Pumps & Systems, November 2007
Here is a proven methodology to use for your bad actor improvement efforts.
A reliability engineer or rotating equipment engineer with a major pump fleet in his care always has some pumps that are as reliable as pet rocks. The only mechanics who know where those pumps are located are the ones charged with lubrication replenishment and routine vibration surveys.
But other pumps seem to spend as much time in the shop as they do in service. For mechanics and operators, these pumps are "bad actors" that should be converted to boat anchors at the first available opportunity. Since these bad actors seriously impact the fleet Mean Time Between Failure (MTBF), chasing them down is the best way to improve MTBF and increase profitability.
For example, in a July 2004 article in Pumps & Systems, Heinz Bloch estimates the average true cost of a pump repair to be $15,000. Using this estimate, assume a typical refinery has 2,000 operating pumps. With a two-year MTBF, that refinery spends $15 million each year on pump repair. But improve that MTBF to ten years and the annual repair costs drop to $3 million per year - meaning $12 million per year is chased to the bottom line.
The following methodology is primarily assembled from experience gained in bad actor improvement efforts on API-610 pumps in various refineries. However, much of what was learned there applies to bad actor pump remediation in any situation.
A Team Effort
Assuming the chase is a one-person responsibility is an error. The responsible engineer, maintenance mechanic, operator, and seal manufacturer each have a different view of the problem that needs to be heard.
The plant rotating machinery engineer or machinery reliability engineer is the logical team leader because he is in a position to accomplish most of the necessary work by himself and has access to the required data. He should also be familiar with the necessary math and spreadsheet software and be able to convert those data into coherent formats.
The most important team member - although he doesn't have to attend team meetings - is the senior management sponsor. The team leader talks to operators and mechanics and needs the assistance of technicians to perform vibration studies and take field measurements of flows, pressures, and amperage. These people all have supervisors that need them to do something else at that time. The management sponsor, who understands how the annual operating cost is connected to the MTBF, must offer his support in all discussions with the relevant supervisor. At the conclusion of the study, he must approve the required expenditures to affect the recommended changes.
There is typically one pump mechanic on the site who the other mechanics turn to for help in solving pump problems. He must be on the team because his knowledge of the bad actor and his unique experience make him invaluable.
The team leader will also need assistance from operators. Different pumps may have different operators with the necessary experience, so this could be a "rotating" team member. However, it is imperative that no team conclusion is reached until the relevant operator gives his approval.
Most large users (such as refineries) have an alliance with a mechanical seal vendor. In return for the mechanical seal business at that site, the seal vendor offers supplies and services that include warehousing spares, a maintained site seal database, and assistance with new application and upgrade projects. Since a significant percentage of failures are usually mechanical seal failures, the site representative for the seal vendor is also a necessary member of the team.
It is not a necessity that all team members attend each meeting, but the five team members mentioned above are critical. Prior to starting these team meetings, however, the team leader can work independently and accomplish a great deal on his own.
Which pumps are bad actors? The team leader has MTBF numbers at his disposal that are the obvious starting point. This number by itself, however, does not necessarily give an accurate picture.
Excluding that rare user that evaluates MTBF based on actual operating hours, the fact is that a pump running 24/7 that has to be rebuilt once a year impacts MTBF more than a pump running 10 hours a week that needs to be rebuilt every two years. However, improving the continuous-duty pump from one year to three years will not help the MTBF as much as improving the intermittent-duty pump from two years to ten years - and the latter problem will most likely be easier to identify and less expensive to solve.
Every pump in the bottom 25 percent of the MTBF spread should be evaluated, giving proper weight to annual hours of operation and severity of service.
Study the Files
After identifying the bad actors, look at the pump files: the pump data sheet, purchase order, the original performance curve and other available file data. The team leader will copy this data for his own files and refer to it several times during the study.
Calculate the actual Suction Specific Speed (Nss). Using the Nss number, calculate the minimum continuous flow. The pump manufacturer probably gave a minimum continuous flow number when he sold the pump. That number is based on stable or thermal conditions, however, not on reliability factors. The minimum continuous flow for reliable long-term operation is the number that is needed, and that number is a percentage of flow at maximum-diameter best efficiency point (BEP) adjusted for Nss.