bilge pump rpoblem

[isha]

I've just fitted a Rule 500 electric bilge pump.Mainly to pump out rainwater which comes in via the keel stepped mast. The bilge is very shallow and pump is situated about 4.5 metres from the transom where it discharges. The lift is about 1 metre. About 300mm from the pump I've fitted a non return valve. It all works fine until I try to use it the next time. The pump is running but nothing happens-no water is pumping out of the boat. I disconnect the join at the non return valve ( the end of the valve furthest away from the pump) and as you would expect all the water in the hose returns to the bilge. Reconnect and try again and it works. try again and its back to not shifting water. Very frustrating to say the least.
The hose runs uphill all the way to the stern locker and there it touches the underside of the side deck and then downhill to the skin fitting which is about 1 metre uphill from the pump and above the water line.
As the hose exits the pump it does go downhill by about 25-30 mm to pass through the drain hole in the framing under the cabin sole and then uphill all the way.
What have I done wrong. Grateful for all advice.
Dave

 

[bobgarrett]

My guess would be that the nonreturn valve is increasing the effective head beyond what the pump can achieve reliably. When you disconnect you push the valve open so its resistance is less until next time.

 

[exfinnsailor]

Strum box will make sure you pick up as much water as possible .. Simple none return valve .. Connect to pump .. Outlet from pump over the side .. This way the pump will always be primed as water will be held in the pump by the non-return valve .. Some Bits HERE

 

[misterg]

I think that the problem is the pump becoming airlocked - once it has pumped the bilge 'dry' and stopped, the water in the pump drains back. When the level in the bilge rises again, the head of water standing on the NRV stops water flowing back into the pump, so the impeller is in a 'bubble'. It probably doesn't help that the hose runs downhill after the pump.

I believe the cure is to drill a small (1/16", say) hole in the hose at the highest point immediately after the pump (but in any case before the NRV). Although this will leak some water back to the bilge, it will release air trapped in the pipe, and allow the pump to prime.

Andy

 

[VicS]

[ QUOTE ]
Strum box will make sure you pick up as much water as possible .. Simple none return valve .. Connect to pump

[/ QUOTE ] Its a Rule 500 a submersible pump!
http://www.jabscoshop.com/includes/resize.asp?path=itemimages/25D-6WC.jpg

I agree with misterg. Its airlocking. A hole in the underside of the pipe may be a better idea than creating a fountain with one on the top.

 

[PembrokeshirePromise]

Third vote for the airlock. I have much the same setup with a larger submersible pump (for emergency use) - unavoidable have to dip down under a cross member before rising up to exit point.

It airlocked until I drilled a small hole in the pipe (I drilled it on the upper side - but slightly off centre) - it does create a small fountain - but as the pump is only activated in the case of severe flooding that would be the least of my worries!

 

[misterg]

[ QUOTE ]
A hole in the underside of the pipe may be a better idea than creating a fountain with one on the top.

[/ QUOTE ]

Agreed, as long as the hole is higher than the impeller. If the pipe dips down immediately after the pump, then the hole may have to be at or near the top of the pipe to achieve this.

Andy

 

[isha]

Thanks to all for this advice. It obviously works as you have experienced.
I will carry out your remedy.
I could not see my way out of this problem, but with your explanation I can make sense of it.
Dave

 

[mikemonty]

Since you have led the discharge up under the deck and the outlet is above the waterline - why bother with the NRV?

The line can't siphon (much) unless the outlet ends up permanently submerged and/or the high point of the pipe run under the aft deck is not near midships.
The NRV adds significantly to the pipe losses, reducing the capacity of the pump and is a potential blockage point with a tendency to snag any fibrous or solid matter that should get through the pump.

Even if the pump is not airlocked - with the NRV you are starting the pump against a head of pressure - quite possible that it might be cavitating. If you eliminate the NRV your pump can take off smoothly from a nice low head.

 

[isha]

I think that without the NRV the contents of 5metres of 19mm pipe will flow back into the bilge, wheras with the NRV it is held in the pipe and not allowed to return to the bilge. Without the NRV it would pump and with less resistance I agree,, but I do not want all that water back in the bilge.
Dave

 

[mikemonty]

But thats what the "hole in the pipe" solution will acheive anyway!
If you want to MINIMISE the volume of back-flow - put the loop near the pump and allow the majority of the pipe to run down towards the transom.
And if you want a completely dry bilge use a sponge!

 

[VicS]

[ QUOTE ]
But thats what the "hole in the pipe" solution will acheive anyway

[/ QUOTE ] Hopefully not. It was suggested that the hole was made between the pump and the NRV.

If the pump is in the lowest part of the bilge it is unlikely to be practicable to run the pipe to a high level initially.

 

[Refueler]

On reading the OP ... I get a strange sense of the NR valve being back to front ?

[ QUOTE ]
I disconnect the join at the non return valve ( the end of the valve furthest away from the pump) and as you would expect all the water in the hose returns to the bilge.

[/ QUOTE ]

If you are disconnecting the exit side of the NR valve as your text suggests - water should NOT flow back down. It should only flow back if you disconnect the INLET side of the NR valve.
Unless your "flows back" - is the water gushing out of the exit pipe ... NOT flowing back down the pipe to pump and bilge.

TBH - 4.5m of pipe on a Rule 500 ? probably small bore pipe ? That's quite a run plus the 1m rise and then add in a NR valve. You are asking a lot of a piddly small pump.

I have a 500 in my bilge, about 2m largest bore hose I can connect that discharges via sink outlet, no NR valve at all. I accept the half litre of water that sits at times. The flow out of the through hull at waterline (about .7m above pump) is not much more than similar to pouring water from a kettle ! It certainly is no jet of water.

As another suggests - I would be inclined to put the NR valve at the strumbox. But in any case ... NR Valves are notoriously unreliable and prone to all sorts of stuck open / stick shut scenarios ...

 

[isha]

Refueller
I am not following you. The pump pushes the water towards the NRV. This valve has an arrow on it and I assume that this indicates the direction of flow. I have the arrow pointing in the direction of flow, from the pump towards the transom.
You say -- 'Unless your "flows back" - is the water gushing out of the exit pipe ... NOT flowing back down the pipe to pump and bilge.'
Thats it in one. This is what happens when I disconnect the exit side of the NRV. Now this is what would happen if there were no NRV fitted, have I got that right?
Without the NRV the bilge would fillup with 5metres x19mm dia of water.i.e. is about 1.6 litres. This is in a very shallow bilge and on top of the residule water the pump cannot get to this out amounts to a fair slop. I can live with a small amount which the pump does not reach. but the extra 1.6 litre- no. The pump discharges the water effectively when the air lock is breached.
This may seem like a sledge hammer to crack a small nut. I have up until now used a cup and a sponge to empty the bilge. However on a holiday cruise with the table in situ, it becomes a frustrating chore to remove the table every time I need to drain the bilge. A keel stepped mast is fine until it rains!
I am inclined to drill the wee hole as advised by the others.
Mikemonty suggests putting the loop closer to the pump. This is a physical impossibility on my boat.
Thanks to all
Regards
Dave

 

[Refueler]

Reason I posted my bit was your post was a little confusing - it implied water was 'flowing back down pipe' when disconnected.

I feel you may have too much resistance in your system. Once you get exit pipe cleared and try again - the pump is not trying to push a head of water already there. It basically is getting a head start and then following through.

I still feel inclined to straighten out run - get free'ist run you can and put NRV after strum suction.... or immediately at pump discharge. If immediate after pump then no debris basically goes through pump and cause NRV to stay open.

 

[mikemonty]

Depends on the boat...
If I needed to do this I could run the loop up beside the engine cooling loop, beside the quarterberth, then back and down towards the transom.

But he doesn't need to do this - if he could live with the few cupfuls of EXTRA bilge that would fall back down the pipe when the pump is swiched off. He's never going to remove all the water with his pump so the difference is really in how much he wants to sponge out.

The popular premise is that the pump is getting airlocked - the suction is presumably submerged but the impeller is not
(I think this unlikely - SURELY Mr Rule introduced a bleed hole in his pump casing so that it couldn't airlock while the suction was submerged? Or else arranged the pump discharge so that air would be naturally be displaced out?)
so a pinhole in the pipe above the level of the pump allows the level in the pump to rise until it can prime.

BUT when he starts the pump with his NRV in place, its like a cars wheels when trying to do a hill start on ice for the pump. It will jump to full speed and then sit there and thrash away against the back-pressure of the head of water behind the NRV.

So he takes out the NRV, the water flows back to the bilge, and hey presto we have a good low discharge head so the pump can generate its flow (very important! we call a stall in centrifugal compressors a surge, and it can destroy million pound machines, I guess its cavitation when applied to pumps) , overcome the NRV, and gradually build up to full discharge head.

Two points re this NRV:
1. If it is a flap valve it must be brand new to actually maintain a complete seal on backpressure - THAT ain't gonna last for long so the problem will resolve itself (though not to the OPs satisfaction)
2. If its some form of spring loaded valve - then Gee! I wonder (hint - pressure drop!) what possible (hint - pressure drop!) factor (hint - pressure drop!) could be creating the problem (hint - CAVITATION!).

For the sake of a few EXTRA cups of water in the bilge he has made his pump unreliable, and even if the pinhole theory turns out to be correct the NRV has compromised the performance and reliability of his pump for the sake of the same few EXTRA cups of water. Unnecessarily.

 

[Refueler]

Which tends to agree with my assessment - pump is already loaded and basically cant pump. Release the head of water and it pumps.

 

[oldharry]

Had this same problem with an NRV on a Rule 500 and it is, as has been suggested being airlocked by the NRV. One point to bear in mind is that the backflow of water from the pipe after the pump stops backflushes the pump strainer and washes any debris out so that it will prime and pump properly next time. Thats more important if using an aoutomatic float switch set up, as the pump would just run on and on until the battery goes flat!

But it does leave rather more water in the bilge than I wanted, so I continued with the NRV and a leak hole to allow the pump to reprime after each use, as suggested above. I also set the NRV a bit further along the pipe so as to allow a bit of backflushing.

 

[misterg]

[ QUOTE ]
SURELY Mr Rule introduced a bleed hole in his pump casing so that it couldn't airlock while the suction was submerged?

[/ QUOTE ]

No he didn't. He felt it better to include the following statement in the installation instructions:

Do not allow the hose to dip or drop anywhere along the hose run, this can cause an air lock. /forums/images/graemlins/smile.gif

Hence the need to introduce one's own bleed hole should the need arise.

I don't like NRVs, either, but can understand the OP's reasoning.

Andy

 

[mikemonty]

Not to make a career out of this, but I went looking and found this:-
Rule pump instructions

If you can't be bothered with the link - or can take my word for it - the salient point is where it states-

"The installation must allow for complete drainage of the hose. All air/water pockets must be eliminated by having the discharge hose running level or continuously upwards."

I'd say that "completely drained" is fairly unequivocable. /forums/images/graemlins/smile.gif
And "water pockets" would include that retained by a NRV. And finally, I'd hazard a guess that the reason for this instruction is to minimise discharge head at start up.
(And yes I do know one shouldn't start a sentence with an "And" /forums/images/graemlins/wink.gif)