Basics of industrial final control elements 16

Prologue:

The articles here which cover the basics of our industrial sensors or final control elements and relevant information may seem to be trivial and primary information in general to most of our esteemed professional colleagues. But these are the basics on which our automation and instrumentation control structure expands upwards. That's the reason it is discussed here to strengthen the base for those who don't have clear idea or for jogging the memory of more professional friends, taking them back in time when we learned these basics. However your continued feedback is important to improve the newsletter articles, and to add missing details where some people find it worth mentioning. Thank you all who are reading the series of articles and providing your valued feedback. One thing I would like to request is that please use the comments section for your input, rather than approaching individually so the concerns are shared among fellow subscribers.

Devices used commonly with industrial control valves

So far we have seen.....

Control valve positioners.

Current to pneumatic (I/P) converters

Volume boosters

Trip relays & Solenoid valves

Next in line of our discussion comes ...

Position transmitters and limit switches

Let's see what is a position transmitter. As the name suggests it just relays the specific position of a pneumatic control valve to a local or remote location. In earlier times pneumatic position transmitters were used which had lever linked to the stem of a control valve and converted the valve travel from zero to full travel in terms of a pneumatic signal of 0.2 to 1.0 bar range or equivalent range in PSI. These are now obsolete items and now in most cases it is an integral part of a smart valve positioners. In some cases still the electronic version is in use where observing the valve position is important. Like in an anti-surge control loop or other critical loops in process. Similar to the older pneumatic position transmitters, a mechanical link connects the transmitter with the valve stem.

With the movement of the valve from zero to maximum travel, the lever movement varies a potentiometer inside the transmitter. This change in potentiometer resistance is calibrated in terms of valve travel and generates an equivalent electronic signal of 4-20 mA (generally).

The position transmitter can also be used for rotary valves, with different mounting configuration. The full travel calibration at the time of its installation is required however to ascertain proper position feedback from the instrument. In some cases capacitive sensors for relaying the position are also used in these instruments.

Whatever the case, it provides continued monitoring of a control valve position independent of other instruments like a smart valve positioner. It hence can be used as a reference source for checking if the valve is responding to a given signal or not. If configured, it can generate alarms or alert operators if the position of the valve deviates more than a preset value.

Next in line are the limit switches. These are also a very important part of a control valve in some cases. It can be used to detect the valve's full open or closed position. Some times these switches are interlocked with other process operatives to make sure the whole process works in a safe manner.

A limit switch though normally is for indication of full opening or closing of a control valve, but can also be used in some instances to relay a particular position of the valve. Most of the time a micro switch is used which switches on/off when valve is moved to the open or closed position. These switches are also used with rotary type of valves, specially ESD valves in most plants. These indicate locally as well as through status switching if a particular ESD valves opened or closed in a process.

As I have mentioned above, these limit switches sometimes are used in safety interlocks. For example a high pressure line, which has a pressure control valve with the limit switch will allow an upstream ESD valve to open ONLY when this PCV is sensed fully closed by using a limit switch on it, which confirms the valve is fully closed.. This will avoid a pressure shock to the system while starting. When the ESD valve status shows open, then this valve can open or close, otherwise it will remain in its fail closed position.

Many other logical scenarios in safe process working can not be achieved without these limit switches. Though the limit switches can also be used for other equipment, but since we are discussing the final control elements here, so we limit our discussion to how these limit switches are used with control or rotary ESD valves.

There is a wide variety of limit switches in the process industry with many variations as well. Some use micro switches, some use magnetic reed switches, and many other type of industrial switches are in use working on low voltages like 5 or 12 VDC to a higher voltages of 120 VDC. These mainly are depending on the type of process or how the system is designed. Similarly in some cases AC supply is also used, but it is the sole discretion of the engineering and design professionals who are building various processes.

Whatever the case, these switches have two states. Either it is ON or having a closed contact, for one position or it will be OFF or in open state for other position.

Here we will conclude our article and in our next publication, we will have a look on how other basic industrial final control elements use different auxiliary devices along-with them to perform their designated tasks in industry.

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