I was thinking of relocating my fresh water pump [not engine] and was wondering whether it makes any difference whether they are located closer to source [where they suck from] or to the destination [where they push to]. As a practical matter should it be closer and closer to the level of the tank, or on the level of the piping the leads to the sink and head? I guess what I'm asking is whether they are better suckers or pushers. [And yes, I am deliberately being Patterson's straight man but, hey, its a dirty job that someone's gotta do]
Pump Theory
- Thread starter wufibugs
- Start date
In general it is best to avoid restricting the suction side of pumps rather than the discharge side.
That is because the only thing forcing liquids into the pump is typically atmospheric pressure (14.7 PSI at sea level) plus/minus the height of the pump below/above the level of the liquid being pumped.
On the discharge side the liquid is being pushed at the pump discharge pressure which may be 40 to 50 PSI for a domestic water ump.
That is because the only thing forcing liquids into the pump is typically atmospheric pressure (14.7 PSI at sea level) plus/minus the height of the pump below/above the level of the liquid being pumped.
On the discharge side the liquid is being pushed at the pump discharge pressure which may be 40 to 50 PSI for a domestic water ump.
just to clarify..... the 14.7 psi of "atmospheric" pressure on the inlet side is "ambient pressure" and is completely balanced out of the equation by the same atmospheric pressure on the outlet side...
in this case, the only thing that would "force" water into the pump would be "head" pressure on the inlet side... which would only be there if the pump were mounted below the tank.... or more correctly, below the surface level of the water in the tank.
but you are correct that any measurable amount restriction on the inlet side of the pump will more than double the loss of output as a result of the restriction.
and a long suction line has more chances of collapse due to the suction/vacuum on the line than does a pressure line, in which the pressure keeps it pushed open so there can be no collapse.... although it can still plug up with debris that either, is in the system or that passes thru the pump.
Put your pump as close to the level of the bottom of your supply tank(s) as possible so that it can self-prime. Select a location where it can be accessed and maintained. Pipe it to an accumulator to reduce cycling, pulsation and improve flow.
"in this case, the only thing that would "force" water into the pump would be "head" pressure on the inlet side... which would only be there if the pump were mounted below the tank.... or more correctly, below the surface level of the water in the tank"
If that were true no pump e.g. the raw water pump or my swimming pool pump or the pump out at the dock could work - all are above the surface level of the water and rely on atmospheric pressure to push the liquid as far as the suction side of the pump.
If that were true no pump e.g. the raw water pump or my swimming pool pump or the pump out at the dock could work - all are above the surface level of the water and rely on atmospheric pressure to push the liquid as far as the suction side of the pump.
technically, your theory can be easily argued....and if you think the atmospheric pressure between the inlet and outlet are different, you are soundly mistaken.
it may be atmospheric pressure doing the pushing, but ONLY because the suction of the pump is making an offset in the pressure differential between the inlet and the outlet... this is called a vacuum. a vacuum is an acute lack of atmospheric pressure, that is only created momentarily by the pump
some pumps CANNOT create a suction, until AFTER water hits the impeller, and you are correct that they cannot work if they are above the water level, because if the pump does not CREATE a vacuum, there will be no atmospheric pressure differential to bring the water in.... so in this case, its the design of the pump that creates a vacuum and sucks the water up to the impeller... after that, the air is out of the system and its a matter of displacement that makes the system work as it does.
atmospheric pressure remains constant, but the vacuum change is the driving force here.
"technically, your theory can be easily argued"
Aint my theory - it is a fundamental principle.
"it may be atmospheric pressure doing the pushing, but ONLY because the suction of the pump is making an offset in the pressure differential between the inlet and the outlet"
Nope, its making a pressure difference between where the liquid is coming from and the suction side of the pump.
Aint my theory - it is a fundamental principle.
"it may be atmospheric pressure doing the pushing, but ONLY because the suction of the pump is making an offset in the pressure differential between the inlet and the outlet"
Nope, its making a pressure difference between where the liquid is coming from and the suction side of the pump.
The water tank in the H31 is beneath the starboard aft berth, toward the stern, which is pretty low in the hull. It would be hard to find a place to mount the pump at the level of the tank outlet. Presently it's about 3 feet distant and about 20" higher than the outlet. I'll have to be creative to see if I can get it closer and lower.
the word atmospheric pressure has a definite and specific meaning...and there is obviously a misunderstanding of the fundamental principal of it.
without the pump to create a vacuum, the water isnt going to move by the atmospheric pressure itself... its not the atmospheric pressure, but the pressure created by the device, and it is NOT atmospheric pressure that is being created. if there is no device creating pressure, there will be no water movement, period.
the atmospheric pressure is the same where the water enters the device as it is where it exits the device, but its the pressure difference being created within the device that causes a suction at one end and pressure at the other end, and again, its not atmospheric pressure.
I feel it is the use of the word "atmospheric" being used in combination with pressure, is where the misunderstanding is/was...
because it doesnt make a difference if the atmospheric pressure is increased ten fold, or if it is reduced to 0, with the pump it will still move the product... atmospheric pressure has nothing to do with it. pressure differential, yes... atmospheric pressure, no...
but I will concede, thats where the argument comes in... because if you remove the word atmospheric from it, its really the same argument that is made by the principal of how an air foil (sail or wing) or water foil works... without some device to create a differential in acute pressure on different sides of the object, there will be no movement.
there are proponents of it being the loss of pressure on the one side of the foil, and then there are the proponents that claim its the increased pressure on the other side that causes the movement.... both side are right. because ultimately, there IS a difference in pressure that causes the movement.
Seems to be some confusion, for a quick refresher here's something.... http://en.wikipedia.org/wiki/Pump
Don't worry about getting it closer, just lower. Avoid the bilges - where it could get flooded.
Aside from all the science, If it has self priming ability, or a check valve, I'd like mine higher than the water level. This way a failure drains back into the supply tank instead of siphoning your fresh water out of its containment and into your bilge.
All "normal" potable water pumps like Shureflo and Jabsco, etc., are self priming; they are positive displacement pumps and will pull air out of the lines and get the water flowing, given not too much rise from the water level to the pump intake fitting. Locating closer and lower are going in the right direction, but not absolutely necessary. It helps to have the pump head wet so the valves seal better if passing gas (?) until the water gets to the pump.. It is imperative that there be no air leaks in the suction piping and strainer and fittings. To start, fill the tank to the absolute top, and continue to fill until the water comes out of the tank vent. Once the water is out the vent, the water "head" will be plenty sufficient (above the pump) to push the water to the pump .. Open the faucets fully to expel all gas.. both hot and cold taps to flood the water heater tank.
On the H-34, there is a 15 or so foot run from the aft tank to the pump; it functions with no trouble..
On the H-34, there is a 15 or so foot run from the aft tank to the pump; it functions with no trouble..
So you would struggle to get your water pump primed because of some concern about siphoning your tanks dry. Let's examine that. Your intake is suction at 14.7psi. Your pressure side is pressurized 40psi connected to an array of fittings, valves, HW tank, faucets, piping runs - all prone to leaks. Guess which side of the pump is prone to draining your tanks? If you want to protect yourself against filling your bilges with your freshwater turn off your freshwater pump when you are not using it. Don't hobble the effectiveness of your pump to fill and pump.
Bingo...!
I fix domestic plumbing leaks with pretty good regularity. Almost always, like 95% of the time, they are in the pressure side of the pump. Inlet / suction side leaks happen too but normally those issues are when a customer has a clogged strainer or leaves all three tank valves open and one goes dry and the pump sucks air.. I have many boats that run 60+ PSI pumps, including my own, and you need good fittings, such as PEX, to be able to deal best with these pressures..
Yes, I would.
Edit, I'm not talking about the masthead, but a couple of inches above the tank seems reasonable. (see the 95% comment above)
The OP would be better served by Googling basic pump theory.
1. There are several types of pumps (diaphragm, centrifugal, vane) and many variations within each group. The characteristics of each is different. Broad statements are difficult.
2. Air leak on the inlet (if the condition is negative in the pipe under flow conditions) are a common problem. Simulates cavitation, so only a tiny bit will kill you, more so the higher the pressure.
3. Inlet filters must be clean. VERY clean for some designs.
4. Lift (suction) is governed by the design. If the rated value is exceeded, the pump will cavitate, like a stalled sail, and you are going nowhere.
5. Bigger suction pipes help. If the suction run is longer, go up one size. Minimize valves and turns (each 90 is typically several feet, depending on flow and size).
6. Even small amounts of debris in a centrifugal can destroy performance.
But read a book or page on pumps. The wiki reference above is good. Add the wiki NPSH page.
http://en.wikipedia.org/wiki/NPSH
1. There are several types of pumps (diaphragm, centrifugal, vane) and many variations within each group. The characteristics of each is different. Broad statements are difficult.
2. Air leak on the inlet (if the condition is negative in the pipe under flow conditions) are a common problem. Simulates cavitation, so only a tiny bit will kill you, more so the higher the pressure.
3. Inlet filters must be clean. VERY clean for some designs.
4. Lift (suction) is governed by the design. If the rated value is exceeded, the pump will cavitate, like a stalled sail, and you are going nowhere.
5. Bigger suction pipes help. If the suction run is longer, go up one size. Minimize valves and turns (each 90 is typically several feet, depending on flow and size).
6. Even small amounts of debris in a centrifugal can destroy performance.
But read a book or page on pumps. The wiki reference above is good. Add the wiki NPSH page.
http://en.wikipedia.org/wiki/NPSH
Very good. I had many years of entertainment and frustration explaining this to new firefighters. Gotta keep it simple.
All U Get