Thermal relief

Thermal relief is necessary in section of liquid piping when it is expected that the liquid will be isolated when the piping is also subject to temperature rises from solar radiation, warm ambient air, steam tracing, fire exposures or other external sources of heat input. 

It is quite common that a liquid-carrying line can be isolated between two isolation valves. The isolated liquid is under thermal expansion due to heat gain from high ambient temperature. Small thermal expansion can be adequate to increase the pressme beyond the design pressure limit of the pipe. The exact calculation of thermal expansion and contingency is complex and is often not required. The volume released is small and a nominal 20 x 25 NB relief valve is installed to protect the system. However, the thermal expansion can be established mathematically as follows  

Process engineering and design using Visual Basic 

it is assumed that the liquid is blocked in by at least one isolation valve. However, the isolation valve can never be fully liquid-tight and will pass some quantity of liquid. This liquid volume leaked out of the system should be considered in calculating the total liquid expansion due to change in temperature. Therefore, effective liquid expansion will be 

Equation 4.16 is the general equation to estimate the pressure increase due to thermal expansion. 

The modulus of elasticity of metals varies slightly with temperature. Values of the modulus of elasticity for commonly used metals at 21°C and 149°C are presented in Table 4.1 [5]. 

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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. 

Blocked Outlets and Inlets for systems, lines or v essels, capable of being filled with liquid and heated by the sun or process heat, require thermal relief to accommodate the liquid expansion (assuming vaporization is negligible). 

Ref [33a] in Appendix C cautions that if the vapor pressure of the fluid at the temperature is greater than the relief set/design pressure that the valve must be capable of handling the rate of vapor generation. Other situations should be examined as the thermal relief by itself may be insufficient for total relief. 

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. 

Special consideration of thermal relief for piping exposure to sunlight (solar radiation) needs to be under taken. This is usually accomplished by painting with reflective paint or burial. Hydrocarbon containing piping is usually painted in a reflective color (i.e., aluminum) for advantages of reflection of solar radiation (heat input) to avoid thermal expansion of fluids in blocked systems. 

Blowdown - The disposal of voluntary discharges of liquids or condensable vapors from process and vessel drain valves, thermal relief or pressure relief valves. 

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. 

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). 

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. 

Thermal Relief Semi Nozzle Full Nozzle Snap Acting Proportional High Performance Metal Seated Sort Seated Steam Valve Liquid Valve Gas Valve conventional Valve Balanced Bellow... 

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. 

A globe valve (which has a typically high pressure drop) mounted under an SRV can in fact only be suitable for liquid thermal relief due to the very low lift of the SRV and the very small amount of product discharged per relief cycle. Other valves may be used under a PRV as long as they are full bore and can be locked open. In Figure 6.2, some L/D values are given for some traditional valve inlet configurations. 

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. 

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