Filter capacity?

Pump repair completed today. Flow rate has improved, but not where it should be. Pool company seems to be stumped. Flow rate has gone up about 30 gpm, but filter pressure has increased and is now running at 30 psi. (He back-washed the filter several times.) They want to change gasket and cover on MP Valve. (We knew the gaskets were bad, as we were getting leaking out the discharge line after back-washing and had to install a valve on that line last year.) But..... is that repair likely to increase the flow rate and drop the pressure??? He also recommended changing sand as a next step (after gasket/ cover replacement). Seems a bit like we are grasping at straws. Thoughts?
 

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Just did the repair to our DMJ3-172 pump. Any thoughts on why we still are not getting the flow rate we should be getting from this pump. Any logical next steps to take to address the low flow rate? Pool company recommends changing multi-port gasket and cover..... then changing sand, if still not meeting required flow rate.
 
The pressure is too high, but the flow should be higher.

Maybe a problem with the sand.

Maybe the flow meter is not accurate.

Maybe replace the flow meter or add a second flow meter.

Maybe there is a problem with the suction being clogged.

Maybe an air leak.

Is there any air building up in the filters?

Check the vacuum pressure.

Check the power being used by the motor.

Maybe a problem with the return line.

Can you show the whole system?
 
What happens when you bypass the filter(s)?
Put the MP valve(s) in Recirculate and see if the flow increases substantially.

Are you only able to measure filter pressure or can you measure pressure downstream of the filter(s)?
 
30 psi is about 69.3 feet of head.

The filter pressure should not go above about 16 psi at full flow, so there might be a problem with the sand or maybe something past the filter is causing a restriction.

Even accounting for the restriction, the flow is too low and you might have a problem with the suction or maybe a suction side air leak or maybe the pump is underperforming for some reason.

Make sure that the pump lid is sealing correctly as that is a common problem with metal lids.

Even a small air leak will cause the pump to underperform.

The flow is about 130 gpm, so that puts the total head at about 115 feet according to the pump performance curve.

115 (Total) - 69.3 (Return) = 45.7 feet (suction).

Suction cannot be more than about 17 feet before the pump will begin to underperform.

45.7 - 17 = 28.7.

34 feet is the maximum possible suction and the pump will begin to cavitate somewhere between about 17 feet and 34 feet.

So, the numbers do not add up.

The gauges might be incorrect or the pump might be underperforming.

Are you getting air into the system?

We need a vacuum pressure.

We need a second way to verify flow.

You should check the total power draw by the pump and check the voltage and current being supplied.

It is a 3 phase motor, so you need someone who knows how to work with 3 phase.

The motor specs are on the motor label.

If we allow up to 17 feet of head for the suction, then that is 86.3 feet total, which should be about 170 GPM.

115 feet (From pump curve at 130 gpm) - 86.3 feet (Estimated) = 28.7 feet unaccounted for.

The TR-140 is rated at 106 GPM (7.06 x 15 gpm/sqft).

2 = 212 GPM.

At 106 GPM, the head loss is about 12 feet from the filter, which is about 5.2 psi.

At 106 GPM, the head loss from the multiport is about 6 psi.

That is 11.2 psi from the filter and valve at 212 GPM total system flow.

30 - 11.2 = 18.8 psi (43.4 feet) for the return line past the filter, which is too high.

If we allow 4.6 feet for the return past the filter, then that is 38.8 feet too much.

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What happens when you bypass the filter(s)?
Put the MP valve(s) in Recirculate and see if the flow increases substantially.

Are you only able to measure filter pressure or can you measure pressure downstream of the filter(s)?
Ran the system in Recirculate this morning. The flow rate did not increase. Any further thoughts after that step?

By "downstream pressure" do you mean the pressure in the plumbing lines after the filter? I'm attaching pics of 2 gauges that are located in the plumbing right after the filter.

Also, technician mentioned 2 returns with eyeballs that are too small... 1/2" vs 1". Could that possibly be a source of the high pressure? I'm going to swap those out asap.
 

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Here are pics of...
  • vacuum gauge (sits right in front of pump)
  • filter pressure gauges (2)
  • pressure gauges located in plumbing after the filter (2)
All are reading about 25 psi.... a little lower on the gauges coming out of the filter.

Does this give any further clues?

Do we need any additional gauges in the plumbing system to help diagnose things?
 

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Also, technician mentioned 2 returns with eyeballs that are two small... 1/2" vs 1".
I would remove those.

You do not need them at all.
Could that possibly be a source of the high pressure? I'm going to swap those out asap.
When did the pressure and flow change from normal to being off?

What was the pressure originally?

What was the original flow?
Any concern about possible damage to the system with filter pressure running at 25-26?
The filters are rated for the pressure, but anything over about 20 psi is going to lead to premature failure.

Equipment life decreases exponentially based on pressure.

If a filter has a standard life of 40 years at 10 psi, then the life will be about 10 years at 20 psi.
 

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The air pressure at sea level is about 14.7 psi, which is about 34 feet.

When the impeller spins, the pressure in the suction line goes below 14.7 psi and the air pressure pushes water towards the impeller.

The pressure that is in the suction line is considered to be zero relative to the air pressure before the pump is on, but it is 14.7 psi or 34 feet absolute.

When the pump is on, the suction is considered to be negative relative to air pressure.

This is also called a vacuum, but the absolute pressure is always positive.

The remaining positive absolute pressure is called net positive suction head or NPSH.

Every pump has a specification for NPSH that is required to prevent cavitation.

To figure out the NPSH, calculate or measure the head loss and subtract it from 34 feet at sea level or the local air pressure at higher altitudes.

Make sure that the NPSH is always greater than the NPSH specified by the pump manufacturer.

For most pool pumps, use 17 feet as the NPSH unless you have the actual number.

If you keep the suction flow velocity below 6 ft/sec, you will usually be fine for pumps at ground level for plumbing under 100 feet in length.

For pumps above the pool, subtract the height.

For pumps over 100 ft away, use a head loss calculator or chart to calculate the head loss.

For existing systems, you can use a vacuum gauge to see what the vacuum pressure is to see if it is excessive.

The required NPSH of a water pump at rated capacity is 17 ft.

Water Temperature 85° F. Elevation 1000 ft. above sea level.

For an elevation of 700 feet, the vacuum gauge should never read higher than 19" Hg.

Realistically, to provide the required NPSH, you would want the vacuum to be at 15" Hg (17 feet of head loss) or lower.

You can contact Pentair to inquire about the NPSH for that exact model, but I would use 17 feet unless you get the actual number.

So, you need to figure out why the vacuum pressure is at 25" Hg and get it down to 15" Hg or lower.

Is the strainer basket clean?

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Since the pressure didn't change when the MP valve was in recirculate, that pretty much rules out a filter media issue.
But there could be an issue with the MP itself.

As @JamesW suggested, remove the small eyeballs on the return side.
Then let us know if flow/pressure changes.
 
The NPSHR is 17 feet and the NPSHA is only 6 feet.

The suction is definitely restricted and that needs to be resolved.

Does the vacuum gauge go to zero when the pump is off?

What were the previous reading for the vacuum when the pump was performing correctly?

NPSHR = Net Positive Suction Head Required.

NPSHA = Net Positive Suction Head Available.
 
25” Hg is about 28 feet of head, which is in cavitation range.

If the gauge is accurate, then the pump is probably cavitating.

Is the pump loud?

Does it sound like it is pumping gravel?

Are all of the suction valves open?

If the gauge is accurate, then it can explain the loss of flow.

Cavitation can cause impeller wear.

If the impeller really was worn, then cavitation is the likely reason that the impeller was damaged.

Did they give you the old impeller?

If yes, can you show it from all sides?

If no, then you should ask for it so that we can inspect it.

Always get parts like that from the service company to verify the part was actually bad and was actually replaced.
 
Since the pressure didn't change when the MP valve was in recirculate, that pretty much rules out a filter media issue.
But there could be an issue with the MP itself.

As @JamesW suggested, remove the small eyeballs on the return side.
Then let us know if flow/pressure changes.
I am going out there now and will remove those eyeballs. Only the smaller (1/2' ones), right? Should I leave the larger 1" eyeballs in place?

Also... here is the latest response from the pool company about how they want to proceed:

I want to install the gauges and check the system one more time before doing the backwash valve tops/gaskets. I advise installing the gauges, $30.00 each, and spending a little time trying to figure out the system problem and then installing the backwash valve tops only if we still deem it necessary. I won’t know the next step until we dive deeper into the system, we may want to check the filter internals and look into the actual plumbing of the system as sometimes when a bunch of 90 degree fittings are installed close to the pump discharge problems with flow/backpressure arise.

Make sense?
 
The suction is definitely restricted and that needs to be resolved.

Does the vacuum gauge go to zero when the pump is off?

What were the previous reading for the vacuum when the pump was performing correctly?
Going down to check the vacuum gauge now to see what happens when I shut off the pump.

Not sure about previous readings. Never really checked that gauge before, but I am going to check past records (Daily Operational Reports) to see if I can find out when the flow rate dropped. I think it has been running below 100 gpm for some time now.
 
Just the 1/2" ones for now.

But I believe @JamesW may be on to something with reference to the suction side.
As he asked - does the pump sound like there is gravel in it when running and do you have the old impeller or pics of it to share?
 
I removed the 1/2" eyeballs. Seems like pressure dropped a bit. I will post pics of gauges in next post. Pump sounds pretty smooth.... no gravel sound (check attached link). I asked for the parts that were removed from the pump, but haven't heard back yet. I did take some pics yesterday, but wasn't really sure what I was looking at (see attached pics).

 

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