Sizing relief valves

A vessel may be subject to more than one condition under different failure scenarios. For example, a low pressure separator may be subject to blocked discharge, gas blowby from the high pressure separator, and fire. Only one of these failures is assumed to happen at any time. The relief valve size needs to be calculated for each pertinent relieving rate... 

A vessel can only be overpressured if the upstream vessel has a higher pressure than the vessel in question. A compressor scrubber with a MAWP of 285 that gets flow from a 285 MAWP separator does not need to have a relief valve sized for blocked discharge. The upstream relief valve will keep the upstream separator pressure from going higher than 285, so there is no way it can oveipressure the downstream scrubber. The scrubber PSV only needs to be sized for fire. 

In summary, the back-pressure for relief valves should be limited to the following values unless the valve is compensated. We do not recommend using a relief valve with higher back-pressure than shown below without consulting a person knowledgeable in relief valve sizing and relief system design. 

For relief valve sizing, refer to manufacturer s tables at 2.0 oz/in. pressure setting and 3.5 oz/in. maximum allowable tank pressure. 

Wong, W. Y, Safer Relief Valve Sizing," Chem. Engr., May 1989, p. 137. 

In the catalyst preparation area where the fire occurred, aluminum alkyl and isopentane are mixed in a batch blending operation in three 8000-gallon kettles. The flow rates of components are regulated by an operator at the control room. Temperature, pressure, and liquid level within the kettles are monitored by the control room operator. The formulated catalyst is stored in four 12,000-gallon vertical storage tanks within this process unit. Aluminum alkyl is a pyrophoric material and isopentane is extremely flammable. Each vessel was insulated and equipped with a relief valve sized for external fire. 

Rather less freedom is allowed in the construction of mechanical flowsheets. The relative elevations and sizes of equipment are preserved as much as possible, but all pumps usually arc shown at the same level near the bottom of the drawing. Tabulations of instrumentation symbols or of control valve sizes or of relief valve sizes also often appear on P I diagrams. Engineering offices have elaborate checklists of information that should be included on the flowsheet, but such information is beyond the scope here. 

The minimum required relieving area (in2) can now be calculated using the following equation for gas and vapour relief valve sizing (lbs/h) ... 

Density of liquid - useful in design of storage vessels and shipping containers, useful in many engineering applications involving liquids, helpful in relief valve sizing for safety... 

The relief valve selected should be one with equal or greater area than calculated using equation 13.105. Relief valve sizes are given in API Standard 526. Sizing equations for subcritical flow of vapors, liquids, steam, and two-phase mixtures are given in API RP 520. 

A toluene surge drum has capacity 500 gal and is normally operated 60% full at 100°E under 300psig of hydrogen in the head space using a level-controlled outflow The normal flow rate into the vessel is 30,000 Ib/h. Determine the vessel dimensions if the vessel is vertically mounted. Evaluate the relief loads for the blocked outflow and external fire cases and hence determine the relief valve size. 

Not only must PSI be maintained up-to-date, but it must also be available to employees who work with or on the covered equipment. Many companies use electronic media Only one copy has to be maintained, and all employees who have access to the organization s computer system should have access to the PSI. In some cases, a real-time electronic P ID is available in which employees can access a particular piece of equipment on the P ID and, from there, can access the details of that equipment, such as the MSDS, vessel thickness, the pump curve data, pressure relief valve sizing, design basis, etc. The regulation does not require electronic information hence hard copies maintained in the control rooms or the supervisor s office are considered acceptable. Irrespective of the format, it still must be maintained up-to-date. ... 

The required orifice area for a relief valve sizing for air at constant back pressure is ... 

Relief Valve Sizing for Vapor Service Program Inputs and Outputs... 

FORMAT(20X, VAPOR RELIEF VALVE SIZING ) WRITE(1, 110)... 

THIS PROGRAM PRINTS THE RESULTS OF LIQUID RELIEF VALVE SIZING WITH VISCOSITY GREATER THAN lOcP. 

FORKAT(20X, RELIEF VALVE SIZING FOR STEAM CONDITION )... 

Example 6. Find the relief valve size given IF =... 

---