Thursday, August 9, 2007
A good chemical & Process engineer should read this...
Case 1
API RP520, 7th Edition, January 2000 is commonly used in Selection & Sizing of Pressure Relief Valve (PRV).
In this code, figure 30—Back Pressure Correction Factor, Kb, for Balanced-Bellows Pressure Relief Valve (Vapors and Gases) and Figure 31—Capacity Correction Factor, Kw, Due to Back Pressure on Balanced-Bellows Pressure Relief Valves in Liquid Service, both use the ratio of gauge pressures.
However, figure 35—Constant Back Pressure Correction Factor, Kb, for Conventional Pressure Relief Valves (Vapors and Gases Only) use ratios of absolute pressures.
Case 2
In this code, figure 30—Back Pressure Correction Factor, Kb, for Balanced-Bellows Pressure Relief Valve (Vapors and Gases) and Figure 31—Capacity Correction Factor, Kw, Due to Back Pressure on Balanced-Bellows Pressure Relief Valves in Liquid Service, both use the ratio of gauge pressures.
However, figure 35—Constant Back Pressure Correction Factor, Kb, for Conventional Pressure Relief Valves (Vapors and Gases Only) use ratios of absolute pressures.
Although the differences between ratio of gauge pressures and ratio of absolute pressures are minimum, it still different.
Case 2
An engineer wrote in the report : “I have a control valve, the upstream pressure and downstream pressure are 1.5 bar and 1 bar respectively. Pressure drop is 0.5 bar”. If the upstream pressure is understood as 1.5 barA (mean 0.5 barg) and downstream pressure is interpreted as 1 barG, the pressure drop should -0.5 bar, not 0.5 bar.
Does it make sense ?
Case 3
One operator reported to supervisor. “I have checked the tank water temperature. It is 25 deg and the pump is running smoothly.”. If the statement mentioned in one of the plant in US, supervisor will scratch his/her head, isn’t the water already freeze at 20 deg F ?
Thus, whenever we specify any unit, it shall always be clear on the unit to avoid any confusion.
posted by Webworm, 4:17 AM
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