Jim McKee (deceased)
Alabama Electric Motor Service
Sheffield, Alabama
Technical Education Committee Member
There seems to be a long and never ending list of equipment and facilities needed by most EASA service centers. But often we are constrained by cost, available space, and more urgent priorities that keep us from fulfilling some of these needs. One such (almost mandatory) need is a vacuum pressure impregnation (VPI) system.
Normally there are two ways to provide this service. One way is to have another EASA service center do the VPI process for you. Another is to buy your own system at a considerable capital investment.
There is also another option. We decided that the way to provide VPI processing was to build our own system. Most EASA service centers have a talented collection of people with numerous skills. Ours is no exception. We felt that we had the skills in house to do the job. All we needed was some hardware and to do a bit of research into how the process works.
The major components of a VPI system are a pressure vessel (or process tank), a vacuum pump, a storage vessel, varnish or resin, and a control and piping system.
For any service center contemplating installing a VPI system, several questions need to be answered. The size and depth of the tank and the type of resin are probably the most important.
While each service center will have to make its decisions relevant to its needs, the process I went through may be helpful to others.
Most of the jobs we undertook that required VPI processing were form wound DC armatures and long, thin mush wound stators. From an earlier bad experience with polyester dip and bake resin, we knew that we needed a sufficient turnover rate to keep our resin in good shape. Most manufacturers of resins recommend adding at least 50% of the tank capacity per year. Our total resin usage was about 250 gallons per year.
This put an upper limit on tank size of about 500 gallons provided we used the same resin for both VPI and dip and bake. A 4-foot diameter tank, 5 feet deep, requires slightly less than 500 gallons to fill. Almost all of our armature work was less than 4 feet in diameter but often longer than 5 feet. That gave us the dimensions that we needed for the tank. We do bigger work than this, but not often enough to justify a bigger tank. Bigger jobs are sent to another service center for VPI processing.
You should talk to your insulating materials supplier about which resin to use. My choice was polyester since I feel it is better as far as storage is concerned. We use our regular dip and bake tank as a storage tank and it is not chilled. It is in-ground so remains a little below room temperature in a part of the facility that is air-conditioned.
Pressure vessel
Finding a good pressure tank may take some time and a bit of luck. Surplus equipment dealers who specialize in autoclaves and pressure vessels are a good place to start. You may need to modify something to suit your situation. A compromise on the exact diameter or depth may be required. I had hoped to find something closer to 5 feet in diameter, but finally settled on a 43-inch tank. The tank should have a working pressure rating of a least 100 psi. It will be hydrostatically tested at 150 psi. A nearly complete vacuum will be applied to the tank as well. Be sure that the seal between the lid and the tank is suitable for both vacuum and pressure.
A tank with lid that is easy to open is a must. The two most common methods to open the lid are to have it hinge up on supports mounted to the side of the tank or to lift the lid slightly and then swing the lid off to the side. Some lids are secured with tension bolts as shown in Figure 1. Others are secured with a locking mechanism similar to a pressure cooker lid, called a Hodge Door.
The lid must have a seal (gasket, o-ring, etc.) to withstand the pressures involved. The Hodge Door is faster, but considerably more expensive. One factor in your decision should be how often you expect to use the tank. If you process a lot of VPI work, the higher initial cost may be justified by the reduced labor required for each job.
If your facility does larger horsepower armatures or wound rotors, a shaft well in the bottom of the tank may be desirable. This will allow processing of longer armatures and wound rotors without having such a deep tank. My shaft well extends 2 feet below the tank and is 10 inches in diameter. Remember that this shaft well will be part of the pressure vessel, and must therefore be added by a certified welder and included in the hydrostatic test.
In order to monitor the process, especially tank fill, two sight glasses must be provided in the tank top. One is to look through and the other is to shine a flood light through for illumination of the inside of the tank.
A 3-inch visible opening is a good size. I do not recommend trying to fabricate the sight glasses. While expensive, it is best to buy them. Remember, they must withstand full tank pressure. While we’re on the subject of seeing inside with the lid closed, it is best to paint the inside of the tank with a white epoxy compatible with the resin you will be using. This greatly enhances the ability to see what is going on inside the vessel.
The tank will need several pipe fittings in various locations. Most tanks will already have some fittings that may be usable. The chances of finding a tank with all of the required features, fittings, etc., are not likely. Some modifications and additions will be necessary. If you don’t have people in your service center with the required skills, find a certified welder with experience on pressure vessels to make the modifications and welds for you.
Starting at the bottom of the tank at the lowest point, provide a fitting for the resin piping. Keep in mind that the resins, whether epoxy or polyester, are quite viscous at tank temperature, so do not skimp on pipe size here or it will extend fill and discharge times considerably. Two-inch pipe is probably a minimum unless your tank is quite small. Other fittings in the side of the tank are required to connect to the vacuum and pressure lines as well as a pressure discharge line. In theory, these should be in the top of the tank. But for practical purposes they can be in the tank side as close to the top as possible. Any connections to the top of the tank must be flexible or removable since the top moves.
Two very useful features for a VPI system are to be able to monitor part (stator or rotor) temperatures and capacitance during the process. Either of these requires an electrical connection that can be screwed into a pipe connector and must be pressure tight. We monitor temperature but not capacitance. Unless you plan on doing very large work, capacitance monitoring is probably not worth the expense.
Two fittings that should go in the top of the tank are for a pressure relief (pop-off) valve and for a vacuum/ pressure gauge. You will have a separate vacuum and pressure gauge at the control panel. But having one at the tank is quite handy and is a good cross check just to be sure what is going on in the tank. A good Bourdon tube type gauge with a dual range from 30 inHg vac/150 psi should be mounted in the tank.
A relief valve rated at about 100 psi is a good size, but the relief setting must be below the hydrostatic test pressure for safety, and above the maximum process pressure. Typical processing pressure will be 90 to 95 psi. The fitting for the pop-off valve should be located at the highest point on the top of the tank. This fitting will be used for two purposes: to permanently install the pressure relief valve and to perform a hydrostatic test. Any pressure vessel should be hydrostatically tested before putting into service and this fitting will be used for this purpose. There is more on testing later.
Vacuum pump
Vacuum pumps are readily available on the surplus market or you may consider purchasing a new one. The pump should be capable of pulling a vacuum of a least 1 Torr or 1000 microns mercury absolute pressure. Required pump capacity in cubic feet per minute (CFM) is determined by the size of the pressure tank, the desired pump down time, and a pump down factor. For a terminal tank pressure of 1 Torr, use a factor of 8. The following formula will get you close.
Once you have some approximate numbers, either contact a pump manufacturer for additional help with the size or see what is available on the surplus market. My pump is a 7.5 hp Kinney rated at 165 cfm. It is much bigger than I needed, but the price was right and I get a pump down time of about four minutes.
You will need to find a good vacuum gauge with an approximate range of 0-150 Torr. This will only be used to measure vacuum after the tank is partially evacuated and should be valved to protect against tank pressure during that part of the cycle.
Storage tank
If you purchase a VPI system, the normal storage vessel will be a tank of approximate equal volume as the pressure tank. It should be capable of being under vacuum and possibly pressure and equipped with a stirring device. And it will come with a chiller system to maintain tank temperature around 65-68F. This is ideal, but I believe there are other ways to accomplish similar results. If you want to use the storage tank for dip and bake, it must of course be an open vessel.
Let’s look at the ideal features one at a time and point out alternatives. Storing the resin under vacuum does two main things: It degasses the resins and reduces the chances of moisture being absorbed by the resin. At least once a week, we pull a vacuum in our tank and transfer all the resin so that it is subject to the vacuum for a couple of hours and then push it back into the storage tank. This also tends to thoroughly stir and mix the resin. Some storage vessels include a low-rpm paddle mixer to stir the resin. This should be placed on a timer, rather than allowed to run continuously. By using a 24-hour timer, the mixer can run 10-15 minutes per day to keep the resin mixed.
Having a chilled tank to maintain the resin at reduced temperature is certainly best, but if you choose a good stable resin and have high turnover, you can get by without the chiller. Be sure to talk to your resin supplier about this point before starting construction on your system. Some resins require a chiller, while others do not. That might influence your choice of resins. For improved VPI results when a chiller is used, turn off the chiller and allow the resin to be at room temperature when introduced to the VPI vessel. A cold resin will not penetrate as well as a warmer resin.
Putting the system together
Refer to Figure 2 to see how all the parts fit together. Black iron pipe and fittings were used for all plumbing. Manually operated ball valves seem to work fine. The resin valve at the bottom of the tank could use a remote actuator since it is hard to reach. We just extended the handle up through the floor grating. Pipe size is not really critical as long as it is not so small that it slows down the process. As mentioned earlier, the resin pipe should be at least 2 inches. Some “active” metals such as zinc can behave as an accelerator for certain resins causing premature gelling in the pipe or at a valve. For this reason the use of galvanized pipe and fittings and brass or bronze valves should be avoided, particularly in the resin piping.
The dry nitrogen is used to break (or release) the vacuum on the process tank before the pressure cycle starts. It is not used for pressurization. Check with your resin supplier; a nitrogen break may not be required.
It is convenient to have all valves in one location. We used an old slant front control cabinet, appropriately modified, to place all valves, temperature monitor, vacuum and pressure gauges, and electrical controls for the pump. Include a timer with an alarm or bell to alert the operator when each stage of the process has been completed. That helps prevent someone forgetting there is a job in the vessel.
Testing
For safety, any pressure vessel must be hydrostatically tested before being put into service. This means completely filling the tank with water, including the space in the lid. It is difficult to get all the water back out of the system, so the best time to do this test is before all the piping work is done but after all the work and modifications to the tank are completed. This includes installation of viewing ports, welded fittings, or any other tank welding or machine work.
Temporarily remove the pop-off valve and install a good analog pressure gauge with a range of 0-150 psi. Once the tank is completely full of water you can then use air to pressurize the tank.
Slowly increase the pressure reading while looking for leaks or any sign of mechanical movement. If any leaks are detected, they should be repaired and the tank refilled with water before pressure is reapplied. Final test pressure should be 150% of working pressure or about 150 psi. This test will also let you know how good the seal is between the lid and tank.
Do not skip the hydrostatic test. A failure of the tank when it is full of water may get you wet. A failure when full of air under pressure can be deadly. If you work with an outside firm to do the modifications to the pressure vessel, be sure to specify a hydrostatic test as part of the acceptance criteria.
If you decide that you want to try building a system, do so with care and caution at your own risk. I will be happy to answer questions and provide additional information.
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