Cooling paranoia or common sense?

[GILow]

Hi all,

I was just perusing the xylem systems site, thinking about engine cooling (as you do) when I stumbled on the following remark.

"DO NOT fit gearbox or engine oil coolers in the pump suction system. Always install them after the pump."

This was on the following page:

https://www.jabscoshop.com/advice-an...ling-pumps.htm

So, this is a bugger because my transmission cooler is most certainly on the input side of the pump.

Does anyone know if this bit of advice is just a general rule to cover the risk of introducing air into the suction side of the pump and thus stuffing up the flow characteristics or is there something more subtle I should be aware of?

Matt

[Pete7]

Probably more to do with pressure, since the pump will create a partial vacuum which is only filled by air pressure forcing sea water into the cooling circuit from the outside. Whilst fitting the gearbox cooler afterwards means the pump creates a positive pressure which will be much higher to force water through the cooling circuit. You could see a bad layout with long pipes and bends having too much restriction to work properly.

That said my Volvo 2003 draws water from the seacock and then pulls it through the gearbox first before going on to the pump and the rest of the engine.

Pete

[GILow]

Thanks Pete. So is your Volvo setup "standard" as far as you know? My setup seems to be a standard Diecon setup (Diecon were a company that did marine conversions of engines in Australia, they no longer do so).

Matt

[Oliver L.]

Hi Matt,

it's a fairly simple matter. A watertight connection is by no means airtight - air will get into a standard hosebarb connection with ease. So when you design hydraulic systems you try to have as few connections as possible below ambient pressure, because the vacuum will pull in the air. The easiest way to do this is to put the pump first into the loop and then the remainder of the system has positive pressure and is safe.

What can happen? The only thing you have to watch for is airlock. When the pump draws in air, you can lose prime and thereby lose cooling. If your system has been working fine, I would not worry.

Oliver

[svseachange]

Our Volvo Penta AQD21B (Marinized Peugot 504/505) has a transmission and oil cooler enroute to the pump.

A few weeks ago I replaced the raw water strainer and hoses and was also wondering about the long path from the raw water inlet to the pump.

[a64pilot]

Any pump, pumps better than sucks. Max suction possible is about 14 PSI, but max pressure, is well usually a lot higher. Unless very shallow, a well pump is down in the well for example.
As a general rule always put "things" after a pump, it's why I wonder why seemingly all fuel filters are on the suction side? Perhaps we don't want to force fuel through s blocked filter, damaging it and allowing dirt through?

[TeddyDiver]

There's a difference between water and oil or fuel. Water molecule is quite small, as small as most molecules in the air. Diesel fuel molecules are 10x larger and lubrication oil even more..

[thinwater]

Actually, in a cooling system the main problem not air at all, it is that water can be near its boiling point. Like having air in the system, the pump will fail, and it will fail when you need it most.

So long as everything stays below ~ 150F and there aren't too many bends the problem is not serious, but it can get bad fast.

It's just a good engineering thing that is often missed when installation guys get their hands on it. I promise the design engnieers did not draw it that way. I once started a recall of thousands of transit buses when a similar installation goof started ruining engines.

Worth fixing? Probably not, but the details (plumbing and cooler) matter.

[NorthCoastJoe]

From my experience teaching pumping in the fire service, pumps push much more than they can suck. I would have nothing between the pump inlet and the seawater strainer and keep the run as short as you can, no extra fittings to create friction loss. One 90 degree elbow creates a large friction loss, anything disturbing the flow will make the pump work harder. I would place everything down stream of the pump and use high quality hose and fittings.

[B_Wieberdink]

On a boat the gearbox cooler is normaly below waterlevel, so suction is normaly not a problem.

I allway wonder why most diesel-installations have the filters at the suction side, every leak sucs in air and the engine will stop. I mostly put in a separate pump immadiatly at the fuel tank.

[Kokanee]

Restrictions such as the gearbox cooler installed upstream of the pump will slow the circulation rate more than if installed on the downstream side. Especially if the impeller is slightly worn and not sealing 100%. The pump isn't acting in a fully positive displacement mode.

My new Vetus M4-45 is installed with the gearbox cooler upstream. I'd guess this may be to keep a standard engine hose configuration instead of having different setups for each available gearbox.

It has worked ok so far, but I noticed recently that my temperature is starting to climb a few degrees at higher revs. My impeller may be getting a bit of wear. I may look at reconfiguring the system.

[Pete7]

Originally Posted by GILow

Thanks Pete. So is your Volvo setup "standard" as far as you know? My setup seems to be a standard Diecon setup (Diecon were a company that did marine conversions of engines in Australia, they no longer do so). Matt

Matt yes standard set up. The VP 2003 gearbox has a small copper pipe that runs through it. You can see the circuit in this photo (not my engine). After the gearbox the black hose goes to the saw water pump on the front of the engine.

Pete

[sailorchic34]

As a general Rule all heat exchangers should be down stream of the raw water pump and not on the suction side. Reason being is there is a pressure drop across any heat exchanger and that will effect inlet suction pressure in what is essentially a unpressurized system.

As A64Pilot indicated, pump inlet suction pressure available is always lower then discharge head pressure.

While the typical raw water impeller is positive displacement, it is a low head pump and it works better with all the major head losses (from heat exchanger pressure drops) on the discharge side.

This is a much larger issue with centrifugal pumps which have generally very low NPSH (Net Positive Suction Head) Many an engineer has been caught, not understanding that very issue.

[sailorchic34]

My yanmar as a transmission cooler, but the cooler inlet is connected to the discharge of the raw water pump and transmission cooler outlet is connected to the coolant tank heat exchanger inlet.

[Stu Jackson]

Originally Posted by a64pilot

..........it's why I wonder why seemingly all fuel filters are on the suction side? Perhaps we don't want to force fuel through s blocked filter, damaging it and allowing dirt through?

Exactly. In all proper pump applications, there is a filter before the pump. It "saves" the pump from damaging materials that don't belong inside the impeller.

For fuel systems, most factories plumbed tank>pump>filter. The little filters on the bottom of those Facet fuel pumps then ended up getting clogged. Most of us have corrected those poor installations.