Why is my pump losing prime?

[thefost]

As you can see in the pictures below, I have a Chugger pump that is used to recirculate and whirlpool my boiled wort. The last few times I've used it at the end of the boil, it starts cavitating immediately. I will then turn off the pump, throttle back the flow and the pump starts to work. As soon as I start increasing the flow to about 50%, the pump will lose prime again. So, looking at my pics, my question is... why?????

 

[mattd2]

To hazard a guess, you are possibly getting cavitation due to trying to pump near boiling liquid. In front of the pump suction the pressure drops to pull liquid into the pump and it is then forced out. In the right (or wrong depending on how you view life ) case the pressure in this region will drop below the vapour pressure of the liquid and it will vapourise (boil) and cause cavitation issues. By throttling the pump you are effectively reducing the pressure across the pump and the Net Positive Suction Head required (NPSHr) to avoid cavitation. The NPSHr must always be less than the Net Positive Suction Head available (NPSHa)
A good explaination is here

From there to reduce cavitation the recommendations for you are:
•Raising the tank, or lowering the pump helps,
•You can cut your frictional losses by:
- Increasing the size of the suction piping or decreasing the length
- Reducing obstructions such as valves, strainers, and other fittings.

I would start with dropping the pump another 2' if you can.

 

[LordUlrich]

My best guess is too much inlet restriction. Try getting rid of the elbow on the inlet and instead using a sweeping curve with your hose on the pump inlet.

 

[mattd2]

My best guess is too much inlet restriction. Try getting rid of the elbow on the inlet and instead using a sweeping curve with your hose on the pump inlet.

Or possibly a 3/4" elbow, to a 3/4"x1/2" bush, to the female camlock - and get rid of the 1/2" street elbow

 

[thefost]

Thanks for the info! Can't drop the pump unfortunately. I'm going to try just getting rid of the street elbow first since that's easy and go from there.

 

[Black Island Brewer]

As you can see in the pictures below, I have a Chugger pump that is used to recirculate and whirlpool my boiled wort. The last few times I've used it at the end of the boil, it starts cavitating immediately. I will then turn off the pump, throttle back the flow and the pump starts to work. As soon as I start increasing the flow to about 50%, the pump will lose prime again. So, looking at my pics, my question is... why?????

As was said above, the near boiling liquid boils if you put it under a vacuum. I have found, using my pump to recirculate while chilling, I can gradually increase the flow as the wort chills, and by about 170 can open it wide open.

 

[Owly055]

The problem is pretty obvious...... very obvious actually. Did you notice perhaps that the inlet on the pump was larger than the outlet port??? Why did you choke it down? Your inlet plumbing needs to be larger than the outlet, and there really is no other way to fix it properly. You need a larger port in the keggle, and a larger hose and fittings. The larger the better, but they should at a minimum be the size of the inlet port. you also need to get rid of the sharp elbow. That isn't helping matters either. A nice sweeping turn in the hose is what should be there instead of the elbow. Move your pump assembly to the left as far as is practical so you can run your new larger suction hose in a nice radius curve right to the pump, and use larger couplers.

I've worked with pumps and fluids most of my adult life (among other things). Look at a hydraulic system, or an irrigation pump, and you will instantly see the intake is far larger than the outlet. With heated water this is even more important.

H.W.

 

[Owly055]

P.S.

Consider pointing the pump inlet directly at the port and moving it as close as possible. Nothing says these pumps must be oriented flat and horizontal as you have it. When you upgrade to a suitable inlet port size, think about having the pump inlet quick couple directly to the port and hang there. You might even be able to do that with the reduced inlet size you have and get a significant improvement. Every inch of line adds flow resistance. Taking out the elbow and the plastic line may be enough to make it work. The pump is light, and the fittings are strong. This might be a quick and easy solution.... but you really do need a full size inlet.

H.W.

 

[NathPowe]

As was said above, the near boiling liquid boils if you put it under a vacuum. I have found, using my pump to recirculate while chilling, I can gradually increase the flow as the wort chills, and by about 170 can open it wide open.

This. I recirculate for the last few minutes of the boil and during my whirlpool/hopstand. For that I just throttle the output down to the point where its not cavitating. It still gives me enough flow to whirlpool and effectively chill. As the wort cools you can keep increasing the flow - eventually to the point where its full blast if that's what you want.

Cheers.

 

[thefost]

The problem is pretty obvious...... very obvious actually. Did you notice perhaps that the inlet on the pump was larger than the outlet port??? Why did you choke it down? Your inlet plumbing needs to be larger than the outlet, and there really is no other way to fix it properly. You need a larger port in the keggle, and a larger hose and fittings. The larger the better, but they should at a minimum be the size of the inlet port. you also need to get rid of the sharp elbow. That isn't helping matters either. A nice sweeping turn in the hose is what should be there instead of the elbow. Move your pump assembly to the left as far as is practical so you can run your new larger suction hose in a nice radius curve right to the pump, and use larger couplers.

I've worked with pumps and fluids most of my adult life (among other things). Look at a hydraulic system, or an irrigation pump, and you will instantly see the intake is far larger than the outlet. With heated water this is even more important.

H.W.

You're right, it's pretty obvious that I should be using 3/4" fittings and tubing on my inlet. I'm using he reducer because, well, that's what it come with when you buy the pump from brewhardware. I think most people use a 3/4 to 1/2 reducer so I figured that it would be fine. In fact, on my old setup I didn't have any problems, but the pump was a foot or two further from the kettle. I started this thread because I though that perhaps that my tubing and fitting layout might somehow flawed, but now I guess that the big difference is just the distance to the pump and the street elbow.

 

[Rivenin]

i can't run my pump at boiling liquid either very much.

the other problem too, your output is right next to the element which is bubbling, so if you pump too fast you might be pulling air down

 

[tjr3000]

I think owlys point is that the wort out port on your kettle is much smaller than the wort in. The out on the kettle needs to be bigger than the in to maintain flow (easier said than done, I suspect). The other parts don't help, but this looks to be the biggest constraint.


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[Owly055]

Flow resistance is a function of the size of line and the distance, just like electrical resistance. My suggestion of hooking the pump up right to the outlet with minimal distance would probably help a great deal. It wouldn't be as neat, but with fluids, a single very short restriction often does not cause nearly the problems that a more lengthy restricted line. Common sense would suggest that if the line in was equal to the line out, it should work, however it is much easier to push fluid than to pull it.
Before I would do anything else, I would try this as it is simple and cheap, and there is a more than even chance that it will work.


H.W.

I think owlys point is that the wort out port on your kettle is much smaller than the wort in. The out on the kettle needs to be bigger than the in to maintain flow (easier said than done, I suspect). The other parts don't help, but this looks to be the biggest constraint.


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[thefost]

Thanks everyone for their suggestions. I brewed over the weekend and tried it in this configuration:

It.. still didn't work that great =(

Slightly better, but still having trouble recirculating at hot temps.

So I guess my options now are to increase the fittings and tubing diameter to 3/4 of an inch, which would be a giant pain, or to lower the pump even more, which may be problematic for my CIP setup.

I'm not even sure if increasing the tubing and fittings to 3/4" will make much of a difference. I noticed bobby posted in another thread that the inlet inner diameter is only .55mm, and I measured it and it looks like he's right, so it wouldn't be a huge difference.

 

[Bobby_M]

Everyone in the thread has reasonable ideas why things go wrong when you try to pump boiling wort. As you've noted, there is very little effect in reducing down on the inlet because the actual inlet ID is pretty small already. Just because it has 3/4" male NPT threads means very little. I have seen elbows close to the inlet have a bigger problem than straight connections. Centrifugal pumps just can't deal with air bubbles very well and trying to pump wort at boiling temps is very difficult to do.

In testing, we have been able to run the center pump a LITTLE faster at 200F when the inlet was plumbed with 3/4" ID tubing clamped directly over the inlet. However, you reach the point of diminishing returns when you consider the cost of larger tubing and fittings and it was the difference between getting 1.75 GPM with 1/2" tubing and just under 2 GPM on the 3/4" tubing. Note that these were the flow rates we could maintain over several minutes without cavitation and we dialed that in with a valve on the output of the pump. I didn't pay very close attention to the valve position for this but it was somewhere around half open.

 

[eric19312]

Everyone in the thread has reasonable ideas why things go wrong when you try to pump boiling wort. As you've noted, there is very little effect in reducing down on the inlet because the actual inlet ID is pretty small already. Just because it has 3/4" male NPT threads means very little. I have seen elbows close to the inlet have a bigger problem than straight connections. Centrifugal pumps just can't deal with air bubbles very well and trying to pump wort at boiling temps is very difficult to do.

In testing, we have been able to run the center pump a LITTLE faster at 200F when the inlet was plumbed with 3/4" ID tubing clamped directly over the inlet. However, you reach the point of diminishing returns when you consider the cost of larger tubing and fittings and it was the difference between getting 1.75 GPM with 1/2" tubing and just under 2 GPM on the 3/4" tubing. Note that these were the flow rates we could maintain over several minutes without cavitation and we dialed that in with a valve on the output of the pump. I didn't pay very close attention to the valve position for this but it was somewhere around half open.

So bottom line is choke it down to start and gradually increase flow as wort chills? I've been dealing with same problem so appreciate the thread.

 

[Owly055]

It will make a difference running 3/4 all the way, in fact moving your pump further away as you did increases flow resistance over being close. Eliminating the elbow definitely was necessary, but you are farther away. Gravity doesn't gain you anywhere near what the small hose size costs you. Moving down is NOT the answer, moving the pump right to the port would give you the most bang for your buck.

Take the hose out and connect the pump directly to the coupler on the pot. You will be surprised at how much difference it makes. Better yet, put a larger fitting in the pot, and use larger couplers, and hook direct. The first thing I would do is take the reducer bell off the pump and put it right on the fitting coming out of the pot. I would then upsize to a single larger coupler, and connect direct. There is no obvious reason why you can't do this from the photos... except the cost of a coupler. If you upsize from the pot, you will need larger couplers anyway. You will be surprised how much difference it makes. All centrifugal pumps have larger inlets than outlets for good reason.

H.W.

Thanks everyone for their suggestions. I brewed over the weekend and tried it in this configuration:

It.. still didn't work that great =(

Slightly better, but still having trouble recirculating at hot temps.

So I guess my options now are to increase the fittings and tubing diameter to 3/4 of an inch, which would be a giant pain, or to lower the pump even more, which may be problematic for my CIP setup.

I'm not even sure if increasing the tubing and fittings to 3/4" will make much of a difference. I noticed bobby posted in another thread that the inlet inner diameter is only .55mm, and I measured it and it looks like he's right, so it wouldn't be a huge difference.

 

[thefost]

Thanks everyone again for your help in this matter.

I noticed the internal diameter of the cam locks was only 3/8", so I went ahead and tried the direct connection method you recommended.

So now we have a full 1/2" connected theoretically as good as it gets. Only increasing to 3/4" will get me very marginal gains anymore.

In this configuration I was able to open the ball valve slightly more than 50% at 185 degrees (hop stand temp). From what Bobby and you all are saying, I suppose I have reached the limits of this pump.

I'm getting a bit stronger whirlpool now than I was originally, but it's not a night and day difference. I guess the final answer to my original question is that these pumps just aren't great at very hot temps.

Mystery solved, thanks all!

 

[Owly055]

Thanks everyone again for your help in this matter.

I noticed the internal diameter of the cam locks was only 3/8", so I went ahead and tried the direct connection method you recommended.

So now we have a full 1/2" connected theoretically as good as it gets. Only increasing to 3/4" will get me very marginal gains anymore.

In this configuration I was able to open the ball valve slightly more than 50% at 185 degrees (hop stand temp). From what Bobby and you all are saying, I suppose I have reached the limits of this pump.

I'm getting a bit stronger whirlpool now than I was originally, but it's not a night and day difference. I guess the final answer to my original question is that these pumps just aren't great at very hot temps.

Mystery solved, thanks all!

If you do upgrade to 3/4 all the way, I think you will be surprised how much difference it makes. In irrigation and hydraulics we have grossly oversize plumbing right to the pump for that reason. The real question is "is it worth it"? You said you believed increasing to 3/4 would only get you marginal gains....... 3/4 pipe has vastly less resistance than half inch pipe. There are a number of charts online that show this clearly. It is worth noting that if you weld a 1" pipe and a 1/2" pipe to a container with a particular head of water, the 1" dia pipe will flow 16 times as much water. That should give you an idea how much difference it makes. Eliminating the hose made a significant difference..... it was what you could do without making major changes. Do NOT make the mistake of thinking increasing the diameter to 3/4 will have a minimal effect. It will have a dramatic effect.

Howard

 

[thefost]

If you do upgrade to 3/4 all the way, I think you will be surprised how much difference it makes. In irrigation and hydraulics we have grossly oversize plumbing right to the pump for that reason. The real question is "is it worth it"? You said you believed increasing to 3/4 would only get you marginal gains....... 3/4 pipe has vastly less resistance than half inch pipe. There are a number of charts online that show this clearly. It is worth noting that if you weld a 1" pipe and a 1/2" pipe to a container with a particular head of water, the 1" dia pipe will flow 16 times as much water. That should give you an idea how much difference it makes. Eliminating the hose made a significant difference..... it was what you could do without making major changes. Do NOT make the mistake of thinking increasing the diameter to 3/4 will have a minimal effect. It will have a dramatic effect.

Howard

The reason I was thinking it wouldn't make much of a difference is because the internal bore size of the chugger pump inlet is only .55". Since the 1/2" fittings I'm using have an internal bore size of about .5", I wouldn't imagine it making too much difference. Bobby even mentioned testing it out with 3/4" pipe and not seeing a much difference.

Or maybe I'm missing something here? I may still have to figure out a way to test this for myself...

 

[mattd2]

If you do upgrade to 3/4 all the way, I think you will be surprised how much difference it makes. In irrigation and hydraulics we have grossly oversize plumbing right to the pump for that reason. The real question is "is it worth it"? You said you believed increasing to 3/4 would only get you marginal gains....... 3/4 pipe has vastly less resistance than half inch pipe. There are a number of charts online that show this clearly. It is worth noting that if you weld a 1" pipe and a 1/2" pipe to a container with a particular head of water, the 1" dia pipe will flow 16 times as much water. That should give you an idea how much difference it makes. Eliminating the hose made a significant difference..... it was what you could do without making major changes. Do NOT make the mistake of thinking increasing the diameter to 3/4 will have a minimal effect. It will have a dramatic effect.

Howard

In this case I would not expect much of a difference if the diameter is increased due to the short length of the inlet piping. For an approx. flow rate 4GPM (0.25l/s) and 5" of pipe you are talking the difference between 0.039 PSI and 0.014 PSI (or nothing and really nothing). The biggest issue I see is the temperature. At 185°F the vapour pressure is approx. 8.4 PSI
To calculate the NPSHa you use the formula NPSHA = HA ± HZ - HF + HV - HVP
For this case (approx.):
HA = 14.7 PSI (atmospheric pressure)
HZ = 1.1 PSI (head pressure)
HF = 0.039 PSI (friction loss)
HV = 0.11 PSI (Velocity head)
HVP = 8.4 PSI (Vapour pressure)
So NPSHa = 7.47 PSI (@4GPM)

If the pipe is increased to 3/4"
HA = 14.7 PSI (atmospheric pressure)
HZ = 1.1 PSI (head pressure)
HF = 0.014 PSI (friction loss)
HV = 0.05 PSI (Velocity head)
HVP = 8.4 PSI (Vapour pressure)
So NPSHa = 7.43 PSI (@4GPM)
So no theoretical benefit to going to a 3/4" inlet (I know minor losses haven't been accounted for but you get my point... don't you?)

If you dropped the pump another 1.5' still with 1/2" inlet the NPSHa would be:
HA = 14.7 PSI (atmospheric pressure)
HZ = 1.78 PSI (head pressure)
HF = 0.18 PSI (friction loss)
HV = 0.11 PSI (Velocity head)
HVP = 8.4 PSI (Vapour pressure)
So NPSHa = 8.01PSI (@4GPM) A marginal increase

Basically for pumping out of atmospheric tanks with reasonably short/simple pump inlet pipework the main factors that affect cavitation are fluid head and vapour pressure

Sources:
https://www.byko.is/media/mapress/DesignTable_for_PipeSys_-_Pressure_drop.pdf - section 2.2
http://antoine.frostburg.edu/chem/senese/javascript/water-properties.html

 

[Bobby_M]

The reason I was thinking it wouldn't make much of a difference is because the internal bore size of the chugger pump inlet is only .55". Since the 1/2" fittings I'm using have an internal bore size of about .5", I wouldn't imagine it making too much difference. Bobby even mentioned testing it out with 3/4" pipe and not seeing a much difference.

Or maybe I'm missing something here? I may still have to figure out a way to test this for myself...

To clarify, when I tested the flow on the 3/4" ID silicone hose, I was still running my 5/8" Diptube and I clamped the hose over the 1/2" NPT threads because I didn't have a barb. The diptube has a 9/16" ID and the bulkhead fitting is similar so that was a choke point that was not a full 3/4" ID.

If you were willing to rework your drain to 3/4 and a 3/4 full port valve, it would be another marginal improvement. The big pain in the butt with running 3/4 on the inlet of the pump and 1/2" on the outlet is that you now have dedicated hoses for either side of the pump and you lose flexibility on your from and to ports.

 

[Esmitee]

Im no pro about pumps and piping. I agree that the street elbow on the inlet is a no no too. But I also sometimes have the same type of "Keeping the Prime" at BOILING temps. I recirculate at the near end of my boils to sanitize my CFC for about 5 minutes and then whirlpool with my IC in my keggle to quickly drop temps to around 120 - 140,I try to get to at least 100, then pump through the cfc to pitching temps into my fermentors.

I use at the moment a chugger "Inline" pump. I do have another center inlet pump I'm planning on useing soon. When I recirc, the wort flows through my 1/2" copper dip tube, through my 1/2" full port ball valve with cam loc fittings 1/2" ID silicone hose to more 1/2" cam loc's attached to the input of chuggler pump. THEN.........

Straight output with 1/2" fullport valve to cam loc fittings, 1/2" silicone hose up to a very high (On the Keggle) thru port with a male cam loc on it. Attached to the inside of the thru port is a SS 90 elbow to 1/2" 1/d copper pipe going straight down to about 3 inches above the keg bottom with a copper 90 0n the end to direct flow sideways. This is all infor To give all an idea of my set up to understand.

What I found and also agree with is, I think while Im boiling the wort, On the very bottom of the keggle, there are many many many little and big bubbles being made from the boil. I think, when I have my valve wide open, I'm basically sucking those BUBBLES ......."AIR" .... right down into the pump intake, causing the cavitation problems .
I find if I dont have the throttle wide open, say about 1/2 way open. I dont get the cavition. A few minutes after flame out, I seem to never have cavitation problems?

WELLL Thats just MY 2 cents! ! !

 

[mattd2]

What I found and also agree with is, I think while Im boiling the wort, On the very bottom of the keggle, there are many many many little and big bubbles being made from the boil. I think, when I have my valve wide open, I'm basically sucking those BUBBLES ......."AIR" .... right down into the pump intake, causing the cavitation problems .
I find if I dont have the throttle wide open, say about 1/2 way open. I dont get the cavition. A few minutes after flame out, I seem to never have cavitation problems?

WELLL Thats just MY 2 cents! ! !

If you think about boiling as when a liquid vaporises at the specific temperature & pressure it at then boiling in a pot and cavitation in a pump are the same thing. The reason that you can pump "boiling" liquids is due to the fact that on the way to the pump they are cooled slightly and they hydraulic pressure increase so it is no longer boiling. Then in the pump head the pressure may drop to the point where it again boils (cavitates). The lower the flow, then lower this pressure drop.

 

[Esmitee]

If you think about boiling as when a liquid vaporises at the specific temperature & pressure it at then boiling in a pot and cavitation in a pump are the same thing. The reason that you can pump "boiling" liquids is due to the fact that on the way to the pump they are cooled slightly and they hydraulic pressure increase so it is no longer boiling. Then in the pump head the pressure may drop to the point where it again boils (cavitates). The lower the flow, then lower this pressure drop.

WOW.....It is very apparent.........Your a "PUMP PRO"

It's very apparent.........I'm NOT LOL!

 

[Owly055]

In an ideal world, we would weld a flat stainless steel plate right into our keggle, and drill about a 1" hole through the plate, as well as holes for our screws, and we would remove the entire front plate from the pump, and simply bolt it to the plate on the keggle, thus eliminating ALL the plumbing on the suction side, and giving us a nice big suction hole. It would actually be quite easy to do when you are building your system. You have to weld fittings in anyway. To take the pump apart, you simply reach down inside the keggle and unscrew the screws that hold the "new cover" to your pump. It would require a bit of machining in many cases, as there is often an O-ring groove and O-ring for a seal.

H.W.

 

[Spellman]

I only browsed this thread but one more thing to try. I have the same pump, much different setup though. One of my two pumps kept loosing it's prime and I couldn't figure out why as it had always worked fine before. I took the head of and cleaned it really well. It did have some minor buildup of old wort/beer stone in there. It wouldn't have been enough to restrict any practical flow, but I suspect it was causing the magnet to slip. After the cleaning it has worked like new. May be worth giving it a shot.