DCS

Please turn off Caps Lock. Many readers consider capital use to be akin to SHOUTING, and consider it rude.

The position of the valve needs to be determined somehow. If an analog signal is not available then limit switches are only able to detect four possible states:

1. Valve fully closed
2. Valve not fully closed and not fully open, i.e. midway
3. Valve fully open
4. Valve both open and closed at the same time, which means the limit switch setting is wrong.

Is this what is needed?

Yes, by looking at the process variable the valve is intended to modify. Say, for example, the valve were intending to fill a tank. The logic goes something like:

• Open valve
• Start timer
• calculate the change in level over time
• bound the change rate within low and high limits, and alarm if outside those limits.

Further thoughts:

It would be unusual to configure an analog control system to maintain a certain valve position using analog position feedback, for that is what the valve-top positioner is there to do. It is more usual to maintain a process variable by modulating the valve position so as to take into account all the process conditions that affect the process variable.

Limit switches can be very useful for route-proving arrangements where a fluid has to leave a number of sources and arrive at a number of destinations using only a low number of prime movers. It it is akin to the arrangements for modern railway signalling, in many respects.

Hi,

I suppose u are tatking about conventional 4 -20ma Signal. In that system we need to consider the limit switch to know the Valve open & Close status. But-----

Here quostion is asked about Foundation Fieldbus Positioner...which is different then the conventional positioner.

In FF positioner u will get Valve status detail without connecting any externallimit switch. U will get Valve "Open" & "Close" status also u will get information about the intermediate position also with out adding any external instrument. One more thing all these type of data will be communicated to DCS via Single Twisted Pair cable. U don;t have to consider any extra cable also.

Many actuators have positioning slidewires.

http://hpsweb.honeywell.com/Cultures/en-US/Products/Instrumentation/ActuatorsHercuLine/default.htm

By "FF", I assume that you mean "Foundation Fieldbus".

To answer your question, "is it possible for the DCS to read the open and close position of the valve without taking separate inputs from the valve with the help of limit switches", the answer is "yes".

Just because you had to ask this question, I can tell that you've never seen the data that is available from a Fieldbus positioner.

Fieldbus instruments have lots of data that you can use. In a valve positioner, one of those data is the actual position of the valve as read from the feedback arm. There's no need for external limit switches or separate analog feedback signals.

Good..

I am 100% agree with you..