Centrifugal Pump selection criteria 3

[maintennance]

When 3 or 4 vendors are technically acceptable , and when we are asked to rank the vendors , it strikes me only the Rated power of the offered pump and the similar supply to other users . Thanks to clarify what are the other such factors to be considered which are critical from the aspect of life cycle cost analysis.

 

[BigInch]

NO NO NO. A rated (motor) power too high over that required will just cause further inefficiency when it runs at a lesser partial load than a more appropriately sized motor. A pump with its own power capacity too high, might not have an economic construction for your lighter weight purposes. A pump runs best when the flowrate and power capacity matches the load, but some amount of oversizing, 10 to 15% over actual load is often necessary for one reason or another. But it should normally be kept within that range.

There are many other factors to consider, many depending on the exact type of pump that is required such as, Operating speed motor efficiency, pump efficiency (power use calculated from pump efficiency at the exact operation point), bearings, L/D ratio and vibrations, suitability for off BEP operations if your operating conditions require such, general construction, basically everything listed on the spec sheet will have at least a small impact on overall life cycle cost from a reliability perspective. From a power cost perspective, high motor efficiency and high pump efficiency within the flowrate operating range are usually the MOST important aspects as far as energy costs are concerned in typical installations where power cost over the installation's lifetime can be 100 times the initial pump cost.

CORRECT PUMP SIZING IS NOT A CASE OF BIGGER IS BETTER! You should try to hit your target power and flow rate(S), with high efficiencies an only an adequate excess power margin, nothing less and nothing more.

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[JJPellin]

I read the question a little bit differently. It sounds as if we are being asked if the only important part of lifecycle cost is horsepower. I interpret this to mean that we would always select the pump with the highest efficiency and thus the lowest horsepower consumed. If so, I would once again answer "No" but with a few qualifications.

Lifecycle cost for a new pump consists of the cost to buy it, install it, operate it and maintain it. If cost is my only criteria, it is a simple economic analysis. I can pay twice as much to get a pump with higher efficiency (lower energy cost) and higher reliability (lower maintenance cost). Allowing for the time value of money, the economic analysis program would tell you which option is the better value. But, in my experience, these costs are not the only criteria.

If a pump is fully spared and downtime has no production penalty, pure lifecycle cost may be of high importance. If the pump is unspared and failures cause a loss of production, reliability may be much, much more important than lifecycle cost. If the product in the pump is flammable or has environmental impact, then failures may carry a high safety or environmental risk. So, reliability could be critical. I can assign value to all of these variables and bring them back to an economic analysis.

For example: In my plant, we bought a 1000 HP motor driven charge pump that runs unspared. When it shuts down, the unit shuts down. It cost US$1,000,000 to purchase and install the pump. Running the 1000 HP motor costs us about US$400,000 per year in electricity. If the pump shuts down, the lost production costs us US$100,000 per day. Each seal or bearing failure of the pump costs us US$25,000 to repair and takes two days. Each complete overhaul of the pump costs us US$150,000 and takes 5 days. Would it be worth it to pay an extra US$100,000 to get a pump that only uses 900 HP? Would it be worth it to pay an extra US$250,000 to get a pump that can run twice as long between major overhauls? If each seal leak carries a 1/100 chance of a fire that will cost us an additional US$500,000 in damage and downtime, I can account for that, too.

All I have to do is put the economics into a spreadsheet and let it calculate the Net Present Value of each option. In my example, reliability is worth more to me than efficiency. In an application with low safety risk (not flammable), no downtime cost (fully spared) and high energy costs, perhaps efficiency would be worth more.


Johnny Pellin

 

[maintennance]

Thanks Biginch and JJ. My query is for a Heater charge pump in a delayed coker unit where there are two pumps . One supplier pump absorbed power is 100 kW more than the other and thereby the pump efficiency is 15% lower . The NPSHmargin for the lower absorbed power supplier is less than 1.8 metres where as for the other is morethan 2 metres.

Similarly the Rated flow as the % of the BEF flow for the lower absorbed power supplier is 70 % where as for the other supplier is 84% .

This is where I got struck. When we say that one supplier is giving higher efficiency pump but with lower NPSH margin and % of the BEF for the rated pump, which one is best to be ranked.

Thanks to clarify.

 

[BigInch]

The NPSH question is a pass-fail criteria. You MUST be able to operate without cavitation, so put a check mark in the pass or fail box next to NPSH, now consider the best efficiencies of any remaining pumps.

If you have only one flowrate, simply score the remaining units by their % Eff x 100

If you have to opearate at many different flowrates, its slightly more complicated; the time you are pumping each flowrate must be considered.

Take the sum of what amounts to an indicator of total power consumed, Power Consumption Indicator ( PCI) when considering all flowrates,

PCI = [∑]_{1 to n} HP_Q_n / Eff_Q_n * %T_Q_n

Where,
n = number of discrete flowates you are evaluating,
HP is the hydraulic pumping power required at each flowrate n,
Eff is pump efficiency at each flowrate n,
%T is the percent of total operating time (100%) you will pump at each flowrate n.


Pick the pump with the lowest total power consumption indicator.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

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[JJPellin]

I don't see the NPSH questions as such a simple check box. As we all know, the NPSHr is generally expressed at the point where cavitation is reducing the total pump head by 3 percent (some specify 1%). That is pretty significant cavitation. The pump manufacturer does not tell you the point of insipient cavitation. There is much debate regarding the amount of NPSH margin needed to avoid significant cavitation. Many sources talk of NPSH margin ratios with NPSHa 2 or 3 times higher than NPSHr.

As we all know, running closer to BEP will generally result in better reliability. And, even if fully spared, the failure of a coker charge pump is a very risky thing. It is risky to the process. A momentary loss of flow could end up with a coked off heater, unit shut down and major equipment damage. A significant seal leak of the pumped product (not the seal flush) would absolutely result in a large fire since the product is above auto-ignition.

With that said, a 1.8m NPSH margin is pretty good. The pump with the lower margin would likely have a lower suction specific speed which will result in better down-turn. For that critical service, I would look closely at bearing arrangement, shaft L/D ratio, etc. It very well may be that the more efficient pump is the best option, overall. But, coker charge is arguably the most important service in our refinery. Our cokers run full, so a lost barrel of coker charge cannot be easily made up. And with coker incentive that can be more than US$40 per barrel, a single lost day of production on one coke unit would pay for more than 20 years worth of energy efficiency benefit.

I would check references on either option. Make sure that the model proposed has been used successfully in coker charge service. Buy the pump that is going to give you the best reliability.


Johnny Pellin

 

[BigInch]

JJ, I agree that everything isn't black and white like a pass or fail box. Just I see it more like this. That was a simplification of one of the easier aspects of pump selection that its ususally possible to simplify quite easily. You might also be able to reduce your required flowrate and head a little, accept a slightly worse L/D ratio, or even a higher speed pump, etc., but in design/purchase work you usually have to eventually separate the pigments of those gray areas into white and black buckets and see which one weighs more and give someone an anwer. Then, even though it was temporarily separated into black and white, everything eventually becomes a new shade of gray that operations must live in forever after.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com