Pumps and Compressors

The compressor emergency shutdown system (ESD) has tripped the natural gas compressor off-line three times in the past 24 hours. Each time the operator goes to reset the compressor interlock, she notices the graphic display panel on the interlock system says “Separator boot high level” as the reason for the trip. After this last trip, operations decides to keep the compressor shut down for a few hours until your arrival to diagnose the problem. Your first diagnostic test is to look at the indicated boot level in the sightglass (LG-93). There, you see a liquid level appears to be normal:

First, explain why this first diagnostic test was a good idea. Then, identify what would your next diagnostic test be.

Finally, comment on the decision by operations to leave the compressor shut down until your arrival. Do you think this was a good idea or a bad idea, from a diagnostic perspective? Why or why not?

The compressor automatically shut down last night, tripped by LSHH-231. The control system alarm log showed a high level alarm LIR-92 about 15 minutes prior to the shutdown:

Identify the likelihood of each specified fault in this process. Consider each fault one at a time (i.e. no coincidental faults), determining whether or not each fault could independently account for all measurements and symptoms in this process.

$$\begin{array} {|l|l|} \hline Fault & Possible & Impossible \\ \hline 2-inch~line~plugged~at bottom~of~separator~vessel & & \\ \hline LT-92~failed~with~high~output~signal & & \\ \hline Air~supply~to~solenoid~valve~shut~off & & \\ \hline Solenoid~vent~line~plugged & & \\ \hline PSV-11~stuck~open~& & \\ \hline LSHH-231~failed~with~high~output~signal & & \\ \hline \end{array}$$

This amount of vacuum (negative pressure) in this knock-out drum is controlled by varying the compressor’s bypass valve:

An operator tells you there is a problem with this system, though: the vacuum gauge near the pressure transmitter registers -6.9 PSI, even though the controller faceplate registers -8.0 PSI which is the same as the setpoint. The same operator notes that the control valve position is approximately 30

Another instrument technician happens to be with you, and recommends the operator place the pressure controller in manual mode to “stroke-test” the control valve. Explain why this test would be a waste of time, and propose a better test for helping to pinpoint the location of the fault.

{\bullet} A valuable principle to apply in a diagnostic scenario such as this is correspondence: identifying which field variables correspond with their respective controller faceplate displays, and which do not. Apply this comparative test to the scenario described, and use it to explain why the technician’s proposed test was probably not the best first step.
{\bullet} A problem-solving technique useful for analyzing control systems is to mark the PV and SP inputs of all controllers with “+” and “-” symbols, rather than merely label each controller as “direct” or “reverse” action. Apply this technique to the control strategy shown here, identifying which controller input(s) should be labeled “+” and which controller input(s) should be labeled “-”.
{\bullet} Predict the effects resulting from one of the transmitters in this system failing with either a high or a low signal.
{\bullet} For those who have studied level measurement, explain how the level transmitter (which is nothing more than a DP transmitter) senses liquid level inside the knock-out drum.