Were you the student at the back of the hall, the one that actually paid attention ? God bless.YEAmy professor would be pleased that I listened to his lectures... and can still remember.
Bilge Hose size problem
- Thread starter ontherocks83
- Start date
Static head pressure is not normally referred to as Lift, but is calculated the same way.
Height is the difference in vertical Height from pump discharge [ if not submerged] to overboard discharge point.
Sorry, but Static Head is that HEIGHT , no matter , vented loops, loop de loops, P-Traps or whatever piping configurations.
Static pressure is calculated by the Density of fluid [specific gravity] times the that Height.
Sea water specific gravity [density] or fresh water is a constant for all practical purposes.
Static means no flow or velocity.
Bernoulli's principle - Wikipedia
Height = z in that equation.
Jim...
I suspect that may be saying the same thing but I'm unable to understand it in this format. For any given bilge pump installation, the static head will be from the bilge pool surface to the highest atmospheric vented "open" system point. If it's a vented loop, then the top of the loop is the highest lift point. If the loop is not vented, then it is the discharge elevation. However, the bilge pump must have sufficient lift to overcome the highest point, even in a closed loop, to establish system flow in the first place.
In our business, it is common to refer to static head as lift, but we are probably not normal people.
By nature, the static head figure changes constantly as the bilge pool level changes (assuming that the pump is working and water is being removed from the bilge...). We could argue the point whether the change is significant or not, but it would be difficult for me to understand how it could be constant. The pump flow decreases as the bilge level gets lower. Whether it's significant or not depends on the pump characteristics.
Hope I haven't spooled you up. That was certainly not my intent. New guy out.
In our business, it is common to refer to static head as lift, but we are probably not normal people.
By nature, the static head figure changes constantly as the bilge pool level changes (assuming that the pump is working and water is being removed from the bilge...). We could argue the point whether the change is significant or not, but it would be difficult for me to understand how it could be constant. The pump flow decreases as the bilge level gets lower. Whether it's significant or not depends on the pump characteristics.
Hope I haven't spooled you up. That was certainly not my intent. New guy out.
A Vented loop is to stop a siphon or back flow.
If the discharge is to the air, no siphon.
So we are saying the same. Height is pool surface on a submersible to the discharge over board.
Maximum Static Height is when the pool surface is at its minimum or the pump discharge point.
Whew it was terminology.
Jim...
A bilge pump is not a safety item, it is merely a convenience item designed to discharge incidental water that collects in the bilge without having to stoop down to scoop it up manually. With the addition of a "float switch" or an automatic pump the discharge can take place even when the boat is unattended. For the intended purpose a pump of 500 to 750 GPH will suffice. Get one to match the desired size of existing discharge hose. Bilge pumps are bench rated by the manufacturer; that means there is no discharge hose attached to the pump and there is no load. In the real world once you add a discharge hose of certain length and having to push water up heads of elevation and that coupled with voltage drops and heat from the motor that their actual performance is closer to 25% of the manufacturers rated capacity. It is for this reason that bilge pumps are not designed or intended as a safety item. Basically they do not have the capacity to handle any significant intake of water. You want safety, correct any leaks that you may find and make sure your hoses and clamps are in good condition and inspect them frequently. As a safety pump it is recommended you install a manual, self priming gusher pump that can be operated from the cockpit. Also some sailors have installed 120V electric sump pumps for use at dockside in case a hose or a head valve fails. I guess some people prefer to install a pump rather than conduct frequent inspections. In my case after going through a number of float switches and finding dead batteries I went back to the old ways and installed a manual switch at the panel to operate the bilge pump. When I first go aboard I will turn on the pump and allow it to run until it starts sucking air and I will then turn it off. When underway I will turn it on a couple of time in a 24 hour period. If I find that the run time is longer than usual I will look for an explanation for it. Obviously any installation depends on your particular circumstances of having or not shorepower, not being able to visit the boat frequently and not having the boat in a location where neighbors could notice in a timely manner whether the boat is taking on water. But even with a 2000 GPH pump do not expect any more than perhaps buy you a few additional hours. It is the smaller boats that really would require the larger pumps as they will fill up faster. Been doing this for a while, hope any of this might be of help.
............................. and keep an eye out for semi-submerged shipping containers
! ! ! !
..... and wait to shave until after you have plugged all holes in your hull.............................. and keep an eye out for semi-submerged shipping containers
Interesting thread. If you extrapolate the "gold" system curve to the pressure axis, it will intersect at the static head (lift) pressure value. So the static head is the origin for the friction-generated system curve (the point that I miserably failed at making earlier). The green line represents pumping into an infinitely large dia cylinder about 7 feet tall (all static head, no friction). For system where the intake pressure changes (due to level changes), there are an infinite number of system curves, one for each level. Your point of designing at the lowest level is spot on. Oh well, as my daddy always told me, I'm not the sharpest knife in the drawer...
One point we haven't covered for @ontherocks83 is the need to route discharge hose or piping without significant bumps or trap points (since he's contemplating adding another Rule bilge pump). It would be a bummer to swamp or sink the boat while the bilge pump was running airlocked...
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The "Air Lock" on the video has 2 issues that should not occur in a good centrifugal bilge pump install.
1) The discharge line is BELOW the pump suction BLUE area on pump. Say what?
If that "trap" bottom was 1" higher or slightly above the discharge point, NO AIR LOCK!!
2) To create that "Air Trap" you would need 5 elbows. How many elbows would you put in your hose line? Maybe 2?
The Friction loss [Capacity reduction] for 5 - 90° Elbows is about the same as 6.5 feet of hose.
The video is a marketing ploy to say it is NOT their pump problem.
In centrifugal pump/piping design, the discharge is NEVER below the suction or it will Never Prime and pump. Period!
This is why there is always water in a bilge, using centrifugal bilge pumps. The Blue Area of that pump.
____
I had heard of the "Air Lock" and was always puzzled by that term.
How in the Heck could you have an "Air Lock" on the discharge line? The pump should just blow out that air.
So...
Just leave the bilge pump discharge with horizontal or upward hose routing. "P" traps could hold yucky water to allow algae growth and big time maintenance.
Otherwise dips or sags should not be a big issue.
Jim....
1) The discharge line is BELOW the pump suction BLUE area on pump. Say what?
If that "trap" bottom was 1" higher or slightly above the discharge point, NO AIR LOCK!!
2) To create that "Air Trap" you would need 5 elbows. How many elbows would you put in your hose line? Maybe 2?
The Friction loss [Capacity reduction] for 5 - 90° Elbows is about the same as 6.5 feet of hose.
The video is a marketing ploy to say it is NOT their pump problem.
In centrifugal pump/piping design, the discharge is NEVER below the suction or it will Never Prime and pump. Period!
This is why there is always water in a bilge, using centrifugal bilge pumps. The Blue Area of that pump.
____
I had heard of the "Air Lock" and was always puzzled by that term.
How in the Heck could you have an "Air Lock" on the discharge line? The pump should just blow out that air.
So...
Just leave the bilge pump discharge with horizontal or upward hose routing. "P" traps could hold yucky water to allow algae growth and big time maintenance.
Otherwise dips or sags should not be a big issue.
Jim....
Respectfully, I disagree. Again, I'm not trying to get sideways with you. I really enjoy your posts. So, pretend we're two old friends having a lively discussion over beers (or coffee if you prefer).
In my opinion, If you create a dip that will hold a static pressure seal equal to the impeller submergence plus the impeller air pressure, the pump won't prime. The sag could be anywhere in the discharge line. Doesn't have to be below the impeller level, just happened to be the way they configured their demo stand. Also, elbows are not required to trap water. Laying flex hose over big floor timbers, etc. and letting it droop between can create a trap. I think they are simply trying to prevent a DIY installer from a) sinking his boat through a poor install; and b) creating a future lawsuit issue for them. Kind of like serving a customer hot coffee and getting sued because they spilled it in their own lap...
I agree that it is an unlikely scenario, but if he's putting in a new pump and asking the type of questions he's asking, it might be good form to tell him not to put any dips or sags in the line, and slope upward to the discharge point - just in case... Which, is exactly what you almost did in your wrap up, but for the dips and sags aren't a problem remark.
You have to admit, these exchanges are more fun than which windex to buy or bolt rope shrinkage.
In my opinion, If you create a dip that will hold a static pressure seal equal to the impeller submergence plus the impeller air pressure, the pump won't prime. The sag could be anywhere in the discharge line. Doesn't have to be below the impeller level, just happened to be the way they configured their demo stand. Also, elbows are not required to trap water. Laying flex hose over big floor timbers, etc. and letting it droop between can create a trap. I think they are simply trying to prevent a DIY installer from a) sinking his boat through a poor install; and b) creating a future lawsuit issue for them. Kind of like serving a customer hot coffee and getting sued because they spilled it in their own lap...
I agree that it is an unlikely scenario, but if he's putting in a new pump and asking the type of questions he's asking, it might be good form to tell him not to put any dips or sags in the line, and slope upward to the discharge point - just in case... Which, is exactly what you almost did in your wrap up, but for the dips and sags aren't a problem remark.
You have to admit, these exchanges are more fun than which windex to buy or bolt rope shrinkage.
Correct!
________
As long as the Centrifugal Pump is "flooded", it will produce the maximum Static Pressure head at ZERO flow at pump discharge point. The video example showed the pump was NOT flooded [ point ≈1.42 in video]
Even if you close a discharge valve fully, a flooded pump will develop "Dead Head Pressure" [zero flow]
Even the smallest Rule bilge pump will develop 12+ feet of water pressure head, at Zero Flow.
Most larger ones, 21+ feet.
So...
What what kind of Sag's would be greater than a few inches of water?
My point...
Sags will reduce flow, full of water or not, but not a big issue, if your bilge pump size is picked correctly.
Jim...
PS: My boat's Static head is ≈ 3.5 feet of water. See Gold curve.
PSS: Minimum hose length will give maximum flow.
PSSS: "Air Lock" myth busted.
@JamesG161, you are absolutely correct that IF the impeller is flooded, the pump would be capable of shutoff head which would easily blow out any dip or sag that produced a water trap. The fact is, the impeller didn't flood, nor will it, due to the trapped air. As soon as you start the pump, the air is centrifuged to the eye of the impeller displacing enough water to prevent total impeller flooding. The trap holds just enough back pressure to prevent it. It would be very sensitive to the specifics of the particular install and pump characteristics. If the trap isn't too deep, and the batteries last long enough; and the pump doesn't burn up; the bilge water level may continue to rise enough to build adequate pressure to flood the impeller. But, I wouldn't want to count on it.
The trap in their demo stand is separated from the pump by a column of air. How would the relative elevation of the trap assembly to the pump be significant (referring to the argument that the trap occurs below the pump inlet)? What properties change if we move the trap 1" higher in the configuration? It appears that the pump is generating about 1-1/2" w.c. of air static pressure when airlocked. So, any dip in centerline over about 3/4" will stop that particular pump from priming with that "sump" level if there is adequate rise between the trap and discharge point.
I will agree that modeling the system where the discharge drops to a level that might be outside of the hull in a normal boat is not great for visualization. I contend, however, that because the trap is isolated by two columns of air; the relative elevation of the trap has a negligible impact on the numbers. Always better to make your models look as much like reality as you can.
You are now considered a true friend. I admire anyone that can look straight at the answer and still argue against it. If we ever meet up, I'm buying the first round!
The trap in their demo stand is separated from the pump by a column of air. How would the relative elevation of the trap assembly to the pump be significant (referring to the argument that the trap occurs below the pump inlet)? What properties change if we move the trap 1" higher in the configuration? It appears that the pump is generating about 1-1/2" w.c. of air static pressure when airlocked. So, any dip in centerline over about 3/4" will stop that particular pump from priming with that "sump" level if there is adequate rise between the trap and discharge point.
I will agree that modeling the system where the discharge drops to a level that might be outside of the hull in a normal boat is not great for visualization. I contend, however, that because the trap is isolated by two columns of air; the relative elevation of the trap has a negligible impact on the numbers. Always better to make your models look as much like reality as you can.
You are now considered a true friend. I admire anyone that can look straight at the answer and still argue against it. If we ever meet up, I'm buying the first round!
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Deal...
What they did in the video was to put the bottom of the trap just at the right height to prevent "flooding" or priming of their mini-bilge pump.
Even an amateur can develop his Gold System Curve for existing piping system.
Tools 3 gallon bucket and stop watch.
1) Get some water from shore source , in 3 gallon bucket
2) Dump the 3 gallons in the bilge
3) Start your timer.
4) When bilge stops, stop the watch. Compute minutes.
Compute... 3 Gallons in X minutes = GPM
Knowing which size Rule Pump, you can get your Gold Curve.
Go to Link of
Dual bilge pumps
for selection of Rule Pumps on my post #23 for curves.
Jim...
Although I don't exactly agree with the specifics of your statement, I will whole-heartedly agree that they constructed the demo to have an adequate "sag" model to show that installers need to pay attention when running discharge hose - keeping it continuously sloping upward to the discharge location. And, if you don't keep sags and dips out of the discharge, you risk creating a situation where the pump only works for the first cycle.
PS: Thanks for a great technical sparring session!
PS: Thanks for a great technical sparring session!