Why use HART Protocol rather than Fieldbus in some applications?

  • Thread starter Mirza Vaseem Raza
  • Start date
M

Thread Starter

Mirza Vaseem Raza

Why HART Protocol is used in DCS rather than Fieldbus in some applications? While mostly in my plant Fieldbus is used in DCS and HART for ESD (Emergency Shut Down) - PLC. but I've seen HART is also used in DCS in some areas in my plant. Why?
 
W
There are a number of reasons, all of which will get you an argument from somebody, somewhere.

Here's my take.

1. There are about 50 times as many devices that are HART enabled as there are Foundation Fieldbus devices. ANY brownfield plant will have a very large investment in HART that would have to come out and be duplicated with FF.

2. HART connects to the DCS via the same wiring whether it is using the "fieldbus type" data or just the 4-20. FF needs different wiring configurations.

3. HART is easier for technicians to troubleshoot and visualize than Foundation Fieldbus, without a goodly amount of training that many employers don't want to provide.

4. HART works exactly the same way when it is wired or when it is wireless (IEC62591-WirelessHART).

5. The PV and diagnostic data from a HART transmitter is essentially the same as from a FF transmitter. In fact, for years, vendors have often made HART transmitters and added a FF board to them to make them FF.

Walt

Walt Boyes, FInstMC, Chartered Measurement and Control Technologist
Life Fellow, International Society of Automation
Editor in Chief, Control and ControlGlobal.com
[email protected]
 
B

Bob Peterson

As best I can tell it is for several reasons.

1. HART is compatible with the existing wiring in older plants.

2. HART is understood, accepted, and appreciated by virtually everyone involved.

3. Fieldbus costs a lot more to implement, especially for existing equipment.

4. Not much in the way of benefits of Fieldbus over HART for most applications.

5. A lot of instruments are not as readily available with Fieldbus.

It is much the same reasons that few people want to use Profibus instruments.
 
M

Mirza Vaseem Raza

Thank you Walt for the information.

In my plant I've seen HART is mainly used for RTD's for DCS..Why Fieldbus is not used for RTD's for DCS?
 
M

Mirza Vaseem Raza

Why HART instead of Fieldbus in DCS for some transmitters like RTD & Vibration monitoring?

In almost many applications Fieldbus is used for DCS (Invensys Foxboro) but for some applications mainly RTD's HART is used. In my plant HART is used for ESD (Triconex). Please help me ...
 
R
I've found that even though the 1200 baud rate is fairly slow for today's standards, HART does give me the ability to monitor multiple parameters from a single transmitter without specialty cable or modules (other than using a HART compatible analog input). An example, I am monitoring a mass flow transmitter to control the additions of ingredients in a batch application. I can also monitor the temperature and density of that material over a standard single 2/c shielded cable. Since the temperature and density does not change rapidly, receiving the data every few seconds is fine for me.

I can also confirm that the flow signal is scaled correctly by reading the digital value in the transmitter and comparing it to the 4-20 mA signal and the scaling factor in the PLC I am using. Along with the remote calibration capabilities using a handheld device, the value seems to be good vs. the cost of implementation.

Russ K
Surprise, AZ
 
J
HART goes hand in hand with hardwired 4-20 mA. 4-20 mA is used for control. HART is used for device configuration and diagnostics. Usually by handheld field communicator, but it can also be done centrally from intelligent device management software.

The ESD PLC uses 4-20 mA and on/off signals for the safety function, but HART can be used for transmitter diagnostics

FOUNDATION fieldbus (FF) is primarily used in new plants built from scratch. Existing plants that use 4-20 mA tend to be upgraded to AI and AO card with HART since HART-enabled devices and the hardwiring are already in place. There have been cases where very old plants using pneumatics are upgraded to FF.

A new plant can be built with digital FF networking instead of hardwiring. Using Fieldbus enables a completely digital control loop, from sensor to actuator.

HART and 4-20 mA go hand in hand. 4-20 mA is used for control. HART is used for configuration and diagnostics. If there is an issue with the 4-20 mA the technician troubleshoots that with a multimeter. But if there is a problem with HART (or Fieldbus) the technician must be familiar with a computer to do the diagnostics - but many are these days.

FF is used for temperature transmitters (on RTD sensors) all the time. You should do it in your plant too. One of the beauties of Fieldbus is that it is not limited to one signal per pair of wires, so you have 8-channel fieldbus temperature transmitters like the Rosemount 848T which takes the place of 8 conventional transmitters, 8 pairs of wire, and an 8 channel AI card.

The Rosemount 3144P Fieldbus temperature transmitter (accept RTD and other sensors) provides statistical process monitoring (SPM) diagnostics not found in non-FF transmitters. SPM alerts provide process engineers with a better view of what is going on in the process. It may be used to detect hydrate formation in natural gas lines, scaling formation, and thermowells coating etc. This is possible because two-wire Fieldbus devices are not limited to 4 mA so they can use the extra power to do more stuff. Similarly, in a Fieldbus valve positioner you will find performance diagnostics such as continuous friction monitoring etc. like in the Fisher DVC. But it is not just transmitters and control valves. Fieldbus provides digital integration and diagnostics from discrete devices such as two-wire intelligent on/off valves etc. as well as tank gauging systems, gas chromatographs, and process gas analyzers etc.

The price for a Fieldbus device is not that much higher than a hardwired transmitter, and in a new plant you need to lay much less cable using fieldbus and can reduce cabinet footprint, particularly using the new DeltaV H1 card with integrated power where the fieldbus cable lands directly on the H1 card eliminating the need for an intermediate marshalling cabinet. When upgrading an existing plant the 4-20 mA wiring is already in place, so tearing that out and putting fieldbus networking in its place would be more expensive. For this reason Fieldbus is primarily used for new plants where the savings can be achieved.

So a new plant using Fieldbus can reduce wiring and I/O, since on average devices may have perhaps 3 signals each; thus saving 3 pairs of wire and 3 I/O channels per device, and you can put several of such devices on the same pair of wires. But in an existing plant these wires are already there so you won't have any wiring savings in an existing plant and might as well continue to use 4-20 mA, but HART-enabled since HART is compatible with the 4-20 mA wiring. If the existing plant has aging temperature multiplexers for which you cannot find spares (a quite common scenario), it may make sense to replace it with multi-channel temperature transmitters using either Fieldbus or WirelessHART:
http://issuu.com/easterntrademedia/docs/iaa_dec2010-jan2011/search?q=EDDL

If the existing plant is dependent on a single vendor for electric actuators / motor operated valves (MOV) due to the use of proprietary bus protocol, it may make sense to replace with standard open FOUNDATION fieldbus:
http://www.ceasiamag.com/article-7594-thevalveconnection-LogisticsAsia.html

Just about all categories of devices are available in a Fieldbus version, including intelligent on/off valves, gas chromatographs, tank gauging systems, electric actuators / motor operated valves (MOV), and process gas analyzers etc. This covers a vast majority of devices around the plant.
http://www.fieldbus.org/index.php?option=com_mtree&task=search&Itemid=324&ff
bstatus=Registered&ffbmfg=&ffbcategory

However, if a particular category of device, such as turbidity or humidity transmitter is not available, a HART-enabled transmitter can be used instead because systems that support FOUNDATION fieldbus also support 4-20 mA/HART. Thanks to the magic of EDDL (www.eddl.org) diagnostics from 4-20 mA/HART, Fieldbus, and PROFIBUS devices (such as motor starters and drives) integrate into the same intelligent device management software.

<b>Moderator's note:</b> When copying and pasting URLs, please delete any spaces entered by the forum software.
 
HART was developed for enhanced maintenance, not for direct process control. In most applications HART provides additional information and remote access to the instrument but the process feedback is transmitted on the control system protocol (4-20 mA, Fieldbus, etc.)

While it is probably okay to use HART for transmitting non-critical process information (i. e. display only) to the control system, I would never use it for direct process control in any process where failure would create a safety issue. As far as I know, HART has never been tested for use in automatic process control schemes.

> Why HART Protocol is used in DCS rather than Fieldbus in some applications?
> While mostly in my plant Fieldbus is used in DCS and HART for ESD (Emergency
> Shut Down) - PLC. but I've seen HART is also used in DCS in some areas in my
> plant. Why?
 
R
As you pointed out HART is used on your SIL rated safety shutdown system. I don't think Foundation Fieldbus is approved for that yet.
Actually I think it's the 4-20 mA that the ESD uses not the HART portion; it's just there as an extra convenience.
 
J
That's right Roy. The SIS only uses 4-20 mA for safety. HART is not approved for safety so it is only used in the IDM software to manage the devices.

Fieldbus does have an FF-SIF protocol which is TUV type approved but devices are not available yet.

Cheers,
Jonas
 
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