26. Back Pressure - Example Superimposed Back Pressure = 8/(2.31*12) = 0.3 psig constant Total Built-up Back Pressure = 15 psig Total Back Pressure = 15.3 psig
49. Fire Case Sizing Example Fire wetted area A= dL + 1.084 d 2 = *5*5 + 1.084 *5 2 = 105.5 ft 2 Heat absorbed Q = 21000 F A 0.82 =21000 *1*(105.5) 0.82 = 957825 BTU/hr Relief rate W = Q / = 957825 / 560 = 1710.40 #/hr
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54. Automatic Control Failure – Vapor PV-001 fails in open position: Relief rate = max flow through PV-001 - normal V2 flow PV-001 fails in closed position, evaluate blocked outlet for PSV1 L1 L2 V2 PV-001 PSV2 100 psig PSV1 200 psig FC PIC PT PY P I D1 D2
55. Automatic Control Failure – Liquid FV-1 fails in open position: Relief rate = P-1 Capacity at Relief Pressure FV-1 fails in closed position, evaluate liquid overfilling for PRV-1 V-1 V-2 PRV-1 Set @ 10 barg PRV-2 Set @ 5 barg P-1 FV-1
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57. Automatic Control Failure – Vapor Blow-Through LV-1 fails in open position, Liquid Inventory Drains Into V-2 Relief rate = Flow Capacity of LV-1 for V-1 Vapor (Provided V-1 Liquid Inventory Cannot Overfill V-2) LV-1 fails in closed position, evaluate liquid overfilling for PRV-1 V-1 V-2 PRV-1 Set @ 10 barg PRV-2 Set @ 5 barg LV-1
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59. Exchanger Tube Rupture - Example E-4 E-5 PSV-005 SET @ 60 PSIG V-5 T-5 DP = 60 PSIG E-5 tube side design pressure = 300 psig E-5 shell side design pressure = 231 psig PSV-005 must be evaluated for tube rupture even though E-5 is designed per the 0.77 rule P des = 300 psig P des = 231 psig
Welcome. In this section we will be discussing overpressure protection and relief valves. Overpressure protection is probably the most important function that is performed by Engineering as it has a direct impact on plant safety.
This course will be broken into two sections. In the first section we will cover: (Read Slide)
This course will be broken into two sections. In the first section we will cover: (Read Slide)
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This sketch is a simple depiction of a typical relief system used at KBR. The horizontal drum and tower are protected by relief valves while the vertical drum is protected by a rupture disk. We will learn more about these devices later in this section. The outlets from these devices are collected in a flare header, routed to the flare knockout drum and then sent to the flare for disposal. Note that a single relief valve is used to protect the tower, shell side of the exchanger and the tower reflux drum.
In this section we will discuss the applicable codes, standards, and references used in overpressure protection. Standard KBR work process and methodology will more than satisfy all applicable industry codes and standards however there are sometimes county or project specific codes and standards that need to be addressed on a project basis
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This is the work process for flare system design
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In this section we will go over the common terms and definitions of several terms used in relief device determination and selection.
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This slide shows some of the important pressure levels for a system with a design pressure of 100psig. Including hydrotest pressure, piping system allowable pressures, ASME section 8 fire case allowable overpressure (21% above design pressure), ASME section 8 multiple relief valve overpressure (16% above design pressure), Allowable overpressure for all single relief valves for cases other than fire (10% above design pressure) allowable overpressure for boiler code ASME section 1 vessels (6% above design pressure). The amount of overpressure is very important in determining the correct relief valve size.
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This slide illustrates the different backpressure terms associated with a relief valve set at 100psig. The constant backpressure is 0.3psig due to the water seal in the flare KO drum. The variable backpressure is 15psig, 10psig for losses in the flare header and 5psig for losses in the individual relief valve outlet. This yields a total backpressure of 15.3psig. The spring differential would be adjusted to account for the constant backpressure and would be set at 99.7psig.
Let’s examine the causes of overpressure as well as some allowable capacity credits. This is covered in great detail in SEM 1-303, before performing a relief load calculation you should familiarize yourself with SEM 1-303. The hardest part of relief valve sizing is coming up with the load, this is the section that we will be coving now.
The following cases are looked at for each protected system in determining the relieving load. We will go through what to look for in each of these causes, and some examples. The intent of this class is not to go through each equation that relates to these causes, these equations can be found in the SEM 1-303. (Read Slide)
The following cases are looked at for each protected system in determining the relieving load. We will go through what to look for in each of these causes, and some examples. The intent of this class is not to go through each equation that relates to these causes, these equations can be found in the SEM 1-303. (Read Slide)
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Now lets discuss another cause of overpressure blocked outlet. (Read Slide)
Thermal Expansion: (read Slide)
This is the equation for determining the required relief rate for thermal expansion, however through experience the required relief rate is always much lower than the smallest relief valve available. For thermal protection of piping KBR generally provides a ¾” x 1” relief valves and does not perform any calculations.
Looking at external fire (Read Slide)
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In determining the relieving load first we use the following equations to determine the heat input to the system. These equations are empirical. The heat input depends entirely on the geometry of the system and insulation type. Typical applications at KBR do not utilize fireproof insulation.
Once the heat input (Q) is found the relieving rate can be determined. This is dependent on the latent heat of the fluid. Vapor expansion can also occur in vapor filled vessels in a fire, for details on this please see the SEM 1-303.
Let’s go through a simple example of determining the relieving rate required due to external fire of a vessel. First the wetted area is determined, note that the wetted area is only considered up to the fire height, not to the normal liquid level. Once we know the wetted area we can find the heat absorbed and knowing the latent heat, at relieving conditions, we can find the relief rate.
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Consider this system, two vessels the overhead of vessel one is sent via pressure control to vessel 2. Evaluating both failure positions of the pressure control valve PV-001 then: If it were to fail open the relief rate for PSV2 would be the maximum flow rate through the control valve PV-001 minus the normal outlet flows from vessel 2. If the pressure control valve were to fail closed then it creates a blocked outlet scenario for vessel 1.
Consider this system, two vessels the overhead of vessel one is sent via pressure control to vessel 2. Evaluating both failure positions of the pressure control valve PV-001 then: If it were to fail open the relief rate for PSV2 would be the maximum flow rate through the control valve PV-001 minus the normal outlet flows from vessel 2. If the pressure control valve were to fail closed then it creates a blocked outlet scenario for vessel 1.
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Consider this system, two vessels the overhead of vessel one is sent via pressure control to vessel 2. Evaluating both failure positions of the pressure control valve PV-001 then: If it were to fail open the relief rate for PSV2 would be the maximum flow rate through the control valve PV-001 minus the normal outlet flows from vessel 2. If the pressure control valve were to fail closed then it creates a blocked outlet scenario for vessel 1.
Exchanger tube rupture…(Read Slide)
Consider this system… Exchanger E5 the reboiler in this system is designed for the 0.77 rule. However a tube rupture in exchanger E5 will cause the vessel to overpressure, requiring relief by PSV-5. (Read Slide)
Exchanger tube rupture…(Read Slide)
(Read Slide) At KBR was must assume that human error only result in one contingency for example only one block valve is accidentally closed or opened.
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Utility failures must be considered in determining required relieving flow rates. These types of failures normally set the overall flare load even though they may not be the governing case for an individual relief device. For this reason the resultant load from a utility failure must be determined for each device so that it’s overall effect on the flare can be determined. (Read Slide)
It is important to look at the concept of cascading failures.. (Read Slide)
It is important to look at the concept of cascading failures.. (Read Slide)
It is important to look at the concept of cascading failures.. (Read Slide)
It is important to look at the concept of cascading failures.. (Read Slide)
It is important to look at the concept of cascading failures.. (Read Slide)
It is important to look at the concept of cascading failures.. (Read Slide)
It is important to look at the concept of cascading failures.. (Read Slide)
It is important to look at the concept of cascading failures.. (Read Slide)
This course will be broken into two sections. In the first section we will cover: (Read Slide)
