An operating case study: elemental sulfur deposition on gas regulator internals.
Discussion is presented on the problem of elemental sulfur deposition inside flow control valves, methods to diagnose the problem, and solutions to operating problems presented by the deposition problem.
Elemental sulfur has been a problem for natural gas transmission systems ever since natural gas transmission systems have been around. It is not localized to any one geographical location and most companies around the world have had to deal with its problematic properties at some point. (1, 3, 5, 6)
The most obvious problem encountered with elemental sulfur is with the fouling of nozzles or orifices as well as the build-up on regulator cages and even gas turbine blades. Adding to the problems associated with deposits of elemental sulfur is the typical frustrating experience of trying to find the source of the elemental sulfur and identifying the particular pipeline conditions that facilitate its deposition.
There is no convenient or field-efficient method to monitor for the presence of elemental sulfur in the gaseous phase, so the first indication of the presence is an operating condition issue such as a restriction or plugging problem.
Transversely, the deposition problem often "goes away" or simply stops occurring seemingly on its own so the exact source or cause of the sulfur is never found. Therefore the solution to the deposition problem is either no longer needed or cannot be worked to completion since it cannot be verified as having an effect.
Sources Of Elemental Sulfur
Elemental sulfur is naturally occurring and is one of the most common elements on earth. In natural gas, elemental sulfur is most often found in the vapor phase and can easily "drop out" or de-sublimate.
There also seems to be evidence that elemental sulfur can be created by a chemical reaction within the natural gas pipeline by the presence of oxygen and a sulfur compound. The oxidation of the compound, at the correct temperature and pressure, can reduce the sulfur compound to its elemental form.
The exact amounts required for the oxidation are not exactly known, but evidence seems to indicate that the oxygen in water and the sulfur found in compressor oils, for example, is enough to create elemental sulfur. Most often, elemental sulfur in vapor phase is in the low ppb level which again makes detection extremely difficult. (1, 3, 5, 6)
Flow Control Valve Problems
The control cages in valve regulators are often targets for elemental sulfur deposition because of the JT effects from the pressure drops as well as the large volumes of gas that flow through them.
In many cases with high-volume control valve regulators, some noise reduction is obtained by using cages for that purpose. These cages with slim or small openings can become fouled and quickly plugged by sulfur as it deposits onto first, the leading edges, and eventually over the entire surface of the opening. Soon thereafter, restrictions are noticed and the regulator must be torn down to investigate the restriction problem.
Restriction problems on other equipment on the same gas source may or may not experience a fouling problem due to different operating conditions or because of a smaller volume of gas flow through that equipment.
The amount of elemental sulfur that desublimates, of course, depends on concentration, but a relatively small amount can create a large issue if the cage openings are small enough; this is typically the case with reduced noise-type cages.
Regulator cages such as linear control cages will have fewer problems with restriction but the pressure drop and subsequent desublimation of the elemental sulfur can still occur.
However, considering the large volumes of gas flow required to create a buildup issue, along with the relatively small amount of sulfur that actually deposits on the surfaces, fewer restriction problems are experienced with this type of cage. But this cage does have noise increases which must be dealt with.
Methods To Diagnose The Problem
Diagnosing the elemental sulfur deposition problem is usually made by a physical inspection of a piece of equipment such as a regulator or being found during the scheduled cleaning of a metering tube, for example.
There currently is no standard instrument in the industry that can analyze for and detect elemental sulfur in vapor phase. Even if there were, the sampling system of the instrument would have to be almost as technically involved as the instrument itself.
Detecting the presence can be done relatively easily but quantifying the amount of sulfur present is much more difficult.
Qualitative assessment of the presence of elemental sulfur by using a simplified sampling method is relatively easy to conduct using standard gas sample cylinders.
By minimizing any transport piping and then inducing a pressure drop across the bottle, desublimation of any elemental sulfur vapor onto the interior cylinder wall should occur. The cylinder can then be washed with a liquid reagent and the subsequent reaction analyzed for sulfur.
Quantification of elemental sulfur is much more involved and difficult to perform. By bubbling the gas through a liquid reagent, the sulfur can be reacted and then analyzed using a sulfur chemiluminescence detector. This is a known analysis method but by no means could be efficiently used for online or continuous monitoring on a natural gas transmission system. (2, 4)
Solutions To Operating Problems Presented By The Deposition
Historically, there have been a limited number of solutions to the elemental sulfur deposition problem. In no particular order, these have consisted of: (1, 3, 5, 6)
* Elimination or a change of the operating conditions that are conducive to desublination;
* Finding and eliminating the source of the problem
* Removal of the elemental sulfur by processing
Changing or modifying the operating conditions that are conducive to desublimation is typically the most effective method of eliminating the elemental sulfur deposition issue.
Existing as a vapor, filtering is not possible and processing the elemental sulfur out of the gas is a costly and involved process.
One method that can be used is to reduce the amount of pressure drop across any equipment element in order to change the conditions.
Multi-stage regulation results in less of a temperature loss which brings the operating condition out of the desublimation zone for elemental sulfur. It is noted, however, that the elemental sulfur is still present but is kept in vapor phase so it is very important that it is held in this state until ultimately being consumed.
Additionally, line heaters have been used to keep the flowing gas at a temperature above the point of elemental sulfur desublimation.
Again, this is a change in operating condition and does not remove the sulfur from the gas but rather keeps it in vapor phase.
Elimination of the elemental sulfur source is the ultimate solution to the problem but poses many obstacles. First, the elemental sulfur must be found and then through a process of elimination, the actual source must be identified. This seems to be difficult given that the desublimation typically presents itself for a period of time and then disappears without any cause or source being identified.
The methods for qualitative and quantitative analysis to determine the source of the elemental sulfur are somewhat involved and not always feasible in field operations. Even after the actual source has been determined, there are limited options for correction of the problem. The source gas can be eliminated in its entirety but is not always possible or feasible. Processing the source gas can be performed but is a costly solution.
A field-level method of detecting and quantifying elemental sulfur in vapor phase is greatly needed. Given the unpredictability of the presence and then the possible desublimation of the deposition issues, almost all efforts in handling the problem are reactionary.
By the time a restriction or other operational issue is experienced, the deposition problem may be at a level where equipment damage on other parts of the system may be starting.
Additionally, by the time the deposition is noticed at any one location or area, it is extremely difficult to pinpoint the exact cause or source of the contamination. All of the elemental sulfur may not necessarily desublimate at any given point, thus remaining in vapor phase and may fall out at another location. This makes pinpointing the source very difficult, if not impossible.
Until a feasible, pro-active approach for monitoring and quantifying elemental sulfur content in natural gas is developed, the problem will continue to be handled after its occurrence, requiring clean-up as well as source investigation.
This article is based on a presentation made at the 2010 AGA Operations Conference held in May 2010 in New Orleans, LA.
(1.) Pack David J.: Elemental Sulphur Formation in Natural Gas Transmission Pipelines. Doctoral Thesis, University of Western Australia, 2005.
(2.) Clark Peter D. and Lesage Kevin L.: Quantitative Determination of Elemental Sulfur in Hydrocarbons, Soils and other Materials. Journal of Chromatographic Science, Vol. 27, May 1989.
(3.) Clark P.D., Davis P., Simon J., Fitzpatrick E., and Lau C.S.C.: Recent Developments in the Mitigation of Sulfur Deposition in Sour Gas. Facilities Department of Chemistry, The University of Calgary
(4.) Davis Paul M., Lesage Kevin L., Clark Peter D.: An Analytical Strategy and Test Protocol for the Determination of Low Levels of Elemental Sulfur in Natural Gas presented at the IGT Combined International Conference and Exhibition on Natural Gas Odorization and Natural Gas Quality & Energy Measurement, Chicago, IL, July 26-28, 1999.
(5.) Pack David J.: Elemental Sulfur Formation in Natural Gas Transmission Pipelines Paper 31 from 14th Biennal Joint Technical Meeting on Pipeline Research, Berlin 2003.
(6.) Pack David J., Chesnoy Andre, Bromly John, White Richard: Elemental Sulfur Formation in Natural Gas Transmission Pipelines The Australian Pipeliner January 2000.
By Robert Runyan, Measurement Supervisor, El Paso Pipeline Group, Houston, TX
Author: Robert Runyan is Measurement Supervisor for El Paso Pipeline Group where he has been employed for ten years. His responsibilities include supervising the Gas Analysis and Metrology Laboratory and Chromatograph Support group in Houston as well as providing analytical and technical support to the Gas Quality group.