Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Odd pump behavior - off the curve

Odd pump behavior - off the curve

Odd pump behavior - off the curve

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!

RE: Odd pump behavior - off the curve

The pump curve would be nice to see but have you checked that the rotation is correct. I mean really checked yourself.

A strewn sketch or diagram would be nice to see but a very good description of your issue

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

Here's the pump curve. The triangle is the design point, 80GPM at 50ft TDH, which intercepted the smaller impeller curve. We're seeing right about 48-53ft of TDH, give or take, depending on conditions at time of reading. The original thought was a little system variance put us a few GPM shy and it was cutting it too close, stick in the bigger impeller and away we go. Well, flow increased by about 4 GPM moving to this impeller.

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

And here's the system diagram excerpt. I removed most of the brand/model info.

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

Ok. Bleeding obvious questions.

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?

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

Solid lines = things my company built. Dashed lines = everything else, field piping, customer devices, etc.

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

OK, You seem to have covered all the bases.

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

How did you check the flow?

RE: Odd pump behavior - off the curve

To be fair to the OP, he mentions a number of times where the flow figure comes from and the accuracy he is obtaining.

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

Are you sure that the impeller IN THE PUMP ... is the same as that shown on your pump curves ?

Could someone somehow have changed out the expected impeller for a smaller one ???

Sr. Process Engineer

RE: Odd pump behavior - off the curve

Yes, my mistake.

RE: Odd pump behavior - off the curve

The 25 psi I originally mentioned was a round number. We're seeing between 20.8 and 22.9psi - 48 to 53 ft. As I said, it was cutting the ragged edge for the original configuration, but moving to the bigger impeller, which has since been verified by the manufacturer, should have put us at the very least above 80gpm.

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

Ok, looks like I'm running out of ideas. The only thing I can suggest further which you've probably done is to check every last connection and valve to make sure there isn't some flow going somewhere it shouldn't. if the pump should be putting out more flow, but you're not spotting it on the flow meter D/s the heat exchanger where can it possibly go?

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

Can you run the existing pump/s in the system on an isolated section of the pipework to verify if the pumps are performing to curve, if they are then the problem is the system and not the usual call "the pumps aren't performing", alternately have an off-site test undertaken.

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

There's nowhere for the flow to go. Even if we had a leak or a stuck valve 40gpm of flow is hard to miss, unless we have several smaller leaks. I find that hard to believe given the scrutiny this has been given.

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

The very last thing I can see is the integrity of the Plate Heat Exchanger.
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

Isolate the bladder tank, stop circulation and check for air at the vent. There can be flow discrepancy when there is air in the system.

RE: Odd pump behavior - off the curve

The plate heat exchanger is a standard setup. It's pressure tested as part of our factory runoff.

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

Part of the pump flow is recirculated through the side stream filter (and shot feeder if valves are open). If pump puts out 80 gpm and 10 gpm goes throught the side stream filter then only 70 gpm will go to the heat exchanger. The 10 gpm goes from pump discharge to side stream filter then back to pump suction.

RE: Odd pump behavior - off the curve

lilliput - the side stream items were closed off. They take about 3gpm combined when opened. I'm missing 40 gpm from the rated performance of the pump, 50% of my current performance. That's not making it through a 3/4" line when the system is setup in 2.5"

RE: Odd pump behavior - off the curve

I observed strange things in the past with air in the system. Pressure reading fluctuation is one indication of air in the system but I won't take that for granted. If you close bladder valve, stop the system, check for air (and do it multiple times) and every time you observe air then it is not properly being vented out. You are directly pressurizing air vent with a line from bladder tank and the location where you connect bladder tank to vent may not allow the air to go up (if your P&ID denotes actual connections).

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 reactor does have a high point vent. As far as we can figure, all of the air is out of the system.

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

Last thing I can think of here is whether there is any contamination or additive which is causing the density to decrease even by 5 or 10%. If there is a lot of dissolved air in the system caused by air being churned through the pump and then kept there by the pressurisation system, this could mean that the head figures we're all using for water from pressures are incorrect.

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.


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

Valve off the city water emergency supply. The city water pressure is working against the pump. Put in a NO solenoid valve powered close so on loss of power both the city water NO (Normally Open) supply valve and NO drain valve will open.

RE: Odd pump behavior - off the curve

Terminology used here is a bit confusing - you say the expansion drum is fitted with a diaphragm ? - A typical closed loop recirculation stream expansion drum would not have a diaphragm fitted to allow air in the recirculation stream to escape - you would only pressurise the vapor space with N2 or some inert gas with a forward sensing (sensing expansion drum pressure) and install a backpressure regulator on the vent exit to release any excess pressure.
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

Say 25 psi is 57.75 ft of water, not 50 so the pump can only put out about 86 gpm not 120 per the system curve. However the pump curve must intersect the system curve. The system required head at 80 gpm is 57.75 x (80/73)^2 = 69.35 ft wg

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

lilliput - the emergency water supply do have NO solenoids to only operate during a power loss. They also have manual bypasses, but those are service valves only. I'm only showing one on the drain as the customer is running a specialty valve on the inlet.

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

If your highest point (at the reactor line) has an autovent, then I have nothing more to add. Post us updates.

RE: Odd pump behavior - off the curve

Ok absolute last shot here.

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

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

Well, the expansion drum arrangement I described earlier is commonly used in oil/gas installations in closed loop heating / cooling circuits, and none of them have diaphragms or air release valves. And in a well laid out design, (a) the main recirc stream feed nozzle at the expansion drum is the high point in the return header (b) the return header slopes continously upwards in the direction of flow back to the expansion drum.

RE: Odd pump behavior - off the curve

You need to plot the system curve on the pump head vs flow chart. Where the system curve intersects the pump curve will be the point of operation. If you measure 73 GPM at 57.75 ft head with the pump running you need to plot the system curve using the formula H = 57.75 x (GPM/73)^2
Plot H ft wg at each corresponding GPM to get the system curve.

RE: Odd pump behavior - off the curve

Rputvin; Just checking. This pump isn't on a VFD is it?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Odd pump behavior - off the curve

LittleInch - the suction conditions aren't ideal with elbows close to the pump suctions. It's not an atypical arrangement for our designs, it may be contributing enough to cause an issue this time around.

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

I get the impression you've gone for this diaphragm type arrangement in order to eliminate the need for pressurisation gas at a conventional diaphragm less expansion drum. If you have the opportunity to redesign this for a conventional expansion drum, it is possible to also almost eliminate the need to top up pressurisation gas. You have to select the right size drum with adequate mechanical design pressure, with adequate vapor space between LAL and LAH so as to keep the pressure swings(between these 2 levels) in this drum below the setting of the outbreathing PCV - good luck.

RE: Odd pump behavior - off the curve

Give us your measured flows and corresponding pressure differential across the pump with new impeller.

RE: Odd pump behavior - off the curve

Could be. Maybe instead of having say an L shape going into you pump you could have more of a longer flat U shape to give you 5 -10D of straight pipe U/s the pump.

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 new pump is in, it's a 3hp model in the same product family. At the same pressure it's rated for over 100 gpm. We're now running 73ft (31.75psi across the pump) and pushing 78 GPM. Matches the curve 100% through all of the flow conditions they can generate. It still feels like we're hitting a wall at 80gpm, I have to wonder if some of their flow balancing devices aren't restricting the flow and the new pump is just pushing harder against whatever is in the system.

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

"some of their flow balancing devices " - what are these? Are they in the customer piping?

"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

Every line in the customer piping has a balancing valve of some kind on it. We were told they're nothing special, I had assumed they were globe valves or circuit setter variable orifice devices. There are some units that react differently as flow increases, multi-orifice units that also limit pressure and play games with flow and pressure in the system, electronic variable units, etc. I don't have a rundown of every device they're using, but we asked if there was anything other than manual balancing valves and if there were any devices like those mentioned that don't like to behave consistently - we were told no.

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 all makes sense alright - in something with multiple branches, flow restriction devices etc it's going to be never ending game to get the flow balanced exactly how they want. Many people don't really understand what the valves they have actually do.

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

Yeah, I suppose I'm all set. Unless we get back into it with them we likely won't have much contact outside spare parts or a checkup call in a year or so.

I appreciate the time and thought everyone put in.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close