Calculate the VOC in your vapor space
How do you know the level of saturation of the vapors in a tank? That is a problem because none of the hand held meters are very accurate when measuring the high end of the vapor concentration. We sometimes use a MSA Gascope to give us an indication because it has a scale that registers 0 – 100% by volume. Laboratory grade analysis in the field is virtually impossible. And if you want to ask a laboratory to analyze for concentration in the vapor space then you have to deal with bag samples and huge expense and wait many hours. The MSA meter is helpful and it has indicated that a number of gasoline tanks we have worked on were 50% rich, which is virtually at the point of saturation. The amount of recovered cargo confirmed that.
- If you ever wondered how much VOC might be in the vapor space of a tank, then here is a simple spreadsheet calculator to estimate the VOC amount in a vapor space. It is only an estimation because there are other factors not considered (for instance the amount of air in the vapors) that can affect the exact amount of cargo in the tank. But this will give you an order of magnitude. Pounds of cargo vapor in the gas of a tank – excel format (xls)
Another way to get an indication of concentration is by checking the oxygen level in the tank. If oxygen is less than 20.9%, and the tank is not under a nitrogen pad then the vapor space is very likely rich or saturated.
If you are not using the PURGIT closed loop vapor recirculating system, the biggest problem you have is getting a representative sample for testing. There is no mechanism or apparatus inside big tanks that could reach out into the tank to get samples at 2 or 3 places and at various heights in the vapor space. Representative sampling is impossible.
PURGIT, on the other hand, mixes tank vapors by circulation in our proprietary technique. That makes PURGIT samples highly representative of the actual tank vapor.
Keep in mind that unless your tank is confirmed to drain all the cargo, it will most likely have puddles of cargo on the floor and those puddles are evaporating and they will continue to evaporate into the vapor space, regenerating the VOC concentration in the vapor space. Please note that testing the vapor space right after the tank has been emptied will probably not be accurate because of the replacement air drawn into the tank. The mechanics of acquiring a representative sample of the tank vapor is a problem. There is no good way to get an accurate sample, unless you use the PURGIT system.
PURGIT mixes the vapors in the tank. The PURGIT closed loop system eliminates dead air and increases the rate of evaporation from the floor. That makes the tank vapor homogeneous for testing and VOC is removed efficiently.
PURGIT is a company in Houston Texas that does tank vapor control as a service and sells tank vapor control equipment. We can be reached at telephone # 713-201-7517.