Control Strategy for PCV (Local, Remote, Auto, Manual states)

ChristianCasas,

There are two ways that I can suggest for you to get the position of the valve.

• Install another sensor that will detect the position and send it back to the PLC. This can be done with a potentiometer and a resistance-to-current (R/I) converter. However, seeing as you're in an oil field or refinery, there may be issues with potential explosion triggering sources. Your R/I converter will need to be close to the valve and be supplied with power. This means that you'll have to house it in an explosion-proof enclosure. Next alternative...
• Purchase a positioner that has a 4 to 20mA, position-feedback, output. We use Yamatake positioners and this product has the features you're looking for. You can order them with the pressure regulators built-in. They have a switch on them so that when switched to manual mode, the pressure regulator can be used to control the position (ignoring the 4-20 signal). They can also be ordered so that they have a 4 to 20mA position-feedback. It's perfect for your application. What you need is an AVP301.

Good luck.

Something isn't clear. The valve is being used for pressure control ('PCV'), correct? In which case the measuring device will be a pressure sensor somewhere else, with the control algorithm in the PLC trying to correct this signal with the loop in auto. With integral action in the controller and a bit of tuning, so much the good.

The need for a feedback signal from the valve positioner is not clear. Apart from feeding back the position of the valve so that someone can see it remotely, and verify that it is actually changing position in repsonse to changes in the demand signal, its need is largely academic; the system pressure is being controlled in response to changing process load, not the valve position.

In order to use valve position as the measuring element the relationship between the system pressure and the valve position needs to be known consistently. Chances are that this consistency is very low.

No, what he's doing is that the PLC controls the positioner when the system is in Remote mode. I believe the remote mode selector is a physical selector switch. When the system is put in Local mode, the control valve's position is controlled by a pressure regulator instead of from the positioner. When the system is in local mode, the positioner is only there for the ride. It's not controlling anything.

His problem is that when the system is in local mode, they do not have any information as to what the actual position is.

His method of control is typical in a petrochemical plant or any plant where there may be an explosive atmosphere. They don't want to have a 4-20mA signal being generated at the field as opposed to it being generated at the control room (where intrinsic safety devices are located).