Hi, we're a knife gate valve manufacturer. We usually sell valves with positioners 4-20 mA. We send the valve configured with:
- 4 mA open valve (in a knife gate rod inside the cylinder)
- 20 mA closed valve (in a knife gate rod extended)

Is this the standard for programming PLC for process control or is it more usual to use 4 for close and 20 for open?

Is it a bit headache to change in the PLC if they see that the open and close positions are changed?

Regards,
Manolo

 

No Manolo, in the areas I'm involved with it's more to do with what is the safest valve position if components in the loop fail (wiring, p/s, modules). Its easy to change in a decent PLC, but that does'nt make it safer.

Hugo

 

I would say that about 80% of valves would be fail closed i.e. 4 mA = closed, 20 mA = Open.
It would be best if you have the option of changing the valve action to suit application.
Having said that it's not common to use a gate valve in throttling service, perhaps for low pressure drop water applications.

Roy

 

Ah You have stumbled upon something that not many think about (any more). All should but in todays downsized world with retirements and knowledge walking out the door this is overlooked. Should never BE overlooked but is none the less.

Failsafe mode (fail open)(fail close). and opeating mode "air to open", "air to close"......ie 4-20 to open or 4-20 to close.

You have to evaluate each situation for what will happen if air is lost and what will happen if electrical is lost...........is the valve spring return close or spring return open or no spring......and on and on.

In other words there is no answer to your question, although "normal" is fail shut and 4-20 to open (4mA closed,20mA open).

Dave Ferguson

Control Systems Engineer

 

Minalo,

Many other system use 0-20 mA or 0-10 VDC. As you may already know they are not quite common due to "live zero". How you distinguish the real zero and wire break ?

Most of common PLC terminal use 250 ohm resistor. Therefore, 4 mA equal to 1 Volt and 20 mA equal to 5 volt. This 5 volt value is the common supply for PLC terminal.

There is no specific standard says about this 4-20 mA rule.

You can change the rule easily by using PLC. You may consult to process engineer to know what is the desired safe state. open or close?

If you have forward PID, 4 mA is used to close the valve. In reverse PID, 4 mA is used to open the valve.