Is my salt water pump too big?

[capnward]

6 years ago I installed the Moyer heat exchanger. My previous heat exchanger was much bigger, about 18" long and 5" across. A friend told me it was big enough for a 90-foot boat. Since then I have been using the same saltwater pump, a Jabsco 777-9001, about 3"diameter impeller, running off a belt to a pulley on the flywheel at the same speed as the engine, giving I estimate something like 15 to 25 GPM. I know this pump is much bigger than the pedestal pump Moyer sells to run salt water through the heat exchanger. Moyer's pump pushes as much water as the fresh water pump does. Last year my Moyer HX developed a leak where salt water exit tube is attached to the body of the HX. I did a repair with epoxy and silica, but it didn't hold up so I got a local guy to silver braze it. He said there wasn't much left to work with. I brought it home and wire brushed it, but he didn't build up material at the joint underneath the exit tube, so I opened another hole there maybe a millimeter across. I fixed that with epoxy. I may have to buy another Moyer HX. I wonder if the force of the bigger pump is eroding the HX too much. Or maybe I just let the pencil zinc in it get too eroded.
Any thoughts?

[BunnyPlanet169]

Completely Wild Speculation: I think you may have a few things going on and there's no way to know which is most causal. Too many variables.

I suspect the inherent back-pressure of the HX is one issue, assuming that the flow after through rest of the exhaust is relatively larger.

At 1750, your pump (from specs on the interwebs) can move 23 GPM at 4 PSI, the Oberdorfer 202M7 can move about 6 GPM at the same back pressure. Both pumps kind of max out at 26PSI back-pressure and 15.5 and 2.75GPM respectively. So, your pump will push more water a lot harder than the HX normally sees. It would be an interesting experiment to put a temporary pressure gauge in the lines, and measure the pressure drop across the HX.

But I don't think it's just pressure.

The fact that the pinhole is at a sharp corner near the exit fitting suggests to me it's partly a physical erosive process - the volume and focus of flow is contributing. You may have a similar situation developing on the entrance. (Note that the bypass exit inside the thermostat housing is the common place for erosion. Also according to lore - the side of cylinder #3 inside the water jacket where the water inlet is often directed.)

Last, I can't evaluate your zinc, but your comment suggests maybe you think you should have replaced more frequently. I think the rule of thumb is no more than 50% loss to replacement.

Again - all of this is wild speculation.