High Differential Pressure Doesn’t Always Mean A Plugged Filter

Case-in-point: RIG recently performed a high velocity, hot oil flush on a small compressor and electric motor package. After achieving an API-614 acceptable screen based on piping size and a lubricant ISO cleanliness code of 15/13/9, we helped the customer install filters in their on-board filter housings and install screens with cotton sheets in the supply headers to run their auxiliary pumps for verification through the bearing headers.

It was determined after start-up by the plant that the 10 um filter used plugged, at which time it was changed out for a new element. Knowing the system was clean, but not seeing the differential and operating factors, we knew something was not right about the findings.

Later, we performed the same flush on a similar system at which time we monitored flow and differential factors to repeat the process and use data to determine the performance factors of the filters.

Step One: RIG removed the filters from the housings, installed jumpers at the bearing supply headers and began flushing using our 400 GPM flushing skid. This proved a 19 psid differential across an empty filter housing with 30 psi system header pressure and oil temperature of 70°F. This would show a 63% filter life used giving the filter change out differential of 30 psid.

Step Two: After the system was cleaned by flushing and achieving the API-614 pipe cleanliness standard, along with the targeted ISO Fluid Cleanliness spec, the decision was made to add a filter and use the on-board aux pump in conjunction with our flushing skid. Based on understanding of flow differential, we knew we should gain anywhere from 1–6 psid with the new 10 um filter installed after start-up.

Step Three: The pumps were started and the bearing headers were still jumped so the only resistance we would encounter from the previous step was the filter and it would prove to be slightly less since the lubricant was at 150°F and the viscosity had dropped to 17.7 cSt. This proved out and we achieved a system pressure of 34 psi and a differential pressure of 23 psid, which would show 76% of used filter life with a change out differential requirement of 30 psid. This was a gain of 4 pounds of differential from adding a filter to the housing even while dropping the viscosity of the oil from 46 cSt to 17.7 cSt.

Step Four: After the system was circulated, using this process, the decision was made to install the bearing headers to the equipment and install a 100 mesh screen with cotton bed sheets to the supply piping after filters. This is where data helps provide a logical explanation of why the filter is not plugged. After installing the bed sheets and screen, we now add more resistance immediately after the filter housing, this will make the system header pressure increase to nearly 99 psi and the post filter pressure at 60 psi which proved a 39 psid across the filters with the lubricant at 80°F, which showed 132.64% of filter life used. After a few minutes, the operators shut down the system and wanted to change the filter, but it was explained that the screen with linen sheets were installed which created the differential from pump supply to filter outlet and asked them to continue running the pump for the duration of an hour so we could check the screens.

Step 5: The screen was pulled from the supply header and the system was restarted for circulation without any restrictions except operational flow orifices. After reading the header pressure and differential. It was concluded that the filter only had a differential of 8.7 psid and a system pressure of 36 psi.