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MCSF 0 ?
- Thread starter hemvig
- Start date
NGLENGR
Mechanical
- Jul 8, 2010
- 48
What happens below the MCSF point is large amounts of recirculation which results in cavitation and overheating of the pump. This in turn results in shaft deflection, vibration, and a whole host of other issues.
You will not be able to have a reliable pump if you run below the MCSF for extended periods of time.
You will not be able to have a reliable pump if you run below the MCSF for extended periods of time.
- Thread starter
- #4
WHAT? "MSCF" in my world stands for "thousands of cubic feet at standard conditions". Do any of you measure centrifugal pump capacity in volumes of gas at STP? The adjustment for changes in pressure and temperature for a liquid is pretty small and I've always mostly ignored it except in really high discharge pressure applications (over 5,000 psig I start being concerned about compressibility).
David
David
NGLENGR
Mechanical
- Jul 8, 2010
- 48
MCSF is Minimum Continuous Stable Flow, parlance for the lowest flow allowed by the pump OEM for reliable operation.
It's a highly controversial topic (between myself and my operators) that is scattered with junk science and hearsay.
To my operators, MCSF is whatever you need it to be!
It's a highly controversial topic (between myself and my operators) that is scattered with junk science and hearsay.
To my operators, MCSF is whatever you need it to be!
I do not know your vendor but beleive me there are always people who fill technical data in proposals without knowing what it means. If seen proposals with negative NPSHR values, power requirements for which the vendor would get the Nobel prize (> 100 % efficiency) and so on. I just guess the person who wrote the proposal does not fully understand what MCSF means. From my understanding a pump must or at least should not be operated continuously below MCSF. If that definition is true, how can MCSF be zero?
About 30% of BEP for most centrifugal pumps, closer to 50% for axial flow pumps. Special consideration for fluids close to vapor pressure, there will be localized heating at the bearing surfaces which will cause flashing and bearing/shaft damage.
Remember all the inefficiency of the pump will go straight into heat, vibration, and noise.
If it's under warranty, simplifies your control system, and you don't mind constant repair/replacement, then sure run it at shutoff constantly.
Remember all the inefficiency of the pump will go straight into heat, vibration, and noise.
If it's under warranty, simplifies your control system, and you don't mind constant repair/replacement, then sure run it at shutoff constantly.
hemvig,
I realize this may be an entirely different situation, as you're probably not in nuclear. But for pumps that are required to operate to provide nuclear core cooling following an accident, the US NRC has recommended, via a couple of information notices and bulletins, minimum flows of between 10 and 25 percent.
Could some pumps operate with a lower minimum continuous flow? Possibly. What it comes down to is how critical your system is if the pump stops working? If you don't mind the down time and have spare pumps sitting around, take a chance on that "0" flow. If it's going to cost you significant money and time, ignore what the vendor said and design your system with a minimum recirculation line.
Patricia Lougheed
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I realize this may be an entirely different situation, as you're probably not in nuclear. But for pumps that are required to operate to provide nuclear core cooling following an accident, the US NRC has recommended, via a couple of information notices and bulletins, minimum flows of between 10 and 25 percent.
Could some pumps operate with a lower minimum continuous flow? Possibly. What it comes down to is how critical your system is if the pump stops working? If you don't mind the down time and have spare pumps sitting around, take a chance on that "0" flow. If it's going to cost you significant money and time, ignore what the vendor said and design your system with a minimum recirculation line.
Patricia Lougheed
******
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.
There are some centrifugals with straight vanes which have very low mcsf, but in 20 years I have never seen one with 0.
There are two main effects to running at low flow: the energy input goes to heat generation, which I have seen happen and cause the pump to catastrophically fail (luckily not fatal to the worker who had just left the pump to get some tools), and also that the shaft bending increases, which will usually cause either a bearing or shaft failure.
There are two main effects to running at low flow: the energy input goes to heat generation, which I have seen happen and cause the pump to catastrophically fail (luckily not fatal to the worker who had just left the pump to get some tools), and also that the shaft bending increases, which will usually cause either a bearing or shaft failure.
Most centrifugal pump manufacturers will state that their pump can take deadhead or 0 flow for 60 seconds. After 60 seconds water starts to heat up and can damage the pump. However, I have found that with centrifugal pumps moving cool water, the MCSF can usually be much lower than is stated on the curve. With an end suction centrifugal pump there are no bearings or bushings in the pump to cause friction. The seal is the only surface that touches anything and can produce heat. So even with fairly large pumps, as long as a couple of GPM of cool water is being washed through the pump case, heat is not a problem. A strong shaft and a heavy radial bearing in the motor can prevent problems with radial deflection.
With split case, submersible, and turbine pumps there are bushings on both sides of the impeller(s) that can generate friction heat. It still takes very little flow of cool water to keep this heat from becoming a problem. The warmer the water and the greater number of impellers or bushings the more flow is required to keep the pump cool. However, the bushings on both sides of the impeller(s) limit problems from radial deflection.
With split case, submersible, and turbine pumps there are bushings on both sides of the impeller(s) that can generate friction heat. It still takes very little flow of cool water to keep this heat from becoming a problem. The warmer the water and the greater number of impellers or bushings the more flow is required to keep the pump cool. However, the bushings on both sides of the impeller(s) limit problems from radial deflection.
API 610 indicates the 60% of BEP flow as allowable (although running below 70% is not recommended).
I suggest you to read the following thread.
Take also a glance at the attached paper (Minimum flow)
I suggest you to read the following thread.
Take also a glance at the attached paper (Minimum flow)
Go ahead and run your pump(s) at zero flow for extended times. Your mechanics will enjoy the overtime pay (until it becomes burdensome on their personal lives).
For a more serious answer, see other postings dealing with this general topic.
In short, operating a pump at zero flow more than momentarily is never a good idea, and it is likely to be very expensive.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
For a more serious answer, see other postings dealing with this general topic.
In short, operating a pump at zero flow more than momentarily is never a good idea, and it is likely to be very expensive.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
I would think it is a reasonable assumption to assume that there are hundreds if not thousands of centrifugal pumps running at CV (dead headed) round the world at this very minute and the planet hasn't shifted on its axis as a result. Having made this statement doesn't mean that I am endorsing such a pratice as I am with the majority in saying DON'T DO IT.
However, having seen many installations in which this does happen my observations are: pumps running at maximum design ie, at maximum design speed and full impeller diameter are a disaster waiting to happen, whereas pumps at reduced speed are more tolerant and less likely to destruct although they are subject to many problems as already detailed by others.
Consider any centrifugal pump designed for 2pole 60hz, running at 4 pole 60 hz, the max. power is 1/8 and the maximum disharge pressure is 1/4 - even less at 50hz and less again if operating at 6 pole - so damage/disaster/ recirc. cavitation /shaft deflection/bearing loads etc comes down to a function of speed.
But to write a spec accepting or requiring operation at zero flow is ridicules or it has been writen by someone who doesn't know their rear-end from their elbow.
However, having seen many installations in which this does happen my observations are: pumps running at maximum design ie, at maximum design speed and full impeller diameter are a disaster waiting to happen, whereas pumps at reduced speed are more tolerant and less likely to destruct although they are subject to many problems as already detailed by others.
Consider any centrifugal pump designed for 2pole 60hz, running at 4 pole 60 hz, the max. power is 1/8 and the maximum disharge pressure is 1/4 - even less at 50hz and less again if operating at 6 pole - so damage/disaster/ recirc. cavitation /shaft deflection/bearing loads etc comes down to a function of speed.
But to write a spec accepting or requiring operation at zero flow is ridicules or it has been writen by someone who doesn't know their rear-end from their elbow.
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