Water supply system steep vs flat curve 3

[desann]

Hi. this is a follow up on my previous post. I think it would be better to make a new thread because there is a clear, specific question now.

My project is about supplying water to our fogging system which is basically another pump and also end user flow.

The requirements from the device's manufacturer are 12 m3/hour at 3-4 bar. However real flow at which is the system operating is 4 or 8 m3/hour, depending on whether single string is on or both. Please note, that the flow is always restricted to 4 or 8 m3/hr by the system.

I would like to use 2 pumps in series of which the second pump is supposed to be Ebara either Matrix or 3M. First pump will be submerged in the water tank, supplying Ebara which is supposed to act as a pressure booster. The supply line will be regulated by VFD and pressure control loop. There will be a pressure tank and high flow filter unit in the system.

Please find below our system curve along with the pump characteristics. The dotted lines, barely visible are standalone pumps, the bold lines are pumps in series and system curves.
I created system curves for 3, 3,5 and 4 bar that is a range required for the end user. Also I created the characteristics for 10 and 20% speed reduction.

I can see that the Matrix pump has a much steeper line than 3M. By looking more closely I would say by going for the "steep" pump it will need more precise speed tuning but I can get the output want.

Also important to mention,the steep one is significantly cheaper.

I would be very interested in getting a more detailed view what are the real advantages and disadvantages of both solutions and which one fits our system better.
Due to the lack of practical experiences I cannot predict that, so I would like to ask you for advice. Is it all about the VFD setting and fine tuning in my scenario or do I miss something?







Thank you.

 

[goutam_freelance]

Too many curves and legends. It will be helpful to show only a) Single pump characteristics b) combined pump characteristics. c) system resistance curve.

 

[EdStainless]

How different is the efficiency?
Will the savings on the pump be consumed in power within a year or two?
And more efficient pumps tend to be more reliable.

 

[desann]

Too many curves and legends. It will be helpful to show only a) Single pump characteristics b) combined pump characteristics. c) system resistance curve.

Here is a simplicited graph. The system curve is now for the system pressurized to 3,5 bar. So there's dead band of +-0,5 bar.

 

[desann]

How different is the efficiency?
Will the savings on the pump be consumed in power within a year or two?
And more efficient pumps tend to be more reliable.

Interesting and very important point! I have to look at that as well.

Other than that, in terms of performance and behaviour, do you see any pros and cons of either system?

 

[goutam_freelance]

There will be a pressure tank and high flow filter unit in the system.

How do you pressurize the system? Do you have any overhead tank to create a static head? A flow diagram will be useful.

 

[LittleInch]

Advantage of a steep curve is that the flow range between your system curves is relatively low so if you have a fairly fixed flow then this would only need a small amount of control, be it VFD or control valve.

A flatter curve gives you a wider flow range but would make precise flow control using a VFD more difficult.

All depends on what flow range you want to see.

 

[Artisi]

is anyone else as confused with the data supplied.

1. is the X axis in M3/H?
2. what does the Y axis represent?
3. what is Leo?
4. what is Pumpa blv?

following on to what goutam_freelance has suggested, besides a flow diagram, a sketch and detail of what's what might help a little.

 

[LittleInch]

is anyone else as confused with the data supplied.

1. is the X axis in M3/H?
2. what does the Y axis represent?
3. what is Leo?
4. what is Pumpa blv?

following on to what goutam_freelance has suggested, besides a flow diagram, a sketch and detail of what's what might help a little.

Keep up!

X is m3/hr, y is metres head

Leo is a type of pump with low head as is the pumpa blv

The OP is then running a second pump in series and his flow range requirement is between 8 and 12 m3/ hr with a variable fixed pressure requirement between 3.5 and 4.5 bar pressure at the end point of the system.

I've forgotten why he or she doesn't just get one pump to do the duty range but hey, that's what we're given.

Flow control by VFD of the higher head pump.

 

[goutam_freelance]

The OP is then running a second pump in series

The point is that how he can get a constant head at 0 flow unless there is a overhead tank to provide a constant head at all flows.
If you have two pumps in series without any static head then system resistance curve should have 0 head at 0 flow.

 

[desann]

is anyone else as confused with the data supplied.

1. is the X axis in M3/H?
2. what does the Y axis represent?
3. what is Leo?
4. what is Pumpa blv?

following on to what goutam_freelance has suggested, besides a flow diagram, a sketch and detail of what's what might help a little.

1. yes
2. head in m
3. first pump submerged in the tank for Ebara Matrix alternative
4. also first pump in series with Ebara 3M alternative

How do you pressurize the system? Do you have any overhead tank to create a static head? A flow diagram will be useful.

The pressure will be created by restricted flow by the fogging system, which will always limit the flow to either 4 or 8 m3/hr depending on whether both or single pumps are operating.

 

[Artisi]

1. yes
2. head in m
3. first pump submerged in the tank for Ebara Matrix alternative
4. also first pump in series with Ebara 3M alternative
View attachment 6269



The pressure will be created by restricted flow by the fogging system, which will always limit the flow to either 4 or 8 m3/hr depending on whether both or single pumps are operating.

Great, now clear as to what's going on.

Is the tank below grade, requiring a submersible?
Why have a pressure tank?
Have you considered using 1 pump in lieu of 2?

 

[Artisi]

The point is that how he can get a constant head at 0 flow unless there is a overhead tank to provide a constant head at all flows.
If you have two pumps in series without any static head then system resistance curve should have 0 head at 0 flow.

Yes, confusing overall.

 

[goutam_freelance]

The pressure will be created by restricted flow by the fogging system

At which point are you restricting the flow, or where the FCV is located?

 

[Snickster]

I believe the OP is modeling the system with the pressure at the fog generation skid as if a constant pressure such as pumping into a vessel at constant pressure since no matter what the flow is the terminal pressure will be the same required at 3, 3.5, or 4 barg. If you look at the intersection of the pump curves with the system curves there is always 0.4 bar above the pressure of given system pressure curve. For instance the intersection is at 4.4 bar for the 4.0 system curve and 3.9 bar at the intersection of the 3.5 bar system curve. So the system friction loss is about 0.4 bar giving 4.0 bar or 3.5 bar available at the inlet of the fog skid pumps.

That being said, I see that the Ebara 3M and pumpa combination will not work as it is below minimum flow per the attached manufacturer's data pump curves for the 4 m3/hr flowrate. The minimum flow for the Ebara 3M is 6 m3/hr per pump curve on page 209 of catalogue.

The Ebara Matrix plus Leo LSW150a is on the borderline of not working since the 4 m3/hr flow is almost at minimum flow of about 3.5 m3/hr for the Ebara Matrix per page 210 of catalogue although the Leo has a minimum flow what looks like about 2 m3/hr, so it may work but is a poor choice. I would find a combination that you can get the 4 m3/hr minimum flow that has a better margin to minimum pump flow but still can get the required design point of 3 to 4 barg at 12 m3/hr requested by the fog equipment manufacturer. Also I would select a combination of pumps such that the head at 4 m3/hr (67 meters) and 8 m3/hr (60 meters), which is the true system flow in operation, is not such a huge pressure rise from the combined pressure head at 12 m3/hr flow, since I don't believe this is a real flow but just something the vendor wants to handle some issue with the fog skid but only very seldomly for whatever reason.

I would chose pumps so that the 8 m3/hr flowrate falls closer to the middle of the combined pump curves while still being able to get the 12 m3/hr requested by the manufacturer towards the right of the curves, and discuss with the manufacturer if you could just give him the 12 m3/hr at the lower pressure of 3 barg but not 4 barg so as to get a better selection/fit of pumps/curves. I am thinking that he is allowing for margin for his fog nozzle if they develop a higher flow at given pressure then his pumps will run out a little on the curve so he wants some margin in flow in case of this since he don't know for sure what the nozzle Kv values are for absolute sure or don't know exactly how many there will be because possibly the quantity of nozzles may be something that is field determined by configuration of system in the field. I am sure he has some reason he wants 12 m3/hr available but his system is fixed at 8 m3/hr.

Also not sure why you are using a Leo sewerage type submersible pump rather than a Leo general submersible pump.

 

[Snickster]

Also I question if you really need VFD control of pump for maintaining discharge pressure. Seems like you are trying to compensate for a dirty filter additional pressure drop so as to maintain 4 barg max. with dirty filter at inlet of the fog skid? If you chose the pumps to output 8 m3/hr at 4 barg (and 12 m3/hr at 3 barg) then you will allow for dirty filter by adding the filter loss to friction loss of piping. Say clean filter is negligible and dirty filter is 0.5 bar and piping loss is 0.4 bar then you will size your pump for 8 m3/hr at 4.0 + 0.5 + 0.4 = 4.9 barg.

With clean filter the pumps will still flow at 8 m3/hr at 4.9 barg since the flow is really controlled by the fog system. I imagine based on a given amount of fog nozzles of a given Kv value at a given nozzle upstream pressure as developed by the fog skid pumps the flow will basically remain constant at a set rate of 8 m3/hr regardless of whether you have a clean filter, as nozle flow does not change significantly with slight change in upstream pressure. it just that the pressure upstream of the fog skid pumps with a clean filter will be 4.5 barg instead of 4 barg so you are not controlling the pressure anyway with your pressure controller. As long as you size your pumps to give you a head available of your piping losses plus your dirty filter loss the system will automatically adjust the pressure if the flow is really limited by the fog skid and fog nozzles.

 

[desann]

At first I would like to thank you all for having an interest in my problem and especially to Snickster for so extensively looking through it.

I believe the OP is modeling the system with the pressure at the fog generation skid as if a constant pressure such as pumping into a vessel at constant pressure since no matter what the flow is the terminal pressure will be the same required at 3, 3.5, or 4 barg. If you look at the intersection of the pump curves with the system curves there is always 0.4 bar above the pressure of given system pressure curve. For instance the intersection is at 4.4 bar for the 4.0 system curve and 3.9 bar at the intersection of the 3.5 bar system curve. So the system friction loss is about 0.4 bar giving 4.0 bar or 3.5 bar available at the inlet of the fog skid pumps.

Exactly. This is right. There is an additional 30-45 meters of head in the curve representing the need for a pressure. If you subtract this, you will get friction loss + static head

The Ebara Matrix plus Leo LSW150a is on the borderline of not working since the 4 m3/hr flow is almost at minimum flow of about 3.5 m3/hr for the Ebara Matrix per page 210 of catalogue although the Leo has a minimum flow what looks like about 2 m3/hr, so it may work but is a poor choice. I would find a combination that you can get the 4 m3/hr minimum flow that has a better margin to minimum pump flow but still can get the required design point of 3 to 4 barg at 12 m3/hr requested by the fog equipment manufacturer.

My assumption was that the VFD would decrease the pump speed when there is demand for 4 m3/hr, and so the pump will be able to go below its minimum limit flow / or being at the borderline since it will be operating somwhere around 70%.

I am thinking that he is allowing for margin for his fog nozzle if they develop a higher flow at given pressure then his pumps will run out a little on the curve so he wants some margin in flow in case of this since he don't know for sure what the nozzle Kv values are for absolute sure or don't know exactly how many there will be because possibly the quantity of nozzles may be something that is field determined by configuration of system in the field. I am sure he has some reason he wants 12 m3/hr available but his system is fixed at 8 m3/hr.

I was also wondering about it and asked about it. They answered that when they were designing the general layout for fogging systems they had a pump engineer who recommended 1,5* flow of the system. I know the flow at 100 bars (which is my operating pressure) of one nozzle, number of nozzles and if I multiply this, I get 8 m3/hr. We dont know Kv values of nozzles, but by my common sense I would say higher p = higher flow through nozzle (its the same way of working as our spraying robot).

But on the other hand the manufacturer of the pump used in the fogging system also needs higher flow. They say double, but at the lower p. IDK the reason. I discussed that with the supplier of fogging and as above they said for our design, 1,5*flow at 3-4 bar is recommended.

Also not sure why you are using a Leo sewerage type submersible pump rather than a Leo general submersible pump.

They were more available on the market here.

Also I question if you really need VFD control of pump for maintaining discharge pressure. Seems like you are trying to compensate for a dirty filter additional pressure drop so as to maintain 4 barg max. with dirty filter at inlet of the fog skid? If you chose the pumps to output 8 m3/hr at 4 barg (and 12 m3/hr at 3 barg) then you will allow for dirty filter by adding the filter loss to friction loss of piping. Say clean filter is negligible and dirty filter is 0.5 bar and piping loss is 0.4 bar then you will size your pump for 8 m3/hr at 4.0 + 0.5 + 0.4 = 4.9 barg.

Also this. but together with the reason above and also because the fogging system is operating in short cycles.