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Join Date: Sep 2008
Posts: 5

Reverse / Direct acting valves

09/19/2008 8:30 AM

Chaps,

What is the definition of a direct and a reverse acting valve? By valve I mean the body with actuator.

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Join Date: Dec 2007
Location: texas
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#1

Re: Reverse / Direct acting valves

09/19/2008 5:24 PM

Hi, a pneumatic direct acting valve will travel to the open position as the air signal to the valve increases.A reverse acting valve will travel to the close position as the air signal increases.A direct acting valve(air to open) will fail close.A reverse acting valve(air to close) will fail open... regards

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Anonymous Poster
#2
In reply to #1

Re: Reverse / Direct acting valves

09/20/2008 12:14 AM

That is exactly what I thought up until a week ago. When we had a few disagreements at work I then checked the Fisher Control Hand Book and on page 174 in that it states, "On direct-acting valves (push-down-to-close action)" and the opposite for indirect acting valves. Again, on page 62 it shows two actuators where a air to push down is direct and air to push up is reverse. Again, the opposite to what I thought!

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#8
In reply to #2

Re: Reverse / Direct acting valves

09/05/2011 7:31 AM

Hello Dear can you send the link for that control valve hand book

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Commentator

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Location: texas
Posts: 92
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#3

Re: Reverse / Direct acting valves

09/20/2008 9:52 AM

Hi, Guest. the question was asked "By valve I mean the body with actuator" now looking at the actuator by it self the book is correct.On a direct acting actuator(air on the top) as air increases the "shaft" will extend out .On a reverse acting actuator (air on the bottom) as air decreases the "shaft" will extend out...regards

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Join Date: Apr 2007
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#4

Re: Reverse / Direct acting valves

09/23/2008 7:02 PM

Direct acting valve : Air to close -- Spring to open --- Failed Open (FO)

Reverse acting valve : Air to open -- Spring to close --- Failed Close (FC)

The key is Air pressure act on diaphragm & spring ...

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Posts: 5
#5
In reply to #4

Re: Reverse / Direct acting valves

09/24/2008 12:13 AM

conflicting answers......

What I take as being correct now is that the term direct / reverse in this day and age usually refers to the control algorithm. I know I asked initially about the valve body / actuator but it is all interrelated. But for a direct controller the operator selection when the DCS is manual results in a 4~20mA signal for a 0~100%. i.e. the current "directly" follows the % selection, 100% being fully open. For Reverse controllers the signal 0~100% results in a 20~4mA signal. This principle is the same for the valve body actuator combination. 3~15 psi (0.2~1bar) results in a 0~100% position for a direct acting valve and for a reverse acting a 0~100% signal results from a 15~3 psig signal.

HOWEVER for actuators only, a direct actuator is increase air to push down and for a reverse acting actuator it is increase air to push up for stem type valves.... This is what I am considering to be the correct use of the terms and is in agreement with our main contractor and client on my present job.

Do you think I am correct or are we all completely wrong!

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Posts: 12
#6

Re: Reverse / Direct acting valves

04/02/2010 6:19 AM

Well about the acutators and valves, i belive both are completely different bodies, although both are connected to control the process

when we refere to the actuator, the reverese and direct acting come from the supply of air pressure. if the air is supplied to open the acuator (it means open the valve), the actuaotor is called direct acting, and similalry if the pressure of air is supplied to close the actuator, it is called reverse acting.

Howver, when you refer to the valve, the reverse and dirrect acting come from the motion of stem of the valve, if the stem (spring) rises to open the valve, the valve is reffered as directing acting valve, and similarly if the stem rises to close the valve, then it is called reverse acting valve. i hope this would have helped you..

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Anonymous Poster
#7

Re: Reverse / Direct acting valves

10/10/2010 12:28 PM

hello guys, as far as the direct and reverse acting actuator is concern, the direct acting actuator is that which closes the valve when air is supplied i.e air to close type and reverse acting actuator is that which opens the valve as air is supplied i.e air to open type. Now for valve bodies the direct acting body is that whose stem when goes down closes the valve and reverse acting body is that whose stem when goes down opens the valve. Now i also have a parting query for you friends can you post any photo which shows that stem moving down open the valve because i am not able to visualise that arrangement. Waiting for reply and Thanks in advance.

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