Polyethylene Production: Valve Essentials

Introduction

Polyethylene (PE), A polymer material known for its simple structure and wide application. It is formed by addition polymerization of ethylene (H2C=CH2) and consists of repeated -CH2- units. Polyethylene is currently the largest production of synthetic resin, mainly including low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and other special products. LDPE has high melting point, stiffness, hardness and strength, low water absorption, good electrical insulation, radiation resistance and other advantages, suitable for the manufacture of corrosion resistance, insulation parts. HDPE has good flexibility, elongation, impact strength and permeability, and is an ideal material for making films. Ultra high molecular weight polyethylene, known for its high impact strength, fatigue resistance and wear resistance, is used in shock absorption, wear resistance and transmission components. Therefore, polyethylene is widely processed into films, wire and cable jackets, pipes, fibers, and used in agriculture, electronics, electrical, mechanical, daily use and other fields.

Properties of Main Media in Polyethylene Plant

Overview of Valve Applications

Given that the main mediums in polyethylene equipment are PE and hydrocarbons, which are minimally corrosive, most pipelines are designed for temperatures below 200℃. Valves are primarily made of carbon steel and stainless steel. For pipelines carrying slurry or powder, the inner wall needs polishing. For utilities and auxiliary systems, valve applications are similar to other devices. Below are some unique valve applications in polyethylene equipment:

1. Valves for Powder or Pellet Conveying Pipelines

Two main features of these pipelines are:

• The material flows by gravity or air, requiring low pressure drop across the system. Valves should have good flow characteristics and reduced resistance.
• The transported material is solid, so the system must avoid dead corners where material can accumulate, especially in powder conveying pipelines to prevent blockages.

Therefore, the inner walls must be smooth. Shut-off valves should be full bore ball valves. Gate valves, globe valves, and check valves are not suitable. Slide valves are commonly used at equipment outlets for isolation.

2. Valves for Polymer Slurry Pipelines

Polyethylene plant’s polymer slurry, whether low, medium, or high-pressure, tends to agglomerate and clog pipelines if obstructed. Thus, the inner wall should be polished to Ra3.2um. Valve selection should ensure no dead corners for material accumulation, and the valve bore must align with the pipeline. Full bore fixed ball valves are recommended, avoiding gate, globe, and check valves. For 1/2in nominal diameter, plug valves can be used. Plunger valves are often used in high-pressure slurry systems. For catalyst slurry, the inner diameters of pipes, fittings, flanges, and valves must be consistent, with ball or plunger valves being preferred.

3. Valves for TEAL Pipelines

Triethylaluminum (TEAL) is a cocatalyst in polyethylene plants, highly reactive, self-igniting upon air contact, and burns violently with water. Pipeline system integrity is critical, requiring increased component strength, rigidity, thicker walls, and higher valve pressure ratings to prevent accidents from impacts. Typically, XXS reinforcement grade pipes are used, with welded connections preferred to minimize leak points. Valves should be non-metallic sealed ball valves to control internal and external leakage. The ball valve stem only rotates without moving up and down, easing leakage control. Non-metallic seals ensure good internal sealing. Ball valves should be fire-safe and anti-static, with welded connections. For DN40 and below, three-piece ball valves with sufficient short pipe lengths are used to avoid damaging the soft seal during welding. Strict leakage grade limits are imposed on these valves.

4. Valves for Cr Catalyst Pipelines

The dry catalyst in polyethylene plants is a Cr catalyst, which is highly hazardous and carcinogenic, especially Cr6+. Valve sealing performance is paramount due to the solid powder nature of the catalyst, necessitating wear-resistant valve types. For catalyst preparation and polymer slurry systems containing Cr catalyst, full bore fixed hard-seal ball valves are used, avoiding gate, globe, and check valves, with strict leakage limits.

Conclusion

Selecting the appropriate valves in polyethylene production is crucial for ensuring operational efficiency, safety, and minimizing maintenance. Full bore ball valves and polished inner surfaces are essential to prevent clogging and ensure smooth operation, especially in slurry and powder applications. Valves must be carefully chosen based on the specific requirements of each application, with particular attention to sealing performance, wear resistance, and pipeline integrity.

FAQs

1. What are the primary considerations for valve selection in polyethylene production?
   • The main considerations include minimizing pressure drop, preventing material accumulation, ensuring smooth flow, and matching valve materials to pipeline conditions.
2. Why are full bore ball valves preferred in polyethylene plants?
   • Full bore ball valves provide minimal flow resistance and prevent material build-up, which is essential for slurry and powder transport.
3. How is the inner surface finish of pipelines related to valve performance in polyethylene production?
   • A polished inner surface finish (Ra3.2um) helps prevent material accumulation and blockage, ensuring efficient flow and reducing maintenance.
4. What special requirements are there for TEAL pipelines in polyethylene plants?
   • TEAL pipelines require reinforced components, welded connections to minimize leaks, and non-metallic sealed ball valves for safety and leakage control.
5. How do Cr catalysts impact valve selection in polyethylene production?
   • Due to their carcinogenic nature, Cr catalysts require valves with excellent sealing performance and wear resistance to handle the solid powder safely and effectively.