Impeller trimed and Flow increased
Impeller trimed and Flow increased
(OP)
Hi,
We have trimed the impeller of a centrifugal pump and after triming the impeller this pump flow has increased and load has reduced.
I'm confused with this as I know by triming the impeller will reduce flow and reduce load on motor.
Could any one share his experience with me if have this type of problem.
thanks
We have trimed the impeller of a centrifugal pump and after triming the impeller this pump flow has increased and load has reduced.
I'm confused with this as I know by triming the impeller will reduce flow and reduce load on motor.
Could any one share his experience with me if have this type of problem.
thanks





RE: Impeller trimed and Flow increased
Sounds like you have significantly affected the curve, perhaps even too much. Load is essentially the product of head x flow.
Head will reduce with a trimmed impeller. If you reduce it too much, even more flow at such a reduced discharge head could easly result in a reduced total load.
RE: Impeller trimed and Flow increased
Also if a better underfile was performed, you could trim a little and potentially increase head.
If you trimmed because you were close to overloading the motor, it's possible that your speed increased.
If no speed increase, then for some reason your efficiency has gone up. Not sure I would classify that as a problem.
RE: Impeller trimed and Flow increased
RE: Impeller trimed and Flow increased
1. it is a poorly designed impeller and there is too much overlap of blades effecting the discharge off the impeller into the volute, trimming back on diameter may have overcome this problem, ie opened up the discharge area.
or
2. Trimming back the impeller has altered the discharge angle of the blade which has changed the flow characteristics.
A little more info might help- pump size and type, impeller diameter and trim amount.
RE: Impeller trimed and Flow increased
RE: Impeller trimed and Flow increased
And BigInch, can I get eye burn from looking at your signature?
rmw
RE: Impeller trimed and Flow increased
You have not mentioned the type of drive for your pump, and I suspect that you have no useful data on the actual shaft speed for your pump either before or after trimming the impeller. If you don't know the shaft speed accurately, it is very easy to make erroneous evaluations of pump performance.
I share rmw's doubts regarding the likely accuracy of flow measurement. It would be no surprise to find that instrument and test inaccuracies could indicate an increased flow rate when reality is just the opposite.
I've seen seemingly careful tests of pump performance carried out over a period of years with the intention of evaluating deterioration of pump performance due to wear, but review of the resultant data showed performance deterioration followed by performance improvement with no repairs or modifications to the pump. The only certain conclusion that could be drawn from the tests was that the pump was actually running, and it was performing tolerably well. The apparent variations were all within a range that was obviously due to the reasonable uncertainty inherent with the instrumentation that was used (all excellent quality and carefully calibrated). Simply stated, if your instrumentation is good for +/- 5%, don't try to conclude anything from an apparent difference of 3% between two tests.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Impeller trimed and Flow increased
Never look into the welding flash!
Don't stare at a solar eclipse!
Don't go outside when it's lightning!
Listen to your mother!
PPE is now required in this and a few other forums.
RE: Impeller trimed and Flow increased
RE: Impeller trimed and Flow increased
Pump Data is here,
V=60m^3/hr
DeltaP=8.83 barg
Shut off pr=14.8 barg
BHP=34kw
Drive HP=39kw
Suction temp=355^0c
Service = Gas oil
Before triming pump was not able to pump more than 35m^3/hr and after triming pump is able to pump 50m^3/hr before the trip load that is 65 amps.
Hope you will make some more clear assumption which I can't do it for understanding this problem.
One more thing biginch asked about amount of material (weight) that idon't know because my company did this job with prevate workshop.
Thanks
MIANCH
RE: Impeller trimed and Flow increased
Impeller dimension before trim,
and after trim?
Operating speed?
would help.
RE: Impeller trimed and Flow increased
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RE: Impeller trimed and Flow increased
However, I suspect you have a system that is more like the type I see in a refinery that use some sort of flow control. In these systems, they don't run to the intersection of the pump curve and the system curve. They run to some constraint. The constraint might be a flow target set by the process engineers. It might be holding a constant level upstream or downstream. Or, it might be something like motor amps. If your operators have a meter to check motor amps, they might be controlling the flow to keep from tripping the motor off. If the flow is controlled by pinching on a valve, then this makes perfect sense. When the impeller was trimmed, the performance of the pump would change based on the affinity laws. As impeller diameter is reduced, the brake horsepower drops as the cube of the diameter. The pump head drops as the square of the diameter. So, if the operators were pinching down on a valve to keep from tripping the motor off, they would be able to get more flow after the cut because of the reduction in BHP.
I could describe this another way. With the larger impeller, you were building more head than you needed and dropping it down across a pinched valve. This is wasteful and inefficient. After the impeller was trimmed, the valve was opened further because the motor horsepower was lower, even at the same flow. Less energy was being wasted by the pinched valve. This energy would then be available to push more flow through the system.
Please confirm if the flow was being pinched back before and after the impeller was cut down.
Johnny Pellin
RE: Impeller trimed and Flow increased
From this, I gather that the limit on flow is due to overloading the motor. Pretty important piece of information if I read this right.
Trimming has reduced your head and power required at a given flow. You are now running further out on the curve before hitting your 65 amps. No mystery here.
RE: Impeller trimed and Flow increased
From the quantitative information in your second posting, it certainly appears that JJPellin's and 1gibson's responses provide the most coherent explanation of the situation. Such a dramatic variation in output could only be be explained by something external to the pump.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Impeller trimed and Flow increased
RE: Impeller trimed and Flow increased
And we all know what happens when we ASSume.
RE: Impeller trimed and Flow increased
IMO It seems strange the OP has not taken into account any discharge line modification
RE: Impeller trimed and Flow increased
The other consideration I took into account is human nature. If the operators are charged with moving as much product as possible, they will tend to operate the pump up to some hard constraint. If the constraint is changed, they will tend to run up to the new constraint. If the motor trips, this is probably an inconvenience to the operators. They will tend to try to find a way to avoid this inconvenience. If they can do this by pinching on a valve, it would not surprise me that they would do so.
Johnny Pellin
RE: Impeller trimed and Flow increased
RE: Impeller trimed and Flow increased
Guess it keeps us occupied and exercise the old brain.
RE: Impeller trimed and Flow increased
I got information on impeller triming,'ie' oreginal impeller dia meter was 366mm and 10mm is trimed.
Now the question is why manufacturers are not consulting with process engineers and validating the design parameters before for manufacturing the pump or any another equipments.
Thanks
MIANCH
RE: Impeller trimed and Flow increased
RE: Impeller trimed and Flow increased
How about a by-pass or minimum flow regulator?
However, 10mm in 336 isn't a huge reduction.
rmw
RE: Impeller trimed and Flow increased
No presure regulator on this pump, this kero/ gas oil fractionator bottom pump for the bottom side pump. After triming the pump,the only change is control of FV increased around OP 2%.
Only apparent change is only FV out put and no other change.
Hope you will make some more assumption about this meyth.
Thanks
MIANCH
RE: Impeller trimed and Flow increased
RE: Impeller trimed and Flow increased
It seems that we misunderstand everything. The pump manufacturers and others are expected to provide the proper pump and advice based on inadequate and misleading information. I'm sure that a variation of this philosophy is behind the attraction of VSD's to pretend to compensate for the effects of misinformation while providing assurance of optimal pump performance and dramatic energy savings (reality being both insignificant and unimportant).
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Impeller trimed and Flow increased
1. it is nearly always the pump at fault.
2. if not the pump, then it's entirely the manufacturers fault.
3. there are no other causes which can affect pump performance as the pump should always fit the application perfectly.
RE: Impeller trimed and Flow increased
There is no difference found in differential head.
Thanks
MIANCH
RE: Impeller trimed and Flow increased
RE: Impeller trimed and Flow increased
2. No change in total differential head
3. Brake horse power decreased.
BHP = Q*Ht*SG/(3960*pump efficiency)
The pump efficiency has changed, so the operating point is not on the same system curve as that it was before trimming
RE: Impeller trimed and Flow increased
my pump head is 179m and differential head is 3.7m. Deisgn flow is 60m^3/hr and efficiency is 43%.
After triming why BHP increased and remaining power where is getting utilized.
Thanks
MIANCH
RE: Impeller trimed and Flow increased
1. Show us exactly how you did the head calculation in your spreadsheet
2. Is your flow meter calibrated
3. Are your pressure gauges calibrated
4. Send a picture of your installation showing the pressure gauges
What you are saying is occurring is impossible. To engineers, who understand pumps, you are not providing much information for anyone to comment on