THIS MONTH'S PUZZLER
We bought an old refinery with a history of safety and reliability problems. One area we're particularly concerned about is the current light slop oil system (Figure 1). The knock-out drum has a level transmitter that only covers part of the drum diameter, the containment area around the drum drains to the storm water sewer, and the slop-oil and sour-water level control valves are fail-closed. In addition, the minimum flow orifices of the knock-out-drum pump only are sized to prevent over-heating of the bowls. Then, there's the sump that the sour water empties into: it uses a simple bubbler for level control; there's no conservation vent or emergency vent, nor vapor space control in the sump pump room and the sump overflows to the ground. How should we establish our budget to modernize this system? Will we have trouble getting new permits? How can we improve the reliability of the equipment?
START WITH THE LEVEL METER
First, consider the slop-oil-system instrumentation/controls/safety. If the inflow to the slop oil knock-out (KO) drum contains significant solids, the impulse legs of a typical dP transmitter may plug. Consider a capillary-filled system. Alternatively, you may want to look at guided wave radar (GWR) or nuclear level measurement. However, before considering a change in technology, it may be helpful to identify what specific problems you have with the existing level meters. It may be that a relatively simple change may solve the problem.
Level transmitters based on liquid head are susceptible to changes in liquid density. Bubblers are also affected by density changes. If there are large changes in density, you should consider other techniques such as GWR, nuclear or capacitance.
You stated that the level meter in the oil phase does not cover the entire tank. To have it cover the diameter of the KO drum, relocate the low-pressure leg of the transmitter on the top of the drum and adjust the span accordingly. Keep in mind that a larger span may reduce accuracy. However, level control typically is not a fast process; this should not be a problem.
There may be a problem with the level meter/transmitter installation. Poor installation could render a level meter ineffective because of heavy turbulence close to its sensor. In the drawing shown with the problem statement, the pump minimum flow line seems to enter close to the level on the oil side. If feasible, relocate the minimum flow line away from the level meter. If this is not possible, you should consider an arrangement where level is measured in a bridle.
Longer-term issues, i.e., safety and environmental permits, also exist. Consider doing the following:
• Conduct a gap analysis (survey) of all systems to identify safety and environmental deficiencies — via a team-based approach where a critical assessment is made of deficiencies in the existing system components;
• Perform a safety lifecycle study — such an analysis takes into account that even carefully selected and installed instruments, if not maintained periodically, can lose their precision and accuracy;
• Standardize the instrument systems in the long run — i.e., using the same instrument systems at all your refinery or plant locations;
• Carry out a hazard and operability (HAZOP) study or a layer of protection analysis (LOPA) and safety integrity level (SIL) assessment of the systems, depending on the safety and severity of the problems you currently see, to help quantify your risk and develop appropriate solutions; and
• Establish a common database for all your plant instrument and control systems in the long run, to avoid or eliminate "islands of information."
GC Shah, Senior HSE Advisor
Mustang Engineering, Houston, Texas
RESIGN YOURSELF TO MAJOR REVISIONS
There are a number of environmental and safety problems that must be top priority; also, several reliability issues require attention (Figure 2).
Let's begin with the dike. Federal aboveground storage tank (AST) requirements apply to vessels larger than 1,320 gallons with a reasonable expectation of discharge to a waterway or adjoining shoreline: 40 CFR Part 112. Unless the refinery is in a desert, AST rules apply. Secondary containment must have a capacity of 110% of the largest tank and freeboard, i.e., rainwater capacity for a "25-year, 24-hour storm event" and must be locked to prevent accidental release. Refer to this EPA document or this one.