Level alarm

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

Implement operating instructions to maintain liquid level above heating surface at all times Install automatic level control with low level alarm and shutdown of liquid withdrawal system to ensure liquid is above heating surface at all times... 

Blocked liquid Provide level switches for effluent collection effluent line result- vessels ing in flooding.. provide high level alarm in liquid effluent line Implement preventive maintenance checks ... 

Operations The QRA team will need specific data on how the system is actually operated. For example, are the bypass valves normally left open to increase throughput, what happens when the high level alarm sounds, or do operators bypass interlocks to continue production Human actions/errors are usually dominant contributors to the real-world risks, and truthful data on actual process operations are vital to credible QRA results. Expect to commit one full-time equivalent for the life of the project. 

The reservoir may be either pressurized or atmospherie. It must have suffieient eapaeity to eontain all oil during drain-baek or shutdown. It must be equipped with an oil level indieator, a low-level alarm switeh, safety relief valve, a pump for oil makeup during operation, drain valve, heater, mist eliminator, strainers, and required valves. Expander reservoirs must be designed and eonstrueted in aeeordanee with applieable ASME eodes. Reservoir retention time is typieally between 5-18 min depending on turboexpander size and manufaeturer s sizing eriteria. This is an area where the owner/purehaser should ask for the manufaeturer s assistanee. 

A typical lubrication oil system is shown in Figure 15-1. Oil is stored in a reservoir to feed the pumps and is then cooled, filtered, distributed to the end users, and returned to the reservoir. The reservoir can be heated for startup purposes and is provided with local temperature indication, a high-tempera-ture alarm and high/low level alarm in the control room, a sight glass, and a controlled dry nitrogen purge blanket to minimize moisture intake. 

The minimum alarms and trips reeommended for eaeh major driver and driven maehine should be a low oil pressure alarm, a low oil pressure trip (at some point lower than the alarm point), a low oil level alarm (reservoir), a high oil filter differential pressure alarm, a high bearing metal temperature alarm, and a metal ehip deteetor. See Table 15-1. 

The single contingency which results in the largest vapor load regardless of any associated liquid load, is used to determine the maximum required vapor space C-F, and a high level alarm is placed at point C. 

If HjS is continuously present in the flare gas or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to a sour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normal loop seal arrangement. Two pumps are provided one motor driven for normal use, and the other having a steam turbine drive with low pressure cut-in. The seal drum level is controlled by LIC with high and low alarm lights plus an independent high level alarm. 

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. 

If the level indicator measures weight, it is good practice to fit a high-level alarm, which measures volume. 

The fire was more serious than it would normally have been because the inventory in the plant, about 70 tons, was about twice the usual amount. Some of the overheads from a reactor were collected in a slops drum and recycled. The inventory in the drum was usually small. At about 9 a.m. on the day of the fire, the recycle pump failed. As a result, the level in the drum rose, and the level in the reactor fell. The operator noticed the fall in the reactor level (but not the rise in the drum level) and recycled product to maintain the level. At 8 p.m. the supervisor noticed that the high-level alarm on the slops drum was lit he found that the recycle pump had failed, and he changed over to the spare it leaked 25 minutes later. Section 3.3.1 describes another occasion when operators failed to notice unusual readings for 11 hours. 

Later in the night shift the control room operator received a pump trip alarm from pump B. Soon after, the second stage separator high oil level alarm sounded in the control room. The operator, needing to reduce the level. 

At about 22 10 received pump trip alarm. He then watched level increase in separators, then high oil level alarm sounded. Operator then switched to pump A and started it up. 

Figure 5.10.3, Vessels—level control on outlet. Legend FIC, flow indicator controller LG, level gauge LIC, level indicator controller LAWL, level alarm for water level, M, motor.

Provide liquid low level alarms to prevent loss of liquid by evaporation, entrainment, leaks, or failure of the makeup liquid system. 

As an alternative, sampling BW from a point just below the extra-low-level alarm point usually is also suitable. However, the BW is not always at its most concentrated point at the water column connection. A sample taken from this point may contain a mix of BW and steam, which leads to errors in sampling and hence, testing, reporting, interpretation, and subsequent actions. 

A water-level alarm system located inside the BW column. 

Consequences—low level in vaporiser (this should activate the low level alarm), higher rate of flow to the reactor. 

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