Q & A > Question Details
Regarding centrifugal compressors in hydroprocessing units, if the suction strainer goes high we need to shut down the compressor manually to avoid vibrations, surging. There are no automatic trips provided on high Delta P. Is there any reason for not providing an automatic trip on suction strainer Delta P?
 
Answers
25/11/2019 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
The setting for the trip would depend on many factors including molecular weight of the gas, shaft speed, operating pressure etc. Deviations in these variables from the assumptions made when setting the trip point could render the trip ineffective, or cause unnecessary trips when there was no risk to the compressor.
A spurious trip of a hydrocracker recycle compressor could cause a process safety incident (stagnant reactor leading to a temperature excursion), hence compressor trips should not be added without weighing up the risks as well as the benefits.
It is more common to include an anti-surge controller than a trip. The anti-surge controller would calculate the approach-to-surge from suction & discharge pressure, flow-rate etc, and would open the anti-surge valve if necessary. Because the route through the anti-surge valve has lower resistance than the route through the process, the anti-surge valve would protect the compressor from surge caused by the strainer unless the strainer became completely plugged. If this system is included in the design, the suction pressure measurement should be downstream of the strainer.
It is also common not to have a suction strainer, or to include one to catch debris from initial construction but remove it at the first maintenance shutdown.