I had always heard that a plug valve should be used for manually balancing hydronic systems instead of a butterfly valve due to the poor throttling ability of the butterfly valve.  However, I am noticing more and more valve manufacturers offering and manual balancing valve with venturi and P/T ports utilizing a butterfly valve.  Have butterfly valve's throttling characteristics changed over the years or are these just more sub-par products offered up by companies trying to save a buck?

There seem to be a lot of manufacturers trying to shoehorn Butterfly valves into every posible niche.  They are pretty lousy block valves for the most part, they've always been really terrible throttle valves, so manufacturers are spending a bunch of R&D money to try to fit them SOMEWHERE.

I won't use the new butterfly valves for throttling service, there are too many good products on the market to use a marginal valve.

That being said; there is no application on this earth where I would recommend a plug valve.  I know someone will follow this post with examples of where plug valves are the perfect answer and they may even be right, I've just spent way too much time with a 36-inch pipe wrench, 20 ft cheater, 12 lb hammer, and a whole roustabout crew to try to shut plug valves that haven't been serviced in a couple of decades.  I know that the problem could have been mitigated with a bit of PM, but I find that in bad times the valve service program is the first thing cut and then it is slow to be re-implemented.  An unserviced plug valve is a disaster.

If you want to throttle a process why not use a throttle valve?  Globe valves, V-balls, and dozens of linear-flow chokes all work really well in throttling service.

OK, I'll be the devil's advocate.  
Things that EVERYBODY knows:  
1.Butterfly valves are bad for throttling.  
2.Plug valves are bad for throttling.
3.The earth is flat.
5.There is a vast international conspiracy from the trilateral commission.
6. Engineers wear calculators on their belts and have tape on their glasses

1. Butterfly valves have an equal-percentage characteristic out to about 60 degrees open.  They throttle just fine BUT THEY ARE LIMITED IN THEIR APPLICATION.  (Every valve has limits)
Butterfly valves have a huge Cv for their size.  Thus they have a reputation for having a quick-open characteristic.  They don't- but if you just slap a line-size butterfly valve in a line without benefit of the thought process, you will likely swamp the system and say "Dagnabit! I tol' y'all them butterfly valves wasn't no good" or other similar intellectual analysis.  If a typical globe valve for control is a size smaller than the line it is in, isn't it reasonable to expect that a much higher capacity valve would be even further reduced?  
Limitations of butterfly valves: They have a low value of Fl- also characteristic of other high-capacity valves.  Put too much differential pressure across them in water service and they are likely to begin cavitating before a globe valve would.  Is this a problem in a balancing valve? Probably not-since a balancing valve is just for...balancing, which is just creating a small restriction to match a similar small restriction elsewhere in the system.  
Rangeability can be similar to a globe valve, but it is necessary to use a gear operator..A handle with a 10-notch plate doesn't really allow for much fine tuning.  Automated valves neet to hae the actuator torque characteristic matched to the valve's torque characteristic.  And a rubber-seated butterfly valve (typical for a balancing application) requires a lot of torque to break the vane out of the seat.  I mentioned 60-degrees earlier: a good place to stop opening the valve for reasons other than capacity.  There is a dynamic torque peak at about 75 degrees where the flow becomes attached to the vane.  The center of pressure is ahead of the shaft-so the valve wants to close.  Don't open past 60 degrees for modulating servixe and this won't be a problem.  Past the torque peak the torque drops off rapidly to zero at 90 degrees open. Some sources report this as a torque "reversal" but the reversal occure in the first derivative of the torque function.   If your actuator is too "soft" it is possible to overshoot and go into a nasty dynamic.   Hence few butterfly valves offered with 3-15 pneumatic diathragm actuators these days, since a diaphragm actuator has the dynamics of a waterbed.  Nice stiff rack and pinion, Scotch yoke, or electric actuators work fine for butterfly valves.  High-Performance (aka double-offset) butterfly valves have reduced breakout torque as well as reduced dynamic torque.  Triple offset valves have even less breakout torque but require huge seating loads to compress the metallic seal ring.  The big advantage of a triple offset valve is that there are no elastomers in the valve so it can withstand extended temperature range.  

Plug valves:
You did not mention that there are 2 types of plug valves: Lubricated and nonlubricated.  "Lubricated" plug valves have a metallic plug that bears directly against the inside of the metallic body.  High-pressure sealant (almost universally referred to as "grease" is stored inside the valve, and injected between he plug and body by turning a screw BEFORE ATTEMPTING TO TURN THE PLUG.  The sealant acts as a lubricant and the high pressure causes the plug to float clear of the body so it can be turned.  The sealant should be selected so that it is both compatible with and non-contaminating to whatever is flowing through the valve.  Leave a lubricated plug valve in place without stoking it for a long time, and corrosion products of system mung  can render it very, very difficult to break loose when you do want to reposition it. Lubricated plug valves are intended for isolation and I know of no vendor who offers them for throttling.  

"NONLUBRICATED" plug valves are actually better lubricated that the lubricated types.  In a nonlubricated plug valve the plug rides in a (usually) PTFE sleeve. Set-torque of a nonlubricated plug valve peaks at around 72 hours, as the TFE sleeve cold-flows to match whatever microscopic imperfections exist in the surface of the plug.  Set-torque is around double run-torque.  Lubricated plug valves ARE offered with characterized ports for throttling, and the port capacity is comparable with a globe valve of the same size. Characterized plugs have equal-percentage characteristics. Rangeability can exceed that offered by a globe valve.  THere is NO effect of pressure on the torque required to turn a nonlubricated plug valve.  THe torque is known, actuator sizing addresses the torque, so automated plug valves can be offered to modulating applications and can meet the ENTECH dynamic specifications.
Limitations of Nonlubricated plug valves:  Also low Fl, therefore can begin to cavitate at lower pressure drops than globe valves.  Limited pressure drop for modulating to around 100 psid due to exposure of the TFE sleeve to the pressure drop.  Limited temperature range due to elastomers in contact.  Is it good as a balancing valve? Sure! Better resolution and rangeability than a butterfly, matches the line size better for a given Cv, and tends to stay where you leave it because of the friction of the plug in the sleeve.  

Nobody mentioned them, but Gate valves really are unsuitable for mudulating in all but a very tightly defined applications window.

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