Crude oil flow through a custody-transfer pipeline rarely remains perfectly constant from the beginning of a batch to the end. Start-up, pump changes, valve movements, process conditions and shutdown can all change the instantaneous transfer rate.
If an automatic sampler collects increments only at fixed time intervals, a low-flow period and a high-flow period can contribute the same number of increments even though very different volumes of crude oil have passed through the pipeline. Flow-proportional sampling is designed to address this mismatch.
The central principle is simple: the contribution to the composite sample should follow the transferred oil volume, not elapsed time alone.
This article explains how that principle works, why it matters in crude oil custody-transfer sampling, and what buyers and engineers should consider when configuring an automatic sampling system.
What Is Flow-Proportional Sampling?
Flow-proportional sampling links sampling activity to the quantity of crude oil moving through the pipeline. In the documented ZHYQ-II-MJ arrangement, the sampling controller can receive instantaneous oil-transfer flow from the DCS and use the project-configured control strategy to determine sampling activity.
When the individual increment volume is held constant and sampling frequency or interval is the control variable, the practical relationship is:
- Lower transfer rate → fewer sample increments
- Higher transfer rate → more sample increments
The individual increments are collected throughout the transfer and accumulated in a sample receiver. The objective is to build a composite sample whose contribution from each part of the batch more closely corresponds to the crude oil volume transferred during that period.
Flow-proportional sampling does not mean that one universal frequency is correct for every installation. The final strategy depends on batch throughput, minimum and maximum flow, desired composite-sample volume, increment volume, transfer duration and the control signals available at the site.
Why Fixed-Interval Sampling Can Bias the Composite Sample When Flow Varies
Consider two equal one-hour periods during the same transfer:
- During Period A, the flow rate is relatively low.
- During Period B, the flow rate is three times higher.
A sampler operating once every fixed minute would take the same number of increments during both periods. However, Period B transfers three times as much crude oil. Giving both periods equal influence can make the accumulated sample less representative of the total transferred volume.
A flow-proportional strategy instead gives the higher-flow period a greater contribution. The exact number of increments is project-specific, but the underlying relationship remains the same: more transferred oil should contribute more to the final composite sample.
Fixed-time operation is not automatically unsuitable in every process. Its appropriateness depends on how stable the transfer rate is and how the complete sampling plan is designed. For a batch with meaningful flow variation, however, time alone may not provide the most representative basis for increment collection.
The Goal Is a More Representative Composite Sample
Custody-transfer sampling is not simply the act of removing oil from a pipeline. The purpose is to obtain and preserve a sample that can represent the transferred batch for subsequent evaluation.
Two different questions must therefore be addressed:
- Is the fluid at the sampling point representative of the pipeline cross-section?
- Does the accumulated sample reflect the volume transferred over the complete batch?
Pipeline mixing and correct probe positioning help address the first question. Flow-proportional increment collection helps address the second. These functions are complementary rather than interchangeable.
A sampler can follow the flow signal accurately and still collect a poor sample if the crude oil at the sampling point is not sufficiently mixed. Conversely, a well-conditioned pipeline stream can still be represented poorly over time if the sampling schedule does not account for major changes in transfer rate.
How Flow Data Reaches the Sampling System
In a modern automatic sampling installation, the sampler can operate as part of the wider custody-transfer measurement and control architecture rather than as an isolated mechanical device.
The ZHYQ-II-MJ reference documentation describes a control system capable of receiving instantaneous oil-transfer flow and other process values from the distributed control system (DCS). The documented communication arrangement includes RS485 and Modbus RTU. Project implementation can vary, so the available signal type, scaling, update rate and communication requirements should be confirmed during engineering.
A simplified control sequence is:
- The metering or control system provides instantaneous flow information.
- The sampling controller processes that information according to the configured sampling plan.
- The sampler takes discrete increments at the required points during the transfer.
- The increments accumulate in one composite-sample receiver for the batch.
The flow signal controls sampling activity; it is not part of the liquid sample path. This distinction is important when reviewing system diagrams and control interfaces.
What Does the Automatic Sampler Collect?
An automatic crude oil sampler normally builds the final sample from many small, discrete increments. It does not divert the full process stream into a sample container and it does not fill the receiver in one continuous operation.
For the documented ZHYQ-II-MJ reference configuration, the individual increment volume can be adjusted from 1.0 to 10 mL, and the separator is documented with a maximum sampling frequency of 30 increments per minute. These are reference product values, not the correct settings for every project.
The required increment volume and sampling activity must be selected together. A project team should consider:
- Total transferred volume
- Minimum and maximum operating flow
- Expected transfer duration
- Required final composite-sample volume
- Available receiver capacity
- Minimum practical increment size
- Maximum permissible sampling frequency
- Available instantaneous flow and control-system signal
The objective is not simply to maximize the number of increments. The objective is to distribute appropriate increments across the transferred volume while remaining within the operating limits of the sampling equipment and sample receiver.
Sampling Frequency and Increment Volume Work Together
In a typical frequency-based arrangement, increment volume remains constant while the time between increments changes with the transfer rate. The exact control logic and signal interface remain project-specific and should be confirmed during engineering.
Whichever strategy is selected, the engineering calculation should connect four quantities:
- Transferred batch volume
- Number of planned increments
- Volume of each increment
- Required final sample volume
If these quantities are not considered together, the receiver could collect too little sample for the intended analysis or reach its configured limit before the transfer is complete.
Flow-Proportional Sampling Does Not Replace Pipeline Mixing
This distinction deserves special attention. Flow proportionality describes how the batch is represented over transferred volume. Pipeline mixing describes whether the fluid presented to the probe is sufficiently representative at the selected sampling plane.
The ZHYQ-II-MJ can be project-configured with jet mixing or static mixing. The appropriate method depends on pipeline size, operating-flow range, pressure-loss limits and the required mixing performance. Our detailed comparison of jet mixing and static mixing explains the selection considerations.
For broader guidance on automatic pipeline sampling principles, see our ISO 3171 crude oil sampling overview.
What Happens as the Transfer Rate Changes?
A simplified batch profile might include the following stages:
Start-up → increasing flow → normal transfer → reduced flow → shutdown
During start-up and shutdown, less oil may pass through the pipeline per unit of time. During the main transfer period, the flow may be substantially higher. A flow-proportional strategy allows the high-volume portion of the batch to contribute more increments while lower-volume portions contribute fewer.
This relationship is particularly relevant where pump changes, operational constraints or terminal procedures produce a wide or variable flow range. It helps prevent equal time periods from being treated as if they always represented equal transferred volumes.
The Role of the Sample Receiver
The increments collected throughout the batch accumulate in a closed sample receiver. The ZHYQ-II-MJ reference documentation lists portable pressure-resistant receivers in 5 L, 10 L and 20 L configurations, with a documented pressure rating of 0.3 MPa. The standard reference arrangement includes one receiver in service and one standby receiver.
Receiver selection should be based on the planned number and volume of increments, the required laboratory sample quantity and the complete transfer duration. The receiver must have adequate capacity without allowing the sampling plan to become unnecessarily sparse.
The sample must also remain suitable for later preparation. Flow-proportional collection can improve how the batch is represented, but it cannot prevent the accumulated crude oil from separating after collection. Sample handling and homogenization therefore remain part of the complete sampling process.
Sample Homogenization After Collection
The ZHYQ-II-MJ can be equipped with an optional sample mixing and splitting system. In the documented arrangement, stored sample is circulated from the bottom of the receiver, passed through a six-unit static mixer and returned to the same receiver through jet piping. A typical documented mixing time is approximately 5–20 minutes, depending on sample volume and crude-oil properties.
This stage takes place after the increments have been accumulated. It should not be confused with mixing in the main transfer pipeline before the sampling probe.
Representative sampling begins in the pipeline, continues through proportional increment collection, and must be preserved during sample storage and laboratory preparation.
Monitoring the Sampling Process
The documented ZHYQ-II-MJ arrangement includes weighing-based sample monitoring. Sample count and sample weight can be communicated to the DCS, helping operators follow the progress of sampling during the batch.
If the measured sample weight reaches a configured limit because of an unexpected condition, the weighing function can stop sampling and generate an alarm. This does not replace correct sampling calculations, but it provides an additional monitoring layer during operation.
During project design, engineers should confirm which status, alarm and process values need to be exchanged between the sampling controller and the site control system.
What Project Data Is Needed?
There is no single flow-proportional setting that is appropriate for every custody-transfer installation. Before configuration, the engineering team should receive reliable project data, including:
- Pipeline size, schedule and material
- Process medium and expected crude-oil properties
- Minimum, normal and maximum operating flow
- Total batch throughput and expected transfer duration
- Operating and design pressure
- Operating and design temperature
- Density and viscosity range
- Required sample volume and receiver size
- Hazardous-area classification and explosion-protection requirements
- Power supply and environmental conditions
- Available DCS and communication signals
- Required operating-status, alarm and sampling data
This information supports selection of the sampling plan, mixing method, probe arrangement, receiver capacity and control interface. The ZNYB Project & Ordering Data Sheet can be used to organize the required parameters before technical evaluation.
Questions Buyers and Engineers Should Ask
- How will the sampling controller receive and process the instantaneous transfer-flow signal?
- How will sampling activity respond during start-up, peak flow and shutdown?
- What increment volume and total increment count are planned?
- Will the receiver capacity cover the complete batch?
- How is sample count or sample weight monitored?
- Is the fluid sufficiently mixed at the sampling point?
- How will the composite sample be homogenized before a laboratory portion is taken?
- Which project data and control-system signals are required before configuration?
Frequently Asked Questions
Is flow-proportional sampling the same as fixed-interval sampling?
No. Fixed-interval sampling is primarily based on elapsed time. Flow-proportional sampling links increment collection to the oil-transfer rate. If the flow varies, the two methods can give different contributions to the composite sample.
Does higher flow always mean a higher sampling frequency?
Conceptually, higher-flow portions of a batch should make a greater contribution to the composite sample. The exact control method can vary by project and must account for increment volume, receiver capacity, equipment limits and available signals.
Can the flow signal come from the DCS?
Yes. The ZHYQ-II-MJ reference documentation includes instantaneous oil-transfer flow among the process values that the sampling cabinet can receive from the DCS. The actual communication method and signal requirements must be confirmed for each installation.
Does flow-proportional sampling eliminate the need for pipeline mixing?
No. Mixing and proportional sampling solve different problems. Mixing helps make the fluid at the probe representative of the pipeline cross-section, while proportional sampling helps the accumulated sample represent the transferred volume across the batch.
What is the documented maximum sampling frequency?
The ZHYQ-II-MJ reference documentation lists a maximum separator sampling frequency of 30 increments per minute. This is an equipment reference value, not a recommended operating setting for every project.
Can the collected sample volume be monitored?
Yes. The documented configuration includes sample weighing, sample-count reporting and an alarm function that can stop sampling if the configured collection limit is exceeded.
Flow-Proportional Sampling Is Part of a Complete Strategy
Flow-proportional sampling should not be viewed as an isolated controller feature. Its value depends on the complete sampling chain:
Pipeline conditioning → Correct probe location → Flow-proportional increment collection → Controlled accumulation → Sample monitoring → Homogenization → Laboratory preparation
The goal is not simply to collect more increments. The goal is to build and preserve a composite sample that better represents the crude oil actually transferred during the batch.
Final sampling settings and system configuration must be engineered for the actual pipeline, fluid properties, flow envelope, batch conditions and site requirements.
For project-specific evaluation, review the ZHYQ-II Automatic Crude Oil Sampling System, download the Project & Ordering Data Sheet, or contact the ZNYB technical team.