Analysis of the application and sizing of pressure safety valves for fire protection on offshore oil and gas installations Annex I Article draft The annex contains an article draft, based on an investigation of the effectiveness of a PSV, including casy studies conducted as part of the associated master thesis. Annex to master thesis by Jacob G. I. Eriksen and Michael S. Bjerre. PECT1-1-F15 Project period for P1: February 2nd 215 - June 9th 215 Supervisor: Matthias Mandø Rambøll supervisors: Anders Andreasen & Carsten Stegelmann MSc in Process Engineering and Combustion Technology Aalborg Universitet Esbjerg Niels Bohrs Vej 8 67 Esbjerg
Journal, Vol. XXI, No. 1, 1-5, 215 Article DRAFT Analysis of the application of pressure safety valves for fire protection on offshore oil and gas installations Michael Bjerre 1, Jacob G.I. Eriksen 1, Anders Andreasen 2, Carsten Stegelmann 2, Matthias Mandø 3 Abstract In this paper, the effectiveness of a fire PSV has been investigated when offshore process equipment is exposed to a fire. According to API 521, care should be taken when a fire scenario is affecting the unwetted part of a pressure vessel. A case study is performed in order to map for which fire scenarios a PSV provides additional safety. Simulations of seven different pressure vessel from an offshore oil and gas separation train, are performed in the state of the art simulation tool, VessFire. The pressure vessels are exposed to a small jet fire, large jet fire, and a pool fire. Rupture times for the vessels are derived from stress curves from the vessels. Rupture times are compared for the vessels, with and without a PSV, in order to see the effect of the installed PSV. It is found that when a flame affects the unwetted part of a vessel, the PSV only offers minor additional protection. When a flame affects the wetted part of a vessel, the PSV relieve the inventory as designed. Blowdown and passive fire protection are found to be good alternatives to a fire PSV. Keywords PSV fire pressure vessel oil and gas offshore 1 MSc student, Aalborg University Esbjerg, Esbjerg, Denmark 2 Rambøll Oil & Gas, Esbjerg, Denmark 3 Department of Energy Technology, Aalborg University Esbjerg, Esbjerg, Denmark Contents Introduction 1 1 Methods 2 1.1 VessFire............................. 2 1.2 Process equipment..................... 2 2 Results 3 3 Discussion 3 3.1 PSVs effect on the unwetted part........... 3 3.2 PSVs effect on the wetted part............. 4 3.3 BDV and PFP......................... 5 3.4 Effect of a fire PSV...................... 6 References 6 Introduction It is normal industry practice to apply pressure safety valves (PSV) on offshore oil and gas installations to protect pressure vessels from rupturing in a fire. The main concern is that an oil spill pool fire exposing a vessel will cause a BLEVE upon rupture of the pressure vessel leading to a significant escalation in consequences. Standards such as API 521 [1] (ISO 23251) and API 14C [2] (ISO 1418) discusses the requirement for PSVs for fire protection and how to size such PSVs. The use of fire PSVs in accordance with API 521 has been developed for pool fires on refineries. However on offshore oil and gas installation a more likely fire scenario will often be a jet fire. According to API 521, fire PSVs does not offer proper protection against jet fire, but other measures such as shutting down the jet fire source and depressurising process inventories should be considered the primary protection against jet fire. Pool fire heat loads applied in API 521 for offshore applications has also been questioned [3]. Even in the case of pool fire a fire PSV may not provide adequate protection in accordance with API 521 if the pool fire exposes the unwetted part of the pressure vessel. Despite this, fire PSVs are often installed on offshore oil and gas installations to protect even completely gas filled pressure vessels. Installing safety equipment such as a PSV that does not provide any or insignificant protections does not seem sensible. Not only does the PSV represent a significant lifecycle cost, but it may also increase risk of operating the offshore installation if it does not provide any significant protection. The PSV will require testing, inspection and maintenance which will expose personnel to risk, the PSV will be a potential leak
Analysis of the application of pressure safety valves for fire protection on offshore oil and gas installations 2/6 source, and human errors in connection with the PSV can lead to risk etc. In fact there have been numerous examples in the offshore oil and gas industry where incidents and accidents have occurred in relation to use of PSVs. The best example, illustrating the risk associated with PSVs, is the Piper Alpha disaster where a PSV was taken out for maintenance together with a condensate pump without positive isolation. As the condensate pump was brought online, a major condensate spill occurred where the PSV should have been, ultimately resulting in 167 fatalities and total loss of the installation. Experiments carried out by Birk et al [4], demonstrates that a pressure vessel, which is exposed to a flame on the unwetted part, will rupture before the pressure is high enough to trigger the PSV. As discussed by Dalzell [5] nothing is safety critical and safety systems should ideally only be installed if they provide some safety benefit and not just because it is recommended in standards and is the normal industry practise. Another problem with installing a fire PSV that cannot prevent rupture is that the PSV may be regarded as a sufficient safety barrier in accordance with standards and best practice, when in fact, it provides no, or no significant, barrier. The purpose of this article is to investigate in which cases a fire PSV is expected to provide a safety benefit offshore by performing a case study with the state of the art software tool VessFire [3]. Different typical offshore process systems will be analysed for large jet fire, small jet fire and pool fire exposing wetted and unwetted part of vessels respectively. Based on this some general conclusions and recommendations for the use of fire PSVs offshore is provided. 1. Methods Case studies are conducted for various fire scenarios for pressure vessels in offshore installations, using VessFire as the simulation tool. The simulations are done with and without a PSV, in order to see if the PSV provides an additional benefit, in terms of a prolonged rupture time of the vessel. The rupture time is defined as the time, at which a rupture occurs. Rupture is defined to occur when the calculated von Mises stress is above the Ultimate Tensile Strength (UTS) of the vessel wall material. The von Mises stress is proportional to the vessel pressure, and the UTS is a function of wall temperature. It is assumed that rupture is acceptable if the rupture time exceeds 6 min, as all personnel can be evacuated from the process platform in this time period. The fire scenarios investigated are the standard small jet fire, large jet fire, and pool fire, as defined by the Scandpower guidelines [6].The three fire scenarios are affecting the wetted as well as the unwetted part of the vessel. The intensity of the flames can be seen in table 1. When the flame type is assumed to be impinging and engulfing the vessel, the background heat load affects the entire vessel surface area. The background heat load thus causes the vessel pressure and inventory temperatures to increase. The peak heat load is the maximum local heat load, which occurs within a flame, and affects a small surface area of the vessel, breathing a hot spot where the material weakens. The peak heat load is assumed not to significantly affect the vessel inventory, and is purely assumed to increase the vessel wall temperature. An increase in the vessel wall temperature will decrease the UTS of the wall material. Table 1. Fire intensity for a fire case of a small jet fire, large jet fire, and a pool fire from Scandpower [6]. Peak heat load Background heat load kw/m 2 kw/m 2 Small jet fire 25 Large jet fire 35 Pool fire 15 1.1 VessFire VessFire is a software package used to study depressurisation scenarios in a process vessel, and can be used to simulate a pressure vessels response to a fire scenario [3]. VessFire solves thermodynamic properties of the fluid inventory, heat transfer, conduction, and stress calculations, in a transient model, in order to predict temperature, pressure and rupture of the vessel. Peng-Robinson is used as the equation of state to solve the state variables of the fluid inventory. The fluid is divided into two separate phases, a gas phase, and a liquid phase. The liquid phase may consists of both water and oil. The temperature and state variables are calculated for each phase separately. The heat transfer through the vessel wall is calculated in three dimensions, while the liquid phases are assumed to be uniform in all spatial direction within the phase. VessFire has been validated in accordance with several articles as shown in Validation of VessFire [7]. VessFire predictions of pressure and temperature of gas, liquid and maximum temperature of the vessel wall, have been compared to experimental results from Moodie et al [8], in which a LPG tank was exposed to an engulfing kerosene pool fire. Wall temperatures predicted by VessFire have been compared to experiments by Berge and Olstad [9]. 1.2 Process equipment The process equipment used in the simulations are a 1st stage separator, 2nd stage separator, 3rd stage separator, test separator, Intermediate Pressure (IP) suction drum, and a Natural Gas Liquid Knock-Out drum (NGL KO drum). The case study is also done for a pressure vessel containing a mixture of propane and n-butane, in order to investigate a variation in inventory composition. The vessels selected are representative for typical pressure vessels located on offshore production platforms in the North Sea. Data for the pressure vessels can be seen in table 2. The vessel material is carbon steel with a UTS of 441 MPa, including 1% safety margin. The PSVs used on the pressure vessels are sized according to API 52 [1].
Analysis of the application of pressure safety valves for fire protection on offshore oil and gas installations 3/6 Table 2. Input data for the different offshore process vessels used in the case study. Pressure Temperature Outer diameter Length Thickness Liquid level Water level Orientation [Bara] [C] [m] [m] [mm] [m] [m] - - 1st stage separator 3. 8 3.58 1.7 32 1.9 1.5 H K 2nd stage separator 12. 8 2.632 1.4 32 1.55.38 H L 3rd stage separator 5. 75 2.532 9. 16 1.. H K Test separator 3. 8 2.15 6.8 25 1.23.45 H J IP suction drum 32.2 4 1.88 3.5 4.95. V G NGL KO drum 62. 51 1.8 3.1 5.77. V F Butane vessel 8. 4 2.532 9. 16 1.25. H K PSV size VessFire simulations have been done for seven different vessels, as indicated in table 2. For each vessel the three different flame scenarios have been defined, with and without a PSV. The total number of possible combinations results in 42 unique case scenarios. The vessel designated as 1st stage separator has been used to investigate the effects of a BlowDown Valve (BDV), and Passive Fire Protection (PFP) using rock wool. BDV and PFP could be an alternatives to the use of a PSV as a safety barrier. 2. Results The 1st stage separator has been chosen as a representative case for the simulations. The other pressure vessels show similar results. Figure 1 compares the UTS and von Mises stress for the 1st stage separator exposed to a small jet, large jet, and a pool fire on the unwetted part of the vessel, without and with a PSV. Some of the simulations have been terminated after rupture has occurred, which is why there is not the same amount of data in all the plots. In a similar fashion, figure 2 compares the UTS and von Mises stress when the flames affects the wetted part of the vessel. Blowdown was done for the 1st stage separator with a BDV which had a diameter of 35 mm. Blowdown was able to prevent rupture in all fire cases except for when the large jet fire affects the unwetted part of the vessel, where the rupture time was 15.3 min. The result is shown in table 3. PFP was also applied to the 1st stage separator in insulation layers of 5 mm, mm, and 2 mm. Again, the PFP was able to prevent rupture in all fire cases except for the large jet fire when it affects the unwetted part of the vessel. Table 3 compares the rupture time of the 1st stage separator when affected by a large jet fire on the unwetted part, without PSV, with PSV, with BDV, and with various thicknesses of PFP. 3. Discussion 3.1 PSVs effect on the unwetted part Figure 1 shows the UTS and the von Mises stress which is used to determine if the vessel ruptures. In the case of a small Table 3. Rupture times for the 1st stage separator when affected by a large jet fire on the unwetted part of the vessel, including a base case without PSV, and then with PSV, only a BDV, and variations of PFP insulation. Rupture time [min] Without PSV 6.2 With PSV 6.6 BDV 35 mm 15.3 PFP 5 mm 37 PFP mm 58 PFP 2 mm >6 jet fire on the unwetted part, it can be seen that the PSV does not influence the rupture time. By analysing the VessFire results, it is evident that the peak heat load of the small jet fire is not large enough to influence the inventory of the vessel and cause a pressure increase. The PSV will thus not lift before rupture, and it will not have the opportunity to provide any protection from rupture. This is also noted in figure 1, as the stress due to vessel pressure does not increase. At the same time, the peak heat load affects a local part of the wall, causing a temperature increase, since the convective heat transfer of the gas is not large enough to transport the heat away from the wall. This causes the UTS to drop below the von Mises stress, which leads to a rupture after 11.4 min. Similarly to the case where the small jet fire affects the unwetted part of the vessel, the large jet fire also increase the wall temperature, but since the peak heat load is greater for the large jet fire, the temperature increase happens faster. The large jet fire also has a background heat load that affects the inventory of the vessel causing the pressure to build up. The combined effect of the faster temperature increase and the pressure build up, means that the UTS and von Mises stress intersects faster, effectively leading to a short rupture time. When a PSV is equipped, the background heat load of the large jet fire increases the pressure just enough that the PSV opens just as the UTS and von Mises stress is about to
Analysis of the application of pressure safety valves for fire protection on offshore oil and gas installations 4/6 Without PSV With PSV Small jet fire 2 UTS von Mises 1 2 3 2 1 2 3 Large jet fire 2 1 2 3 2 1 2 3 Pool fire 2 2 1 2 3 1 2 3 Figure 1. Plot of UTS and von Mises stress for the 1st stage sparator when exposed to a small jet fire, a large jet fire, and a pool fire. Results are shown for when the flames affect the unwetted part of the vessel, both with and without a fire PSV. intersect. This extents the rupture time with a few seconds, but the peak heat load of the large jet fire is still so large that UTS becomes lower than the von Mises stress leading to a rupture at 6.6 min. A correctly sized PSV is therefore not sufficiently to protect the vessel against rupture for a large jet fire on the unwetted part of the vessel. For a pool fire affecting the unwetted part of the vessel, figure 1 indicate a rupture time of 14.5 min when a PSV is not equipped. The rupture time is extended to 21.5 min when a PSV is equipped. This is similar to when the large jet fire affects the unwetted part, where the peak heat load lowers the UTS of the wall, and the background heat load increases the pressure of the vessel. The PSV in these cases limits the pressure increase until the UTS becomes lower than the von Mises stress and rupture occurs. In the case of a pool fire, the PSV has a larger effect, measured in rupture time, than for a large jet fire, because the peak heat load is less, and the UTS therefore takes a longer time to drop and intersect with the von Mises stress. 3.2 PSVs effect on the wetted part Figure 2 shows the UTS and the von Mises strees for when the fire scenarios affects the wetted part of the vessel. For the small jet fire, the case is similar to the scenario where the small jet fire affects the unwetted part of the vessel, and the pressure does not increase, and in fact, it decreases a little bit because of a drop in temperature. The difference here, is that the fire affects the wetted part of the vessel. The liquid has a high heat transfer coefficient which removes the heat from the wall, keeping the wall relatively cool. The heat from the peak heat load is transferred from the wall to the liquid, and
Analysis of the application of pressure safety valves for fire protection on offshore oil and gas installations 5/6 Without PSV With PSV Small jet fire 2 UTS von Mises 2 4 6 2 2 4 6 Large jet fire 2 2 4 6 2 2 4 6 Pool fire 2 2 2 4 6 2 4 6 Figure 2. Plot of UTS and von Mises stress for the 1st stage sparator when exposed to a small jet fire, a large jet fire, and a pool fire. Results are shown for when the flames affect the wetted part of the vessel, both with and without a fire PSV. the liquid is then cooled by the ambient conditions, which affects a much larger area of the vessel. The vessel enters a state of heat transfer equilibrium, and would therefore be able to withstand this fire exposure indefinitely. It should be noted that the PSV does not have any significance on the outcome of the result. In the case of the large jet fire affecting the wetted part of the vessel, there is a rupture time of 23.9 min when a PSV is not equipped. The convective heat transfer of the liquid is high enough to limit the drop in UTS by the peak heat load, but the background heat load causes the pressure to increase rapidly, causing the von Mises stress to increase and intersect with the UTS. No rupture occurs when a PSV is equipped. In this case, the PSV relieve the pressure from the vessel, and prevents rupture of the vessel. This means that when a large jet fire affects the wetted part of the pressure vessel, a PSV will prevent a rupture by limiting the pressure. The case for a pool fire affecting the wetted area is similar to the case of a large jet fire affecting the wetted part of the vessel, and the only difference is the peak heat load of the flame. In both cases, the vessel will rupture due to a pressure increase if a PSV is not equipped, with rupture times of 23.3 and 23.9 respectively. The slight difference in rupture time is due to different peak heat loads for a pool fire and a large jet fire. If a PSV is equipped, then there will be no rupture. 3.3 BDV and PFP Cases for the 1st stage separator was done with a BDV and another case where PFP was utilised. Blowdown was able to prevent rupture of the vessel in all but one case, which was for the large jet fire affecting the
Analysis of the application of pressure safety valves for fire protection on offshore oil and gas installations 6/6 unwetted part of the vessel. This shows that in a majority of the fire scenarios, blowdown can be an effective way of depressurizing in order to prevent rupture. Similar, PFP was able prevent rupture for all fire scenarios except for a large jet fire affecting the unwetted part of the vessel. This also shows that PFP can be a way to avoid rupture in a majority of the fire scenarios. When a large jet fire affects the unwetted part of the 1st stage separator, both a PSV, a BDV, and PFP was unable to protect the tank from rupture within a 6 min limit. Table 3 shows the rupture time for the 1st stage separator exposed to a large jet fire on the unwetted part of the vessel. The rupture times indicate that a PSV offers very little additional protection compared with a vessel which does not have any protection. A blowdown process does not prevent rupture, but it does extend the rupture time. PFP is able to significantly extend the rupture time and by applying a thick enough layer of insulation, or using a different type of insulation, it is possible to avoid rupture within the 6 min limit. This indicates that PFP is generally a more effective and more widely applicable safety barrier compared to a PSV. [5] G. Dalzell and A. Chesterman. Nothing, is safety critical. Process Safety and Environmental Protection, (957-582), 1997. [6] Scandpower. Guidelines for the Protection of Pressurised Systems Exposed to Fire. Scandpower Risk Management AS, 24. [7] VESSFIRE. Validation of VessFire. Petrell, 29. [8] K. Moodie, L.T. Cowley, R.B. Denny, L.M. Small, and I. Williams. Fire engulfment tests on a 5 tonne LPG tank. Journal of Hazardous Materials, (2), 1988. [9] Geir Berge and Harald Olstad. Experimental study and simulation on heat exposed liquid filled process equipment. 24th International Conference on Offshore Mechanics and Arctic Engineering, 25. [1] API Standard 52. Sizing, Selection, and Installation of Pressure-relieving Devices. American Petroleum Institute, 214. 3.4 Effect of a fire PSV From figure 1 and 2 it is shown that a PSV will not always remove or adequately reduce the risk of rupture, when a flame is impinging on and engulfing a vessel. This becomes evident when the fire affects an unwetted part of the vessel. Subsequently when other safety barriers are designed, it should be taken into consideration that the PSV may in reality not be an effective safety barrier. Furthermore, in the cases in which the PSV is ineffective, the installation of the PSV will still have accompanying costs of added weight, installations costs, maintenance costs, and possibly added flare capacity and pipe infrastructure. As suggested by API 521 [1], each scenario problem should be individually assessed and engineering experience should be applied. A well designed blowdown process or even PFP could be considered as an alternative to a fire PSV. References [1] API Standard 521. Pressure-relieving and Depressuring Systems. American Petroleum Institute, 214. [2] API Standard 14C. Recommended Practice for Analysis, Design, Installation, and Testing of Basic Surface Safety Systems for Offshore Production Platforms. American Petroleum Institute, 27. [3] VESSFIRE. A Calculation System for Blowdown of Process Segments and Process Equipment Exposed and Unexposed to Fire. Petrell, 25. [4] A. M. Birk, D. Porier, and C. Davison. On the response of 5 gal propane tanks to a 25% engulfing fire. Journal of Loss Prevention in the Process Industries, (19), 26.