4. STRUCTURAL CHARACTERISTICS OF NUOVO PIGNONE'S CENTRIFUGAL COMPRESSORS

The first section of this paper has described the various types of centrifugal compressors at present manufactured by N.P.

Let us consider now their various components, particularly as concerns their constructional technology, the sizing criteria and materials used.

4. 1 CASINGS

4.1.1 Horizontally split casings (MCL, 2MCL, 3MCL, DMCL)

Both half-casings are obtained from conventional castings.

The material is chosen depending on operating pressure and temperature, size, gas handled, and regulations provided by API stds.

Generally Nuovo Pignone use Meehanite GD cast iron with 25-30 Kg/mm2 tensile strength and 70 Kg/mm² compressive strength (i.e. best than many steels).

When steel has to be used to cast these casings, ASTM A 216 WCA steel is employed; should the compressor operate at low temperatures ASTM A 352 steel is used in one of its four grades depending on the operating temperature; lastly, we use ASTM 351 Gr. CA15 steel (13% Cr) or Gr. CF8 in case of corrosive media.

The usual test these castings undergo is the magnetic particle inspection. In particular cases, when a more careful check is required, the ultrasonic test is carried out.

Sometimes the radiografic inspection is required; we do not consider it useful as stresses affecting these elements are limited and the flaws surely existing in castings, yet acceptable and not detrimental to such castings, can be displayed in this way.

The latest tendency is to use welded casings, this solution being more advantageous than casting: in this way danger of rejections, repairs etc., is reduced, avoiding to lay models aside, which calls for large room and great care in case of diversified production line such at Nuovo Pignone's, to prevent any damage to them (see Figure 4.1).

4.1.2 Vertically split casings (BCL, 2BCL, PCL)

Both casings and end covers are obtained from forgings so that material might be as homogeneous as possible, hence more resistant, taking account of the high pressures these compressors have to meet with.

ASTM A 105 qr. II carbon steel is generally used for the barrel, supports and end covers: the carbon content applied (0.2-0.25% instead of 0.35 as per tables) is enough to get good mechanical cha- racteristics, thus granting also characteristics of weldability.

Alloy steel with higher mechanical characteristics is used for compressors running under very high pressure (BCL/c, BCL/d).

Suction and discharge nozzles are welded to the casing, generally forged in the same material; as to PCL's, owing to their complicated structure hence not suited to be forged (see pict. 4.2), another material is used for casting.