Standby pumps reliability
Standby pumps reliability
(OP)
Are there any studies showing how swapping pumps with their standby spares impacts reliability? I realize different pump designs, services, fluids, etc. may require different strategies. In general, its a question of what is the desired benefit, what are the costs and potential risks, and then is it worth it. This will vary for different applications, and organizations - I get that. There are arguments for both sides, so what I'm curious is if there are any documented studies on the topic that can be shared.





RE: Standby pumps reliability
I did an analysis for a project (power gen) for various pump configurations (2x100%, 3x50%, 2x50%) on two large water systems. Since it was a power plant, I used lost generation revenue versus capital cost and additional O&M expense to determine a payback period. Payback period was quite long, so we went 2x50% on one system and 3x50% on the other.
RE: Standby pumps reliability
But not quite sure what you mean by " swapping pumps with their standby spares impacts reliability" though. Can you expand?
ultimately it becomes risk versus cost.
I know one company that commonly uses 2 x 50%. they work that on the basis that one unit will probably do 70% of the requirement so the losses are reduced. occasionally it hurts them, but overall they save money.
However things like pumps might be operationally more efficient as a single unit so overall costs including OPEX might be better with 2 x 100%
All in the detail....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Standby pumps reliability
Both of you left out any information about the Operation protocol for the different pump configurations.
Example:
2-100% Pumps - Alternate with each 100% pump on a 48 hour on and 48 hour off.
Pro: This tends to maximize your operation life for both pumps. It also gives you a 48hour for maintenance window for routine or simple repairs.
Con: this tends to be a higher initial installation cost
3-50% Pumps - Alternate pumps with two pumps on and one off on a 48 hour cycle
(A & B on w/C off, then A & C on w/B off, then B & C on w/ A off)
Pro: This tends to extend the working life of all three pumps
Con: This tends to require more space
2-50% Pumps - Both pumps run 100% of the time This might work good for a batch operation.
Pro: This tends to be less initial cost
Con: This tends to place your operation at risk
Sometimes its possible to do all the right things and still get bad results
RE: Standby pumps reliability
I didn't leave it out of my analysis. I just didn't discuss it here. Instead I gave a brief example of what I have done without diving into details since he is asking about studies. To me, one would need real operational data with service intervals and maintenance costs (corrective and preventative), which would be after the equipment was in service for several years. My analysis was on the front end, so I made assumptions (since that data does not exist) based on maintenance schedules recommended by the manufacturers, anticipated operational profile of the unit, and maintenance outage durations and costs for other similar units.
RE: Standby pumps reliability
swapping pumps with their standby spares: Say you have 2 X 100% ANSI pumps that feed a process. To ensure the spare pump is available when needed every month operations shuts down the running pump and starts the idle pump. This also prevents bearings from brinelling, seals from sticking, tests the electrical power supply. The reason I hear most often is to ensure the spare pump will start.
On the other hand, motors send a high inrush current during starts which over time eat away at the life of bearings and couplings. If the pump swap is done manually, there is also risk of operator error. In some cases, the cost of swapping is too high and not worth it. So the counter argument I hear is you are shortening the life of both pumps, decreasing the reliability, and there may come a time when whichever is on standby mode does not start due to a failure induced by increased run life or too frequent starts.
And there are variations of this: A primary/secondary for instance. Where the primary pump runs for 3 weeks, then the secondary runs for 1 week. The intent here is to separate life so that they are not both due for major overhauls at the same time. Another version is to jog the idle pump or turn by hand. But with LOTO and work permits, this is not always practical. A third version is to swap equal run time once a week.
Various arguments for why one should and why one shouldn't alternate pump run time exists. I accept that each site has to consider their own priorities. Still, I was curious if anyone had actually studied the practice and shared their findings with the general public.
RE: Standby pumps reliability
You can start a centrifugal pump against a closed valve, so the switch over is fairly painless. In a plant it would be programed into the controls.
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P.E. Metallurgy, Plymouth Tube
RE: Standby pumps reliability
We have other plants owned by our company that have extended the interval between pump switching to 60 or 90 days. We hired a consultant with 40 years experience with one of the largest oil companies in the US. They are emphatic that they improved their pump reliability by extending pump switching intervals to as much as 1 year in some services.
We are going to a 60 day switching interval currently and plan to extend further to 90 days in some services.
Johnny Pellin
RE: Standby pumps reliability
I find it hard to believe that if your pump will last for 5 years, that there is a significant difference between 6 starts and 60 starts. Maybe if we were talking about 600 starts it would start to have an effect. The cases that I know of where they switch every week or two are severe service and the pumps only last 2 years. I am sure that he size and style of pump is a factor, as well as the service. Heck in may cases the seals may well be the limiting factor.
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P.E. Metallurgy, Plymouth Tube
RE: Standby pumps reliability
Johnny Pellin
RE: Standby pumps reliability
I have found that pump #1 actually last longer because it is running 24/7/365. And pump #2 is still like brand new 20 years from now when I finally need a backup pump. I no longer have to be in a hurry to repair pump #1, as pump #2 is in good shape and makes a much more reliable backup.
On municipal type systems I like 2 X 100% plus a jockey of about 10-20% capacity. As these type systems are at low flow conditions a lot of the time, the jockey pump greatly increases the life of pump #1. Along with using the exercise clock for pump #2, this makes a very dependable system.
RE: Standby pumps reliability
Where I have seen pumps alternated they usually run one of them 2-5 times as much as the other. That way they don't wear out together, and you can prove that they both work. I like to see the second pump actually doing the job so that you can get accurate motor readings, flow readings, and vibration numbers. Better to find something wrong when you know that you can switch back to the other pump.
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P.E. Metallurgy, Plymouth Tube
RE: Standby pumps reliability
I would feel more trust in the reliability of a pump which has run 24/7 for five years than a pump which set idle for five years other than 15 minutes of operation against a pinched valve once per week.
I have spoken to engineers at refineries that extended pump switching beyond 30 days and experienced increased pump reliability. Some of the pacesetters in the Solomon study are switching their pumps less often. I do not have study results to back up this position. But, the evidence I have seen is sufficient to drive me to extend switching intervals in our refinery.
Johnny Pellin
RE: Standby pumps reliability
RE: Standby pumps reliability
Interesting point on having the standby pump in offline mode ALL the time. It is a novel concept to me, but it does sound intriguing. I was ready to bet on my 15 years of experience that keeping the standby pump out of service longer than 2-3 months will definitely cause problems like shaft bending, seal deformation, corrosion in some cases etc.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Standby pumps reliability
RE: Standby pumps reliability