TPT July 2007

110

uly

ugust

2007

5.1 Major system features

Master speed control enables the coordinated increase and

decrease of motor speed references. This speed control is carried

out by always ensuring comparative balance of the set drive speeds

during the rolling process.

An essential element of the system is wall thickness measurement;

gauging of the thickness of the rolled material is provided and the

set of the proper roll gap opening performed. The process control

system analyses the data produced by the wall thickness gauge

located at the extractor stand exit. Based on the results, the mill

mathematical model updates the speed and gap parameter set

points to improve the quality of the rolled pipes.

Impact drop compensation acts to minimise the length

of pipe rolled at speeds that are different from pre-set

values by using auto-adaptive parameters, relying on

real-time calculation algorithms.

The core of the process control is the installation of

hydraulic capsules, which are fully managed by the

system. On each roll cartridge three independent

hydraulic capsules are mounted, one per roll. The main

function of the control system of the hydraulic capsules,

controlled by servo-valves, is to move the stand rolls.

The position of the cylinders and pressure of the chambers

are measured in real time by dedicated transducers and

used by a dedicated high-speed control system, capable

of controlling and maintaining the set position.

5.2 Control system functions

• Synchronised position control maintains the symmetry between

rolls and the rolling centreline, to avoid damages to chocks and

bearings

• Separation force measurement computes the average and

differential values of the rolling forces using the pressure

transducers installed on the hydraulic capsules

• Capsule position control regulates the position of the rolls by

comparing position pre-set and position feedback and set-up

for roll changeover

• During rolling, automatic variation of the roll positioning

takes place in accordance with a calculated rule, in order to

compensate the temperature disuniformity over the tube length

• Impact compensation increases the gap between the rolls

during the entrance of the shell into each stand, thus reducing

excessive thickness on the tube head end. The impact peak

compensation on mechanical components limits the stress

on the roll bearings and reduces the mandrel and rolls

consumption

• A general damage prevention system (automatic roll emergency

opening), based on automatic gap increase, limits the overload

and enables the tube end to be rolled. There is also an

emergency capsules control to avoid strong deformation of the

pipe and consequent damage to mechanical parts, mandrel

and rolls

6. FQM™ integration in the rolling line

A typical material flow, together with the process control installed,

is a central part of the system. The billet, cut into multiple lengths

of the rolling length, are re-heated to 1,280°C in the re-heating

furnace, rotary hearth type or walking beam type.

After the re-heating, the piercing of the billet is undertaken in a cross

rolling mill to form a round hollow shell. The cross rolling mill mainly

consists of two opposite and equi-rotating rolls, suitably shaped in

order to rotate and advance the billet against a plug. The external

deformation of the material is contained by means of lateral rotating

disks or fixed shoes (at 90° in respect to the rolls). The internal plug

determines the internal material deformation.

The cone shape and divergent orientation of the work rolls require

no abrupt change of ovality and twist direction of the material during

rolling. They help achieve the following benefits:

Figure 10

Material flow during rolling in a 5-stand FQM

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Figure 8 and 9

A side view of a 5-stand FQM (top and above)