Grab sampling systems are ubiquitous throughout processing plants, refineries and other industrial facilities. They play a key role in quality and process controls, compliance with regulations and verifying the performance of process analysers. To perform grab sampling, operators must capture a sample in a container, typically either a cylinder or a bottle, at a collection point and then promptly transport it to a remote laboratory for offline analysis. The sample must be representative of the process conditions at the time it is collected, and it must remain as close to those conditions as possible when it is analysed.
Specifying a grab sampling system
When configuring a grab sampling system, designers must first decide which sample transport vessel to use – either a sealed metal cylinder or a glass or polyethylene bottle. Both the system pressure requirements and the sample phase influence this decision. Cylinders may contain pressure and can be used for gas or liquid samples, while bottles cannot contain pressure and are used for liquid samples only, as they are not airtight. Other key criteria driving system design considerations include:
- Pressure: To ensure safe operation, the maximum rated pressure of a grab sampling system must not be exceeded. Additionally, a rupture disc or relief valve should be fitted to enhance safety when using chemicals that can rapidly expand and pressurise due to temperature changes.
- Temperature: The sampling system’s maximum fluid operating temperature must not be exceeded, to protect the integrity of its seats and seals. Conversely, the system must operate above its minimum operating temperature, so that the process fluid flows at a sufficient rate for timely analysis. If the supply temperature exceeds 60°C, the option of cooling the sample should be considered in order to reduce the risk to operators retrieving samples, while keeping in mind that this lower temperature could affect the sample’s representativeness.
- Material compatibility: The materials
used in a grab sampling system must be compatible with the process fluid as well as with the operating environment. The standard construction material is 316 stainless steel, but system requirements may dictate the use of alternative materials such as alloys 400 and C-276.
- Purging needs: Some chemicals may contaminate or corrode grab sampling lines, or possibly pose a hazard to the operator if not flushed from the system. In such cases, designers should add a purge setup to remove residual process fluid from the lines.
Capturing samples in cylinders
There are a variety of options for configuring grab sampling systems that capture gases or liquids in a sealed cylinder. Perhaps the most efficient design is a closed-loop system with the sample continuously circulating through the cylinder while the operator takes a sample. A closed-loop system pulls samples from a positive-pressure process and returns them back to the process at a lower-pressure location – for example, upstream of a pump – using the differential pressure to drive the fluid through the sample system. Such a design can reduce or eliminate system purging needs, as the sampling system becomes an extension of the process system. Upon opening the inlet valve of the Swagelok GSM-G-2(-N) grab sampling system with continuous flow and without purging, for example, process fluid will flow through the system tubing and the sample cylinder before flowing out via a hose to the outlet port. Any older process fluid remaining in the short inlet line will quickly move through the closed loop path and return to process as the cylinder fills. When the sample vessel is ready for removal, the operator can simply close the cylinder’s inlet and outlet valves and turn the system to vent, isolating the supply and return lines and allowing fill lines to vent. Finally, the operator will close the system’s inlet valve to halt all flow and remove the cylinder for lab analysis. The fluid in the cylinder remains under the same process conditions that existed at the time of the sample, except for temperature, and is therefore always representative of the process.
For gas and liquid samples
While grab sampling systems with cylinders can be used for gas or liquid samples, design considerations differ between the two types. Most notably, the flow path should be different for liquids and gases to purge out-of-phase fluid from the cylinder. Gases should flow from the top of the cylinder down, pushing out any liquid or condensate from the sample cylinder as it fills, to ensure liquid doesn’t collect in the cylinder and skew lab analyser readings. Liquids should fill from the bottom up to displace the vapour space and ensure the cylinder is full. Designers may also add an outage tube to cylinders capturing liquid samples to keep expansion space in the cylinder, as the trapped vapour may compress under pressure. Swagelok offers a variety of sampling cylinder (GSC) configurations, including options for purging bypass tubes, quick couplings with plug or body caps, outage tubes and specific certifications.
Capturing liquid samples in bottles
Liquid-only grab sampling systems draw fluid into a non-pressure-containing bottle. Operators draw the fluid directly from the process – perhaps after some conditioning – and then transport the vessel without risk of spillage or evaporation. Such systems can be used in a number of liquid applications, where the process fluid is not at risk of fractionating or evaporating when stored at atmospheric pressure to ensure the sample remains representative. This precaution allows designers to use less expensive glass laboratory bottles for samples, with the added benefit of gaining immediate feedback on the visual quality of the sample stream.With bottle sampling, the sample is taken at atmospheric pressure. Any increase in internal pressure may cause loss of sample through the lid or septum cap seal. Therefore, liquid grab sampling systems with bottles are typically used with water or other low vapour-pressure liquids. If bottle sampling is suitable, designers must determine whether continuous flow and purging are required or whether a fixed-volume option would be more appropriate.
Continuous flow during sampling
Continuous flow is useful during sampling when the sample requires constant motion – to keep it from freezing, for instance – or if a long tubing run leads up to the sample point. The sample flow runs continuously through a bypass loop in the grab sampling system, as shown in the Swagelok GSL3 “Continuous Flow Sampling System Design”. This ensures that the sampled fluid remains representative of the process, as it will not be sitting in the tubes for an extended period of time. An operator can then fill the sample bottle, using a spring-loaded sampling valve to retrieve the sample. If the sampled fluid has the potential to solidify in place, a purge assembly is advised to aid in cleaning the dispensing needle and internal tubing. If the sampled fluid is under high pressure or hazardous, designers should consider a fixed-volume system, which effectively isolates the process pressure from the user while limiting the volume of dispensed fluid. In a fixed-volume system, the sample first fills a metal cylinder and is then gently pushed into the sampling bottle by a low-pressure purge gas. This precaution helps prevent accidental overfilling.
Outage tubes as a safety mechanism
System designers should consider using an outage tube as a safety mechanism for any grab sampling system that captures a liquid sample in a cylinder. The outage tube enables a defined volume of vapour space to remain inside the cylinder when capturing a sample. This vapour space will allow the liquid in the cylinder to expand if the temperature increases. Without enough space, even a small temperature increase can cause the liquid to expand and the pressure to increase dramatically. During sampling, the cylinder is held vertically with the outage tube at the top, as shown. The length of the outage tube determines the amount of vapour space, which is expressed as a percentage of the cylinder’s total volume.
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