Thermal relief valve

Themtal. Thermal relief is needed in a vessel or piping run that is liquid-packed and can be isolated, for example pig launchers and meter provers. Liquid is subject to thermal expansion if it is heated. It is also incompressible. The thermal expansion due to heating by the sun from a nighttime temperature of 80°F to a sun-heated temperature of 120 F can be enough to rupture piping or a vessel. The required capacity of thermal relief valves is very small. 

A simple manifold is illustrated in Figure 40.41. This manifold contains one pressure inlet port and several pressure outlet ports that can be blocked off with threaded plugs. This type of manifold can be adapted to systems containing various numbers of subsystems. A thermal relief valve may be incorporated in this manifold. In this case, the port labeled T is connected to the return line to provide a passage for the relieved fluid to flow to the reservoir. 

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. 

Where the fluid is the same on each side of the isolation means, and no contamination will result this is the optimum choice for thermal relief release. Consideration of the possibility of backpressure onto the thermal relief valve, rendering the valve ineffective, should be evaluated before an analysis is finished. 

Furnace tubes, process piping, and heat exchangers may also have to be protected by relief valves. Incidentally, the small %- or 1-in relief valves you see on many tank field loading lines and on heat exchangers are not process relief valves. They are there for thermal-expansion protection only. This means that if you block in a liquid-filled exchanger and the liquid is heated, the liquid must expand or the exchanger will fail. That is what the thermal relief valve is there to prevent. 

The flows required for thermal relief are very small, and there are special thermal relief valves on the market that accommodate this specific application. Oversizing a thermal relief valve is never a good idea, and orifice sizes preferably below API orifice D are recommended. 

Types of spring-operated SRVs 5.2.6.1 Thermal relief valves... 

Thermal relief valves are small, usually liquid relief valves designed for very small flows on incompressible fluids. They open in some proportion of the overpressure. Thermal expansion during the process only produces very small flows, and the array of orifices in thermal relief valves is usually under the API-lettered orifices, with a maximum orifice D or E. It is, however, recommended to use a standard thermal relief orifice (e.g. 0.049in2). Oversizing SRVs is never recommended since they will flow too much too short, which in turn will make them close too fast without evacuating the pressure. This will result in chattering of the oversized valve and possible water hammer in liquid applications. 

Thermal relief valves are usually of rather simple design. A few are designed to resist backpressures. However, there are so many designs on the market, it is impossible to treat them all in this book. 

The main requirement for thermal relief valves in cryogenic conditions is to reduce their freezing risks at any cost (Figure 11.2). Therefore, we must select valves with low simmer, a rapid pop/snap opening and high seat tightness. We need to reduce unnecessary product loss, so again low simmer and preferably a short blowdown is required. 

Most metal-seated thermal relief valves are designed to operate proportionally and most have a fixed blowdown. 

Thermal relief valves specified with threaded ends shall have internal, NPT-type, female-tapered threads. 

Mechanical equipment that performs an action to relieve pressure when the normal operating range of temperature or pressure has been exceeded. Physical relief devices include pressure relief valves, thermal relief valves, rupture disks, rupture pins, and high temperature fusible plugs. 

For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of valve necessary to provide protection from excess pressure caused by thermal expansion. In this case, a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure. 

Shell and tube exchangers do sometimes have thermal relief valves to protect... 

Thermal relief valves are found in services where a system, such as a long length of pipe, is liquid full and is subject to the heat of the sun or some other low intensity heat source. Thermal relief valves are also used for low or ambient temperature pumps that can be blocked in. 

The following guidelines are normally followed in installing a thermal relief valve ... 

The thermal relief valve is set at the maximum possible pressure. In most cases, the relief valve is used only to protect the pipeline, and if the pipeline is designed according to ASME B31.3 [5], the set pressure can be 1.2 times the design pressure of the pipe. 

In some cases, the thermal relief valve discharges into a closed system in such a situation, the back pressure should be considered in the design of the relief valve. 

A thermal relief valve that releases toxic chemicals must disci ... 

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