In the process industries, sampling is commonly undertaken for two reasons:
To monitor the production process and maintain quality control of the product.
To test the incoming raw materials and outgoing finished products to ensure that they meet contractual or other specifications.
Therefore, there are commercial as well as technical reasons behind the need for sampling, but in all cases it is essential that the sample is representative of the whole batch or parcel. The technology of sampling has been developed most strongly in the oil industry where huge quantities of crude oil and refined products are transported by ship and pipeline. Traditionally samples have been taken manually from ships' tanks or from storage tanks on shore. It is accepted that this method has its limitations, particularly when sampling crude oil which contains water and other impurities which are not distributed uniformly within the tank.
As a consequence the industry has developed technology for sampling from one or more pipelines throughout the transfer of a cargo or parcel of crude oil. During ship loading or discharge, sampling equipment (which may be permanently installed or may be connected for each operation) generally withdraws small sample "grabs" (see Grab Sampling) from each line to produce a total sample of typically 20 liters. The process is normally computer controlled and the sampling equipment may be powered electrically or pneumatically. The ideal sampling location from the commercial point of view is at the custody transfer point, which for ship loading or discharge is at the ship's manifold. However, that is not always feasible and samplers are often installed in pipelines in refineries, in storage terminals or on jetties.
Several types of pipeline sampling system are available but the most common are in-the-line and external loop systems (see Figures 1 and 2). The former involves a specially designed probe, which is inserted directly into the line and is capable of isolating and removing grabs of about 1 or 2 ml each time it operates. The latter requires a small proportion of the fluid flowing through the line to be withdrawn via a probe and circulated through an external or bypass loop before re-entering the line. The sample to be collected is extracted from the bypass loop.
Three important criteria must be satisfied by any pipeline sampling system:
Adequate mixing must be available in the pipeline to ensure that a homogenous distribution of oil, water, etc. exists across the line at the sampling location. If the natural turbulence generated by the flow is insufficient to achieve this then static or powered mixers may be employed (see Mixers and Mixers, Static).
The rate of sampling should be proportional to the pipeline flowrate. A flowmeter must therefore be installed to monitor the flowrate throughout the operation (see Flow Metering). It is generally argued that a measurement accuracy of ±10% of actual flow rate over the working range is acceptable. The signal from the flowmeter is used to control the rate at which the sample is withdrawn by the grab sampler or from the bypass loop.
The fluid velocity at entry to the sampling probe must be close to the local pipeline velocity. This isokinetic condition ensures that the fluid streamlines are not distorted as they approach the probe and that representative sampling is maintained. To satisfy this condition the external loop of a bypass system must be pumped.
It is important that the sample should be collected in a sealed receiver which does not allow the volatile components of crude oil to escape. Similarly when the sample is being transferred from the receiver and prepared for analysis, care must be taken to minimize the loss of these components.
The design and operational requirements of pipeline sampling systems for use in the petroleum industry are described in considerable detail in the relevant standards, which are listed in the references.
Institute of Petroleum, Petroleum Measurement Manual, Part VI, Sec. 2, Guide to Automatic Sampling of Liquids from Pipelines.
American Petroleum Institute, Manual of Petroleum Measurement Standards, Ch. 8, Sec. 2, Standard Practice for Automatic Sampling of Liquid Petroleum and Petroleum Products.
ISO 3171: 1988. Petroleum Liquids, Automatic Pipeline Sampling.