3.1.1 Overpressure Protection Basic Rules
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Understand the philosophy regarding protection against over and under pressure. This includes aspects such as:
Simultaneous occurrence of events (related vs. unrelated) Role and validity of various safeguarding systems (operator response, automatic control, IPF, relief valve etc.)
Design basics for proper functioning of safeguarding systems
Understand the selection of control valve failure action
Know how to determine a pressure system.
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We take simultaneous (sequential) occurrence of “over” and “under” pressure events into account if it can be that: the events are
one failure does not lead to another
(Note: above definition also accounts for “over” and “under” temperature events)
Example : a fire and the loss of Cooling Water (CW) is normally unrelated. However, a power failure followed by a CW failure are normally sequential events.
What remains are only
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Below controls are generally available to prevent “over” and/or “under” pressure beyond concept design parameters (design Pressure and material selection): (1) Control of process (a) Operator response (manual controls by operation of valves, start- stop buttons, etc) b) Automatic continuous control by instruments (e.g. from the Distributed Control System (DCS))
(2) Instrumented protective functions (IPF) Alarm Switch Trip Depressuring
Last line of defence against overtemperature and, by exception, against overpressure (e.g. high integrity backflow protection)
(3) Mechanical protection Sufficient design pressure Relief valves and/or rupture disks (Inter-)Lock systems
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For operator response (control 1a listed on slide 4) to be a valid control, the following shall be in place: (process surveillance)
Operator observation (control room + unit rounds) Alarms
(Panel) Operator needs to be able to recognise offset/cause of being outside operating window/loss of containment/etc.
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(operator response)
Need for sufficient time to take effective action:
Phenomenon develops over time.
Time to respond to situation before LOC typically 30 minutes (no less than 15 min.)
Requirement for clarity on effective action:
Unambiguous corrective action is possible. Appropriate means of to intervene are available. Proper instructions. In case of complicated process upsets > automated action. (Run-away reactions, integrated equipment (furnaces/compressors), follow-up events (Domino effects).
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Automatic control (control 1b listed on slide 4)
For instance, liquid level control of a vessel or column to prevent overfilling (= overpressure)
IPF (control 2 listed on slide 4)
For instance, high pressure or high level alarm/trip.
Shall be independent from other barriers.
SIL classification provides architecture for reliability and availability.
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In general relief valves (control 3 listed on slide 4) are the last line of defence against overpressure. Operator response, automatic continuous control and IPF are normally taken into account for relief valve sizing (design flow). Reasons:
Operator intervention could be ineffective,
Automtatic control can be on manual, has a wrong setpoint initiation or could fail open,
IPF could have a failure on demand.
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The failure position of control valves shall allow each system to reach an inherently safe condition during a total instrument air failure scenario (e.g. overpressure) Select spring action to:
Maximise cooling
Minimise heat input
Minimise relief quantity
Keep material where it belongs; do not export problems which may cause a relief to elsewhere
In case of instrument air failure, the control valves will be assumed to have taken their spring action.
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Different nomenclature for valve failure action:
FO AFO ATC STO
Failure Open Air Failure Open Air to Close Spring to Open
FC AFC ATO STC
Failure Close Air Failure Close Air to Open Spring to Close
Lock-up device (usually mechanical). Allows valve not to completely open or close:
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A pressure system is an individual equipment item or groups of equipment including their associated piping that, within its boundaries, remains under all conditions. This means: Valves (block valves, control valves).
See next slide for some acceptance criteria
Blockage in piping/equipment, e.g.
fouling, freezing/solidification, debris, salts, damaged internals and elicit objects
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Role of valves in a pressure system
In general, relief valves cannot be taken to be open (or closed) to prevent a relief. However deviations are allowed. This depends on the region. This should be approved by the principle. (Shell interpretation)
Single valve with or without lock (i.e.: locked open “LO” valve or interlock system) is normally as an open system closure of that valve can create an overpressure case > 110% of the DP. (ASME 8 – Rev. Appendix M)
Single valve is normally as an open system closure of that valve cannot create an overpressure case > 116% of DP (MAWP). Single valve with lock (i.e..: “LO)” as an open system closure of that valve cannot create an overpressure case > corrected hydrotest pressure (yet >116% of DP) (MAWP) Copyright of Shell Projects & Technology
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A pressure system has virtually the same operating pressure throughout. A pressure system can be considered to be exposed from one or more pressure sources of which one governs the pressure. A pressure system can be designed for this governing pressure (by design pressure) or can be protected at one point against the governing overpressure by a pressure relief device. Hereby taken into account:
Hydraulics to be evaluated to assure that no equipment exceeds its design pressure (see note 1). Low flow interconnecting piping to be checked for potential restriction for relief flow. Consider actual elevations that may result in different design pressures due to liquid head (see note 2).
Note 1: If all equipment items in a system have equal design pressures with the PRV located on the last piece of equipment, then due to the pressure drop, the set pressure of the PRV shall be below the design pressure to ensure that the upstream equipment is not overpressured before the PRV opens. Note 2: When the system is liquid-full, the design pressure of the accumulator vessel downstream of a cooler of a distillation column (vessel located at significant lower elevation with respect to the location of the relief v alve near top of distillation column) shall be increased for extra liquid head. Copyright of Shell Projects & Technology
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Note: Pressure system II is based on P1 pump shutoff. Pressure system III is indicative is case of a potential run away of R1 (higher pressure than P1 shut off pressure). Copyright of Shell Projects & Technology
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Relief valve is OK to be used to protect the entire system
It is accepted that a check valve will open in the flow direction
Note that pressure drop over the non return valve should be taken into account (3% rule).
Relief valve cannot be used to protect the entire system
Due to the non return valve there is no open connection from the downstream vessel to the relief valve.
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DP column = (OP x 1.05) x 1.10 Relief scenarios to consider blocked outlet DP column = (1) + (2) + (3) – (4) Relief scenarios do not have to consider blocked outlet for flow ex pump (1) (4) (2)
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Is the single relief valve good enough for protection of both LP separators against failure of either control valve?
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Is a relief valve on cold LP separator good enough for protection of both LP separators against failure of either control valve?
No guarantee that NNF line is open and free of blockage (less attention paid, or at least not the amount that is paid to dead ends like to the RV) No guarantee that that the NNF line is large enough to enable suitable path to RV (see rules in slide pack 3.1-4 associated with this lesson)
So, typically
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Hydrotreater and hydrocracker systems may have a flat (new systems) or a sloping pressure profile (typically for existing systems)
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Overdesign pressure events are only based on
scenarios
Operator response can be a valid barrier to prevent overpressure. Automatic control is not. Sufficient design pressure or a relief valve is the last line of defence against overpressure Control valve failure action should be selected with care to prevent an unsafe (e.g. overpressure) situation A pressure system shall remain and no blockage allowed)
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under all conditions (no valves
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