The Journey Towards Zero-Exposure
One of the recent trends we have observed with rising emphasis is attention to and reduction of H2S personnel protection monitor “hits” at refineries and other facilities processing H2S laden, “sour” media. In 2010, the American Conference of Governmental Industrial Hygienists (ACGIH) – one of a handful of organizations informing benchmarks for jurisdictional agencies such as OSHA – changed the recommended exposure limits for airborne H2S exposure down to a threshold limit value (TLV) of 1 ppm over an 8-hour time-weighted average and a short-term exposure limit (STEL) of 5 ppm. Previously these values were 10 ppm and 15 ppm, respectively. While the enforceable OSHA limits remain slightly higher, many Health, Safety & Environmental (HSE) teams at processing facilities have taken note and are keen to comply with lower thresholds.
To a large degree, similar such low threshold emphases have been in place for some time. Still, in many facilities, there has also existed a reasonable detente for what is deemed to be a somewhat inevitable and/or unavoidable risk of exposure. After all, there is risk in everything we do; experienced and trained operations staff are entrusted to take proper precautions and utilize supervision. Each industry, unit and situation is unique, having its own distinct history of design, construction, and operation. Compromise – coupled with common sense – prevails. One of the most prominent risks of exposure occurs when an operator takes a sour vapor or liquid grab sample from a unit and it is transported to a lab for analysis. The life of the sample cylinder here is also worth noting, as it changes hands typically from an operations team to a lab team and then back again. In a refinery, for example, the sour samples and sample points are well-known by both operations and lab personnel. The appropriate level of care and precaution is taken when handling these samples in order to keep people safe, but depending upon the facilities available – both in the field and in the lab – the risk for H2S exposure remains, and consequently there is the occasional “hit” (and subsequent paperwork).
Fundamentally, any further mitigation of this risk (other than suiting up with fresh air, which is never popular) must be done by venting / purging the temporary connections (i.e., quick connects or similar) used with the sample cylinder whenever connecting and disconnecting the cylinder in the field and – often – in the lab as well. While certain lab analyses are performed underneath a vacuum hood with sufficient shield protection for lab personnel, there are also many sample cylinder extractions (i.e. – GC injection) that may not always have the luxury of a hood.
Altogether the journey towards a “zero-exposure” world requires additional facilities in the form of inert gas supply for purging and a vent or drain connection for sour process media to flow. This seems straightforward, and there are many ways to do this, but for, say, dozens of sour sample points in a given plant, this task grows into a larger project to manage, particularly across multiple units in a refinery and the lab.
The P&ID right (Figure 1) shows a generic template for a sampling arrangement in which a purge bypasses the sample cylinder. A check valve in the bypass helps force sample flow through the sample cylinder until the sample cylinder valves are isolated and the purge is initiated in the reverse direction. Often overlooked, pressure indication also lets the operator verify – very basically – that the system is depressurized and the sampling quick-connects are safe to uncouple.
However, this often only addresses half of the problem. The same – or similar – facilities are required in the lab to ensure that the sample cylinder can be safely decoupled and brought back out into the field. Sour media will reside behind the quick connect and build pressure if not vented/purged prior to being sent back out to the field. As the quick-connects see service life and start to leak, this only increases the risk of exposure.
Sample valves – both at the sample station and on the sample cylinders – often utilize elastomers in the stem packing. It is important that these elastomers consider the process media, and if it is sour, then a Kalrez variety is often recommended. A similar tenet may apply with quick connects. It is important to also pay attention to the minimum ambient temperature rating required for reliable operation. For example, QC series quick-connects, which are common, are only rated to 60°F with Kalrez, so we often suggest the QTM series as an alternative because they are rated to 0°F and contain no o-ring. This change has a ripple effect in the lab as well as it changes the interface for the sample extraction.
The process of handling sour sample cylinders and maintaining a zero-exposure approach requires a balanced view of the entire process between individual process units, operations teams, lab personnel and the purging/venting resources available at all points. Moreover, a managerial commitment is necessary to enact the necessary changes in order to limit H2S exposures, and we see this more and more. Keeping people safe remains an industry trend, and this is an all-around positive.