| John Olson, Truck Product Manager for Satellite Industries, gives some maintenance tips in regard to pressure relief valves and how they can affect the longevity of your tank. |
| The tank shown above demonstrates what the additional forces of vacuum can do to an older tank if the vacuum relief valves fail to limit vacuum levels. As you can see this tank has been well cared for and may have provided a few more years of service had vacuum levels stayed at an acceptable range for efficient pumping. |
Small seemingly insignificant components can make a big difference in the performance and longevity of your
pumper truck. Take your vacuum relief valve for example. Most operators pay little attention to this seemingly
insignificant component but to do so may eventually make you very sorry. Ignoring relief valve function actually
can rob system performance, waste money and even worse, cause catastrophic system failures.
Vacuum pumps form the heart of every vacuum system. Left uncontrolled, vacuum pumps will draw down the
vacuum level in a tank to the point where the pump physically can not draw out or evacuate any more air from the
tank. The total amount of vacuum drawn from the tank depends upon the pump condition determined by rotor and
end plates clearances inside the pump. The condition of the vanes and their sealing capability with the inner bore of
the pump, also affect vacuum levels. In addition, the condition system components such as hoses and fittings and
the amount of leaks present also influence vacuum levels. Most pumps in good condition draw at least 25 inches of
mercury if no relief valve is present and some will draw close to 27 inches of mercury. One common
misconception about vacuum pumps is that bigger displacement pumps create higher vacuum levels. This is not
true. Actually, small displacement pumps, in an equivalent condition, produce the same vacuum levels as larger
pumps, it just takes them longer time to accomplish the job.
As vacuum levels increase, pumps work harder to evacuate the tank and all components experience increased
forces on their outer surfaces due to atmospheric pressure. To limit these forces and control vacuum levels, all
properly designed and maintained vacuum systems rely on a well maintained and accurately adjusted vacuum relief
valve. For servicing portable restrooms, a vacuum level set to maximum of 18 – 20 inches of mercury provides an
adequate level for efficient pumping. As we will see later, controlling vacuum levels becomes a vital factor for the
life and performance of all the components on your pumper truck.
So what’s the problem with a few extra inches of mercury vacuum level? Reviewing some basic laws of science
answers this question. Normal atmospheric pressure runs about 14.7 PSI. One (1) inch of vacuum equals .489
PSI. Therefore 18 inches of vacuum equals a pressure on the outside of the tank of 8.8 PSI. If the vacuum level is
allowed to reach say 22 inches of mercury the pressure on the outside of the tank reaches 10.8 PSI or an increase
of 2 PSI. On a 1600 gallon tank with a 60 in diameter there are about 29,000 square inches of area. Performing
the math, we find that an increase of just 4 inches of mercury level puts an extra 38,000 pounds or 19 tons of
additional force on the outside surface of the tank. If the vacuum relief valve in the vacuum systems fails and the
vacuum level reaches 26 inches of mercury an additional 38 tons of force become exerted on the outside of the
tank. That’s a surprisingly large number that can produce catastrophic results on an aging tank should this situation
occur.
Don’t let this happen to you! Keep a close eye on vacuum levels and adjust the relief valve to assure your levels do
not get too high. Also, make sure the vacuum gauge operates properly indicating the correct vacuum level. If not,
replace the gauge immediately.
As one can readily see, maintaining the components in your vacuum system, especially the vacuum relief valve, can
pay big dividends by keeping your system running in an optimum manner. Also, remember this, it pays to spend a
little extra for a good quality relief valve, one that opens over a small differential in vacuum level. This allows
precise adjustment of the vacuum level and saves money by pulling air out of the tank rather than opening early and
drawing air from the outside just when the system has begun to reach the optimum pumping vacuum level range.
pumper truck. Take your vacuum relief valve for example. Most operators pay little attention to this seemingly
insignificant component but to do so may eventually make you very sorry. Ignoring relief valve function actually
can rob system performance, waste money and even worse, cause catastrophic system failures.
Vacuum pumps form the heart of every vacuum system. Left uncontrolled, vacuum pumps will draw down the
vacuum level in a tank to the point where the pump physically can not draw out or evacuate any more air from the
tank. The total amount of vacuum drawn from the tank depends upon the pump condition determined by rotor and
end plates clearances inside the pump. The condition of the vanes and their sealing capability with the inner bore of
the pump, also affect vacuum levels. In addition, the condition system components such as hoses and fittings and
the amount of leaks present also influence vacuum levels. Most pumps in good condition draw at least 25 inches of
mercury if no relief valve is present and some will draw close to 27 inches of mercury. One common
misconception about vacuum pumps is that bigger displacement pumps create higher vacuum levels. This is not
true. Actually, small displacement pumps, in an equivalent condition, produce the same vacuum levels as larger
pumps, it just takes them longer time to accomplish the job.
As vacuum levels increase, pumps work harder to evacuate the tank and all components experience increased
forces on their outer surfaces due to atmospheric pressure. To limit these forces and control vacuum levels, all
properly designed and maintained vacuum systems rely on a well maintained and accurately adjusted vacuum relief
valve. For servicing portable restrooms, a vacuum level set to maximum of 18 – 20 inches of mercury provides an
adequate level for efficient pumping. As we will see later, controlling vacuum levels becomes a vital factor for the
life and performance of all the components on your pumper truck.
So what’s the problem with a few extra inches of mercury vacuum level? Reviewing some basic laws of science
answers this question. Normal atmospheric pressure runs about 14.7 PSI. One (1) inch of vacuum equals .489
PSI. Therefore 18 inches of vacuum equals a pressure on the outside of the tank of 8.8 PSI. If the vacuum level is
allowed to reach say 22 inches of mercury the pressure on the outside of the tank reaches 10.8 PSI or an increase
of 2 PSI. On a 1600 gallon tank with a 60 in diameter there are about 29,000 square inches of area. Performing
the math, we find that an increase of just 4 inches of mercury level puts an extra 38,000 pounds or 19 tons of
additional force on the outside surface of the tank. If the vacuum relief valve in the vacuum systems fails and the
vacuum level reaches 26 inches of mercury an additional 38 tons of force become exerted on the outside of the
tank. That’s a surprisingly large number that can produce catastrophic results on an aging tank should this situation
occur.
Don’t let this happen to you! Keep a close eye on vacuum levels and adjust the relief valve to assure your levels do
not get too high. Also, make sure the vacuum gauge operates properly indicating the correct vacuum level. If not,
replace the gauge immediately.
As one can readily see, maintaining the components in your vacuum system, especially the vacuum relief valve, can
pay big dividends by keeping your system running in an optimum manner. Also, remember this, it pays to spend a
little extra for a good quality relief valve, one that opens over a small differential in vacuum level. This allows
precise adjustment of the vacuum level and saves money by pulling air out of the tank rather than opening early and
drawing air from the outside just when the system has begun to reach the optimum pumping vacuum level range.
The Sanitation Journal is a monthly trade publication that caters to the portable restroom industry. Each month
thousands of portable restroom operators utilize information from the Journal to aid in business decisions.
Subscribe today and make sure you never miss an issue.
thousands of portable restroom operators utilize information from the Journal to aid in business decisions.
Subscribe today and make sure you never miss an issue.
 | |  | |  | |  | |  | |  | |  | |  | |  | |  | |  | |  |
 | |  | |  | |  | |  | |  | |  | |  | |  |
Serving the American Liquid Waste Industry Since 2002