Cavitation issues

[warno]

that chart is more confusing the more i look at it. It has many different models on there with there own flow rates but it doesn't show the 90 plate at a lower flow.

 

[Bob Rohr]

Another suggestion to increase flow would be to remove the strainer basket from this Y strainer. Even brand new, with a clean screen they have a few ft of pressure drop. With any crud in there it adds more pressure drop. Leave that basket in it for a day or two after any piping work to catch debris, then remove the strainer.

Always good to have isolation valves around components that will need service. One valve at each boiler connection and on the other side of the last component.

 

[warno]

Another suggestion to increase flow would be to remove the strainer basket from this Y strainer. Even brand new, with a clean screen they have a few ft of pressure drop. With any crud in there it adds more pressure drop. Leave that basket in it for a day or two after any piping work to catch debris, then remove the strainer.

Always good to have isolation valves around components that will need service. One valve at each boiler connection and on the other side of the last component.

When I change the piping around I plan to add in a couple valves in some areas i didn't think about. The strainer is one of those areas.

 

[heaterman]

Reading through this thread and thinking back over all the times I have seen this topic and others related to it discussed without end just makes my head hurt.
I should write a book or something to spare people the pain of doing things 5 times and still not understanding why it does not and will never work...

Here's a few bullet point comments regarding open systems, pumps and pipe size.

Fact 1
Open systems such as commonly found in factory built and home made wood burners present some issues that must be understood BEFORE attempting to pipe a system up.
Fact 2
These issues are related to simple laws of physics and if you try to cheat or circumvent them you will most definitely lose. EVERY TIME.
Fact 3
Many of these issues stem from undersized pipe/tube and oversizing pumps to try and compensate for the same.
Fact 4
There is never time or money to do it right the first time. There will ALWAYS be money and time to do it over.
Fact 5
Open systems have issues not present in a pressurized (real boiler) system which will cause you to learn more than you ever thought existed about piping and pumping

There are 2 basic solutions.
*Seal the system and pressurize it...which opens another can of worms... but hey, your pumps wouldn't cavitate.
*At a very elementary level... Double the size of tube you think you need and cut the size of the pump in half.
You'll spend more on tube, less on replacement pumps and your system will work.

 

[warno]

@heaterman oh trust me I've learned a huge amount of information I never thought existed. Anyone that talks about building their own boiler I cannot stress enough to them they need to do the research. I unfortunately did not when I started building mine. Which is why I'm here proving your fact 4 correct.

 

[jwise87]

@heaterman oh trust me I've learned a huge amount of information I never thought existed. Anyone that talks about building their own boiler I cannot stress enough to them they need to do the research. I unfortunately did not when I started building mine. Which is why I'm here proving your fact 4 correct.

Reading this thread makes me realize I must have gotten lucky building my first owb, it ate pumps, but other than that it worked good. It had 3/4" piping and an 0013 Taco, proving one of @heaterman 's other points.

 

[Marshy]

Reading through this thread and thinking back over all the times I have seen this topic and others related to it discussed without end just makes my head hurt.
I should write a book or something to spare people the pain of doing things 5 times and still not understanding why it does not and will never work...

Here's a few bullet point comments regarding open systems, pumps and pipe size.

Fact 1
Open systems such as commonly found in factory built and home made wood burners present some issues that must be understood BEFORE attempting to pipe a system up.
Fact 2
These issues are related to simple laws of physics and if you try to cheat or circumvent them you will most definitely lose. EVERY TIME.
Fact 3
Many of these issues stem from undersized pipe/tube and oversizing pumps to try and compensate for the same.
Fact 4
There is never time or money to do it right the first time. There will ALWAYS be money and time to do it over.
Fact 5
Open systems have issues not present in a pressurized (real boiler) system which will cause you to learn more than you ever thought existed about piping and pumping

There are 2 basic solutions.
*Seal the system and pressurize it...which opens another can of worms... but hey, your pumps wouldn't cavitate.
*At a very elementary level... Double the size of tube you think you need and cut the size of the pump in half.
You'll spend more on tube, less on replacement pumps and your system will work.

Very enlightening and comical, thanks for the laugh.

Warno, I didn't bail on you. I m in the works of installing my own boiler and the whole family decided to get sick yesterday. I will be back.

 

[warno]

Very enlightening and comical, thanks for the laugh.

Warno, I didn't bail on you. I m in the works of installing my own boiler and the whole family decided to get sick yesterday. I will be back.

That's fine I've got little ones that tend to catch the bugs going around. I understand, family first!

My one question I'm hoping to get answered now is what kind of pressure drop at 10gpm I'll see if I go with that 90 plate HX I posted earlier. Also if the delta T will be excessively low. I mean the surface area is close to the one selection from flat plate, but a small dimensional HX i would think would have a higher pressure drop. I'm probably wrong but want to make sure.

What is the formula to figure pressure drop in a HX? I do appologize if it's been posted, we've discussed alot in this thread.

 

[Marshy]

No easy way to calculate the pressure drop from the htx. It usually comes from the manufacture. It seems common for the eBay htx's to be missing that sort of data. The rest of your system is fairly low head loss so I wouldn't worry too much about it but, depending on what low head pump you choose it could be a concern. Double edge sword.

 

[maple1]

Isn't there a way to extrapolate the drop in the table to a different flow?

 

[warno]

Ok so another development here, just for fun I took a temp reading from the bottom plate of my water jacket. The bottom is hotter then the top.

Here's the back of the boiler just under my lower port.

And heres the front of the boiler right next to the fire box.

So with that in mind I don't think pulling from the lower ports would be a good idea for the cavitation fix. Lol

I'm thinking the only option is keep pulling from the top and then dropping the pump lower to the floor. What do you guys think?

 

[Marshy]

Isn't there a way to extrapolate the drop in the table to a different flow?

There is only one data point, Im just not certain head loss is proportional to flow. It may be, I'd have to research a little. Even if not it's probably close enough for what we are dealing with.

The chart says 0.3 psi head loss at 26.6 gpm. That's like 8.3" of water column. Lower flow would result in less head loss. Thsts minor incimparison to the head loss in the rest of the system. I really want to do some calcs when I get time.

 

[jwise87]

Ok so another development here, just for fun I took a temp reading from the bottom plate of my water jacket. The bottom is hotter then the top.

Here's the back of the boiler just under my lower port.

View attachment 221220

And heres the front of the boiler right next to the fire box.

View attachment 221221

So with that in mind I don't think pulling from the lower ports would be a good idea for the cavitation fix. Lol

I'm thinking the only option is keep pulling from the top and then dropping the pump lower to the floor. What do you guys think?

According to this illustration from the document someone posted earlier that will do the same thing as pulling from the bottom. Heck, you could put the circulator in a hole if you wanted even more pressure.

 

[warno]

I understand the concept in that drawing. Basically you draw up like a siphon then it goes down to the pump and you gain that much more column of water. But how well does it work I wonder? If it works great I'd be willing to try it beings how I'm going to be replumbing the suction anyway. I'd like to hear what others think on that illustration.

 

[Marshy]

I understand the concept in that drawing. Basically you draw up like a siphon then it goes down to the pump and you gain that much more column of water. But how well does it work I wonder? If it works great I'd be willing to try it beings how I'm going to be replumbing the suction anyway. I'd like to hear what others think on that illustration.

No, it doesn't work the way you wrote it. Any of the piping above the atmospheric water level in the boiler does not add head to the pump, only friction loss from the lengrh of pipe. The head is negated by itself. The reason is you are lifting (rather, sucking) the water up on the one side and then adding it on the down side. Net change in ststic head is zero, only more fruction loss from extra pipe. That is why the reference is from centerline of the pump to the water level exposed to atmosphere. Only way to circumvent this is raising the water line exposed to atmosphere.

 

[warno]

No, it doesn't work the way you wrote it. Any of the piping above the atmospheric water level in the boiler does not add head to the pump, only friction loss from the lengrh of pipe. The head is negated by itself. The reason is you are lifting (rather, sucking) the water up on the one side and then adding it on the down side. Net change in ststic head is zero, only more fruction loss from extra pipe. That is why the reference is from centerline of the pump to the water level exposed to atmosphere. Only way to circumvent this is raising the water line exposed to atmosphere.

That makes sense. back to the original plan of just dropping the pump.

Did you happen to come across anyway to tell if the pressure drop through that 90 plate HX would kill me at 10 gpm feeding it?

 

[Marshy]

If it's proportional to flow then you can expect 0.1 psi drop at 10 gpm.

 

[maple1]

That makes sense. back to the original plan of just dropping the pump.

Did you happen to come across anyway to tell if the pressure drop through that 90 plate HX would kill me at 10 gpm feeding it?

I poked around here:

https://www.brazetek.com/products/d...changers/90-plate-brazed-heat-exchanger-1-1/4

This is still pretty darned fuzzy. My head started to spin looking at all the combos in the selecting guide. Pressure drop isn't linear with flows, and it also depends on dT thru the plate, and temps. But the bottom line is that if the plate gives 0.3psi head loss at 26gpm, it will be less loss at lower gpm. In some of those chart values, doing a simple linear calc is close, but others are a bit off. I would guess that a guess of 0.1psi at 10gpm would be ballpark. Or 2-3" of head. Which isn't very much.

 

[warno]

So the added head loss from that HX wouldn't be a huge deal then if that is correct. Because in my calculations of my ~20ft of head we rounded up to get there.

 

[leon]

Your boiler is going to steam, not good!!
If you lower the high limit to 160 and the low limit to 140 being summer operating temps
your going to have many fewer problems.

If you do that you can leave your circulator and its suction on the bottom using a much
larger circulator at a lower pressures and temperature.

When your circulator suction is not flooded you are inviting quick circulator failure.

You need at least one set of isolation valves with gauge ports.
This is to to keep track of outlet pressures P.S.I.G. on your circulator and you need to
have an isolation valve with a port for a vacuum gauge to be sure you are not cavitating
which will destroy the circulator.

You have to remember that if your circulator inlet is that high in the water column it will love to suck in
air bubbles that have not dissolved as air is lazy and will go everywhere.

 

[warno]

Your boiler is going to steam, not good!!
If you lower the high limit to 160 and the low limit to 140 being summer operating temps
your going to have many fewer problems.

If you do that you can leave your circulator and its suction on the bottom using a much
larger circulator at a lower pressures and temperature.

When your circulator suction is not flooded you are inviting quick circulator failure.

You need at least one set of isolation valves with gauge ports.
This is to to keep track of outlet pressures P.S.I.G. on your circulator and you need to
have an isolation valve with a port for a vacuum gauge to be sure you are not cavitating
which will destroy the circulator.

You have to remember that if your circulator inlet is that high in the water column it will love to suck in
air bubbles that have not dissolved as air is lazy and will go everywhere.

Not sure where you read my boiler is streaming unless you are implying about my temp readings of over boiling point. and those readings were taken right next to the firebox, hottest spots, in the water jacket. But I can assure you I'm not losing any water due to stream. And what summer temps are you talking about? My boiler is shut down during the summer. And it's been 40s here lately, hardly summer.

If I drop my high limit to 160 then charging my 750 gallon storage tank is basically pointless. I can hit 160 in my storage on about 3-4 splits and a pile of kindling in the boiler.

The circ suction is always flooded. There is 8-9" of water above the suction port at all times.

My caviation is not from sucking air in it's from flashing to steam at the impeller inlet. We've already established this.

I'm not sure you've read through this whole thread.

 

[leon]

I use Summer operating temperatures in my coal stoker all winter and they are set
at 140 low 160 high with a fifteen degree differential.

If your flashing to steam at the impeller that always indicates cavitation problems due to
lower inlet pressures(vacuum) you need to move the circulator to the bottom to have
better flooded suction without having it go to steam at the impeller inlet and lower your
operating temperatures.

With storage you already have thermal mass and it takes less energy to heat the
water up to temperature and using summer temperature settings allow the system to
respond faster too as the high limit is lower.

You could use 160 low and 180 as the high limit with no issues.

 

[warno]

I use Summer operating temperatures in my coal stoker all winter and they are set
at 140 low 160 high with a fifteen degree differential.

If your flashing to steam at the impeller that always indicates cavitation problems due to
lower inlet pressures(vacuum) you need to move the circulator to the bottom to have
better flooded suction without having it go to steam at the impeller inlet and lower your
operating temperatures.

With storage you already have thermal mass and it takes less energy to heat the
water up to temperature and using summer temperature settings allow the system to
respond faster too as the high limit is lower.

You could use 160 low and 180 as the high limit with no issues.

Once again I'm not sure you've read this thread. I understand the low inlet pressure and high temperatures is my problem. I cannot move my pump to pull from a lower port in my boiler because the bottom water is hotter then the top. Due to the design of my boiler. If you scroll up in this page of the thread you'd see that. That's why I'm going to move the circ to the floor of the boiler shed and keep pulling from the top port but with bigger piping to the suction of the circ. By doing this I will gain a taller column of water above the circ inlet.

I'm using a plate heat exchanger to charge storage because storage is pressurized and boiler is not. So I need hotter boiler water then 180 to get storage water close to 180.

 

[maple1]

I use Summer operating temperatures in my coal stoker all winter and they are set
at 140 low 160 high with a fifteen degree differential.

If your flashing to steam at the impeller that always indicates cavitation problems due to
lower inlet pressures(vacuum) you need to move the circulator to the bottom to have
better flooded suction without having it go to steam at the impeller inlet and lower your
operating temperatures.

With storage you already have thermal mass and it takes less energy to heat the
water up to temperature and using summer temperature settings allow the system to
respond faster too as the high limit is lower.

You could use 160 low and 180 as the high limit with no issues.

This has all been hashed over already in this thread. It's all here for your reading pleasure - all 8 pages (so far) of it.

[Is there a forum record for that kind of thing? ]

 

[Marshy]

Maybe they thought we were sharing chocolate chip cookie recipes for the last 7 pages?