Modern refinery operations process an incoming crude oil stream into multiple high value product streams, along with byproducts streams that need additional processing. Both the high value and byproduct streams require periodic sampling to ensure that the refinery processes are operating within specified parameters. Due to the often high pressure and chemical nature of the processes, obtaining a representative sample safely is essential.
In the petroleum refining process one of the main byproducts is wastewater rich in hydrogen sulfide (H2S) and ammonia, commonly referred to as sour water. The term sour water is used because – in addition to contaminants such as NH3, solids, chlorides, mercaptans and phenols – H2S, or hydrogen sulfide, is present in the water stream. Plants sample their sour water to measure these chemicals; they must be removed to prevent contamination and corrosion. Hydrogen sulfide is considered a broad-spectrum poison, meaning that it can poison several different systems in the body, although the nervous system is most affected. OSHA has established a permissible exposure limit (PEL) (8 hour time-weighted average) of 10 ppm.
Accurate measurement of hydrogen sulfide and ammonia concentrations at the outlet of the H2S stripping unit, also called the stripper, provides critical process validation information and allows for corrective action in the event of stripper failure. The presence of hydrogen sulfide in the sour water stream requires special handling and/or equipment when taking a sample. A sampling system must use exhaust and ventilation equipment to reduce hydrogen sulfide levels. The system also needs to be non-sparking, grounded, corrosion-resistant, separate from other exhaust ventilation systems and explosion-proof.
In the absence of this type of sample station, the operator taking the sample needs to follow stringent PPE (personal protective equipment) requirements, which include respiratory protection and other personal protective equipment (PPE), such as eye protection and possibly fire-resistant clothing. Respiratory protection includes:
- For exposures below 100 ppm – An air-purifying respirator with specialized canisters/cartridges for hydrogen sulfide. A full face respirator will provide eye protection.
- For exposures at or above 100 ppm – A full face pressure demand of a self-contained breathing apparatus (SCBA) with a minimum service life of 30 minutes or a combination full face pressure demand supplied-air respirator with an auxiliary self-contained air supply. Exposures at or above 100 ppm are considered immediately dangerous to life and health (IDLH).
Obviously, both options are problematic for end users. While competitor sample systems are complex and expensive, if they do not meet special design requirements, the operator taking the sample must suit up in PPE gear, which is time-consuming, and the gear can be hot, cumbersome and uncomfortable to use. Most of all, taking a sample in this manner is not necessary, as better, safer and more effective solutions are available for refinery sampling.
For several customers, our engineers evaluated available options and decided a new approach was possible. A standard, time-proven sampler design, the Sentry® MVD manual low-emission sampler, was adapted to provide a safe, yet straight-forward sampling solution. The MVD is a closed loop manual sampler with a double block valve configuration that assures an extra degree of safety for operators. Samples are collected in a bottle with a septum. The MVD uses non-coring needles to pass through the septum, which seals around the needles and isolates the sample from the environment.
For our refinery customers, the MVD design was modified to locate the sample container portion of the MVD within a windowed enclosure. This allows the operator to operate the valves and deposit the sample into the contained environment. The enclosure is provided with an eductor system that allows any H2S gas to be purged from the enclosure before the operator opens the enclosure to remove the sample. The MVD model can be designed with a wide variety of bottle, septum and needle/tube stub configurations and sizes. Because some customers have issues with plugging, a common solution is a half-inch MVD tube stub design with an enclosure and educt system.
Although the bottle septums are self-sealing, some operators believe more protection is needed during transport, so they remove the cap with the pierced septum and replace it with an intact cap, which risks the escape of the volatile gases that threaten the safety of the operator and the environment and compromise the sample integrity. The new Sentry No-E Double Cap offers takes care of this issue with a cap-on-cap design that features external threads on the existing cap to allow placement of another cap on top of it – so the bottle can be completely and safely sealed.
This allows the user to cap and transport the sample bottle safely, protecting the safety of the operator and environment while preserving sample integrity. Together, these solutions simplify operation and provide protection from H2S exposure at an affordable cost. Operators do not need to suit up in extensive PPE, reducing the time and hassle of sampling and increasing safety.
Because the MVD sampler design is based on a standard design, many options – such as sample cooling, nitrogen purge, enclosure air make-up, carbon vent scrubbing or special coatings for wetted process components – are available.
A special thank you to Kevin Kirst for contributing significantly to this article.
Learn more about the MVD sampling solution.