FOCUS ON PUMPS: A maintainer’s guide to Treating Centrifugal Pumps
A pump is a wonderful thing when operating properly. You never give it a second thought. But when it's not running properly, you hear about it right away. People on the receiving end are not getting h...
February 1, 2001 | By MRO Magazine
A pump is a wonderful thing when operating properly. You never give it a second thought. But when it’s not running properly, you hear about it right away. People on the receiving end are not getting heat, cooling, process fluids, etc., and they’re quick to let you know, usually very loudly.
So here are some service procedures for centrifugal pumps that will help keep your customers in hot water and you out of it.
But before we launch into the actual service, a few words about safety. After all, being rushed to the emergency ward doesn’t make the job go any quicker.
When working on any type of equipment, safety should always be your first concern, and centrifugal pumps are no exception. You need to follow the proper procedures setup at your shop. There are several areas with which to be particularly concerned.
First, make sure the pump can be taken out of service. It may be providing services that are extremely important. Removing a pump from service that’s providing chilled water to the operating suite at a hospital, without permission, for example, will not earn you that bonus you deserve.
Before you even start, you’ll want to remove power to the equipment. All shops have some type of lock-out/tag-out procedure. Follow them completely–all the way through to tag removal. This is absolutely necessary, because if you occasionally forget to remove your tags, someone else will get in the habit of removing them for you, usually while you’re still working on the equipment.
Make sure you remove all sources of power. Some motors, such as at hospitals, also have an emergency power source. Isolate that source from the pump too. More safety tips will be mentioned as we go through the servicing procedures.
We’re not quite ready yet to start disassembling our pump. To ensure that the whole process runs smoothly, let’s take a few steps before proceeding. It would be nice to have an exploded diagram of the pump before we begin. One reason for this is so we’ll know where to put all the extra parts we have left over when we’re done. They have to go somewhere. Manufacturers don’t usually add parts that don’t have any function.
An exploded diagram is especially helpful. It not only illustrates the pump, it also gives us the proper part names. Therefore, if we need to order a replacement, we know what to call it besides a “thing-a-muh-jig.” Several manufacturers have made these diagrams available over the Internet. Just check their web sites and you’ll be surprised at all the information that can make your job easier. You will even find valuable service instructions.
Points of service
We’re going to look at the three main areas for service. They are:
Lubrication is probably the single most important step in a maintenance program to keep a pump running without problems. Obviously, the manufacturer’s recommendations as to the type of lubricant and proper lubrication intervals are the best place to start. It’s a starting point; not the final word. The reason is that the manufacturer doesn’t know how his equipment is being used. How it’s used will affect how much you lubricate it.
A couple of factors that affect the lubrication schedule are how often you run, or don’t run the pump. What type of environment is it in? Is it hot, dirty, and dusty? Or is the pump lucky enough to reside in a cool, dry, location? You can see that the answer to “How often should I lubricate a pump?” is, “it all depends.” Not much of an answer. So here’s where experience comes into play. The more you know about the system, the better you’re able to protect it. If you’re lacking in experience, don’t worry, you have something else to rely on: common sense.
To help you determine a lubrication schedule, check your records to see how the system is performing. What have you done in the past to maintain the system? Where have you been successful, or unsuccessful in the past? If you don’t know, you don’t have a good set of records. So now, you need to start a log. There are a few things you want to put in that log: suction pressure, discharge pressure, how the pump is running (hot? noisy?), when was it lubricated?
You’re going to lubricate the bearings on occasion. Generally there are two types of bearings: sleeve bearings and ball bearings. Everything about them is different. One of the best ways to determine when to lubricate either type is based on how hot they’re running. If you have a way to measure the temperature, great. If not, you’ll want to err on the side of lubricating them too often. After all, the cost of grease or oil is small compared to the time and cost of replacing the bearings.
Let’s start with the ball bearings. Before you start pumping grease into the grease fitting, check to see if there’s a plug in the grease relief hole. If there is, remove it. You don’t want to pump new grease into the bearings with no way of removing the old grease. Too much grease can be just as harmful as too little grease. You want to keep pumping new grease in until you start to see all the old grease come out.
You’re done greasing that bearing, but do you want to put the grease plug back in? Many people will simply throw the relief hole plug away for fear the next person will forget to remove it before they start to pump in grease. In fact, some manufacturers won’t even thread the hole to keep the plug from getting jammed in the hole.
For sleeve bearings you’re going to want to add oil. Not just any oil, mind you. Once again, use what the manufacturer recommends. This will be a non-detergent oil. There’s an important reason for this. Oil evaporates–detergent doesn’t. As the oil evaporates, the concentration of the detergent becomes stronger. You can see that each time you replenish the oil, you add even more detergent to the oil.
Once the oil is added, it is absorbed by a wick that deposits it where it’s needed, between the sleeve bearing and the shaft to maintain a thin film of oil between the two. The wick has an important job. Therefore, you’ll want to inspect it. If it’s scored or burnt, replace it. If it feels waxy, it can no longer do its job effectively.
The most likely reason for disassembling a pump is to replace the seals. Even if seal replacement isn’t your reason for disassembly, it’s a good idea to replace the seal while you have the pump apart. That seal kit you bought contains all the parts you need. Replace the whole thing and not just an individual part.
When you remove the seal, put on your detective’s hat. Ask yourself why the seal wore the way it did. Were there grooves in the seal? This could be a sign of a high concentration of suspended or dissolved solids. Suspended solids of less than 10 microns will get between the two seal faces. Maybe you need to choose a seal that is more tolerant of these solids. Maybe you need to determine if there’s a way to remove these solids from the system.
Your chemical concentration may be too high. A seal has two highly polished surfaces mated up to each other. Sure, no liquid is supposed to leak between the two surfaces, but it does–it has to. The thin film of liquid between the two surfaces heats up due to friction. The heat is then removed when the liquid evaporates. You never see that evaporation because it’s extremely small. But, like the oil in the sleeve bearing, the chemicals are left behind. They can then cause grooving in the seal.
If you find that you’re replacing seals often, perhaps you need to look at the type of seal you’re using. If you can’t remove the high concentration of chemicals, perhaps you need a seal that is more tolerant of the high concentration. Other factors that will affect seal selection are pH levels and temperature.
The last word on seals is that they’re most often damaged during their installation. Be especially careful when putting the seal in. Don’t touch its surface with dirty hands. Use soapy water to slide the seal on the shaft; if you use a petroleum product for lubrication, it may adversely affect the seal. Try not to chip the seal when placing it on the shaft. And don’t run
the seal dry. Open the service valves to flood the pump cavity before running the pump.
As you disassemble the pump, you’ll have to deal with gasket surfaces. Whenever you’re dealing with a gasket surface, be careful not to scratch these machined surfaces. It may cause a leak path. And always use new gaskets when putting the pump back together.
Alignment and coupled pumps
Alignment is a concern for flexibly coupled pumps. The coupler’s job is to transmit power from the motor shaft to the pump shaft. It can also take up minor misalignment. The key word here is minor. Exactly how much has been determined by the coupler manufacturer, based on several factors, so use its recommendations.
How you check alignment is basically determined by rpm. The lower the rpm, the less sophisticated a method you can use. At 1,750 rpm you can get away with aligning the shafts with just a straight edge and taper gauges. At 3,500 you’ll want to use a dial indicator. For higher rpm and greater accuracy, you may need a laser.
Check that the pump and motor shafts rotate around the same axis. This means you have to check the parallel and angular alignment. To change these alignments, you’ll have to place or remove shims from under the motor and move the motor from side to side. After you’ve placed shims under the motor, tighten the motor down to check alignment again. It’s always better to use fewer, larger shims than a larger number of thin shims. After you’ve placed the pump in service, recheck the alignment when the pump reaches its operating temperature.
Obviously, there’s a lot to cover when servicing a centrifugal pump. Here, we’ve just touched on some of the main areas of concern. Along with that, it’s important to use common sense and be methodical when working so as to not damage the pump–or yourself.
This article was prepared by ITT Industries. It previously appeared in the company’s TechTalk newsletter in June 1999. It is an abridged version of a Little Red Schoolhouse staff article.