THIS MONTH’S PUZZLER
A contractor installing a new motor starter for a crude pump in a motor control center (MCC) received permission to trip the breaker controlling the old bucket. With the trip, we lost the crude pump, the gas oil pump and several recirculation pumps in an atmospheric crude distillation unit. This breaker was supposed to go only to the pump being replaced. The operator responded quickly, bringing the unit back up although there were severe swings in column temperature and pressure. An investigation later found no loop sheets or inspection records. The wiring diagram for the MCC didn’t show any common wiring for the starters, but then no as-built drawing was produced and no commissioning documentation exists. What can be done to avoid this problem in the future and address what may be a refinery-wide problem?
MODELING CAN HELP
Drawings often fail to detail the looping of power supplies and this is not the only area where drawings have omitted information or have errors. This incident was due to deficient documentation.
The usual solution is to perform a site survey of the wiring compared with the drawings and then to revise the drawings. However, this is usually impractical as it is: 1) a high cost exercise; 2) hard to check the results without testing the circuits which is risky for both personnel and plant.
Our company specializes in replacing control systems on live plants and platforms. We’ve been in business for 17 years. In that time, we have transferred more than 100,000 I/O successfully without incident.
Our solution is to use connections data modeling rather than drawings to represent the components and circuits. Utilizing a model is nearly 100% accurate even with very poorly documented systems and the cost relative to drawings is a savings of 50 to 80%. Also, modeling is much faster.
Carole Williams, business development manager
I&E Systems, Perth, Australia
IT’S THE DCS LOGIC
The distributed control system (DCS) may have been programmed to shut off the other equipment when the breaker tripped. The easiest way to find if the DCS caused the other pumps to shut down is to ask one of the control room operators to look at the programming. You may also want to read the control narrative or review the cause and effect diagrams, if this documentation is available. Both of these methods can be done safely, offline, without affecting operations. It could have been that the first pump going offline caused a low or high level, tripping the rest of the pumps.
If none of these options are available, or if it is determined that the DCS did not cause the shutdown, the cause requires more work. You will have to physically trace the wiring from the MCC out to the pumps. The power wires may have been connected in parallel or series (“daisy chained”). This would be a good time to make “as built” drawings. In any case, a critical piece of the puzzle that was not present are loop drawings. Both electrical and control systems loop drawings must be present in order to see what power and signals are going to the pumps. Since these were not available for this scenario, it’s a good bet that the rest of the plant also suffers from poor documentation.
Many plants just ignore this problem until something is upgraded and the contractor is forced to clean up the plant’s mess. Documentation is provided from the contractor when the plant is built.
Having an electronic document management system and keeping it updated is the key to preventing these situations. The key to keeping the system updated is having proper plant procedures any time work is done on equipment. Checklists and requiring employees to sign off work documents puts “ownership” back in the picture.
Syed Imran Akhter, control systems engineer
Jacobs Engineering Group Inc., Houston