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Design minimum flow line

Design minimum flow line

Design minimum flow line

(OP)
In my opinion you have three options for minimum flow protectin.
1. minimum flow line with orifice
2. minimum flow line with control valve
3. minimum flow line with automatic return valve.

When do you decide which type of minimum flow protection is the best?

RE: Design minimum flow line

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.

RE: Design minimum flow line

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.

RE: Design minimum flow line

(OP)
Even with a VSD is the minimum flow required lower than the minimum flow of the pump. Therefore a minimum flow line is required.

Maximum flow is 180m³/h and minimum is 20m³/h with almost same discharge head.

RE: Design minimum flow line

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.

RE: Design minimum flow line

What is the TDH?

RE: Design minimum flow line

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.

http://virtualpipeline.spaces.msn.com

RE: Design minimum flow line

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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RE: Design minimum flow line

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.

RE: Design minimum flow line

Why is the discharge head "nearly the same", is it nearly all static head.

RE: Design minimum flow line

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.

http://virtualpipeline.spaces.msn.com

RE: Design minimum flow line

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.

http://virtualpipeline.spaces.msn.com

RE: Design minimum flow line

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?

RE: Design minimum flow line

(OP)
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.

RE: Design minimum flow line

(OP)
We are planning to install a VSD to control the discharge pressure. Is this wise in this situation?

RE: Design minimum flow line

Why would you fit a VSD drive if the discharge head is always the same?- I suggest you read the information that has been suppleid by some of the posters regarding the use of VSD.

RE: Design minimum flow line

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

RE: Design minimum flow line

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.

RE: Design minimum flow line

Here is a radical idea.  Ask the pump manufacturer what to use.

RE: Design minimum flow line

Asking the pump manufacturer what to use is like letting the fox guard the hen house.  Most pump and motor manufacturers are promoting VSD.  However, I have learned that this is not to make pumps and motors last longer and also does not save energy as they claim.  It seems pump and motor manufacturers like to sell pumps and motors.  Therefore, they are not in the business of showing you what makes pumps and motors last longer.  A simple pump control valve is your best option.  A PCV will maintain a constant pressure by varying the flow from the pump.  It will do this without any of the negative side effects associated with VSD controls, that actually shorten the life of your pump system.

RE: Design minimum flow line

There is no where near enough information provided in the question to even start discussing practical solutions.  Who set the minimum flow requirements and for what reason? What type of pump is this and what are the duty conditions? Is cold water being pumped? What size electric motor is being used?  Is this a single pump system?

Assuming min flow conditions are set by pump manufacturer to protect against excessive heat rise under low flow conditions, then the equation to calculate min flow requirements is relatively simple provided you have enough data (which we don’t).  For pumps in this size range, actual low flow limits are typically about 5% of design flow (or only about 9m3/h in this case).  

RE: Design minimum flow line

Valvecrazy, I feel the same way about people selling valves!

Once again, valves and drives have their place.  Drives are not a fad nor are they a gimmick. They work very well in the right application. True, overzealous vendors can give them a bad name.  

A well applied and programmed VSD will not shorten the life of a pump, and it will extend it.  Excessively throttling a pump WILL shorten the life of a pump.

I would use a large pressure tank to add some capacity to the system with a high and low pressure switch to control the pump, if they make them that big.

RE: Design minimum flow line

Ouch!!  I never said drives were a fad or a gimmick.  Just that they are not the cure for all ills as the overzealous salespeople would like for you to believe.  Many even get very defensive if you bring up a few facts.

I do not see how a drive can extend the life of a pump or motor when compared to a correctly sized pump running across the line.  Considering the harmonics, voltage spikes, partial discharges, resonance frequencies, motor shaft currents, and many other characteristics of drive control, across the line will always last longer.

Excessive throttling can destroy a pump but, I have seen controlled throttling increase pump life by more than 400% many times, as compared to cycling with a pressure tank and pressure switch.
  
I don't think they can even make pressure tanks large enough to solve the problem.

RE: Design minimum flow line

I just don't understand why its so hard for "engineers" to believe there is nothing more efficient than a properly sized pump!

http://virtualpipeline.spaces.msn.com

RE: Design minimum flow line

Sorry Valvecrazy, I had flash backs of my old boss when I read your post.  He hated drives and fought tool and nail to keep them out of his plant, even when it was clearly needed.

If you throttle back to prevent runout conditions, yes you will extend the pump's life.  Also, I have seen 5000 gallon pressure tanks in the entertainment industry but they are probably custom made and very expensive.

I have installed hundreds of VSD's on pumps and have never seen a problem with anything you suggested.  Of course, they were all properly applied. I have VSD's on air compressors that have been running for 10 years.

BigInch, I agree. However when flows vary dramatically (heating plants, sanitary water, water shows, etc), VSD's are sometimes a good option.

Sorry to beat the point in. I feel like VSD's get a bad rap on this forum.  These sales "engineers" are giving them a bad name.

RE: Design minimum flow line

Variable flow is a bit of a different animal, but then there's still nothing more efficient than a properly sized pump ... with a tank.

http://virtualpipeline.spaces.msn.com

RE: Design minimum flow line

(OP)
I was trying to find out if there were any rules of thumb on minimum flow protection.

dickon17 is right about the information I gave about my problem is not sufficient to get to a good fundamented sollution.

There is an expansion needed of the current condensate supply in the plant, because a new unit will be operated parallel of the existing ones. A new condensate tank will be placed of 55m3. This is an atmospheric tank and the condensate temperature is 90°C. If all the units are in operation the condensate demand is 140m3/h, but every 8 hour an extra demand of 40m3/h is required (so 180m3/h total demand) Several months per year not all units are running and then the demand will drop to 15-30m3/h.
Each unit require condensate supply @ about 4 bara.

1)
The main question is that I require a pump which can do all these cases. The pump vendor specified a pump with a minimum flowrate of 35m3/h, but this is less than the minimum requirement of the process.
2)
How do I keep a constant supply pressure of condensate the eacht unit?

RE: Design minimum flow line

You mean minimum process flow is less than minimum pump flow.

On Aug 17 you said, "Maximum flow is 180m³/h and minimum is 20m³/h with almost same discharge head."

If minimum pump flow is 35 and min process is 20, then when you are running minimum process flow, you must recycle 15 m3/h to maintain a total flow through the pump of 35.

The requirement you have already stated for "minimum is 20m³/h with almost same discharge head". Will prohibit a VSD from working.  As already has been explained, dropping the speed will drop the head and you will not be able to get the process minimum flow into the vessel (or wherever its going).

You must get a pump with a curve showing that minimum flowrate at vessel entry pressure (or head equivalent) is possible and the pump curve must also show capability to do maximum flow at that (apparently "almost the same") head.

You should use a recirculation line and a possibly a discharge control valve to control those flows.  A VSD is not possible, given what you have said. You must look at the pump curve you have, the line sizes, the control valve and recycle valve options, the pipe sizes, the minimum pump flow and the pressures involved and design accordingly.

http://virtualpipeline.spaces.msn.com

RE: Design minimum flow line

The rule of thumb on min flow protection is: Don't operate below min flow.  I wouldn't suggest running close to it either.  "Good practice", according to Flowserve, is to run -30% to +15% of the BEP.

You need a PCV, as Valvecrazy and BigInch suggested.  These types of applications give VSD's a bad name. Someone will try it and it won't work.

RE: Design minimum flow line

At 90C I would also be concerned about significantly increased pump NPSH requirement at minimum flows.  Check your pump curve.  If min flow of 35m3/hr is too much for your system, check out another pump supplier.  

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