Unplanned downtime is one of the biggest challenges and greatest expenses for processors. Principal reasons for unplanned downtime include unexpected equipment maintenance and failure. In process industries such as chemicals, the average annual cost of unplanned downtime is estimated at $20 billion, or almost 5 percent of production. In the petrochemicals industry, unplanned downtime is responsible for production losses of 2 to 5 percent.
In such critical process industries, plate-and-frame heat exchangers offer advantages over shell-and-tube designs for a few reasons. First, plate heat exchangers typically have a larger coefficient of heat transfer because they have more agitation of the fluids (more turbulent flow). As a result, they can respond to an increase or decrease in demand while reducing energy consumption and maximizing heat recovery in the process. Second, plate heat exchangers see less fouling than shell-and-tube designs due to higher turbulence inside the channels of the latter.
Several types of plate heat exchangers are available. Traditionally, gaskets have been used in plate heat exchangers, and the industry standard is a high strength, form-shaped rubber gasket. Gaskets can pose several problems, however. The most common are leakage and corrosion.
Conditions that can cause gasket leakage include:
External leakage can lead to wasted product and unplanned downtime. It also can result in environmentally dangerous spills, which are a safety concern for facility employees and the neighboring community.
Stress-corrosion failures also can occur with rubber gaskets. All rubber gaskets have a maximum temperature, and operating above it should be avoided to maintain gasket performance. If the operating fluid temperature runs near or above the maximum temperature for an extended period of time, gaskets can melt or become brittle. This condition also can cause the gaskets to flatten, leading to leakage, which causes unexpected maintenance costs and unplanned downtime.
Reliability experts estimate that unplanned downtime costs 10 times as much as planned downtime in the process industries. While unplanned downtime decreases productivity and profitability, the impact on safety and environmental efficiency can be even more damaging. A single unplanned shutdown that lasts hours can lead to the release of months’ worth of emissions into the atmosphere.
A welded plate heat exchanger is better able to withstand chemicals that can damage gaskets and extreme temperatures. Welded plate heat exchangers are hermetically sealed with TIG welding seams, without filler metals. Benefits of TIG-welded exchangers include virtually no leaks. They also offer good strength, exceeding many times the crushing force limit of gaskets.
Frequently, TIG welding is used in high tech industries such as aerospace and automotive due to its ability to produce strong, quality welds on thin materials. In addition to producing high quality welds, defects are rare using TIG welding. Many construction materials such as stainless steel, acid-resistant steels, titanium and nickel alloys can be TIG welded.
At the same time, fully welded plate heat exchangers retain the benefits — efficient plate design and optimized flow — of gasketed plate heat exchangers. Units engineered for countercurrent flow are designed such that the fluids move anti-parallel to each other within the heat exchanger. This is inherently more efficient than a crossflow heat exchanger. The countercurrent flow arrangement creates a more uniform temperature differential between the fluids throughout the length of the fluid path, thereby enabling optimum heat transfer.
In addition to maximizing thermal output via countercurrent flow, fully welded plate-and-frame units perform well under challenging circumstances and conditions for a number of reasons.
First, fully welded plate heat exchangers can withstand extreme temperatures and are resistant to thermal shock. Any of these conditions can damage a gasketed heat exchanger, leading to unplanned downtime.
Second, maintenance is minimal for welded plate heat exchangers because there are no gaskets to be serviced or replaced. The plate arrangement of welded plate heat exchangers can be optimized to the process. This is true whether it is a single-pass arrangement with all inlet and outlet nozzles on one side, or a multi-pass arrangement to achieve small temperature approaches between the hot and cold fluids. In any case, piping connections are minimized to simplify maintenance.
Third, welded plate heat exchangers have narrow flow channels and reduced liquid-volume needs. Due to their efficiency, they use less coolant, which can decrease operational costs.
Finally, welded heat exchangers are small and compact. They take up minimal floor space and are lighter in weight than shell-and-tube designs. This eases expand plant expansions when needed.
Welded plate heat exchangers have been shown to be reliable and efficient in refineries and other critical processes. Two brief case studies demonstrate their role in refining operations.
Gas Purification. One oil-and-gas company installed two welded heat exchangers in a European plant more than a decade ago. Operating in tandem, one of the exchangers stays in operating mode while the other is on standby.
The plant uses the Alkazid process for gas purification. This process primarily is used to treat gas of high sulfur content before it goes to other steps for more complete purification. The process treats gas containing up to 10 percent hydrogen sulfide (H2S) and removes H2S to between 0.07 and 0.10 percent.
The heat exchanger’s countercurrent flow achieves temperature profiles down to 1 Kelvin between the fluids, and the unit effectively withstands the high pressure differential. The narrow temperature profile generates energy savings on the heating side in the regenerator as well as in the cooling water cycle.
Management values the heat exchanger’s reliability and the fact that cleaning and maintenance of the units is minimal. They also like the welded heat exchanger’s compact design, which they say is beneficial for future upgrades as well as for the company’s offshore platforms.
Sulfur Recovery. In another European refinery, three welded plate heat exchangers are installed in a sulfur-recovery unit. Two of the three heat exchangers run while a third remains in standby mode.
Before startup, the units had to meet several codes, including those set by National Association of Corrosion Engineers (NACE), American Society of Mechanical Engineers (ASME), European Pressure Equipment Directive (PED) and American Petroleum Institute (API). In addition, the TIG welding without filler material had to meet special project welding specifications in wet H2S service for the highest security level in the refinery.
Management is pleased with the performance and efficiency achieved by the welded plate exchangers. Its fishbone design works especially well for the lean/rich amine exchanger, and the edged vertical-plate design yields excellent heat recovery.
In conclusion, European facilities have trusted welded plate heat exchangers in manufacturing for years; however, U.S. companies have been slower to recognize their performance and benefits.
Process facilities, especially those with critical processes such as chemical, pharmaceutical and oil and gas, should consider welded plate heat exchangers. They are a reliable, durable means of transferring heat for industrial applications due to their thermal output with minimal energy consumption.