Page 166 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 3 .8 Compressor Best Practices

Best Practice 3.8Practice 3.8Practice 3.8
Best
Best
Centrifugal horizontal split casing compressor nozzles use of a horizontal split centrifugal compressor with top
should always be located on the bottom of the casing for nozzles will result in internal component removal times
lowest mean time to repair (MTTR). that are 2 to 3 times longer.
Be sure to justify either horizontal split with bottom nozzles or radial
split casing with top nozzles to minimize time to repair (MTTR) during Benchmarks
the pre-feed phase of the project: Since the 1990s, I and Forsthoffer Associates, Inc. have used the fol-
The time to disassemble a horizontal split case with bottom nozzles is lowing best practice for selecting centrifugal compressor casing types.
1 e 2 days maintenance time. It saves 4 e 5 days maintenance time, and therefore increases daily
The time to remove the inner casing from a radial split compressor is revenue for this time, potentially in excess of 10MM additional revenue.
2 e 3 days maintenance time. If the driver is a condensing steam turbine use bottom nozzles since
The time to disassemble a horizontal split case with top nozzles the height of the compressor platform will be set by the condenser
considering piping removal and re-assembly without exceeding size and condensate pump NPSH requirements.
external piping forces and moment limits is 7 days or more For all other types of drivers that are planned to be installed at
maintenance time. ground level, use barrel type compressors with top nozzles e the
capital cost compared to horizontal compressors is approximately
10% increase.
Lessons Learned
In order to minimize construction costs on motor drive,
back pressure steam turbines or gas turbine drivers, the

B.P. 3.8. Supporting Material caused by the process piping or foundation can cause bearing,
seal and rotor failures. Positive containment of the process fluid
In this section details concerning the compressor case and the is accomplished by the design and manufacture of leak-tight
stationary internals will be covered. joints. All casings are hydrostatically tested at 1.5 times their
We will first define the functions of the casing as follows: maximum working pressure prior to shipment. Gases containing
more than 50% H 2 require a gas test at rated pressure, in ad-
- Flow guidance
- Positive sealing dition to the hydrostatic test.
- Full rotor support Casing joints are either horizontal or vertical (radial). Hori-
zontal joints are usually metal to metal, as opposed to vertical
The different types of turbo-compressor cases will be joints, which always incorporate an ‘O’ ring, although some
defined: manufacturers do incorporate an ‘O’ ring in their horizontal
design. In my opinion, metal to metal horizontal joints are
- Single stage preferred to ensure that a leak will not develop if the ‘O’ ring is
- Multi-stage
damaged.
- Horizontal split An effective design feature or modification for horizontal
- Radial joints is to ‘relieve’ the joint. That is, to minimize the joint
- Integral gear
contact area, and therefore increase the joint contact stress by
Nozzle orientation will also be covered. Emphasis will be on machining the horizontal faces in certain areas of the casing
minimal disturbance to the process piping during compressor flange. The machined areas are usually 0.032"e0.062"
disassembly. Casing fabrication types will be discussed high- (0.8e1.5mm) deep. A plan view of a typical relieved casing is
lighting cast and fabricated casings. Casing stresses and de- shown in Figure 3.8.1. The relief can be in either the top or
flections due to internal forces and external loads will be bottom half of the casing joint.
covered. Barrel or radially split casings are used for gases with high H 2
content, or for pressures above approximately 600 PSI. The
advantages of the barrel type compressor are:
Casing functions
- Minimum casing joint area
- Removal of internals with removal of process piping
The major functions of any compressor case, regardless of type
are: The disadvantages are:
- To provide support of all bearing seals and stationary - Special tools are required for disassembly
internals - Additional maintenance space is required for removal of
- To provide positive containment of the process fluid internals
- To provide efficient guidance of the process fluid
When a two-case application uses barrel type compressors, it
Often, the support function of the casing is not mentioned is advisable to design the driver with double shaft extensions for
when stating the major casing functions. Excessive casing forces ease of maintenance.

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