Overfill

Second, when filling a pipet or volumetric flask, set the liquid s level exactly at the calibration mark. The liquid s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware s calibration mark (Figure 2.6). The meniscus should be adjusted with the calibration mark at eye level to avoid parallax errors. If your eye level is above the calibration mark the pipet or volumetric flask will be overfilled. The pipet or volumetric flask will be underfilled if your eye level is below the calibration mark. 

Opera.tlons. Overfill of tanks, owing to any number of reasons, is a common occurrence. This results from inoperative or failed equipment... 

Bucket Elevators. In a bucket elevator, a series of buckets attached to an endless belt or chain are filled with material and lifted vertically to a head pulley or sprocket, where the material is dumped. The buckets are then returned back down to a tail pulley or sprocket at the bottom. Bucket elevators are not self-feeding. They must be fed at a controlled rate to avoid overfilling the buckets and damagiag the machinery. In the usual arrangement of a bucket elevator, the chain or belt path is vertical or steeply inclined ia a single plane. Special chain supported bucket systems that can travel ia two and three planes have been developed. 

Location of Vacuum Relief Device (Carl Schiappa, Michigan Engineering, The Dow Chemical Company, Midland, Mich., personal communication, March 20, 1992.) If a vacuum relief device is used, locate the device at the highest point on the top of the tank. If the vacuum relief device is not installed in this location and the tank is overfilled with liquid, the relief device will be sealed in liquid and will be ineffec tive in protecting the tank. This is especially true for the part of the tank above the vacuum relief device if it is sealed in liquid, tne liquid level is lowered, and the tank goes into a partial vacuum. 

Overcharge/overfeed of reactants Possibility of overfilling vessel, or initiating runaway reaction. 

Install level device interlocked to prevent overfill... 

Install independent high level alarm with instructions to prevent overfilling... 

Thermal expan- Drum at proper temperature sion due to liquid, Keep drum away from heat source overfill leading to loss of reaction is complete before drumming containment. Allow adequate freeboard for each material CCPS G-3 CCPS G-14 CCPS G-22 CCPS G-29... 

Overfill drum due Calibrate weighing devices and maintain equip-to operator error ment in good working order or valve failure,. metering pumps can lead to opera- tor exposure, slip- fill operation with weighing device pery floors, spread of flammable liquids. CCPS G-3 CCPS G-15 CCPS G-22 CCPS G-29... 

Overfill by plugging, blinding cloth, failure to start underflow pump, loss of vacuum or by operator error. 

API RP 2350, Overfill Protection for Storage Tanks in Petroleum Facilities, 2nd ed., January 1996... 

Historical releases during transfer of materials into or out of storage Discharges of wash water or other liquids during operation or maintenance Overfills... 

Accidental release, spillage Transport incidents Overfilling of containers Equipment failure Unexpected reactions Runaway reactions... 

Speeifieation of a storage eapaeity and a proeedure to avoid overfilling. 

A eheek that it is not overfilled and is within its safe operating limits. 

Whether a high-level alarm is needed depends on the rate of filling and on the size of the batches being transferred into the receiving tank. If these are big enough to cause overfilling, a high-level alarm is desirable. 

Spillages resulting from overfilling should be retained in tank dikes (bunds). But very often the drain valves on the dikes—installed so that rainwater can be removed—have been left open, and the spillage is lost to drain (see Section 5.5.2 c). 

Alarms and Trips Can Make Overfilling More Likely... 

A high-level trip or alarm may actually increase the frequency of overfilling incidents if its limitations are not understood. 

Liquid is sometimes transferred from one tank to another b gravity. Overfilling has occurred when liquid flowed from a tall tank to a shorter one. On one occasion, an overflow occurred when liquid was transferred from one tank to another of the same height several hundred meters away. The operators did not realize that a slight slope in the ground was sufficient to cause the lower tank to overflow. 

Suppose a tank is designed to be filled at a rate of x mMrr. Many tanks, particularly those built some years ago, are provided with a vent big enough to pass v m /hr of air but not x m /hr of liquid. If the tank is overfilled, the delivery pump pressure will almost certainly be large enough to cause the tank to fail. 

Many leaks have been discussed under other headings, including leaks that occurred during maintenance (Chapter 1), as the result of human error (Chapter 3), or as the result of overfilling storage tanks (Section 5.1). Other leaks have occurred as the result of pipe or vessel failures (Chapter 9), while leaks of liquefied flammable gas are discussed in Chapter 8 and leaks from pumps and relief valves in Chapter 10. 

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