Does the expansion tank require for a booster pumping with VFD system? 4

[moideen]

Does the expansion tank require for a booster pumping with VFD system for water distribution for multi-storey building? As we know the primary purpose of expansion tank to reduce the excessive staring of pumps in booster pumping. Yesterday, I visited on of building complex and found the all system built with expansion tank and then I was told pumps are only varying based set point without any cut off condition. thanks for your comments

 

[TugboatEng]

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.

 

[1503-44]

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.

 

[Valvecrazy]

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.

 

[LittleInch]

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.

 

[TugboatEng]

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.

 

[Valvecrazy]

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!

 

[itsmoked]

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:

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TugboatEng]

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.

 

[Valvecrazy]

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.

 

[bimr]

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

 

[Valvecrazy]

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.

 

[Valvecrazy]

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.

 

[1503-44]

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.

 

[pierreick]

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

 

[itsmoked]

How can I show that on the PID?

LOL You got me...?

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

 

[Valvecrazy]

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."

 

[Valvecrazy]

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.

 

[Valvecrazy]

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

 

[Valvecrazy]

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.