HART Digital Signal Required
Replacing a flowmeter with an equivalent HART transmitter will not necessarily improve flowmeter performance because an analog signal is still generated by the transmitter and converted by the control system (See Figure 2).
To eliminate the need for the transmitter and control system conversions and the errors associated with them, the control system and HART flowmeter must communicate digitally. Dick Caro, a consultant with CMC Associates in Acton, Mass., and author of the book The Consumer Guide to Fieldbus Network Equipment for Process Control, adds, “Existing wiring can still be used to power the HART transmitters, but the existing control system interface will have to be upgraded to include the capability of HART digital communication with the transmitter.”
HART digital transmission was generally considered too slow to be effective for flow control loops. However, Wally Pratt, the chief engineer with HART Communication Foundation, says, “The new HART C8PSK specification allows C8PSK-compatible transmitters and control systems to operate at higher data rates that are sufficiently fast for most flow control loops.” Caro explains that once this is done, “HART C8PSK-enabled systems can have HART and HART C8PSK-enabled transmitters communicating digitally on the same HART segment wiring.”
Profibus-PA and FF transmitters are digital fieldbuses that do not use analog signals for data transmission (See Figure 3). Each type of transmitter must be used on its own segment with their respective interfaces to the control system. “This means that once a fieldbus is selected for an area, the user is limited to transmitters available for use with that fieldbus,” says Dick Caro.
Not being able to mix fieldbuses on the same fieldbus segment can limit flowmeter selection. Flow measurement performance can be reduced when a superior flowmeter is not available for use with the selected fieldbus. Fortunately, many flowmeters are available with Profibus-PA or FF transmitters. If this is not the case, Profibus-PA and FF segments can be installed to accommodate both fieldbuses, however this will add some complication and expense since most control systems will not accept both fieldbuses.
Can Perform Flow Computations
The capabilities of Profibus-PA and FF transmitters are similar with regard to flow measurement. However FF transmitters can contain standard field control blocks that can be configured to control and perform calculations. This means that FF transmitters such as linearization and pressure/temperature compensation that would otherwise be located in the control system or in a flow computer. Profibus-PA transmitters cannot perform these functions, but they can be performed by the control system.
With field control, flow transmitters can contain proprietary algorithms that may be superior to conventional algorithms. Locating flow totalization in the transmitter can sometimes improve performance because the totalizer block is usually more accurate if since it usually runs more often in the transmitter than it would in the control system.
Flow Computers Inside
Flow measurement systems often use flow computers to incorporate information from various measurement devices, such as from a flowmeter, pressure transmitter, and temperature transmitter to compensate the flowmeter measurement for the operating pressure and temperature.
Hans Kwee, an instrumentation/process control engineer with Gaz de France in the Netherlands, notes, “The performance of flow computer systems would be improved if the measurement devices could communicate with the flow computer using fieldbus.” He laments the lost opportunity because “flow computers are not compatible with fieldbus.”
Rick Heuer, sales manager for the Flow Technology Division of Emerson Process Management in Houston, replies, “Flow computers that communicate with instruments via fieldbus are available for process applications. However, the use of fieldbus in flow computers is dependent upon the availability of a flowmeter that can communicate via fieldbus.” For example, Coriolis mass flowmeters and differential-pressure flow transmitters that communicate via fieldbus are readily available, whereas turbine flowmeters are generally not available. In these applications, flowmeter selection may be limited by the fieldbus communication requirement.
Ed Morrison, a consultant with Automated Services, Houston adds, “Flow computers using fieldbus instruments have the capability of being used for custody transfer in North America.” However, these flow computers must be certified by a third party to ensure compliance with applicable standards. Morrison continues, “No such flow computer has yet been certified for custody transfer.”
The First to Comply
The Merluza off-shore oil platform is located approximately 200 km off the coast of Brazil. Carlos Moura and Nerivaldo MarangĂ£o, technical consultants with Petrobras in Rio de Janeiro, Brazil, say, “Using fieldbus transmitters to retrofit this platform reduced the down time and its associated lost revenue.” The window for equipment being out of service averages only about 4 hours per day and could be less. They continue, “Using fieldbus transmitters helped Merluza to be the first Brazilian offshore platform to comply with ANP (Brazilian National Petroleum Agency) standards for custody transfer.”