DYNAMIC SIMULATION FOR DESIGN IMPROVEMENTS, COST REDUCTION AND OPERATIONAL STABILITY IN LNG PLANT DESIGN

Transcription

1 DYNAMIC SIMULATION FOR DESIGN IMPROVEMENTS, COST REDUCTION AND OPERATIONAL STABILITY IN LNG PLANT DESIGN SIMULATION DYNAMIQUE POUR L AMELIORATION DU PROCEDE, REDUCTION DES COUT, ET STABILITE DE L OPERATION D UN PROCEDE D UNE INSTALLATION DE GNL Satish L. Gandhi, Ph.D. Director - Process (LNG) Felix F. de la Vega, M.S. Vice President - Petroleum (LNG) Fluor Daniel, Inc. Sugarland, Texas, , U.S.A. ABSTRACT This paper discusses some of the dynamic and steady state simulation studies performed for LNG plants utilizing the APCI propane (C3) precooled mixed refrigerant () cycle which have resulted in or indicated the way towards design improvements and reduction in plant costs while ensuring stable operation. The dynamic studies presented include (a) reduction in flare header size and cost by minimizing the potential for release from refrigeration compressors; major failure scenarios considered are total and partial loss of power to C3 compressor s air-cooled exchanger fans, inadvertent closure of block valve at the outlet of C3 compressor, and inadvertent closure of block valve at the outlet of C3 receiver, and (b) elimination of pressure surge dampening equipment in LNG loading lines. The steady state design studies presented include (a) optimization of C3 condenser size for the specified availability of gas turbine power, and (b) sensitivity of LNG production to variations in compressor performance characteristics. RESUME Ce papier présente quelques études de simulation en régime dynamique et permanent du procédé APCI à cycle aux réfrigérants mélangés pré-refroidi résultant en ou menant dans une voie d une amélioration du procédé avec une réduction des coûts de l installation tout en assurant la stabilité de l opération. D.3 1

2 Les études présentées incluent; (a): une réduction de dimension du collecteur de torche ainsi que le coût en minimisant les fuites potentielles du compresseur de réfrigération, en considérant les scénarios de failles majeurs telles que: perte totale ou partielle de l énergie motrice du ventilateur de l échanger à air du compresseur de propane, fermeture inattentive de la vanne de sectionnement du refoulement du compresseur propane, et fermeture inattentive de la vanne de section de la ligne de soustirage du ballon accumulateur de propane, et (b): l élimination de l équipement d élimination de surpression dans les lignes de chargement de GNL. Les études de calcul en régime permanent présentées incluent, (a): optimization de la dimension du condenseur de propane pendant la disponibilité de l énergie motrice de la turbine à gaz, et (b) l analyse de sensibilité de la réponse du compresseur de réfrigération aux variations des caractéristiques de la performance du compresseur. INTRODUCTION This paper presents several studies performed for LNG plants which have resulted in or indicated the way towards design improvements and reduction in plant costs while at the same time ensuring stable operation. Most of these studies have been carried out utilizing an integrated dynamic process simulation model of the APCI propane precooled cycle in LNG plants. Since it is an integrated model, the effects of interactions between the C3 and refrigeration-compression systems are considered which is essential for addressing LNG plant dynamics and design issues. A schematic drawing of an APCI propane precooled cycle in an LNG plant with air cooling is shown in Figure 1. The dynamic process simulation model is built from a library of customized unit operation models of individual equipment and control systems specifically developed for LNG plant design. The dynamic simulation model converts into a steady state modeling tool by simply setting the time derivatives of state variables to zero. The model allows analysis of a wide range of problems related to the process design of LNG plant, either dynamic (time dependent) or steady state. The design issues addressed in this paper include the following: Flare header sizing and cost reduction Hydraulic surge analysis of the LNG loading system Optimum design of the C3 condenser Sensitivity of LNG production to variations in compressor performance curves FLARE HEADER SIZING AND COST REDUCTION The relief flows associated with major plant upsets are usually derived from steady state analysis. Such analyses ignore the dynamic interactions between various parts of the process. This approach also precludes a way to assess the impact of process or control system modifications that could be implemented for mitigating the maximum relief D.3 2

3 flows. These limitations lead to designing relief and flare header systems that are significantly oversized and costly. The dynamic process simulation model of the LNG plant developed at Fluor Daniel allows us to evaluate the actual dynamic response of the system to any potential failure or upset scenario within the LNG plant. The model also provides an efficient tool for evaluating design alternatives, such as high integrity instrumentation systems, for mitigating or eliminating the impact of plant failures. Three major plant upset scenarios usually considered for sizing the relief and flare header systems during the process design of an LNG project (assumed to be an air-cooled plant) are discussed below. FIGURE 1 LNG EXPANDER G M FLASH GAS COMPRESSOR TO FUEL GAS MERCURY DEHYDRATION REMOVAL UNIT UNIT HHP HP MP LP G EXPANDER LNG RUNDOWN TO STORAGE MAIN CRYOGENIC HEAT EXCHANGER GT M SYSTEM TO/FROM FRACTIONATION UNIT RECEIVER F LP MP HP HHP PROPANE SYSTEM GT M PROPANE RECEIVER LP MP HP HHP Loss of Power to C3 Compressor s Air-Cooled Exchanger Fans The dynamic response of the relief system to the total loss of power to the C3 compressor s air-cooled exchanger fan motors is shown in Figures 2 and 3. Even with the total loss of power, about 1% of air flow is maintained to account for natural convection. The flow and the back pressure at the relief valve are shown as % of the maximum allowable relief rate and back pressure requirements. The back pressure limitation at the relief valve is based on the lowest set pressure of the critical relief valves in the flare header system. The dynamic response of the relief rate at the flare tip is somewhat damped and delayed as compared to that at the relief valve due to the effect of packing in the flare header piping. The dynamic response shows that the total loss of power to the exchanger air fans would result in maximum relief rate and back pressure at the relief valve about 25% higher than the maximum allowable values. This would require increasing the pipe diameters for both the relief valve outlet header piping as well as the long main flare header piping to the flare stack resulting in a significant increase in the cost of the flare header system. D.3 3

4 An alternative for reducing the maximum relief flow is providing emergency power to some of the air fan motors in case of total loss of power. A dynamic simulation analysis was performed to evaluate the sensitivity of the relief rate and back pressure to the availability of emergency power. The results for 75% loss of power (or availability of emergency power to 25% of air fan motors) are shown in Figures 4 and 5 and for 5% loss of power (or availability of emergency power to 5% of air fan motors) are shown in Figures 6 and 7. It is seen that the relief flows as well as the back pressures at the relief valves are only about 5% and 25% of the maximum allowable value for the 75% and 5% loss of power, respectively. The analysis confirmed that providing emergency power to 25% of the air fans instead of increasing the size of the flare header piping is a cost effective solution to the total loss of power scenario. flow - % max Total Loss of Power flare tip relief valve rv back pressure - % max Total Loss of Power Figure 2 Figure 3 flow - % max 2 75% Loss of Power flare tip relief valve rv back pressure - % max 2 75% Loss of Power Figure 4 Figure 5 D.3 4

5 flow - % max 2 5% Loss of Power flare tip relief valve rv back pressure - % max 2 5% Loss of Power Figure 6 Figure 7 Inadvertent Closure of Block Valve at the Outlet of C3 Compressor with Bypass Relief Valve Similar to the loss of power scenario discussed earlier, dynamic simulation analysis of the inadvertent closure of the block valve in the discharge line of the C3 compressor showed maximum relief rate and back pressure at the relief valve to be higher than the allowable values. The block valve is located between the C3 desuperheater and the C3 condenser after the recycle line take-off point. A proposed solution for minimizing the potential for release is to provide a modulating relief valve as a bypass around the block valve. The set point for the bypass relief valve is lower than that of the relief valve which releases into the flare header at the discharge of C3 compressor. The dynamic response of the pertinent system variables to the closure of the block valve over a period of 15 seconds in presence of the bypass relief valve is shown in Figures 8 to 12. The response in Figure 8 shows the flow rate through the bypass relief valve as % of the initial flow before the closure of the block valve. The C3 compressor discharge pressure in Figure 9, as % of the initial discharge pressure at time zero, shows that the bypass valve is able to maintain the discharge pressure below the relief valve set pressure causing the relief valve to the flare header to remain closed. The C3 and compression power, as % of their respective initial values at time zero, are shown in Figure 1. In this case the gas turbines had enough spare power to avoid trip during the power peaks observed in the transient response. The dynamic response of the anti-surge system in preventing each of the four stages of the C3 compressor from surge is shown in Figures 11 and 12. The simulation results indicate that the transient response of the system stabilizes after about 1 minutes and the C3 compressor is able to operate through the bypass around the closed block valve, although less efficiently and at a higher discharge pressure and consequently higher temperature levels. Thus, the overall dynamic response of the system validates the proposed design modification of using a modulating relief valve as a bypass around the block valve in eliminating the potential for relief due to the inadvertent D.3 5

6 closure of the block valve. The dynamic simulation analysis of the system was also used to determine the appropriate size of the bypass relief valve. flow - % initial C3 Compressor Blocked Outlet 12 2 block valve bypass rv Figure 8 C3 discharge P - % initial C3 Compressor Blocked Outlet rv set pressure Figure 9 power - % initial C3 Compressor Blocked Outlet C3 Figure 1 anti surge valve - % open C3 Compressor Blocked Outlet LP 2 Figure 11 MP D.3 6

7 anti surge valve - % open C3 Compressor Blocked Outlet HP 2 Figure 12 HHP Inadvertent Closure of Block Valve at the Outlet of C3 Receiver Another major failure scenario with potential for a significant release to the flare header is the inadvertent closure of the block valve at the outlet of the C3 receiver. The release would occur if the C3 receiver and condenser become filled with liquid before depletion of liquid C3 inventory from the propane vaporizers. The dynamic response of the system to such a failure scenario is depicted in Figures 13 to 18. The propane condenser gets completely flooded in less than 4 minutes after the closure of the block valve and causes the release to occur. The maximum relief rate and back pressure at the critical relief valve, shown in Figures 13 and 14 respectively, are essentially the same as the maximum allowable values, i.e., %. The C3 compressor discharge pressure, as % of the initial discharge pressure at time zero, is shown in Figure 15. The C3 and compression power, as % of their respective initial values at time zero, are shown in Figure 16. The dynamic response of the anti-surge system in preventing each of the four stages of the C3 compressor from surge is shown in Figures 17 and 18. The dynamic simulation response confirmed the current design of the relief and flare header system to be adequate in handling the relief load associated with this upset scenario assuming enough spare power is available. If not, the compressor will overload and trip shortly on or after the initial relief. An alternative to avoiding such a trip would be a modulating relief valve bypass around the block valve. D.3 7

8 flow - % max 12 2 relief valve flare tip rv back pressure - % max 12 2 Figure 13 Figure 14 C3 discharge P - % initial rv set pressure 11 9 power - % initial C3 Figure 15 Figure 16 anti surge valve - % open 2 LP MP anti surge valve - % open 2 HP HHP Figure 17 Figure 18 D.3 8

9 HYDRAULIC SURGE ANALYSIS OF THE LNG LOADING SYSTEM A hydraulic surge analysis of the LNG loading system was conducted to determine the need for pressure surge dampening equipment to maintain the surge pressures below the pipeline design pressure. The system comprised of several LNG storage tanks and loading pumps and dual LNG loading lines from the storage tanks to the LNG loading jetty. The study involved evaluation of several potential failure scenarios. One of the potential worst case scenarios considered activation of an emergency trip due to the closure of the trip valves at the LNG loading jetty with a subsequent trip of all loading pumps except one in each tank failing to trip. The emergency trip initiates a simultaneous closure of the trip valves at the jetty (with a closure time of 5 seconds) and the trip valves at the plant boundary (with a closure time of 15 seconds). The closure profiles of the trip valves and associated surge pressure profiles are shown in Figures 19 and 2. The hydraulic transient response showed formation of vapor cavities at the emergency trip valves located at the jetty and certain high elevations in the loading lines. This results in spikes in pressure profile, such as the one seen around 16 seconds in Figure 2 due to the collapse of the vapor cavity at the jetty trip valves. The hydraulic surge study determined that pressure surge dampening equipment was not required for the LNG loading system. 12 Hydraulic Surge Analysis pressure (% max) valve (% open) 12 Hydraulic Surge Analysis pressure (% max) valve (% open) % value Emergency Trip Valves - Plant % value Emergency Trip Valves - Jetty time (sec) Figure time (sec) Figure 2 OPTIMUM DESIGN OF THE C3 CONDENSER SIZE The LNG plant simulation model is an effective tool for optimizing the sizes of the C3 and compressor discharge and inter-stage coolers and C3 vaporizers. Results from an LNG plant study conducted for optimizing the C3 condenser size for a fixed total available power from the C3 and gas turbines and helper motors are shown in Figures 21 and 22. A plot of LNG production capacity versus condenser size is shown in Figure 21. Both the LNG production capacity and the condenser area are shown as % of the base design case values. The LNG production capacity increases and the rate of increase in LNG production capacity represented by the slope of the curve decreases with increasing C3 condenser area. Plots of variations in C3 and compression power, as % of the total fixed available power, are shown in Figure 22. The C3 compression power D.3 9

10 requirement decreases while the compression power requirement increases with increasing C3 condenser area. Based on the above information, an economic analysis was performed to determine the most cost effective size of the C3 condenser. C3 Condenser Size Optimization 14 C3 Condenser Size Optimization LNG prod - % base design power - % total C condenser size - % base design Figure condenser size - % base design Figure 22 SENSITIVITY OF LNG PRODUCTION TO VARIATIONS IN COMPRESSOR PERFORMANCE CURVES The compressor performance curves available during the initial process design of an LNG project furnished by compressor vendors are based on preliminary information. Since the compressor performance curves affect the LNG production capacity, it is worthwhile to assess the sensitivity of LNG production to variations in head-flow performance curves for each stage of the C3 and compressors. This information is very helpful to compressor vendors in optimizing the compressor performance. The LNG plant simulation model provides an effective tool for conducting a study of this type. Results from an LNG plant study on the sensitivity of LNG production to variations in the head-flow performance curve for the HHP stage of the C3 compressor are provided in Figures 23 and 24. The analysis is based on a fixed total available power from the C3 and gas turbines and helper motors. The effect of (+/-) 1% variations in the head value for the HHP stage on the LNG production capacity, as % of base design case value, is shown in Figure 23. The LNG production capacity decreases with an increase in the HHP stage head. Plots of variations in C3 and power, as % of total fixed available power, are shown in Figure 24. These plots show that the C3 compression power requirement increases while the compression power requirement decreases with an increase in the HHP stage head. Similar information developed for each stage of the C3 and compressors can then be furnished to compressor vendors for use in their own design optimization effort. D.3 1

11 LNG prod - % base design Sensitivity to Variations in Head- Flow Performance Curve C3 HHP head - % base design Figure 23 power - % total Sensitivity to Variations in Head- Flow Performance Curve C3 HHP head - % base case Figure 24 C3 CONCLUSIONS State-of-the-art modeling tools developed by Fluor Daniel are being used in the performance of dynamic and steady state simulation studies in LNG plant design. These studies are very helpful in addressing a multitude of process design issues during various phases of LNG projects including basic design, detailed engineering, and plant startup and operation. The studies result in improved, cost effective and reliable design of LNG plants. D.3 11