Pressure, temperature and flow measurements around a heat exchanger can tell you exactly how well it's working, how fast it's fouling and when it needs to be cleaned—if you have the time and resources to pay attention. But as Emerson Exchange attendees observed in Session 1B-4851 on Tuesday morning in Grapevine, Texas, many engineers are too busy to put a high priority on getting and analyzing exchanger data, so plants use more energy, run at lower productivity and even experience unplanned outages due to exchanger neglect.
Such was the case at PEMEX's Cadereyta refinery, in operation since 1979, where combined feed/reactor effluent exchangers on diesel hydrotreating units were fouling unpredictably due to variations in the composition of different feedstocks. The plant's existing system of using an Excel spreadsheet to keep track of exchanger cleaning intervals was not keeping up with the process dynamics.
As Mexico's sole refiner and supplier of oil, natural gas and refined products, the company has long had a lock on its market, but it's also "now making very important changes," said Miguel Angel Camacho, operations coordinator, PEMEX, resulting in "a new culture for a more reliable process, requiring renovation of a number of facilities."
Exchanger fouling in cooler areas of refineries also is an increasing problem in the United States due to the varying qualities of shale oils, according to Gary Hawkins, senior refining consultant for Emerson Process Management.
The Cadereyta refinery has six exchangers on each of two hydrotreaters. "Heat losses due to fouling—in this case, by salts—must be made up by heaters and were costing $400,000 to $2 million dollars a year," said Marcelo Carugo, director, refining and oil terminals, Latin America, Emerson Process Management.
PEMEX wanted real-time monitoring of overall exchanger health, fouling factor and rates, lost energy cost, exchanger coefficients and notification when cleaning was required. The exchangers are equipped with wireless temperature and pressure measurements, and flow is available through an OSI PI database. "Our solution takes the exchanger design data and online measurements, delivers the calculated value and displays the needed information," said Nikki Bishop, senior application consultant, Emerson Process Management.
Heat duty is automatically calculated, which can be done for both the hot and cold sides of an exchanger. The heat exchanger coefficient, or rate of heat transfer, is determined using heat duty and surface area. The application calculates the observed coefficient (corrected for process flow), compares it to an established baseline for the specific exchanger and determines the extent of fouling.
The heat exchanger process display shows temperature, pressure and flow readings, as well as calculated values and a "fouling factor" of 0% to 100%. It also can show the cost associated with the current level of fouling. The fouling factor tells when it might be time to clean the exchanger. If the differential pressure across the exchanger rises high enough, it tells the operator that cleaning is required.
"Readings are highlighted if the fouling rate has accelerated, so you can see that something has changed," said Carugo. "Before, operators had no alert. Now they can see and make a decision about the best timing to clean the exchanger."
The application, Asset Monitoring for Heat Exchangers, is an AMS product, "not a snap-on," added Bishop, and can be used on any kind of exchanger. The algorithms run in Asset Graphics, engineers can set alarms, and maintenance personnel can see results on the maintenance console. It can also be connected to other systems via OPC. She added, "It uses a common dashboard, gives continuous analysis and provides notification when attention is needed."