Pre-tension in Actuator Spring of Pneumatic Control Valve

[Daren Samy]

In Fisher control valve, there is no OEM instruction to pre-tension the actuator spring by rotating the handwheel before calibrating the actuator travel against pneumatic pressure. Is it right practice to pre-tension the spring ?

Daren!

 

[SPDL310]

No it is not the correct practice. The IOM for diaphragm actuators linked below specifically mentions how springs should be pre-tensioned in the Discussion of Bench Set section on page 7. This procedure is to be performed before mounting, and once it has been performed addition spring pre-tension is not required.

[URL unfurl="true"]https://www.emerson.com/documents/automation/instruction-manual-fisher-667-diaphragm-actuator-sizes-30-30i-76-76i-87-en-122368.pdf[/url]

 

[Daren Samy]

What it says "some spring compression is required to move the diaphragm to the bottom of its travel" It indicates some pre-tension of spring before applying air pressure?

Spring Verification , Page 7

Daren!

 

[SPDL310]

Yes, it is stating that for the particular actuator, which is a spring close actuator, it requires some spring tension to be applied in order to reach the full down travel. The diaphragm will push the actuator stem slightly upward without spring tension. Refer to Figure 2 on page 3. I believe it would be the opposite for a spring open model.

 

[gerhardl]

As SPDI310 says it is quite simple. It's all about a balance of forces in each direction. Think about is as a regulating valve that sometimes, without pneumatic pressure, has to go to closed position. No pre-tension, no force to hold the valve in this end poition.