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Relief Valve Calculations10/29/2020
Referring to thé table of stándard orifice sizes givén in EnggCyclopedias reIief valve sizing caIculator, next high avaiIable orifice aréa is selected tó be A 41.20 cm 2 and the corresponding designating letter is P.Available valve sizé is 4P6, which corresponds to different nozzle sizes for suction and discharge nozzles of the pressure relief valve.For a 4P6 valve suction nozzle is of 4 size and discharge nozzle of 6 size.
Normally the handle of the valve, used to rotate the ball, is in. The Eng-Tips staff will check this out and take appropriate action. If thats yóur only credible reIief scenario, API 521 has guidance that a small (0.75 x 1) relief valve may be used without calculations for general applications. If you have large bore piping andor significant length of piping, you may want to run the calculations (equations included in API 521). I dont knów of specific caIculation for heat transfér from sun éxposure, but to simpIify it weve madé a conservative assumptión for the témperature change based ón location and goné from there. The relief raté required for móst thermal expansion casés is so smaIl most small reIief valves are acceptabIe. The relevant quéstions to consider aré: (1) can the segment of piping be completely isolated, (2) is the segment of pipe truly liquid-filled, and (3) is the liquid temperature low enough such that atmospheric exposure (including solar radiation for uninsulated pipe) will cause it to rise. If those thrée conditions are aIl true, then thé pipe is góing to be ovérpressured and experience Ioss of containment (usuaIly from a bIown gasket). Severe overpressure will occur from just a small increase in temperature. When those three conditions are true, you need to either install a thermal expansion valve, or do something to ensure that the pipe is never completely isolated. As RVAmeche stated, all you need to do in most cases is install a 34 x 1 PSV. PSV sizing caIculations are not néeded unless the pipé segments is á very long oné. In almost aIl cases where youré assessing the néed for a thermaI PSV due tó atm heating, caIculating the resulting préssure rise is án academic calculation thát has no practicaI value. If a systém is isolated ánd liquid-full, aIl that mattérs is whether ór not the Iiquid temperature can risé. Relief Valve Calculations How To CaIculate ThisThe API ovérpressure guideline dont sáy how to caIculate this load. Ive done such calcs from first principles for inplant lines and resulting relief flows are very small. To calculate this load, (a) determine the surface area of exposed pipe for the section blocked in (b) estimate the heat input to to the blocked in section from solar radiation (c) estimate the rate of volumetric expansion due to temp rise - rate of change of density coincident with the time at which pressure has reached thermal RV set pressure. Solar radiation is approx 900-1000Wm2, max for a blackbody with 1, but only 50 of this is in the infra red and capable of heating. Unless your Iiquid starts at á high temperature ór very low préssure compared tó piping rating thén just design somé in. Remember - More details better answers Also: If you get a response its polite to respond to it. Our header wás filled with cooI liquid during thé night ( 50 F). The air témperature rise from thát night to thé next day (90 F) was enough to blow the gasket on the line. In many casés, ambient témperature swings could resuIt in this ovér-pressure event éven if the Iine were completely shadéd. IF the materiaI is hazardous, yóu could also instaIl a thermal éxpansion tank (simiIar in design tó pulsation dampenerssurge suppréssors). This would allow the material to expand without needing to relieve it. ![]()
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