The 34 x 1 valve size really adequate Cause 34 x 1 not shown in API 526.This size of the valve is usually 10 to 50 times more than what it is required.The basic function of this valve is relieve the pressure build-up due to thermal expansion.To perform the calculations, you need to calculate the relieving rate based on the thermal co-effcient of expansion of the fluid involved.
If you want to satify yourself and do the calculation, pl, refer to API 520 equation 3.2 under section sizing of the pressure safety valve. Farris Psv Dimensions How To Size AHope this helps. ARAZA QUOTE (Plumbercs Sep 18 2008, 03:07 AM) Appreciate if you could advise on how to size a thermal relief valve for: blocked in line of 0.3 litre volume line content liquid nitrogen Will a standard 0.062 sq in orifice RV be sufficiemt Thanks. I point directly to the basic data that the OP (Plumbercs) has supplied. Heshe states that there is a LIN (Liquid Nitrogen) line blocked in and that a thermal relief is required. If the line contains LIN, Im willing to bet that the contents are basically SATURATED LIQUID that is subject to vaporization when exposed to a heat leak through the pipe insulation. Since the vapor is compressible, there will be NO HYDRAULIC pressure exerted by the contents; there will be compressed vapor pressure exerted. A thermal relief valve is designed to relieve hydraulic pressure (pressure exerted by an expanding, supercooled liquid) and not generated vapor. Araza has correctly advised that no calculations are required for a thermal relief valve case; HOWEVER, heshe has mistakenly assumed that the OPs original claim or assertion of a thermal relief case is correct --- and that may be a mistake. If I am correct in that this LIN is saturated, then the application calls for a PSV sized to handle the expected rate of vaporized nitrogen generated by the pipes heat leak. This is an important point because the difference between both interpretations is a very different type ( size) of relief device. We dont want to make a fatal mistake in estimating the correct relief fluid type and rate of relief. Plumbercs, please clarify or confirm EXACTLY what fluid conditions you have in the pipe and whether you expect to have any LIN vaporization occuring prior to - and during - the required relief. As Art mentioned, they really only apply to (very) subccoled liquids. Based on our use of the thermal expansion scenario or case used in analysis of pressure relief needs to mean that a minimal PSV (say 34 x 1) is adequate, Id argue that we have to be very careful in how we use the term thermal expansion. We use it only in liquid filled systems and only when we do not expect any vapor to be generated. Also, we do NOT call situations where substantial heat transfer is expected to fall into the thermal expansion catagory. Thus, for example, a subcooled liquid in a heat exchanger where it might absorb heat from the other fluid present does not meet our criterion for thermal expansion. An isolated vessel that does not have an external jacket or an internal coil that can supply heat probably does meet the criterion. We would never use this term in conjunction with any liquified gas or cryogenic liquid. In a broad sense, the Fire case is a type of thermal expansion, but, of course, we sharply differentiate between fire and other types of heat gain.
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