Control ratio 2

[koubi]

What is typical control ratio of a control valve for HVAC application with metal to metal sealing? I understand that a control valve with metal to metal sealing should have higher range ability compared to the one with soft sealing.

 

[gerhardl]

.. I am afraid you have to be more specific....

 

[JLSeagull]

For a throwdown ratio, for control valves compare the valve capacity sizing coefficient at 10% and 100% travel; or 10 and 90 degree for a rotary valve. This is largely a function of the valve characteristic such as equal percentage, parabolic, linear, quick opening, etc. Using this criteria you will not observe any difference between metal and soft seat. I don't know that you will find such data if you are addressing duct balancing dampers etc.

 

[koubi]

Thanks JLSeagull but wwhat about below 10% of the valves travel? It is about dual valve high load - low load arrangement application.
Lets say that we have two control valves with linear characteristic, one with metal to metal sealing and one with soft sealing. It is assumed that soft sealing element will cause an initial jump at the very begining of the valve`s opening.
What about metal to metal sealing? Does this valve in above mentioned case performs better?

 

[JLSeagull]

I prefer to avoid operation near the seat. I don't know that you would see a significant jump as the plug comes off a soft seat. I also prefer equal percentage trim for most applications so that the flow rate changes little with travel near the seat. My experience suggests that even hard trim in water service can wire-draw the control valve plug. This is more significant with high pressure drop. I often suggest level controls with gap action so that the level must increase somewhat before the valve comes off the seat; then immediately moves to about 20%. However, I don't configure the controls these years and don't know whether anyone actually implements this suggestion.

 

[JimCasey]

Traditionally HVAC applications use 1/3-2/3 split range control for broader rangeability. Small valve with about 1/3 capacity for "sumertime" loads, and a parallel valve twice as large for heavier demand. Many times the same steam used for building heat is used for heating bath and dish water, but that's a small fraction of the building's winter heat load.

I don't know why you assume soft seats cause an intial jump in valve position. That is very much a property of the valve design. Maybe double-offset butterfly valves with TFE seats, but that's about it. I have seen a LOT more metal seated valves with poor positioner calibration that had a jump at initial opening.

I have had success in using eccentrically rotating plug valves for this application without resorting to the 1/3-2/3 split. The ERP valve has greater than 100:1 rangeability. With better than class IV leakage, it barely leaks enough to keep the pipes warm...and thermal cycling the pipes is noisy as they pop and creak as the temp changes. THermal cycling the pipes causes fatigue-related problems, too.

Pay attention to the steam trap installation also. Poorly installed steam traps lead to slugs of water in the line and steam hammer in the pipes....which is really "condensate hammer"

 

[belowzero]

I don't know why you assume soft seats cause an intial jump in valve position. That is very much a property of the valve design. Maybe double-offset butterfly valves with TFE seats, but that's about it. I have seen a LOT more metal seated valves with poor positioner calibration that had a jump at initial opening.

The 'Jump' being discussed in the soft seated valve is the characteristic of the material rather than design, PTFE based seats are prone to 'cold flow' i.e. the small movement of the material at high load points, so at the lip or sealing point of a valve there is typically a higher load which enables the material to provide the seal against the line pressure (approx 2000psi for rtfe) this load deforms the material and causes a sticking point. If a valve is moved within a short time then the this is less of a problem, if there exists either long dwell times before movement or elevated temperatures (because PTFE and like materials will move more quickly the warmer they are and ultimately the reason for their failure at higher temperatures) then there will be a rise in torque greater than normal to enable the seat to be released, ultimately once movement starts then the high torque will cause overtravel and the controller will have to compensate, consequently there are a lot more 'ripples' in the medias flow as the controller brings the valve to the correct position.
This will exist in all ball and butterfly valves to some extent or other.
B

 

[bcd]

I've got to disagree with the previous statement. Jump is a function of the design. Not the material. Metal seated butterfly valve can jump out of the seat just as well as a soft seated. Same with ball valves, but not a severe. Jumping is due to the difference between unseating and running torque and is related to operation with pneumatic actuators. With offset type butterfly valves, once the disc breaks contact with the seat, the torque necessary to turn the valve the rest of the way open in only a small percentage of the unseating torque. Unless you have very high flow rates. Material cold flow or not, overshoot can happen. Ball valves are not as bad since the ball drags across the seat and the running torque is closer to the unseating torque. You need to build up enough pressure in the actuator of overcome the unseating torque. Once the disc breaks contact with the seat, the air prressure inside the cylinder is greater than needed to continue rotation of the disc so the air rapidly expands causing the disc to rotate past the desired control position.

You won't see a difference in jumping between metal and soft seated globe valves.

Rangeability varies with the valve type and in general, you can say that metal seated valves tend to be more tolerant to throttling close to the seat. However, you really need to look into the specifics of the valve to be sure of its capabilities.

 

[belowzero]

bcd,

I've got to disagree with the previous statement. Jump is a function of the design. Not the material

Hmmm

You need to build up enough pressure in the actuator of overcome the unseating torque. Once the disc breaks contact with the seat, the air prressure inside the cylinder is greater than needed to continue rotation of the disc so the air rapidly expands causing the disc to rotate past the desired control position.

Unseating and breakout of metal or soft is another word for 'jump' sorry if the terminology was a little simplified.

Globe valves don't have the same sealing characteristics as butterfly and ball.
Metal seated valves are more tolerant to working near the closed position as they have a far greater resistance to errosion when higher flows exist as the valve closes.
B