DEMCO valve

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... 

Figure 1-2. The take-off branch was dismantled with the Demco valve open. (Illustration courtesy of the U.S. Department of Labor.)...

The accident investigation evidence showed that this safe procedure was not followed specifically, the product takeoff valve was removed, the DEMCO valve was in the open position, and the lockout device was removed. This scenario was a serious violation of well-established and well-understood procedures and created the conditions that permitted the release and subsequent explosion. 

The OSHA investigation13 found that (1) no process hazard analysis had been performed in the polyethylene plant, and as a result, many serious safety deficiencies were ignored or overlooked (2) the single-block (DEMCO) valve on the settling leg was not designed to fail to a safe closed position when the air failed (3) rather than relying on a single-block valve, a double-... 

Ethylene is dissolved in isobutane. This ethylene reacts with itself to form polyethylene particles that gradually settle out of solution and are collected in one of six settling legs. Particles pass through a main 8-inch (21 cm) block valve called a DEMCO valve to collect in a chamber. The particles are periodically isolated from the reactor, the volatiles are removed, and the polyethylene is routed to other equipment for further processing. See Figure 5—7, which appeared in OSHA Reports and has been reproduced with the permission of OSHA. 

In the Phillips reactor, the plastic material frequently clogged the settling legs. When this happened, the DEMCO valve for the blocked... 

After the explosion it was found that the DEMCO valve was open at the time of the release. The leg to be cleaned had been prepared by a Phillips operator. The air hoses that operated the DEMCO valve were improperly connected in a reversed position such that a closed DEMCO valve would be opened when the actuator was in the closed position. In addition, the following unsafe conditions existed ... 

The DEMCO valve did not have its lockout device in place. 

The air supply valves for the actuator mechanism air hoses were in the open position so that air would flow and cause the actuator to rotate the DEMCO valve when the hoses were connected. 

The DEMCO valve was capable of being physically locked in the open position as well as in the closed position. The valve lockout system was inadequate to prevent someone from inadvertently opening the DEMCO valve during a maintenance procedure. 

In the Phillips reactor, the plastic material frequently clogged the settling legs. When this happened, the DEMCO valve for the blocked leg was closed, the leg disassembled, and the block removed. During this particular maintenance process was going on, the reaction... 

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