By Norm Lieberman
The use of the coke drum motorized switch valves in new delayed cokers has become pretty much standard. They have replaced the older Wilson-Snyder manual valve.
Problems and Pluses
Advantages of the motorized switch valve are obvious:
- Speed in switching. The valve can be swung over from one drum to the other in ten seconds. The Wilson-Snyder valve would take a minimum of ten minutes.
- Reduced manpower. The Wilson-Snyder valve was a two man operation. The motorized valve requires only the use of a switch key and a push button.
- The motorized valve essentially eliminates the manual effort associated with the Wilson-Snyder three-way switch valve.
Problems with the Motorized Valve:
On several new cokers, and on one retrofitted unit, we have observed a number of inter-related and rather serious drawbacks with the motorized switch valve. It is our view that the nature of these problem calls into question the actual benefit of using the new automated switch valve, in comparison to the old Wilson-Snyder three-way valve. That is, newer is not always better:
- Steam Requirements: There are eight steam purge points on the switch valve. Should an adequate flow of steam not be kept to these points, the switch valve may stick. If this happens, the valve may still be manually cranked, but this is a difficult and time consuming procedure. The steam flow to these eight points is controlled through restriction orifices.
- Typically, several thousand pounds per hour of steam is required. The cost of the steam is not an important issue. It is the effect of the fractionation tray vapor load and the overhead condenser duty that is of concern. A rough rule of thumb for a delayed coker limited by fractionation tray capacity is that each 1,000 pounds per hour of steam entering the fractionator displaces 400 BSD of coked feed.
- Switching Speed: On at least two new delayed cokers, operating personnel have attempted to switch the coke drums in three steps (i.e., stopping twice for ten minutes at each interval). However, this practice was abandoned when the motorized switch valve was found to stick part way through the switch which then necessitated manual cranking. The valve now is switched in ten seconds, which avoids the sticking issue, but which may be promoting coke drum foam-overs. Very likely, the sticking on the valve, when the switch was stopped for ten minutes, was a consequence of inadequate steam purge flow to several of the eight purge points. Regardless, the tendency of the motorized switch valve to get stuck, and then have to be manually cranked-over, is an operating problem which must be considered.
- Switch Valve Delta P: The automated switch valve creates a substantial pressure drop during its travel. This causes an appreciable increase in the coker heater passes outlet pressure. In one new delayed coker, the operators find that they must place the heater feed pass control valves on manual, to prevent over-pressuring the heater passes during switching. The resulting temporary loss of flow through the heater coils will promote heater tube coking. Using the quick switch option of the motorized valve, rather than a conventional 20 minute switch, largely eliminates the problem. However, the quick switch also can cause a loss in pressure and a foam-over from the full coke drum, unless the panel board operator intervenes to hold back-pressure on the coke pressure drums, by raising the fractionator operating pressure.