development of a user friendly computer program for designing conventional oilfield separators.pdf

7/27/2019 development of a user friendly computer program for designing conventional oilfield separators.pdf

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degree of gas turbulence. The secondary separation section is designed to utilize the force of gravity to enhancethe separation of entrained droplets from the gas phase. Smaller liquid droplets that do not settle out of the gasstream in the secondary separation section are removed in the mist extraction section. Liquid collection sectioncollects all the liquid separated from the inlet stream. The external controls are used to maintain fluid levels andcontrol the pressure inside the separator. Separators are commonly designed to meet certain specifications andrequirements imposed by the purchaser. Therefore, in addition to the features described above, separators maybe equipped with more and other features, if required by the purchaser. Table 1.0 in the appendix gives asummary of the comparisons between horizontal, vertical and spherical separators. This study is on the design of

three-phase horizontal and vertical design procedures only. Spherical separators are rarely ever used in oilfieldfacilities, installation and operation of level controls on such separators are difficult and very few are still employedin oil production facilities today.

Conventional separator design procedures are labor-intensive, involving a lot of manual trial and error methodswith wide table look-ups and require the application of many rules of thumb. Depending on the amount of production data available and number of iterations to be performed it becomes difficult to establish an acceptabledesign that is readily available. This paper presents four (4) developed computer programs for sizing horizontaland vertical conventional oilfield separators using the basic equations and procedures established by Svrcek andMonnery in 1994 and Arnold and Stewart in 2008. These two design methods carefully outline a step by stepprocedure employed in the design of conventional oilfield separators.

Separator Design Theory

The primary tasks of conventional procedures employed in designing oilfield separators are to accuratelypredict size (i.e. separator diameter and height or length) and shape, configuration of separator internals, and fluid

levels inside separator. The inlet fluid stream from the well head into the separator is a mixture of liquid droplets of different sizes suspended in a continuous gas phase. The liquid droplets have to be separated from the gas phaseemploying a combination of physical separation mechanisms. The most widely employed separation mechanismsused in designing conventional oilfield separators are: Gravity settling Centrifugal force separation Impingement

The basic equations used in designing separators are widely known and have remained the same since inception.These equations originate from the work of Souders and Brown in designing fractionating columns. They re-arranged the force balance equation to calculate the settling velocity of the moving particle and hence size avertical fractionating column. Fig. 2 is an illustration showing the three forces acting on a particle in vertical motion.Refer to Fig. 2 in appendix. Based on their work and analysis, separator designers over the years have developedand published procedures for sizing conventional oilfield separators. Traditional separator design methods are

based on (1) using stokes law and a single droplet size to estimate separator size, (2) setting a residence/retentiontime, and (3) cost of equipment.

……………………...………….. (1)

v t = settling velocity, ft/sg = acceleration due to gravity, ft/s

2

d p = droplet diameter, ftC d = drag coefficient

g = gas density, lbm/ft3

l = liquid density, lbm/ft3

Calculating the settling velocity of the liquid droplets based on a uniform droplet size enables separator designers

to estimate the length or height liquid droplets will travel inside the separator before falling to the liquid collectionsection. The time required for the gas and liquid mixture to reach equilibrium inside the separator and ensureeffective gas liquid separation is defined as the residence time. For the mixture to reach equilibrium a certainstorage capacity is required; the residence time enables separator designers to accurately estimate the storagecapacity of the separator

Basic fluid properties (such as pressure, temperature, gas, oil and water flow rates) are also essential todesigning and selecting the appropriate separator for a hydrocarbon stream. The separator designer has to beprovided data such as the minimum, maximum, and operating pressures as well as the, minimum, maximum, andoperating temperatures. The densities and viscosities of the fluids present in the inlet stream are also required. It is



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Refer to Fig. 8 for horizontal separator Dimensions

Arnold and Stewart:

………………………… (4)

d max = maximum separator internal diameter, in (ho )max = maximum oil pad thickness, inβ = fractional height of liquid inside the separator

Refer to Fig. 9 to estimate β.

Gas capacity:

………………………… (5)

LSS = seam to seam separator lengthLeff = separator effective length, ft

Liquid capacity:

…………………………… (6)

Vertical Separator Design

Svrcek and Monnery:

⁄ 12 ………………… (7)

Qg = gas flow rate, ft3/min, MMscfd

v g = gas velocity, ft/s

.. (8)

H T = total vessel height, ftH H = liquid hold up heightH L = height difference between the outlet of the light liquid and the heavy liquid interface, ftH R = height difference between the outlet of the light liquid and the baffle, ftH BIN = height difference between the baffle and inlet nozzle, ftH A = height difference between baffle and liquid level, ft H D = disengagement height, ft

(Refer to Fig. 7 in appendix to see vertical separator dimensions).

Arnold and Stewart:

Separator diameter based on liquid droplet size:

5040 ………… (10)

d p = droplet diameter, micronp = pressure, psiaT = temperature,

oR,

oF

z = gas compressibility

Separator diameter based on water droplet size:



SPE 167578 5

6690 ∆ ……………… (11)

d p = droplet diameter, micronQo = oil flow rate, BPD ∆SG = difference in specific gravities of the water and oil μ o = viscosity of the oil, cp

Separator diameter based on oil droplet size:

6690 ∆ ……………… (12)

d p = droplet diameter, micronQw = water flow rate, BPD ∆SG = difference in specific gravities of the water and oil μ w = viscosity of the water, cp

For diameters ≤ 36 in

………………………… (13)

ho = height of oil pad, in

hw = height of water outlet to interface

For diameters > 36 in

……………………… (14)

The preliminary results obtained are void of the designers past experience on separator design. However these results provide a baseline design that serve as a basis for the final design dimensions.

Output

Refining the generated preliminary designs using experience and sound engineering judgments’ results in asecond set of generated designs from which the designer could select the most suitable choice. API standarddimensions for conventional oilfield separators are reported as follows: Separator diameter is given in multiples of 12 inches and the separator height is in multiples of 5 feet. The final design dimension selected should follow APIguidelines and satisfy all constraints of stability, strength and cost.

Case Study 1This case study is adapted from one of the reviewed literature (Arnold and Stewart, 2008). It is used to validate theresults obtained in the literature and also to compare the results obtained from using two design procedures. Table3 shows the input production data and Tables 4 and 5 displays the generated results. Tables 3 through 5 areshown in the appendix.

The results displayed in Tables 4 and 5 using the Arnold and Stewart Design procedures are the same as theresults shown in the literature and this validates the computer program. The results using both design proceduresare similar and a final dimension can be selected that will satisfy both procedures and all other constraints. Fromthe results displayed in Table 4 and 5, most suitable separator dimensions are as follows:

Final Vertical Separator Dimensions:

Arnold and Stewart: 84-in (OD) X 20-ft (S/S) Length; SR = 2.9Svrcek and Monnery: 84-in (OD) X 20-ft (S/S) Length; SR = 2.9

Final Horizontal Separator Dimensions:


The final results are presented in API format and satisfy the constraints of cost, stability and strength. Thenew dimensions ensure that the separator is properly configured for the appropriate installation of separator internals while enabling dimensions to be reported in API format.



Case Study 2

This case study is also adapted from one of the reviewed literature (Arnold and Stewart, 2008). This case studyis used to further validate the computer program and also to compare the results obtained from using the twodesign procedures. Table 6 shows the production input data and Tables 7 and 8 displays the generated results.Tables 6 through 8 are displayed in the appendix.

Final Vertical Separator Dimensions:


Final Horizontal Separator Dimensions:


The final results are presented in API format and satisfy the constraints of cost, stability and strength. It can beobserved that the results shown in Tables 7 and 8 are similar and further validate the computer program. Theresults also show a similar trend with the results obtained using case study one. An increase in diameter corresponds to a decrease in separator diameter height or length and this is due to the direction of the travellingparticle. If the given design data for a vertical separator is accurate, the results always show a decrease inseparator height as the separator diameter increases. If the separator diameter the liquid particles (i.e. liquid, oiland water) have to travel is sufficiently large, the height required for particle settling would reduce. This trend isillustrated in Figs. 4 and 5.

The final separator dimensions shown above are as a result of the design procedures modeled into a simulator and designer experience. Estimating separator size for a given production data using only the basic equations or design experience could lead to a bottleneck of the entire crude oil processing facility. Hence it is very important toapply both in separator design.

Conclusions1. A user-friendly computer program was developed to facilitate the design of three-phase oilfield vertical and

horizontal separators using two design procedures (Arnold and Stewart, 2008 and Svrcek and Monnery, 1994). Automating the design process enable separator designers save time and ensures the reliability and reuse of the design procedure.

2. The design procedure proposed by Arnold and Stewart is more suitable for the design of separators. This isbecause it involves the use of less design variables and constants, and does not rely heavily on the use of charts and graphs to estimate vessel dimensions. Less guesswork is involved, computation time is reduced,and the calculation procedures are easy to understand and less prone to human errors.

3. Horizontal separators provide greater fluid stability for the same production input data. The results show thatthe predicted horizontal separator dimensions will allow for higher separation efficiency than vertical separators

for the same production input data.

Recommendations1. The liquid particle sizes to be removed from the inlet stream have to be specified. During the course of this

study, it was observed that failure to specify the particle size to be removed led to separator dimensions thatwere not feasible. The results shown in this study are based on the following liquid particle sizes: liquids = 100microns; oil = 200 microns; and water = 500 microns.

2. Due to uncertainty within the reservoir the production input data can’t remain the same forever and changeswould always occur with the original design data. It is suggested that the developed programs should be usedwith a reservoir simulator program to ensure separator dimensions that will be equipped to handle suchchanges in the production data.

ReferencesSpec. 12J, AP1, API Specification for Oil and Gas Separators, seventh edition, 1989. Washington, DC: API

Arnold, K. and Stewart, M. 2008. Surface Production Operations Vol. 1, third edition: Gulf Publishing Company. Arnold, K. and Stewart, M. 1989. Surface Production Operations Vol. 1: Design of Oil-Handling Systems and

Facilities, first edition: Gulf Publishing Company. Arnold, K. and Stewart, M. 1986. Surface Production Operations Volume 1: Gulf Publishing Company. Arnold, K. and Stewart, M. 1984. Designing Oil and Gas Production Systems. World Oil Journal, 199 (7) 87-98.Halvorson, M. 2010: Visual Basic 2010 Step by Step: Microsoft Press.Bradley H. B. 1987. Petroleum Engineering Handbook: Oil and Gas Separators, Chapter 12 (Smith, H. V).

Richardson, Texas, SPE



SPE 167578 7

Souders, M. J. and Brown, G.G. 1934.Design of Fractionating Columns: Entrainment and Capacity. Industrial andEngineering Chemistry Journal, 26 (1): 98 – 100.

Svrcek, W.Y. and Monnery, W.D. 1993. Design Two-phase separator within the Right Limits, ChemicalEngineering Progress, 89 (10): 53-60.

Svrcek, W.Y. and Monnery, W.D. 1994. Successfully Specify Three-phase Separators .Chemical EngineeringProgress 90 (9): 29-40.

Walkenbach, J. 2010: Excel 2010 Power Programming with VBA: Wiley Publishing Inc.

Nomenclature Ag = separator area occupied by gas, ft

2

AHL = Interface area occupied by heavy liquid, ft2

ALL = Interface area occupied by light liquid, ft2

AT = separator cross-sectional area, ft2

C d = drag coefficientd = separator diameter, ind max = maximum separator internal diameter, ind p = droplet diameter, micronF B = buoyancy force, N, lbf

F D = drag force, N, lbf

F G = gravity force, N, lbf

F i = sum of forces acting on the particle, N, lbf

H A = height difference between baffle and liquid level, inches, ft

H BIN = height difference between the baffle and inlet nozzle, inches, ftH D = disengagement height, ftH H = liquid hold up heightH L = height difference between the outlet of the light liquid and the heavy liquid interface, ft, inchesH R = height difference between the outlet of the light liquid and the baffle, ft, inchesH T = total vessel height, ftho = height of oil pad, inhw = height of water outlet to interface(ho )max = maximum oil pad thickness, ing = acceleration due to gravity, ft/s

2

LSS = seam to seam separator lengthLeff = separator effective length, ftL/D = slenderness rationp = pressure, psia

Qg = gas flow rate, ft

3

/min, MMscfdQo = oil flow rate, BPDQw = water flow rate, BPDSg = gas specific gravitySGo = oil specific gravitySGw = water specific gravityT = temperature,

oR,

oF

(t r )o = oil retention time, min(t r )w = water retention time, minV H = holdup volume, ft

3

V S = surge volume, ft3

v g = gas velocity, ft/sv t = settling velocity, ft/sz = gas compressibility

Greek Symbols

∆SG = difference in specific gravities of the water and oil

g = gas density, lbm/ft3

l = liquid density, lbm/ft3

μ g = gas viscosity, cp μ HL = viscosity of the heavy liquid, cp.



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μ LL = viscosity of the light liquid, cp. μ o = viscosity of the oil, cp μ w = water viscosity, cp

Appendix A

Table 1: Comparisons between the three types of separators (Petroleum Engineering Handbook: Oil and Gas Separators, Chapter 12)

Consideration Horizontal Vertical Spherical

Efficiency of separation 1 2 3

Stabilization of separated fluids 1 2 3



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Figure 2: Forces acting on a moving liquid droplet in a gas phase

FD

FB

FG

Fi

FB

= buoy

FD

= drag

FG

= grav



SPE 167578 11

Figure 3: Computer Program Outline

Table 3: Production Input DataTable 4: Vertical Separator Dimensions

Table 5: Horizontal Separator Dimensions

Oil flow rate, Qo 5000 BPD

Water flow rate, Qw 3000 BPD

Gas flow rate, Qg 5 MMscfd

Operating pressure, p 100 psia

Operating Temperature, T 90oF

API 30o API

Water specific gravity, (SG)W 1.07

Gas specific gravity, (SG)g 0.6

Oil specific gravity, (SG)o 0.876

Water retention time (tr )w 10 mins

Oil retention time (tr )o 10 mins

Oil viscosity, μo 10 cp

Water viscosity, μw 1 cp

Gas density, ρg 0.3 lbm/ft3

Water density, ρw 62.11 lbm/ft3

Oil density, ρo 54.7 lbm/ft3

Drag Co-efficient, Cd 2.01

Surge time 5 mins

Arnold and Stewart Svrcek and Monnery

d (inches)LSS(feet) SR d (inches) LSS (feet) SR

84 18.2 2.6 60 18.5 3.7

90 17.7 2.4 66 16.7 3

96 17.4 2.2 72 15.5 2.6

102 17.2 2 78 14.6 2.2

108 17.1 1.9 84 14 2


d (inches) LSS (feet) SR d (inches) LSS (feet) SR

84 21.5 3.1 90 16.5 2.13

96 16.4 2.1 96 15.4 1.93

108 13 1.4 102 12.8 1.51



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Table 6: Input production dataTable 7: Vertical Separator Dimensions

Table 8: Horizontal Separator Dimensions


d (inches) LSS (feet) SR d (inches) LSS (feet) SR

60 36.2 7.3 78 25 3.8

72 25.2 2.6 84 25 3.6

84 18.5 3.1

Figure 4: Vertical Separator Dimensions Figure 5: Horizontal Separator Dimensions

Oil flow rate, Qo 3900 BPD

Water flow rate, Qw 3000 BPD

Gas flow rate, Qg 1.6 MMscfd

Operating pressure, p 455 psia

Operating Temperature, T 90oF

API 30o API

Water specific gravity, (SG)w 1.07

Gas specific gravity, (SG)g 0.6

Oil specific gravity, (SG)o 0.876

Water retention time (tr)w 10 mins

Oil retention time (tr)o 10 mins

Oil viscosity, μo 10 cp

Water viscosity, μw 1 cp

Gas density, ρg 1.43 lbm/ft3

Water density, ρw 62.11 lbm/ft3

Oil density, ρo 54.41 lbm/ft3

Drag Co-efficient, Cd 1.93

Surge time 10 mins


d (inches)LSS(feet) SR d (inches) LSS (feet) SR

72 18.6 3.1 60 18.7 3.1

78 17.7 2.7 66 17.5 2.6

84 17.1 2.4 72 16.6 2.2

90 16.7 2.2 78 16.0 2.0

96 16.5 2.1 84 15.6 1.8



SPE 167578 13

Figure 6: Down-comer allowable flow chart (Svrcek and Monnery 1994)

Figure 8: Horizontal separator (Monnery and Svrcek 1994)

Figure 7: Horizontal separator (Monnery and Svrcek 1994)

Table 9: Empirical Constants for Cylindrical height and area conversions (Svrcek and Monnery, 1993)

WD/D → A/AT

a = -4.7593 X 10-5

b = 3.924091 c =0.174875

d = -6.358805 e = 5.6668975 f= 4.018448

g = -4.916411 h = -1.801705 i = 0.145348



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Figure 7: Coefficient β for a cylinder half filled with liquid (Arnold and Stewart, 2008)

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