Odd pump behavior - off the curve
Odd pump behavior - off the curve
(OP)
I've got an interesting set of circumstances that is confounding all of the engineers at my company and I'm looking for someone that may have some idea of how to troubleshoot this.
Less than a month old, we ran off at our facility, but don't record flow values, only pressure and current on the motors. Closed loop, running a stamped stainless close-coupled centrifugal pump at ~73 GPM and ~25psi differential pressure. The pump curve for this pressure should be giving us ~120gpm of flow (selected slightly oversized), and we're sitting short of the customer's required flow rate of 80 GPM. The system pressure is almost dead-on the calculated drop for the system.
We're moving city water between 75 and 100°F. Using standard pressure gauges at multiple points in the system, before and after the pump. Using IFM Efector flow meters with proper installation runs, one after the pump discharge before branching, and one on each branch before collecting to the return header, all flow meters downstream equal the discharge meter before the header.
We're on the curve through about 40 GPM, then we start getting deviations and we're sitting 40% or so off the curve. The pump manufacturer built us another pump and ran it off through the entire curve to make sure their stamping and impellers hadn't drifted since the curve was established - their pump matched the curve values, albeit with much more ideal and controlled conditions than what I'm dealing with.
Our first step was to change the impeller to the largest available trim, which improved the numbers slightly to where I'm at now, but still significantly lower than expected flow.
The system is pressurized with a diaphragm expansion tank, we started at 12psi and have increased it to 20psi. There's an air separator installed, which is operating as best as we can tell, it does not have a strainer in it. The make-up line is city water connection regulated to 17 psi. The rest of the system is the customer's piping that I'm short on details on, but should follow a standard flow/pressure drop relationship and not figure into the conundrum.
The pump will make deadhead (shutoff/no flow) pressure. As we open the throttle valve (butterfly valve used for isolation mainly) we see a 1-2psi drop in suction pressure. The motor is drawing near nameplate amperage, suggesting we're to the far right on the curve. Required NPSH is all of 2.5psi
Any ideas as to what might cause a pump to... not pump? We're thinking suction restriction is a possible culprit, but there's nothing there to impede flow. Would a small amount of air in the system wreak this kind of havoc?
Any help is appreciated!
Less than a month old, we ran off at our facility, but don't record flow values, only pressure and current on the motors. Closed loop, running a stamped stainless close-coupled centrifugal pump at ~73 GPM and ~25psi differential pressure. The pump curve for this pressure should be giving us ~120gpm of flow (selected slightly oversized), and we're sitting short of the customer's required flow rate of 80 GPM. The system pressure is almost dead-on the calculated drop for the system.
We're moving city water between 75 and 100°F. Using standard pressure gauges at multiple points in the system, before and after the pump. Using IFM Efector flow meters with proper installation runs, one after the pump discharge before branching, and one on each branch before collecting to the return header, all flow meters downstream equal the discharge meter before the header.
We're on the curve through about 40 GPM, then we start getting deviations and we're sitting 40% or so off the curve. The pump manufacturer built us another pump and ran it off through the entire curve to make sure their stamping and impellers hadn't drifted since the curve was established - their pump matched the curve values, albeit with much more ideal and controlled conditions than what I'm dealing with.
Our first step was to change the impeller to the largest available trim, which improved the numbers slightly to where I'm at now, but still significantly lower than expected flow.
The system is pressurized with a diaphragm expansion tank, we started at 12psi and have increased it to 20psi. There's an air separator installed, which is operating as best as we can tell, it does not have a strainer in it. The make-up line is city water connection regulated to 17 psi. The rest of the system is the customer's piping that I'm short on details on, but should follow a standard flow/pressure drop relationship and not figure into the conundrum.
The pump will make deadhead (shutoff/no flow) pressure. As we open the throttle valve (butterfly valve used for isolation mainly) we see a 1-2psi drop in suction pressure. The motor is drawing near nameplate amperage, suggesting we're to the far right on the curve. Required NPSH is all of 2.5psi
Any ideas as to what might cause a pump to... not pump? We're thinking suction restriction is a possible culprit, but there's nothing there to impede flow. Would a small amount of air in the system wreak this kind of havoc?
Any help is appreciated!





RE: Odd pump behavior - off the curve
A strewn sketch or diagram would be nice to see but a very good description of your issue
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RE: Odd pump behavior - off the curve
I haven't physically been on site, but I believe the rotation was checked when our service team was on site to do the commissioning and performed the impeller swap.
We've swapped gauges and verified against a calibrated master to ensure we're not off by 3psi or some amount on a gauge.
RE: Odd pump behavior - off the curve
We're running a simple loop, removing heat from a reactor (non-nuclear). The customer's somewhat complex piping is more or less the big circle. As I said, I'm short on details.
We're moving forward with sticking a bigger pump in place, trying to get them running with enough flow to meet their needs. I'd still really like to know if we can figure out what's wrong either before we start cutting into the piping, or purely academically in case we run into this scenario again.
RE: Odd pump behavior - off the curve
Is the standby pump isolated or sitting in the non return valve?
What is the 3m filter?
What is the chemical shot thing?
Where is your flow meter on that diagram?
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RE: Odd pump behavior - off the curve
The standby pump is checked to prevent backflow. The higher pressure on the discharge side would force the check valve shut. We've run both pumps simultaneously with similarly underwhelming results.
3M filter is a fairly simple bag filter with a differential pressure switch to signal a dirty filter. The chemical feeder is a 2 gallon drum that allows easy checks of the water health and dosing of chemicals as needed to combat untreated city water coming into the system - mostly biocides and rust inhibitor. Both were isolated during measurements, they consume a total of ~3 GPM combined when open, flowing from high to low pressure. So our target is technically 83-84 GPM on the pump discharge, but as I said we're oversized and should be pushing >100 GPM, even if the piping conditions give us added inefficiency and loss to move 10-20% off the curve.
The flow meter is downstream of the heat exchanger in the customer piping. It exits our pump skid and the first transition is to a straight pipe run with 10 diameters upstream and 5 downstream of the flow meter. From there it flows into a header to feed the individual sections of the reactor. Post-reactor flow meters are on every branch, then the branches collect into a return header and loop back to the pump skid.
We tend not to trust flow meters, but all of them are setup perfectly according to the install guides, and have a published error of less than 2% (+/- 0.8% + 0.5% of flow value) and are from a reputable company.
RE: Odd pump behavior - off the curve
Only thing left to check as far as I can see is to check the actual voltage on the motor connectors when running to see if you're getting 460V and 60 htz when operating at the max flow.
Looking at the information supplied though it seems to me that in reality you've just got a pump that has the wrong pump curve shape ( far too flat at duty point) and also far too susceptible to small changes in pressure.
You say ~25 psi differential. I can see on the diagram a pressure gauge? rather than transmitter on the pump discharge, but not sure where you're taking inlet pressure from?
If we take this 25 as correct, 25 psi is 58 ft (1psi = 2.31ft of water) head. Even with your max impellor size that's only 90 pgm, not the 120 you state.
If you allow say 3psi for extra losses between your inlet pressure gauge, wherever it is, and the pump inlet, that gives you 65 ft differential and a flow of about 60 GPM.
To have such a small differential pressure (+10%) to make such a difference in flow (-35%) will result in the position you find yourself in, IMHO.
So what to do?
1) buy a larger pump with a steeper pump curve at your duty point so you're not so badly affected by small changes in differential pressure. At the duty point I would try and make sure that +/- 5 psi )~10 ft) only makes +/- 10 GPM difference. your current pump 10ft makes 40 GPM difference.
2) buy a VFD or invertor and run the motor at 70 htz or 80 htz. However if the motor is rated at only 2HP (you don't say what the nameplate of the motor says?) then you might as well just get the next size up pump.
3) buy a PD pump of some sort where you don't have to worry about the pressure, if its the flow rate you really want to achieve. At that size of pump there are many pulseless PD options ( Screw, Gear, sliding vane...)
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
Could someone somehow have changed out the expected impeller for a smaller one ???
MJCronin
Sr. Process Engineer
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
A gauge was added to the pump suction during install to troubleshoot the flow numbers.
You may be correct, and error, drift, tolerance, etc is just all stacking the wrong way on me, but it doesn't explain the lack of performance after making the impeller swap that should have added nearly 40gpm at the same pressure and we saw around a 5gpm increase.
Positive displacement is a bad plan in this situation. Mostly due to a few circumstances with the reactor that I can't really get into.
One of the planned options was to over-spin a 3hp with a VFD, but there's a large concern that we're going to do more work and get nowhere given the changes we've already made and the results we've gotten.
We're moving forward with a new pump. It requires a lot of piping changes, electrical changes, etc. I posted here in hopes there was something we were overlooking to avoid the expense of replacing everything.
MJCronin - the original impeller was removed by my lead service tech and swapped for the new one, measured and verified at the 200mm max size for the case per the manufacturer. The test pump built for verification was made with the same parts lot. The original setup spec'd a 1.5hp motor, but our set of pumps was built with 2hp due to availability. The larger trim requires 2hp, so there was a bit of convenience.
RE: Odd pump behavior - off the curve
Through the NRV - but you've tried isolating the second valve
Through the other filters etc
Through some other small line not shown on the diagram
Otherwise pump and motor
check rotation
check impellor clearances / wear rings / sealing
check power supply and frequency / voltages
Check seals / stuffing boxes for excessive frictional losses from the motor ( heat gun would be good when its running to spot heat where it shouldn't be) 2HP isn't much and it wouldn't take much to reduce shaft power.
Let us know if you find anything....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Odd pump behavior - off the curve
Rotation has been checked. The pump case is stamped stainless and doesn't use wear rings. One pump impeller is slightly tight and rubbing, but the other isn't and both perform the same. When it was opened no blockages, debris, etc was noted that could be impeding flow through the vanes. The motors and pumps were gone through when they were pulled to service the impellers.
The one thing that I don't believe was checked was actual motor speed and incoming frequency. But if we were under-spinning we wouldn't be on the curve at lower flows and wouldn't make the rated shutoff pressure.
The other trouble with this is the customer's facility is a few hundred miles away and our next visit is to perform the changeover to the new pumps. I doubt they'll want to spend another half day trying to track things down, they're not running production and it's costing them money to be down.
Artisi - due to the nature of the system we can't do a bypass or a bucket test or anything. The branching circuits split from a 2.5" pipe to over a dozen lines of varying size, none of which can individually handle the flow. Doing a physical check of flow would have been best, but we can't pump to an open bucket since the circuit is pressurized and we'd quickly be running dry.
A third pump was made at the manufacturer's facility and was verified that the performance curve is accurate. No, it wasn't done with the actual pumps, but if this company can't make 3 "identical" pumps from the same parts lot we've got different issues. We could have swapped in that pump to the circuit, but our customer is getting increasingly impatient and doesn't want to spend days on what-if investigation and wants it fixed so they can move on.
RE: Odd pump behavior - off the curve
And all those city water inlets and outlets need to be isolated during your testing I think or disconnected to make sure they're not passing.
What actual pressures are you getting into the pump, not the differential?
The thing I was taking about voltage was that at higher current levels you'll get higher voltage drops but agree the frequency one is a bit left field.
If there was some sort of cross over going on or leak into the void and then a drain out? difficult to see if happening with 20-40 GPM I'll grant you and would need the city water supply to be constantly feeding water in. Stranger things have happened....
BTW what does the temperature Sensor control?
But remember your current 80 odd GPM is still only 26 psi differential compared to your 21 to 23. If your pressure gauges on the pumps were inboard of the valves then there's very little chance of them being affected by other things.
Nope - time for a bigger / steeper pump curve I think.
Please keep us informed as to progress - these sorts of posts are the most interesting and we can all learn from anything you eventually find out or don't find out....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
As far as I know the city water is not influencing the system. I was told they were isolated before recording the values.
Suction pressure is 19 psi. I don't have the numbers in front of me, but we're charged to 20.1 psi I believe and when a pump starts running we drop to 19.3psi if memory serves.
All of the branches are independent and all have flow meters installed. Shouldn't be any cross flow to anything else. And if there was we'd see a discrepancy between upstream and collective downstream flow meters.
The temp sensor is operating controls on the opposite side of the heat exchanger to modulate cold water flow. Over-cooling is a concern for the reactor, so it needs to be monitored and modulated to ensure we're not causing condensation or allowing thermal gradient to cause undue stress in a few key areas.
quark - this was done when our tech was originally on site. The customer claimed it was fully purged of air, but their venting was inadequate. The system was re-vented and is at a "satisfactory" level. We'll never get it 100% bled, but we're not getting surging, no variation to the numbers when everything is steady, no noise from the pipes that can indicate air, no movement in the flexible hoses on start/stop, etc. The increase in the bladder pressure was an attempt to see if there was still any large pockets trapped, but once the pressure was increased the vent didn't let any additional air out of the system.
Is it possible that air in the system can cause behavior like this? We know there's some air in there, just form the way it's designed and every branch doesn't have venting, with multiple elevation changes as different systems route along the reactor. But the balancing is steady enough that any air in the system isn't moving. I'd understand pressure increases due to the restrictions air causes, but would air in some other part of the system cause the pump to drop flow and no longer match the curve as you move along to the right on it?
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
You further stated that there is a reactor 20 ft above the loop line. Does it have an air vent? If you have one air vent at the top most point, that should be sufficient.
RE: Odd pump behavior - off the curve
The commissioning report notes when our tech arrived the system was showing unstable flow and pressure, the hoses were having a lot of movement (especially on flow start/stop), and air was audibly in the system. He spent the first four hours of startup purging air to the point we're at now. I can't say it's 100%, but it should be to a level that's acceptable for operation.
RE: Odd pump behavior - off the curve
Maybe what you should do is reduce the pressurisation level (when cold) to only say 5psi then run the system for a few volumes through the air eliminator and see if lots of air comes out? plus bleed the system at that low pressure.
I still think though for pumps of this size and rating, the accuracy of the pump curves from the vendor even for "identical" pumps is often not much better than 5%. Maybe you've got a slightly different casting or some manufacturing tolerance issue which is enough to cause an issue.
So a bigger pump with higher head and steeper curve still sounds like the best solution.
If you had a photo of the pump installation it might be interesting as sometimes diagrams don't match what was actually built or something stands out that you can't see from the diagram.
Good luck, hope we've been of use and let us know how it goes.
LI
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Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
And how do you operate the makeup water to this drum? - is there a level guage installed? All expansion drums are fitted with level guages or LT to ensure adequate level (by addition of makeup coolant) for pumping operations. And the tie in point for this makeup water is also nontypical - it should also feed into the expansion drum; not downstream of the drum as you have shown. Could the RV on this makeup water line be leaking?
RE: Odd pump behavior - off the curve
If you draw the system curve, the intersection with the pump curve would be at about 74 gpm and 59.3 ft wg so the pump is undersized and can not meet the system requirement of 80 gpm at 57.75 say 58 ft wg.
RE: Odd pump behavior - off the curve
There are a lot of posts, but I corrected that 25psi figure - that was a "round" number I used in my OP as I didn't have the exact figures in front of me. Actual pressure is a bit lower than that.
georgeverghese - That style of setup, from what I understand, has been phased out in this type of system for a while. The drum itself pressurizes the system and allows for thermal expansion. The air separator handles the venting, the drum is not the high point of the system. A sight glass is not used on a diaphragm, there's a makeup line regulated to 17psi to ensure the system volume doesn't drop, and a main relief on the reactor set at ~34psi to handle over-fill.
Is the makeup regulator leaking? Possibly. City water is at a higher pressure, so it would only be coming into the system. Would this not add to the main flow and not subtract?
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
What is the actual piping layout of these pumps for inlet especially. Lots of elbows and tees can generate a lot of flow disturbance and swirl. This will get worse as flow increases so your low flow will follow the curve but then start to diverge.
Not sure what the vendor recommends but you usually want min 5D of straight pipe before the pump free from valves etc.
What does it physically look like?
You could try a flow conditioner plate of you've got room
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Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
Plot H ft wg at each corresponding GPM to get the system curve.
RE: Odd pump behavior - off the curve
Keith Cress
kcress - http://www.flaminsystems.com
RE: Odd pump behavior - off the curve
georgeverghese - I'm familiar with the setup. It has several drawbacks, and in this instance the pump skid is ~20ft lower than the high point in the system, we'd have to roof-mount the tank if using the traditional arrangement.
lilliput - I'm not sure how the system curve is going to help me. I know several operating points in flow and pressure and could plot a new curve, the question is why this system doesn't want to follow the pump curve at all as flow is increased. It's like we're hitting a wall.
itsmoked - no VFD. It was being considered as a fix to add a VFD to overspin the motor, but that was abandoned.
I need to call my contact at the customer facility today. We shipped them new spools and pumps to be installed on Saturday and my office was shut down with power issues yesterday. We did get a request for another system yesterday, so that's a plus.
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
RE: Odd pump behavior - off the curve
It is probably a mix of issues just enough to cause the few feet head difference. Might be that particular pump model is more sensitive than others or is just operating too close to its limits.
Will be interested to see if the mods work or not.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
The customer is satisfied, so I'm thinking they're going to be changing some line sizes in the system to alleviate some of the restrictions. I haven't asked about getting those pumps back yet, there had to have been some conditions on the inlet that was causing the strange behavior.
Lesson learned with this build though, we're definitely going to make sure we nail the next build.
RE: Odd pump behavior - off the curve
"At the same pressure" it's rated for 100GPM - what pressure?
Does now sound like you're coming up against some other restriction as that diff pressure and flow is a steep system curve from what you had before.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
The pressure in the system has to be artificially increased to a certain degree by restricting the main/largest branches in the system to allow flow to spread everywhere. Otherwise the large branches would flow and the small lines feeding different aspects would have none. It's a tricky procedure to balance a heat treat furnace/oven/reactor, etc that has jackets, power supplies, quench coils, pumps, and other items that are all water-cooled. Nothing that unfamiliar, 90% of the units we build are attached to a system like this and is our specialty niche in the market.
At the system pressure we had previously of 20-some-odd psi across the pump this new pump should be pushing a lot more water. But we're riding higher on the curve with a roughly 10psi increase. I'm not sure if they're able to further restrict the main branch to force more flow out to their vacuum pumps that were the biggest concern and overheating with the previous pumps, or if something else was changed. I'm also not entirely sure they know what they're doing. I've been working with their main engineer and he a stickler for published values. We weren't far off of his pressure figures, but now we're running above 70ft of head.
The original selection we had was a 5hp model that was 80gpm at 80ft, we were told to scale back and select an operating point of 80gpm at 50ft. That's what got us into this issue, and also caused delays in the build as we had to change the design to accommodate the smaller pump.
RE: Odd pump behavior - off the curve
That scale of pump and pressure losses etc just has far too many variables and tolerances. Just remind your customer engineer that the line on the pump curve for a 2HP pump is infact a fairly fuzzy band where you can be 5% out and still in tolerance from the vendor.
A steeper pump curve wouldn't go amiss either.
If you ever get to the bottom of the strange pump performance let us know but this sounds like the end of the story?
Hope we were all able to help (sometimes you need confirmation you haven't missed something obvious).
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Odd pump behavior - off the curve
I appreciate the time and thought everyone put in.