Focus on Pumps: It’s all in the connections
They're all around us. We each have one, the cars we drive have at least three, and according to the Guinness Book of Records, after the electric motor they are the most used item in our industrial wo...
November 1, 2001 | By MRO Magazine
They’re all around us. We each have one, the cars we drive have at least three, and according to the Guinness Book of Records, after the electric motor they are the most used item in our industrial world. They are pumps. They come in all shapes, sizes and price ranges. They supply the lifeblood to our industries.
Try to imagine how many pumps are used in the chemical industry alone. Other large users of pumps are steel mills, paper mills and mines. One paper mill I visited had over 600 pumps.
Not surprisingly the industry that manufactures and supplies pumps is large and complex. It takes a lot of highly skilled people to design, manufacture, market and technically support the pumps that suppliers sell. The cost of all of this is in the purchase price of a pump. If you have been involved in acquiring a pump recently, you will know that they are not inexpensive. But the cost of the maintenance of pumps can be much higher than their original price tag.
The greatest influence on the amount of maintenance that will be required for a new pump is not the amount or type of pumping it is doing but rather how it is installed. In fact, it comes right down to how you connect the pump and what you connect it to.
If you think about it, a pump that is sized properly and is made out of the correct material for the job should pump forever. The majority of pump repairs are not made because the impeller has worn out, but because the seal is leaking. Pump bearings do fail, mainly due to misalignment and contamination, but the pump is usually pulled because the seal is leaking and — many of you probably think that you might as well overhaul it as well, while you have it out. This is why the bearings get replaced. This is obviously over-maintenance and I must admit I used to be one of those “better safe than sorry” guys.
The mounting connection
Pumps are great workhorses but it’s what you connect them to that creates problems. The first connection is to the base. You can either make your own or buy a base that is designed for the pump. The latter usually will be very well made and likely be very expensive, but there is no guarantee that it will not have soft foot, which has a huge influence on premature pump failure.
If we bolt a base to an inadequate foundation, it will take on the same characteristic as the foundation. If the foundation is not flat, the base will not be flat. A twisted base can create internal misalignment between the bearings of a pump or motor. It can also deflect the shaft, which will influence the alignment. The base and foundation are critical to long service life, as they are the cornerstone of your pumping unit. They both have to be flat and free of defects.
The next connection is the piping. If you have spent a lot of money on a new pump base, there will probably not be much left over for buying a flexible joint to fit between the pump and the pipework. However, it would be a great investment. One benefit is that if you use a flexible joint, it means that you will have to support the pipe. Nobody leaves a flexible joint supporting a pipe — but they will allow a pump alone to support a pipe. Pumps are not designed to handle this kind of load.
Piping can be very heavy, especially when it is full of product. Unsupported, it will have a detrimental effect on the longevity of the pump.
Another benefit of flexible joints is that they eliminate pipe strain. Many of us have seen instruments such as pry bars, wrecking bars and come-alongs used to move pipes into position so they can be bolted to the pump. After the pipe is released, the strain has a significant impact on the pump life. If you don’t think so, try placing a magnetic base dial indicator under the pump shaft before you release the come-along. You may be surprised at what you see, as it will prove there is strain on the pump.
The coupling connection
The next connection is the coupling. Before we even attempt to connect it, we have to be sure that there is no soft foot or pipe strain, because both of these can cause shaft defection. If the shaft defects, we will not be able to achieve the results we need.
The recommended tolerance used should be that of the pump manufacturer, not the coupling manufacturer. These tolerances are designed so that you get the optimum efficiency out of the pump.
While some tradesmen talk about “coupling alignment,” what they actually mean is “shaft alignment.” It’s the centre line of the shaft that we are trying to align, not the outside edge of the coupling. Unfortunately, this is a mistake that is still made by many. If the coupling and the shaft are not concentric and you align the coupling edges, you will never get the shafts aligned. You must check the runout of the coupling even if you are doing shaft-to-shaft alignment. This will determine if the coupling is out of alignment and also if the shaft is bent (it does happen).
The coupling’s job is to transmit the power from one shaft to the other, as well as compensate for a small amount of misalignment. That’s because the chances of getting the shaft centre-lines collinear (in a straight line) at operating conditions range between slim and none.
There are a wide variety of couplings to choose from, and the chances that the style installed is the same as what you keep on the shelf also are between slim and none. But the coupling is a very important part of the machine unit, so you have to choose it wisely. You can use mechanical styles such as gear, chain or grid. Maybe a metal membrane (disc) type or an elastomer style will be effective.
Your choice should be influenced by how much horsepower, torque and speed the coupling can handle, not on how much misalignment it can tolerate. Coupling manufacturers usually state the tolerances of fatigue limits of couplings. This has nothing to do with the tolerance of the equipment it is driving. Because a coupling has a high tolerance does not mean it is the best coupling for your application.
For example, you may have a high-speed pump, complete with a balanced impeller and shaft. You wouldn’t want to install a heavy, unbalanced cast iron coupling just because it had a rubber tire for an insert.
Shaft alignment is the easy part of the job, as long as you have the right tools. The right tools are not straight edges, as they only get you a close alignment, but not close enough. The right tools do not include home-made bracket sets that have excessive bar sag, nor are they old, loose mag-based dial indicators.
The proper tool should be either a laser system or a dial system that is designed for shaft alignment. You wouldn’t do pipe work without a pipe wrench and it should be the same for alignment work. It’s the price you pay to do the job right the first time. If you have done all the other preparation work, you’ll find alignment is a simple process.
You are now connected, except for the power, but as long as you don’t have an electrician who decides to move the motor closer to his cable, you should be all right. Start the pump and see how it runs. It’s best to use a vibration meter to check it. I use a simple little instrument that I equate to a digital vernier gauge, but instead of measuring the size of an object, it measures the movement. I can determine the ISO standard on a small card I carry. It shows what class the pump is and what it should be vibrating at.
If you follow these techniques, your pump will be up and running and within spec. The only other thing that could affect the pump would be dynamic movement, but I will save that topic for a future article.MRO
Reader Service Card No. 398
It’s all in the connections
By John Lambert