Fixed volume sampling applies mainly to liquid sampling, where a specified volume of liquid is transferred to the sample container during the sampling procedure.
A fixed volume system is particularly useful when:
- Sampling from high pressure process lines
- Spillage is of concern
- 20% free volume needs to be maintained to allow for thermal expansion
When sampling at relatively high pressures, >5 barg, there is a likelihood of high flow or velocity occurring at the sample exit point, even with the use of needle or globe valves. When sampling, or purging, using relatively small sample containers, splash back is likely to occur, this is both undesirable and does not conform to many site safety standards.
Specified fixed volume sampling avoids spillage and ensures a repeatable amount of “free volume” above the sample when it is secured in the sample container. It is recommended for a liquid sample to have at least 20% ullage.
Fixed volume sampling ensures no over-spill or situations where samples are left flowing unintentionally. As a safety reinforcement, each fixed volume sample system is provided with dual needles, one longer than the other. The shorter needle functions as a secondary ullage tube, in case of failure upstream. The ullage/vent outlet is also vented safely into the correct drain or flare sometimes via a carbon filter.
"During a grab sample evaluation at a UK refinery, Swagelok Bristol witnessed dangerous sampling procedures, resulting in splash back of a very hot sample and sample containers filled to more than 80% capacity."
This dangerous splash back occurred due to a combination of coarse flow control, delivered from the current sampling valve and high (18barg) process pressure. The actual sample temperature was measured at 160°C.
Overfilling of the sample containers occurred because of a substandard sampling procedure. An accurate sampling procedure would specify to the operator the amount of vapour space required, aided by fill line markings on the sample containers. It's worth pointing out that the site standard referred to the vapour space requirement as; “fill the sample bottle to the shoulder of the bottle – do not overfill”. This is somewhat inadequate as “to the shoulder” would only provide approx, 10% vapour space, and it is subjective where the shoulder of the bottle is.
The recommendations were to;
- Upgrade hazardous open liquid sample points to fixed volume systems for safety.
- Update local procedures to include specific ullage volume.
- Where non-fixed volume bottles are still used, replace bottles with ones that indicate max fill line.
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