Throttle Service

[BobPE]

I am interested to see what other engineers are doing when they throttle butterfly valves in potable water applications. I know throttling is not the optimum practice and that butterfly valves are not the best valve for throttle service but I would like to know the acceptable closure percentage range one would expect to use for this type of valve out there in the real world. I would also like to know what spec valve you have found to be most usefull in helping the valve survive in throttling service.....AWWA, ANSI, etc.


Thanks in advance...

BobPE

 

[quark]

For chilled water and cooling water applications I extensively used butterfly valves ranging from 3" to 36". As far as my experience is concerned, they control flow upto 70% close condition and below that it is too bad . The main drawback is when you control the disc opening two streams form in opposite directions and create much turbulence downstream side.

There was a debate long back in this forum but I couldn't get the link. Globe valves are best control valves though pressure drop is more.

 

[hacksaw]

"Butterfly valves" work fine in throttling services, especially in the water industry.

The biggest difficulty is developing reliable sizing data for the valve and actuator requirements; and you have to work to the strengths of the particular design your working with. It is not a spec and forget exercise.

There are so many design options and improvements in the "rotary disk"/ butterfly valve designs that general application rules have not really emerged other than seek a cost effective solution that works and is reliable.

 

[saxon]

BobPe, Straight from the Fisher Control Valve Handbook:
1. Convential Countoured discs provide throttling control for up to 60 deg. of disc rotation. Patented, dynamically streamlined discs available for applications requireing 90 deg. rotation.

2. A dynamically contoured disc such as the patented, "FISHTAIL" by Fisher, permits control through full 90 deg. of disc rotation. Convential disc's are usually limited to rotation of 60 deg. Butterfly valves exhibit an equal-pecentage flow characteristic and can be used in throttle, or on-off control.

Hope this helps.
saxon

 

[hydrae]

BobPE
I have used AWWA butterfly valves for thottling in drinking water applications only when the pressure differential is low, less than 20 psi. AWWA valves have a short life when throttling large differential pressures. If the app has large variations in flow I will use a large and small in parrallel. At 10% open or less on the big valve switch to the small valve, at 50% open or more on the small valve switch to the big valve.

For operators, MOV's and hydraulic Piston, and I avoid the position controllers and use the PLC programming directly with open, close, and hold input controls, for outputs include full closed and a position indicator. The position verses flow is nolinear so PID loop on the desiried parameter not valve position. I have been lucky to always had PLC programmers who knew how to tweek the PID loops. Another pitfall with b-flys is the gear lash, again compensated by the programing.

I would not use butterfly's to throttle without the PLC.

I normally use diaphram operated globe valves for normal throttling service, but they do not work well in low diff pressure apps without a high pressure control water source, spilling control water, or investing in a double chamber operator.

Going to an non AWWA valve you have to justify to the gov why you are not using NSF61.

Hydrae

 

[ilan7]

I'm using butterfly valves for throttling control for years and they works fine for us. Usually controling geothermal hot (180 to 400°F) water and steam.
As long as the pressure drop across the valve is low and the valve is at least High performance type with two or three offsets design.
We usually choose Fisher control High Performance valves (e-Disc type) or Vanessa (Tyco) type 33000 zero leak valves.

 

[oxilume]

Dear Bob,

Found your thread. I can help you in this area, since of course I spent many years down on the deckplates in the engine room of several Navy Ships. Likewise, I have consulted with superintendents for dewatering(pump selection) projects and worked with consultants with respect to pump problems in the oilfield - rigs, platforms, etc. Finally, I have seen so many mistakes made in pump packages in World Class Refineries and Petrochem plants that it is scary.

I helped solve a $100,000 pump cavitation problem with about $50 of pipe and a few 2" PVC valves. As the water treatment salesperson, they wanted to place the blame on scaling and erosion in the pipe and consequently plugging up the pump due to my chemicals. In addition, they claimed the impeller was being destroyed by the scale.

I traced the system, hand over hand, pulled out my tools from my company car and solved their problem. Different operators would throttle the butterfly valve based upon their own experience. You can imagine what type of control occured from 3 shifts per day.

The first problem I noticed - that is common in every plant (but rarely on Navy Ships/Subs) - is that the pump did not have a recirculation line. Since I had a tap on the pump discharge line to inject my chemicals and a truck backed into the injection line and cracked it, I replaced it and tied back into the suction side of the pump. I left written instructions to not shut the chemical injector feed valve.

About a year ago I ran into one of the engineers who worked at the plant(incident was in 1992). He told me that my solution saved the plant $100,000 in downtime and pump repairs.

I am amazed at how many pumps do not have recirculation lines. If you are going to throttle with a butterfly valve, just install a recirculation line. It prevents the pump from cavitating and destroying itself.

The plant engineers did not know why the pump sounded like marbles were rolling around inside - cavitation!

Sincerely,
Todd Foret