Suction pipe from fountain pump to pool

If I understand correctly the suction pipe rises 5m above the water level and then runs horizontally for 15m

This will exceed the NPSH requirements by a great margin. Air valves will not solve any suction problems.

You should lower the pump but that don't seem to be possible without major changes to the pump station.

Your options seem to be to redirect any pressure available from the waterfall into the suction of the pump or install a low pressure submersible in the pool to lift the water up-to the pump.

It might be more economical to use the full 20 m available from the waterfall and install a booster pump for achieving the required pressure for the pumped water. Your generated power will reduce according to the water subtracted from the flow.

From Reading the post again it seems that the suction pipe is again dropped to 1.1m above water level. (A 5m syphon)

This type of suction will not work even with air valves.

Try and change the suction so that it rises all the way from water to pump.

Using the available pressure or a booster pump is still an option.

Dear Hendrik,

Thanks alot for your reply.

We will try to provide a check vave at suction headline and prime water to ensure suction line filled up without airlock and hopefully it will work for the time being. I think your idea of submersible pump in front of suction seems good but how practically to work as it may have to interlock work the original fountain pump to boost the preesure up to 20m. Also the lower pool seems to have problem to house submersible pump in it.

Thanks alot again for your reply.

Hi wsh4102

Sorry i saw water Fall instead af water wall.

The problem with your current setup is that atmospheric pressure must lift the water to 5 m above the water level in the pond. This can not be done. The pump will expel the water in the down pipe (5m - 1.1m) and no water can be replaced.

Your options are.

Reroute the suction line to eliminate the hump. This don't seem to be possible.

Install a submersible in the pond to supply the main pump.

With only 20m to the top sub can be used to do all the work.

Another alternative is to move the pump to another position and hid it underground next to the pump with a flooded suction.

It was a bit wild of me to say it will not work.I have done more with a syphon / pump combination over a dam wall. Admitted that the pump was far below water level.

Your idea of a foot valve may work depending on the friction losses.

With a 45kw motor lifting to only 20m your flow may be quite high. If the pipe is too small friction losses will be high.

What is the design flow?

Let us know about the outcome of your test.

Dear Hendrik,

Thanks for your response. Appreciated.

We will test out first the situation and see the outcome. This is a huge indoor fountain in a shopping mall with 20m high and length of about 30m long with water cascading down along the slanted feature wall to create thick white water effect. (I think should be very noise but this is the architect design intent)

Sorry, I may not understand what your meaning of "20m to the top sub can be used to do all the work", I wonder are you mean using the discharge pressure to boost the suction line as shown in below "red line" to be controlled by pressure differential valve?

Thanks

The valve there will supply positive pressure where a 5m+ vacuum is needed. It can be used to prime the pump and then closed.

Your first test would be to add a suction non return valve at the pool end of the suction side, (foot valve)

You would also need a priming device (the suction line should be filled with water before the pump is started. A funnel on a T piece with valve may do. A pumping device will be better.

The white water effect make me think of plenty of bubbles in the pool which may be sucked into the system and prime will be lost.

The sub stands for submersible. (The pump and motor is submerged into the pool.)

Dear Hendrik,

Thanks a lot for your advice.

Calvin

A couple things, including a clarification question. First the question: How is water to be pushed out onto the wall? With spaced nozzles? This (Point 2) awaits your answer.

Point 1. In the sketch, I'm seeing a siphon action (apart from/supplementary to) pump suction delivery of pool water to the pump inlet; such self-perpetuating action which could be "broken" by the introduction of air...as by cavitation at the pump(?). Might it be possible to provide alternate escape for any pump-induced air introduction in the suction (siphon) line? For example, an unconfined plenum on the "siphon outlet" side from which pump could draw water, and air could escape to atmosphere...combined with additional lift/starter pump on the pool side of the siphon? The idea, basically, would be to exploit (as opposed to overcoming) the architecture of the overall system.

Will await reply about the water wall cascade method.

I would first clean the line. Water line get dirty and have slim in them after a while.

This slim inside the line will slow/drag down your flow.

First: I have no knowledge or experience with this sort of thing so please ignore me if this is rubbish.

Would it be possible to fit an inverted 'P' trap somewhere along the suction line:-

Then as routine maintenance: turn off the pump; close the valve at B; open the valve at A, and bleed the air at C.

The height you have, if there are no places for air to hide in the line like filters, should be enough t drive any air up an out as the water is spumed thru the line.

I believe the p trap would be a waste of time, effor and money if that drawing repsersent ther line accurately.

Clean the line the re evaluated the flow. You may need to test the pump for worn parts too,

Maybe I am misunderstanding the numbers and what is trying to be done. But it appears from this:"This is a huge indoor fountain in a shopping mall with 20m high and length of about 30m long with water cascading down along the slanted feature wall to create thick white water effect. (I think should be very noise but this is the architect design intent)" that it is a brand new fountain that has not been up and running yet. That wsh4102 is having problems getting it to work.

Another thing, to me, a 10" (250mm) pipe is a big pipe to be opening and closing valves on and taking a bucket to prime it through a hole.

A 10" (250mm) foot valve? How much does that weigh/cost? Trying to drag the water up 15' (5m) and then over to the pump 45' (15m) away and then dropping back down to the pump inlet (which is 4' / 1.1m above the pool) causing an 11' / 3.9m negative.

I don't see how that could work, you get some air in that 10" / 250mm pipe right at the pump inlet and I don't think there would be any way to pull water out of the pool over that 11' / 3.9m hump.

I don't know if the mall is open yet, or what is under that hump, but if at all possible it would be to drop the suction pipe into the floor. The hundreds or thousands of dollars you spend to do that just might be cheaper than trying to get it to keep prime.

It wouldn't matter if you needed to go a meter or more below the floor to get to the pump, since gravity feed from the pool would keep the pipe full and it wouldn't change the 4' / 1.1m you have to lift the water from pool level. I think... I'm no water pump expert. But common sense tells me that is true.

Would still need some type of a foot valve in case the pump went down for any reason so it wouldn't lose prime. Wouldn't take long for that 60' 20m pipe full to run right back into the pool without the pump running and is it normal for a pump pulling through a 10" / 250mm pipe to be able to suck water straight up 4' / 1.1m with no prime? And where do you find a foot valve that works upside down? Have to be a really tight pump I would think. When I was a kid we only had a 1 1/2" / 38mm 12' / 3.6m suction line on our house well, but it always had to be primed to get it going. That was back in the 1950s, but I don't think things have changed that much, have they?

Seems like the engineer/person who designed the plumbing needs to take another look at it. That big of a pipe, you are moving a lot of water a minute.

Ken

I think you can probably make this system work with fairly minor modifications.

First check the net positive suction head available/required (NPSHa and NPSHr) and make sure you have what you need. Any good pump manual will tell you how to calculate NPSHa. The pump curve will tell you the NPSHr. NPSH is essentially the absolute pressure available at the suction of the pump minus the vapor pressure of the flowing fluid. It is origin pressure minus the sum of accelleration losses, friction losses and vapor pressure of the fluid. All pressures are absolute, not gauge.

You will also have to ensure that the pressure at the high point in the suction piping is above the vapor pressure of water by an analagous calculation.

To get the pump started, the suction line must be liquid full. I would find a swing check valve and install it on the suction line inside the pool. You must then create the ability to fill the suction line before starting the pump.

To keep the pump running, you must have a way to get rid of air that will accumulate at the high point in the suction piping. Without this, the line will vapor lock and flow will be lost. Float type devices are made for just this purpose (they vent air to atmosphere and close when they are full of liquid). Unfortunatlely, this device cannot be simply vented to atmoshpere in your application because the line is below atmpsheric pressure where it needs to be vented. YOu can install a pair of these float valves and attach the outlet to a small liquid ring vacuum pump and it should then work.

I do not think the valve "A" or the Feature "C" will aid your cause. Valve "B" should be a check valve with low pressure drop.

This is the cheapest way out, and provide the needed pressure add line shown

How do you get the system to expel the air upon start up?

In fire department pumping operations, when it is required to pump water from a lake or river that is below the level of the truck pumping the water, a priming device is used until the air is removed from the suction portion of the pumping system. At that time the pump in the fire truck will pump the water as long as needed. The most common priming pump in use today is a rotary vane vacuum pump. By lowering the pressure at highest point in the suction piping, the water level in the suction pipe will rise to meet the impeller eye. An electrically operated valve mounted between the priming pump and suction pipe, that is opened only when the priming pump is running would be needed.The priming pump and valve could be remote mounted as needed. I have never seen performance figures on the priming pumps used, but they are plumbed with 3/4" pipe. The minimum vacuum allowable for FD use is 20.0"Hg. (Memory straining) This setup is used for suction piping sizes of 6" with 10' of lift. The time allowed to prime the pump is 20 to 30 seconds (memory failure on this one) Trucks with a front mounted suction sometimes use an additional priming pump to shorten the time to prime.

A suitable vacuum pump should allow the system as first shown to work if equipped with a priming device. Good luck.