Unplanned downtime is one of the biggest challenges and expenditures facing processors. The main causes of unplanned downtime include unexpected equipment maintenance and breakdowns. In the chemical and other process industries, the average annual cost of unplanned shutdowns is estimated at 20 billion U.S. dollars, accounting for almost 5% of production. In the petrochemical industry, unplanned shutdowns cause 2% to 5% production losses.
In such a critical process industry, for some reasons, plate and frame heat exchangers are more advantageous than shell and tube heat exchangers. First, plate heat exchangers usually have a larger heat transfer coefficient because they have more fluid agitation (more turbulence). As a result, they can respond to increases or decreases in demand while reducing energy consumption and maximizing heat recovery in the process. Secondly, the plate and shell heat exchanger has less fouling than the shell and tube design, which is due to the greater turbulence in the plate and shell heat exchanger channels.
There are several types of plate heat exchangers available. Traditionally, gaskets have been used in plate heat exchangers, and the industry standard is high-strength shaped rubber gaskets. However, the gasket may cause some problems. The most common are leakage and corrosion.
Conditions that may cause gasket leakage include:
• The board is not installed correctly.
· Pressure fluctuations from the pump or other components of the system.
• The wrong gasket material is used.
·Long-term use in high pressure or high temperature applications.
External leaks can lead to product waste and unplanned downtime. It can also lead to environmentally harmful leaks, which is a safety hazard for facility employees and nearby communities.
The maintenance cost of welded plate heat exchangers is extremely low because there are no gaskets to repair or replace.
Rubber gaskets can also experience stress corrosion failure. All rubber gaskets have the highest temperature, and operation at the highest temperature should be avoided to maintain gasket performance. If the working fluid temperature is operated near or above the maximum temperature for a long time, the gasket will melt or become brittle. This condition can also cause gaskets to flatten, leading to leaks, which can lead to unexpected maintenance costs and unplanned downtime.
Reliability experts estimate that the cost of unplanned downtime is 10 times the cost of planned downtime in the process industry. Although unplanned downtime reduces productivity and profitability, it has an even greater impact on safety and environmental efficiency. An unscheduled shutdown that lasts several hours can cause emissions to be released into the atmosphere for months.
The fully welded plate heat exchanger retains the efficient plate design and the optimized flow rate of the plate heat exchanger with gaskets.
Welded exchangers have advantages
The welded plate heat exchanger is better able to withstand chemicals that can damage gaskets and extreme temperatures. The welded plate heat exchanger is sealed with TIG welds and has no filler metal. The advantage of TIG welded heat exchanger is almost no leakage. They also provide good strength, which exceeds the crushing limit of the gasket many times.
Generally, TIG welding is used in high-tech industries such as aerospace and automotive because of its ability to produce strong, high-quality welds on thin materials. In addition to producing high-quality welds, TIG welding rarely exhibits defects. TIG can weld many building materials, such as stainless steel, acid-resistant steel, titanium and nickel alloys.
At the same time, the fully welded plate heat exchanger retains the advantages of the gasket plate heat exchanger-efficient plate design and optimized flow. The unit is designed for counter-current flow, so that the fluids move anti-parallel to each other in the heat exchanger. This is inherently more effective than cross-flow heat exchangers. The counter-current arrangement creates a more uniform temperature difference between the fluids over the entire length of the fluid path, thereby achieving the best heat transfer.
Welded plate heat exchangers do not use elastic gaskets to seal each plate. Instead, the hot plate is sealed by TIG welding.
In addition to maximizing heat output through counterflow, fully welded plate-and-frame equipment also has good performance in challenging environments and conditions for a variety of reasons.
First of all, the fully welded plate heat exchanger can withstand extreme temperatures and has the ability to withstand thermal shocks. Any of these conditions can damage the gasketed heat exchanger, resulting in unplanned downtime.
Secondly, because there are no gaskets to be repaired or replaced, the maintenance of the plate heat exchanger is minimal. The plate layout of the welded plate heat exchanger can be optimized for this 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 with a small temperature close between the hot fluid and the cold fluid. In any case, pipe connections should be minimized to simplify maintenance.
Third, the welded plate heat exchanger has narrow flow channels and reduced liquid volume requirements. Because of their efficiency, they use less coolant, which can reduce operating costs.
Finally, the welded heat exchanger is small and compact. Compared with the shell-and-tube design, they take up the smallest floor space and are lighter in weight. This can simplify plant expansion when needed.
Case: Welded heat exchanger used in an oil refinery
Welded plate heat exchangers have proven to be reliable and efficient in oil refineries and other critical processes. Two short case studies demonstrate their role in oil refining operations.
More than ten years ago, an oil and gas company installed two welded heat exchangers in a European factory. When running in series, one of the switches is in operation mode, while the other is in standby.
The plant uses the Alkazid process for gas purification. This method is mainly used to treat high-sulfur gas, and then perform other steps for more complete purification. The maximum hydrogen sulfide content in the gas processed by this process is 10%, and the removal amount of hydrogen sulfide is between 0.07% and 0.10%.
The countercurrent flow of the heat exchanger reduces the temperature distribution between the fluids to 1 Kelvin, and the device effectively withstands the high pressure difference. The narrow temperature curve can save energy on the heating side of the regenerator and in the cooling water circulation.
Managers value the reliability of the heat exchanger and the fact that the possibility of cleaning and maintaining the equipment is extremely low. They also like the compact design of the welded heat exchanger, which they say is beneficial for future upgrades and the company’s offshore platform.
Sulfur recovery .
In another European refinery, three welded plate heat exchangers are installed in a sulfur recovery unit. Two of the three heat exchangers are running, while the third is still in standby mode.
Before being activated, these devices must meet a number of specifications, including those established by the American Society of Corrosion Engineers (NACE), the American Society of Mechanical Engineers (ASME), the European Pressure Equipment Directive (PED) and the American Petroleum Institute (API). In addition, TIG welding that does not use filler materials must meet the welding specifications for special items in the wet H2S service to achieve the highest safety level of the refinery.
The management is satisfied with the performance and efficiency of the welded plate exchanger. Its fishbone design is particularly effective for dilute/rich amine exchangers, and the vertical plate design with edges produces excellent heat recovery.
All in all, European factories have trusted the production of welded plate heat exchangers for many years. However, American companies have been slower to understand performance and earnings.
Process equipment, especially those with critical processes, such as chemical, pharmaceutical, and oil and natural gas, should consider the use of welded plate heat exchangers. Due to their heat output and minimal energy consumption, they are reliable and durable heat transfer methods for industrial applications.
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