|2. Butterfly Valve|
|4. Special design|
|5. Important design points|
|6. Common problems with butterfly
The most common valve found in bulk solids handling is without doubt the butterfly valve. This type of valve is simple, robuts and works globally well with the flow of bulk solids.
Butterfly valves are the valve of choice for the isolation of an equipment as they are used to close inlet and outlet of hoppers, silos and containers which handle powder or granular materials. Once open, they allow the unloading of the container, control the flow or can even be used to dose the product if they have specific features like a positioner on the actuator.
This page is giving details on the design of butterfly valve and their actuator.
The constitution of a butterfly valve is quite simple : the product flow is stopped or allowed thanks to a butterfly, a thick gasket is ensuring the tightness of the valve and an actuator (most of the case pneumatic) is allowing the valve to move.
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Figure 1 : Butterfly Valve drawing
The actuator can be basically of 2 types, either single effect or double effect. Single effect actuators have only one chamber to be pressurize with compressed air, a spring ensuring the movement in the other direction when no compressed air is applied. Double effect actuators have on the contrary 2 chambers, pressurized alternatively, for rotation in both directions.
Double effect actuators are usually able to supply greater torque and therefore be more reliable for difficult applications.
Recently, butterfly valves equipped with special actuators have been introduced to the markets. These actuators are equipped with a positionner, allowing to open precisely the valve to a defined percentage, but the actuator is also equipped with a vibrator. Such vibrating butterfly valves have proved successful in Dosing and weighing precise amounts of powder, thus replacing more complex equipment like Screw Conveyor or Vibrating Tube when the layout allows it.
The vibration allows to make the powder flowing even at small opening, thus reaching a reliable and precise feeding.
To be noted that, in some cases, the use of a positioner can be sufficient, when a very rough dosing - to a vibrating sieve for instance - has to be achieved.
For butterfly valve, the part submitted to wear is the gasket, which must be monitored regularly as, in case of damage, the valve may not be tight anymore or may release some rubber chips to the product flow constituting a foreign body.
In a general rule, butterfly valves should not be used in Pneumatic Conveying line as the butterfly will cause some damage to the product being conveyed, while the product flow at high speed will also increase the gasket wear.
Butterfly valves must be sized large enough at the outlet of the hoppers in order to ensure a Mass flow out of the hopper, without bridging or ratholes.
Butterfly valves are usually reliable equipment, however a certain number of issues may require some corrections :
Table 1 : common problems with butterfly valves
|Issue||Root cause and action|
|Wear of gasket||Incorrect installation of the gasket protruding on the inside of the valve and flow of product|
|Valve blocked||Too low torque from the actuator
The valve is activated on static powder, prefer to close the valve when the product is flowing