Demister Pad - Maintenance, Classification and Specification

Profiles, working principle and features of demister pad

Demister pad, also called mist pad, wire mesh demister, mesh mist eliminator, catching mist, mist eliminator, is used in gas entrained mist separation column to guarantee the filtering efficiency.

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DPZ-01: Working principle of demister pad.

History of demister pad

The late fifties, the foreign used an apparatus which made of the special wrinkle woven mesh to remove inclusions of tiny liquid foam in the gas. Then it is found that is very efficient and the resistance is very small, which is unmatched by other gas-liquid separation device, so it is widely accepted and used in a wide range of applications.

Wire demister standard was formulated by the Ministry of Chemical Industry in China on December 20, , the basic implementation of the Shanghai municipal government standards Q/YSZZ-88-91 wire mesh industry was in , began to fully implement HG/T- national unity standard after the year . The new standard demisters are divided into HG5--81 riser type, HG5--81 necking type and HG5--81 full bore wire mesh device.

Demister pad is commonly placed at the top of the packing tower, which can not only ensure the mass transfer efficiency, but also reduce the board spacing. Demister pad is widely used in chemical, petroleum, pharmaceutical, light industry, metallurgy and other industrial production in the vertical cylindrical equipment vapor-liquid separation device. It can also be used in the desulfurization and other productions to remove oil mist, toxic and poisonous gas.

Working principle of demister pad

When the gas with mist rises at a constant speed and passes through the wire mesh, the rising mist will collided with the mesh filament and attached to the surface filament due to the inertia effect. The mist will be diffuse on the filament surface and the droplet will follow along the filaments of the two wire intersection. The droplet will grow bigger and isolate from the filament until the droplets gravity exceeding gas rising force and liquid surface tension force while there is little gas passing through the demister pad.

Separate the gas in the droplets can improve the operating condition, optimize process indicators, reduce corrosion of the equipment, extend equipment life, increase the amount of processing and recovery of valuable materials, protect the environment, and decrease air pollution.

DPZ-02: Round shape embedded demister pad.

DPZ-03: Circle shape embedded demister pad.

Features of demister pad

• Simple structure.
• Lightweight and easy to transport.
• High porosity, less pressure drops.
• Large surface area and high separating efficiency.
• Easy to install, operate and maintain.
• Durable and long service life.
• Corrosion and rust resistance.

Selected wire mesh form of demister pad and domestic standards

Selected wire mesh form

Selected demister pad forms have four categories: standard type, efficient type, high penetration type and shock absorber type.

Specifications of standard type demister pad Item Wire type Wire size
(mm) Wire width tolerance
(mm) Bulk density
(kg/m3) Specific surface area
(m2/m3) porosity STWM-01 Flat wire 0.1 × 0.4 +10/-0 150 475 0.981 STWM-02 Round wire 0.23 320 Note: every 100 mm thick mat has 25 layers wire mesh.

DPZ-04: Flattened type standard demister pad.

DPZ-05: Ginning type standard demister pad.

Specifications of efficient type demister pad Item Wire type Wire size
(mm) Wire width tolerance
(mm) Bulk density
(kg/m3) Specific surface area
(m2/m3) porosity ETWM-01 Flat wire 0.1 × 0.3 +10/-0 182 626 0.977 ETWM-02 Round wire 0.19 484 Note: every 100 mm thick mat has 32 layers wire mesh.

DPZ-06: Rectangular shape efficient demister pad.

DPZ-07: Round shape efficient demister pad.

Specifications of high-penetration type demister pad Item Wire type Wire size
(mm) Wire width tolerance
(mm) Bulk density
(kg/m3) Specific surface area
(m2/m3) porosity HTWM-01 Flat wire 0.1 × 0.4 +10/-0 98 313 0. HTWM-02 Round wire 0.23 Note: every 100 mm thick mat has 20 layers wire mesh.

DPZ-08: High penetration type demister pad detail.

DPZ-09: High penetration type demister pad.

Specifications of shock absorber type demister pad Item Specification Filament specifications
(mm) STWM-01 33-30 type 0.08 STWM-02 38-40 type 0.08 STWM-03 20-40 type 0.22 STWM-04 30-40 type 0.1/0.12 STWM-05 30-50 type 0.16 STWM-06 40-60 type 0.12 STWM-07 30-80 type 0.22 STWM-08 50-120 type 0.22 STWM-09 40-120 type 0.22

DPZ-10: Shock absorber type demister pad detail.

DPZ-11: Shock absorber type demister pad.

Domestic standards

Wire mesh demister pad domestic standards are divided into HG5- (, ) - 81, HG/T -, HG/T -98.

• HG5-(, )-81
  The standards are divided into the riser-type (), the necking type () and full bore type ().
  The diameter of demister is from 300 mm to mm and height is 100 mm and 150 mm.
  Mark example: HG5--81-11, 304 refers to the 304 stainless steel of grating and wire mesh, dull bore coat type, mm diameter, 100 mm height demister pad.
• HG/T -
  New standard merges riser type, reducing necking type and full bore type installation form and divide into SP, HP, DP and HR four kinds of network types and expanded the type of material section.
  Mark example: HG/T wire mesh demister S-100 SP 304 indicates that 304 stainless steel of gratings and wire mesh, coat type, mm diameter, 100 mm height, SP screen demister grid.

  DPZ-12: SP type demister pad.

  DPZ-13: DP type demister pad.

  DPZ-14: HR/HP type demister pad.

  Type Standard wire specification SP 0.1 mm + 0.03 mm × 0.4 mm DP 0.1 mm + 0.02 mm × 0.3 mm HR 0.23 mm + 0.03 mm × 0.4 mm. HP 0.1 mm + 0.03 mm × 0.4 mm
• HG/T -98 (drawer mesh demister device)
  This standard is appropriate for the demister element, which is often needed to be replaced or cleaned, for example sulfuric acid drying tower and absorber device. The nominal diameter has 56 kinds ranging from DN 30 mm to DN mm.
  Mark example: HG/T .11-98 WME -100 NS-80/Q235-A refers to the wire mesh using NS-80 carbon steel grill, mm in diameter, 100 mm in height drawer type demister pad.
• Other non-standard wire mesh demister
  Wavelike type, embedded type, external loading type, SWD1 type, square, rectangular and irregular and so on.

Using method of demisters

Mesh pad installation

There are two kinds of wire mesh demister pad, which are disk shaped demister pad and bar type demister pad.

According to different using condition, it can be divided into upload type and download type. When the opening is located in the above of the demister pad or when there is no opening but has flange, you should choose the upload demister pad.

When the opening is in the below of the demister pad, you should choose the download type demister pad.

DPZ-15: Upload type demister pad.

DPZ-16: Download type demister pad.

Schematic of demister pad

DPZ-17: Horizontal separation tower.

DPZ-18: Spherical separation tower.

DPZ-19: Scrubber.

DPZ-20: Distillation column.

DPZ-21: Vertical separation column.

DPZ-22: packed tower.

Maintenance of demister pad

After using for a long time, the demister pad will clog or prone to flooding and the pressure drop will increase. All these because of the particles in the gas blocks the demister pad. So you need maintain the demister pad properly and regularly.

Maintenance method: clean the demister pad. As we all know, the dilute acid on the demister pad will shorten the service life of demister pad, so we should place the demister pad in the natural state or under the sunlight