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Actuator Retrofitting (Existing Actuator with a new valve)

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Jo26

Mechanical
Aug 29, 2012
1
Good Day Everyone,

Breif Background: Recently, we requested a new valve to replace a valve that we have which was beyond repair. This valve was automated with a pneumatic actuator connected to it since the 1980s. When we recieved the new valve, which has the same size (10 in.) and pressure rating (150#) we found that it has different stem dimensions as well as a different manufacturer (I guess that's what happens when you don't go for a single source ..)

so the issue now is that we have an existing actuator that needs to be connected to a new valve. Since no one was around, I was handed the job. From my mechanical background, I knew that Torque is the factor affecting the sizing and I researched about the subject to find out what I need although no clear way was found. moreover, I stumbled upon this great website which I think you guys can help a lot.

Technical Info Collected:

New Valve Torque Requirements:

Break-To-Open Torque: 704 N.m
MAST: 3038 N.m

Existing Actuator Torque Capabilities:

@ Min Operating Pressure of 50 PSI: 150 m.kg = 1471.5 N.m
@ Max Operating Pressure of 125 PSI: 375 m.kg = 3678.75 N.m

Based on what I researched, this actuator can operate the valve safely given that the pressure doesn't exceed 100 psi (Based on linear interpolation of the actuator data, since Torque = Area * Radius * Pressure, so if we consider Area & Radius to be constant, the relation would be linear [correct me if I am wrong, my mechanical background might be wearing].
From my interpolation, I found out that at 100 psi, the actuator gives 2940 N.m which is around 100 N.m less than MAST)

Also, from my research, I have read that to size an actuator, the BTO Torque is usually multiplied by a factor of 1.25 - 1.3 to determine the minimum torque requirement of the actuator. Now this actuator is about double the BTO Torque at its minimum operating pressure, will that be a problem ?

Supporting Info:

The old valve manufacturer was contacted to specify the torque requirements of the old one and this info is what I received:

BTO Torque (@ Max DP): 1073 ft.lb = 1455 N.m
MAST: 2011 ft.lb = 2726.5 N.m

When I received this info, I was a bit hesitant about my conclusion. I mean if this actuator was able to rotate the old valve easily given that it had higher BTO Torque, it will rotate the other one for sure to the extent it might break it ! (That I am not sure about)

Can you please help me in this issue ? That is, is my conclusion that the actuator will safely rotate the valve given the fact that the supply pressure doesn't exceed 100 psi ?

since the actuator torque @ min operating pressure is double the new valve BTO Torque, will that create a problem ?

Thanks a lot for your help

Best wishes,

Joseph
 
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Hello Jo26,

Let me try to answer some of your questions:

a) First choice, when an actuator and valve is that old, is always to change both valve and actuator together, and (as you say) from sources that can supply products that fits together. Preferably supplied as ready mounted and tested unit. The reason for this is expected lifetime of the new actuator, and that mechanical fitting and adjusting can be a problem, or at least needs skilled and experienced mechanics and correct tools and testing equipment.

b) If you do have a valve and suitable actuator not fitting mechanically, you will probably need a yoke between the two, and a cmechanically adapted and fastened connection piece between the two stems. The question then is if the (very solid) yoke can be connected to valve and actuator mechanically strong enough to withstand max torque. Max torque will be limited to the necessary torque to loosen and move the valve (which will vary under wear and standstill, fluid type etc) if the actuator travel is limited.

c) The trick is of course to have mechanical end stops mounted and adjusted on the actuator to limit the stroke, and limit switches to stop (and hold) the pneumatic air. End switches on pneumatic actuators gives seldom sufficient repeated accurracy alone. (In comparison: Electric actuators of high quality will have limit switches cutting off current in fairly accurately travel position with very high repetitivety. In addition the electrical actuator will be equipped with torque switches.)

In my opinion your problem is the mechanical fitting, mounting, adjusting and present equipment and possibillities your challenge. Very often mechanical special adaption and extra equipment will cost more than a new, ready to fit actuator.

 
You should be fine re-using that actuator.

But like gerhardl said above, the most important thing is to ensure the mechanical stops are set on the actuator, such that they limit the valve's movement, not the valve's stops, otherwise you'll likely break the valve stop, shear a pin, or something worse.
 
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