ISO 3171 Petroleum liquids - Automatic pipeline sampling
7 Sampling probe design
7.1 The sampling probe and/or the separating device should be of sufficient strength to withstand the bending moments imposed by the maximum flow in the main pipeline and to withstand the vibrations caused by vortex shedding. Although the actual determination of strength is difficult, conservative approaches tan be made by treating the member as a cantilevered beam.
7.2 The sampling probe should be designed to create the minimum of disturbance to the main pipeline flow. One approach is a pitot-tube type probe entry with a chamfered edge. The entry should face upstream into the direction of the flow.
7.3 lt is recommended that the diameter of the sampling probe opening be not less than 6 mm.
8 Sampler design and installation
8.1.1 Automatic samplers should be designed to procure representative samples of the liquids flowing through pipelines and to store these in one or more receivers. Samplers may be of the continuous or intermittent type.
NOTE - There are two types of intermittent automatic sampling system. One system locates the separating device directly in the main line [see figure 9a)], whereas the other system locates the separating device in a Sample loop [see figure 9b)].
8.1.2 The petroleum liquids to be sampled may contain wax, abrasive particles and corrosive components such as sulfur compounds and water. They may, in certain cases, possess poor lubricating properties. These facts should be taken into account in designing an accurate, reliable and durable sampling system.
8.1.3 The installation of piping and valves between the separating device and the receiver should be so designed that there is no separation of components, such as water and crude oil, at any point. It is desirable to keep the volume to a minimum.
8.1.4 A sample loop usually consists of a circulating loop, a suitable pump and a separating device [sec figure 9b)]. Any connection between the loop and the separating device should be of minimum volume.
8.1.5 The Sample loop entry velocity should be as near as possible to the maximum expected velocity in the main pipeline. If the separating device is not in the immediate vicinity of the sampling probe, care should be taken ensure that the flow rate within the sample loopis high enough to produce fully turbulent flow and thus avoid water settlement.
8.1.6 The inclusion of a device to indicate that there is a flow in the sample loop is recommended. If there is no circulation in the sample loop and sampling continues, erroneous sampling will result.
8.2.1 The separating device and the sample receiver should be installed as close as possible to each other in order to minimize dead volume. Connections should be made with short lengths of pipe consistent with the application, and the number of bends should be kept to a minimum. Where possible, the sample receiver should be at the lowest point in the system.
8.2.2 Where for practical reasons it is not possible to install the separating device and the sample receiver close to each other, then a sample loop is recommended. Precautions should be taken to ensure that both the sample in the sample loop and the sample taken from the sample loop are representative.
8.2.3 Automatic samplers should be installed with suitable valves and connections to enable equipment to be flushed automatically or manually either with the liquid being sampled or with some suitable solvent. Proper arrangements should be made for the disposal of the flushings and solvents used. All solvents should be removed from the receiver and the circulating lines to avoid contamination of the next sample.
8.2.4 If necessary, a non-return valve or equivalent device should be installed in the sampling line to prevent any possibility of the sample being returned to the pipeline from which it was drawn. This should not restritt the free passage of sediment into the receiver.
8.2.5 The method of installation of an automatic sampler should provide easy access for maintenance and cleaning; it may be essential that this tan be carried out without shutting down the pipeline.
8.2.6 The sampling probe should be installed through a stub-mounted full-bore valve and a suitable insertion and withdrawal mechanism should be provided. If the probe has to be withdrawn without depressurizing the pipeline, means, such as safety chains, should be provided to prevent the probe being ejected by the pipeline pressure. If this system is installed, sufficient space should be allowed for fitting the mechanism and withdrawing the probe.
8.3.1 There may be a tendency for water or heavy particles to drop out in the sampler tubing and components, and appear in the sample receiver during subsequent sampling, To minimize this, the system should be free from pockets or enlarged sections and should be pitched downwards towards the receiver. Care should also be taken to ensure that there are no pockets in which vapours or water tan be trapped.
8.3.2 To prevent solidification of high-pour-point crude oils or products, the separating device, associated tubing and components should be heat-traced and insulated. Care should be taken not to over-heat the sample liquid, although the temperature should be high enough to keep the material in the liquid phase to ensure proper operation of the automatic sampler system. Thermostatic temperature control may be necessary if self-limiting heat tracing is not used.
8.3.3 If sampling lines and systems which are not in use are likely to block through cooling and wax deposition, flushing oil should be used to fill such lines and systems.
8.4 Special characteristics
The general recommendations on automatic samplers for crude oil are applicable to automatic samplers for refined products. However, certain products have specific characteristics which may require special sampling conditions. Some examples are given in table 1. In these cases, the automatic sampler should be adapted so that it fulfils the required sampling conditions.