Can a centrifugal pump evacuate a closed tank?

[redboat219]

I want to evacuate water out of a closed ballast, would a centrifugal pump work. Not looking to evacuate all of the water out , say just 30-40% total volume.

Why a centrifugal pump and not a positive displacement one? Well I dont want to buy something I need to order overseas. I plan to use an ordinary windshield washer pump, readily available here.

Think Subculture and Albion are the best ones to answer this.

[He Who Shall Not Be Named]

No. Gotta be a positive displacement type pump.

M

[Subculture]

Haven't tried pulling against a vacuum with a centrifugal pump. Can't imagine you'll get much if any water out.

You could use a pressurized ballast system, which fills about 60% of a sealed tank. The air compresses to about 40psi, so the tank needs to be sturdy, and you need baffles to reduce sloshing upsetting the stability of the boat.

This is the system Sheerline use in their range of kits and they use centrifugal pumps.

[redboat219]

Haven't tried pulling against a vacuum with a centrifugal pump. Can't imagine you'll get much if any water out.

You could use a pressurized ballast system, which fills about 60% of a sealed tank. The air compresses to about 40psi, so the tank needs to be sturdy, and you need baffles to reduce sloshing upsetting the stability of the boat.

This is the system Sheerline use in their range of kits and they use centrifugal pumps.

Ok. Had an idea in my head about doing an aspirated-pump ballast system similar to what Norbert Bruggen has in his book. It's a simple system but has a drawback that it won't work if the snorkel head is submerged so I was thinking of adding a flapper valve to the snorkel head. Instead of sucking in additional water the pump should pump out water already in the tank. Once the snorkel breaks the surface the seal opens letting air in.

[Subculture]

You can get little diaphragm air pumps off ebay for a couple of quid. they tend to be a lot more efficient than washer pumps, which are usually fairly large and fairly thirsty.

Just stick with the basic snort system, it's about as simple and reliable as you can get.

Oohh and finish you kilo!!

[roedj]

Just for the record, what kind of pump, PD or centrifugal, are the two pumps used in the Merriman/Caswell SNORT or SAS system?

Dan

[Subculture]

They're positive displacement diaphragm based air pumps.

[roedj]

They're positive displacement diaphragm based air pumps.

Thank you. That's what I thought.

But I have a follow-up question, please, as I'm a little fuzzy on this whole pump thing.

It's my understanding that both Sheerline and OTW use centrifugal pumps which require a shut-off valve, of some sort, to trap the water that's been pumped into a holding tank else the water will just flow out again around the pump impellers when the pump is turned off. Is this correct? The upside to this is that if the pump is inadvertently left on it will continue to pump up to the maximum pressure for the pump design but no more thus it cannot, under normal circumstanxes, lead to a burst hose system and a disastrous flooding of dry areas.

On the other hand, the Merriman SAS/SNORT system uses a PD type of pump which will deliver a set volume of liquid regardless of opposing head pressure and thus, if left on, can lead to a burst hose with subsequent flooding. Merriman gets around this by having the discharge into the outside and not the inside of the WTC thus if the pump is left on no harm is done. Is my understanding correct?

If you have the time, please answer this before you-know-who gets on here and hands me my posterior for even questioning his system.

Dan

[redboat219]

You can get little diaphragm air pumps off ebay for a couple of quiud. they tend to be a lot more efficient than washer pumps, which are usually fairly large and fairly thirsty.

Just stick with the basic snort system, it's about as simple and reliable as you can get.

Oohh and finish you kilo!!

Yes. But the basic snort wouln't work with the snort head underwater. Was thinking along the lines of this http://www.gentoosjournals.co.uk/Lin...-S-Winans.html

just swapping out the gear pump with a centrifugal one.

Principle is actually similar to David's SAS.

[Subculture]

To answer Dan's question first.

The OTW and Sheerline systems are both water pump systems, and they both compress air inside the boat, but that is where the similarity ends.

The OTW uses a Shurflo nautilus pump, which uses a kind of slinger disc, so it relies partly on centrifugal principles, but it's slightly different to conventional centrifugal pumps. The design has the advantage of pumping bi-directional, but it is unable to make more than about 15 psi pressure (at which according to the manufacturer the flow is about zero). The OTW system is configured to vent pressure into the dry areas, this a permits the system to fill the tank to about 80-90% full, and also keep the pressure level down, which I think will operate at about half an atmosphere (7-8psi). Metal probes mounted inside the tank detect the water level at full and empty preventing the pump from operating when the tank has been emptied and when it is full and in danger of pushing water into the dry spaces. An electronic board mounted underneath the pump deals with all of this and also has a built in failsafe. The pump is run when filling and emptying the tank, and a solenoid actuated valve prevents backflow through the pump as you stated, as the shurflo pump is not self-checking.

The sheerline system uses a car type centrifugal windscreen washer pump, and a sealed ballast tank which doesn't vent into the dry spaces. These pumps can make about 40psi, which is necessary to get about 60% of the tank filled. Overfilling of the tank isn't really possible, the pump simply stalls out if you try and fill the tank too much. To empty the tank a pinch valve is released and 40-ish psi pushes the water out fairly easily. Electronics are very simple with this system, all you need is a servo and microswitch, which operates both the pinch valve and the pump.

Each system has pros and cons. The Sheerline is cheaper and uses less electronics, but you need a 20-30% larger ballast tank for equivalent ballast volume. With the OTW, if the pump stuck on (e.g. the electronics go ape) then it is possible to flood your dry areas. Also the air is humid, so you need to vent the WTC after a run. Clearly with the Sheerline system you can't flood the dry areas under the same circumstances as the tank is isolated from the dry spaces.

Both systems have tanks with exposed water surfaces, so they are baffled to cut down on sloshing and the subsequent effect on a shifting C.G. My own personal experiences of operating subs is that systems of this type tend to be a little less razor sharp than boats which have no exposed water surface. A little like the difference between a car that has rack and pinion steering versus worm and nut. With boats which have a larger metacentric height the difference is less noticeable.

With Dave's system, you can't over pressure it, if you leave the pump on, you'll just blow bubbles in the water! In the case of the SAS if pumping from with the cylinder, the pump will eventually reach a point of maximum vacuum (which I suspect will only be a few inches of mercury at best).

[Subculture]

To answer Romel's question. Colin Gross's boat used a geared pump because he needed bidirectional pumping. The servo valved snorkel allowed the tank to be aspirated when above the surface. You could use two centrifugal pumps, set-up to pump in pump out, but really that is going to take up a lot of room, and these days with air pumps so cheap and plentiful, why bother?

[He Who Shall Not Be Named]

Andy gave the best over-view of the operating principles of the OTW, Sheerline, and my ballast sub-systems I've seen to date. Little to add other than to explain why the positive displacement type air-pumps we use won't get hammered when pumping into a dead-header:

Our diaphragm type pumps won't mechanically stall (stop) when the vacuum gets to the critical value at the suction side, or the discharge side of the pump is presented with a dead-header; situations were air/water will no longer be pumped from point-A to point-B. This is a unique feature to almost all positive displacement type gas and/or liquid compressors/pumps which will lock-up at a high differential pressure.

Our little positive displacement LPB's employ an elastic diaphragm(s) -- as the force increases with differential pressure (pressure difference between suction and discharge sides of the pump/compressor) instead of transferring the force to the motor, instead the diaphragms bellow out; they shift the motors energy from gas/liquid compression, to rubber diaphragm distortion. Gas/liquid flow may all but stop, yet the pump will not lock-up and physically stall the motor -- it will keep running, but to no useful effect. Our LPB will not stall, even when pushing a non-compressible fluid into a dead-header.

M

[roedj]

Andy,

Yes. thank you. That was a very clear explanation of the pump system types and answered a lot of my questions.

Dan

[roedj]

Our diaphragm type pumps won't mechanically stall (stop) when the vacuum gets to the critical value at the suction side, or the discharge side of the pump is presented with a dead-header; situations were air/water will no longer be pumped from point-A to point-B. This is a unique feature to almost all positive displacement type gas and/or liquid compressors/pumps which will lock-up at a high differential pressure.

Our little positive displacement LPB's employ an elastic diaphragm(s) -- as the force increases with differential pressure (pressure difference between suction and discharge sides of the pump/compressor) instead of transferring the force to the motor, instead the diaphragms bellow out; they shift the motors energy from gas/liquid compression, to rubber diaphragm distortion. Gas/liquid flow may all but stop, yet the pump will not lock-up and physically stall the motor -- it will keep running, but to no useful effect. Our LPB will not stall, even when pushing a non-compressible fluid into a dead-header.

M

DM,

So are you saying that the Merriman PD pumps could be used in a trim tank fashion?

One would use the normal Sub Driver to flood the boat down to a slightly positive buoyancy and use the PD LPB pumping water in and out of a separate small trim tank (perhaps 5% to 10% of the main ballast tank) in order to control overall density pf the boat and thus its buoyancy? I fully realize I'm sailing into fairly treacherous waters here so be gentle (who am I kidding?).

Dan

[He Who Shall Not Be Named]

DM,

So are you saying that the Merriman PD pumps could be used in a trim tank fashion?

One would use the normal Sub Driver to flood the boat down to a slightly positive buoyancy and use the PD LPB pumping water in and out of a separate small trim tank (perhaps 5% to 10% of the main ballast tank) in order to control overall density pf the boat and thus its buoyancy? I fully realize I'm sailing into fairly treacherous waters here so be gentle (who am I kidding?).

Dan

Other than these are one-way pumps, no problem using it (them) this way. Use either two pumps or employ a re-direction valve to select direction of flow between forward and after trim tanks. Run a vent balancing line between the tanks so you don't get any air compression/vacuum issues within the tanks as they fill/empty.

Do'able. But .... why? Another Plumber's nightmare.

M