Key points
Enhancing Analyzer Accuracy and Operator Safety for Better Results
Grab sampling for offline process fluid analyses isn’t necessarily as easy as grab and go. You must first ensure the sample is representative of the process property or composition that you are trying to measure, so analyser measurements are accurate and meaningful. You also need to play it safe, as grab sampling carries more inherent risks for system operators compared to online sampling. To help improve your plant’s grab sampling operations, here are eight guidelines for enhancing system accuracy and safety.
Improving accuracy
At the time of lab analysis, captured grab samples must be as representative of the process conditions as possible. That means ensuring the sample itself was fully mixed within the process line and that it won’t become contaminated along its route to the sampling vessel. The following tips will help you improve sample representativeness – and therefore analyser accuracy.
Use Sample Probes
When drawing samples from a process line, faster-moving fluid near the centre of the line will likely be better mixed than slower-moving fluid near the line’s wall. The faster-moving fluid has a turbulent flow that encourages mixing, while the slower-moving fluid has a laminar flow that may contain higher amounts of solid particles (with a liquid sample) or liquids (with a gas sample) than the overall process stream (Figure 1).
Therefore, samples drawn from the laminar flow area may not be representative of the process. Instead, industry standards recommend installing a probe at every grab sampling tap. [1, 2] The probe will poke through the tap into the centre of the process stream to draw samples from the turbulent, better-mixed fluid.
Avoid Sampling from the Bottom of Process Lines
Particles within liquids flowing through process lines are prone to collecting and moving slowly along the bottom of the lines. The same is true for liquids in gas streams. If you mount a grab sampling tap to the bottom of a line, this settled matter will end up in the sampling vessel, creating a non-representative sample. Ideally, all grab sampling nozzles should be mounted horizontally – with a probe – to ensure samples are well mixed and representative. [1, 2]
Remove Deadlegs
Deadlegs (Figure 2) represent dead spaces in sampling systems that trap old sample material, which can then bleed into new samples being drawn for analysis.
The result is a mixed sample that isn’t true to real-time process conditions. Deadlegs may eventually dissipate without intervention or not. They do not behave in a predictable manner.
Generally, deadlegs become more problematic as the ratio of length to diameter increases. In addition, lower flow in the analytical line increases the degree of the deadleg’s effect. Deadlegs may occur at tees, pressure gauges, temperature indicators, and other areas.
Minimise or avoid installing such components upstream of the sample bottle or cylinder to prevent new sample contamination. If practical, relocate any upstream tees downstream of the sample vessel, as the resulting deadlegs won’t affect sample integrity there.
Use the Right Hoses
Some hoses shouldn’t be used in certain areas of sampling systems. For example, a corrugated hose used to connect a liquid sampling station to a bottle runs the risk of old sample material remaining in the corrugations – even following extended purging.
That old material can contaminate newly extracted samples, skewing analyser results. Use a smooth-bore hose instead to ensure drainability and purgeability. In addition, ensure hoses are compatible with system media to promote longevity.
Fully Purge Cylinders Before Sampling
Plant laboratories may not empty grab sampling vessels before returning them to service. Any material remaining inside will not be representative of the current process and must be purged prior to collecting a new sample.
In doing so, the sampling system must have a separate purging option or sufficient flow to fully purge the old sample to a vent or a disposal system. Therefore, it’s helpful to install indicators within the system to verify flow.
However, the best practice to avoid sample contamination is to follow industry standards that recommend cleaning and drying sampling cylinders before reuse. [1, 2] Even then, it may be wise to purge the empty cylinder to ensure sample representativeness.
Enhancing safety
The personal safety of people in and around the plant is the highest priority for fluid processing plants. To address operator safety, plants need to reduce risks associated with both drawing samples and transporting sample vessels. They can also perform preventive maintenance to boost safety. Here are a few ways to enhance safety for your operations.
Mount Sample Vessels Properly
Capturing and storing samples in cylinders or bottles and transporting those vessels to a lab for analysis carries inherent risks. Both types of vessels often hold materials at elevated temperatures, increasing the risk of burns when grabbing and transporting samples.
In addition, cylinders often capture samples at high pressures, exposing operators to potential risk during sampling and analysis.
To enhance safety, mount cylinder supports or bottle shrouds and stirrups in grab sampling stations. These supports securely hold vessels in place, so they don’t hang freely from sample lines.
Free-hanging vessels place stress on quick-connect couplings, which could potentially lead to a severed connection that blasts an operator with hot, high-pressure fluid. Clamping vessels in place reduces stress on quick connects, increasing their service lives, while also improving operator safety.
Use Outage Tubes in Sample Cylinders
Operator safety is especially critical when filling and transporting cylinders containing liquified gas samples. The cylinders must have a sufficient volume of vapour space inside to account for expansion of their contents.
Without enough space, a small temperature increase can cause the liquid to expand and the pressure to increase dramatically, creating a potential safety hazard. Refer to local regulations and other appropriate guidelines for safe cylinder filling limits for your application.
Installed in the top of vertically mounted cylinders, outage tubes enable a defined volume of vapour space to remain inside cylinders when capturing samples. This vapour space allows the captured liquid to expand if the temperature increases.
Without using an outage tube, an operator may overfill a cylinder, presenting the risk of a pressure release during transport, analysis, or reinstallation into the sampling system.
Properly Install and Maintain Hoses
When using hoses to connect a sample vessel to a sampling panel, avoid installations that place undue strain on hoses, which can shorten their service lives or cause failures.
Follow these best practices – and inspect hoses frequently for signs of wear – to both enhance operator safety and reduce hose maintenance costs:
| Follow Minimum Bend Radius Guidelines: Installing a hose with too small of a bend (Figure 3a) may cause it to kink, reducing its life. Bending a hose too close to the hose/fitting connection (Figure 3b) may cause it to rupture or leak. Install a longer hose to fix either issue. |  Figure 3a.  Figure 3b. |
| Avoid Undue Hose Strain: Failing to use elbows and adapters to relieve hose strain on horizontal end connections can lead to leaks (Figure 4). When adding these accessories, you may need to adjust the hose length to maintain the minimum bend radius. |  Figure 4. |
| Bend Hoses on One Plane: Avoid twisting a hose or bending it on more than one plane to prevent undue stress on the hose. For a compound bend, use multiple hose pieces or other isolation methods (Figure 5). |  Figure 5. |
| Replace Damaged Hoses: Replace any hose that has been kinked or has a damaged reinforcement layer. Such hoses could leak or burst at any moment. If damage is limited to the cover and does not reach the reinforcement layer, you can likely leave the hose in place until the next scheduled replacement interval. |
Improving operations
With high stakes for ensuring accurate analyser readings and plant and operator safety, processing plants should consider following the above recommendations to enhance their grab sampling operations.
Through careful system design and operation, they can ensure sample compositions are representative of the original process conditions at the time of analysis, as well as reduce a number of risks to system operators.
