ISO 3170 Petroleum liquids - Manual sampling
6 Safety precautions
Careful consideration shall be given to the nature and known hazards of the products being sampled, which will affect the detailed nature of the precautions to be observed. Guidance on safety precautions is given in Annex A.

7 Procedures for homogeneous petroleum liquids
7.1 Introduction
This clause specifies the general procedures that shall be applied for sampling homogeneous liquids. Additional procedures for sampling crude oil and non-homogeneous liquids are described in Clause 8.

Unless otherwise specified, multiple-spot samples shall be collected using the detailed procedure relevant to the particular application. Typically they are upper, middle and lower; or, upper, middle and suction (outlet) samples.

The minimum number of samples shall be in accordance with Table 1.

NOTE When the liquid level is less than 4.5 m, it may be permissible to take fewer spot samples than shown in Table 1.

7.2 Precautions
7.2.1 General
7.2.1.1 A sample shall not include any product other than that to be sampled and, if it is necessary to transfer a sample from a primary sampler to a secondary sample container, appropriate precautions shall be taken to preserve the integrity of the sample. Thus, steps shall be taken to avoid sample contamination (e.g. by rainwater or perspiration), and the sampling method shall, whenever practical, avoid sample transfer by permitting the sample to be transported to the laboratory in the container it was originally obtained in (the primary sample receiver).

NOTE The transfer of a sample will often have the following effects:
a) loss of light ends (affecting density and vapour pressure);
b) changes in the relative proportions of oil and contaminants such as water and sediment.

7.2.1.2 Sampling personnel shall be fully instructed in the relevant procedures for the particular sampling application. Specific precautions are necessary when drawing samples for certain tests and the correct sampling procedures shall be closely followed to ensure that the test results are meaningful.

These additional precautions do not form part of this International Standard, but should be set out in the test method or product specification concerned.

7.2.1.3 Samples shall not be drawn from unperforated or unslotted still-wells, guide poles or stand pipes since the contents of an unperforated pipe are not normally representative of the bulk contents of the tank at the same depth or location outside the pipe.

Still-well, guide pole or stand pipe samples shall only be drawn from pipes that have perforations or slots that allow the free flow of product into and out of the pipe.

NOTE A row of perforations, typically with a diameter of 25 mm and spacing of 300 mm, or two rows of overlapping slots of similar width are normally sufficient to allow the free flow of the product into and out of the pipe.

7.2.1.4 For handling samples, use sampling equipment, containers, receivers or samplers that are impervious to and resistant to solvent action by the product handled (see 5.1).

7.2.1.5 Thoroughly inspect all sampling equipment, including closures, to ensure that it is clean and dry.

7.2.1.6 Leave a minimum of 5 % ullage in the container to allow for expansion. If spot samples are drawn from a tank, some of the sample has to be removed from the sample container; this shall be done immediately after the sample container has been withdrawn from the tank.

NOTE 1 Decanting to obtain the ullage is not good practice because it may invalidate the representativity of the sample, particularly if any free water or an emulsion layer is present; however, it cannot always be avoided.

NOTE 2 The ullage requirement only applies to fixed volume sample receivers and containers, not to variable volume sample receivers.

7.2.1.7 Immediately after filling and closing the sampler, sample receiver or container, examine it closely for leaks.

7.2.1.8 If large-volume samples are required which may not, because of volatility or other considerations, be obtained by the bulking of smaller quantities, mix the tank contents thoroughly by the means available (e.g. circulation, tank side mixer). Confirm the homogeneity by tests on samples taken at sufficiently different levels, as described in 4.2 and 8.2.1. The container shall be filled using a sample inlet extending to near the bottom of the container, from tank-side taps, or from a circulation-pump bleed-valve sample point.

7.2.2 Samples for special analysis
If samples are taken for the determination of trace elements, for example lead, specially prepared sample containers may be required. Take such samples directly in the prepared container. Ancillary equipment and sampling cord used shall in no way contaminate the sample. Special care is required to prevent accidental contamination (e.g. by sampling cord being wetted with sea water on a ship's deck prior to being used to sample a product, where the sodium content is a critical analysis in the purchasing specification).

If the tests for which the sample is intended include certain specific requirements, such as copper or silver strip corrosion, take the sample in dark-coloured glassware (or containers of other suitable material), and protect the sample from light prior to testing.

NOTE Any other method of obtaining the sample could affect the results of the corrosion test.

If tests for such properties as water-separation characteristics, oxidation stability, existent gum, etc., are required, then take care to ensure that any sample container has been suitably prepared and is entirely free from contaminants such as flux or other chemicals.

7.2.3 Volatile products
7.2.3.1 When taking samples of volatile crude oils and products, if it is necessary to avoid the loss of light ends, for example for determination of density, vapour pressure or distillation, do not transfer, composite or bulk oil from the original sample container(s). Transport and store the sample in an inverted position to avoid loss of light ends through the closure.

7.2.3.2 Depending on the nature and temperature of the liquid, the ambient temperature and the purpose for which the sample is required, some or all of the following precautions may be necessary:
a) passing the sample through a sample-cooler at the point of sampling;
b) cooling the sample container to a suitable temperature before sampling;
c) keeping the sample container cool until it has been sealed;
d) keeping the sample container cool until it is delivered to the laboratory.

Sample containers may be cooled, if necessary, by immersion in a cooling medium, e.g. crushed ice, but care is required if cooling the sample could result in the partial separation of wax and/or other heavy components.

Cooling of such samples may result in wax or heavy components being deposited on the walls of the sample container so that subsequent subsamples may no longer be representative of the total original sample. Crude oil samples should not be cooled below a temperature of 3 °C above their wax-appearance temperature. In instances where the wax-appearance temperature is greater than ambient, it would be necessary to warm an original sample before subsampling.

7.2.4 Tank side and pipeline sampling
If a tank side or pipeline sampling point is used, adopt the following additional sampling precautions.
a) Before taking tank side or pipeline samples, completely flush the sampling line to ensure removal of all previous contents of the line.
b) When filling fixed volume sample containers, the sample line outlet shall be designed to extend to near the bottom of the sample container during sampling. When filling variable volume sample receivers, facilities shall be available to flush the whole sample line volume to the receiver inlet valve.
c) If the product being sampled is volatile, cool the sample container to a suitable temperature and use a sample cooler (see 5.7), if necessary and appropriate (see 7.2.3.2).
d) If the oil being sampled has a high pour point, it may be necessary to insulate the sample line thermally or to provide means of heating the sampling connections in order to prevent solidification.

7.2.5 Labelling and transport
7.2.5.1 Clearly label sample containers; tie-on labels are preferred. Use indelible marking on the labels.
It is recommended that the following particulars be included in those recorded:
place at which sample was drawn;
date;
initials or other identification mark of operator;
description of the product;
quantity represented by sample;
tank number, package number (and type), name of ship;
type of sample;
sampling device or sampler used;
any additional sampling details.

7.2.5.2 If samples are to be dispatched, all appropriate transportation regulations shall be adhered to. Care shall be taken to ensure that any packaging material does not contaminate the sample when it is subsequently opened.

7.3 Tank sampling
7.3.1 Shore tanks
7.3.1.1 Vertical cylindrical tanks
7.3.1.1.1 Spot samples
Lower the sampling device until its opening is at the required depth, open it in the appropriate manner and maintain it at the required level until it is filled. Retrieve it and either decant a small portion back to the tank to create an ullage space before sealing the sample, or carefully transfer the complete sample to a secondary sample receiver.

In hot climate conditions or whenever there is a significant difference in temperature between the sampler and the product being sampled, the sampler should be conditioned to the temperature of the tank contents by gently raising and lowering it through approximately +/- 300 mm for 1 min to 2 min before operating the opening mechanism.

When sampling at different levels, take the samples in sequence from top to bottom, in order to avoid disturbance at a lower level.

In the case of a zone sampler (with essentially full bore top and bottom valves that allow the tank contents to flush through the sampler as it is lowered), lower the sampler in a controlled manner until it is at the required depth. Close the valve(s) as soon as the lowering operation ceases, and retrieve the sample immediately. Carefully transfer the complete sample to a secondary sample receiver.

Where the design of the zone sampler does not permit full flushing during lowering, it is recommended that the sampler be raised and lowered two or three times after reaching the spot sampling location, before closing the valve(s). The raising and lowering shall be through a distance of at least the height of the sampler.

In the case of a top sample, lower the open sampler/container carefully until its neck is just above the surface of the liquid, and then allow the sampler to fall sharply 150 mm below the surface. When the sampler is full, as indicated by the cessation of air bubbles, withdraw it and proceed as for regular spot samples.

7.3.1.1.2 Composite samples
A composite sample may be prepared from representative subsamples of spot samples obtained from within a single tank (e.g. by combining subsamples from the upper, middle and lower spot samples), or may be prepared by combining subsamples representative of individual tanks to provide a composite for a larger oil quantity (e.g. several ship or barge tanks of the same product). Composite samples shall include all the material collected in the primary sampling device without subdivision. The volume collected in the primary sampling device shall be chosen to allow the entire contents of the device to be added to the volume of the other subsamples in the transport container. Compositing of subsamples smaller than the entire contents of a subsample shall only be performed in a laboratory with facilities to assure adequate mixing and measurement of the subsamples.

To prepare either type of composite sample, transfer subsamples of representative individual samples into a composite-sample container, and mix them together gently. The subsamples shall be volume weighted in proportion to the quantity that each represents.

When the subsamples that are to be combined originate from a tank of non-uniform cross-sectional area (or from multiple tanks), the compositing operation shall require careful calculation and measurement of the subsamples to maintain the representative nature of the sample. These operations shall, if practical, be conducted under controlled laboratory conditions.

NOTE Evaporation of light ends, and adhesion of water/sediment to the wall of the original sampler, may influence the representative nature of composite sample(s) (see also 7.2.3).

Do not prepare composite samples for testing unless they are specifically requested, and agreed by the interested parties. As an alternative to physical compositing, individual spot samples shall be tested and a mean value calculated from the individual test results in proportion to the bulk represented by each sample.

7.3.1.1.3 Bottom sampling
Lower the bottom sampler until it rests in an upright position on the bottom of the tank so that the valve opens and the sampler fills. After withdrawing the sampler, examine it closely for leaks. If any are detected, discard the sample, clean the bottom sampler and re-sample. If necessary, transfer the contents into a secondary sample container, taking care that all the sample is properly transferred, including any water or solids that may adhere to the inner walls of the primary bottom sampler.

7.3.1.1.4 Interface sampling
Lower the sampler with the valves open to permit the liquid to flush through the device. At the level desired, close the valve(s) and withdraw the sampler from the liquid.

If a transparent tube is used, any existing interface can be detected visually through the wall of the sampling tube and its position within the tank determined by measurement on the graduated sampling tape. Check that the valves have properly closed; otherwise re-sample.

NOTE The sample may be retained for testing.

7.3.1.1.5 Tap (tank-side) sampling
This is not a preferred method for custody transfer or yield inventory purposes and therefore shall be applied only if no other method of sampling is possible.

The sampling point valves shall be a minimum of 12,5 mm in diameter and shall be fitted to the side of the tank at regular intervals, by connections extending at least 150 mm into the tank, except on floating-roof tanks, in which this is impossible. The lower connection shall be level with the bottom of the suction pipe (see also 7.2.4).

Before a sample is taken, flush the tap or valve connection with the product to be sampled, after which draw off a sample into a container or receiver.

CAUTION - Open the taps with care when sampling under pressure. Make no attempt to clear a blocked connection by rodding through an opened valve.

If the contents of a tank fail to reach the upper or middle sample connections on a tank equipped with three connections, take the sample for the tank as follows.
a) If the level of the contents is nearer the upper sample connection than the middle one, take two-thirds of the sample from the middle connection and one-third from the lower one.
b) If the level is nearer the middle connection than the upper, take one-half of the sample from the middle connection and one-half from the lower one. If the level of the contents is below the middle sample connection, take all the sample from the lower connection.

7.3.1.1.6 All-level sampling
Refer to 5.2.6 for descriptions of the different types of equipment that are available. All-level samplers can be of the "top-down" or "bottom-up" type. As the sample receiver is filled while travelling in a single direction through the tank's contents, different procedures are required for "top-down" and "bottom-up" techniques.

Follow the manufacturer’s instructions when using proprietary devices.

To obtain a (bottom-up) all-level sample with a bottle fitted to a weighted sampling cage (or with a weighted sampling can), proceed as follows. Stopper the bottle or can, and lower it to the tank bottom (avoiding any bottom-lying free water). Jerk the cord to remove the cork, and raise the sampler back to the surface at a uniform speed without any pause or hesitation. The hauling speed should be chosen so that the bottle or can is about 80 % full, but not more than 90 % full, when withdrawn from the liquid. Cap or stopper the bottle immediately, or carefully transfer the complete sample from the weighted can to a secondary transportation receiver (see also 5.2.6).

If a fixed volume all-level sampler is less than 90 % full when retrieved from the liquid, it may be assumed that oil was flowing into it from all depths during its passage through the tank's contents. If the sampler is more than 90 % full when retrieved from the liquid, the sample may not be representative and should be discarded before re-taking the sample using a faster raising speed.

NOTE 1 The use of fixed volume all-level samplers is not a preferred method for custody transfer or yield inventory applications, as such devices may not fill at a uniform rate. Additionally, the operator may not be able to lower or raise the sampler at the rate required for proportional filling, which is approximately proportional to the square root of the depth of immersion.

Variable volume all-level samplers shall be designed to fill in proportion to the distance travelled through the tank contents.

Refer to 5.2.6 for sample acceptance criteria.

NOTE 2 Care is required with the operation of bottom closing "top-down" types of all-level sampler if a layer of free water can be present at the tank bottom. Some designs of proprietary sampler include an adjustable extension "foot" to trigger closure just above the level of any free water.

7.3.1.1.7 Running samples
Refer to 5.2.5 for descriptions of the different types of equipment that are available.

NOTE The use of fixed volume running samplers is not a preferred method for custody transfer or yield inventory applications, as such devices may not fill at a uniform rate. Additionally, the operator may not be able to lower or raise the sampler at the rate required for proportional filling which is approximately proportional to the square root of the depth of immersion.

To obtain a running sample with a bottle fitted to a weighted sampling cage (or with a weighted sampling can) equipped, if necessary, with a suitable device to restrict the filling rate, proceed as follows. Lower the open can or bottle/cage from the surface of the liquid to the tank bottom (avoiding any bottom-lying free water), and raise it back to the surface at the same uniform speed and without hesitation when changing direction. Select the inlet restriction orifice size and/or the rate of raising and lowering so that the bottle or can is about 80 % full, but not more than 90 % full, when withdrawn from the liquid. Cap or stopper the bottle immediately, or carefully transfer the complete sample from the weighted can to a secondary transportation receiver (see also 5.2.5).

To obtain a running sample with a proprietary running sampler, select the inlet restriction orifice size required for the liquid depth and viscosity, and follow the manufacturer's instructions. If the sample quantity is more than 95 % of the sampler capacity, discard the sample and select a different size inlet orifice and/or change the speed of lowering and raising to achieve the target sample volume.

If a fixed volume running sampler is not more than 90 % full when retrieved from the liquid, it may be assumed that oil was flowing into it from all depths during its passage through the tank's contents. If the sampler is more than 90 % full when retrieved from the liquid, the sample may not be representative and should be discarded before retaking the sample using a smaller orifice restriction and/or a faster raising and lowering speed. Care is required during the operation of running samplers if a layer of free water is present at the tank bottom.

Free water should not normally be included in such samples, but should be quantified separately by gauging or by bottom sampling with an interface sampler.

7.3.1.2 Horizontal tanks with circular or elliptical cross-sections
Except when noted otherwise, samples shall be taken as spot samples as described in 7.3.1.1.1 from the levels indicated in Table 2. If they are to be combined to give a composite sample as described in 7.3.1.1.2, combine them in the proportions given in Table 2.

NOTE By mutual agreement, a single spot sample at the location corresponding to 50 % of the contained volume may be considered sufficient. Alternatively, it may be acceptable to use one of the other methods described in 7.3.1.1.

7.3.1.3 Tanks with other geometrical shapes
Sample spherical tanks and tanks of irregular shape by taking spot samples as described in 7.3.1.1.1. Determine the actual levels at which the samples are to be taken to allow for the volume distribution over the height of the tank.

7.3.1.4 Tanks fitted with vapour-lock valves
7.3.1.4.1 Use a portable sampling device (PSD) which is compatible with the vapour-lock valve that is installed on the tank, or use a suitable gas-tight adapter. Select the correct sampler for the type of sample that is to be taken (i.e. spot, zone, bottom, interface, running, or all-level) and attach it to the hanging device on the PSD tape/cable. Set any associated sampling valve triggering device and, in the case of running or all-level samplers, select the desired inlet orifice restriction device.

7.3.1.4.2 Confirm that the vapour-lock valve is fully closed before removing the protective cover/cap. Check that the datum surface of the valve and the matching datum surface of the PSD are both clean and free from any foreign material which could prevent the PSD from seating correctly and providing a gas-tight seal.

NOTE Earth continuity between the PSD and the tank structure will also normally be provided via this contact, but a separate earthing connection may also be provided.

7.3.1.4.3 Fit the PSD to the valve, and ensure that it is seated correctly before tightening the coupler device to lock it in position. Where the PSD is provided with a separate earthing connection, connect it to a suitable part of the tank structure to ensure earth (grounding) continuity.

7.3.1.4.4 Open the vapour-lock valve fully and carefully lower the sampler into the tank by unwinding the PSD tape or cable winding handle.

Depending upon the type of sampler used, (i.e. spot, running, all-level, etc.) take the sample(s) in accordance with the procedures specified in 7.3.1.1. When retrieving the sample, ascertain that the sampling device is fully withdrawn above the vapour-lock valve, before closing the valve. After retrieving the sample, ensure that the vapour-lock valve is fully closed before opening the PSD and/or transferring the sample to a secondary receiver.

7.3.1.4.5 When the sample has been obtained with a restricted system PSD, it shall be handled in exactly the same way as the equivalent open samples (see 7.3.1.1) and transferred to a secondary transportation receiver if necessary.

When the sample has been obtained with a closed system PSD, it shall be transferred completely via the gas-fight PSD housing to a fixed or variable volume transportation receiver.

NOTE It is important to verify the cleanliness of the internal components of the PSD housing (as well as the sampler) before re-using either a restricted or closed system PSD.

7.3.1.5 Pressurized tanks with valved sample points
Some pressurized tanks such as LPG Spheres, Bullets, etc. may be fitted with probes to enable sampling from different depths within the tank contents. Alternatively, other valved connections to pressurized tanks may be suitable for taking samples. In either case, use one of the pressurized receiver sampling methods described in 7.4.3.

7.3.2 Tanks on ships or barges
7.3.2.1 General
If open sampling procedures are acceptable, use the procedures detailed in 7.3.1.1 in conjunction with 7.3.2.2 to 7.3.2.4. If restricted or closed system sampling is required, follow the procedures detailed in 7.3.1.4 in conjunction with 7.3.2.2 to 7.3.2.4.

Safety and environmental regulations may limit the release of hydrocarbons to the atmosphere during ship and barge cargo operations. This has resulted in the restriction and, in some cases, the prohibition of traditional methods of obtaining cargo samples via open gauge-hatches or sighting ports. Thus, it is now a common condition specified in ship chartering agreements that ships should have facilities for restricted or closed system measurement and sampling, and that access to the cargo tanks should only be via vapour-lock valves.

The installation of vapour-lock valves should be in accordance with the requirements of the ship's Classification Society and the appropriate port authorities.

NOTE The total load capacity of a vessel is normally subdivided into a number of compartments which may vary in size and geometry. Some tank compartments may not have a uniform volume-to-height ratio, and thus some types of sample may not be representative. In these circumstances, spot samples from each compartment are preferred; however, in practice, time restraints associated with shipping operations normally necessitate the taking of all-level or running samples.

7.3.2.2 Sampling non-inerted, non-pressurized vessels
If open sampling procedures are acceptable, use the procedures described in 7.3.1.1.

If restricted or closed system sampling is required, use one of the procedures described for sampling tanks fitted with vapour-lock valves (see 7.3.1.4).

7.3.2.3 Sampling inerted, but depressurized vessels
For sampling inerted but depressurized vessels, the same procedures and precautions apply as detailed in 7.3.2.2.

7.3.2.4 Sampling inerted and pressurized vessels
For sampling inerted and pressurized vessels, use an appropriate restricted or closed system sampling device (see 5.2.7 and 7.3.1.4).

7.3.3 Railcars
For railcars, if open sampling is acceptable, use the procedures described for sampling horizontal cylindrical tanks (see 7.3.1.2). If restricted or closed system sampling is required, use one of the procedures described for sampling tanks fitted with vapour-lock valves (see 7.3.1.4).

If it is agreed that it may be acceptable to sample from a limited number of railcars in a train that all contain nominally the same material, samples should be drawn from railcars that are selected using a sampling plan in accordance with the general procedures described in 11.1.4.

7.3.4 Road vehicle tanks
For road vehicle tanks, if open sampling is acceptable, use the procedures described for sampling horizontal cylindrical or other geometry tanks as appropriate (see 7.3.1.2 and 7.3.1.3). If restricted or closed system sampling is required, use one of the procedures described for sampling tanks fitted with vapour-lock valves (see 7.3.1.4).

7.4 Pipeline sampling
It is often necessary to obtain manual dynamic pipeline samples, e.g. for instrumentation verification and quality control purposes. It should be noted that such samples are spot samples which may or may not be representative of the bulk transferred quantity.
7.4.1 Non-homogeneous liquids
For sampling non-homogeneous liquids, see Clause 8.

7.4.2 Homogeneous liquids
Sampling of homogeneous liquids shall be performed using a suitable pipeline sampling apparatus (see 5.4.2). Before a sample is drawn, flush the sample line and valve connection with the product to be sampled, after which draw off a sample into a sample container or receiver, taking into account the precautions given in 5.4, 7.2.4 and 7.4.3 as appropriate.

The contents of pipelines may be under considerable pressure and therefore special procedural precautions and equipment may be necessary (see Clause 6). It is recommended that a pressure gauge be provided in the line at each sampling point to enable the pressure to be read before sampling. The line service should be clearly labelled and updated on any change of service.

7.4.3 Spot sampling of high vapour pressure liquids
7.4.3.1 Sampling into single-piston variable volume sample receivers
7.4.3.1.1 Principle
The liquid is sampled at line pressure and maintained at line pressure (or above) during transportation and subsequent subsampling. A typical single-piston receiver is illustrated in Figure 13. The sample is accumulated against one side of the floating piston by carefully reducing the pressure of an inert gas buffer on the opposite side of the piston.

7.4.3.1.2 Receiver selection and leak testing
Select a receiver with the required capacity and a rated working pressure which exceeds the pipeline pressure. Confirm that the piston seal elastomer is compatible with the pipeline liquid, and is capable of operating effectively at the pipeline temperature. Ensure that the receiver is clean and dry.

Pressurize both sides of the receiver with inert gas to at least 100 kPa (1 bar) more than the anticipated pipeline pressure, and test for leaks. Test the receiver piston seals for leakage by pressurizing each side in turn to the same pressure while the other side is open to atmosphere. If a valve, fitting or seal is found to leak, replace it and re-test, or use another receiver.

7.4.3.1.3 Pre-charging the receiver
Open the sample inlet valve. Connect the inert gas side of the receiver to a supply of the appropriate inert gas and slowly pressurize it to at least 100 kPa (1 bar) above the pipeline pressure, so that the piston is fully displaced against the sample inlet end plate. Close all valves and disconnect the inert gas supply. Transport the pre-charged receiver to the sample point.

7.4.3.1.4 Purging the sample point lines and receiver
Connect the pre-charged receiver sample inlet valve to the sampling point.

If the receiver has only a single sample inlet connection, keep the sample inlet valve closed and flush the sample probe and line up to the inlet valve to a closed drain connection (or other safe disposal route such as a flare line).

Some receivers are provided with an additional end plate connection which enables the purging of the receiver sample side dead volume. With this type of receiver, purge the total hold-up volume by connecting the second valve to the disposal line and opening both valves. When the system is fully purged, close the second valve (i.e. the outlet valve) and control the filling by gently reducing the inert gas pressure from the other end of the receiver (7.4.3.1.5).

7.4.3.1.5 Filling the receiver
When filling a single sample inlet receiver, slowly open the inlet valve. A double sample inlet receiver will be ready to fill as soon as the second sample end valve is closed on completion of purging.

The piston shall not move at this stage due to the buffer gas pressure being higher than the pipeline pressure. If the piston does move, reject and use another receiver. Cautiously crack open the inert gas end valve to gradually reduce the inert gas pressure and allow the receiver to fill to no more than 80 % of nominal capacity.

The pressure difference between the sample and inert gas buffer sides of the piston should not exceed 100 kPa (1 bar) at any time during sampling.

Close the sample inlet valve and de-pressurize the sample line to the drain before disconnecting the receiver from the sample point. Finally connect the inert gas inlet valve to an inert gas supply and increase the inert gas pressure to at least 100 kPa (1 bar) above the pipeline pressure. Disconnect the receiver and transport it to the laboratory as soon as possible.

7.4.3.2 Sampling into double-piston variable volume sample receivers
7.4.3.2.1 Principle
The liquid is sampled at line pressure and maintained at line pressure (or above) during transportation and subsequent subsampling. A typical double-piston (internal mixing) receiver is illustrated in Figure 14. Sample is accumulated against only one of the floating pistons by carefully reducing the pressure of the inert gas buffer on the opposite side of that piston. The inert gas pressure on the second piston is kept significantly greater than the pipeline pressure throughout the sample accumulation stage to ensure that the receiver is not overfilled.

Prior to subsampling the primary sample for analysis, it may be homogenized by forcing the total sample quantity repeatedly through fine-bore transverse holes in the central baffle plate. This is achieved by alternately reducing the pressure of one inert gas buffer relative to the other (while maintaining the sample pressure above its bubble point). The efficiency of this multi-phase sample homogenization may be verified by controlled injection/recovery tests. Consequently, these double-piston receivers are recommended for taking time-synchronized spot samples that are used for the calibration and verification of automatic pipeline water content analysers operating on high pressure pipelines (e.g. unstabilized crude oil or condensate production, fiscal or allocation accounting applications).

7.4.3.2.2 Receiver selection and leak testing
The working (sample) volume of a double-piston variable volume receiver is only 50 % of the total volume. Select a receiver with the required capacity and a rated working pressure which exceeds the pipeline pressure. Confirm that the piston seal elastomer is compatible with the pipeline liquid and capable of operating effectively at the pipeline temperature. Ensure that the receiver is clean and dry. Leak test all components as detailed in 7.4.3.1.2. If a valve, fitting or seal is found to leak, replace it and re-test.

7.4.3.2.3 Pre-charging the receiver
Open the sample inlet valve. Connect both inert gas inlet valves in turn to a supply of the appropriate inert gas and slowly pressurize it to at least 500 kPa (5 bar) above the pipeline pressure, so that both pistons are fully displaced against the central sample inlet/baffle plate. Close all valves and disconnect the inert gas supply. Transport the pre-charged receiver to the sample point.

7.4.3.2.4 Purging the sample point lines and receiver
Connect the pre-charged receiver sample inlet valve to the sampling point. Connect the sample outlet port/septum adapter valve to a closed drain or other safe disposal route.

Open the receiver sample outlet port valve fully and partially open the receiver sample inlet valve (1/4 turn). Carefully open the pipeline sample point valve to thoroughly flush the sample line and receiver dead volume. Control the rate of liquid flow through the receiver by carefully opening the inlet valve and the sample point valve further until the held-up volume has been thoroughly flushed.

7.4.3.2.5 Filling the receiver
Close the receiver sample outlet port valve fully on completion of the flushing operation. Neither piston should move at this stage due to the buffer gas pressure being higher than the pipeline pressure.

Cautiously crack open one (but not both) of the inert gas end valves to gradually reduce the inert gas pressure on one side of the receiver. The reduction in the inert gas pressure will allow sample to accumulate into the working side of the receiver, under the action of the pressure differential between the pipeline and the inert gas buffer. The inert gas valve shall be carefully adjusted to ensure that the corresponding sample inlet flow rate is properly controlled. The pressure difference between the pipeline and the working inert gas buffer should not exceed 100 kPa (1 bar) at any time during the sample accumulation period.

Close the valves when the piston position indicator shows that the receiver has been filled to approximately 90 % of the filled side of the receiver (i.e. about 45 % of the nominal total capacity).

De-pressurize the sample line to the drain before disconnecting the receiver from the sample point. Finally connect the working inert gas inlet valve to an inert gas supply and increase the inert gas pressure to at least 100 kPa (1 bar) above the pipeline pressure. It is not necessary to equalize the pressure between both inert gas buffers, but this may be done provided that the resulting final inert gas pressure is maintained above the pipeline pressure. Disconnect the receiver and transport it to the laboratory as soon as possible.

7.4.3.3 Sampling into fixed volume sample receivers
7.4.3.3.1 Principle
The liquid is sampled at line pressure and maintained at a pressure close to the line pressure during transportation and subsequent subsampling. It is essential to create an ullage volume within the fixed volume receiver immediately after taking the sample, in order to prevent any unsafe increase in sample pressure due to thermal expansion effects during transportation or storage.

Where the vapour pressure of the liquid being sampled is close to the line pressure, the reduction in sample pressure associated with the creation of the ullage space may result in phase separation that can make subsequent representative subsampling difficult. In these circumstances, a variable volume sample receiver (7.4.3.1 or 7.4.3.2) should usually be used to ensure that the sample is maintained at sufficient pressure to prevent phase separation. However, if the liquid is also refrigerated, the effect of low temperatures on the receiver piston seals should also be considered.

Four main designs of fixed volume receiver are available depending on whether they are provided with
a) one or more inlet/outlet valves, and
b) an internal ullage tube or not.

Two-valve receivers are easier to purge prior to sampling, while an ullage tube simplifies ullaging the receiver by the correct amount (20 % of the total fixed volume) after sampling.

NOTE An internal ullage tube within a fixed volume sample receiver may also be known as an outage tube or dip tube.

7.4.3.3.2 Receiver selection and leak testing
The working (sample) volume of a fixed volume sample receiver is typically 20 % less than the nominal volume to allow for the necessary ullage space. Select a receiver with the required capacity and a rated working pressure that exceeds the pipeline pressure. Verify that the pressure test certification for the sample receiver and the bursting (rupture) disk is current. Ensure that the receiver is clean and dry.

Pressurize the receiver with inert gas to at least 100 kPa (1 bar) more than the anticipated pipeline pressure, and test for leaks. If a valve or fitting is found to leak, replace it and re-test, or use another receiver.

7.4.3.3.3 Purging the sample point line
If the receiver has an internal ullage tube fitted, determine which receiver valve supplies the tube and connect this valve to the sampling point. If the receiver has a second valve, connect that valve to a closed drain or other safe disposal route. Confirm that all valves are initially closed.

Earth continuity should usually be provided to the receiver via the metallic pipework connection to the sample point, but in some cases it may be necessary to provide a separate earth (grounding) connection.

Purge the sample transfer line by displacing at least 150 % of the line's volume (from the sample probe in the pipeline to the sampling point) to the vent immediately prior to the sample point valve. Then close the vent valve.

7.4.3.3.4 Purging the sample receiver
For a one-valve receiver, open the receiver inlet valve to partly fill the receiver. Then close the sample line control valve, and open the line vent valve to purge the receiver. Close the vent valve and repeat the partial filling and venting process at least twice more to purge the receiver as fully as possible.

For a two-valve receiver, open the receiver inlet valve to partly fill the receiver. Then slowly open the receiver outlet valve to its vent. Close the sample line control valve, and allow part of the receiver contents to escape to vent through the receiver outlet valve. Close the receiver vent valve and open the sample line vent valve to allow further venting. Close the sample line vent valve and repeat the partial filling and venting process at least twice more to purge the receiver as fully as possible.

7.4.3.3.5 Filling the receiver
On completion of the purging operation, open the sample line control valve to fill the receiver, then close the receiver inlet valve. Then close the sample line source valve (at the pipeline), and open the sample line vent valve to depressurize the sample line. Finally, close the remaining sample line valves and disconnect the receiver.

7.4.3.3.6 Providing a safe ullage space within the receiver
Immediately after taking the sample, a 20 % ullage shall be provided in the receiver. This may be achieved by either using an integral receiver ullage tube (where fitted), or by weighing the receiver to determine the sample quantity.

If using the ullage tube procedure, position the receiver upright with the sample inlet valve (and ullage tube) at the top.

NOTE 1 Electrostatic safety considerations may mean that it is advisable to fit the receiver with an earth (grounding) connection before proceeding to the next partial venting step.

Cautiously open the receiver inlet valve slightly until liquid is observed to escape. Allow the excess liquid to escape, but, as soon as it is observed that the escaping liquid material changes to vapour quickly, close the valve.

If no liquid escapes initially then the receiver was not filled sufficiently and the sample will not be representative. In these circumstances, the suspect sample should be discarded, and the sampling procedure repeated.

If using the weighing procedure, weigh the filled receiver and deduct the tare weight to determine the total weight of sample that has been taken. Calculate the weight of sample that represents a 20 % ullage, and vent this amount by cautiously opening the receiver inlet valve slightly.

NOTE 2 Electrostatic safety considerations may mean that it is advisable to fit the receiver with an earth (grounding) connection before conducting the venting operation.

Close the receiver inlet valve and re-weigh the receiver to verify that a safe ullage space has been created. If the gross weight still exceeds the tare weight plus 80 % of the original sample weight then the partial venting operation should be repeated.

If the receiver weighing cannot be performed at the sampling location, it is important to ensure that a small quantity of the liquid phase sample is vented immediately to prevent excessive pressure build-up as a result of sample expansion due to any subsequent increase in temperature. The full weighing and ullaging procedure should then take place as soon as possible after transportation to a suitable location where the facilities are available.

7.4.3.3.7 Sample handling
On completion of the preceding steps, immediately check the receiver for leaks with a proprietary leak detecting fluid, with soapy water or by immersing in water. If any leaks are detected, discard the sample and repair or replace the receiver before obtaining another sample.

Label the sample receiver clearly, and prepare it for transportation by packing in a suitable container as required by the appropriate transportation regulations. Transport the receiver to the laboratory/test location without delay. Where intermediate storage is necessary, the sample should be protected from extremes of temperature.

7.5 Dispenser (nozzle) sampling
This procedure is applicable for sampling light fuels from retail-type dispensers. Fit a nozzle extension to allow fuel to be dispensed to the bottom of the sample receiver, without splashing. Where the nozzle is fitted with a vapour recovery system, a spacer will be needed to hold back the nozzle sleeve. Fill the sample receiver slowly, through the nozzle extension, until it is approximately 85 % full. Remove the nozzle and extension and close or cap the receiver immediately.

If the sample is to be analysed for vapour pressure, chill the receiver prior to filling.