Pump efficiency operating cost evaluation 1

[elquesifilma]

Hello All,

I want to evaluate 2 pumps for the following application.

S.G fluid: 1.05
Flow: 350 gpm
P: 600 ft head

I found 2 pump curves that work for my application.
-Curve 1: shows the design point requiring 75 HP with a pump efficiency of 80%. Pump would require 100HP motor to cover the complete curve
-Curve 2: shows the design point requiring 100 HP with a pump efficiency of 55%. Pump would require 150 HP motor to cover complete curve

Both pumps will be controlled with a control valve downstream.

Obviously my Curve 1, should be the obvious pick for the application, the problem being cost.

Curve 1 (pump/ 100 HP motor skid) : $60,000
Curve 2 (pump/ 150 HP motor skid) : $16,000

What information do I require , and how do I calculate operating cost for each pump, to look at a possible payout time in less energy costs by selecting the more expensive pump/motor.

Thanks in advance to all!

 

[BigInch]

Hardly requires "evaluation", but here goes,
neglecting interest rate and even assuming maintenance costs were equal, even though you would probably do more maintenance on the cheap one, neglecting electric line losses (higher for 150 HP motor for same cable size) AND neglecting environmental cost!,

convert HP to kW
assume just 1 year of operation
pay for the kWh
at some power cost
If power cost is 14¢/kWh
You can see that you an pay the difference in motor cost with savings in power cost in ONLY 1 year.
If power cost was 0.07¢/kWh it would take 2 years.
You can calculate your payback time using your electric cost.

motor1 motor2 difference
motor cost 60000 15000 45000
HP 100 150
HP to kW 0.7457
kW 75 112
hrs/yr 8760
cost/kWh 0.14
power cost/yr 91453 137179 45726

It appears to be a very short payback period, and a very good deal in general to buy the high effiency motor. If you are going to run this for more than 2 years, as I would think you will, go hi eff.



**********************
"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/

 

[elquesifilma]

Thank you BigInch.

Allthough if i have a 100 HP motor on the skid, but my curve shows the operating point only using 75 HP, should i run the numbers with 75 HP?

What am i consuming when i run at design point, the full skid 100HP or just what the rated 75 HP?

 

[BigInch]

Yes. I didn't check the HP numbers and thought they didn't include efficiency.

So, let's look at these HP again.
This time I did the HP numbers and get 66 HP and 97 HP
And those are Brake HP (power at pump shaft).

I also included some imagined motor efficiency values
eff1 = 0.94 for pump1 and eff2 = 0.87 for pump2

Makes me wonder why the motors have such high ratings.
100 HP for 66 BHP is only 66% load. No 75 HP motors?
150 HP for 100 BHP is only 64% load. No 110, or 125 HP motors?

Anyway, 66 BHP at 94% motor eff = 52.6 kW for pump 1
and 82.7 kW for pump 2.

Use those kW x hours x cost to get power cost.
This time I give you the spreadsheet. That's easier.
Check it. Its 2:00 am here.

Pump 1 pays for itself in about 2.5 years w/o interest, etc.







**********************
"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/

 

[JJPellin]

One of the things you can get out of the calculations that BigInch provided is a simple value for cost per year per horsepower. Once you have that number, you can do a quick and easy check for the viability of power saving projects. Just be sure to update the number every year or so for changing power costs. And once you get down to the final decision point, verify the savings with a full blown analysis.

Our area must have lower electricity costs than most. Our value is US$350 / HP / Year. So, for your example, saving 31 BHP would only produce savings of $10,850 per year. In our current economic climate, I would not be able to justify an extra $44,000 expense on this savings. I would be forced to go with the lower cost option. For the time being, capital is king and a payback longer than 4 years is not going to be funded.


Johnny Pellin

 

[BigInch]

It seemed to me to be a rather big diff in base cost between the 2 options. Is one used, one API, or is it just a real cheapo? Might work, as long as it holds together.


**********************
"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/

 

[JJPellin]

Good point, BigInch. I was assuming that the two options were equivalent in terms of reliability, operability, maintainability, etc. With this big cost difference, that is probably a poor assumption. On top of the energy savings, you need to account of these other differences. There is more involved in the decision that just energy.

Johnny Pellin

 

[waross]

From an electrical point of view, I wouldn't begin to evaluate this without seeing the utility tariff. There is too much variation in costs to assume anything in relation to energy costs/savings. In the tariff, check also for demand charges. Demand charges, when applicable, may double the monthly charge for energy.
If you get down to comparing pennies, check the motor efficiency and PF at the expected loadings. uncorrected PF typically drops as the load drops, possibly incurring penalty charges. Motor greatest efficiency is often at less than 100% load.
Step one, check the tariffs and estimate the savings. If the result is acceptable great. If you are close, look at PF, PF correction and the points of greatest efficiency.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigInch]

If we get to comparing pennies, or in other words, whenever the difference looks reasonably affordable, shouldn't we just be responsible engineers and select the least energy using option? That could also be a big hedge on your bets on your future energy cost estimates. If energy costs ever start rising again (did they stop over there? And IMO not a real big if), investing in the more efficient pump today just might save you a hell of a lot more money than you ever thought possible later on. Take a look at the utility bills over the last years, extend the curve a bit and it doesn't take a lot of gray matter to see how much that might be. Hedge your bets and invest in a greener future as well? If the difference is only pennies, why not? Looks like a sure shot to me.

**********************
"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/