|January 20, 2016|
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Stop Swapping Your Pumps and Start Testing Them Instead
By Jay Shellogg
This article appears in the forthcoming January/February 2016 issue of Paper360°.
Does anyone out there run two pumps in parallel so that if one fails you can swap to the other pump with little or no consequence to operations?
I’m sure many of you do, and if you have read the title of this article you probably think I’m crazy because if you don’t swap your pumps then how do you know the “standby” will work when you need it?
Let me explain why most folks are alternating their run time between pumps and why this is wrong.
During my years in the paper industry, I’ve seen only two reasons for an installed spare pumping arrangement: 1) the designers felt that the consequence of failure was too severe to only have one pump, and only with a backup pump could the availability of the function be secured to a tolerable level; and 2) money was no object. (I never actually experienced this second phenomenon.)
There are many functions that require an installed spare pumping arrangement; a waste water pumping station comes to mind as a very unique example. But for the majority of the pulp and paper industry, those installed spare pumping systems are there to maintain simple process availability.
Few of us have seen a greenfield mill come up out of the mud. We have learned our maintenance and operational practice through tribal knowledge and, in most cases, we have not been provided knowledge at all. We have just been given tasks to do with no real explanation as to why.
As it relates to an installed spare pump arrangement, I dare say that the designers of those systems almost never intended for those pumps to be operated in an alternating run cycle. I bet most intended for the installed spare pumping set to be operated in a true duty/ standby pump arrangement where the duty pump is the primary running component and the standby pump sits idle next to it, ready, willing and able to take over in the event the duty pump fails.
So why do we swap pumps? Probably because, sometime in the distant past, the duty pump failed and the standby wouldn’t start up. And when we looked inside the standby pump we found that it was “mudded in” because of leaking isolation valves. So instead of fixing the isolation valves we began a routine of alternating from one pump to another. I have seldom seen an organization that was really that good at swapping their pumps, except in very critical systems like boiler feed water.
Over the years I have seen many frustrated maintenance folks called in to fix both the duty pump and the standby pump because neither was operational. I actually know one maintenance superintendent who had all the standby pumps removed from his operational area because, in his words, “they were always broken and maintenance doesn’t find out about it until the duty pump breaks as well. So [operations] really don’t have a standby pump, so they must really not need them.”
That’s an extreme example, but I can tell you that their uptime actually increased. They were forced to focus proactive maintenance on what became a standalone pumping system. Now I’m not suggesting you remove your standby pumps, but I would say that a strong proactive maintenance program is in order, and alternating run time is not it.
WHAT’S THE PROBLEM?
When you alternate pumps, you stress the wear components equally. So if (when) you have a wear related failure of the first alternating pump, the remaining life on the other pump is only equal to or less than the alternating frequency. Most of the time, this remaining life will not fit into a standard planning and scheduling window, so it becomes an emergency repair.
To resolve this, pick one of the alternating pumps as your duty pump and run it. The other pump becomes your standby and will need to be periodically function tested. But keep in mind that the failure modes of the standby pump are different than those of the duty pump. For instance, since the standby pump is idle most of the time, it is susceptible to failure modes that the duty pump is not, such as: bernilling of the bearing, slumping of the grease, being “borrowed,” or mudding in. If any of these failure modes are likely, you must have proactive plans in place to deal with each one.
To prevent bernilling or slumping of grease, you may just have to “bump” the motor on/off to set the bearing in a new position. As for “borrowing,” a regular visual check will make sure it is still there. To prevent mudding in, fix the leaking valves or run the standby pump to restore its inlet and outlet and then shut it back down.
For bernilling, slumping and mudding, you need to have a reason- able idea of how long each of these degradation processes take. Then perform the restoration at an interval that is less than the degradation process. As for the “borrowing” and the full function test, there is a way to calculate the correct Failure Finding Interval, but that explanation is best covered in a more technical article. Also, whatever proactive maintenance is employed on the stand, it must not induce failure, so care must be taken in these choices.
Making a change to a true duty/standby system is a tough fight. I have lost this battle more times than I have won it, but it does secure greater reliability. Typically, senior leadership will not take the time to fully understand this logic and is frightened to change something that has been culturally ingrained in their practices.
Also, most managers are risk averse and in the case of an alternat- ing pump arrangement they have very little to personally gain from making a change. And despite the fact that this could improve uptime, in their minds the perceived risk does not warrant the effort.
Jay Shellogg spent the last 16 years of his career working at a large pulp and paper mill, primarily as a senior environmental engineer and maintenance/reliability superintendent. During that time he encountered many challenges; in his own words, “some I overcame and some I didn’t.” Contact Jay Shellogg at: [email protected].
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