Centrifugal Pump selection criteria
Centrifugal Pump selection criteria
(OP)
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.





RE: Centrifugal Pump selection criteria
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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RE: Centrifugal Pump selection criteria
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
RE: Centrifugal Pump selection criteria
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.
RE: Centrifugal Pump selection criteria
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_Qn / Eff_Qn * %T_Qn
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.
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.live.com
RE: Centrifugal Pump selection criteria
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
RE: Centrifugal Pump selection criteria
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.live.com