Liquids thermal expansion reliefs

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. 

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. 

Size safety relief devices according to the appropriate safety code (see Table 3-4) for the most demanding of these possible upset conditions —closed valves on line or eqviipment —hydraulic expansion of liquid —thermal expansion... 

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 ... 

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. 

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. 

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. 

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. 

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. 

It is still necessary to have a small relief system to allow for thermal expansion of a liquid-full system. This relief system is also necessary for handling hydraulic overfill and fire conditions, but the system is usually relatively simple. 

A recommended safe practice is to install hydrostatic pressure relief valves in all pipelines wherever the possibility exists that liquid fluorocarbons may become trapped (such as between two valves) to prevent piping rupture through thermal expansion of the fluorocarbon. 

In system design, materials of construction must be compatible with the liquids handled at the low temperatures to be encoimtered. Apparatus must have proper pressure relief. Where liquid can be trapped between valves, the possibility of high pressure due to partial vaporization or thermal expansion must be considered. Again, pressure relief devices may be called for. 

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. 

Within a chemical plant and over distances of several kilometers, chlorine can be transported by pipelines, either as gas or liquid [24], [240]. Every precaution should be taken to avoid any vaporization of chlorine in a liquid-phase system or any condensation in a gas-phase system. Wherever liquid chlorine could be trapped between two closed valves or wherever the system could be overpressurized by thermal expansion, an expansion chamber, a relief valve, or a rupture disk should be provided [241], [242]. 

For liquid full equipment, such as filter, exposed to fire, four stages of relief need to be studied The stage is the liquid relief due to thermal expansion from absorbing the heat fiom fire at 21% overpressure. The 2" stage... 

Liquid relief load from thermal expansion of liquid is calculated by following equation ... 

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 ... 

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). 

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