Relief installation

Figure 8-11 Relief installation practices. Adapted from Eric Jennett, Components of Pressure-Relieving Systems, Chemical Engineering (Aug. 19,1963), pp. 151-158.

Additional pressure limitation system of the reactor pressure vessel Increase of the availability of the pressure relief installations 1994... 

Similarly, instead of installing vacuum relief valves the vessels can be made strong enough to withstand vacuum. In addition, if the vessel contains flammable gas or vapor, vacuum relief valves will often need to admit nitrogen to avoid flammable mixtures. A stronger vessel often may be safer and cheaper. 

Testing, certification, and installation rules for reheving devices are extensive. Eveiy chemical engineer responsible for the design or operation of process units should become veiy familiar with these rules. The pressure-relief-device paragraphs are the only parts of Sec. TII, Division I, that are concerned with the installation and ongoing operation of the facility all other rules apply only to the design and manufacture of the vessel. 

It is very important that a melt-ont riser be installed whenever tank contents are expected to freeze on prolonged shutdown. The purpose is to provide a molten chimney through the ernst for relief of thermal expansion or cavitation if fluids are to be pumped ont or recirculated through an external exchanger. An external heat tracer, properly... 

Rupture Disks A rupture disk is a device designed to function by the bursting of a pressure-retaining disk (Fig. 26-15). This assembly consists of a thin, circular membrane usually made of metal, plastic, or graphite that is firmly clamped in a disk holder. When the process reaches the bursting pressure of the disk, the disk ruptures and releases the pressure. Rupture disks can be installed alone or in combination with other types of devices. Once blown, rupture disks do not reseat thus, the entire contents of the upstream process equipment will be vented. Rupture disks are commonly used in series (upstream) with a relief valve to prevent corrosive fluids from contacting the metal parts of the valve. In addition, this combination is a reclosing system. 

Particularly reasonably priced explosion isolation of systems involves the use of a relief pipe with which the flow direction can be diverted by 180°. It prevents flame jet ignition with precompression in constructionally protected equipment. If suction is present, explosion propagation can occur. To prevent this, the use of an additional extin-gui ing barrier or a rapid-action gate valve is necessary. If a diverter is installed where positive pressure feed is given, then the diverter is, in general, safe against an explosion propagation. 

Location of Vacuum Relief Device (Carl Schiappa, Michigan Engineering, The Dow Chemical Company, Midland, Mich., personal communication, March 20, 1992.) If a vacuum relief device is used, locate the device at the highest point on the top of the tank. If the vacuum relief device is not installed in this location and the tank is overfilled with liquid, the relief device will be sealed in liquid and will be ineffec tive in protecting the tank. This is especially true for the part of the tank above the vacuum relief device if it is sealed in liquid, tne liquid level is lowered, and the tank goes into a partial vacuum. 

Examples of Vacuum-Related Accidents Figure 26-47 shows a jacketed tank, where the jacket was designed for low-pressure steam. When the steam was turned off and the drain valve and trap were closed, the steam condensed, causing the jacket to collapse. The jacket should have been designed for full vacuum, or a suit le vacuum relief device should have been installed on the jacket. 

Install adequately designed emergency relief device... 

Install flame arresters on atmospheric vents to prevent fire on the outside of the tank from propagating back into the vapor space inside the tank. Provide fire resistant insulation for critical vessels, piping, outlet valves on tanks, valve actuators, instruments lines, and key electrical facilities. Provide remote controlled, automatic, and fire-actuated valves to stop loss of tank contents during an emergency provide fire protection to these valves. Valves should be close-coupled to the tank, and must be resistant to corrosion or other deleterious effects of spilled fluids. Vessels should be provided with overpressure relief protection. 

Install emergency relief device/system on vessel and/or utility line... 

Provide for automatic or manual addition of diluent, poison, or inhibitor directly to reactor Install emergency relief device (ERD)... 

Install depressurizing system Install emergency relief device Develop emergency response plan... 

Install weak sections in piping and duct work to provide overpressure relief... 

Install emergency relief devices where appropriate... 

In the third e.vample, the line terminates at 53%. This means DO NOT run this pump at less than 53% of the BEP. 53% of 4500 gpm is 2385 gpm. Because this is a firewater pump and because firemen need to throttle the nozzles on their fire hoses, then we need to install a pressure relief valve on this system with a discharge bypass line so that the pump dumps the restricted water (less than 2400 gpm) back into the suction tank or lake. If not, this firewater pump is likely to suffer bearing failure during an emergency. 

Consideration should be given to the effects of thermal expansion of liquids and pressure-relief valves installed unless ... 

Balanced Bellows Safety Relief Valve - A balanced safety relief valve incorporates means for minimizing the effect of back pressure on the performance characteristics opening pressure, closing pressure, lift and relieving capacity. This is usually achieved by the installation of a bellows. 

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... 

Backup systems which depend upon the action of automatic cut-in devices (e.g., a turbine-driven standby spare for a motor-driven cooling water pump, with PLCI control) would not be considered an acceptable means of preventing a utility failure for normal pressure relief design purposes, even though their installation is fully justified by improved continuity and reliability of plant operations. 

When the control valve size is established by the designer, for example selecting a CV, it is assumed that this size valve is installed. If the control valve size is later increased it may be necessary to recheck the PR valve relief considerations. 

Figure 7. Pressure conditions for safety relief valve installed on a pressure vessel (vapor phase) supplemental valve used for fire exposure only.

This section describes the requirements for the design and installation of pressure relief valve inlet and outlet piping manifolds and valving, including safety valve and flare headers. 

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