Dual relief valves

One practical method to maintain relief valve operability is to design the relief valves for on-stream cleaning and maintenance. As an operating manager, this was my preferred design for relief valves subject to fouling. 

Of course, an isolation gate valve was required underneath each relief valve. This is legal as long as the gate valve is chain-locked open. The bigger problem is that if one relief is removed from service for maintenance, then the remaining relief valve, or valves, must still have full and adequate, capacity. Typically, for larger vessels there are three 50 percent capacity relief valves. 

8 Process Design Responsibility for Relief Valve Design 

Quite often, a higher pressure vessel will vent, not to a flare, but to a lower pressure vessel. For example, a high pressure reactor might relieve to a low pressure fractionator to recover the liquid portion of the relief valve effluent in the larger fractionator. But then the fractionator relief valve must be sized for the vapor venting rate from the high-pressure reactor. A complex engineering problem results. 

I had been injecting a corrosion inhibitor into the depropanizer with the reflux. The inhibitor was supposed to be soluble in liquid propane, but it was not. After several months of using this inhibitor chemical, the relief valve nozzle of the top head of the depropanizer plugged up. But how did this first come to my attention  

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The nearest catalogue standard receiver is 240 mm diameter by 3.25 m long and has a gross capacity of 314 litre or 187 kg. Being over the limit of 285 litre, it must have dual relief valves. 

When a relief valve is removed for testing, the vessel that it was protecting no longer has that safeguard against overpressure. Ideally, the equipment item will be shut down and opened to the atmosphere. In many facilities block valves are installed immediately below the relief valve. The block valve is normally in the open position (either car-sealed or locked). When the relief valve has to be removed for maintenance, the block valve is closed so that the system can remain in operation. (If a gate valve is used, it should be installed horizontally so that, where the gate to fall off the stem, it would not inadvertently block the line.) Some facilities use dual relief valves tied in to a three-way valve so that at least one relief valve remains in service at aU times. 

The basis for design overpressure described in this section is related to the ASME Boiler and Pressure Vessel Codes and ANSI B31.3, Code for Petroleum Refinery Piping. Compliance with these codes is a requirement, or is recognized as the equivalent of a requirement in many locations. Where more stringent codes apply, the local requirements must be met. Therefore, local codes must be checked to determine their requirements. For example, some countries do not permit the use of block valves underneath pressure relief valves, unless dual valves with interlocks are installed. Also, in some cases, 20% accumulation under fire exposure conditions is not permitted, and accumulation allowed may be lower than the ASME Codes. In the United States, the ASME Code is mandatory, since it is a requirement under the Occupational Safety and Health... 

Figure 7-19 illustrates a newer approach at simplifying the dual safety relief valve installation, ASME Sect. VIII, Div. 1, UG-135(b) [1] and API RP-520, Part II Conformance [33]. Note that the SRV valves are mounted on top of each of one dual vertical connections and are bubble tight. Also see cross section view. The flow C, valves for each size device are available from the manufacturer. 

Figure 7-19. Safety selector valve for dual safety relief valve installation with switching. By permission, Anderson, Greenwood and Co. AGGO.

For example, 30 % Oleum is boiled to generate SO3 vapours at about 125 °C which are sent to the condenser to produce liquid SO3. Here the pressure can get reduced to below atmospheric pressure (-100 mm WG) on condensation of SO3. This condenser needs a special dual pressure gauge and pressure relief valve also (to vent of any non-condensibles). 

The safety/relief valves are dual-function valves discharging directly to the pressure suppression pool. The safety function provides protection against overpressure of the reactor primary system. The relief function provides power-actuated valve opening to depressurize the reactor primary system. The valves are sized to accommodate the most severe of the following two pressurization transient cases determined by analysis ... 

Important parts of well-designed tanks include proper dual pressure relief valves, a vapor absorption system, liquid level and pressure gauges, liquid and vapor transfer valves, and an adequate electrical ground. Pipes, fittings, pumps, gauges, and other equipment should be of steel, iron, or other material not subject to corrosion by the methylamines. Storage and... 

The objective was to prevent corrosion in the relief valve headers flowing into the dual flare K.O. drums. I was always very careful, when adding new process services to the relief valve system, to adhere to this successful segregation scheme. 

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