Valves block isolation

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.)... 

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

A three-valve configuration common in shut-off applications to prevent release of a gas or fluid to the area of isolation by providing a positive means of isolation. Two main shut-off valves (block valves) operate on the main process line to stop flow. A third bleed valve in a T-section between the two block valves fitted with a vent or drain can be opened to relieve pressure of the process fluid from the region between the two block valves. In this fashion, passing of the isolation valve on the active side will not allow entry into the isolated equipment. Typically considered as a one out of... 

Does the design provide for isolation by valve blocking, spools, double blocks and bleeds, flanges, and flushing connections ... 

Dead legs in the sample line must be discharged safely to ensure that the sample will ac tuaUy be representative of the material in the unit. Without blowing down the dead leg, samples taken will be erroneous, as they may be representative of some past operating conditions. If the location is nonroutine, the sample leg may have accumulated debris. The debris could partially or totally block the line. Opening the isolation valve to blow down the line could result in a sudden, uncontrolled release, presenting a hazard to the sample gatherer. 

The 1997 edition of the API RP 521 extends the two-thirds rule to include the upstream and downstream system. At a minimum, the inlet and outlet piping up to and including isolation valves must be designed for the two-thirds rule to be able to block in the exchanger. If the upstream and downstream equipment is not designed for the two-thirds rule, relief devices may be required on both the inlet and outlet piping to protect the piping and adjaeent equipment. 

Addition of a small open bypass around one of the isolating valves which could be used to block in the equipment. 

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 ... 

Blocked Discharge. It is assumed that all outlets from the vessel are shut in and the total inlet flow stream (gas and liquids) must flow out through the relief valve. This condition could occur, for example, if the equipment has been shut in and isolated and the operator opens the inlet before opening the outlet valves. 

The valve was operated by compressed air, and the two air hoses, one to open the valve and one to close it, were connected up the wrong way around. The two connectors should have been different in size or design so that this could not occur. In addition, they were not disconnected, and a lockout device on the valve—a mechanical stop—had been removed. It is also bad practice to carry out work on equipment isolated from hot flammable gas under pressure by a single isolation valve. The take-off branch should have been slip-plated, and double block and bleed valves should have been provided so the slip-plate could be inserted safely (Figure 1-1), [16, 17]. 

The correct way to light a furnace (hot or cold) that bums gas or burns light oil is to start with a positive isolation, such as a slip-plate, in the fuel line. Other positive isolations are disconnected hoses, lutes filled with water (if the fuel is gas at low pressure), and double block and bleed valves closed valves without a bleed are not sufficient. Then ... 

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. 

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

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. 

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. 

When a pipe or vessel is totally filled with a liquid which can be blocked in, for instance, by closing two isolation valves, the liquid in the pipe or pressure vessel can expand very slowly due to heat gain by the sun or an uncontrolled heating system. This will result in tremendous internal hydraulic forces inside the pipe or pressure vessel, as the liquid is non-compressible and needs to be evacuated. This section of pipe then needs thermal relief (Figure 2.9). 

One always has to be very careful with the use of isolation valves at the inlet of an SRV. If it is absolutely necessary to use them, always use full-bore valves and ensure that the incoming piping is one size larger in diameter than the SRV inlet (Figure 6.12). Always make sure the isolation valves can be blocked in open position and that they are provided with a clear position indicator. If isolation valves are used in both inlet and outlet, they need to be provided with an interlock system in order to be sure both valves are together in closed or open position. 

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... 

The isolation valve has no locking device. It is always recommended to have a locking device on each isolation valve so it can be blocked in open position. It is also highly recommended that the valve be equipped with a clear position... 

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