Page 29 - Compression Machinery for Oil and Gas

Equipment Overview Chapter 2 19

Table 6-1
Comparison of prime movers*
High-speed diesel Low-speed diesel
Electric motor (900–1800 rpm) (400–600 rpm)
Initial cost, $/bhp 100–150 175–250
20–30
Fuel rate
(energy rate +
demand)
A. No. 2 diesel Ib/bhp/h 0.48 0.59
B. Gas, BTU/bhp/h
0–10,000 10–12,000
Maintenance,† $/bhp/yr 0.75 25–45 5–10
Efficiency,‡ % 95
35 30
Speed, rpm 1200/1800/3600
900–1800 400–600
Availability on load base, %
99.9 90 99
Time between overhauls, h 100,000
30,000 75000
Time between major
inspections, h
25,000 4000 20,000
*1000 to 5000 bhp in base-load operation. †Maintenance includes all parts and contact labor (excludes
fuel) for U.S.A. onshore. ‡Turbine overall efficiency can be doubled or tripled by applying heat recovery.
Source: Oil & Gas Journal 16 February 1981, p. 87
FIG. 2.3 Comparison of prime movers [3].
interstage cooling of the process gas can achieve much higher overall compres-
sion ratios.
Single-stage centrifugal compressors are typically designed to operate with
compression ratios in the range of 1.1–1.4.
Gas Type (Sour Gas, Wet Gas, etc.)
Many natural gas streams carry contaminants in varying amounts warranting
careful consideration to the design, installation, and maintenance of adequate
gas cleaning facilities upstream of compression equipment to adequately pro-
tect them from serious problems resulting from operating with dirt, wet, or sour
gas. Wet or dirty gas entrained with contaminates in reciprocating compressors
can result in compressor valve damage and accelerated wear of compressor pis-
tons by wiping lubrication from critical areas of the compressor. Worst case,
entrained liquids could fill the compressor cylinders resulting in serious damage
of compressor rods, bolting mechanisms, or catastrophic failure of the unit.
Contaminants in centrifugal compressors can quickly cause erosion of impel-
lers, volutes, diaphragms and shaft, and pressure seals. For all of these reasons
gas contaminants must be removed to avoid efficiency losses, breakdowns, and
high maintenance costs.
For wet gas (primarily water), a well-designed system consisting of a com-
bination of separators, scrubbers, filters, and gas dehydrators are the industry
standard to provide a palatable gas quality upstream of the compression process
so as to not risk wet gas detriments. “Wet gas” can also refer to unprocessed gas
streams containing higher quantities of hydrocarbon liquids that require more
advanced processing and treatment equipment to process out or “strip” these
liquids prior to the gas compression process to reduce fallout of liquids in
the compression cycle.