How can you sample the line-fill between the sampler and custody transfer point?
The volume left in a pipeline after a batch between the sampling system and the point of custody transfer (maybe a metering system or ships manifold) can be substantial. In the event that this is large it may be necessary to sample the line-fill to measure the quality, as in the case of ship unloading operations this is often when the worst part of the cargo is discharged.
The normal procedure for sampling line-fill is outlined below:
- The sampler must be capable of taking samples at a high grab rate to ensure that enough sample is taken to be representative of the line-fill volume (normally 10 litres).
- The sampling system must have two or more sample receivers with a capability to automatically change between receivers.
- The line-fill volume is a known volume that is programmed into the controller. When the batch starts the controller rapidly takes samples into a sample receiver. When the line-fill volume has passed the sampler (this is known because it operates flow proportionally) the controller automatically selects a different receiver and starts taking a normal batch sample into that receiver.
- The receiver containing the line-fill sample can now be taken to the laboratory for analysis and the result pro-rated with the batch sample by the batch/line-fill volume.
- A new receiver is placed in the system ready to sample the line-fill when the next batch starts.
- The batch sample is analysed in the normal way.
Using this method it is possible to obtain a representative sample of both the batch and line-fill and avoid the poor mixing and flow profiles problems associated with manifold samplers.