Mass balance is critical for understanding unit performance and troubleshooting. What goes in must come out. In simple equipment like drums or tanks, this is straightforward. However, in more-complex situations such as with reactors, creating a full mass balance may be a challenge. Nevertheless, all components, including trace ones, must mass balance.
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Mass balance also includes an accumulation term. The difference between flow in and flow out shows up as a change in inventory. This simple analysis can lead to powerful troubleshooting techniques.
One example is understanding trace amounts of water in non-aqueous systems. Many systems have water boots that may drain either continuously or intermittently. Often, this is a small waste stream that doesn’t get carefully measured, particularly if it’s one that flows intermittently. Data on flow rate may be captured but not the frequency of the flow. Also, many times local flow instruments handle intermittent flows, and the readings don’t go to a historian.
One simple method of estimating a small flow rate in this type of system is to track the change in level over time. First, block-in the downstream flow from the water boot (for example). Then, measure the change in level in the boot over a defined time to get an average flow rate.
This approach even suits verifying flow rates for larger streams. For instance, many plants rely on a day tank to ensure a constant feed rate to an operation. If the feed tank can be isolated for a reasonable period, such as a day, checking the change in level and calculating the volume can serve to double-check flow meter readings. Of course, this requires knowledge of the density.
Blocking-in flow can reveal other issues as well. Two common findings are (1) tanks aren’t truly cylindrical, and (2) valves leak.
Large tanks, in particular, deviate from true cylinders. They can have a tilt and swell out under hydrostatic load. Their bottoms, if sitting directly on the ground, can flex. While the flex usually amounts to millimeters, in two extreme cases I’ve seen a tank exhibit a bottom flex of five inches when completely filled.
Problems are more common in older tanks constructed under no-longer-accepted standards. To account for this, tanks often have strapping tables that correct for volume based on height and fluid density rather than just assuming a cylindrical shape. Additionally, a tank may have suffered damage or not have been a cylinder in the first place.
Valves can leak. In one troubleshooting assignment, plant staff insisted that no water contamination was entering a process. Operators pointed out they never had to drain water from a water boot on a drum. However, given the upstream feed came from a drum with a boot that did contain water, the feed had to have soluble water in it. The products out of the unit were completely dry. So, where did the water go?
Mainly to humor me, the operators agreed to close a second isolation valve on the line from the water boot. We waited after closing the second valve. Within the hour, the boot showed a water level, with the level increasing over time. Just opening the isolation valve, without opening the official drain valve, made the water level drop and disappear. The manual drain valve was leaking.
By measuring how fast the level dropped, we estimated a leak rate. It exceeded the entire water rate expected. In normal operation, the drain valve was leaking a small but constant flow of hydrocarbons into the water treatment system. This hydrocarbon flow was the problem we actually were trying to find.
Replacing this apparently minor valve with the plant running was impossible. This is a common problem with many valves often seen as of minor importance and serving more of a utility function. Resorting to some special procedures eventually enabled adding a new drain valve in series with the existing one. The existing (leaking) valve then was fully opened and the new one used instead. After this modification, the water treatment system operation greatly improved.
Mass balance is a critical concept often important in troubleshooting, as these examples illustrate. Never forget that mass balance must apply to both bulk and trace components.
