Things to know 
Plunger, Packing, Lubricant – a complex tribological system in LDPE hyper compressors
3. The tribological system

The structure of the tribological system is formed out of the four elements [3]:
- Friction body 1 (seal elements made of special bronze)
- Friction body 2 (plunger out of solid cemented carbide or with carbide coating layer)
- Intermediate material 3 (lubricant mineral oil or synthetic compounds)
- Ambient medium 4 (ethylene)

Depending on how friction body 1 and friction body 2 are contacting each other, via the intermediate material or without intermediate material, the individual kinds of friction can be defined and allocated. The usable generic term for this kind of friction is

– Sliding friction – absolutely.

From a tribological point of view the following limiting factors co-exist or may occur alternately during normal operation

- Fluid friction

The best case is apparently the complete separation of the friction body surfaces, constituting a closed and persisting lubricating film that remains steady during the entire working period of the tribological system. Thickness from 0.25 up to 20 µm is described depending on system conditions [4].

Picture 3: Fluid friction

- Mixed friction

Even when there is an as yet intact, but very thin lubricating film, both friction bodies may already contact each other in the range of micro geometry. This contact is reduced when there is higher surface quality of both friction partners. From these facts it can be assumed that mixed friction is the main component within the operation of hyper compressors in LDPE plants due to the conditions in the tribological system referred to here.

Picture 4: Mixed friction

- Dry friction

If there is absolutely no lubricant film between both friction partners, contact will occur beyond the micro geometry on the entire friction surface showing results such as abrasion, thermal heating, or melting of the surface areas of each friction body possibly leading to complete deformation of one or both these parts. This state should in principle be avoided, as the friction bodies may be irreparably damaged. In [3] it is reported though, that friction pairs damaged by abrasion can re-stabilized by increased lubricant feed and efficient cooling.

Differences between Hot Isostatical Pressing (HIP) and Sinter HIP Procedure
Differences between Hot Isostatical and Sinter HIP
1. Hot Isostatical Pressure
2. Sinter HIP Procedure Pressure

Plunger, Packing, Lubricant – a complex tribological system in LDPE hyper compressors
1. Introduction Pressure
2. Elementary structure of a plant for high-pressure polymerization of ethylene
3. The tribological system
4. Wear process in the tribological system
5. Practical knowledge from a carbide metal producer’s point of view
6. Other partners of the tribological system
7. Outlook
8. Summary
9. Reference
4. Wear process in the tribological system