Common faults and countermeasures in plate heat exchangers: leakage and leakage

Main reasons:

1. Insufficient or off-pressure pressure at L value.

2. Place plates and sealing gaskets upside down.

3. Aging gaskets or improper selection of gasket materials.

4. Poor adhesion of gaskets, foreign objects, or defective gaskets.

Measures:

First, check whether the nuts on the clamping screw are loose and whether the clamping dimensions match the equipment installation drawings. If the nut is loose, the clamping size is generally too large; retighten the nut to see if it is loose and whether the clamping dimensions match the drawings; If leakage persists, open the equipment to check the gasket. If the gasket comes out of the gasket groove, reattach it. If damaged, replace it. If most gaskets are damaged together, be sure to select a sealing gasket made of suitable material.

Common faults and countermeasure faults: liquid mix-in

Inspection method: During normal operation, open the low-pressure side outlet to vent and check for a second type of liquid mixed in.

The main reason

1. Cracks in the plates.

2. Plate corrosion penetration.

Measures: Use the plate inspection method described in section 3.6 to identify problematic plates and remove or replace them. When adding or removing plates, make sure plates A and B are added or removed in pairs.

Common faults and measure failures: increased pressure drop, low heat exchange efficiency

Main reasons:

1. Internal blockage.

2. Scale buildup on the plates.

3. Incorrect plate arrangement.

4. Internal air is not fully vented.

Measures: Do not disassemble for circulation cleaning or disassemble for cleaning; Check and correct disc arrangement errors; Expel the air stored inside.

Methods for descaling tube bundles in plate heat exchangers

1. Manual or mechanical methods

When the tube bundle is slightly clogged or scaled, cleaning is done manually or mechanically using tools such as scraping or wire brushes, and purging is done with compressed air, high-pressure water, and steam. When the pipe is heavily scaled or completely blocked, a tubular flushing drill (also known as a pipe piercing machine) can be used for cleaning.

2. Rinse method

There are two types of flushing methods: the first is countercurrent flushing, usually used during movement or during short stops. The device may not need to be disassembled, but counterflow secondary lines must be pre-installed on the equipment. This method is more effective when scaling is not severe. The second method is high-pressure water gun flushing. Different heat exchangers use different rotating water nozzles, which can be rigid or coiled, with pressure freely adjustable from 10MPa to 200MPa. High-pressure water is used to remove dirt, suitable for pipe intersections, inside pipes, and housings. High-pressure water guns perform well in flushing heat exchangers. Widely used.

3. Chemical Descaling

Scaling in the tube side of plate heat exchangers is mainly due to poor water quality, resulting in coking, sedimentation, and adhesion. To remove scale chemically, the first step is to test and analyze the scaling substances, clarify their properties, and then decide which solvent to use for cleaning. Generally, sulfate and silicate scale is washed by alkaline washing (soda ash, caustic soda, trisodium phosphate, etc.), while carbonate scale is washed with acid (hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc.). For grease and spacing, sodium hydroxide, sodium carbonate, laundry powder, liquid detergent, sodium silicate, and water can be mixed in a certain ratio to form a cleaning solution. Using chemical cleaning methods requires re-piping on site, which is time-consuming.

The basic principle of removing limescale

Dissolving action: Acid solutions easily react with calcium, magnesium, and carbonate scale to form easily soluble compounds that dissolve scale.

Peeling action: Acid solutions can dissolve oxides on metal surfaces. Damage combined with scale. This helps remove scale attached to the surface of the metal oxide. And it fell off.

Loosening effect: For scale mixed with silicates and sulfates, oxides of calcium, magnesium, carbonate, and iron dissolve in acid solutions, causing residual scale to become loose and easily washed away by flowing acid solutions.

Gas stirring effect: After acid solutions react with calcium, magnesium, and carbonate scale, a large amount of carbon dioxide is produced. Carbon dioxide gas during the overflow process. For scale layers that are poorly soluble or dissolve slowly, there is a certain lifting force that causes scale to fall off from the heated surface of the heat exchanger.

Choice of cleaning agent

Currently, pickling is the choice of cleaning agent, which includes both organic acids and inorganic acids. The main organic acids are: oxalic acid, formic acid, etc. The main inorganic acids include: hydrochloric acid, nitric acid, etc. Based on analysis of heat exchanger scaling, process, material, and scale composition, the following conclusions are:

1. The heat exchanger has a small flow area and a complex internal structure, making it difficult to discharge if the cleaning liquid forms precipitation.

2. The heat exchanger is made of nickel-titanium alloy and uses hydrochloric acid as the cleaning solution. It is prone to severe corrosion of the plates, shortening the service life of the heat exchanger.

Through repeated experiments, it was found that formic acid was the most effective choice for cleaning fluids. Adding buffers and surfactants to formic acid cleaning solution improves cleaning results and reduces corrosion of the plates caused by the cleaning solution. Chemical tests on scale samples have shown that formic acid can effectively remove scale. Through acid soaking tests, formic acid was found to effectively remove scale attached to the plates, while also having minimal corrosive effect on the heat exchanger plates.

Measures to prevent scaling in plate heat exchangers

1. Strictly control water quality during operation. Water in the system and the softened water in the softening tank must undergo rigorous water quality testing. Only after passing the water can it be injected into the pipeline network.

2. Throughout the system, in addition to irregular cleaning of the dirt remover and filter, the pipeline network should also be kept clean to prevent heat exchanger clogging.

3. When a new system is put into operation, the heat exchanger should be separated from the system, and after a period of cycling, the heat exchanger should be integrated into the system. This prevents impurities from the pipeline network from entering the heat exchanger.