Page 180 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 169

where there is little chance of liquids leaking from the cylinder into the frame.
Similarly, labyrinth compressors are suited to clean dry gas at 12 or higher MW.
They are poorly suited to hydrogen due to the leakage past the labyrinth causing
poor compression efficiency. Vertical reciprocating designs use less floor
space, however the cylinders are close together with poor maintenance access.
Diaphragm compressors originated in the early 1900s. These hermetically
sealed positive displacement machines consist of two systems: a gas compres-
sion system, and a hydraulic system. The gas compression system includes thin
metal membranes, or diaphragms which are clamped between an oil distribution
and gas cavity plate, and process gas inlet and discharge check valves. The
hydraulic system involves a motor-driven crankshaft connected to a reciprocat-
ing piston. The piston pressurizes a hydraulic fluid which in turn causes the dia-
phragm group to sweep through a contoured cavity, thus moving the gas out of
the compressor through the discharge check valve. The diaphragm group
completely isolates the hydraulic fluid from the process gas.
Diaphragm compressors utilize static seals at the outer circumference of the
diaphragm group providing lubrication free, leak free, and contamination free
gas compression. This method of compression makes diaphragm compressors
ideal for processing hazardous and high-purity gases. In addition, they are suit-
able for compression ratios up to 15:1 per stage, whereas conventional recipro-
cating compressors are limited to approximately 3:1 per stage. Diaphragm
compressors can also produce discharge pressures to 100MPa and are capable
of compressing corrosive gases with minimal modifications to the materials of
construction. The diaphragms are generally manufactured of stainless steel
material, although other high alloy materials such as Inconel and Monel can
be used where process gas compatibility dictates their need. Typical operating
parameters for diaphragm compressors range from laboratory scale (1.1kW
motor) to continuous operation processes in production scale plants (up to
190kW motor). Diaphragm compressors used in production scale plants are
typically manufactured in accordance with API 618 [1]).

Advantages and Disadvantages Compared to Other Compressors
Reciprocating compressors are of high efficiency, they have piston rings which
provide effective sealing of the gas plus the heat transfer is low so that the com-
pression efficiency approximates the adiabatic compression cycle. An adiabatic
compression efficiency of 85%–95% is typical of slow-speed compression and
80%–90% typical of high-speed compression (difference is due to heat transfer
is higher in water cooled slow-speed cylinders, lower valve, and gas inertia
losses), in addition, the mechanical efficiency due to the friction losses inside
the cylinder and in the frame and running gear is typical 97%–98% for slow-
speed designs and 95%–97% for high-speed types. Other positive displacement
compressor types such as screw, single screw, and vane tend to have greater
internal leakage and lower efficiency. All positive displacement compressors