PD pump proposal outside Its curves

[pipertecho]

firstly, sorry if this question duplicates with any of the previous threads. I did some quick search and didn't find any direct resemblance. I have a situation where a PD pump with approx duty point 14 m3/hr @ 1 bar needs to take Diesel oil suction from the two tanks (one of the tank placed in another compartment and pipe needs to climb up the wall). Planed pipe routing can be seen in image with its approx height, length dimensions. For this application, vendor proposed me a pump, which performance curves end/terminate before my duty point starts. The vendor is very confident on his pump selection based on duty point as well as NPSH and claiming that pump will work perfectly fine for my application. Honestly speaking, I am not very confident with this situation and hoping for support from the expert community

 

[pipertecho]

I have attach image file for pump curve and dimensions. it can be found from link

https://files.engineering.com/getfi...baf-a194-4efe37bfbf8d&file=Untitled_skeck.png

 

[bimr]

PD pumps have an operating range, not a duty point. The PD pump will operate anywhere on that performance line as the RPM changes. Ask the supplier to furnish you with a graph showing pump speed versus RPM.

With 10 feet of suction lift, the NPSH_a for diesel is approximately 7.5 meter. Don't believe that NPSH will be an issue based on your pump curve and the sketch.

Here is an article showing how to read a PD pump curve:

Link

Note that with PD pumps, you may need a relief valve on the pump discharge if the discharge piping has an isolation valve.

 

[pipertecho]

Thanks bimr. My understanding was the curve start point and its endpoint shows the pump operation limit, outside operation generally not advisable. Here in my case, vendor proposed something, which is not covered in the performance curve. the curve for 20 cSt starts from 19m3/hr@2 bar and terminates around 16.5 m3/hr@20bar.

I will ask vendor to furnish another curve showing flow, pump rpm, pressure, and NPSH covering my requirement. I will also insist to propose a pump with a built-in relief valve.

Also, there is a strong possibility all my input data might change as per project progress, such as tank dimensions and their locations, especially suction lift. After receiving the new curves, I will go through with NPSH calculation again. Any suggestion on what should be the recommended margin for NPSH for PD pumps? I assume 1.5m should be ok.

 

[bimr]

The NPSH_r is a function of the pump and is supplied by the pump manufacturer. The NPSH_a must be higher than the NPSH_r of the pump

 

[pipertecho]

Thanks bimr

 

[LittleInch]

pipertecho,

The pump will do the work, despite being outside its stated range, but you appear to be being offered a pump which is about 10 times the power rating you need and 1.5 times the flow you need.

Is this just some random pump the vendor has lying around or just the smallest one he produces??

You usually want at least 2m on a PD pump because the thing the pump supplier never tells you about is acceleration head, because this is a system thing, not a pump thing.

You don't say what type of pump this is but if this is a piston pump, the flow actually is quite "jerky" as the pistons suck in fluid then stop then start. This creates losses in the pipework called acceleration head and can become significant especially as the vertical lift increases and the length of pipe increases. Look it up.

But if I was you I would find another vendor who can give you the size of pump you need, not just what he has available. The motor on this pump must be 14-15kW, but you only need about 1.5kW. Think of the extra cost of the wiring, motor controls etc as well as the pump cost.

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

 

[pipertecho]

Thanks, LittleInch.

Correct. This is the smallest pump under his portfolio. I forgot to specify the pump type in my earlier post. The proposed pump is a horizontal 3-spindle screw pump with shaft power ~1.9kw at 20cSt. Currently, assembled with 3kW @ 1450rpm motor. This offer in terms of cost, seems ok but surely I will ask other vendors to offer.

 

[LittleInch]

piper,

Your data is very confusing. The data sheet indicates the pump is good for ~14kW shaft power.

So unless you are seriously reducing the max pressure with a relief / bypass valve to about 2 bar then the motor will be seriously under powered.

Even a 3kW motor when you only need 1.5kW is double the cost once you add in wiring etc.

And if you don't need 15m3/hr don't pay for it.

Get it fixed what flow and head you NEED and then float it around a few pump vendors.

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

 

[pipertecho]

Very sorry LittleInch for the delayed response and my data if it confused you. As mentioned in my original thread, my application involves a diesel pump with 14 m3/hr @ 1 bar. I have given the same details to the vendor and he reverted with his offer and pump curve. In the image shared earlier, my doubt was the blue line (my operating point) which is outside the pump curve. in that curve, P kW is also shown as 2.

As you and bimr suggested to NPSH won't be an issue, which I did check from checalc.com and found ok. Pump NPSHr is 3m and available is 3.78m assuming vapor pressure as negligible. But the only worry is the very less margin. I used DN80 as a suction line and can not increase it further to get more margin

 

[LittleInch]

What flow rate are you using for the NPSH calculation?.

The unit you have, if those curves were for 1450 rpm will flow at about 19m3/hr as there is so little slip at <2 bar discharge for the unit as it's designed for a lot more flow and pressure.

So it will probably work if your NPSH is good at 19m3/hr, but the motor will stall if you ever close the discharge valve.

Far far better to get a pump that does what you need and no more.

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

 

[pipertecho]

I am using 14m3/hr flow rate for my NPSH calculations. I have also floated this inquiry to other vendor. Later have to take decision comparing both pump performance and cost

 

[LittleInch]

Then the pump you have sent us details of may not work at low liquid levels in the tank. That pump is just too big for your duty.

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