Crude oil custody transfer is not only about measuring how much oil moves from one party to another. The quality and composition of the transferred crude also need to be represented by a sample that can be used for subsequent analysis. For this reason, an automatic sampling system must do more than simply remove a small amount of oil from a pipeline.
The sample should represent the condition of the overall oil stream as accurately as possible. ZNYB's ZHYQ-II-MJ product manual describes the system as an automatic crude oil sampler for Trade Transfer Measurement and states that the equipment is designed in accordance with GB/T 27867, ISO 3171, and API 8.2. For buyers and engineers, the key idea is therefore simple:
Taking a sample is not the same as obtaining a representative sample.
A properly designed custody transfer sampling system needs to consider the condition of the crude oil before sampling, the location of the sampling probe, the relationship between flow and sampling frequency, the way the sample is stored, and how the final laboratory sample is prepared.
Why Is Representative Sampling So Important?
Crude oil in a pipeline may not always be completely uniform across the pipe cross-section. When different fluid phases are unevenly distributed, a sample taken from the wrong location or under poor mixing conditions may represent only a local portion of the flow. The ZHYQ-II-MJ documentation specifically states that the sampling equipment should receive fluid that is as representative as possible of the entire pipeline cross-section.
It also states that when the fluid is uneven or multiple fluids are mixed, additional mixing is required before sampling. This is one of the most important engineering principles in crude oil trade-transfer sampling. The objective is not simply:
“Collect oil from the pipeline.”
The objective is:
“Collect a sample that reflects the transferred oil stream.”
This is why pipeline mixing, sampling probe design, flow-proportional sampling, and sample homogenization all need to be considered as parts of one system.
How Does an Automatic Custody Transfer Sampling System Work?
The ZHYQ-II-MJ system is divided into three main sections: Main pipeline mixing system → Automatic sampling system → Sample mixing and splitting system The mixing system may use jet mixing or a static mixer, while the final sample mixing and splitting system is optional.
A simplified sampling process can therefore be understood as: Crude oil pipeline
Pipeline mixing
Sampling probe
Automatic sub-sample collection
Sample receiver
Sample homogenization
Laboratory analysis Each stage can affect the quality of the final sample.
1. Pipeline Mixing Before Sampling
Mixing is one of the most important parts of representative crude oil sampling. If the fluid at the sampling point is not sufficiently uniform, even a highly accurate automatic sampler may still collect a non-representative sample. The ZHYQ-II-MJ system supports two main mixing approaches:
Jet Mixing and Static Mixing
The documentation states that either approach must satisfy the required C1/C2 mixing conditions associated with ISO 3171 and that engineering calculation is required.
Jet Mixing
The jet mixing system includes a jet nozzle assembly, sampling probe, and jet pump. A portion of the process fluid is extracted and injected back into the main pipeline at high velocity upstream of the sampling probe. This creates additional mixing energy and promotes more uniform distribution of the different flow phases across the pipeline cross-section.
The sampling probe then collects from the mixed liquid stream. An important feature of this arrangement is its ability to assist under lower-flow conditions. The documentation states that the mixing system can provide continuous mixing energy when oil flow is low and can stop automatically once natural pipeline turbulence becomes sufficient.
Static Mixing
A static mixer can also be used in suitable applications. Unlike an active jet mixing system, a static mixer depends on the energy of the process flow passing through its internal mixing elements. Its mixing efficiency and pressure loss therefore need to be calculated.
The ZHYQ-II-MJ documentation specifies that the sampling probe is installed approximately 0.5 to 8 pipe diameters downstream of the static mixer. The correct mixing method should therefore be selected according to actual pipeline conditions rather than equipment preference alone.
2. Sampling Probe Selection and Position
After the fluid has been properly conditioned, the sample needs to be extracted from the pipeline. This is the function of the sampling probe. The ZHYQ-II-MJ system supports both:
Fixed sampling probes and Retractable sampling probes
If a retractable probe is selected, sufficient space needs to be reserved for insertion and withdrawal. The installation position of the probe on the main oil pipeline must also meet the applicable sampling requirements. This means probe selection should consider more than just pipeline diameter.
Engineers also need to consider installation orientation, available maintenance space, process pressure, temperature, and the position of the probe relative to the upstream mixing system.
3. Automatic Sub-Sample Collection
Once the fluid reaches the sampling probe, the automatic sampling system collects individual sub-samples and transfers them into a sample receiver. The ZHYQ-II-MJ automatic sampling system can include a skid-mounted cabinet, flow detection device, optional online water-content transmitter, separator, portable pressure-resistant sample container, weighing system, and explosion-proof control box. The separator is responsible for extracting the required individual sample increments.
According to the product documentation: Minimum sub-sample volume: 1.0 mL Adjustable sub-sample range: 1.0–10 mL
Maximum sampling frequency: 30 times per minute These parameters allow the sampling strategy to be configured according to the actual transfer operation.
4. Why Flow-Proportional Sampling Matters
One of the most important functions described in the ZHYQ-II-MJ manual is flow-proportional sampling. The documentation states that the sampling rate can be made proportional to the oil-transfer rate. This matters because crude oil transfer flow does not necessarily remain constant throughout an entire batch.
Consider a simplified example. If the flow is low, fewer sample increments may be required. If the transfer rate increases significantly, more sample increments can be collected.
The principle is: Lower transferred flow → fewer sample increments Higher transferred flow → more sample increments
This helps the accumulated sample better correspond to the volume of oil actually transferred during different stages of the operation. For trade-transfer applications, this is much more meaningful than treating every period of the transfer as if the flow rate were identical.
5. Sample Storage Is Also Part of Representative Sampling
Collecting a representative sample from the pipeline is not the end of the process. The accumulated sample must remain suitable for subsequent laboratory preparation. The ZHYQ-II-MJ uses portable pressure-resistant stainless-steel sample containers.
The documented container design includes:
- Large quick-release opening
- Smooth internal surface
- Internal jet homogenizing component
- Pressure-relief device
- Quick-connect fittings
The internal jet arrangement causes the liquid inside the container to rotate, assisting homogenization. The available design volumes are: 5 L
10 L 20 L with a documented pressure rating of 0.3 MPa.
The standard arrangement includes one container in operation and one backup container. This is important because the representativeness of a sample needs to be maintained after collection as well as during pipeline sampling.
6. Sample Mixing and Splitting Before Laboratory Analysis
The system can also be equipped with a separate sample mixing and splitting unit. According to the ZHYQ-II-MJ documentation, the stored sample is extracted from the bottom of the sample container, circulated through a six-unit static mixer, and returned to the container through jet piping. The resulting circulation causes the sample inside the container to rotate and mix.
Typical mixing time is approximately 5–20 minutes, depending on sample volume and oil type. This highlights an important principle:
Representative sampling does not end when the crude oil leaves the pipeline.
If an accumulated sample separates before the laboratory takes a smaller test sample, the final laboratory result may no longer represent the original collected sample. This is why sample homogenization and splitting should be treated as part of the complete sampling process.
7. Online Water-Content Measurement
The ZHYQ-II-MJ can also be equipped with an online water-content transmitter. The documented configuration uses a capacitance method with: Resolution: 0.01
Repeatability error: 0.05 Because this transmitter is optional, the actual instrumentation configuration should be selected according to project requirements.
8. Control System and DCS Communication
Modern custody-transfer sampling systems are not purely mechanical. They also need to communicate operating and process information. The ZHYQ-II-MJ controller is based on a Siemens PLC and uses a 7-inch explosion-proof color touchscreen.
The documented communication configuration includes: RS485 and
MODBUS RTU The sampling cabinet can send several data points to the DCS, including equipment operating status, fault information, sampling parameters, real-time water-content values, sub-sample count, and sample weight. It can also receive process values such as instantaneous oil-transfer flow, temperature, density, water-content correction parameters, and sampling-operation settings from the DCS.
This means the automatic sampler can become part of the broader transfer-measurement and control system rather than operating as an isolated device.
9. What Happens If the Sample Volume Exceeds the Limit?
The system also includes a weighing-based protection function. According to the ZHYQ-II-MJ documentation, if the sample volume exceeds the configured limit because of an unexpected condition, the weighing device can automatically stop the sampling operation and generate an alarm. This gives operators another layer of monitoring during long transfer operations.
10. Key Technical Parameters
The ZHYQ-II-MJ manual lists the following reference technical parameters:
| Parameter | Reference Value |
|---|---|
| Sampling Performance Coefficient (PF) | 1 ± 0.1 |
| Unit Sample Volume Coefficient (GF) | 1 ± 0.05 |
| Unit Sample Count Coefficient (CF) | 1 ± 0.01 |
| Sub-sample Volume | 1.0–10 mL |
| Sampling Interval | 2–900 s |
| Sample Receiver Volume | 5 / 10 / 20 L |
| Engineering Pressure | 2.5–6.4 MPa |
| Explosion-Proof Identification | Ex d IIB T4 |
| Electrical Protection | IP65 |
| Power Supply | 380 V |
| Main System Power | 2–3 kW |
| Skid Cabinet Size | 800 × 750 × 1300 mm |
| Total Weight | 350–950 kg |
These values are taken from the current ZHYQ-II-MJ product documentation and should be treated as reference configuration data; final project design still depends on the actual application.
What Information Should Buyers Provide?
An automatic crude oil sampler for trade or custody-transfer measurement should not be selected only by model number. The engineering team needs to understand the actual operating conditions. For project evaluation, buyers should be prepared to provide information covering pipeline size and material, process medium, minimum and maximum throughput, pressure, temperature, minimum and maximum operating flow, fluid density and viscosity, sample receiver requirements, electrical conditions, hazardous-area requirements, and required site signals.
The ZHYQ-II-MJ manual therefore includes a dedicated Order Base-Data Sheet for project configuration. This is exactly why your Project & Ordering Data Sheet should eventually become one of the main CTA elements on the ZNYB website.
Frequently Asked Questions
Is custody transfer sampling the same as taking a manual spot sample?
Not in the context described by this system. The ZHYQ-II-MJ is designed as an automatic pipeline sampling system where pipeline mixing, sampling position, sub-sample collection, sample storage, and subsequent homogenization are considered together.
Why does crude oil need to be mixed before sampling?
The product documentation states that the fluid received by the sampling system should represent the pipeline cross-section as accurately as possible. When different phases are unevenly distributed, mixing is required before sampling.
Can the system use either jet mixing or a static mixer?
Yes. The ZHYQ-II-MJ documentation includes both jet mixing and static mixing as possible configurations. The appropriate method depends on actual operating conditions and engineering calculation.
Can the sampling rate follow the crude oil flow rate?
Yes. The system documentation describes flow-proportional sampling so that sampling rate can be proportional to the oil-transfer rate.
Can the system communicate with a DCS?
Yes. The documented controller supports RS485 and MODBUS RTU and can exchange operating and process information with the DCS.
What explosion-proof rating is available?
The referenced ZHYQ-II-MJ configuration lists Ex d IIB T4 and IP65. Final hazardous-area requirements should be confirmed for each project.
Custody Transfer Sampling Is a Complete Process
The key lesson for buyers and engineers is that custody transfer sampling should not be viewed as a single sampling valve or probe. A complete automatic crude oil sampling system needs to consider: Pipeline condition
Mixing
Sampling probe
Flow-proportional sub-sample collection
Sample storage
Sample homogenization
Laboratory preparation
Measurement and reporting Each stage can affect whether the final sample truly represents the transferred crude oil. For this reason, the best way to select an automatic sampling system is to evaluate the complete operating envelope of the project rather than choosing equipment only from a standard model list.
Need Help Evaluating a Crude Oil Custody Transfer Sampling Project?
ZNYB provides automatic crude oil sampling systems for pipeline trade-transfer and related petroleum measurement applications. Provide your pipeline size, process medium, minimum and maximum flow, pressure, temperature, density, viscosity, hazardous-area requirements, and control-system requirements to our engineering team.