Isolating block valves

Procedures Spills, Leaks, Decontamination Process Isolation Block Valves, Fire-Safe Valves,... 

Many relief valves pop open above or below their set point. Such valves have to be removed from the vessel and reset in a machine shop. Often, there are isolating block valves, located beneath the relief valve, that permit the relief valve to be pulled while the vessel is still in service. These block valves are perfectly legal, provided they are chain-locked open. It in unlawful to have an isolating valve below a relief valve that is not chain-locked open or sealed open in some positive manner. 

During a polymer plant shutdown, the operator was supposed to remove the plug from and connect a nitrogen purging hose to a fitting immediately downstream from an isolating block valve connected to a depressurized system. Instead, the employee mistakenly connected to a pressurized fitting immediately upstream from the block valve, into a 350-psi... 

The isolation block valve closes tight and does not leak... 

Isolation Valves for Pressure Relief Systems - Block valves for maintenance isolation purposes are permissible in pressure relieving systems, provided that they are car sealed open and comply with the requirements of CSO valves. The particular locations where such CSO valves are p>ermitted are ... 

You can quickly identify these plant sections by reviewing process flow diagrams and valving arrangements. Isolation points are defined by control valves or powered block valves that can be remotely activated. Process hazard analysis techniques help you identify the maximum credible accident scenarios. (Note that manual valves should not be considered reliable isolation points unless they are located to be accessible following a major accident. However, remotely-activated valves can only be considered reliable isolation points if there are adequate reliability engineering and maintenance programs in place.)... 

Often a system (a group of vessels not capable of being isolated from each other by block valves, or containing restriction to flow and release of pressure) may need a relief valve set reasonably close, sat +15% to 20% when system is below 1000 psig above, typically use 7% to 15% above as set criteria related to normal operating pressure to catch any pressure upswing. Then this may have a backup valve set higher (but within code) to handle further pressure increase. Or, the second device may be a rupture disk. It is not unusual to have two relief de ices on the same equipment set at different pressures. 

This is Acceptable os there is no Block Valve Isolating any Item. If Block Valve Installed ot A,Safety Volve would be Required to Protect Condenser and Receiver. If Additional Block Valve Installed of B, Safety Valve would be Required for Condenser ond also for Receiver. 

Note that the intent of the ASME Code is to ensure that under those circumstances where a pressure relieving device can be isolated by a block valve from its pressure, or its discharge, that a responsible indhadual lock and unlock the block valve to the safe open position and that this individual remain at the block valve the entire time that the block valve is closed. 

This is Acceptable as Ihere is no Block Volve isolating any Mem If Block Valve Installed ot A. Safely Volve would he Reouired to Protect Condenser and Receiver. II Additional Block Valve Installed ot B. Safety Valve would he Required lor Condenser and also > Rtc,iv r-... 

Other guidelines cited in Reference 5 suggest methods for estimating additional release factors such as release duration or inventories. It should be noted that the above methods are again only approximations. Site-specific designs and process conditions should be evaluated. For example, some facilities may have few, if any, emergency block valves available to isolate a release. Other facilities may be designed for rapid isolation. 

There are three main methods to dispose of releases from thermal relief valves. Discharge around a block valve (isolation circumvention) is widely used in most situations, however where this is not practical or economical disposal to a sewer or release to surface runoff can be specified in certain cases. 

Isolation or emergency shutdown (ESD) valves should be installed to stop fuel flow and the process feed flow into the heater in the event of heater tube rupture. These valves can be automatically actuated by controls or safety interlocks or can be manually operated remotely. Remote actuation can be from a control room console or in the field field actuation stations should be located at least 50 ft (15 m) from the heater. It is also common to provide a manual block valve, located at least 50 ft (15 m) from the heater, on each of the fuel and process feed lines. These should be accessible to operators in the event of an incident involving the heater. 

Isolate S2C1 to VI35 with block valve near VI38. 

Isolate IC4 to VI35 with block valve in pipe rack near VI61 with chain operator. ... 

The initial incident involved a pinhole leak on a 6-in. line located 112 feet above grade. The unit might have been shut down at this point, but a decision was made to keep it on line. Efforts to isolate the circuit by closing four block valves failed. The line remained hot and the level in the downstream stripper vessel continued to rise. Several attempts were made to depressure the circuit and free drain all the residual naphtha. At least 15 work orders were issued over a 13-day period and most of these failed to meet the company s own work standards. 

Earlier in this book, we cautioned against using the type of changeover valves or inlet isolation valves related to inlet pressure drops. The main factor here is the Cv (valve flow coefficient) value of the changeover device or block valve used upstream of the SRV. If individual foil-bore valves are used, the same principle applies as for changeover valves. Note once more, however, that if individual valves are used upstream of an SRV, they must be interlocked if used in a switch-over system or locked open if used in an individual isolation... 

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. 

Investigators determined that during a hectic day of operations, the chemical process operator erred. On this afternoon, he inadvertently tried to startup the heater with the burner firing and the heater tubes isolated from the circulating pump by closed blocked valves. Shortly after firing the heater, the lead operator checked the flame pattern but observed nothing out of the ordinary. 

Relief devices Emergencies Process isolation Instruments Adequacy, vent size, discharge, drain and support Dump, drown, inhibit and dilute Block valves, fire-valves and purging Air quality, time lag, reset wind-up and materials of construction... 

It should be possible to easily isolate fluids in equipment and piping when potentially dangerous situations occur. This can be done using emergency block valves (EBVs). An EBV is a manually or remotely actuated protective device that should be used to provide manual or remote shutoff of uncontrolled gas or liquid flow releases. EBVs can be used to isolate a vessel or other equipment, or an entire unit operation. Manual valves are often used on piping at block limits where it is unlikely that there would be a hazard to personnel if an accident occurs. Remotely controlled EBVs are recommended on tanks and on piping in areas where it may be hazardous for personnel in the case of an accident, or where a quick response may be necessary. 

It should be possible to easily isolate fluids in equipment and piping when potentially dangerous situations occur. This can be done using emergency block valves (EBVs). An EBV is a manual or remotely-actuated protective device that should be used to provide manual or remote shut-off of... 

With all due respect to the findings of the investigating team, the root cause appears to me to be a human factors situation. The new piping system looked simple but in fact this unique system was confusing. A check valve (valve no. 2) was installed on the vent header to prevent cross-contamination. A small block valve (valve no. 4) was installed on the vacuum relief impulse line to provide easy isolation. [5]... 

After the explosion, investigators confiimed that the 8-inch DEMCO block valve was open at the lime of the accident. Furthermore, tests showed that the compressed air hoses were improperly connected in a reversed position. This unauthorized step during maintenance activity allowed the DEMCO isolation valve to open when the actuator switch was in the closed position. [7] Over time, somehow, the company s maintenance procedures had been modified and this tragedy resulted. 

Shut-off valves (block valves or isolation valves), whose purpose is to close off the flow ... 

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