Api 521 Summary.docx

Overpressure is the result of disruption of material and energy flows that result in the build up of either the material or energy or both in some part of the system. PRDs are installed to ensure that a process system or its component is not subjected to pressures that exceed maximum allowable accumulated pressure. Double or multiple jeopardy - the simultaneous occurrence of two or more unrelated causes of overpressure is not a basis for design; API 521 describes single jeopardy scenarios as a basis of design The liquid or vapor rates used to establish relief rate are developed by the net energy input – heat (vaporization or thermal expansion) and direct pressure input Pressure and temperature should be considered as they affect the volumetric and compositional behavior of liquids and vapors. Blocked Outlet

Blocked Outlet

Inadvertent closure of a valve on the discharge of a pressure equipment when the equipment is on stream and such closure results in pressure in excess of the design or MAWP. In general, omission of block valves in between vessels in a series or elimination of block valves can reduce the number of PRDs. Centrifugal pumps: prd not required if pump, piping and equipment downstream can withstand maximum shut-off pressure or other pressure sources such as a start-up or warm-up line.

relief load determined at relieving instead of normal operating conditions. Frictional drop in pipelines between source of overpressure and system being protected should also be considered maximum flow capacity of the devices feeding the system determines load

Positive displacement pump – needs PRD to protect the pump and downstream equipment against shutoff Compressors – blocked discharge or loss of interstage cooling or loss of a downstream compression stageheat exchangers

size the relief valve for rated flow rate of compressor only ????

Fractionation Tower (Vapors from 2nd tray from top of tower)

Fractionation Tower

Reflux failure (pump shut down or valve closure) causes condenser flooding (=coolant failure) or loss of coolant in tower: Total condensing Partial condensing

relief rate based on heat and mass balances at relieving conditions; difficult to perform these calculations, so simplified heuristics: vapors from 2nd tray from top incoming vapor rate to the condenser incoming – outgoing vapor rate

Overfilling Evaluate if Source pressure of liquid feed > design pressure of the equipment or PRD set pressure

Automatic Controls Gas blowby = loss of liquid level followed by high pressure vapor flow; it is the discharge of gas from a process equipment through a liquid outlet; occurs due to failure of liquid level control system or an inadvertent opening of cv bypass; can lead to overpressure in downstream equipment. Relief rate = full vapor flow through the liquid cv Inlet control devices and bypass One inlet is in fully open position regardless of CV failure position; relief rate = maximum expected inlet flow – normal outlet flow from valves that are open If manual bypass on inlet cv is partially open, total flow = flow through cv wide open and the bypass Outlet control devices One of the control valves at fail close on outlet, relief rate = maximum expected inlet flow – maximum outlet flow through remaining outlets

Abnormal heat input Reboilers, other process heating equipment: abnormal heat input may cause vapor generation > condensation; relief rate = max vapor inflow – condensation or vapor outflow

Inadvertent valve opening Relief load depends on the maximum operating pressure upstream of the valve and the downstream equipment pressure at relieving conditions Check valve failure When a spare pump is brought online, if check valve has leakage, spare pump may see discharge pressure of the operating pump and lead to overpressure situation Reverse flow through check valves in series is a scenario when the maximum operating pressure of the high-pressure system is greater than the corrected hydrotest pressure of the low pressure equipment Transient Pressure Surges – Water hammer or hydraulic shock waves/steam hammer/ condensate hammer – due to rapid closure of valves – cannot be controlled by typical PRD – avoid the use of quick closing valves Chemical Reactions

Cryogenic fluids and loss of process control – reduction in pressure lowers temperature to minimum allowable design temperature of the equipment = low-temperature brittle failure Exothermic reactions – runaway reaction causes pressure above MAWP – DIERS Methodology for relief rate – if PRDs are infeasible, use reaction inhibitors, depressuring, quench, automatic shutdown Hydraulic Expansion – increase in liquid volume due to increase in temperature Equipment or piping – liquid-filled, blocked and heated up by solar radiation or heat tracing Exchanger – blocked in on the cold side with flow on the hot side; hydraulic expansion relief device on cold side Phase change – If the blocked-in liquid has vapor pressure higher than the relief design pressure, the PRD should be capable of handling the vapor generated Required relief rate is small; relief valve with nominal diameter DN (20) x DN (25) or NPS (3/4”) X NPS (1”) specified Fire Overpressure due to vapor formation or fluid expansion Pool fire (confined or open) – ignited liquid spill; open pool fire – design basis for fire case Jet fire – ignited pressurized leak; occurs when any flammable fluid under pressure is released to atmosphere; PRDs ineffective; focus on prevention of leaks Vapor formation – consider portion of vessel wetted by internal liquid up to a height of 25 ft or 7.6 m above the base of a pool fire – any level at which a substantial spill or pool fire could be sustained; usually ground level; for a sphere, wetted area is higher of the maximum horizontal diameter and 25 ft; column – normal level in the bottom plus liquid holdup from trays upto 25 ft (if reboiler part of column, add reboiler level) For vessels with liquids: Q = 21,000 FA^0.82; F – environment factor = 1 for bare vessel’ A^ 0.82 – area exposure factor/ratio; if no adequate drainage or firefighting, Q = 34500 FA^0.82; at subcritical conditions, relief rate = rate of vapor formation = Q/heat of vaporization; Orifice Area – calculate and select from D to T; rated capacity = relief rate * (selected/calculated orifice area); near the critical point, latent heat of vaporization approaches zero and sensible heat takes over and so 50 BTU/lb taken as approximation; above critical point, no phase change occurs and relief depends on the expansion of the liquid due to heat input Vessels with gas/vapor/supercritical fluids: Effective discharge area formula – page 52

Tube failure

Power failure