Modulated (vsd) pumps versus throttle valves

[ozmosis]

Pump experts
As more and more vsd's/vfd's are controlling pumps, a question keeps cropping up in our (VSD) team " will vsd controlled pumps eventually replace throttle valves?". We make both. Just wondered what real users feel.

 

[quark]

Yes for low static applications and closed loop circuits.

 

[notnats]

I feel that VFDs are a bit fragile and in my experience, if one fails after say 5 years, you find that the original vfd is obsolete and has been replaced by one only half as big with different interface connections and different parameter setups not to mention the confusing navigation through the parameter menus.

I know all vfds now have line filters, but there is still a concern about rubbish from the vfd carrier frequency getting onto analogue signals.

Valves on the other hand don't wear out and when used with a suitable surplus valve and a flow meter (burkert have a nice paddle wheel meter system) can give easy to use flow control.

I have seen valves used with a pressure gauge upstream and a set of pump curves to give a fairly easy flow adjustment.

Jeff

 

[ccfowler]

Both means of control are useful, effective, and suitable for appropriate applications. There is no reason to expect this to change much in the future.

Quark is right that VSD's provide their best advantages in such duties where flow rates need to vary significantly. Where the flow rate can remain substantially constant, a simple induction motor drive should be used without a VSD. (The VSD would present needless additional costs and complications.)

The details of each application's needs will (and should) dictate the specific equipment selections.

 

[CB2]

I would suggest you watch Goulds and how they are doing with their "Pump Smart" accessary product. There are a number of other pump manufacturers about to introduce similar options.

Does that put the control valve future in question?

 

[abcpump]

My opinion is that the popularity of VFD's will increase in direct proportion to increases in energy costs. No matter which way you look at it, throttling a pump is akin to driving your car with one foot on the brake! This is not to say that throttling valves will not maintain a presence in the HVAC balancing market, but already some manufacturer's have small circulating pumps with built-in VFD's to take the place of two way valves in heating systems.

 

[ozmosis]

thanks for your comments so far. It's a discussion we have had for a while. The people who purchase throttle valves are not the ones who would generally purchase vsd's but, as you point out abcpump, more and more manufacturers are integrating the drives into the motor/pump system making the decision easier in a way but also putting the onus onto others in the design chain who perhaps may not have the knowledge in the dynamics of pump control. This, I suspect, is the single challenge of vsd suppliers to make the actual vsd 'invisible' and more the result of modulated pump control the focus.

 

[MortenA]

abcpump

I think your point is true. In Denmark we have very high energy taxes. A kWh costs (approx) 20 cents (US) - so here VFD are more poular than many other places.

Best regards

Morten

 

[PUMPDESIGNER]

VFDs are one more tool in the tool box, use it when it is proper to do so.
Valves are here to stay, simple, reliable, and on low specific speed pumps they are often more energy efficient than VFDs (yes, they save more energy than VFDs can on the correct pump).

But one we often forget is no valve no VFD. I like that one best when it can be done, and it can be done more often than people think.

I have replaced many complex pump stations with a simple and carefully selected pump on systems with well known hydraulic characteristics, even when flow rates vary greatly.

I had an exchange with a consulting engineer for the Feds about energy conservation. I was questioning his approach where the Feds go into a factory and analyze then make suggestions about the motors and pumps to save energy. He surprised me when he said that he first tries to eliminate any controls with a more correct pump, trim impeller, change the process, etc. If that is not workable, then step up to a VFD or Control Valve, whichever resulted in the highest energy savings.

notnats brings up a valid point about durability. There are local areas in the world where VFDs just have trouble due to transient surges. The areas I know of with dead VFDs hanging on walls, replaced frequently, or having been replaced by control valves are Central and South Florida, Houston, Venzuela, and any area where daily thunderstorms simply wear out VFDs. In those areas VFDs are still used of course, but in many cases they continue with VFDs because they can actually bring great benefits in the right situation.

Silicon will never be able to resist serious transients, and at some point protecting them becomes impractical.

PUMPDESIGNER

 

[jraef]

All good points, and I follow the idea that each application must be considered on its own merits so valves will always be viable. I would however like to address notnats' comment about how "fragile" VFDs are in the system. While it was once true, it is no longer so IMHO. Be that as it may, if a VFD saves enough energy to pay for itself in 2 years, and then dies and needs to be replaced in 5, it STILL far exceeds the economic value of using a throtling valve in the same application. The only time it can be considered economically risky is if the VFD dies before it pays for itself and cannot be fixed. Many, if not most, VFD manufacturers are offering 3 year warranties, making that a non-issue as well (assuming a payback in under 3 years).

"Venditori de oleum-vipera non vigere excordis populi"

 

[notnats]

If the vfd dies on a remote mining site, for example, (or a ship at sea) phone calls, faxes and emails may take a day before a replacement is ordered. Even with overnight delivery (not always possible if stock is only held in Sydney and the mine is in NW Australia, or the ship is in mid ocean)it will be another day before the thing is installed, programmed and running. If it has different software or terminal layout the sparky on site might spend hours on the phone before it is properly sorted out.

A $1000 vfd might cost much more in down time of the plant.

To send a 10kg parcel same day by air from Sydney to Darwin costs $500, and you still might have to go another 500km to site.

Don't get me wrong, I like vfd's and am always impressed by how small and cheap each new model is and I know the reliablity is improving all the time, but there are areas where a 5 year life is not very useful.

Jeff

 

[jraef]

That is why people buy spares... or at least should.
What happens in the remote location when the valve breaks or gets clogged or the operator mechanism jams and can't be repaired without a total shutdown? At least with a VFD you can sometimes leave a bypass starter in place and get back running in a pinch.

But you made your point and it is not a bad one. As I prefaced my statement, each application must stand on its own merits.

"Venditori de oleum-vipera non vigere excordis populi"

 

[macmil]

There are alternatives to variable frequency drives which are applicable to pumps. In many instances, these alternatives offer advantages over v.f.d.s, reliability and life expectancy not least.
mac

 

[unclesyd]

There is another speed and torque control system that I’ve seen and the people who were using it were quite pleased.

This was the Gyrol Fluid Drive on was a system that required a variable torque output.

There is nothing to wear out except some bearings, which are readily available.

http://www.howdenbuffalo.com/index.htm