Choosing a Crude Oil Automatic Sampler for Custody Transfer

8 hours ago

3 min read

At a glance. A custody-transfer sampler must take a flow-proportional sample from a well-mixed stream, in enough small grabs to represent the whole batch, and deliver it to the laboratory unchanged. Specify it to API MPMS Chapter 8.2, ISO 3171 or ASTM D4177, confirm the pipeline is mixed at the sample point, choose in-line, fast-loop or power-mixing design to suit the line, and prove the system once it is installed.

Why does the sampler matter so much?

Crude is paid for net of water and sediment, and that figure comes from the sample. On a 1,000,000-barrel cargo, an error of 0.1% in water content is 1,000 barrels, paid for by one party and not the other. A sampler that misses free water at low flow, or a receiver that loses light ends, turns directly into money.

Is the stream mixed at the sample point?

Water settles towards the bottom of a pipe unless flow keeps it dispersed. The standards expect the operator to show that the stream is mixed at the probe, by calculation or a profile test, across the full flow range.

High, steady flow – natural turbulence and a probe in the centre of the line may be enough.
Variable or low flow – a static mixer ahead of the probe disperses the water.
Low flow or large lines – a power-mixing system jets the stream back into the line with a pump, so mixing no longer depends on pipeline velocity.

In-line, fast loop or power mixing?

Design	How it works	Best for
In-line	Sampler head grabs directly from the main line through a probe	Lines of 2 in and larger with good mixing
Fast loop	A pump draws a side stream past the sampler and returns it	Where the head cannot sit on the main line, or analysers share the loop
Power mixing	Pumped jet mixing plus sampling in one package	Low or variable flow, and large lines

How many grabs, and how big a receiver?

Common practice is at least 10,000 grabs over a batch, each typically 1 to 3 ml, paced in proportion to the flow measured by the meter. The receiver is sized so the sample fills it to no more than about 80% at the end of the batch, leaving room for mixing before the laboratory draws its sub-samples. For volatile crude, condensate and NGL, a constant-pressure receiver with a floating piston keeps the sample in a single phase.

How is the system proved?

A new or modified system is checked against the standard's acceptance tests, often by injecting a known amount of water upstream and comparing what the sampler collects. Routine checks then confirm grab volume, pacing and receiver mixing, and the controller records every grab for audit.

Specification checklist

Crude type, flow range, pressure and temperature
Line size and the straight length available
Mixing evidence at the proposed probe location
Batch size, grab volume and number of receivers
Standard to follow: API MPMS 8.2, ISO 3171 or ASTM D4177
Receiver type: fixed, portable or constant-pressure
Hazardous-area zone and controller interface
Proving method and witness tests

Tech Inc. automatic samplers

Tech Inc. builds in-line, fast-loop and power-mixing samplers, pressurised samplers for condensate, NGL and LPG, sampler controllers, receivers and mixing units, constant-pressure cylinders and a retrofit kit for existing probes. Send your line data to mail@techincresearch.com.

Frequently asked questions

Which standard should a crude sampler follow?

API MPMS Chapter 8.2 in most oil markets, ISO 3171 internationally, and ASTM D4177, which is technically equivalent to API MPMS 8.2. Sample handling and mixing follow API MPMS Chapter 8.3.

Why must sampling be flow-proportional?

Because the water content of a batch can change as it is delivered. Grabs paced by the flowmeter give every barrel the same weight in the composite sample.