LTX Units and Line Heaters 111 decrease to well below the hydrate point. Hydrates form, but they fall into the bottom of the separator and are melted by the heating coil. The hydrates do not plug the choke because the choke is inside the separator. The gas, condensate, and free water are then discharged from the ves- sel through backpressure and liquid dump valves. The gas leaving the separator is saturated with water vapor at the temperature and pressure of the top of the low temperature separator. If this temperature is low enough, the gas may be sufficiently dehydrated to meet sales specifica- tions. Dehydration is discussed in greater detail in Chapter 8. The low-temperature separator acts as a cold feed condensate stabiliz- er. A natural cold reflux action exists between the rising warmed gases liberated from the liquid phase and cold condensed liquid falling from the stream inlet. The lighter hydrocarbons rejoin the departing gas stream and the heavier components recondense and are drawn from the vessel as a stable stock tank product. This process is discussed in more detail in Chapter 6. The colder the temperature of the gas entering the separator downstream of the choke, the more intermediate hydrocarbons will be recovered as liquid. The hotter the gas in the heating coil, the less methane and ethane there will be in the condensate, and the lower its vapor pressure. In some cases, it may be necessary to heat the inlet gas stream upstream of the coil, or provide supplemental heating to the liquid to lower the vapor pressure of the liquid. In summary, a colder separation temperature removes more liquid from the gas stream; adequate bottom heating melts the hydrates and revaporizes the lighter components so they may rejoin the sales gas instead of remain- ing in liquid form to be flashed off at lower pressure; and cold refluxing recondenses the heavy components that may also have been vaporized in the warming process and prevents their loss to the gas stream. LTX units are not as popular as they once were. The process is diffi- cult to control, as it is dependent on the well flowing-tubing pressure and flowing-tubing temperature. If it is being used to increase liquid recov- ery, as the flowing temperature and pressure change with time, controls have to be reset to assure that the inlet is cold enough and the coil hot enough. If the coil is not hot enough, it is possible to destabilize the con- densate by increasing the fraction of light components in the liquid stream. This will lower the partial pressure of the intermediate compo- nents in the stock tank and more of them will flash to vapor. If the inlet stream is not cold enough, more of the intermediate components will be lost to the gas stream. 112 Design of GAS-HANDLING Systems and Facilities From a hydrate melting standpoint it is possible in the winter time to have too cold a liquid temperature and thus plug the liquid outlet of the low temperature separator. It is easier for field personnel to understand and operate a line heater for hydrate control and a multistage flash or condensate stabilizer system to maximize liquids recovery. LINE HEATERS As shown in Figure 5-2, the wellstream enters the first coil at its flow- ing-tubing temperature and pressure. Alternatively, it could be choked at the wellhead to a lower pressure, as long as its temperature remains above hydrate temperature. There is typically a high-pressure coil of length Lj, which heats the wellstream to temperature, T\. The wellstream at this point is at the same pressure as the inlet pressure, that is Pj = P in . The wellstream is choked and pressure drops to P 2 . When the pressure drops there is a cooling effect and the wellstream temperature decreases to T 2 . This temperature is usually below the hydrate temperature at P 2 . Hydrates begin to form, but are melted as the wellstream is heated in the lower pressure coil of length L 2 . This coil is long enough so that the outlet temperature is above the hydrate point at pressure, P 2 . Typically, a safety factor of 10°F higher than the hydrate temperature is used to set T out . In fire tube type heaters, the coils are immersed in a bath of water. The water is heated by a fire tube that is in the bath below the coils. That is, the fire tube provides a heat flux that heats the water bath. The water bath Figure 5-2. Schematic of line healer. LTX Units and Line Heaters 113 exchanges heat by convection and conduction to the process fluid. Instead of a fire tube, it is possible to use engine exhaust or electrical immersion heaters to heat the water bath. Fire tubes are by far the most common source of heat. Since the bath fluid is normally water, it is desirable to limit the bath temperature to 190°F to 200°F to avoid evaporating the water. If higher bath temperatures are needed, glycol can be added to the water. In order to adequately describe the size of a heater, the heat duty, the size of the fire tubes, the coil diameters and wall thicknesses, and the coil lengths must be specified. To determine the heat duty required, the maxi- mum amounts of gas, water, and oil or condensate expected in the heater and the pressures and temperatures of the heater inlet and outlet must be known. Since the purpose of the heater is to prevent hydrates from form- ing downstream of the heater, the outlet temperature will depend on the hydrate formation temperature of the gas. The coil size of a heater depends on the volume of fluid flowing through the coil and the required heat duly. Special operating conditions such as start up of a shut-in well must be considered in sizing the heater. The temperature and pressure conditions found in a shut-in well may require additional heater capacity over the steady state requirements. It may be necessary to temporarily install a heater until the flowing wellhead temperature increases as the hot reser- voir fluids heat up the tubing, casing, and surrounding material. It is perfectly acceptable for a line heater to have an L } equal to 0. In this ease all the heat is added downstream of the choke. It is also possible to have L 2 equal to 0 and do all the heating before the choke. Most fre- quently it is found that it is better to do some of the heating before the choke, take the pressure drop, and do the rest of the heating at the lower temperature that exists downstream of the choke. HEAT DUTY To calculate the heat duty it must be remembered that the pressure drop through the choke is instantaneous. That is, no heat is absorbed or lost, but there is a temperature change. This is an adiabatic expansion of the gas with no change in enthalpy. Flow through the coils is a constant pressure process, except for the small amount of pressure drop due to friction. Thus, the change in enthalpy of the gas is equal to the heat absorbed. 114 Design of GAS-HANDLING Systems and Facilities The heat duty is best calculated with a process simulation program. This will account for phase changes as the fluid passes through the choke. It will balance the enthalpies and accurately predict the change in temperature across the choke. Heat duty should be checked for various combinations of inlet temperature, pressure, flow rate, and outlet temper- ature and pressure, so as to determine the most critical combination, The heat duty can be approximately calculated using the techniques described in Chapter 2 once the required change in temperature is known. The change in temperature due lo pressure drop through the choke can be approximated from Figure 4-8, The hydrate temperature can be calculated as described in Chapter 4, and the outlet weilstream temperature selected at approximately 10 °F above the hydrate tempera- ture. The total temperature change for calculating gas, oil and water heat duties is then: Recalling from Chapter 2, the general heat duty for multi-phase streams is expressed as: where q = overall heat duty required, Btu/hr q g = gas heat duty, Btu/hr q 0 = oil heat duty, Btu/hr q w = water heat duty, Btu/hr qj = heat loss to the atmosphere, Btu/hr The amount of heat required to heat the gas produced from the well- stream is calculated using the following equation: where q g = gas heat duty, Btu/hr C g = gas heat capacity, Btu/Mscf °F Q g = gas flow rate, MMscfd TI = inlet temperature, °F T 2 = outlet temperature, °F LTX Units and Line Heaters 115 The amount of heat required to heat any condensate or oil produced with the gas is calculated using the following equation: where q 0 = oil heat, Btu/hr C = oil specific heat, Btu/lb °F (Figure 2-13) Q 0 = oil flow rate, bbl/day SG = oil specific gravity T[ = inlet temperature, °F T 2 = outlet temperature, °F The amount of heat required to heat any free water produced with the gas is calculated using the following equation: where q w = water heat duty, Btu/hr Q w = water flow rate, bbl/day Tj = inlet temperature, °F T 2 = outlet temperature, °F Heat loss varies greatly with weather conditions and is usually the greatest in heavy rain and extreme cold. As an approximation it can be assumed that the heat lost from the heater to the atmosphere is less than 10% of the process heat duty. Therefore: The heat duty may have to be checked for various combinations of inlet temperature and pressure, flow rate, and outlet temperature and pressure to determine the most critical combinations. FIRE-TUBE SIZE The area of the fire tube is normally calculated based on a heat flux rate of 10,000 Btu/hr-ft 2 . The fire-tube length can be determined from: 116 Design of GAS-HANDLING Systems and Facilities where L = fire tube length, ft q = total heat duty, Btu/hr d = fire tube diameter, in, A burner must be chosen from the standard sizes in Table 2-12, For example, if the heat duty is calculated to be 2.3 MMBtu/hr, then a stan- dard 2,5 MMBtu/hr fire tube should be selected. Any combination of fire tube lengths and diameters that satisfies Equation 5-7 and is larger in diameter than those shown in Table 2-12 will be satisfactory. Manufacturers normally have standard diameters and lengths for different size fire tube ratings. COIL SIZING Choose Temperatures In order to choose the coil length and diameter, a temperature must first be chosen upstream of the choke; the higher T h the longer the coil L] and the shorter the coil L 2 . In Chapter 2 we showed that the greater the LMTD between the gas and the bath temperature, the greater the heat transfer per unit area, that is, the greater the LMTD, the smaller the coil surface area needed for the same heat transfer. The bath temperature is constant, and the gas will be coldest downstream of the choke. Therefore, the shortest total coil length (L t + L 2 ) will occur when LI is as small as possible (that is, Tj is as low as possible). Although the total coil length is always smaller when there is no upstream coil (Lj = 0), the temperature could be so low at the outlet of the choke under these conditions that hydrates will form quickly and will partially plug the choke. In addition, the steel temperature in the choke body may become so cold that special steels are required. Therefore, some guidelines are necessary to choose T { for an economical design. It is preferable to keep T 2 above 50°F to minimize plugging and above -20°F to avoid more costly steel. With this in mind the following guide- lines have proven useful. For a water bath temperature of 190°F: 1. Set T 2 = 50°F. Solve for AT and calculate T,. If T, is greater than 130°F, L] will become long. Consider going to the next step. 2. Set T! = 130°F. Solve for T 2 . If T 2 is less than -20°F special steel will be needed. Consider lengthening Lj instead and go to the next step. LTX Units and Line Heaters 117 3. Set T 2 = -20°F. Increase Tj as needed. 4. If L! becomes too long, consider using glycol/water mixture or another heat medium liquid and raise the bath temperature above !9()°E Choose Coil Diameter Volume 1, Chapter 9 explains the criteria for choosing a diameter and wall thickness of pipe. This procedure can be applied to choosing a coil diameter in an indirect fired heater. Erosional flow criteria will almost always govern in choosing the diameter. Sometimes it is necessary to check for pressure drop in the coil. Typically, pressure drop will not be important since the whole purpose of the line heater is to allow a large pressure drop that must be taken. The allowable erosional velocity is given by: where V e = fluid erosional velocity, ft/sec c = empirical constant (dimensionless); 125 for intermittent service, 100 for continuous service p m = fluid density at flowing temperature and pressure, lb/ft 3 The fluid density must be for the combined stream of oil and gas and should be calculated at the average gas temperature. where (SG) = specific gravity of liquid relative to water P = operating pressure, psia R = gas/liquid ratio, ft 3 /bbl S = specific gravity of gas at standard conditions T = operating temperature, °R Z = gas compressibility factor (from Volume 1, Chapter 3) The required pipe internal diameter can be calculated based on the vol- umetric flow rate and a maximum velocity. The maximum velocity may be the erosional velocity or a limiting value based on noise or inability to use corrosion inhibitors. In gas lines it is recommended that the maximum 118 Design of GAS-HANDLING' Systems and Facilities allowable velocity would be 60 ft/sec, 50 ft/sec if CO 2 is present, or the erosional velocity, whichever is lower. (Please note that API Spec 12 K Indirect Type Oil Field Heaters uses 80 ft/sec as a limit.) In liquid lines a maximum velocity of 15 ft/sec should be used. A minimum velocity of 3 ft/sec should also be considered to keep liquids moving and to keep sand or other solids from settling and becoming a plugging problem. The equation used for determining the pipe diameter is: where d = pipe inside diameter, in. Z = gas compressibility factor R = gas/liquid ratio, ft 3 /bbl T = operating temperature, °R P = pressure, psia Qj = liquid flow rate, bbl/day V = maximum allowable velocity, ft/sec Choose Wall Thickness Before choosing a wall thickness it is necessary to choose a pressure rating for the coil. Typically, the high-pressure coil (Lj) is rated for the shut-in pressure of the well, and the low-pressure coil (L 2 ) is rated for the maximum working pressure of the downstream equipment. There are many exceptions to this rule and reasons to deviate from it. If designing LI to withstand the well shut-in tubing pressure is too costly, it is com- mon practice to design the coil above the normal operating pressure of the flow line and install a relief valve set at the maximum allowable operating pressure of the coil. If flow is accidentally shut-in by a hydrate plug or other blockage at the choke, Lj could be subjected to total well- head shut-in pressure unless it is protected by a relief valve. The wall thickness of the coil can be chosen by using any number of recognized codes and standards. In the United States, the most common- ly recognized are American National Standard Institute (ANSI) B31.3 and B31.8, or American Petroleum Institute (API) Specification 12 K. Volume 1 has the tables for ANSI B31.3 and ANSI B31.8. Table 5-1 LTX Units and Line Heaters 119 illustrates the ratings from API Spec 12 K, which uses the calculation procedure from ANSI B31.3, but assumes no corrosion allowance. After the minimum inside diameter and the required wall thickness, a coil diameter and wall thickness may be selected. Very often, the coil length downstream of the choke (L 2 ) is of a different diameter and wall thickness than the length upstream of the choke (Lj). Coil Lengths With the known temperatures on each end of the coil, the heat duty for each coil can be calculated from the heat transfer theory in Chapter 2. Since the bath is at a constant temperature, LMTD can be calculated as: Table 5-1 Maximum Coil Working Pressure from API 1 2K Maximum Working Maximum Working Nominal Pressure* Pressure* Pipe Nominal Grade B Grade C Size Wall Thickness S = 20,000 5 = 23,300 in. in. psig psig 1XS 0.179 5,270 2Std 0.154 2,380 — 2XS 0.218 3,440 — 2XXS 0.436 7,340 8,560 2J4Std 0.203 2,600 — 2!4XS 0.276 3,610 — 2I4XXS 0.552 7,770 9,050 2 1 A 0.750 10,720 12,490 2 1 A 0.875 12,530 14,600 3Std 0.216 2,260 — 3XS 0.300 3,200 — 3XXS 0.600 6,820 7,940 4Std 0.237 1,920 — 4XS 0.337 2,770 — 4XXS 0.674 5,860 6,830 6Std 0.280 1,530 — 6XS 0.432 2,400 — 6XXS 0.864 5,030 5,860 8Std 0.322 1,350 — 8XS 0.500 2,120 — 8XXS 0.875 3,830 4,460 *Maximum working pressure has been rounded up to the next higher unit of 10 psig. No corrosion allowance is assumed; same formula as ANSI B31.3 120 Design of GAS-HANDLING Systems and Facilities where AT] = temperature difference between coil inlet and bath AT 2 = temperature difference between coil outlet and bath The overall film coefficient, U, for the coil can be calculated or read from the charts and tables in Chapter 2. Since U, LMTD, q, and the diameter of the pipe are known, the length of the coil can be solved from the following equation: where d = coil outside diameter, in. Equation 5-12 describes an overall length required for the coil. Since the shell length of the heater will probably be much less, several passes of the coil through the length of the shell may be required, as shown in Figure 5-3, to develop this length. For a given shell diameter there is a limit to the number of passes of coil. Therefore, the correct selection of coil length also requires determining the length of the shell and number of passes. As the shell length decreases, the number of passes increases, and a larger shell diameter is required. For a given shell length the number of passes for each coil can be determined. Since the number of passes both upstream and downstream of the choke must be an even integer, actual Tj and T 2 may differ slightly from that assumed in the calculation. The actual values of Tj and T 2 can be calculated from actual coil lengths LI and L 2 . Once the total number of passes is known, the coil can be laid out geo- metrically assuming that the center-line minimum radius of bends is 1 1 A times nominal pipe size. The required shell diameter is then determined. Other selections of shell length, number of passes and required diame- ters can then be made to obtain an optimum solution. STANDARD SIZE LINE HEATERS The previous procedure is very helpful for reviewing existing designs or proposals from vendors. In most situations, however, it will be eco- [...]... * 7 '6" 1, 500.000 4'4" »2-2"XH 3440 93.4 16 6 17 .9 4. 060 4'4" 12 -r'XXH 7340 85.9 16 6 17 .9 4,725 WXH 3200 948 J13.2 17 .5 4.390 8.3-XXH 68 20 BS9 11 X2 17 .8 5,335 14 -r'XH 3440 34.0 237 28.7 5 ,65 0 48" * re" 14 -T'XXH 7340 20.5 237 28.7 6. 60O 1^ 00.000 48"* 1O-3"XH 3200 4S.O 17 3 .1 28.0 6. 23S 4'4" 7 '6" 1^ 00.000 48" x T6" 1O-3"XXH 68 *0 31. 4 17 3 .1 28.0 7*78 3,000.000 60 "x20'0f 1 -2"XH 3440 7B.7 311 51. 8 10 ,11 0 2,000.000... 8-y'XXH 7340 26. 5 54 2.9 1. 61 0 500,000 30" * 0"0" 8-2"XH 3440 42 .6 76 6.0 2, 210 500.000 30" x 0-0" 8-2"XXH 7340 38.3 76 6.0 2. 510 75O^OO 36" » TO" TO-r'XH 3440 64 .4 11 4 10 .S 2.87S 750,000 3T'« ro- io-r'xxM 7340 580 11 4 10 .5 3.32S 75O.OOO 38" « TO" 6- 3"XH 3200 594 70.9 10 .3 3,030 7SO.OOO 36" » rO" 6- 3"XXH 68 20 S3.B 70.9 10 .3 3. 61 6 1. 000.000 42" » 1, 000.000 42" « 1, 000.000 42"* 1, 000,000 42" x 4'«" 1. 500.000... ie-2-xxn 7340 580 311 51. 8 11 . 360 7.000.000 60 "x20'0" 10 -3"XM 3200 66 .9 19 8 .1 51. 2 10 ,580 2.000,000 60 " x 2O-0'' 10 -3"XXH 68 20 50.4 19 8 .1 SI 2 12 .240 •Subject to change without notice Other combinations are available Figure 5-4 Dimensions of standard line heaters (From Smith Industries, Inc.} LINE HEATER DESIGN EXAMPLE PROBLEM Design a line heater for each of the 10 wells that make up the total 10 0 MMscfd... hydrate formation at 1, 000 psia a From equilibrium values Mole Fraction N, CO, H,S C, C2 C3 iC4 nC, 1C, nC., (V C7+ 0. 014 4 0.0403 0.000 019 0.8555 0.0574 0. 017 9 0.00 41 0.00 41 0.0020 0.0 013 0.0 015 0.0 015 Kv.s Values at 1 ,000 psia 70°F 50°F 0 .60 0.07 1. 04 0 .14 5 0.03 0. 013 0 .14 5 — — — — — 0.38 1. 26 1. 25 0.70 0. 21 1.25 — — b From Figure 4-5 Specific gravity from Table 2 -10 is 0 .67 At 0 .6 gravity hydrate... psig and 224°F From Figure 4 -6, we have: 12 6 Design of GAS-HANDLING Systems and Facilities lb water/MMscf of wet gas at reservoir conditions (8,000 psig and 224°F) lb water/MMscf of wet gas at 1, 000 psig and 75 °F Water to be heated, Ib/MMscf Water quantity = Qw e Total process duty 3 Calculation of coil length a Calculate LMTD Temperature of bath is 19 0°F b Calculate U = 260 = 2£ 232 LTX Units and. .. XX Pipe A -10 6- B D = 2.30 in = 0 .19 2 ft (Table 2-2) 12 7 12 8 Design ofGAS-HANDUNG Estimated U from Figure 2 -11 Systems and Facilities LTX Units and Line Heaters 12 9 U = I06Btu/hr-ft2-°F Use U = 96. 4 Btu/hr-ft2-°F c Calculate Coil Length 4 Calculate fire tube area required For heat transfer to water use 10 ,000 Btu/hr-ft2 flux rate: Estimate shell size: Assuming a 10 -ft shell, then four passes of 3-in,... The sizing of indirect fired heaters for these uses relies on the same principles and techniques discussed for wellstream line heaters 12 2 Design of GAS-HANDLING Systems and Facilities NOMINAL DIMENSIONAL DATA A B c D i V Q Ft In Ft In Ft In Ft In Ft In Ft In In »nly 1t/t6* Mmv ITU HH 280.000 r-tf r PS! Sm Mwn Coil AruSq Ft Appru« Coil Lcn Ft W«nr Fill Vot: Bblt Shippm^ W.^hl Pmjndi 260 ,000 24" > r«" 8-2-XH 3440 29-S S4 2.9 1. 400 290.000 24"... temperature, °F 68 Average temperature, °R 528 T c ,°R(Table2 -10 ) 375 TR = T/TC 1. 41 where qg = gas heat duty, Btu/hr AT "1 _ T ii i — i out T J in Since flow through coil is a constant pressure process, we have: Calculation of Cg where C = gas specific heat, Btu/lb0F From Figure 2 -14 , C at 68 °F is = 0.50 ACp from Figure 2 -15 , (at TR = 1. 41, PR = 1. 49) = 2 .6 c Oil duty: where SG = 0.77 From Figure 2 -13 at 68 °F... is 0 .67 At 0 .6 gravity hydrate temperature is 60 °F At 0.7 gravity hydrate temperature is 64 °F By interpolation, hydrate temperature at S = 0 .67 is 62 .8°F 2 Determine the process heat duty Temperature at outlet of heater should be about 5 to 15 °F above hydrate temperature Choose temperature at heater outlet as 75°F 12 4 Design of GAS-HANDLING Systems and Facilities a Temperature drop through choke Flow . 2O-0'' 8-2-XH 8-y'XXH 8-2"XH 8-2"XXH TO-r'XH io-r'xxM 6- 3"XH 6- 3"XXH »2-2"XH 12 -r'XXH WXH 8.3-XXH 14 -r'XH 14 -T'XXH 1O-3"XH 1O-3"XXH 1 -2"XH ie-2-xxn 10 -3"XM 10 -3"XXH 3440 7340 3440 7340 3440 7340 3200 68 20 3440 7340 3200 68 20 3440 7340 3200 68 *0 3440 7340 3200 68 20 29-S 26. 5 42 .6 38.3 64 .4 580 594 S3.B 93.4 85.9 948 BS9 34.0 20.5 4S.O 31. 4 7B.7 580 66 .9 50.4 S4 54 76 76 11 4 11 4 70.9 70.9 16 6 16 6 J13.2 11 X2 237 237 17 3 .1 173 .1 311 311 19 8 .1 198 .1 2.9 2.9 6. 0 6. 0 10 .S 10 .5 10 .3 10 .3 17 .9 17 .9 17 .5 17 .8 28.7 28.7 28.0 28.0 51. 8 51. 8 51. 2 SI. 2 1. 400 1. 61 0 2, 210 2. 510 2.87S 3.32S 3,030 3. 61 6 4. 060 4,725 4.390 5,335 5 ,65 0 6. 60O 6. 23S 7*78 10 ,11 0 11 . 360 10 ,580 12 .240 NOMINAL . 2O-0'' 8-2-XH 8-y'XXH 8-2"XH 8-2"XXH TO-r'XH io-r'xxM 6- 3"XH 6- 3"XXH »2-2"XH 12 -r'XXH WXH 8.3-XXH 14 -r'XH 14 -T'XXH 1O-3"XH 1O-3"XXH 1 -2"XH ie-2-xxn 10 -3"XM 10 -3"XXH 3440 7340 3440 7340 3440 7340 3200 68 20 3440 7340 3200 68 20 3440 7340 3200 68 *0 3440 7340 3200 68 20 29-S 26. 5 42 .6 38.3 64 .4 580 594 S3.B 93.4 85.9 948 BS9 34.0 20.5 4S.O 31. 4 7B.7 580 66 .9 50.4 S4 54 76 76 11 4 11 4 70.9 70.9 16 6 16 6 J13.2 11 X2 237 237 17 3 .1 173 .1 311 311 19 8 .1 198 .1 2.9 2.9 6. 0 6. 0 10 .S 10 .5 10 .3 10 .3 17 .9 17 .9 17 .5 17 .8 28.7 28.7 28.0 28.0 51. 8 51. 8 51. 2 SI. . 0.875 12 ,530 14 ,60 0 3Std 0. 2 16 2, 260 — 3XS 0.300 3,200 — 3XXS 0 .60 0 6, 820 7,940 4Std 0.237 1, 920 — 4XS 0.337 2,770 — 4XXS 0 .67 4 5, 860 6, 830 6Std 0.280 1, 530 — 6XS 0.432 2,400 — 6XXS