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Design minimum flow line
- Thread starter evdbroek
- Start date
a fixed orifice is simple and cheap, but has the downside that it will always bypass that flow, even when it is not needed for minimum flow protection. In some cases, it will actually mean that you don't get the flow you need for your process, because there is too much being bypassed.
control valves
have the advantage of variable flow, so no more flow is wasted than necessary, but are more complex and require more instrumentation
automatic recirc valves are self contained and therefore less complex, but require someone with some knowledge to size and apply, and are typically not as flexible for changing operations.
control valves
have the advantage of variable flow, so no more flow is wasted than necessary, but are more complex and require more instrumentation
automatic recirc valves are self contained and therefore less complex, but require someone with some knowledge to size and apply, and are typically not as flexible for changing operations.
ClicketyClack
Mechanical
Consider a drive as well. Others may jump on me for suggesting it but, in SOME cases, a VSD will work well. It all depends on the application, pump and system curves.
- Thread starter
- #4
ClicketyClack
Mechanical
That's a significant amount of kw's. I would go the control valve route or even consider a smaller "jockey" pump for the low flow periods.
With the same discharge head at low flows, forget using a VSD. At low speed, you will have NO head.
At lower than 20% through flow (higher than 80% recycle), you will probably have significant and fast heating, so recirculation back to a tank will help delay temperature rise. Recirculation directly back to suction could probably be used, if the lower flow rates are ONLY for starting up. You will have to move up to higher flows asap.
At lower than 20% through flow (higher than 80% recycle), you will probably have significant and fast heating, so recirculation back to a tank will help delay temperature rise. Recirculation directly back to suction could probably be used, if the lower flow rates are ONLY for starting up. You will have to move up to higher flows asap.
evdbroek
I agree with your up-front statement. You really need a minimum flow line and I can't think of any other way to design it.
In regard to systems which use orifices (orifixii?), problems I've seen are ensuring that you're bypassing the "right" amount of fluid -- not too much and not too little -- especially after 20 or 30 years of operation. While you gave your pump flow ranges of operation, you didn't say whether you have a lot of margin in your pump capacity. Can you afford to bypass approximately 20 - 45 m3/hr when you're at the highest flow rate? If you can't, then you need to look at some type of valve which will isolate at higher flow rates.
I agree with BigInch, that it's best to recirculate back to a tank rather than directly to the pump suction, especially if you're using less than 25% flow for your minimum recirculation.
Patricia Lougheed
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I agree with your up-front statement. You really need a minimum flow line and I can't think of any other way to design it.
In regard to systems which use orifices (orifixii?), problems I've seen are ensuring that you're bypassing the "right" amount of fluid -- not too much and not too little -- especially after 20 or 30 years of operation. While you gave your pump flow ranges of operation, you didn't say whether you have a lot of margin in your pump capacity. Can you afford to bypass approximately 20 - 45 m3/hr when you're at the highest flow rate? If you can't, then you need to look at some type of valve which will isolate at higher flow rates.
I agree with BigInch, that it's best to recirculate back to a tank rather than directly to the pump suction, especially if you're using less than 25% flow for your minimum recirculation.
Patricia Lougheed
Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.
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ClicketyClack
Mechanical
I was waiting for you to retaliate on the VSD BigInch!!
But you are absolutely right as usual, a VSD is no good here.
I would determine the power requirements for each possible solution at the different operating conditions. I would then weigh this against the purchase and maintenance costs of each solution. Without the pressure and more info on the application, it is hard to make a good recommendation.
But you are absolutely right as usual, a VSD is no good here.
I would determine the power requirements for each possible solution at the different operating conditions. I would then weigh this against the purchase and maintenance costs of each solution. Without the pressure and more info on the application, it is hard to make a good recommendation.
No "retaliation" Clickety. After you got the head variation info, you immediately recommended going with a CV, so I effectively only seconded your opinion; just including the reason that it should be so.
A CV will usually give more flexibility in control and operation and will generally work well, provided that the pressure drop from discharge to suction is not extreme, in which case a combination of an orifice and a CV might be in order, but now I'm going too far out on the limb for such minimal info.
A CV will usually give more flexibility in control and operation and will generally work well, provided that the pressure drop from discharge to suction is not extreme, in which case a combination of an orifice and a CV might be in order, but now I'm going too far out on the limb for such minimal info.
Sounds like we're all getting on the same page with these VSDs.
No "retaliation" Clickety. After you got the head variation info, you immediately recommended going with a CV, so I effectively only seconded your opinion; just including the reason that it should be so.
A VSD making low flow rpm will not develop the same unbalanced pressure differentials nor heat as running at full speed, so the usual min flow (at rated rpm) can be reduced somewhat, but the problem is still little or no head.
A CV will usually give more flexibility in control and operation and will generally work well, provided that the pressure drop from discharge to suction does not reach extremes.
In the case of extreme pressure differential, a combination of an orifice and a CV might be in order, but now I'm going too far out on the limb with such minimal info.
No "retaliation" Clickety. After you got the head variation info, you immediately recommended going with a CV, so I effectively only seconded your opinion; just including the reason that it should be so.
A VSD making low flow rpm will not develop the same unbalanced pressure differentials nor heat as running at full speed, so the usual min flow (at rated rpm) can be reduced somewhat, but the problem is still little or no head.
A CV will usually give more flexibility in control and operation and will generally work well, provided that the pressure drop from discharge to suction does not reach extremes.
In the case of extreme pressure differential, a combination of an orifice and a CV might be in order, but now I'm going too far out on the limb with such minimal info.
I still am not clear on something - what is the minimum flow required by the process, and what is the minimum flow required by the pump?? That is going to be useful to deciding how to do this. Will the pump ever run when the process demand is zero?
- Thread starter
- #15
The pump will be a new pump in an existing plant. This pump will supply to several end consumers, which will not always require feed. Hence the flowrate has a wide range and the discharge pressure is almost the same. The pump will be pumping from an atmospheric tank of 55 m3.
- Thread starter
- #16
Repeat...
With the same discharge head at low flows, forget using a VSD. At low speed, you will have NO head.
With the same discharge head at low flows, forget using a VSD. At low speed, you will have NO head.
djack77494
Chemical
Using a VSD as a means of "pressure control" will not work. Sounds like your system has almost a fixed discharge pressure, so it would operate at almost a fixed speed. This = no control.
btw, to respond to Artisi's comment, the pump may feed vessels operating at an elevated pressure and would therefore not have much head change as the capacity changes.
Doug
btw, to respond to Artisi's comment, the pump may feed vessels operating at an elevated pressure and would therefore not have much head change as the capacity changes.
Doug
I think you are not posting the right question here.
The core problem is how to design a supply loop to meet POUs pressure and consumption requirements? Is this right?
If yes, you need to take account into all the ideas expressed as above:
1 - What is the minimum flow required by the pump itself?
2 - What is the minimum flow required by the process, that is the minimum flowrate at the same time for all POUs.
3 - the general method in Ultra pure water distribtution loop in my working field is to use VSD which is controlled by the supply pressure near the first POU and then further regulated by a PCV at the end of the loop return.
Hope this is helpful.
Nick.
The core problem is how to design a supply loop to meet POUs pressure and consumption requirements? Is this right?
If yes, you need to take account into all the ideas expressed as above:
1 - What is the minimum flow required by the pump itself?
2 - What is the minimum flow required by the process, that is the minimum flowrate at the same time for all POUs.
3 - the general method in Ultra pure water distribtution loop in my working field is to use VSD which is controlled by the supply pressure near the first POU and then further regulated by a PCV at the end of the loop return.
Hope this is helpful.
Nick.
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