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Old 11-06-2020, 09:51 PM
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Flow Sensor Maintenance

A few months ago, forum member Ando had concerns about growth inside his raw water flow sensor. As a result, he provided his sensor to me for a close examination which did indeed show some growth but not enough to affect its operation, at least that was the case on the bench. I went a step further and removed my own sensor for examination as well. It showed evidence of minor growth too but in my case, it DID affect operation. The effect of the growth inside mine caused the movable magnetic spud to stick closed. It wasn't stuck hard but it did not slide like a new one, the way it should.

Flow sensor autopsy

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During the development of the sensor for our application, added restriction was anticipated, tested and measured. Bench flow testing of an Oberdorfer M3 with and without a sensor showed a 7% flow reduction imparted by the sensor. It's important to know the test was done with a brand new, pristine sensor.

If you added a deck winch you added its maintenance too. If you added fresh water cooling, youíd have another pump and heat exchanger to maintain. Oil filtration system? Same. And so it is with the flow sensor to preserve its original performance. That is, to keep the restriction it imparts as close to the original and expected 7% as possible.

What measures could you take to maintain the sensor? You want to keep debris out of it so a decent quality raw water strainer would be important and Iím not talking about those horrible external thru-hull grates. To prevent growth inside the sensor youíll want to flush it with fresh water as a regular part of boat clean-up following an excursion. With a flushing Tee installed it is very easy to do, takes only a few minutes. Regardless of whether youíre RWC or FWC, a regular fresh water flush of the raw water system is beneficial to the entire system too.

Pump intake flushing Tee

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Here are a couple of other things to consider:
  • Placing the sensor in the discharge side of the raw water pump is preferable to the suction side. It will have less of an effect in that position.
  • If you have a Sherwood or Oberdorfer M3 raw water pump, their performance when they were new is less than the Oberdorfer M7 or MMI 502 pumps, worse yet after 45+ years of service so the 7% flow reduction with the sensor installed will have a greater effect with the original equipment pumps. (Hint-hint)
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Old 11-11-2020, 06:47 PM
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Flow Sensor Problem

Iíd like to raise a related problem. I have the EWDS, which includes the flow sensor. I also have a PSS instead of a stuffing box; and because I canít leave well enough alone, that PSS is fed cooling/lubricating water from the same exhaust from the (FWC) heat exchanger. In other words, the cooling (Bay) water exhausted from the HE is split, with part going to the standpipe and part (what I thought was a small part) going to the PSS/cutlass bearing. But when we test-ran it, a WHOLE lot of water was being emitted from the PSS/cutlass, and relatively little was making it thru the standpipe and out the transom. This didnít seem prudent, so we removed the sensor, and Presto!ó-MUCH more water came thru the transom, and a lot less, but still a pretty decent flow, thru the cutlass bearing. This made me happier than the alternative; but Iím now without the benefit of the flow sensor. I should note that we have the Moyer, not the Oberdorfer, raw water pump. Has anybody else had this problem? Is there a bigger flow sensor I could use, so it doesnít so much restrict flow? Thanks!
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Old 11-11-2020, 10:55 PM
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I am curious to know where in your cooling system the sensor was located? If it was in the raw water pump discharge I am having difficulty imagining why it was causing the asymmetry in water flow so far downstream

Peter
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Old 11-11-2020, 11:22 PM
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Did you have any elevated engine operating temp problem? How long have you had the sensor installed? Has it ever been flushed as described earlier? I'm also interested in your reply to Peter's question.

I'd be happy to pay round trip shipping if you would send me your sensor for careful examination and testing.

As for the question about a larger sensor, most if not all of the flow sensors I've run across have their switching the opposite of ours. That is, normally open contacts (NO) whereas we need normally closed (NC).
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Old 11-11-2020, 11:37 PM
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It definitely was in the raw water discharge. Leaving the heat exchanger, I took a tee off it, which stepped down to a smaller line, perhaps 1/4 inch or 3/8 of an inch, which ran to the PSS. The remaining, main, portion of the raw water discharge (5/8Ē?) ran to the sensor, and then to the standpipe. So my belief is, that because not enough water was making it through the sensor, that it was backing up, upstream, and the only place for it to go was out through that 1/4 inch or 3/8 inch line to the PSS.
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Old 11-11-2020, 11:44 PM
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Just now seeing Neilís post. This was a brand new installation, i.e. a new to the boat A4, never before run. The test was actually conducted while in the slings, using hose water, emptied into a bucket and pulled by the Moyer pump. Thus marine growth was never an issue. For the same reason, I have no feedback on overheating. The engine simply never had a chance to overheat, because when I saw what I saw, we shut things down. There was simply way too much water coming out of the PSS. It just didnít look right.
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Old 11-11-2020, 11:58 PM
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I have a short video of water coming from the PSS, but apparently I lack the technological expertise to post it.
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Old 11-12-2020, 07:36 AM
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So that location will definitely lead to the problem you have described. Try putting the tee after the flow sensor

Peter
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  #9   IP: 104.174.83.118
Old 11-12-2020, 09:36 AM
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I agree with Peter and think it would be worthwhile to place the sensor immediately after the raw water pump, the PSS supply Tee after the HX. Since the PSS flow is actually open out of the boat, a metering valve in the supply line to it would allow you to control its flow - - - and the flow penalty to the exhaust system.
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Old 11-12-2020, 11:21 AM
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Those all sound like good ideas, and I thank you both, and Iíll try them. But somehow they seem to me to be a Band-Aid on the issue, which is flow restriction caused by the sensor. Iíd observe that Neilís earlier posts seem to confirm that the 7% figure comes from a bench test of an Oberdorfer M3. I have the MMI pump, which puts out significantly more flow, as seems generally acknowledged. So right off the bat it seems that the restriction is likely well in excess of 7%, and thatís before any fouling (which is not yet an issue in this brand-new installation.) Now, I donít know that the flow restriction is in fact a problem. Neil asked about overheating, and that isnít even on the table right now. I was just concerned because so little water was coming out the transom and what was coming out felt hotter than I thought it should, and that concerned me. And the video I shot of the flow from the PSS while in the slings shows water shooting out like a showerhead. So it seems to me that if the engine is cooling properly, maybe flow restriction isnít a big issue, although that seems to diminish at least one of the reasons why one would go for the MMI pump in the first place. And I agree that one could put a metered valve on the line to the PSS; but then thereís no way to tune that flow while the boat is in the water, as far as I can tell. Iíd still feel much more comfortable with a bigger flow sensor, which I suspect might also be less susceptible to the fouling issue. Iím aware that too much flow could be a problem; but Iíd rather put that metering valve on the whole cooling line, not just the PSS line, right where Neil suggests putting the sensor: right after the pump and before the HX. That way I could tune the flow at the headwaters, so to speak. It seems to me that the soundest approach would be to allow for max flow at the Ďheadwatersí end of the freshwater circuit but measure it with the sensor at the far end, since what weíre concerned with is ultimate flow falling below a certain minimum. It may be far-fetched, but if the sensor is upstream, and there was a leak downstream, the sensor would say everythingís fine, when in fact it was not. Much appreciation for the responses and suggestions.
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Old 11-12-2020, 05:08 PM
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I'm going to propose that by designing in an open path (PSS line) before the sensor, whatever restriction was added by the sensor (the measured 7%) had a greater effect on the flow to the waterlift exhaust regardless of pumping capacity. The PSS line introduced a path of lesser resistance so that's where the water naturally went and your observations confirmed it. Because the design placed the restriction after the HX it would have had no effect on the engine temperature because all related components in the engine's cooling system had full flow. However, the exhaust system could have suffered component damage due to insufficient cooling.

As suggested before, well designed cooling systems with components in good condition have not displayed ill effects of adding the modest restriction of the flow sensor. Your system with the PSS line and sensor placement was a significant departure from the norm.

There is no quarrel that the sensor imparts a restriction but if it's small enough that it does not impair the overall engine cooling, the benefit outweighs the nominal restriction.
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Old 11-12-2020, 06:04 PM
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Thank you, Neil. Much appreciated. I agree with you that by placing an open path before the sensor, that the water would have taken the path of least resistance. That makes sense to me. Iím less able to agree with your second point, that because the sensor was downstream of the HX, that it had no effect on engine cooling. As I understand cooling, anyway, itís not a static but a dynamic function; it depends on a certain flow past the HX. Yes, the coolant loop is closed, but as it heats up it depends on a flow of fresh water thru the HX to cool the coolant. And if thereís a fresh water restriction past the HX, less fresh water is going to flow thru the HX, i.e. thereís no off-ramp just past the HX where fresh water, having fully flowed, is now accumulating, standing around and smoking cigarettes. To the contrary, if the exit for the fresh water is blocked or restricted, like was happening, it just means thereís less fresh water flowing to soak up the same BTUs. And what fresh water there IS in the HX is staying there longer, having more time to heat up. And certainly that could have an effect on cooling the coolant, and thus the engine. Again, I didnít have a cooling problem; I was just concerned about one. And I agree, some restriction that doesnít cause other problems is probably a worthy trade-off for the peace of mind of having a sensor. But Iím back to my original issue: if Iím going to go to the trouble and expense of a Moyer pump with its greater flow, and the trouble and expense of the EWDS, then why should I give up that greater flow, if a bigger sensor would alleviate that problem? This is not an attack on anyone or anything or any design, just asking what I think is a fair question. And I get it that sensors are off-the-shelf components, not custom; and that a bigger one might not have the same sensitivity. I just donít know, which was why I asked my original question: is there a bigger flow sensor which might not restrict flow so much? So far I havenít heard that addressed, although Neil has pointed out that most sensors seem wired the wrong way for us, which certainly suggests that there might be alternative sensors, which was, after all, the original question.
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Old 11-12-2020, 06:53 PM
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. . . but in your setup the open PSS line took up the difference of the restriction confirmed by your own observation running on the hard. The HX still had the sum of the reduced waterlift flow and the increased PSS flow. All of the flow came through the HX first before it split between the PSS and waterlift.
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Old 11-12-2020, 07:01 PM
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Youíre absolutely right. Thank you for correcting me. Any thoughts on that bigger sensor?
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Old 11-12-2020, 07:15 PM
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Quote:
Originally Posted by Wrsteinesq View Post
Any thoughts on that bigger sensor?
None beyond what has already been said. The typical contact form for most flow sensors regardless of size is normally open, the opposite of what we need. It can be reversed through a relay but that adds electrical complication. I'll repeat that your experience is not shared by the vast majority of members with sensors installed but neither was your installation.

Just trying to look at the whole picture. . . . .
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Old 11-12-2020, 07:54 PM
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I would suggest that if you found a bigger sensor that was wired correctly, the system would work in the sense that it would still give you a warning when the flow was "low". The definition of low would be a little bit different however.

If you could not find a larger sensor with the appropriate wiring, you could install some circuitry (a relay for example) to reverse the logic.

If you are determined to not lose flow and you cannot find a larger sensor that is wired right, you could install a bypass around the sensor to let the missing 7% through the system.

Both of these latter two ideas would make the whole system a bit more complex and therefore introduce more potential failure points.

You may also be able to restore flow rate by changing fittings and/or hose sizes in your cooling system. I have no doubt that some clever engineer will be able to tell us what changing from a 1/2" to a 5/8" hose would do to flow rate.

You do not mention if you are using a thermostat. if you are, unless the reduction of flow leads to the thermostat not being able to regulate the temperature, i think your engine will run at the same temperature with or without the sensor. The thermostat will adjust to get things to the same temperature.

It is true you will have lost a little "reserve" cooling capacity that you might notice under extreme conditions - running into a stiff headwind in warm seas might get your thermostat to the end of its travel.

As for the MMI pump, there are other advantages to such a pump other than its increased flow rate. Bearings versus bushings so no regular greasing required being the most attractive for me.

I guess at the end of the day it is a case of the design process being a situation that requires balancing one thing against the other. The flow rate sensor, I have the impression, was already in use with the existing Cole-Hersee system and presumably Neil determined that any advantages of going to a different sensor were outweighed by the disadvantages of changing sensors - upgrading to the EWDS would not involve buying a new sensor if you already had the Cole-Hersee system for example.

He clearly considered the flow rate issue and in his estimation it was not of sufficient concern to warrant a change. A different designer might have made a different decision but at the end of the day, it is quite likely that both systems would have ended up with an engine running at the same temperature under normal operating conditions with similar, but not identical, reserve cooling capacity.

Phew, hope that is not too much! More so, hope it is useful.

Peter

Last edited by Peter; 11-12-2020 at 07:54 PM. Reason: typos
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Old 11-12-2020, 09:05 PM
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I love this forum. Thank you all for sharing, generously, your time, your ideas, and your thoughts. What a lot of brain power/wisdom/consideration! I think Iím likely to try to find some workaround so as not to lose that ďexcessĒ cooling capacity. To me, anyway, having excess cooling capacity is kinda like having too much money: itís a condition I can generally recognize in others, but somehow have never managed to diagnose myself with. I absolutely see Peterís view of how Neil likely came to use the existing sensor, and sure, that makes sense for the fleet writ large; but selfishly Iím still wanting a solution for my situation alone, and that will likely involve a fairly unique solution. Iíll keep yíall posted as to whatever I come up with. With appreciationó
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Old 11-12-2020, 11:01 PM
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If the real goal is maximizing your cooling capacity and once again looking at the total system, increasing the size of your HX to the largest 4 pass HX you can physically fit makes more of a difference than any other system modification. Other things you can do to achieve the goal are upsize your hoses, upsize your raw water intake thru-hull, upsize your raw water strainer, reduce system elbows to an absolute minimum, get rid of the thermostat and completely close the bypass.

And yes, either upsize or eliminate the flow sensor.
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Last edited by ndutton; 11-13-2020 at 02:53 AM.
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Old 11-13-2020, 04:18 AM
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Respectfully, I think you may be misunderstanding my objectives. I never said that my real goal was to maximize my cooling capacity, although if that were my goal, you have graciously suggested a number of ways that could be accomplished, some more involved than others. I made it quite clear that I do not believe that I have an overheating problem.

But not wanting to sacrifice Ďexcessí cooling capacity which I already have, is a very different thing from seeking to increase that cooling capacity.

Everyone seems to agree that the standard flow sensor does in fact involve some degree of flow impediment. I now have a better understanding of how I might re-order various components, like putting the PSS tee downstream of the sensor, which may help even out distribution, but which doesnít address the actual flow reduction. But it still seems to me that the degree of that sensor-induced flow impediment is greater with a higher flow raw water pump like the MMI one: if you tried to flow a garden hose flow thru a mousehole it would be less impeded proportionally, than if you tried to flow a river thru it. I wasnít asking about how to make a bigger river, but rather how to better utilize the river I already have.

Which is why my question went to, is there a bigger mousehole available?

Again, thanks to all.
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Old 11-13-2020, 08:32 AM
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Quote:
Originally Posted by Wrsteinesq View Post
Everyone seems to agree that the standard flow sensor does in fact involve some degree of flow impediment.
That was never in dispute. Even the product description in the MMI catalog says it does. Another way to look at the 7% restriction is the MMI sensor maintains 93% flow.

edit: there are ultrasonic clamp-on flow sensors (external mount only) available that do not impart any restriction. The one I saw has a pulse output that would have to be translated to be useful for our EWDS.
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Old 11-17-2020, 12:10 PM
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I see two more tests that could be done to figure this out.
Put a valve on the PSS to eliminate it from the equation for testing. (may need to provide alternate cooling source temporarily unless testing at the dock out of gear.)
Test/observe flow rate with and without the flow sensor to determine if it is a problem.
If I had a choice between a spring in my way or an open path, I'd take the open path every time. This same problem was solved with a ball valve in the sideplate bypass years ago.
My $0.02.
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Old 11-17-2020, 01:38 PM
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The PSS does not need cooling water fed through the hose in a sailboat application
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