Expansion tanks are installed to manage thermal growth of the fluid. Head tanks are for NPSHr. Hydropneumatic tanks are used to store energy to reduce pump cycling.

A system with a reliable base flow may not need a hydropneumatic tank. Even with a VFD, a centrifugal pump is going to need to maintain a minimum speed to maintain pressure so it will require some flow for cooling.

It should probably have a minimum flow switch. No point keeping the pump energized when there is no or a very low demand. A tank should provide supply when demand is less than the pump's minimum flow capability.

The minimum flow for a VFD controlled pump is not as low as you would think. The pumps need to be set at a minimum speed that will produce enough flow to satisfy the pump. When demand is below the minimum flow required the pressure tank lets the pump cycle on and off using the draw down from the tank.

moideen,

Terminology can vary, but most people would think "expansion tank" is there to cater for thermal expansion//contraction of the fluid in a closed loop due to thermal expansion or contraction.

What you seem to refer to is normally called a "buffer tank" or hydro pneumatic tank as Tug say.

It all depends how big the building or supply is and how low is the minimum flow at 3 am in the morning.

Even VFDs need to have a certain minimum flow to prevent damage. With no buffer storage, a low flow cut off would result in multiple stop starts.

You would normally expect to see some sort of recycle line if they want to operate like that (VFD only) mode.

But the devil is in the detail of how low the flow goes and how low can the pump go. If you have multiple pumps then it might be really low.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Keep in mind, VFD's are great for heating and cooling pumps but not so good for water supply. The speed of a pump and it's pressure are directly related. Since you want to deliver constant pressure to the taps your pump speed also needs to be constant.

Most water supply pumps for smaller systems are set up to supply constant pressure. You will have the same 50 PSI for filling the coffee pot as for washing the car. This leaves very little speed reduction possible as head is lost by the square of the speed. On larger systems you could have flow based pressure control, but during off peak hours you would have the same low pressure to wash your car as to fill the coffee pot. Water systems that pump from wells or up to an elevation must produce even more head or pressure to buck the static, further limiting any possible speed reduction. While VSD maybe cost effective for closed loop or positive displacement pumps, there usefulness for centrifugal water supply pumps is greatly over rated. The natural reduction in horsepower when restricting a full speed centrifugal pump with a valve to produce constant pressure is only a couple of percent different than when using a VSD. Another way of saying it is that for constant pressure a VSD burns within a couple of a percent as much energy as a control valve. Poof!

moideen; The tank is needed for reduced pump cycling AND to control the pump.

The following thread details how this is done. If you want to be fully informed read the entire thread. It's very educational.

A specific highlight to what I'm referring to above is:

Quote:

² itsmoked (Electrical)9 Oct 17 11:47
Often a small bladder tank is included on the outlet side of the pump in these cases. It allows the water to start flowing the instant it's called for giving the pump some finite time to spool-up. Systems control to pressure looked at next to the little bladder tank which also provides some pressure signal filtering. I've seen the commercial packages turn down the VFD briefly (almost unnoticeably) every 60 seconds or so to see if the pressure drops meaning a demand is really occurring. Since the VFD is PID'ing to the pressure it can not actually tell when there is no longer flow. That's how it's done avoiding flow sensors.

thread830-430849: Water distribution controls for a small plant

Keith Cress
kcress - http://www.flaminsystems.com

Quote:

When a constant speed pump can be operated within between 70% to 110% of its BEP flow, you are usually right.

This shouldn't be challenging to achieve. If you size your piping to put the pump in the middle of it's BEP island while filling the hydropneumatic tank then there will be no efficiency gain from variable speed.

The downstream demand only determines how long it takes to fill the tank provided your pump is properly sized. The pump itself shouldn't see the effects of the downstream system.

There is a pressure window the pump will operate under. Maybe it's 50-70 psi? Would there be benefit to starting the pump at a lower speed and ramping it up as pressure builds in the expansion tank. Maybe maintain a constant pressure differential? I don't know.

Poof! I think you answered your own question. "When a constant speed pump can be operated within between 70% to 110% of its BEP flow, you are usually right. A control valve is about as efficient as a VSD, if not more so."

But here lies the problem,"as long as you can get still get the reduced speed flows through your system with less than 50% of BEP head". With an open system 50% head will not even buck the static pressure. If the pump can be slowed by 50%, buck the static head, and that still fill a coffee pot, the pump is way oversized and wasting energy at every point on the curve.

If you had said VSD was convenient but not more efficient you might be right. But even when utilizing a VSD to reduce the system pressure it is always less efficient than a full speed pump properly sized for that head and flow or a pressure tank system where the pump is either running at BEP or it is off.

A small system is going to have 50 PSI constant all the time. It is not practical to "push a button" when filling a coffee pot or washing the car. Just open the kitchen faucet when filling the coffee pot and open a large outside faucet to wash the car. The coffee pot will just fill quicker with 50 PSI than 10.

Quote (moideen (Mechanical)(OP))

Does the expansion tank require for a booster pumping with VFD system for water distribution for multi-storey building?

The expansion tank is not required. Projects are never exactly the same so it appropriate to share more details on the scope of the project.

Here is an example of a booster station for a multi-story building.

Link

Lol! You are the one trying to sell that VSD saves energy even down to 30% of BEP, and that is not true. When variable flow with constant head or pressure is the goal, which is the case with most open systems, the natural characteristics of a centrifugal pump that causes the power to decrease when using a control valve is very similar to varying the pump speed. Again, in other words, when maintaining a constant head a VSD will burn almost as much energy as a control valve at any flow rate.

As you can see from the typical pump curve attached, when 231' of head is required this 10HP, 3450 RPM pump can only be slowed to a minimum 2886 RPM. Any slower and it will no longer produce the head needed. This greatly limits any reduction in horsepower. The design point set at 5 GPM shows the VSD has slowed this pump form 10HP down to 1.96HP. But a simple control valve will reduce the horsepower from 10HP down to 3.25HP. 1.29HP energy is not much difference between a VSD controlled 10HP pump and an valve controlled 10HP pump. The VSD is wasting almost as much energy as a control valve. At BEP this pump produces 10 GPM per horsepower. Varying the speed reduces the efficiency by 400% at 2.5 GPM per horsepower. Varying the speed may decrease the energy needed to spin the pump and motor, but nearly always increases the energy needed to pump a gallon of water.

• https://files.engineering.com/getfile.aspx?folder=7da3ddd5-5196-4939-b5d6-2

You are the one mixing in the oranges. You can see the performance of that pump from 0% to 100% of BEP in that curve. Not much difference at 70% or any place along the curve. Challenge for you. Show me a pump where the energy to produce a gallon of water does not increase with VSD at any point on the curve.

Which curve? The pump curve or the system curve? Why is whatever you mean important? What is asking that question supposed to prove.

If total energy consumed at a point on the system curve is less than what a constant speed pump consumes, the vsd is more efficient at that point, if not then use a constant speed at that point. Points close to constant rated speed BEP will favor a constant speed. Points away by 30% or more from BEP will favor using a vsd. There you go. I said it again. I can copy print 500 times if you want.

Hi ,
To add to this post , consider the document attached .
Good luck
Pierre

• https://files.engineering.com/getfile.aspx?folder=751fdb9a-5c78-46b2-8329-b

Quote:

How can I show that on the PID?

LOL You got me...?

Quote:

of course using any VSD in a small domestic system is a bad example

I put them into domestics all the time. I prefer them. They're better than *^%##$ bladder tanks IMO.

The Grunfos SCALA2 product line.

Keith Cress
kcress - http://www.flaminsystems.com

I also agree that the 10% closed thing is bunk. Many control valves are designed to open fully so as not to restrict the flow at all when no restriction is needed. Even so, I am not sure that a VSD won't beat a control valve by a couple of percent on everything except max flow. However, a VSD is only a couple percent more efficient than a control valve, which means the VSD is burning almost as much energy as a control valve. The expense of a VSD means a couple of percent better than a control valve would probably never give a ROI for the VSD compared to a control valve.

"However, in systems with high static head, the system curve does not start from the origin but at some non-zero value on the y-axis corresponding to the static head. The reduction in flow is no longer proportional to speed; a small turn down in speed greatly reduces flow rate and pump efficiency. A common mistake is to also use the Affinity Laws to calculate energy savings in systems with static head. Although this may be done as an approximation, it can also lead to major errors."

People need to heed the quote above. Then they would not disagree with Tuboateng when he says....
"Keep in mind, VFD's are great for heating and cooling pumps but not so good for water supply. The speed of a pump and it's pressure are directly related. Since you want to deliver constant pressure to the taps your pump speed also needs to be constant."

I would not be recommending the SCALA. They are piling up in the dumpsters at the pump distributors. High percentage of failures, which I see a lot with most of the domestic size VFD's. The bladder tank and pressure switch is still the most efficient, cost effective, and reliable way to control most water supply pumps. The only problem with a bladder tank system is the repetitive cycling, and there are easy ways to solve that problem. And yes, most domestic size VFD's require a pressure tank.

"I cannot agree with such an over-generalization of what appropriate service is for a VSD." Lol!

A lot of people know VFD waste energy on systems with static head. But most realize it is a waste of time to argue with someone who has already drank the VSD Kool Aid like I am doing right now.

Edit;
Since "1503-44 (Petroleum)" deleted his posts, it looks like I am arguing with myself. It would have been nice if he had left those posts as he is not the only one who falsely believes VFD's or VSD's always save energy. Everyone starts out thinking that way. I also thought that about 30 years ago. It takes most people a few years and a lot of research to understand that a VFD reduces the energy needed to spin the pump and motor, but the pump must run much longer to produce the same amount of water, so the energy needed to pump the water actually increases with variable speed. I learned that from a really smart guy on this forum.

I am crazy about valves because I have been using them to solve the problems of variable speed for 30 years now. When you finally realize a system with a constant pressure valve can be as efficient as a variable speed system it can make life easier. There is a saying that when there are two or more ways to accomplish the same task, the simplest way is always the best. And a mechanical control valve is much simpler than a computerized variable speed drive.

This is related but somewhat off the topic of this thread. So, since I know a little on the subject I will get back to the actual topic.

I've been watching this thread. I can't believe I am the one saying this but 1503 you took a very unprofessional tone early on and I have chosen to not engage as a result. I know some users have issues with my posts in the pub but I reserve my best behavior for the professional forums.

And I agree with everything Valvecrazy has said.

I don't like having to correct anyone on this subject, as they always get mad at me for doing so.

As for the pressure tank, there are many different kinds and sizes of variable speed systems. As was said, if the system is large enough to have a minimum flow that will satisfy a pump, many times no tank is needed. But even then sometimes a small pressure tank can smooth out the pressure giving the VFD time to react to changes in flow.

On smaller systems, many times the minimum system flow is not enough to cool the pump. I don't like recirculation lines in systems like this, as sometimes that gets the water hot as well. A small pressure tank can be used to let the pump/VFD go to sleep when the flow rate is lower than the pump can handle. Then the pressure tank delivers some water as the pressure drops to a lower set point and starts the pump again.

In the smaller domestic systems there are many different types of VSD control. Some use a transducer, others just a switch. Some have sleep mode, some don't. Some like the PID brand/model don't really work like a variable speed pump. As was said it is hard to figure the minimum speed needed for every different pump model and every different setting depth to properly set a variable speed device. So, the PID just goes into sleep mode anytime the flow rate is constant for 60 seconds. If the pressure drops, the VFD knows you are still using water, and ramps the pump up again. But it never really stays running and just varies the speed with the flow. In these type situations a pressure tank is very important, as the PID system works more like a hydro-pneumatic system than a VFD.

There are many problems with VFD's and just as many different band aids and ways to try and solve these problems. After learning that valve control can be as efficient or better than a VFD it was an eye opener. For the past 30 years I have been using valves to replace VFD's and solve a lot of problems. Simple is what makes a pump system dependable, and a mechanical control valve is very simple. 10-15 years ago I was chastised even more than today for saying such things. But after many years a lot of people are finally realizing a control valve is not a bad thing as they were taught, and VFD or VSD is not the magic energy saving device some people are so convinced it is.

Sorry if I got off topic, but much of it is related to the pressure tank question.

Thanks for all the comments. In Dubai, truth be told, I have not seen any booster pumping with CSV (CYCLE STOP VALVE) mechanism. maybe my limited experience in booster pumping. So far before the advent of VFD in booster pumping, SQUAR D type pressure switch was using to shut down and switching on. The diaphragm tank will set 2 psig lower than cut-in pressure set point. I have been carefully reading these heated discussions and very interesting and has refreshed the concept of VFD selection, a trend has everywhere in the industry that running behind VFD whether it is closed loop or open loop irrespective of in-depth analysis of its performance in operation and in the perspective of energy analysis. Still say, I am not expert, and I have to gain more knowledge…
However, why all leading manufactures (Grundfos, xylem.) whose latest published catalogues do strongly recommend VFD in booster pumping. Example, Grundfos latent intelligent control system for booster pumping in dominant static head system…
thank you
Moideen-Dubai

"However, why all leading manufactures (Grundfos, xylem.) whose latest published catalogues do strongly recommend VFD in booster pumping. "

I answered that question several times in the posts I deleted. As this thread could not get past the "VFD for heating and cooling water and constant speed for water supply, I disengaged from it.

If you like, I'd be happy to answer that question again, but you have to send me your email. You will find my addresses here. Up to you. Otherwise I'm finished with this thread.
https://www.dropbox.com/s/degc15s6vw42y8x/email%20...

After all this back and forth, I don't see where any flowrates or head, min /max demands and frequency requirements have been advised. Knowing this would make a difference to the best approach without all the academic round and round.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

When devising an engineering procedure to determine if VFD/VSD or a constant speed/CV, or for that matter both, should be provided, you don't have to know those things. Picking a target head and flow is all the details you need initially. From that you can make an infinite number of possible designs and evaluate each one for minimum to maximum capacity, efficiency, cost, time to construct, shipping and installation costs and operating/maintenance cost. I don't care if its diesel, electric, both or steam driven. You can select the equipment from any all pumps, VSDs, CVs and pipes you can find on the Internet. How will you do it? Devise the procedure that works for all pumping systems from the North Pole to Antarctica. All you need to produce is a flow chart of the procedure. It might just have two paths, "VSD for heating and cooling, CS/CV for water", but that doesn't work for me.

Quote (moideen (Mechanical)(OP)19 Dec 21 11:48
Thanks for all the comments. In Dubai, truth be told, I have not seen any booster pumping with CSV (CYCLE STOP VALVE) mechanism. maybe my limited experience in booster pumping. So far before the advent of VFD in booster pumping, SQUAR D type pressure switch was using to shut down and switching on. The diaphragm tank will set 2 psig lower than cut-in pressure set point. I have been carefully reading these heated discussions and very interesting and has refreshed the concept of VFD selection, a trend has everywhere in the industry that running behind VFD whether it is closed loop or open loop irrespective of in-depth analysis of its performance in operation and in the perspective of energy analysis. Still say, I am not expert, and I have to gain more knowledge…
However, why all leading manufactures (Grundfos, xylem.) whose latest published catalogues do strongly recommend VFD in booster pumping. Example, Grundfos latent intelligent control system for booster pumping in dominant static head system…
thank you
Moideen-Dubai)

Although the Cycle Stop Valve was designed to mimic and replace a VFD on a constant pressure system, any valve will have the same effect on the performance of a pump/motor. I am not trying to promote Cycle Stop Valves. However, having started with VFD’s in their infancy, then making and selling valves for 30 years I have some insights that other people may not.

The first misconception most people have is that the big pump, motor, control manufacturers do R&D, promote, and sell devices to be more efficient and more economical. Nothing could be further from the truth. Big manufacturers promote and advertise what makes them the most money. Profit is the number one design characteristic. Efficiency and economy are the misinformation in the advertisements. No profit loving big company is going to mention, much less promote something that would make their equipment last several times longer than the 84 months they build into it.

A “fluid system” is the goal, and that has nothing to do with water. A “fluid system” is one where the products last a predicted amount of time, can’t be repaired, and must be replaced on a regular basis, keeping the cash flowing “fluidly” through the company. The most profitable products usually come at the expense of efficiency and economy, as that is what makes them profitable. They would not have big money to spend on all those advertisements and hiring droves of salespeople if their products really did last longer and save people money. I have been dealing with companies like the ones you mentioned since the early 70’s, and believe me, any changes they make/made where not to make products last longer and save money. Doing just the opposite of what a company like that is promoting is best for the consumer.

VSD/VFD has been a cash cow for the manufacturers. I will say this again. There are many good applications for a VFD, and many will even save energy. But when constant head or pressure is needed, or even when a slight variation in head is possible, a VFD is wasting energy. However, open systems are such a large part of the pump market they can’t keep from trying to get more of that pie. There is no end to the mis-information available about VFD efficiency in open systems. Once they get someone to drink that Kool Aid, they have a VFD warrior for life. Many people are so convinced that VFD’s always save energy they get mad instead of getting educated, lashing out at anyone who says otherwise. I need pretty thick skin to even mention this as you can see.

One thing they are good at is making things more "user friendly". They say you don’t even need to know the flow rates, head, frequency, or anything. Just pick any old pump and touch the “GO” button on a VFD to make it do any job needed and save energy all at the same time. Don’t worry your pretty little head about things like flow, head, efficiency. They have already taken care of that for you.

We finally learned even though the really big company was promoting it as a safe and effective cure all, it is not good to use Opioids for a headache. Now we need to learn a VFD is not the magic pill to save energy on every application.

Finally you disclose that you are "making and selling valves". Cyclestop valves by any chance?

Sure! Been doing valves for 30 years, after I quit drilling wells and building pumps. Mostly retired now but won't deny the influence. Also what gives me insight into things others cannot see. Just trying to not advertise on the forum. Doesn't make what I said any less accurate. It has given me thick enough skin to post the facts. Many people just won't speak up because they know they will get this kind of harassment from people who don't understand.

Ill post one more here. The last one, I promise.
IMO It also tends to give you tunnel vision, even though you don't notice it.
I have that sometimes. I once said, 35yrs ago, that fully turbulent flow boundary is at Nre = 4000. An aerospace engineer was kind enough to remind me that all the world does not fit inside a pipeline.

I actually don't have any problem with most all of what you say. I didn't actually say I disagreed with anything in any post except the "vfd for Heat/Cool, and CS/CV for water supply" logic, and that was actually Tugboat's comment. My comments were around my central argument, that the choice should be driven by any variation from a constant speed pump's BEP H/Q/P that the final design must allow to meet the system requirements. Functionality requirements often will even take precedence over actual energy efficiency, sometimes not, no matter what type of system is being designed, or what the fluid in the system is, and that's true no matter how huge or tiny the system is. I was once a constant speed CV "valve guy", if you will, just like you. The most economic design, get the most efficiency possible kind of guy. In fact that is still my personal preference, mostly because it is easier to justify a selection based entirely on $$$. It took me a long time to realize that all that engineering economics and power costs can quickly fly straight out the window when you meet up with some "operation savey guy" that values convenience of variable flow and head, rather than running one speed (+10%/-20%) all the time, or switching on and off every day, and/or building tanks everywhere to allow you to run constant speed until the tanks get filled up, then stop and restart 3 days from now. The problem is that the value of "flexibility in operations" is very much more difficult to evaluate on a spreadsheet that awards selection by lowest lifetime cost alone. A lot of guys never get that fact either. IMO, sincerely no offense meant, you could be one of them.

There are so many people who think like that I have considered having "tunnel vision" many times in 30 years. I always go back and check the pump curves to see what I missed. The pump curves never change. Every once in a while I find something from someone willing and brave enough to say VFD's waste energy. I try to go to their defense, because I know they will be treated like this. I don't think it is me that has tunnel vision. You can dismiss what I say because "hey, he sells valves!" Or you can get out a pump curve and see what I am talking about. And remember, the pump curve doesn't add back the losses of the VFD itself or he efficiency at lower speeds. But "I sell valves". So, here is something from someone else for you to tear apart. https://www.aceee.org/files/proceedings/1999/data/...

I was still editing and adding. Sorry. I don't find these tablets very convenient for writing a long post in one go.

"Flexibility in operations beats economic selection alone almost every time". As long as rpm/head is still enough to get the job done. That's what sells so many VFDs.

Quote (1503-44 (Petroleum)19 Dec 21 20:36
I was still editing and adding. Sorry. I don't find these tablets very convenient for writing a long post in one go.

"Flexibility in operations beats economic selection alone almost every time". As long as rpm/head is still enough to get the job done. That's what sells so many VFDs.)

Quote (Ill post one more here. The last one, I promise.
IMO It also tends to give you tunnel vision, even though you don't notice it.
I have that sometimes. I once said, 35yrs ago, that fully turbulent flow boundary is at Nre = 4000. An aerospace engineer was kind enough to remind me that all the world does not fit inside a pipeline.

I actually don't have any problem with most all of what you say. I didn't actually say I disagreed with anything in any post except the "vfd for Heat/Cool, and CS/CV for water supply" logic, and that was actually Tugboat's comment. My comments were around my central argument, that the choice should be driven by any variation from a constant speed pump's BEP H/Q/P that the final design must allow to meet the system requirements. Functionality requirements often will even take precedence over actual energy efficiency, sometimes not, no matter what type of system is being designed, or what the fluid in the system is, and that's true no matter how huge or tiny the system is. I was once a constant speed CV "valve guy", if you will, just like you. The most economic design, get the most efficiency possible kind of guy. In fact that is still my personal preference, mostly because it is easier to justify a selection based entirely on $$$. It took me a long time to realize that all that engineering economics and power costs can quickly fly straight out the window when you meet up with some "operation savey guy" that values convenience of variable flow and head, rather than running one speed (+10%/-20%) all the time, or switching on and off every day, and/or building tanks everywhere to allow you to run constant speed until the tanks get filled up, then stop and restart 3 days from now. The problem is that the value of "flexibility in operations" is very much more difficult to evaluate on a spreadsheet that awards selection by lowest lifetime cost alone. A lot of guys never get that fact either. IMO, sincerely no offense meant, you could be one of them.)

Ok so we finally agree. VFD’s can be a convenient way to control a pump system but is not efficient. Sure, I believe “flexibility in operations” is important. Pump systems need to be able to handle anything the customer wants to do with the water from zero to max flow being drawn for a minute or 24 hours a day. A VFD is an easy way to maintain the pressure and vary the flow. From a touch screen or even your phone you can turn the pump on or off, see the amps, voltage, speed, temperature, history, and will usually tell you what is broke and needs replacement.

But if the pump just comes on when needed, supplies the correct flow rate at the right pressure, and goes off when no longer needed, you don’t need any fancy controls. I have systems that have gone so long without any problems that no one has even been in the pump room for 20+ years. It has been so long since they even thought about their water system, they cannot even locate the well. Generations have come and gone with the new generation having no clue why water just dependably comes out the tap when opened. Funny thing is, even after 30 years of perfect service without any maintenance, when the pump system does fail, some tech will decide to replace the long lasting and dependable valve control with a VFD. The customer will be told the VFD will save enough energy to pay for itself in 7 months and will make the pump last longer. Lol!!

Really? I don’t mind VFD’s being sold as a convenient way to control a pump where you can see everything on your phone, but they do not save energy. I have some fancy equipment where I can see my pump info from my phone. But the pump is still controlled by a simple control valve, a mechanical pressure switch, and a small diaphragm tank. So, I haven’t even looked at any of the pump info on my phone or been in the well house in years. Water has just been dependably coming out the taps when needed for 30 years.

A VFD being used to vary the flow and maintain a constant pressure is fantastic as long as it keeps working and you can afford it. But you can get the same variable flow and constant pressure from a simple inexpensive valve, and simple is what makes pump systems dependable.

The real problem is most people these days can’t read a pressure gauge, amp meter, or even know which end of a screwdriver to use. They can’t even tell if a pump is running unless a green light is blinking on their phone. A VFD is a good way for them not to worry their pretty little heads about things like amps, TDH, minimum and maximum flows, etc. A VFD makes it easier for the operator, but at the cost of efficiency and longevity.

Sorry to have gone off subject on this thread. But a pressure tank is nearly always a good idea on a VFD system as it will help with some of the problems that go with.