impeller trimming calculation 1

[rajm008]

hii everyone,

I have a pump model which underwent UL testing and i got 2 flow points. 50 and 100 gpm , each flow points has got a pressure range too.

50 gpm- (120 to 133 psi)
100 gpm -(112 to 127 psi)

max impeller dia of this model is 272 and minimum is 260.

so to get 100 gpm @ 127 psi i used my maximum impeller.

Now i need to know how much i should trim the impeller to get 50 gpm @ 120 psi. for this i used affinity law. (d1/d2)3= q1/q2 , (d1/d2)2=h1/h2, (d1/d2)5= p1/p2.

multiplying all these equations i got d2 as 241 mm . but it is out of the approved impeller range tht is 260- 272 mm.


Please help me. Please tell me where i went wrong.

 

[rajm008]

please find the calculation i did attached with this .

 

[Artisi]

Don't really understand what you are trying to achieve, you can already achieve 50 gpm at 120 - 133 psi, was this by throttling the discharge?

Are you asking to calculated the flow / head of 50 / 120 as an operating condition?

Think you need to expand your question a little for understanding.

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.)

 

[LittleInch]

From the data supplied, it would seem that the pressure ranges at the given flow points are the min and max impellor sizes.

Affinity law is quite good, but not precise. There are other factors which cannot really be calculated, only by actual testing can you get the data needed.

You don't say what you're using for p1 and p2. p1 needs to be 133 psi, but as said, I think the vendor has already done this for you and 50gpm @ 120 psi is 260mm

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rajm008]

no sir it was not by throttling the discharge. In the UL testing we tested the model with maximum impeller 272 mm to get a curve and then tested the same model with minimum impeller 260 mm to get another curve.

So now we have 2 curves one for maximum impeller and one for minimum impeller. So we can get the intermediate values from the two curves.

I was trying to calculate how much i should trim the impeller diameter to achieve 50 gpm and 120 psi from same model pump. without changing imepller inlet diameter.

 

[rajm008]

dear little inch,

My vendor is saying there is no way to achieve another flow point without changing inlet diameter of the impeller , with the same model pump.

I showed a sample curve of another company showing curves of different diameters of a single model and how they achieved different flows and pressures. but still they cannot get it , because in thier test facility they can only plot one curve in the graph.

So i wanted to show them approximate diameter by affinity law. But when i calculated the impeller dia was coming as 241 mm which is lesser than the minimum impeller dia of that model.

so i came here for help.

 

[rajm008]

p1 in kw =(gpm* head in feet)*.745/(3960*efficiency)

This is the formula i used to calculate power for each flow and pressure points. taking efficiency as 75%

 

[Artisi]

Seems to me that 260mm does what you want.

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.)

 

[rajm008]

thank you sir,

Please tell me if my way of calculating is right? i know i can get only approximate but stil just wanted to confirm.

I have read some where that its better to trim 75% of the value we get by affinity calculations to be one the safer side. Taking that into consideration it comes around 250mm.

Please correct me if i am wrong.

 

[LittleInch]

I think your problem is that you seem to be changing two things at one, i.e. flow and head.

(100 gpm at 127 to 50 at 120??)

Affinity only works when you only change one thing.

Spell out your calc using figures in all the boxes so we can see where

When you say you get a pressure range, what do you mean??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

When testing was a H/Q curve plotted or did you only plot one point for each impeller diameter.
Without a curve it is nearly impossible to plot a single point with any sense.

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.)

 

[rajm008]

please find the test data in excel form from which UL gave us the flow and pressure range. Using these datas we got pressure ranges for both 100 and 50 gpm from the same model.

 

[rajm008]

Dear Artisi,

during testing of one model(say 50 and 100 gpm model) H/Q curve was plotted for both minimum and maximum impellers. Taking readings from shut off, rated point ei 100 gpm and 50 gpm till 150 percent flow. after which power drop was observed.

I am attaching the listing that we got for the end suction models. The model i am talking about is MIN-XBD-80-50-315. You can see in the list we have got pressure ranges for both 50 and 100 gpm from same model.

For 100 gpm point he took pressure range as pressure we got with minimum and maximum impeller at 100 gpm point.

for 50 gpm point he took pressure range as pressures we got with minimum and maximum impeller at 50 gpm point.


So how can i trim the impeller? without considering the gpm?

 

[LittleInch]

That's a confusing set of data. on the first table what are 11 and 12? Higher flow and higher pressure yet less power?

What is the difference between the two - inlet conditions? -5 what? m, bar. discharge seems to be different to your values.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rajm008]

Dear little inch,

So as per what you told, I should not be worried about gpm. I just need to trim the impeller to get the pressure required? using affinity law i can get the power of the motor required too.

Please correct me if i am wrong.

 

[rajm008]

Dear Littleinch,

11 and 12 points where taken to ensure power drop will occur. Its UL requirement to have power drop after 150 % of rated flow.


Sorry about the misleading value. its not actuly -5 it is -0.5 bar.

Its thier requirement to test in both negative suction and positive suction(lift and head condition)

 

[rajm008]

power drop is a safety requirement. If power doesn't drop or stabilizes then in worst condition fire occurs and if power keeps on increasing it may lead to tripping and fire wont be extinguished. I think it happens only in wrong selection of pump or wrong design of the system.

But its safety measure taken in fire pumps.

 

[Artisi]

http://www.pumpfundamentals.com/help7.html

This is basic pump hydraulics 101.
Suggest you plot a pump curve from your printed data and then select a number of new impeller diameters to calculate a curve to suit your Q/H you want.

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.)

 

[rajm008]

Thank you sir for your time.
In this link at first they gave affinity law for fixed speed.. and after the graph . Its showing another equation for reducing the dia. But if we take n1 and n2 as constant then it wil contradict with the first given equation.. it wil be eqaul to affinity law for fixed diameter.

 

[Artisi]

Please explain what is head variation you have given 50 gpm- (120 to 133 psi) and 100 gpm -(112 to 127 psi)

What I don't understand is, the pump was tested at 50gpm @ 120psi, so what is the problem or what is your question?

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.)