Method for controlling a butterfly valve

1. A method of operating a butterfly valve, comprising:

defining the stem stroke of the butterfly valve into a first, a second and a third operation regions, wherein the first operation region corresponds to a lower stem stroke, and the third operation region corresponds to a higher stem stroke of the butterfly valve;

providing a first set of proportion, integration, and differentiation parameters for the first operation region such that the butterfly valve functions as an on/off control valve in the first operation region;

providing a second set of proportion, integration, and differentiation parameters for the second operation region such that the butterfly valve functions as a percentage control valve in the second operation region; and

providing a third set of proportion, integration, and differentiation parameters for the third operation region such that the butterfly valve functions as an on/off control valve in the third operation region,

wherein the first and third sets of proportion, integration, and differentiation parameters produce a greater output level than the second set of proportion, integration, and differentiation parameters.

2. The method of claim 1 , further comprising:

defining the second operation region into a plurality of sub-regions; and

providing a set of proportion, integration, and differentiation parameters for each of the sub-regions such that the butterfly valve functions as a percentage control valve in each of the sub-regions,

wherein the first and third sets of proportion, integration, and differentiation parameters produce a greater output level than each set of proportion, integration, and differentiation parameters provided for each sub-region.

3. The method of claim 2 , further comprising:

determining a target output level for the butterfly valve;

providing a pre-determined output level for each of the sub-regions of the second operation region; and

operating the butterfly valve to attain the pre-determined output level for each of the sub-regions toward the target output level.

4. The method of claim 1 , wherein the first operation region corresponds to stem stroke percentages of approximately 0%-30%.

5. The method of claim 1 , wherein the second operation region corresponds to stem stroke percentages of approximately 25%-80%.

6. The method of claim 1 , wherein the third operation region corresponds to stem stroke percentages of approximately 70%-100%.

7. The method of claim 1 , wherein the first operation region is responsible for approximately 10% of the maximum output level of the butterfly valve.

8. The method of claim 1 , wherein the second operation region is responsible for approximately 80% of the maximum output level of the butterfly valve.

9. The method of claim 1 , wherein the third operation region is responsible for approximately 10% of the maximum output level of the butterfly valve.

10. The method of claim 1 , wherein the butterfly valve is a first valve, the method further comprising:

coupling the butterfly valve to a load operable between a maximum load value and a minimum load value;

coupling the butterfly valve to a second valve; and

controlling an output level of the second valve to ensure that the butterfly valve operates in the second operation region.

11. The method of claim 10 , further comprising controlling the output level of the second valve to ensure that the maximum load value of the load is reached before the first valve enters the third operation region.

12. The method of claim 10 , further comprising controlling the output level of the second valve to ensure that the minimum load value of the load is reached before the first valve enters the first operation region.

13. The method of claim 10 , wherein the load is a heat exchanger including a heat transfer medium.

14. The method of claim 13 , further comprising:

providing the heat exchanger with a rating temperature (T HE );

providing a pre-determined working temperature (T W ) for the heat exchanger; and

keeping the butterfly valve operating in the second region by resetting the pre-determined working temperature T W to follow the relationship: T HE +T M ≧2 T W

wherein T M is the temperature of the heat transfer medium.

15. The method of claim 1 , wherein the step of resetting the pre-determined working temperature T W includes keeping the value of T HE +T M slightly greater than that of 2 T W .

16. A method of operating a butterfly valve, comprising:

defining the stem stroke of the butterfly valve into a first operation region, at least one second operation region, and a third operation region;

providing a first set of proportion, integration, and differentiation parameters for the first operation region;

providing at least one second set of proportion, integration, and differentiation parameters to the at least one second operation region such that the second set of proportion, integration, and differentiation parameters corresponds to the second operation region;

providing a third set of proportion, integration, and differentiation parameters for the third operation region, wherein the first and third sets of proportion, integration, and differentiation parameters produce a greater output level than the second set of proportion, integration, and differentiation parameters;

determining a target output level for the butterfly valve;

providing a pre-determined output level for the second operation region; and

operating the butterfly valve to attain the pre-determined output level for the second operation region toward the target output level.

17. The method of claim 16 , wherein the butterfly valve functions as an on/off control valve in one of the first operation region and the third operation region.

18. The method of claim 16 , wherein the butterfly valve functions as a percentage control valve in the at least one second operation region.

19. A method of operating a butterfly valve, comprising:

defining the stem stroke of the butterfly valve into a first, a second and a third operation regions;

providing a first set of proportion, integration, and differentiation parameters for the first operation region such that the butterfly valve functions as an on/off control valve;

providing a second set of proportion, integration, and differentiation parameters for the second operation region such that the butterfly functions as a percentage control valve, wherein the first set of proportion, integration, and differentiation parameters produce a greater output level than the second set of proportion, integration, and differentiation parameters;

coupling the butterfly valve to a load operable between a maximum load value and a minimum load value;

coupling the butterfly valve to another valve; and

controlling a flow rate of the first valve for the butterfly valve to operate in the second operation region.

20. The method of claim 19 , further comprising:

defining the second operation region into a plurality of sub-regions; and

providing a set of proportion, integration, and differentiation parameters for each of the sub-regions, wherein the first set of proportion, integration, and differentiation parameters produce a greater output level than each set of proportion, integration, and differentiation parameters provided for each of the plurality of sub-regions.

21. The method of claim 20 , further comprising:

determining a target output level for the butterfly valve;

providing a pre-determined output level for each of the sub-regions of the second operation region; and

operating the butterfly valve to attain the pre-determined output level for each of the sub-regions toward the target output level.