1. Introduction

The earliest pumps were waterwheels developed by Romans and Persians to raise water, and then the more sophisticated ‘Archimedes screw’ was invented.

Mining operations (e.g., for coal) in the Middle Ages led to development of the suction (piston) pump. A suction pump works by atmospheric pressure: when the piston is raised a partial vacuum is created in the pump’s cylinder.

Atmospheric pressure outside the pump forces water into the cylinder, where it is permitted to escape by an outlet valve. Atmospheric pressure alone can force water to a maximum height of about 34 feet (10 metres), so the force pump was developed to drain deeper mines.

It was not possible to develop velocities and pressures much greater than those provided by nature until the 19th century, but the invention of pumps brought a vast potential for their application.

Gases and liquids in a pipeline move through the pipeline because a force (pressure) is applied to them. Pressure, and pressure increases, are accomplished through the use of ‘pumps’ and ‘compressors’.

Pressure increases are needed along the pipeline as pipeline pressure decreases as a fluid product flows along a pipeline due to: elevation changes, friction in the product (‘viscosity’), friction between product and the inside wall of the pipeline, offtakes, changes in temperature, etc.. Accordingly, fluids in pipelines will usually need assistance as they progress along a pipeline:

  • gas will not flow without compression;
  • liquids will not flow without pumps.

Consequently. a pipeline transporting a liquid will need pumps to assist flow, and a pipeline transporting a gas will need compressors to assist flow, Figure 1.

Figure 1. Compressor Station.(Photographs copyright and courtesy of Penspen Ltd., UK)

2. Operating Principles

A pump is a device that expends energy in order to raise or transport fluids.

A compressor is a device for increasing the pressure of a gas by mechanically decreasing its volume. Air is the most frequently compressed gas but natural gas, oxygen, nitrogen, and other industrially important gases are also compressed.

Pumps and compressors: what’s the difference?

Gases and liquids are fluids – they can flow. Compressors and pumps help fluids flow.

A pump can move either a gas or a liquid from one place to another. The main purpose of a pump is to move the fluid, and when pumping liquids through a pipe, there is no volume change in the liquid in the pipe.

A compressor compresses (squeezes) a gas into a smaller volume and usually also moves the gas from one place to another. When compressing gases through a pipe, there is a volume change in the gas in the pipe.

You cannot compress a liquid, so compressors will be of no use with a liquid.

The operating principles of pumps and compressors are basically the same: they increase the pressure at the outlet of the component. Gases and liquids are moved through ‘impellers’ in the compressor, or pump. This increases the pressure at the outlet of the component. The amount of pressure increase is dependent on design characteristics of the compressor or pump. In hilly terrain, pressure increases are required more.

3. Pumps and Compressors in Pipelines

The basic components of a mainline natural gas compressor station include: compressor units, scrubber/filters, cooling facilities, emergency shutdown systems, Figure 2. An on-site computerized flow control and dispatch system maintains the operation of the station. Today, most compressor stations are unmanned, and monitored by off-site ‘Supervisory Control and Data Acquisition’ (SCADA) systems.

The three general types of compressors are:

  • positive displacement;
  • centrifugal; and,
  • axial.

The average compressor station in the USA is capable of moving about 700 million cubic feet (MMcf) of natural gas per day, while the largest can move as much as 4.6 billion cubic feet (Bcf) per day [1]

Pumps and compressors have to be driven by motors (‘prime movers’). A driver can be:

  • a ‘reciprocating’ engine (like an automobile);
  • a turbine (like a jet engine); or
  • an electric motor.
Figure 2. Components of a Compressor Station.

Most pumps used in the USA are driven by electric motors, although diesel engines or gas turbines may also be used. Gas turbines and electric motors have become more popular in recent years.

4. Siting of Stations

Pumping stations in the USA are sited at the originating station of the line and at 20 to 100 mile intervals along the length of the pipeline. The actual spacing depends on:

  • pipeline design;
  • topography/terrain; and,
  • capacity requirements.

Compressors are normally located every 40 to 100 miles along the pipeline, again depending on the above factors.

References

  1. Anon., ‘Natural Gas Compressor Stations on the Interstate Pipeline Network: Developments Since 1996’, USA’s Energy Information Administration, Office of Oil and Gas, November 2007.