Document 3JYenbzkMa4pkO4MxEYjjDDGa
(conoco)
Interoffice Communication
to T. H. Martin
From G. G. Draper
Date August 27, 1975
subject
Loading of "City of Tampa" Barge on May 28, 1975
On May 28, 1975 the barge "City of Tampa" was loaded with VCM at the Refinery #3 Dock. In the initial phase of the loading operation, an unusually large amount of VCM was observed venting from the 148 foot vent recovery stack. The purpose of this letter is to outline in writing three potential causes for this VCM release. These potential causes have been previously reviewed in a meeting on July 10, 1975 with Messrs. Martin, Cowden and Jakes prior to a second loading of the "City of Tampa" barge on July 14, 1975.which was accomplished without incident. It should be noted that the ideas presented in this letter are purely speculative since there were no VCM Plant personnel at the dock when the incident occurred.
The first potential cause of the release is on the barge itself. The piping manifold is such that there is a block valve which separates the vent header from the liquid fill line. If this valve was inadverently opened during the loading operation, it would provide a direct route for liquid VCM to reach the vent stack. However, a short time after the incident occurred, this valve was observed to be in a closed position. In addition, the liquid knockout pot (see attached sketch) in the vent recovery unit was observed to have a low level of liquid VCM a short time after the incident occurred. If the VCM release had occurred as a result of the opening of the valve on the barge, the knockout pot would have had to be completely full to allow liquid VCM to reach the stack.
A second potential cause is that the vent recovery compressor capacity was exceeded by a sudden surge of VCM vapors. The normal startup procedure for the vent recovery compressor, as observed on occasion by VCM Plant personnel, is to lower the discharge pressure controller on the Sundyne compressor until the compressor is on line and then gradually increase the back pressure. This procedure is utilized to keep the compressor from surging during the startup operation. The vast majority of the VCM ships loaded at the #3 Dock have been nitrogen purged and as a result the vent recovery compressor for the first few minutes of operation handles mostly nitrogen. This was not the case for the "City of Tampa" barge as it was under a VCM atmosphere and a pressure of 55-60 psig. Therefore, if the back pressure controller was set on a low pressure and the 6" vapor vent valve on the dock opened suddenly, the compressor capacity could be exceeded resulting in pure VCM vapors blowing past the compressor and out the vent stack. However, a short time after the incident occurred the back pressure controller was observed to be set on the normal operating pressure of 90 psig. It should be noted that later on in the loading operation the vent recovery compressor was put in operation with the barge tank pressure at 130 psig without incident.
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T. H. Martin Page 2
The third potential is that one of the valves utilized for clearing the spool sections (see attached sketch) for blind removal on the liquid loading and vent line was inadverently left open. This would provide a direct route for VCM to reach the vent stack. However, on inspection after the incident occurred these valves were observed to be in a closed position. In summary, there is no conclusive evidence as to why the VCM release occurred on the first loading of the "City of Tampa" barge. However, one of the above mentioned three items appears to be the most likely cause. To minize the potential of an accidental release of VCM occurring in the future, the'independent cargo inspector will check each ship to determine if there are any interconnecting valves between the liquid fill and vent lines. If such a valve exists, the inspector will place a seal on the valve to insure that it remains closed during, the loading operation. In addition, I would recommend that the dock operators take extra precaution to insure that the valves utilized in clearing the spool sections on the liquid loading and vent lines are closed tightly during the loading operation. Startup of the vent recovery compressor is also extremely critical when the compressor is taking suction on a tank which contains VCM vapor under pressure. Lowering the discharge pressure on the compressor and opening the suction valve suddenly may exceed the capacity of the compressor. The compressor can be made to operate correctly as was evidenced later in the loading operation of the "City of Tampa" when the compressor was placed on the line with the barge tank pressure at 130 psig. If I can be of further assistance please advise.
6. G. Draper bw CC JAD-CEG-JHM-REL-GAO-RHC
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ACV. 10.1.13
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