Thermal relief liquids

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). 

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. 

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

Physical and thermodynamic property data, such as thermal expansion coeffici t, are important in process engineering. The following brief discussion illustrates such importance. Liquids contained in process equipment will expand with an increase in temperature. To accommodate such expansion, it is necessary to design a relief system which will relieve (or vent) the thermally expanding liquid and prevent pressure build-up from the expansion. If provisions are not made for a relief system, the pressure will increase from die diermally expanding liquid. If the pressure increase is excessive, damage to the process equipment vtdll occur. 

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). 

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... 

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. 

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. 

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. 

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. 

Full liquid containers require protection from thermal expansion. Such relief valves are generally quite small. Two examples are... 

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

PR valves handling materials which are liquid or partially liquid at the valve inlet. An exception to this is made for certain thermal expansion relief valves as described below. 

The ASME code requires every pressure vessel that can be blocked in to have a relief valve to alleviate pressure build up due to thermal expan sion of trapped gases or liquids. In addition, the American Petroleum Institute Recommended Practice (API RP) 14C, Analysis, Design, Installation and Testing of Basic Surface Safety Systems on Offshore Production Platforms, recommends that relief valves be installed at vari ous locations in the production system and API RP 520, Design and Installation of Pressure Relieving Systems in Refineries, recommends various conditions for sizing relief valves. 

Valdes, E. C. and Svoboda, K. J., Estimating Relief Loads for Thermally Blocked-in Liquids, Cheni. Eng., Sept. 1985, p. 77. 

Equation 9-45 describes the fluid expansion only at the beginning of heat transfer, when the fluid is initially exposed to the external temperature Ta. The heat transfer will increase the temperature of the liquid, changing the value of T. However, it is apparent that Equation 9-45 provides the maximum thermal expansion rate, sufficient for sizing a relief device. 

Consider Problem 9-9, part a. This time use alcohol as a liquid medium with a thermal expansion coefficient of 1.12 X 10 3/°C. The heat capacity of the alcohol is 0.58 kcal/kg °C, and its density is 791 kg/m3. Determine the relief size required. 

For a liquid thermal expansion relief device that protects only a blocked-in portion of a piping system, the set pressure shall not exceed the lesser of the system test pressure or 120% of design pressure. 

ITie Henry and Fauske model employs curves similar to Fig. 16. Immediately upon initial contact, they assume that there is rapid pressurization at the interface. Nucleation in this vicinity is then prevented [Po in Eq. (7) is large and so is Dq] until the pressure is acoustically relieved by the wave moving to a free surface and returning. During this period, the thermal boundary layer in the cold liquid continues to develop. At relief, there still may be no intersection of the t-Do curve (in Fig. 16), so until such a time... 

Results for thermal expansion coefficient of liquids are presented for major organic chemicals. The results are especially helpful in the design of relief systems for process equipment containing liquids that are subject to thermal expansion. 

The same example problem as used in 7.6 will be used. A reactor of volume 3.5 m3 has a design pressure of 14 barg (maximum accumulated pressure 16.41 bara). A worst case relief scenario has been identified in which a gassy decomposition reaction occurs. The mass of reactants in the reactor would be 2500 kg. An open cell test has been performed in a DIERS bench-scale apparatus, in which the volume of the gas space in the apparatus was 3800 ml, and the mass of the sample was 44.8 g. The peak rate of pressure rise was 2263 N/m2s at a temperature of 246°C, and the corresponding rate of temperature rise was 144°C/minute. These have been corrected for thermal inertia. The pressure in the containment vessel corresponding to the peak rate was 20.2 bara. The liquid density at 246°C is estimated as 820 kg/m3. The gas generated by the runaway has a Cp/Cv value of... 

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