Question about pumps and physics

[island_pondscum]

I'm sure this qualifies as a stupid question but I am stumped.

In past ponds I have always used submerged pumps. But in the latest version I am planning to use an external pump to run a bottom filter that sits on top of the liner. That will pipe will run up the side (5'), over the edge of the pond / top of the liner, and into a pre-pump strainer.

Now, for the physics question: is it just the pump needs to be below the waterline, or does the pump *and* the piping need to be below the waterline?

If the latter, how do I get the pipe through the liner, and if the former, how do I prime it?

I figure this is not something to learn through trial and error. Thanks!

 

[Waterbug]

Insightful, not stupid,

There are some external pumps that are called self-priming. But these do have limits.

Assuming the pump is not self-priming or placement of pipes and pump aren't within the self-priming pump limits...

...is it just the pump needs to be below the waterline, or does the pump *and* the piping need to be below the waterline?

The answer you want is that the pump and pipes need to be below the water line. That's a bit general. The pump intake has to be below the waterline but the whole pump doesn't have to be. The main issue is the strainer basket. When the lid is off you want the waterline to be so the pot is full but not below the waterline or else water from the pond will flow out until the pond level goes down enough to be at the top of the pot. Too low a pot can be solved somewhat by adding a shut off valve just before the pot but this is not what most installers would call correct.

The pipes also don't have to be all below waterline. It's just that's the easiest and most reliable way. Technically the pipes can't have air in them. So even if the pipes are all below the waterline if there is a long horizontal run in say a trench and there is a hump in the line then air can collect in that high spot and cause problems (loss of flow and even complete shutdown). Even removing air from these high spots is not always a long term solution because water still contains gases which can escape and the air pocket can happen again.

Running a pipe over the side is this hump/air pocket problem in the most obvious form. The pump is unlikely to achieve prime or it will lose prime over time. There is a solution. What you do is put a tee at the apex of the pipe going over the liner, the high spot. Then generally a reducer to whatever 1/2", 3/4" and a short riser and a ball valve. When the pump is all hooked up and ready to go, the strainer pot is closed and sealed, you open the ball valve and pour water in to fill the pipe. You don't want to hook a garden hose directly because that isn't to code, but you can use the hose to manually fill the pipe. When full you close the ball valve and turn on the pump. It should run for days, weeks, maybe months, until a new air pocket forms and the process has to be repeated. You can also add various valves at different places to make life easier.

However, that's a huge pain and not efficient pumping wise. Running the pipe through the liner is normally how this is done. Today most installers use bulkhead fittings to do this. They're pretty cheap. Long ago bulkhead fittings were really expensive and not always easy to find (pre internet). A guy named Greg Bickal (finally remembered his name) invented the Pipe Boot. You can Google that. It works every well for EPDM liner and just uses a cheap and easy to find pipe clamp. I still use this method.

To reduce risk I send the pipe out of the pond through the side just below the waterline rather than the bottom, even for bottom drains. How far below the waterline depends on where the pipe has to go from there. I like to keep some upward slope on the pipe 1/4" over 4', something like that, so air bubbles will travel up to the pump and generally collect in the strainer basket which are cleared every time the strainer basket is cleaned. Humps in the pipe are not good.

Filter on the bottom of the pond? That doesn't sound easy to clean.

 

[island_pondscum]

Wow, thank you! This is a treasure of information and things I have not found searching. The pipe boot sounds like a winner for me, definitely don't want to deal with air pockets in the lines. I obsess about things enough without any encouragement. Thank you again!

 

[phoxtrot]

The pump can be above the water line. Just need to prime the pump once and use a check valve to keep the water in the line

 

[mgmine]

Actually, you need a foot valve. It's the same as a check valve only it sits at the bottom of the pump line.

 

[Waterbug]

Check and foot valves work great in clean water applications. In ponds they have a couple of problems. Clogging is the most common problem. A foot valve addresses this by adding a basket - basically a check valve with a basket. The basket catches debris and has to be cleaned. This basket is what is more often called a "pre-filter" in the pond hobby. If you search this forum and other pond forums you will see the issue with pre-filters - clogging and having to be cleaned. In addition to dead leaves and other stuff getting stuck a bigger problem is string algae growing on them. Bio-film over a longer period can also be a problem.

If you don't mind cleaning these there is still the issue of reduced flow. Generally people only clean these as needed meaning when they notice a waterfall being weak. But there is a reduced flow up until it gets bad enough to be noticeable. So you can be paying for electric just to keep stuff stuck to a pre-filter instead of moving water.

The second issue is bio-film forming on the check valve. This is less of a problem in systems where the check valve closes and opens often as the bio-film can get scraped off a bit. But in most pond applications the check valve may remain open for months and the gasket can get covered in a film preventing a tight seal and it doesn't take much water to leak pass the gasket to cause loss of prime. Depending on the type of check valve the hinge on the flap can also get covered in a bio-film to the degree the flap doesn't even close or closes slowly enough that too much water get's by and prime to lost.

And of course there is the concept of just one more part to have to buy, install and have break.

Imo check valves are not an optimal solution. I've not seen them used even in swimming pool installations where the water is pretty clean. Instead installers simply install the pump at the correct level. It isn't hard to do.

 

[addy1]

I have a check valve on my deck pond pump. It does run on a timer, comes on a few times a day. It is one of the pvc check valves been installed for around 4 years and still working great. Without it I would need a pump that self primes. The deck ponds pump is a clear water pump from harbor freight. It can handle the approximate 100 feet of head pressure to get the water up to the ponds.