Why impeller diameter should not to exceed 95% of the max impeller ??
Why impeller diameter should not to exceed 95% of the max impeller ??
(OP)
Hello,
In API 610 9th edition article 5.1.6 states that :
"Pumps shall be capable of at least a 5% head increase at rated conditions by replacement of the
impeller(s) with one(s) of larger diameter ..."
And I've seen some project specification (for seawater pump service) which is modified from article 5.1.6 by adding :
"The diameter of the impeller(s) provided (rated impeller and the one that gives 5% head increase), shall not be greater than 95% of the
maximum impeller diameter that may be installed in the pump."
Now, my question is why they have to specify the diameter shall not be greater than 95% of max impeller.
What is this margin for ?
As my understanding the smaller impeller means more clearance which is less efficiency so I don't understand the specification above.
Ps. The motor for this pump is sized for the 5% head increase impeller only not cover the max impeller.
Thank you for your help.
In API 610 9th edition article 5.1.6 states that :
"Pumps shall be capable of at least a 5% head increase at rated conditions by replacement of the
impeller(s) with one(s) of larger diameter ..."
And I've seen some project specification (for seawater pump service) which is modified from article 5.1.6 by adding :
"The diameter of the impeller(s) provided (rated impeller and the one that gives 5% head increase), shall not be greater than 95% of the
maximum impeller diameter that may be installed in the pump."
Now, my question is why they have to specify the diameter shall not be greater than 95% of max impeller.
What is this margin for ?
As my understanding the smaller impeller means more clearance which is less efficiency so I don't understand the specification above.
Ps. The motor for this pump is sized for the 5% head increase impeller only not cover the max impeller.
Thank you for your help.





RE: Why impeller diameter should not to exceed 95% of the max impeller ??
After round-off and selection of the next higher common size, its not unusual to find a motor sized at 10-15% over what is actually needed.
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
Thanks for your quick reply.
I understand that 5% head increase is for case that someone miscalculates the head so we have enough space to install the larger diameter impeller to get the required head.
But in this case, if that provision (not exceed 95% of max impeller) is also considered as a margin for someone miscalculates the head then they have to get the new motor for the impeller that exceed 95% of max impeller and the size is quiet big (1100 kW - Motor rating) so this would cost a lot.
Therefore, I'm not sure this is the real reason or not or maybe there are some reasons that I can't think of.
Do you think is there another reason except the one we're talking ?
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
A number of times I have had pumps removed from site for retest - sometimes tested by other pump companies to remove any bias in testing just to prove that the pumps perform as offered.
What must be remembered here also is that all testing is subject to a testing code tolerance and some end users have difficulty in accepting a +- 5% or what ever the code allows. There is a good argument as a supplier or buyer to offer / request certified witness testing prior to delivery as this removes all doubt as to the pumps performance, it could also explain the 95% diameter rule being discussed here.
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
If a pump was purchased for 100% of expected head, and it came up short, it would be very difficult to correct. The foundation, base plate, piping, motor driver, conduit, wire, starter and switchgear were all designed for that pump. If you had to change to the next size larger pump, it could be a monumental project. It is better to allow a little bit of wiggle room for slight underestimations (not necessarily gross errors) such that the original pump and motor could meet the higher head requirement.
P.S. BigInch. Do you ever sleep?
Johnny Pellin
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
Did you know that 76.4% of all statistics are made up...
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
Just a question...
If for any reason, the pump during test or commissioning fail to achieve the required head, so actual head is lower then expected. Let say we move to 100% impeller diameter to recover. Then why the absorbed power at the recovered head should change ? Is it due to some efficiency difference ? what about the fact that 100% impeller diameter will allow reduction of leakages even quite marginal.
Thanks & Rgds
wimple
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
Fitting another impeller at an increased diameter will result in a different hydraulic performance.
If H and Q are meet then the only way power can change is for the impeller / case having a different hydraulic performance.
Remember, performance testing has a code tolerance on Q,H, Eff. which must be acceptable by both manufacturer and buyer.
In many cases a manufacturer having to prove performance on a critical performance test may well fit a slighly oversized impeller for a pre-test prior to any official testing taking place.
If a pump fails on test there are a few things that can be done to try and enhance performance - but this is another story.
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
/Unquoted.
So at the opposite of what is pointed out above, this is definitely not to rise issue on motor power margin. Usual 10% motor oversizing, should be fine.
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
For attaching a VFD and increasing the impeller size to yield a curve equal to the non-VFD impeller curve.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
Why the VFD 100% speed and non-VFD constant speed should be different?
Are you meaning for example some possible slip in the rpm due to VFD design to be corrected by impeller diameter to match the head with 0 negative tolerance or to keep the gearbox aligned if any ?
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
Kindly let me know Whether the pressure drop of 18 MWC is due to?
RE: Why impeller diameter should not to exceed 95% of the max impeller ??
Start a new post to discuss your problem.