Categories
On site cleaning of plate heat exchanger
When there are fibrous and large particles in the heat exchanger, the effect of backwashing is quite obvious. The purpose of backwashing can be achieved by one of the following two ways:
Jul 5th,2021
316 Views
When there are fibrous and large particles in the heat exchanger, the effect of backwashing is quite obvious. The purpose of backwashing can be achieved by one of the following two ways:
(1) Flush the unit with clean water in the opposite direction of normal operation.
(2) Lay out the pipeline and set valves on the pipeline, so as to work in reverse mode at the product side at a fixed time. This special mode is especially suitable for the heat exchanger whose product is steam.
(3) When the water flow contains a considerable amount of solid or fibrous substances, it is recommended to install a mesh filter on the water supply pipeline in front of the heat exchanger. This reduces the number of backwash.
Principle of on site cleaning
(1) Never open the heat exchanger when it is still hot, under pressure, carrying liquid or in operation.
(2) Water must always be used for flushing operations（ The water should be salt free, sulfur-free, chlorine free, or low in iron ion concentration).
(3) If steam is used as sterilization medium, the steam temperature of NBR gasket should not exceed 120 ℃ and EPDM gasket should not exceed 140 ℃.
(4) If chlorine containing solution is used as cleaning medium, the minimum concentration solution should be used at the lowest temperature as far as possible. The time for cleaning the plates with this solution should be as short as possible. The concentration of chlorine in the solution should not exceed 100 × The temperature of the solution must be lower than 37 ℃, and the contact time between the plate and the solution must not exceed 10 min.
(5) Concentrated cleaning solution must be added before water circulates through the unit. Never inject these solutions during water circulation.
(6) A centrifugal pump must be used to keep the cleaning solution circulating.
(7) Do not use hydrochloric acid to clean the plate.
(8) After cleaning the plates with any type of chemical solution, the plates must be thoroughly rinsed with clean water.
Precautions for plate cleaning
(1) Do not use hydrochloric acid or chloride with a concentration of more than 300 × Wash stainless steel plate with 10-6 water.
(2) Do not use phosphoric acid or sulfuric acid to clean titanium plate.
(3) Generally, the concentration of cleaning solution should be below 4% (except for special cases), and the temperature of cleaning solution should not exceed 60 ℃.
003. How to maintain the plate heat exchanger in use
1. When the plate heat exchanger stops running in winter, the medium inside the equipment should be discharged to prevent the equipment from freezing. Other anti freezing measures can also be adopted.
2. The aged gasket should be replaced. When the plate and gasket fall off, use thinner to wash off the adhesive of the plate sealing groove. After cleaning, apply suitable adhesive to the sealing groove and re bond firmly.
3. When assembling / installing the plate heat exchanger, the plates and sealing gasket shall be kept clean, the plates shall be placed in order, and the product assembling / assembling diagram shall be followed. When pressing, the stud should be pressed diagonally, and the stress should be uniform, so as to prevent individual stud from being too tight to damage the stud and plate.
4. The damaged plate should be replaced in time. If there is no spare plate, two adjacent plates can be removed if the operation permits (the two plates should not be reversing plates but plates with four holes), and the clamping size should be reduced accordingly.
5. After long-term operation of plate heat exchanger, the surface of plate will produce different degrees of scale or sediment, which will reduce the heat transfer efficiency and increase the flow resistance. Therefore, the equipment should be disassembled regularly according to the actual situation of water quality, temperature, medium characteristics, etc. The plate can be cleaned with sodium carbonate solution and brown brush. Do not use a metal brush to avoid scratching the surface of the plate and reducing the corrosion resistance. For a small amount of sediment deposition in the channel or debris deposition in the medium, it can also be used for short-term reverse operation in the opposite direction to flush out part of the sediment, which also has a good effect.
004. Main failure factors of plate heat exchanger
As a kind of heat exchange equipment, plate heat exchanger is widely used in many fields of our country, and has brought great economic benefits. However, all kinds of failures and damages of plate heat exchanger also cause certain economic losses to users. The reasons are various. It is related to the short application history of plate heat exchanger in China and the lack of experience in design, use, maintenance and maintenance.
1 seal failure
1.1 pressure
The leakage of detachable plate heat exchanger when it is used within the rated working pressure is mainly related to the abnormal impact load (water hammer, air hammer) in the system, which is not easy to be observed by ordinary operators, except for the quality factors of equipment manufacturing and assembly. The instantaneous pressure peak caused by impact is often 1-3 times higher than the normal working pressure, which makes the rubber gasket installed in the plate heat exchanger shift and leads to seal failure. Because the heat transfer elements of this kind of equipment are made of stainless steel sheet (thickness is 0.5-0.8), the sealing rigidity is relatively poor and the sealing periphery is very long, so the ability of resisting impact pressure is far lower than that of shell and tube heat exchanger. Preventive measures: ① in view of the structural characteristics of plate heat exchanger, the design pressure of the equipment can be increased by 1.5-2 times according to the operating pressure. ② The impact phenomenon in the system should be avoided as far as possible. ③ In the case of such accidents, the thickness of the plate can be increased according to the situation, and the effect is obvious.
1.2 temperature
Rapid changes in temperature can also cause seal failure. When the temperature changes too fast, the linear expansion coefficient of the rubber gasket does not match with the elastic deformation and the sealing preload, which reduces the sealing preload and causes the pressure bearing capacity of the equipment to be lower than the rated design pressure. To solve this problem, the following methods can be adopted: 1. When operating the equipment, the pressure rise and temperature rise should be slowed down as far as possible. ② Compression pre tightening spring can be designed on the clamping stud to effectively compensate the change of sealing pre tightening force and prevent leakage.
2. Equipment blockage and scaling
2.1 blockage
The gap of flow passage of plate heat exchanger is small (2.5-6 mm), and the particles with diameter larger than 1.5-3 mm are easy to block the plate passage, which makes the pressure drop of equipment worsen sharply and leads to equipment failure due to blockage. According to the need, the coarse filter or backwash device can be set at the medium inlet, which can effectively prevent the equipment from blocking.
2.2 scaling
Scaling can reduce the heat transfer coefficient of heat transfer equipment, and even block the plate channel in serious cases. The countermeasures are as follows: 1) plate heat exchanger should not be used in dirty or easy scaling environment (unless other effective measures are added). ② When using unsoftened cooling water as cooling medium, the operating temperature should be controlled at about 50 ℃ or even lower to avoid the sensitive temperature of calcium and magnesium ions precipitation in the medium. For domestic hot water (bath water), it is necessary to design a special temperature control system to control the outlet water temperature (bath water is not allowed to soften or be treated by other chemical methods), so that the equipment can be used normally.
It has been proved that chlorine can effectively prevent stress corrosion.
3 conclusion
With the understanding of the main influencing factors of plate heat exchanger failure, the correct selection, operation and maintenance of equipment can give full play to the high efficiency and energy saving characteristics of plate heat exchanger safely and reliably, so as to obtain better economic benefits.
