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Blue Stocking · 27-05-2012, 04:32

"Suction" is a non-technical nomenclature word, used in every day speech, to describe the phenomenon taking place in a "below atmospheric pressure" evironment created inside a non-porous, open- ended container, when one end is immersed in a liquid, and the other is attached to a sealed- connection device designed to remove such atmosphere.
When the atmosphere within the container is removed, the weight of the external atmospheric pressure (14.7psig) forces the fluid up into the container. The rate of transfer of fluid thru the pump chamber(in this case) maintains the reduced container atmosphere, and the forcing upward of the fluid.
"Suction" does require the pump to have a sealed "reducing volume area", such as the segment between the impellor vanes, when distorted by their travel over the cam.
There are other ways to remove the atmosphere, such as pitot, and exhaust ejector primers, as any fire service pump operator can tell you, and, as I taught them in the latter part of my 17yrs as an instructor in the FD.
Just scratching the surface of fluid dynamics here, FWIW

cas206 · 27-05-2012, 06:10

daddle · 27-05-2012, 06:25

Wiki is wrong. The fluid is not lifted by the pump. It is pushed up by atmospheric pressure. Diameter has nothing to do with the force required. Diameter does affect how much WORK is required to PRIME the system as a great volume of fluid will need to be lifted. But once prime is achieved it is same same.

**sailorchic34** · 27-05-2012, 09:42

Going way off topic now...
Wiki, is not that wrong, but its two different things. When lifting water with a positive displacement pump, be it a diaphram or old fashion hand pump, the lift is generated by the displacement volume of the positive displacement chamber. So in that case using a smaller diameter pipe would make it easier for the displacement pump to extract the fluid, due to leakage issues in the design.

Using too large a pipe say 8-20 inches could cause the positive displacement pump to not operate correctly, depending on the efficiency of the pump, slippage, etc.

Now a water pump">raw water pump in a smaller vessel uses a rubber impeller working as a positive displacement pump. Its also most of a time a flooded suction so NPSH is not really a factor and can be ignored. From an engineering perspective its important, but not relevant to the discussion.

So short answer is everyone is right. Suction head related to pumps is really just the inlet pressure at the pump. For centrifugal (Non-positive displacement) pumps they like to operate at something above 10-15 feet absolute. It really varies with the impeller diameter, rotational speed, volute design, etc. Some need suction pressures above 14.7 psia (0 psig).

Displacement pumps can to a point ignore NPSH and can pump, in theory anyway, to a far greater depth. The limiting factor being efficiency of the positive displacement pump. Using a smaller diameter pipe allows the PD pump to pump, IE fill the displacement chamber easier at higher lifts.

So Ex-cali is right and so is Dennis G. But your talking about two different things...

Probably clear as mud now.

**Paul Elliott** · 27-05-2012, 09:47

Back to the hose diameter -- I would be worried about an oversize hose clamped onto a below-waterline through hull. If the hose slips off, or the clamps break, the boat is going to sink.

cas206 · 27-05-2012, 10:21

First off, no one is off topic. Even though the theory is getting deep and probably in the noise, it's all related to my question (and fun to read). Fun? Ok, I'll admit I was an electrical engineer in a distance past.

So far sailorchic34 and Paul's opinions on the questionable seaworthiness of a hose on hose (no snickering plz) installation below the water line is the best argument I've seen for getting the right size. I'm convinced. But please continue to expound on the hose theories if you wish.

Regards,
cas

Cheechako · 27-05-2012, 10:28

All the armchair engineering aside....:>)..... to the OP: you are on the right track, clean everything out using a pressure hose if required. Did the previous owner have it moored in shallow water? if he ran the engine often with it in gear, he may have stirred up mud and that got it in the system. That's a Raw water cooled engine right?... need a good flow to try to get that fine muck out of there somehow...... or your engine's going to have "hot spots" where the muck has filled up the cooling cavity around the cylinders..... also, beware of using jury rigged hose on a suction side, it needs to be reinforced or may collapse restricting flow while the engine is running!

cas206 · 27-05-2012, 10:56

It is a raw water cooled engine. It was tied up to a pier at a private residence. The water was too shallow for the draft of the boat. He had to wait for the right tide to get it in/out to the gulf. Plus, there is the paper documentation showing multiple towings due to being grounded. He was a want to be sailor who admitted to me he wasn't really a sailor after owning this boat.

I've checked out several online hose articles. I'll probably go with something like this:
http://www.westmarine.com/webapp/wcs...classNum=50651

Oiy. Just looked at the clock. Time for me to head out to the boat for more flushing. The sun intensity should be starting to fade and I still have time before the evening humidity starts to climb.

cas

**David M** · 27-05-2012, 11:10

Quote:

Originally Posted by Dennis.G

I respectfully believe that you are not correct on this.

The static head is determined by the vertical height liquid has be be moved. The volume does not matter. The pressure of a column of water only depends on its height (when static). When pump moves the water you need to add the dynamic or friction loss head pressure to the static, which would total a higher head with a small hose.

Dennis is exactly right, static lift is determined by height and not hose diameter. Dynamic lift is determined by both. The smaller the hose and/or the greater the flow the greater the dynamic resistance.

Florida Mike · 27-05-2012, 13:55

Back flush, I would start off duty each section separately. You may have to take the heat exchanger apart ( in so equipped ) and rod out the passages. If you are trying to get by as cheaply as possible you can go buy a junkyard again a couple of speedometer cable’s and use them like a snake with an electric drill. Incidentally if you have compressed air available while you are back flushing you might figure out a way to inject air into the water stream this will add agitation to flow. Just don’t go to wild with the air. If the engine is raw water cool then you definitely want to remove the thermostat and back flush with their. Just my two cents, Mike.

barnakiel · 27-05-2012, 14:06

Have MD7 here. No oversize items anywhere, just plain & correctly sized fittings and you are fine.

Worth converting to fresh water unless you already have it.

BTW this engine does not push a lot of cooling water thru the system - not as much as some newer Yanmars and other such likes. Just test the thermo and the sensor and you are fine though.

Cheers,
b.

CarinaPDX · 27-05-2012, 14:37

I had a raw-water cooled Sabb 18hp (still have pending more removals, but in storage). It only moved about a liter of water a minute. It did not have a thermostat, so faster water flow would have prevented the engine warming up in cold water. The inside of the engine does suffer from mineral build-up over time, so the insides need to be cleaned when overhauling and the wet cylinder liners are out. If the buildup is allowed to continue for too long the mineral scale will start to flake off and join the flow of water, sometimes collecting in places of restricted flow and blocking the cooling water. I suspect the Volvo is similar. So try to get a peek inside the cooling channels in the block or head and determine the level of scaling. There are probably plugs with zincs on them which can be removed to access the water chambers. Needless to say, these zincs need to be kept current as well.

Greg