Chokes clearing

The drain (blowdown) line on a boiler appeared to be choked. It could not be cleared by rodding (the choke was probably due to scale settling in the base of the boiler), so the maintenance foreman pushed a water hose through the drain valve and turned on the water. The choke cleared immediately, and the head of water left in the boiler pushed the hose out of the drain line and showered the foreman with hot water. Although the boiler had been shut down for 15 hours, the water was still at 80°-90°C and scalded the foreman. 

A similar but more serious incident occurred in a polyethylene plant in 1989. A take-off branch was dismantled to clear a choke. The 8-in. valve isolating it from the reactor loop (the Demco valve in Figure 1-2) was open, and hot ethylene under pressure came out and exploded, killing 23... 

Other incidents due to trapped pressure and clearing chokes are described in Sections 17.1 and 17.2. 

How should we clear chokes in small bore lines ... 

A sample point on the suction line of two water pumps became choked, and a maintenance worker was asked to clear it. He was not told how to do so—craftsmen dislike people from other departments telling them how to do their Jobs—but the operators assumed he would use water under pressure or a rod. Instead he used compressed air at a gauge pressure of 115 psi (8 bar), and a pocket of air caused the pumps to lose suction (Figure 1-19). 

An operator had to drain water from a 1,200-m spherical storage vessel nearly full of propane (Figure 8-1). He opened valves A and B. When traces of oil showed that the draining was nearly complete, he shut A and then cracked it to complete the draining. No flow came. He opened A fully. The choke—presumably hydrate, a compound of water and a light hydrocarbon with a melting point above 0°C—cleared suddenly, and the operator and two other men were splashed with liquid. The handle came off valve A, and they could not get it back on. Valve B was frozen and could not be moved. Access was poor because the drain valves were immediately below the tank, which was only 1.4 m above the ground. 

Many operators find it hard to grasp the power of compressed air. Section 2.2 (a) describes how the end was blown off a pressure vessel, killing two men, because the vent was choked. Compressed air was being blown into the vessel, to prove that the inlet line was clear. It was estimated that the gauge pressure reached 20 psi (1.3 bar) when the burst occurred. The operators found it hard to believe that a pressure of only twenty pounds could do so much damage. Explosion experts had to be brought in to convince them that a chemical explosion had not occurred. 

Rodding out narrow bore lines is sometimes necessary. But before doing so, a ball valve or cock should be fitted on the end (Figure 17-lb). It is then possible to isolate the flow when the choke has been cleared, even if the original valve will not close. 

On another occasion, a 4-in.-diameter vertical U-tube, part of a large heat exchanger, was being cleaned mechanically w hen the cleaning tool, which weighed about 25 kg, stuck in the tube. A supply of nitrogen at a gauge pressure of 3,000 psi (200 bar) was available, so it was decided to use it to try to clear the choke. The... 

Figure 17-1. The wrong (a) and right (b) ways of clearing a choked line.

Gas pressure should never be used for clearing choked lines. 

Clearing the choke should not have been attempted until the temperature of the water was below 60°C, the foreman should have worn protective clothing, and if possible a second valve should have been fitted to the end of the drain line as described in (a) above. The accumulation of scale suggests that the water treatment was not adequate [3]. 

Service lines are often not labeled. A mechanic was asked to fit a steam supply at a gauge pressure of 200 psi (13 bar) to a process line in order to clear a choke. By mistake, he connected up a steam supply at a gauge pressure of 40 psi (3 bar). Neither supply was labeled and the 40 psi supply was not fitted with a check valve. The process material flowed backwards into the steam supply line. Later the steam supply caughtfire when itwas used to disperse a small leak. 

Liquid-velocity correlations are generally based on one of two concepts, and it is not always clear which one is the basis of a given correlation. One concept relies on the principle that vapour should not be entrained to the tray below. The other is grounded on the phenomenon of downcomer-inlet-choking caused by the inability of the low-density froth... 

Another criterion sometimes used is to provide sufficient residence time in the downcomer to allow adequate disengagement of gas from the descending liquid, so that the liquid is relatively gas-free by the time it enters the tray below. Inadequate removal of gas from the liquid may choke the downcomer. Kister (loc. cit.) reviewed various published criteria for downcomer residence times and recommended those by Bolles (private communication, 1977) and Erbar and Maddox (Maddox, Process Engineer s Absorption Pocket Handbook, Gulf Publishing, Houston, 1985). Both sets of guidelines are similar and are summarized in Table 14-8. The residence times in Table 14-8 are apparent residence times, defined as the ratio of the total downcomer volume to the clear liquid flow in the downcomer. 

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