Cavitation caused by poor intake design?

[FernTheBrute]

I've got a new 1.5 hp 2-speed Carvin pump for my above ground pool which is making a lot of noise when running at the highest setting. It sounds like gravel in the pump, but there is absolutely no air in the strainer or anywhere before or after the pump (don't see any bubbles coming out in the pool). It's dead quiet when running at low speed.

From what I gathered, it's probably cavitation, since there's no air in the system and it only occurs at high speed. My strainer basket is clean and so is the skimmer filter.

I'm running 1.5" piping in and out of the pump. The connexion from the pump to the filter is a nice arc, but I have this mess entering the pump, with a 90 bend right in front of it, The 17 y/o who was hired for the summer to install this clearly hasen't majored in... well anything !



If I were to cut the intake line to install a new straight fitting instead of the 90 elbow, what are reasonably my chances of getting rid of the cavitation?

Is it more likely the issue is with the filter itself or the plumbing coming out of it, to the water heater and then back to the pool? It also has a lot of bends.

Thanks!

 

[ajw22]

Just follow a process of elimination and simplify your plumbing one elbow at a time and see if you can get enough water flow to make the pump happy.

Check that no pipes are collapsed internally or kinked with the flex hose.

 

[Bill1974]

Most pool pumps state the inlet pipe should be straight for a foot or so. Sweep 90° also help smooth out flow. Oversizing the plumbing helps too.

From the install manual.

 

[1poolman1]

That 90 right at the inlet is most of the problem. With a pump that large and 1.5" plumbing, the cavitation can actually be happening in that fitting at high speed. Most install manuals call for 5 - 10 X the inside diameter of the pipe in straight pipe into the pump for just this reason.

 

[FernTheBrute]

That 90 right at the inlet is most of the problem. With a pump that large and 1.5" plumbing, the cavitation can actually be happening in that fitting at high speed. Most install manuals call for 5 - 10 X the inside diameter of the pipe in straight pipe into the pump for just this reason.

Should I also remove the 90 bend at the exit of the skimmer while I'm at it, or is it sufficiently far from the pump to not cause an issue?

 

[1poolman1]

Should I also remove the 90 bend at the exit of the skimmer while I'm at it, or is it sufficiently far from the pump to not cause an issue?

Why is there a 90 at the skimmer? The fewer the fittings the better the waterflow. Picture?

 

[FernTheBrute]

Why is there a 90 at the skimmer? The fewer the fittings the better the waterflow. Picture?

Well that's a good question... the installers probably always do it that way to make the pipe point downwards as quickly as possible.
Here's a pic :

 

[Bill1974]

Interesting! where does the black poly line go?

 

[1poolman1]

Well that's a good question... the installers probably always do it that way to make the pipe point downwards as quickly as possible.
Here's a pic :
View attachment 575474

Not so much a 90 as a valve. What does it do? What is the other line for?

 

[FernTheBrute]

The black line is the drain at the bottom of the pool. The valve alows me to either pump only from the skimmer, only the bottom drain or 50/50. There's a 90 bend screwed into the valve, but the pump is 5-6' further

 

[Darin]

If you have to have an elbow to hide under the bush then get a 10 or 12 inch nipple and screw in between basket and elbow, if you don't then straighten it as stated above.

 

[1poolman1]

The black line is the drain at the bottom of the pool. The valve alows me to either pump only from the skimmer, only the bottom drain or 50/50. There's a 90 bend screwed into the valve, but the pump is 5-6' further

Would have been better if the hoses were reversed.

 

[Fugacity]

I've got a new 1.5 hp 2-speed Carvin pump for my above ground pool which is making a lot of noise when running at the highest setting. It sounds like gravel in the pump, but there is absolutely no air in the strainer or anywhere before or after the pump (don't see any bubbles coming out in the pool). It's dead quiet when running at low speed.

From what I gathered, it's probably cavitation, since there's no air in the system and it only occurs at high speed. My strainer basket is clean and so is the skimmer filter.

I'm running 1.5" piping in and out of the pump. The connexion from the pump to the filter is a nice arc, but I have this mess entering the pump, with a 90 bend right in front of it, The 17 y/o who was hired for the summer to install this clearly hasen't majored in... well anything !

View attachment 575330

If I were to cut the intake line to install a new straight fitting instead of the 90 elbow, what are reasonably my chances of getting rid of the cavitation?

Is it more likely the issue is with the filter itself or the plumbing coming out of it, to the water heater and then back to the pool? It also has a lot of bends.

Thanks!

gravel sound is definitely cavitation. The 90's is a likely culprit an pretty easy fix. It's not the filter or the discharge piping. More loss there actually would improve cavitation by slowing down flow.

Also, make sure you're not running way to fast. Probably unlikley, but if you don't have enough resistance in your discharge, you'll be running way on the end of your pump curve, and that will cause potential cavitation. What's your filter pressure gauge read?

 

[FernTheBrute]

gravel sound is definitely cavitation. The 90's is a likely culprit an pretty easy fix. It's not the filter or the discharge piping. More loss there actually would improve cavitation by slowing down flow.

Also, make sure you're not running way to fast. Probably unlikley, but if you don't have enough resistance in your discharge, you'll be running way on the end of your pump curve, and that will cause potential cavitation. What's your filter pressure gauge read?

Fixed the 90 this weekend and installed a straight connector. Still the same sound :/. I might try replace the 90 at the valve with a sweeping 90 but I doubt it will do much since it's 6-8 feet from the pump.

I might be running too fast, thing is I don't have a lot of control as the pump is only 2 fixed speeds. On low, it hums quietly but outputs about 2-3psi at the filter and the current isn't strong enough to keep the pool clean - I use it at nigh. On high, I'm at about 14 to 16 psi depending on how long it's been since my last backwash.

What I don't really get is that, according to this chart, the pump can output like 90 gpm if I read it correctly? But isn't 1.5 tubing limited at like 45 gpm? If so, why on earth do they fit those with 1.5 adapters???

 

[mas985]

Fixed the 90 this weekend and installed a straight connector. Still the same sound :/. I might try replace the 90 at the valve with a sweeping 90 but I doubt it will do much since it's 6-8 feet from the pump.

Neither 90 should be causing cavitation. They don't have than much head loss especially with a small pump like that.

Cavitation is the boiling of water and the pressure would have to drop by over 14.7 PSI in the pump basket for that too occur and that pump/plumbing should not do that unless there is something else wrong like a valve that is fully closed off.

I might be running too fast, thing is I don't have a lot of control as the pump is only 2 fixed speeds. On low, it hums quietly but outputs about 2-3psi at the filter and the current isn't strong enough to keep the pool clean - I use it at nigh. On high, I'm at about 14 to 16 psi depending on how long it's been since my last backwash.

Again, that pump and plumbing should not be cavitating especially when the pressure is that high.

Are you sure you don't have something rattling inside of the pump basket or inside the pump?

What I don't really get is that, according to this chart, the pump can output like 90 gpm if I read it correctly? But isn't 1.5 tubing limited at like 45 gpm? If so, why on earth do they fit those with 1.5 adapters???

No limits on flow rate in pipe only recommendations. However, the flow rate from the pump is dependent on the plumbing head loss and visa versa. It is a set of two equations that cross at the operating point.

Can you post a video and audio of the pump in operation so I can hear it?

Have you removed the pump basket and felt inside the impeller inlet for any debris or objects?

 

[mas985]

At 14 PSI filter pressure, that means the return head loss is 32' of head which also means the flow rate must be below 56 GPM. Allocation 3' of head loss for the suction side, the flow rate should be around 50 GPM. These conditions are not conducive to cavitation.

[EDIT] Forgot to mention that to get cavitation, the suction head loss would have to be over 33' of head and that pump has a maximum head of 48' so it is just not possible.

 

[FernTheBrute]

Neither 90 should be causing cavitation. They don't have than much head loss especially with a small pump like that.

Cavitation is the boiling of water and the pressure would have to drop by over 14.7 PSI in the pump basket for that too occur and that pump/plumbing should not do that unless there is something else wrong like a valve that is fully closed off.


Again, that pump and plumbing should not be cavitating especially when the pressure is that high.

Are you sure you don't have something rattling inside of the pump basket or inside the pump?


No limits on flow rate in pipe only recommendations. However, the flow rate from the pump is dependent on the plumbing head loss and visa versa. It is a set of two equations that cross at the operating point.

Can you post a video and audio of the pump in operation so I can hear it?

Have you removed the pump basket and felt inside the impeller inlet for any debris or objects?

I did try to feel for debris, couldn't feel anything. Plus it's super quiet on low, I would guess if something was rattling it would also make some noise at that lower speed.

Here's a video with sound. First part is on Low.

I've watched a video with a variable speed pump. The person filming just increased the speed until it began making exactly that noise, explaining it's cavitation because the pump is too powerful for the rest of the piping and filtering system. Hence my guess it is cavitation, but my CS degree didn't prepare me for the kind of calculation you made, so I guess I'll have to trust you on that one

 

[mas985]

A VS pump is much more powerful than AG pump so cavitation is more likely. But cavition is very rare in pool pumps and even more rare with AG pumps.

Sounds more like something is stuck in the impeller. Have your removed the wet end from the motor to inspect the impeller.

 

[FernTheBrute]

A VS pump is much more powerful than AG pump so cavitation is more likely. But cavition is very rare in pool pumps and even more rare with AG pumps.

Sounds more like something is stuck in the impeller. Have your removed the wet end from the motor to inspect the impeller.

No I haven't. I ruled out this possibility since I can't hear anything at low speed,but I might bring it to a service center.
I'll post another video later. After listening to this old one, I realize the sound may have changed a bit since I modified the piping. It's less gravely.

 

[duraleigh]

I hear a very small suction leak (air) on high speed.

I think I would "fix" it by reducing pump speed until bubbles stop and make that my "new" high speed.