They become upset when their bilge pump stops and the water in the hose to the outside rushes back to the bilge, raising the water level.
People tend to hate water in their bilges.
I hate check valves and here are four reasons why. The pump may have some air in it when it shuts off and the check valve closes. The pump can become air locked the next time you want it to run. "Air lock" means that there is air trapped in the pump below bilge water level keeping the impeller from getting traction on the water. Centrifugal pumps are not self priming, meaning they must have water at the impeller to start pumping. An air lock can last long enough to sink your boat (presumably any way), and it does not develop every time. One solution is to drill a small hole in the hose below the check valve. This will eliminate the airlock until the hole gets plugged up with lint. In the mean time, it will also squirt a thin stream of water across your bilge which can be unnerving the first time you see it. The new solid-state augmented float switches extend the pump run time after they actually trip off. This gets the pump trying to suck air at the end of it's cycle and increases the possibility of airlock with a check valve in the system.
The second problem with check valves is that you can trap so much water weight above the valve with a high head that the pump can't open the check valve from a standing start. Centrifugal pumps do not like to be stalled out. With the inevitable air cushion below the check valve, the pump sees a gradually increasing back pressure culminating in complete blockage, The check valve can stick shut a little too. The pump may not develop enough static pressure to open the check valve. The inertia of the water moving through the discharge pipe aids the pumping action, if you think about it. An operating pump is a dynamic system with water moving through it. If you stop the water from flowing, you have changed the system conditions enough that water may not always start flowing again. Bilge pumps are puny little things and do not always tolerate unusual conditions.
The third problem with check valves is they leak. The leakage increases as they age. This means that the check valve you installed to stop your little oscillating pump system now just oscillates with a longer period as the water drains back to the bilge at a slower rate. If the pump only runs every 15 minutes this may be tolerable.
The fourth problem with check valves is that they inhibit a good thing. All that water rushing back down the discharge pipe backflushes the debris screen(s) at the intake to the pump. Mostly these screens do not get clogged and this is why.
6/13/08