When to use I/P and why 5

[adiva]

I am just learning more details on control valves and its components. Since now I'm starting to see the real objects instead of on paper (P&ID), I've noticed things that I never thought of before such as:

1. Positioner on control valve. My understanding is so it's there to release/add air to the actuator so the stem/valve opening position will match with the controller output. (please correct me if I'm wrong) When do we know to add a positioner to a control valve?

2. Some control valve has a positioner that has: instrument goes to the regulator, then from regulator to the positioner, then to the actuator. But I also saw control valve that has an I/P before the regulator, then from regulator to the positioner, then to the actuator.
When do we need/know to add I/P? Is it depends on what kind of signal the controller put out (current or pneumatic)? But that also back to my question, when & why use I/P.

3. Do all I/P also need a valve with a positioner?

Thanks in advance

 

[CJKruger]

adiva,

Here's my take on it, but I assume others will correct me.

1) Without a positioner
Suppose you send a signal of 50% (range 0-100%) from control room. Wire to field carries 12 mA (range 4-20 mA). I/P converts it to 9 psi (range 3-15 psi). Puts 9 psi on valve diaprahm and valves open to anywhere from 0% to 100% (you hope for 50%, but don't know for sure).

2) With positioner
Same as above, except I/P converst to 9 psi and put that on the positioner. The positioner see you want 50% valve opening. Positioner then put IA pressure (range 0-25 psi) on the diaprahm till the valve opening is 50%.

3) I/P location
I think you get positioners that can take the 4-20 mA signal directly.

 

[dcasto]

A valve positioner was never my favorite item, it requires calibration, maintenance, and cost and what did I get, oh maybe a 1 psi closer approach to set point or something.

The real benefits of a positioner is that you can send a 3 to 15 psi signal to the positioner and get a 6 to 30 psi out if you need it. The positioner can be feild changed from forward to reverse so you don't have to mess with controller. The positioneer can "speed up or slow down" the valve responce if needed. Finally, the positioner will compensate for hysterisis on a valve caused from the newer very tight packing the is used to reduce VOC's.

On simple loops like level controls I don't specify postioners unles it has some exotic packing, a little over/under shoot on the level isn't always bad. Stabil system that have very slow changes, again no positioner. (most vales and systems can have a positioner added in the field for a few bucks (not including the positioner itself).

The I/P is just the interface between the electronic signals and pnuematic. It can be a seperate device or built into the positioner. Some I/P can be calibrated such that a 4 - 20 mA signal is 3 to 15 psi or 6 to 30 psi or even 4.87 to 31.6 psig, whatever you want.

You use I/P or P/I to interface any electronic device with a pnuematic device. A PLC with a air operated valve. A3 to 15 psi output device with a PLC.

 

[bimr]

Just to add the the preceding.

Air is used as the motive force to turn the valve since air is powerful, inexpensive, and reliable. You can also use electrically operated valves instead of pneumatically operated valves. In the past, electrically operated valves were more expensive and less reliable, but they have improved over the years.

The basic valve is an on-off type of function. Adding a positioner along with a controller, allows the valve to operate partially open or closed.

 

[adiva]

Thanks for all of your resonses.

As someone mentioned above, that "i/p is just the interface between the electronic signals and pneumatic." So, is the answer to the "when do I need I/P" is really: depends on what kind of actuator we select and what kind of signal the controller put out?

A controller that puts out a PNEUMATIC signal WON'T NEED an I/P if it feeds to a pneumatic actuator.

A controller that puts out an ELECTRONIC signal WILL NEED an I/P if it feeds to a pneumatic actuator.

And the control systems engineer or the electrical engineer who decides what kind of controller is used and the type of signal it puts out?

 

[Oldliar]

Your I/P converter converts I (current) into P (air signal). So, when the system (PLC) gives a analog output based on 4-20 mA and you have a pneumatic operated valve, you use the I/P converter to convert current into air (typical 3-15 psi). A valve positioner can be pneumatic (and then you need the I/P converter between the control system and the valve positioner) or you can have the I/P converter ingtegrated into the valve positioner (such as Fisher FieldVue). It would be your control system engineer deciding, as your electrical engineer typically doesn't care about the 24 volt loops (instrumentation power).

 

[JimCasey]

In Olden days, some valves were operated with a straight pneumatic 3-15psi signal. Valves that needed more force got a different spring pack and were given a 6-30 psi signal.
This got to be a lot more complicated than it appears because they wanted the valve to respond to 3-15 IN SERVICE, but when calibrating the valve on the bench without the process pressure in the valve the bench range had to be different. If the valve was unbalanced and intended, for example, to be installed with a 250 psi system pressure, the bench range might be vastly different than the desired installed range because of the process forces on the installed plug. Globe valve companies that use diaphragm actuators still use vast and complicated spring charts to allow the matching of the spring to the application. That works until the process pressure changes and then the installed range changes. It makes Mr. Calibration unhappy.

Introduce the positioner. This is a pneumatic amplifier with a feedback from the valve position. Give a positioner a signal, and it adds or bleeds air from the valve actuator until the valve position corresponds to the signal. In spite of almost every valve spec I have ever seen, the output of a positioner is 0 (zero) to full supply pressure. The output of a positioner is NEVER 3-15 psi, although it is frequently specified that way. The open loop gain of a positioner is somewhere between 100 and 1000, meaning that for a 1% change in signal, the output of the positioner will go to as much as full supply pressure and the valve will be brute-forced to the desired position regardless of friction, change in process pressure, or other extenuating circumstance.

When electronic control became available, transducers were created to convert the electronic signal (usually 4-20 madc) to the pneumatic signal required at the valve ( 3-15 psi). Although some operators persisted in trying to run this 3-15 into the valve actuator without a positioner, the more productive use of the transducer was to load its 3-15 output into the signal port of a valve positioner.

Combined positioners evolved shortly thereafter incorporaing an integral I/P transducer into the positioner. It's a much cleaner installation. There is only one air supply required, one conduit connedtion required, and only one device to fail instead of two.

In the last few years, true digital positioners have evolved. The signal comes in and is handled digitally. The positioner opens or closes discrete valves to control air to the actuator, so there is no internally intermediate 3-15 pneumatic signal at all. With the digital technology they have incorporated other features such as stroke counters, timers, valve signature genrators, and other information services that are useful for preventive maintenance so you can actually have malfunctions identified before the valve fails.

 

[code1]

I have found out a lot about control valves from Emerson's Book:

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

Regards