Understanding vacuum and pressure can improve the operation of your oil pumps
By George Lanthier
In my travels on the Internet, in seminars and just talking to so many people in this industry, it never seems to amaze me how many people have problems with oil lines, fuel pumps and, specifically, vacuum. When I was thinking about this article and what it would take to explain vacuum, I began to realize that it really wasn’t that easy for most of us who are not rocket scientists or physicists. There’s a lot that has been put on paper on the effect of gases and liquids and what became laws by guys from the past like Messieurs Boyle, Charles, Dalton and Henry, but I’m not a rocket scientist and this can get downright confusing, but here is my spin on it anyway.
Pressure is basically thought of as a force that will push things against a resistance, like pipes and containers, or through a small opening like a nozzle orifice. But, pressure is really anything that is greater than the atmospheric pressure around us. That atmospheric pressure is 14.7 pounds per square inch absolute (psia) or 0 pounds per square inch gauge (psig). Some of the things we measure in psig are oil, steam, air, water, etc. The beauty of pressure is that you cannot only see it on a gauge, you can feel it and in some cases even hear its effects.
Vacuum is just the opposite of pressure, its alter-ego, so to speak. Vacuum is a pressure, too, but it’s a negative pressure and it is lower than atmospheric pressure. With the exception of outer space, the good news is a vacuum only occurs in closed systems. Why is that good news? Keep reading. Knowing that vacuum is really a negative pressure can work for you and is proven in a device like an oil safety valve shown in the top part of Figure 1. Owning a vacuum gauge allows you to measure this negative atmospheric pressure, and that allows us to see its presence just like a positive pressure.
Vacuum is commonly expressed in inches of mercury (inches Hg) and the absolute highest vacuum that you can record is 29.92 inches Hg, which is the reason why vacuum gauges only need to go to 30 inches. You must recognize that indisputable fact about vacuum. In reality, I’ve found that you have about one-third of that 30 inches to work with on oil systems and everything beyond that should only be done in outer space, because it just isn’t gonna work on this planet.
One of my favorite expressions in teaching is that “If you want to move oil, you must use pressure; vacuum just isn’t going to cut it. You can push oil all the way to China using pressure, but you’re not going to do it with vacuum.”
Remember, vacuum gauges only go to 30 inches Hg, but you can buy pressure gauges up to 10,000 psi. What’s important is that everyday I see people exceeding not only the limitations of pumps and the laws of hydraulics and physics, but they also just throw out a lot of other important rules, too. I want to go over those rules in as simple a way as possible and so I’m going to get really basic.
First of all, let’s make a few observations.
Did you ever notice that pump manufacturers always tell us to use a bleeder hose when bleeding a pump? Have you noted that they recommend that you only open that bleeder a half turn? Have you also read somewhere that they want the end of that hose in a supply of oil? Well, here’s why.
The Ulster school received a Peerless boiler plus Taco and Honeywell controls. The school also received testing equipment and supplies necessary to learn simple skills like working with copper tubing. Bruno estimated they had received some $5,000 in equipment. The 104-hour oilheat course meets NORA standards, he said.
When I first came into the business in the late 1960s, quick bleeders and hoses were the newest big rages along with the so called mini-pump. Everyone couldn’t put the pumps or bleeders on fast enough. To that point, pumps were all 1725 rpm and were bled by removing a pipe plug and pointing the hole at a pail. Whenever you bought a pump, new or rebuilt, or even a bleeder, the guys at the counter would give you another hose. It got to the point that I had dozens of them.
The idea behind the bleeder, the hose and the smaller hole was simple: keep the pump and the liquid from picking up anymore air. Putting the end of the bleed hose into the liquid also keeps the air out and the problem away, vacuum. If you have a hole and it’s open to 14 psi, do you think air is trying to get in as you are trying to get air and oil out? Think about it.
The proper and only way to bleed a pump line, and everything else all the way back to the tank is the following: First of all make sure the pump is full of oil, if you have to fill it! Place the hose over the bleeder. A device like the one shown in Figure 2 works great for this and is available on my Web site.
Open the bleeder only a half turn. Make sure that the open end of the bleed hose is immersed in oil in your pail or bucket. Close the inlet valve at the pump and start the burner. Wait until the pump starts to whine. If you have a vacuum gauge inserted at this point it will show 20 to 25 inches of vacuum. By the way, if the hose starts to have white smoke coming out of it, you didn’t fill the pump and it’s burning up. Oooooops! The old-timers used motor oil to start and prime new pumps and stubborn jobs, and, as usual, they were right, more lubrication and viscosity.
Open the inlet valve and watch what comes through the bleeder for several minutes. Normally, if this was a tuneup you would see some oil, then lot’s of bubbles and then air-free oil. Once it’s bled out, close the bleeder with the pump running and you’ve now done a “vacuum power bleed.” I’ve used this method for a lot of years and I can get even the most troublesome jobs to work.
A lot of people will tell you overhead lines won ’t work, that’s just BS or the ranting of an idiot, it’s just a siphon, and it has to work. If the oil tank level is at or higher than the pump, it has to work, that’s physics, but bleeding the system this way and using half-inch tubing for your burner line makes the pump work a lot easier.
Some of the same idiots are the one ’s that will also tell you that oil safety valves (OSVs) don’t work. If the oil industry doesn’t do something to stop oil line leaks, there are people out there that will take care of the problem for us, permanently.
If you have gravity flow, an OSV will work; if you have a pure lift job, you don’t need one. There are some basic guidelines for the proper use of the OSV and all of them come from the people who make and approve them.
First, the OSV can’t be located in freezing temperatures. Second, it can’t be more than three feet above the lowest point in the oil line to the burner. Third, the line has to be oil and air tight, vacuum-proof and the way to get there is the vacuum power bleed. Suntec has even put a little pin or flag at the top of their PRV to tell us what’s going on (see Figure 1). If the pin is up, you’ve lost your vacuum; if it’s down, everything is as it should be. Think of that pin as a friend or service tool because that’s just what it is.
Imagine a straw in liquid, say a clear glass of cola. If you have a clear straw you can see the liquid level in the glass and in the straw, right? Now put your thumb over the end of the straw. Lift the straw out of the liquid and what happens? The liquid is trapped, right? That’s vacuum! Take your thumb away, break the vacuum and you have a leak.
Want to have some fun? Take a drink of that cola through the straw, there’s no leak and that’s vacuum at work. Now take the straw out of the liquid, big vacuum leak, no liquid. Then, take the straw and put a hole in it towards the bottom of the straw. Put it back in the liquid and drink down to the hole, guess what happens?
The next thing is exceeding pump lift or the ability to move oil with vacuum working against us. But, it’s not just vacuum, it’s chemistry, the chemistry of the oil. Oil has changed and the old rules about pumps just don’t work anymore and in my opinion many of them never did because of one other thing, temperature.
Temperature and its effects on viscosity or the thickness of the oil is going to have a lot to do with how a pump moves the oil and that is an indisputable fact. Most of today’s single-stage pumps are advertised to be able to work on two-pipe systems up to six inches of vacuum and most two-stage pumps up to 12 inches of vacuum, but at what oil temperature?
I live in Greater Boston, but teach all over the country. I would never dare teach the 6-inch/12-inch rule in Rutland, Vt., or Idaho Falls, Idaho, Madawaska, Maine, or Pittsfield, Mass., because it gets cold there, very cold. Ever seen oil gel to the consistency of Vaseline or Jell-O? That’s viscosity, baby, and you got to plan for it. How about outside tanks on north walls, think they get cold? Think the oil gets more viscous, or is it vicious?
I won’t design or leave a system under these conditions or at OEM design levels in what I call a cold-oil design; it just doesn’t seem to work for me. If I can get the vacuum capability up by using a two-stage pump, fine, if not, it’s booster time, Figure 3. I love boosters, but I am not going to go there today. I already did two articles on that. If you need them go to our Web site. The articles are in “The Mine” at www.FiredragonEnt.com.
When it’s a cold-oil design and you have to make it work in these conditions, if you keep your single-stage vacuum limited to four inches and your two-stage vacuum to 10 inches, and don’t forget your valves, fittings, etc., you probably will have a lot less problems in the future. The continued use of a pour-point depressant starting in September seems to make sense, too.
One of the other things to consider about vacuum and chemistry is the oil itself, as I’ve already said. Engineers in the early 1980s showed me that when oil gets over 12 to 13 inches of vacuum, the oil actually starts to separate and create problems. As the vacuum goes up, it breaks down the oil, hydrogen and carbon, and the lighter and heavier components separate. This is called hydrogenization. The pump can handle the heavier parts, but the lighter parts end up creating even more vacuum and the pump essentially goes gas-bound. That gas is air and hydrogen and the pump won’t move it, period.
In Figure 4, we show a pump at 10 inches of vacuum and at 20 inches of vacuum in Figure 5. At 10 inches, the pump is full of oil and pumping merrily along, but at 20 inches it has become a full-blown oil separator and that just won’t work and by the way, at zero vacuum, it’s just filled with gas-free liquid.
When troubleshooting vacuum problems, gauges are an absolute necessity. It’s the only way to see this negative pressure. If you’re reading this you probably already believe in gauges and test equipment; that’s been my experience anyway. The guys who keep struggling and ruining our industry are the hacks that don’t test and don’t believe in it. But I’d like to make a suggestion to you professionals, if I may. Upgrade your gauges to liquid-filled; they’re just better, like oilheat. Liquid-filled gauges, Figure 6, are less sensitive to small quirks that will make dry gauges bounce like crazy and that can drive you nuts. The newer vacuum gauges also read backwards or negative, and why not? You are measuring a negative pressure, right?
Finally, is there anyway to check a leaky pump seal in the field? Sure, and it’s called a pump vacuum test. If you want to go over it again, it’s on my Web site, too, in an article called “That Tough Leak.”