Hi,

In one of my applications there are are ESD valves with limit switches. These limit switches will be connected to ABB DCS. These inputs will be line monitored since they are ESD valves. I have following questions:

1. What is meant by monitored inputs? How are they defined?
2. What kind of hardware arrangement is required on field instrument side?
3. Is any special software required on DCS side for this?

Thanks in advance.

 

There are four possible conditions associated with a filed contact:

* Open contact
* Closed contact
* Open wiring
* Shorted wiring

Generally an ESD system or DCS can't differentiate between open contact and open wiring or closed contact and shorted wiring. Some ESD systems such as Triplex Trusted (or PlantGuard from ABB), by adding line monitoring devices, can set thresholds for the four possible conditions and identify the fault.

I am not sure whether your DCS has this feature. If not, you may have to connect the contact to an anlog input through a divider network of resistors. Size the resistors in such a way that you can set current thresholds for the possible fault conditions. You will have to use comparators in DCS to generate logic signals for these conditions.

 

1st of all there is no need for line monitoring. Why ESD valve limit switches will require line monitoring? Failure of the limit switches is not creating any safety hazard and therefore, no line monitoring is required. Simply take normally closed contacts (NC) and connect to DCS.
Line monitoring will be required for those inputs which are normally de-energised and failure of which will create a safety hazard.

Imagine you have a pressure switch in the instrument air line which will activate the deluge valve. Now if you take a normally closed contact, then a break in the cable will unnecessary activate the deluge, which is not desirable. If you take a normally open contact then if there is a break in the cable, you will not know and when the demand comes your deluge will not operate and will create a dangerous situation. In such a case you need to monitor the cable break and a line monitoring is recommended.

Now your other questions:

1. What is meant by monitored inputs?

-I think I have already explained the reason for the requirement. Line monitoring simply means that the control room equipment will monitor the input for any short circuit or open circuit fault and will be able to detect them.

>How are they defined?
-Not a relevant question.

>2. What kind of hardware arrangement is
required on field instrument side?

- No special hardware arrangement is required. Most of the vendors connects two resistors at field. One parallel with the contact and another in series with the contact. You need an analog input card for this arrangement.
Some vendors use a special line monitoring arrangement consisting of zener diode and resistor. In this case a digital input card can be used.

Be careful that most of the applications you need four terminals at the field contact end instead of two terminals.

>3. Is any special software required on
>DCS side for this?
-No special software is required.

 

>1. What is meant by monitored inputs?
>How are they defined?

Monitored inputs are usually inputs that have at least 4 states that can be distiguished - Signal on, signal off, loop open circuit and loop in short circuit.

A standard 4-20ma loop is capable of doing this but there are other forms for say fire and gas detectors also.

>2. What kind of hardware arrangement is
>required on field instrument side?

There are a few variations but the general principle is that the components adjust the current/voltage sensed. You could dream up a circuit using Ohms law, 2 resistors and a switch on a 2 wire circuit. Proximity switches (solid state) in conjuntion with signal conditioners are another alterative.

>3. Is any special software required on
>DCS side for this?

Whatever does the processing of the different signal levels in the loop just needs to be able to translate between the signal level and the required state. e.g. signal < 3.5mA means short circuit therefore system takes appropriate action, signal > 4mA && < 10mA means signal off therefore take appropriate action.

Hope that helps
David