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RATED POINT

RATED POINT

(OP)
After i trim my impeller and test my pump again . how can i know the new rated point?

RE: RATED POINT

I guess that rated point is defined by/requirement from process. If rated flow has been specified, then after trimming the impeller, you head should decrease, so the new head at rated flow as specified is the point you are looking for.

RE: RATED POINT

maybe you should ask, how can I calculate the trim needed to meet the required flow/head.

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: RATED POINT

The "rated point" comes from you, the client / end user. So if you don't know then no one does....

Rated point is simply a point chosen on the particular curve / diameter of impellor to match what the requirement is in the data sheet, i.e. xx m3/hr @ yy m head

Do you mean something else like BEP?

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

RE: RATED POINT

If the pump has a diameter that is too big for the specified duty, it will move the operating along the "system curve" - that is to say the pump will deliver extra head and extra flow compared to what is required by the process. You trim the impeller to move the operating point back to the desired flow and head as specified.

But in trying to understand what is behind the OP request... could it be that you are doing the trimming using testing set up and using trial an error until you arrive to a certain desired pressure. or am I off the mark ?

RE: RATED POINT

(OP)
Thank you for your answers. To be more precise . For every model there is a max. impeller and a minimum impeller. I am getting the pumps with the max. impeller. Then i trim to get the intermediate points as per customer requirement.

Lets say my 1500 gpm model i am trimming to a flowrate of 1250 gpm at some bar. With relation between impeller dia, head, discharge and power i can find out the diameter to get after trimming.

But after trimming i want to test and ensure that the trimmed impeller is matching the required flow 1250. So when i do testing i get points from shut of to 150 flow including 1250 point. But how to know if its the rated point or if i have trimmed more or trimmed less?

RE: RATED POINT

Because during your test at the 1250 gpm point ( the rated duty for flow) , you measure the differential head and see if matches the required differential head asked for by the customer.

There will be two outcomes.

1) At 1250 gpm, your differential head is lower than asked for. you've trimmed too much - Oh SH1T

2) At 1250 gpm your differential head is higher than asked for - Then trim a bit more or if the head is within the tolerance in the specification (often 3-5%) then deliver it.

Aim for 2)

LI2thumbsup

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

RE: RATED POINT

(OP)
Dear Rotw,

As you said i am using a test set up. I get my models with maximum impeller. I do calculation if find out how much i should trim to bring down the rated flow and head as per customer requirement. Then i do retesting. But how can i ensure from the test result that the trimming i did was correct or if i should trim more or if i over did the trimming ?

RE: RATED POINT

rajm008,

I think that LittleInch has summarized for you the procedure...

If after trimming and testing - the readings show that the required rated flow and head (see LittleInch above post / step 2) are achieved than you are done. The only thing I want to add is this: You seem to calculate the target (trimmed) diameter using some theoretical equations, is this correct? Are you improvising some sort of approach using fan laws or technical literature derived formula OR are you using an OEM proven procedure? I am not specialist of pump testing, but I suspect any OEM's (if that is your case) have procedure in place for performing this operation.

I know it is not rocket science, but If you use some of your own formula, I would keep a margin just in case, because the equations do not model exactly the real behavior, you don't want to end up in the scenario highlighted in the above post - per step 1.... Just out of my common sense here, I would use/invest in some intermediate step(s) of trimming - bottom line is to not overshooting the final trimming diameter...

RE: RATED POINT

(OP)

These are the datas i got for minimum impeller.

Impeller Size: Minimum
Condition: Head

Suction Pressure Delivery Pressure Flow RPM Power
3.5 bar 12.2 bar 0 M^3/hr 1497 RPM 60.14 kW
2 bar 10.8 bar 91.2 M^3/hr 1496 RPM 74.76 kW
2 bar 10.8 bar 136.87 M^3/hr 1497 RPM 82.9 kW
2 bar 10.8 bar 181.05 M^3/hr 1495 RPM 92.54 kW
2 bar 10.8 bar 228.16 M^3/hr 1494 RPM 103.51 kW
2 bar 10.6 bar 283.93 M^3/hr 1495 RPM 116.77 kW
2 bar 10.6 bar 317.97 M^3/hr 1495 RPM 125.05 kW
2 bar 10.4 bar 341.96 M^3/hr 1494 RPM 129.57 kW
2 bar 10.2 bar 385.74 M^3/hr 1492 RPM 138.94 kW
2 bar 10 bar 423.21 M^3/hr 1492 RPM 146.29 kW
2 bar 9.4 bar 510.68 M^3/hr 1491 RPM 163.33 kW
2 bar 8.6 bar 610.32 M^3/hr 1491 RPM 181.07 kW
2 bar 8.2 bar 700.53 M^3/hr 1488 RPM 193.15 kW
1.5 bar 7.2 bar 758.37 M^3/hr 1489 RPM 201.53 kW
1 bar 6.2 bar 831.07 M^3/hr 1489 RPM 207.73 kW
1 bar 5.4 bar 870.2 M^3/hr 1488 RPM 213.04 kW
1 bar 5.2 bar 889.04 M^3/hr 1487 RPM 214.47 kW
1 bar 3.8 bar 920.7 M^3/hr 1487 RPM 213.21 kW


below is the datas is got for the same pump but with the maximum impeller

Impeller Size: Maximum
Condition: Head

Suction Pressure Delivery Pressure Flow RPM Power
3.5 bar 1.54 Mpa 0 M^3/hr 1496 RPM 95.63 kW
3 bar 1.5 Mpa 45.38 M^3/hr 1496 RPM 109.69 kW
2.5 bar 1.46 Mpa 90.18 M^3/hr 1495 RPM 121.03 kW
2.5 bar 1.46 Mpa 137.09 M^3/hr 1495 RPM 131.49 kW
2.5 bar 1.44 Mpa 227.94 M^3/hr 1493 RPM 152.35 kW
2.5 bar 1.42 Mpa 280.79 M^3/hr 1492 RPM 169.7 kW
2.5 bar 1.42 Mpa 316.93 M^3/hr 1492 RPM 181.64 kW
2.5 bar 1.4 Mpa 346.78 M^3/hr 1489 RPM 190.98 kW
2.5 bar 1.4 Mpa 384.19 M^3/hr 1491 RPM 203.86 kW
2.5 bar 1.4 Mpa 425.57 M^3/hr 1488 RPM 216.69 kW
2 bar 1.32 Mpa 509.77 M^3/hr 1487 RPM 240.73 kW
2 bar 1.3 Mpa 550.29 M^3/hr 1486 RPM 251.47 kW
2 bar 1.24 Mpa 602.57 M^3/hr 1484 RPM 265.57 kW
2 bar 1.08 Mpa 701.37 M^3/hr 1481 RPM 289.27 kW
2 bar 0.94 Mpa 780.96 M^3/hr 1478 RPM 310.87 kW
2 bar 0.9 Mpa 814.65 M^3/hr 1477 RPM 320.64 kW
1 bar 0.46 Mpa 853.15 M^3/hr 1481 RPM 282.66 kW

The rated gpm for the abouve model is 1500 gpm.

from these two datats i get two curves for max and min impeller. and from that graph i can plot for 1250 rated point with an intermediate impeller dia?


Or lets make it simple is there any way i can know the rated flow and bar of a pump with test set up if pump is without a name plate to show rated flow and head

RE: RATED POINT

rajm008, you know the trim was correct by the flow and head generated by the pump when you test it, or are we missing something in our reading of your posts

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: RATED POINT

(OP)
sorry for the earlier post i have attched the data with this one.


So is there any way one can identify the pumps rated flow and bar with just testing if he as no clue about the same? lets say no name plate is there in the pump to show its rated point and i want to test and sell it with its rated point and all i have is a test set up to get the pump curve is it possible?


Please forgive me if i am wrong i am not an expert in pumps. I think i know only a very little about it.

RE: RATED POINT

Quote (Please forgive me if i am wrong i am not an expert in pumps. I think i know only a very little about it. )


I think we've worked that out.....

"So is there any way one can identify the pumps rated flow and bar with just testing if he as no clue about the same? lets say no name plate is there in the pump to show its rated point and i want to test and sell it with its rated point and all i have is a test set up to get the pump curve is it possible?"

You have a fundamental issue here which you need to understand.

Testing like you're doing for a variety of flows and measuring inlet pressure and discharge pressure will generate a pump curve. Convert the DIFFERENTIAL pressure to m head of your test fluid ( presumably water) and draw the graph.

Your "rated duty" can be virtually anywhere on that pump curve, but is usually between 80% to 110% of the Best Efficiency Point, usually on the right hand third of the pump curve. Note that in your data noted above 1500 gpm (342 m3/hr) looks to be quite along way left on the curve and is probably not very efficient.

Try thinking of it this way - you have a motor car. The car can go from zero to 100 MPH (=flow) and carry up to five people (=head) up to 50 MPH, 4 people up to 90 MPH and three people up to 100 MPH and you test it to make sure it can d

The car can be "rated" to travel at any constant speed with the number of people as noted above, but in practice you would choose a constant speed between 60 MPH to 90 MPH carrying 4 people.

Does that help??

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

RE: RATED POINT

rajm008,

Looking at your spreadsheet...I made the following steps so you can think about it :

Lets assume the required pressure and (rated) flow is P0 and Q0 respectively.
- Step 1: Select from the data set @ Max Diam. the starting point (Ps, Qs) that most closely satisfies: Qs / Q0 = (Ps / P0) ^ 1/2 (edit)
- Step 2: Select from the data set @ Min Diam. the final point (Pf, Qf) that most closely satisfies: Qs / Qf = (Ps / Pf) ^ 1/2 (edit)
- Step 3: Calculate the slope and offset coefficients a,b of the line crossing the two points above, i.e.: P = a x Q + b
- Step 4: Set an initial trimming diameter ratio. Say 0.95 and calculate the corresponding Flow Qt based on: Qt/Qs = 0.95 (edit)
Calculate the pressure using: Pt = a x Qt + b
- Step 5: Check (Pt, Qt) vs. (P0, Q0) (within the contract tolerance); adjust diameter ratio as needed.

DISCLAIMER: DO NOT use this to perform your work. This an attempt aimed at giving an initial estimate of the trim. I also suppose your curves min/max need to be nicely subdivided (pair of data equally spaced relative to the min/max curves correspondingly). If this is not available in the original data sets then I guess you have to derivate / create those points.

RE: RATED POINT

I had to edit the affinity laws in my previous post. Reason is I picked the formulas from wikipedia (conveniently), but I just realize that wikipedia is stating the affinity laws wrong!

https://en.wikipedia.org/wiki/Affinity_laws



RE: RATED POINT

(OP)
Dear Littleinch/rotw,

Thank you for your help. Yes it did help alot. I think its clear now.

Now my pump department is just about to start in few months so may b piratical experience while doing the test and trimming will give me idea.

As you said this model for 1500 is a not very efficient. My company dint manufacture the pumps instead we get from another company. It had to pass many tests . So these people to be on safer side considered a bit bigger sized pumps.On the time of testing the power drop for 2000 gpm model didn't happen. So in order to get atleast 1500 gpm rating. We reduced the impeller inlet dia and and some other adjustments so at end we got rating for 1500 gpm but the pump was bit bigger.

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