ISO 3170 Petroleum liquids - Manual sampling
4 Principles
4.1 To ensure that samples submitted for examination are as representative as possible of the liquid being sampled, the necessary precautions are given. These depend on the characteristics of the liquid, the tank, container or pipeline from which the sample is being obtained, and the nature of the tests to be carried out on the sample.
Two basic manual sampling methods are available:
tank sampling (static sampling);
pipeline sampling (dynamic sampling).
When a batch is received or consigned, either tank or pipeline sampling, or both, may be possible. However, if both methods are used, the two sets of samples shall not be mixed.
4.2 Tank sampling is commenced when the contents of the tank are at rest. The following types of samples are normally taken for analysis:
a) upper, middle and lower samples, or
b) upper, middle and suction-level (outlet) samples.
If tests on these samples show that the contents of the tank are homogeneous, they may be combined, in proportion to the volume that each sample represents, for further tests.
If the tests on these samples show that the contents of the tank are non-homogeneous, it may be necessary to draw spot samples from more than three levels and either a composite sample is prepared for analysis or, if blending would impair the integrity of the sample, each sample is analysed separately and the composition corresponding to the composite sample is calculated. In this calculation, allowance is made for the proportion of the oil represented by each sample. Examples of spot sample positions are shown in Figure 1.
Other methods are a running sample or an all-level sample.
As both these methods only result in a single sample, they cannot be used to assess the homogeneity (or otherwise) of a tank's contents. Running and all-level samples are commonly taken and used to determine the average quality of a tank's contents.
NOTE 1 Safety and environmental regulations may restrict tank sampling operations which can result in the release of hydrocarbons or other volatile organic compounds (VOCs) into the atmosphere. In these circumstances, it will not normally be feasible to use traditional open sampling procedures via an open gauge hatch or gauging access point. If the tank ullage space is pressurized, and/or the tank forms part of a vapour balancing/recovery system, it will normally be necessary to use closed or restricted sampling procedures to avoid de-pressurizing the tank and minimize the consequent loss of VOCs. If the vapour from the tank contents is hazardous, it will also normally be necessary to use closed or restricted sampling procedures to minimize the risk of environmental impact.
NOTE 2 Closed sampling is the process of taking samples within a tank using closed sampling devices under closed system conditions. A closed system exists when the operations do not permit the direct exposure and/or release of any tank contents to atmosphere. Manual closed sampling is therefore normally carried out via a vapour-lock valve, using a closed sampling device that provides a gas-tight seal when in use. In order to ensure that no residual vapour is released from a closed system, special facilities may be provided to displace any vapour held up within the device prior to disconnecting the sampling device from the vapour-lock valve.
NOTE 3 Restricted sampling is the process of taking samples within a tank using a restricted sampling device that is operated via a vapour-lock valve. Restricted equipment is designed to substantially reduce or minimize the vapour losses that would occur during open sampling, but may still allow a small quantity of vapour to escape because the equipment is not completely gas-tight.
4.3 To obtain a representative sample of a batch/parcel transfer quantity being pumped in a pipeline, the sample is drawn using an automatic sampling device in accordance with ISO 3171. On occasions, it may be necessary to take dynamic pipeline samples manually. These are spot samples and may not be representative of the bulk (see 7.4).
