Three Phase Separators - Inlet Devices

Saeid Rahimi

28-Jan-2013

Introduction

For many, three phase separator sizing is a challenging job . This is mainly because o f the number o f process parameters involved, the variety o f internals and possible internal configurations. In addition, the numbers o f parameters that have to be checked to ensure proper separator sizing are relatively high and sometimes a combination o f these criteria adds to the complexity o f the calculation. That is why some believe that there is as much art as there is science to properly designing a (horizontal) three phase separator. ^

Nevertheless, 1 believe separator sizing is a simple set o f calculations when you know the basic sizing principals such as gas-l iquid separation theory, l iqu id - l iqu id separation fundamentals and the definitions o f different terms and their importance. The next step is to obtain the required input data and try to find a size which satisfies these requirements and criteria. Without having the whole picture o f what is going to be done, any simple exercise can turn into a cumbersome and complex iterative problem.

1 am going to develop a series o f notes to cover the basics o f three phase separator sizing. This note reviews different types o f inlet devices, their effects on the gas-liquid separation, and sizing and selection details.

Inlet Device Importance

A n inlet device should perform the fo l lowing functions:

f ' %

• Separate B u l k L i q u i d s ^ " ' W " I * '

O f the main functions o f the inlet device is to improve the primary separation o f l iqu id from the gas. A n y bulk liquids separated at the inlet device w i l l decrease the separation load on the rest o f the separator and thus improve the efficiency. Good bulk separation w i l l also make the separator operation less sensitive to changes in the feed stream. When mist extractors (mesh or vane pad) are ut i l ized to enhance the l iquid droplet separation, the amount o f l iquid in gas in the face o f mist extractor ( l iqu id loading) adversely affects the performance o f the mist extractor. Therefore using an appropriate inlet device plays a major role in achieving required separation.

• Ensure Good G a s and L iqu id Distribution

A properly sized inlet device should reduce the feed stream momentum and ensure the distr ibution o f the gas and liquid(s) phases entering the vessel separation compartment, in order to optimise the separation efficiency. Mal-d is t r ibut ion o f l iquid can lead to a large spread in residence times, decreasing the separation efficiency. Also a gas mal-distribution at the entrance o f the mist extractor or cyclone deck can locally overload the demister and cause severe carryover.

• Prevent Re-entrainment and Shattering

Re-entrainment o f l iqu id droplets can be caused by b lowing gas down or across the l iqu id surface at very high velocities. This phenomenon often occurs when vessels w i t h deflector baffles or ha l f pipes are operated at the higher gas flow rates than what they were designed for. L i q u i d shattering inside the inlet device can also happen in a vessel w i th no inlet device or wi th a deflector baffle when the feed stream's l iquid smashes into the plate and is broken up in extremely small droplets. This can create smaller droplets than were present in the feed stream, making the separation in the rest o f the separator even harder. Selecting a proper inlet device and fo l lowing common design guidelines for setting the distance between the bottom o f the inlet device and highest l iqu id level inside the vessel should minimize this problem.

• Facilitate De-foaming

I f the feed stream has a tendency to foam, an inlet device that prevents or even breaks down foam can significantly improve the separation efficiency o f the vessel, reduce the size o f the vessel and the use o f chemicals.

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Inlet Device Type

The fo l lowing section provides some information about different types o f inlet devices.

• No Inlet Device

The simplest form o f vessels has no inlet device on the inlet nozzle.

• Deflector Baffle

Deflector baffles are historically one o f the most common types o f inlet devices in o i l and gas industries before inlet devices w i th higher separation efficiency become so popular. This device s imply uses a baffle plate in front o f the inlet nozzle to change the direction o f the inlet stream and separate the bulk o f the l iqu id from the gas. However, an increasing number o f contractors and operators are mov ing away from traditional types o f inlet devices towards more advanced designs wi th higher separation efficiencies.

• 90° Elbow

This inlet device is used in the horizontal vessels to direct the inlet stream towards the vessel dish end. Long Radius (LR) elbows are normally preferred for this purpose and there is no straight run o f pipe downstream o f the elbow. However, Short Radius (SR) elbows can be used i f install ing LR elbow increases the height o f the vapor space. They can be also provided wi th a straight am o f pipe w i th a length equal to two times o f the inlet nozzle diameter (2d | ) to direct the feed to the dish end rather than the surface o f l iqu id inside the vessel and minimize the l iquid re-entrainment.

• Ha l f Open Pipe

H a l f open pipes are the modified versions o f 90° elbow devices, suitable for both vertical and horizontal separators, wi th slightly improved bulk l iquid removal and reasonable gas distribution. In this type, a piece o f pipe w i th a length up to three times the inlet nozzle diameter (3d | ) is welded to the inlet 90° elbow.

1 1 1 fey]/ lil 1 fey]/ lil

l a . Horizontal Vessel - Top E n t r y lb . Vertical Vessel I c . Horizontal Vessel - Side Entry

Figure 1 - Ha l f Open Pipe Installation Configuration in Horizontal and Vert ical Vessels

In horizontal vessels, the last section o f the ha l f open pipe should be horizontal, point ing opposite to the f low direction in the vessel and w i t h its opening directed upward (Figure l a ) . In vertical vessels, the last section is closed and its opening is directed downward (Figure l b ) . The same configuration is used when the hal f open pipe is used for a horizontal vessel wi th a side nozzle (Figure I c ) .

• Vane Type Distributor

The simplest form o f the vane distributor is the dual vane inlet device (shown in Figure 2.a) which offers a reasonable flow distribution w i th low shear and pressure drop. In horizontal vessels, it is suited for top entry only. The benefits o f this device compared wi th simpler deflectors such as deflector plates include reduced agitation and hence improved phase operational performance, more stable level control , and reduced foaming. For l iquid slugging applications, usually where there is a long incoming flow line, this device provides excellent mechanical strength. The dual vane works by smoothly d iv id ing the incoming flow into two segments using curved vanes lo suit the overall geometry o f the inlet nozzle. The gas phase readily separates and disperses along the vessel, whilst the l iquid phase velocity is reduced and the flow directed to the vessel walls where it further disperses and falls into the bulk l iquid layer at relatively low velocity.

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Figure 2 - The Different Types o f Vane inlet Devices

For ser\'ices where there is a high gas f low relative to the l iquid flow, the multi-vane inlet device provides excellent vapour distribution a l lowing a reduced height lo the mass transfer or mist el iminator internals. The vane distributors work by smoothly d iv id ing the incoming flow into various segments using an array o f curved vanes to suit the overall geometry o f the inlet nozzle and distributor length. To achieve this effect the vanes start wi th a wide spacing and gradually reduce the gap, g iv ing the unit its characteristic tapering shape. It can be installed in both vertical and horizontal (top and side entry) three phase separators. Figure 2.b shows the internal details o f multi-vane inlet distributor.

Some vendors have tried to employ the multivane distributor benefits together w i t h tangential entry (which provides considerable centrifugal force) to improve the bulk separation. Figure 2.c shows a typical type o f vane developed for vertical separators only.

• Slotted Tee Distributor

The slotted T-shaped distributor consists o f a vertical pipe extended inside the vessel to bring the distributor to the right elevation and a slotted pipe wi th large holes or rectangular slots (perpendicular to the inlet pipe) ensuring a reduced feed stream veloci ty and min imized flow turbulence. It can be used in both vertical and horizontal (top entry on ly) separators.

The openings o f the slots are usually 120 ° (±60°) and towards the dish end and l iquid interface in horizontal and vertical vessels, respectively.

Tangential Inlet With A n n u l a r Ring Figu re 3 - Tee distributor

Tangential inlet devices have been exclusively developed for vertical vessels. The feed flow radially enters the vessel and accelerates passing through the inlet device, the cyclonic action o f the inlet device helps the l iquid droplets flow on the inner wal l o f the vessel and the stripped gas to flow through the central section o f the inlet device (annular ring) to the gas outlet nozzle. -•••t̂ ^

There are two options w i t h regards to the inlet nozzle arrangements shown in Figure 4. The type depicted in Figure 4a generates higher centrifugal force and sl ightly better separation efficiency. However, it is not recommended for pressures higher than 5.0 bar due to its construction difficulties at high pressures. Furthermore, both types can have a circular or rectangular inlet nozzle. A larger cross sectional area can be provided when a rectangular (wi th height larger than the width) nozzle is used.

• Cyclone

The cyclonic inlet device is used in horizontal and some vertical separators where there is a requirement for high momentum dissipation, foam reduction and high capacity. They work on the principle o f enhanced gravity separation by accelerating any incoming stream to a high g-force, which particularly helps foam to break down into separate l iquid and gas phases. Unl ike most inlet devices that are positioned in the gas phase, the inlet cyclone is partly submerged in the l iquid phase. The l iquid phases are also separated cenlrifugally through the perimeter o f the cyclone lubes and fall down in lo the bulk l iquid layers, whils t the gas forms a central vortex core and escapes through a top outlet hole into the gas space. The mix ing elements on top o f the cyclone outlet section usually provide a proper distr ibution o f the cleaned gas to downstream devices. The device has a high pressure drop associated wi th it.

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4a. Round E n t r y 4b. Straight E n t r y

Figure 4 - Tangential Inlet Entry Arrangements

The designs o f the inlet cyclones have evolved over the past decades from short single (conventional cyclones) or dual cyclones into multi-cyclone arrangements (Figure 5). A main characteristic o f the cyclone inlet device is its high f low capacity, meaning that more throughput is possible through any given size separator.

Selection Criteria Figure 5 - M u l t i Cyclone Inlet Device

In order to make a proper selection, you need to know how different types o f inlet devices perform in similar conditions. Table I evaluates to what extent they fu l f i l the functions described earlier in this note.

Table 1 - Comparison of Performance of Different Inlet Devices

Inlet Device Functions No Inlet Device

Deflector Baffle

H a l f Open Pipe

Vane Type Distributor

Cyclone

Separate Bulk Liquids Poor Poor Average Good Very Good

Ensure Good Gas and L i q u i d Distribution

Very Poor Very Poor Average Good Good

Prevent Re-entrainment of Liquid from the G / L Interface

Very Poor Very Poor Poor Good Good

Minimize Droplet Shatter Poor Very Poor Average Very Good Very Good

Facilitate De-foaming Very Poor Very Poor Poor Average Very Good

It may be concluded f rom Table-1 that it wou ld always be necessary to install a sophisticated inlet device such as vane type distributors or cyclones. But, it should be noted that Table- I compares the performance o f different inlet devices in similar conditions. For example, the abi l i ty o f ha l f open pipe to prevent re-entrainment o f the l iqu id from G / L interface is poor compared to the vane type distributor when they are installed at the same distance from the gas/liquid interface. However, this deficiency can be improved by providing enough height between the ha l f open pipe and the interface level. In other words, the weakness o f the inlet device can be compensated i f proper engineering practices are taken into consideration.

Another example is the cyclone which is exceptionally effective in breaking the foam. But the fact is that using the cyclone is particularly essential when the amount o f l iqu id is considerable. I f the l iquid flow rate is not high, most probably it is more reasonable to de-foam the l iqu id by provid ing the adequate residence time (not necessarily much more than what is required for the process operation or l iqu id - l iqu id separation) rather than an advanced technology device lo enhance the de-foaming.

Furthermore, performance mentioned in Table I is expected from a correctly designed inlet device. Otherwise, using a wrongly designed cyclone can cause gas b low-by or l iquid carry over which w i l l increase foam formation instead o f stopping it. Or having a high mixture veloci ty at the exit o f the annual ring w i l l result in re-entrainment o f the l iquid film which has been already collected on the separator wa l l .

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In summary, the selection o f the opt imal inlet device differs from case to case. Therefore, it is important to understand how inlet device functions and the effect it has on the separation efficiency o f the rest o f the vessel. The fo l lowing section describes other important parameters in selecting a proper inlet device.

• Separat ion Requi rements

The first parameter to check is whether a high performance inlet device is required from a process point o f view or not. I f there is no specific separation requirement other than bulk l iquid separation and the major port ion o f the l iquid droplets are relatively large ( typical ly larger than 150-200 microns), most probably a vertical K O D wi th one o f the simplest inlet devices (may be N O inlet device) is sufficient.

On the other hand, using a high performance inlet device is essential in some other applications where the ratio o f l iquid to gas in the feed stream is high and mist extractors (mesh or vane pads) are used to enhance the quali ty o f gas leaving the vessel. In such applications, the amount o f residual l iquid in the gas stream in the face o f the mist extractor plays a major role in achieving the desired separation efficiency in the mist extractor. Therefore, a high performance inlet device which efficiently performs the primary separation and uni formly distributes the gas across the vessel area prevents the mist extractor f rom being overloaded w i t h the l iqu id . - ' - ^ t l j , ^ly'

Therefore, depending on the ratio o f l iquid to gas in the feed stream and the selected gas-liquid separation device (permissible l iquid loading in the face o f mist extractor) a proper inlet device should be selected. Otherwise, to take in to account the l iquid overloading effect into vessel sizing, the mist extractor K value should be de-rated as pointed out earlier in "Three Phase Separators - Time Def in i t ion" . To calculate the amount o f l iquid at the entrance o f the mist eliminator, the feed stream's l iquid fiaction can be mul t ip l ied by the factors specified in Table 2. ^

Table 2 - Efficiency o f Different Inlet Devices for Bulk L i q u i d Removal

Tnlet device N o inlet device Deflector baffle H a l f open pipe Mult i -vane distributor

Cyclone

Separation efficiency < 0 . 5 0.5 0.8 0.95 > 0 . 9 5

Space and Cost Considerations

In some applications where vessel size is significantly important (such as offshore installations or where the size o f equipment is l imi ted by transportation restrictions), reducing the size o f the vessel can be a great advantage. Ut i l i z ing a higher performance inlet device is one o f the methods. For example, i f the diameter o f vertical K O D mentioned above is large, installing a hal f open pipe on Ihe inlet nozzle can cause a great reduction in the length o f the vessel. On the other hand, adding a hal f open pipe in horizontal vessel w i th top nozzle may result in a larger vessel diameter as accommodating the hal f open pipe w i l l need the larger vapor space (the distance between H H L L and the top o f the vessel).

Furthermore, it should be noted that theses k ind o f changes may not always be economically attractive. For example, according to Table 3, where the effects o f different inlet devices on the length o f a vertical vessel have been summarized, replacing the hal f open pipe w i th vane distributor in a vertical K O D o f 2000 mm diameter can result in about 0.55D (0.25D in the distance between H L L and inlet device + 0.3D in the distance between the inlet device and top T L ) which is about 1000 mm. Therefore, i f the only reason for this upgrade is to reduce the vessel size, the equipment cost saving due to 1000 m m reduction in the length can be offset by the higher cost o f the inlet device, vendor and contractor engineering costs and delay in procurement, etc.

• Other Parameters

There are some other parameters to look into whi le selecting/designing an inlet device for a three phase separator:

Flow regime: annular dispersed and mist flow regimes in the inlet pipe need special attention and inlet devices wi th higher surface area such as mult ivane inlet distributor o f tangential inlet wi th annular r ing are recommended when low l iquid carry over is essential. Almost all inlet devices can be made mechanically appropriate to handle l iqu id dominant flow regimes such as bubble and intermittent (slug or plug) flow regimes without vibration. The most desired flow regimes in the vessel inlet pipe are stratified (smooth or wavy) flow regimes. The design o f inlet p ip ing to achieve this flow regime should be considered for special services in which a low l iquid entrainment rate is required but mist extractors are not allowed due to any reason (i.e. the possibil i ty o f mist pad plugging).

Pressure d r o p : the pressure drop o f the inlet device increases as the number o f internals and changes in Ihe direction o f inlet flow increases but it usually remains in the order o f a few Pascals. Inlet cyclones usually create higher pressure drop than other inlet devices, therefore they are not recommended i f the allowable pressure drop is very l imited.

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Table 3 - The Effect o f Different Inlet Devices on the Length o f the Vert ical Vessel

Inlet device H H L L to Inlet device Inlet device height

(Note 1) Inlet device to top T L

( K O D ) Inlet device to

wiremesh

No inlet device 0.5 D (min 300mm) d, 1.0 D (min 1200mm) 0.7 D (mm 900mm)

Deflector Bafne

0.3 D ( m i n 300mm) 2 d , I . 0 D ( m i n 900mm) 0.5 D ( m i n 600mm)

Half open pipe 0.3 D (min 300mm) di 0.9 D (min 900mm) 0.45 D (min 6 0 0 i T i m )

Vane type d i s t r i b u t o r

0.05 D (mm 150mm) d| + 2 0 mm 0.6 D (min 600mm) d, (min 300 mm)

Notes:

1) The inlet nozzle size ( d i ) also varies w i th the type o f inlet device. Using high performance inlet devices allows the designer to use higher momentum for the inlet nozzle sizing. This leads to the reduction in the size o f the inlet nozzle which directly contributes to the length o f the vertical vessel and can be important (as discussed above) in the size o f horizontal vessels. Refer lo the nozzle sizing section for further information.

2) The effect o f the inlet device on the size o f a horizontal separator is not that straight forward. On one hand, using a high performance inlet device (as illustrated in Table 2) can reduce the size o f the vapor space through improving (not de-rating) the separation efficiency o f mist extractors. On the other hand, the vapor space may be governed by other factors, such as maximum f i l l i ng level or accommodating the gas outlet device (refer to the paper "Three Phase separators - Gas Internals"). For example, some o f the gas outlet devices need vapor space o f at least 4 0 % o f the vessel diameter in which any inlet device o f ordinary size can be accommodated. But in any condit ion, the vapor space should be sufficiently high to accommodate the feed inlet device plus m i n i m u m 150 mm between the bottom o f the inlet device and H H L L . See the fo l lowing section for the size o f the inlet device.

Foam services: cyclones are the only inlet device w i th proven capability to break the foams. The primary purpose o f the inlet cyclone is that o f foam el iminat ion inside a separator. M a n y crude oils exhibit moderate or severe foaming tendency and the traditional approach to these problems is through a combination o f oversized equipment using foam breaking packs and chemicals.

Nozzle size: using a high performance inlet device enables the designer to use higher momentum for inlet nozzle sizing. This can reduce the nozzle size and subsequently the length o f the vessel. It can also be o f great advantage especially when the entire vessel mechanical design can be adversely affected by an extraordinarily large opening.

Sol id ; a vessel wi th no inlet device is preferred. Cyclones are not suitable for this ser\'ice as they further accelerate the inlet f luid for separation purpose which can create an extremely erosive environment. I f an inlet device is inevitable, it should have 1-2 mm extra thiclcness for erosion.

Fouling services: Inlet devices wi th narrow openings such as slotted tee distributors should be used wi th extra precaution in the foul ing services. They are not recommended i f the feed's wax, solid, scale or asphaltene content are high.

Sizine Considerations

The fo l lowing section covers the sizing requirements o f the various inlet devices:

• Deflector Baffle

Figure 6 shows the typical dimensions on a deflector baffle.

9 0 ° Elbow

The diameter o f the elbow should be the same as the inlet nozzle. Figure 7 shows the m i n i m u m height o f vapor space to accommodate LR and SR elbows. Furthermore, the elbow should be installed as close as possible to the tangent line considering reinforcement and fabrication requirements (150 mm).

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A n impingement baffle should be installed opposite to the elbow to protect the drum shell. The impingement baffle diameter should be twice the elbow diameter, as o f a m i n i m u m .

• Hal f Open Pipe

Similar to 90 degree the diameter o f the ha l f open pipe is the same as that o f the inlet nozzle. However, the length o f the device is usually 3 times the inlet nozzle diameter (3d | ) out o f which 2d | is dedicated to the opening cut.

A ha l f open pipe can also be fitted downstream o f a L R or SR elbow when it is used in horizontal vessels - top entry depending on available vapor space (Refer to Figure 7 for details). For a horizontal vessel, an impingement baffle should be installed opposite to the elbow to protect the drum shell. The impingement baffle diameter should b twice the e lbow diameter, as o f a m i n i m u m .

• Vane Type Distributor

In a horizontal vessel, the length o f a multi-vane distributor is between 3 to 5 times the inlet nozzle diameter.

For a vertical vessel, the length o f a multi-vane distributor can be as high as the vessel diameter. However, a shorter vane distributor is needed to accommodate the required number o f vanes whi le obtaining a reasonable opening between two adjacent vanes. /

In other words, a specific number o f vanes should be installed wi th particular pitch in order to achieve the targeted performance. This can be a trial and error exercise, starting wi th an assumed length, fol lowed by specifying the size and number o f vanes, and f inal ly checking i f this number o f vanes can meet other manufacturing requirements (consult w i th vendor) in the assumed length or not.

The height o f the vane is the same as the inlet nozzle. Addi t iona l height for the inlet elbow should be added to the vane distributor height when it comes to determining the m i n i m u m vapor space in horizontal vessels to accommodate the inlet device. T

Vane distributors are not recommended i f

^ Vessel diameter is less than 500mm. > Inlet nozzle diameter is smaller than 150mm. > Inlet nozzle diameter is larger than 1/3 o f the vessel diameter.

To prevent vibration o f the vanes, special considerations should be given to the design and construction o f the vane distributors when the vane height is in excess o f 800mm or exceptionally high f low rates are possible or l iquid slugging is expected. » --fev-

• Slotted Tee Distributor

The slotted tee distributor can be sized based on the superficial velocity o f the inlet f luid in the slot through the fo l lowing equation ( I ) :

Vslot = MG

Other details and dimensions o f inlet distributor can be calculated using the formulas below:

N , M = Q T / ( C 2 X A „ „ , x v „ J

Figure 6 - Deflector Baffle Dimensions

150 mm

1.5 dl (LR) or 1.0 dl (SR)

0.5 dl

150 mm LAHH

Figure 7 - Inlet Elbow Dimensions

Figure 8 -Tee Distributor Details

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L,,„,= 1 2 0 n d , e e / 3 6 0

Q T = Q G + Q L

Tee distributor diameter, d,ee. is tlie same as the inlet nozzle size and its length can be calculated by the equation below:

Ldistributor = d ,e ,+ N„„, X W,,,, + (N,,,,, + 2) X Pitch

Where

0 ; l iquid surface tension. m N / m PL : l iquid density, kg/m3

PG: gas density, kg/m3 [iQ-. gas viscosity, cP C , : 7 X 10 ' ' (Met r i c unit) C 2 : 2 when the f low is split between the two sides o f the distributor (tee type) and 1 i f a slotted pipe distributor is used. Wsi„,: width o f slot, 15mm

Pitch: the distance between two adjacent slots, 25mm

• Tangential Inlet With A n n u l a r Ring

Inlet nozzle is usually sized by determining the velocity required to satisfy the fo l lowing requirements:

> L iqu id bulk separation

> Inlet stream momentum (pV~) > Foaming separation requirement > Prevent erosion i f erosive materials are present

Figure 10 - Tangential Inlet wi th Annular Ring

The annular r ing w id th is usually the same as the inlet nozzle diameter. The ring height should be 2.5 times the inlet nozzle diameter. The diameter o f the vessel is also l inked w i t h inlet nozzle size and it is the larger o f

> 3.0 and 3.5 times o f mlet nozzle width/diameter for rectangular and circular inlet nozzles, respectively. > Vessel diameter required to maintain the gas velocity in the vessel cross sectional area below 3m/sec.

Furthermore, one solid circular baffle w i t h diameter to al low superficial velocity o f 45mm/sec through the annular gap (wi th min imum gap o f 50 mm) should be installed at least 150 mm above H H L L . Four vertical anti-swirl baflles should be provided below N L L . These baflles should be extended from 150 m m below the N L L to the bottom T L . The baffle width should be about 10% o f the d rum diameter.

• Cyclone

There are some general guidelines about cyclone inlet device sizing; however, the best approach is to ask the vendor to provide the size o f this device.

Nozzle Design Details

A feed stream can enter a horizontal vessel from top or dish ends. When the feed nozzle is on top o f the vessel, the inlet device is usually faced toward the dish end to make the most use o f the space available in the head cap section.

The diameter o f the inlet nozzle is a function o f the feed flow rate and the pressure. The criterion for nozzle sizing is that the momentum o f the feed should not exceed an allowable l imi t . The max imum allowable inlet momentum can be increased by fi t t ing inlet devices into the inlet nozzle. The fo l lowing table shows the allowable p V ' cri terion for different inlet devices which can be customized based on the inlet device vendor recommendation.

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Table 4 - Inlet Nozzle Sizing Criteria

Inlet device No inlet device

Deflector baffle

Ha l f open pipe

Mul t i -vane distributor

Cyclone (conventional)

Mul t i -cyclone

Pn>V„,\s (pa) 1,000 1,400 2,100 8,000 1,0000 35,000

Where

is the mean density o f the mixture in the feed pipe in k g / m ' Vn, is the velocity o f the mixture in the inlet nozzle in m/s.

Where the inlet feed is practically gas phase, pV" can be increased to 8000 Pa regardless o f the type o f inlet device. Erosional and 80% o f sonic veloci ty are other l imitations that should be met for inlet nozzle design.

Contact

Please visit www.linked]n.com/uroups/Chem\vork-3822450 should you have any comments, questions or feedback or feel free to contact S.Rahimi(iijgmail.com.

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APl-521 Flare K O D Design and Even More (Part 1)

Saeid R. Mofrad

03-Jan-2014

Introduction

A P I 521 provides tlic principles o f flare Knock out Drum ( K O D ) sizing, some guidelines on selecting the type and orientation, number o f inlet/outlet nozzles, internals and the basis for sizing the gas and l iquid sections o f the K O D . It also provides the sizing procedure and sample calculation for a typical horizontal K O D .

Though K O D sizing seems to be a simple task, the problem usually rises due to number o f emergency eases for wh ich the vessel needs to be sized, the variation o f sizing parameters such as l iquid and gas f low rates, densities and viscosities and K O D pressure and temperature at different emergency cases. Furthermore, the interpretation o f design companies from A P I requirement is different. Specific or additional requirements are sometimes imposed by the Client. For instance, 1 faced some problems recently when in the detail engineering o f a project I tried to set (f i t ) different l iquid levels inside a horizontal flare K O D already sized in the F E E D stage based on A P I standard but not taking the required process control , alarms and trips into account.

This note intends to discuss the A P I requirements in detail, review some o f the debates about flare K O D sizing parameters and show how to meet the API guidelines as wel l as the actual project and process needs. H o w to specify the K O D sizing cases along wi th a stepwise sizing procedure and a case study w i l l be discussed in the part 2 o f this paper.

A P I Requirements ''^^n^h

The fo l lowing section outlines the main A P I guidelines for the flare K O D sizing and selection:

• The economics o f drum design can influence the choice between a horizontal and a vertical drum. I f a large l iquid storage capacity is desired and the vapour flow is high, a horizontal drum is often more economical. Also, the pressure drop across horizontal drums is generally the lowest o f all the designs. Vertical knockout drums are typical ly used i f the l iquid load is low or l imi ted plot space is available. They are wel l suited for incorporating into the base o f the flare stack.

• Al though horizontal K O D s are available in many configurations, the differences are mainly in how the path o f the vapour is directed. The various configurations include the fo l lowing:

1. the vapour entering one end o f the vessel and exi t ing at the top o f the opposite end 2. the vapour entering at each end on the horizontal axis and a centre outlet (split entry) 3. the vapour entering in the centre and exit ing at each end on the horizontal axis (split exit)

• Configurations 2 and 3 can be used to reduce the drum diameter (but increase the length) for large flow rates and should be considered i f the vessel diameter exceeds 3.66 m (12 ft). Careful consideration should be given to the hydraulics o f split entry configuration to ensure the flow is indeed split in the desired proportion.

• The dropout veloci ty in the d n i m may be based on that necessaiy to separate l iquid droplets from 300nm to 600nm in diameter.

• The effect o f any l iqu id contained in the drum on reducing the volume available for vapour/ l iquid disengagement should be considered in K O D sizing. This l iquid may result from:

a) Condensate that separates during a vapour release, or b) L i q u i d streams that accompany a vapour release.

The volume occupied by the l iqu id should be based on a release that lasts 20 to 30 minutes. Larger hold­up volume may be required i f it takes longer to stop the flow. A n y accumulation o f l iquid retained from a prior release should be added to the l iquid indicated in items a) and b) above to determine the available vapour disengaging space.

vapour and liquid releases

^ minimum vapour space for dropout velocity

liquid hoid-up from safery relief valves and other emergency releases

slop and drain liquid

pumpout

Figure 1 -Flare Knock out Drum Sizing Basis as per APl-521

• The max imum vapour release case does not necessarily coincide w i th the max imum l iqu id . Therefore, the K.OD size should be determined t l i rough consideration o f both the max imum vapour release case as wel l as the release case wi th the max imum amount o f l iqu id .

The overall size o f flare K O D can be specified using the gas droplet settling velocity and l iquid hold up time mentioned above. For a vertical vessel, this is quite straight forward as the gas f low rate determines the vessel diameter and l iquid hold up time is used to calculate the max imum l iquid fill and finally vessel length. However, for horizontal vessel this needs to be done through a trail-and-error calculation.

L i q u i d H o l d - u p T i m e

In theory, the l iqu id holdup time o f flare K O D should be higher than the summation o f the t ime required for the operator to take action and the action to become completely effective. A properly designed K O D should provide enough time for operator to respond and for the pi'ocess system to react in the favorable direction to cease the re l i e f (i.e. re l ief valve to close).

• A commonly accepted t ime range for the operator action is between 10 to 30 minutes, depending on many parameters including operator experience, the complexi ty and extent o f automation and instrumentation o f the plant. For example, i f the process configuration is pretty simple or few sources can release the l iqu id to the flare system, i t . w i l l be relatively easy for operator to diagnose the problem, analyse the situation and take proper action. Furthermore, i f the corrective action can be executed from the control room rather than site, it w i l l minimize the response time.

• H o w fast the response becomes ful ly effective depends on the process dynamics. For example, the effect o f opening an inadvertently closed S D V at the discharge o f a reciprocating pump is sensed by pump re l ie f valve in a relatively short time span i f the blocked valve is near the pump. However, it w i l l take several minutes for the discharge pressure to fall and rel ief valve to start closing i f the S D V is at the other end o f a long pipeline. Keep in mind that the rel ief valve won ' t be fully closed unless the pressure falls wel l below the rel ief valve set pressure ( typical ly 9 3 % o f the set pressure for conventional re l ief valve).

M i s c e l l a n e o u s V o l u m e

APl-521 defines the miscellaneous volume as any accumulation o f l iquid retained from a prior release (from pressure re l ief devices or other sources). Then it uses the mysterious value o f 1.89 m3 (500 US gal) o f storage for miscellaneous drainings in a sample calculation which has become sort o f standard value for many these days, regardless o f the actual plant condit ion.

The actual intention o f dedicating a part o f the K O D volume to the miscellaneous liquids becomes clear when we understand few points:

• Flare K O D is sometimes used as the drain drum in onshore plants and often in offshore applications. Therefore, most probably small volumes o f l iquid during normal operation and larger ones during plant overhaul and maintenance are drained into the K O D .

• The number o f streams connected to flare are such versatile (control valves, instrument devices' drains and vents, maintenance vents, sampling points, analyzers, pump and compressor seal vents, etc) that it is almost impossible to guarantee that no l iquid w i l l be sent to the K O D during normal operation.

• Unless there is very strict plant housekeeping practices in place (to drain the K O D l iqu id content every so often), otherwise the l iqu id collected dur ing normal operation can go unnoticed t i l l the l iqu id level hits the H H L alarm level.

Therefore, in order to specify the miscellaneous volume, a detailed review o f the project drainage philosophy, the sources and amount o f l iquid/t \vo phase/condensable gas release to the flare net^vork and finally K O D during normal operation, the type o f l iquid handling system, its design and operating philosophy is required. The purpose o f this study should be to estimate a practical volume so that the frequency o f l iquid handling operation becomes reasonable dur ing normal operation.

L i q u i d H a n d l i n t ; S y s t e m

The fo l lowing section reviews the design o f various l iquid handing systems as they can affect the flare K O D sizing. The l iquid collected in the K O D can be evacuated in one o f the fo l lowing ways:

• Pump

The most common l iqu id handling system employs the pump to send the l iquid back to the process for repossessing, slops or product tanks, etc. Different instrumentation configurations have been used in past for K O D pumps:

1. The simplest control configuration which is usually fol lowed for all rotating machines is to provide a local manual start and stop for the K O D pumps so that site operator does the routine checks to ensure that everything is alright before putt ing the pump in service. In this configuration, it is only possible to stop the pump form the

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control room. Using the pump wi th local start is in line wi th the tlare K O D design intent which assumes that K O D is large enough to accommodate the largest l iquid release for 20-30minutes wi thout any need for pump operation.

2. For this specific service, some companies prefer to have the start provision at control room in addition to above.

3. The pump is automatically started and stopped based on the l iquid level in the K O D . W i t h this configuration, the first pump starts at the level 1 call First Pump Start Level (FPSL corresponding to H L L - 1 in Table I ) which is s l ightly below H L L . I f the l iquid level keeps rising, the second pump starts at Second Pump Start Level (SPSL corresponding to F ILL-2 in Table 1) which is slightly higher than H L L . Both pumps are stopped at the level called Both Pumps Stop Level (BPSL corresponding to L L L - 1 in Table I ) which is sl ightly above L L L .

In all above configurations, i f control system or operator fails to stop the pump due to any reason, both pumps are tripped at L L L L through Emergency Shutdown system (ESD).

The pump capacity is usually selected to drain the l iquid content o f the K O D in a specified time ( typical ly two hours).

G r a v i t y

A connection from the bottom o f K O D to the closed drain system wi th a control valve operating w i t h i n a l iqu id band can be an ideal l iquid handling system i f there is an adequate dr iv ing force (basically differential head between K O D and destination vessel) for the l iqu id inside the K O D to flow. In this configuration, the drain control valve opens at a level slightly below H L L and closes sl ightly above L L L . A n d finally at L L L L , ESD system w i l l act to close the drain line shutdown valve in order to prevent the gas b low-by from K O D to the drain system.

F igure 2 - Gravi ty Drain - Siphon Type

The siphon drain (shown in F i g w e 2) is a type o f gravity drain where the l iquid level inside the K O D is maintained by the l iquid seal height out o f the vessel wi thout any need for control system. A shutdown valve acting at L L L L however is needed to prevent the gas blow-by. In a properly designed l iquid seal for flare K O D , the top o f l iqu id seal matches the desired l iqu id level in the vessel and the height (depth) o f seal is specified based on the 175% o f the drum's max imum operating pressure. This requirement w i l l restrict the application o f this configuration to very low pressure Hares; otherwise the depth o f l iquid seal w i l l be too high (most probably below grade) which outweighs the auto-draining advantage (s impl ic i ty) o f this configuration. ,-

Siphon Breaker

To Drain

Seal Height

Heater : > >

The heater/coil inside the K O D is usually used because o f two reasons; win te r iza t ion ' or l iquid vaporizer. The winterization is out o f the scope o f this article but the heater can be used as l iquid handling system to vaporize the collected l iquid inside the vessel to the stack when there is no system available for draining the l iqu id into, the l iquid is relatively light, and the long term fiaring is not an issue, and at remote locations (i.e. portable flare KODs) .

However, the effect o f exposing l iquid to the high temperature which can result in coke formation, polymerization, solid deposit, chemical reaction, etc. should be taken into account whi le selecting this method.

The heater can ut i l ize electricity or any hot fluid (stream, hot water or o i l ) as heating medium depending on the avai labi l i ty o f heating medium and the temperature required to vaporize the l iqu id . It is sized to vaporize the K O D content w i t h i n particular t ime which can be as long as 8 hours i f sizing for shorter duration (say 2 hours) considerably increases the heater size/ power consumption.

The heater is usually switched on by a gap controller at a level lower than H L L and switched o f f at a level higher than L L L . The heater is usually tripped at L L L L by ESD system. The importance o f t r ipping (he heater before l iquid falls below the top o f the heater is vi ta l for the electrical heater where the heater elements it can be damaged as a result o f overheating. The need for t r ipping (closing the heating medium S D V ) in all other types o f heater should be evaluated on case by case basis. Un l ike rotating machines, heaters can have start provision from control room which minimizes the

' keeping the l iquid content above a specific temperature to prevent it from becoming an undesirable fluid as a result o f cold weather or auto-refrigeration

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site intervention and subsequently improves the possibil i ty o f fast start on demand. However, they are usually sluggish systems compared to pump or even gravity drain systems as l iquid heating and vaporizing are relatively slow processes.

A K O D equipped wi th heater should have an additional provision such as truck out connection for evacuating the residual l iquid below L L L L .

Table 1 - Control and Shutdown Systems' Actions

Level Action through Pump Heater Gravi ty

H H L L Shutdown system L A H H - plant trip L A H H - plant trip L A H H - plant trip

H L L - 2 Control system Start the second pump N A N A

H L L Control system L A H L A H L A H

H L L - 1 Control system Start the first pump Svvitch on the heater Open the drain control valve

L L L - 1 Control system Stop both pump Switch o f f the heater Close the drain control valve

L L L Control system L A L L A L L A L

L L L L Shutdown system T r i p both pumps Tr ip the heater Close the drain S D V

Note: having separate settings for the control actions than the l iquid level alarms is highly recommended for monitor ing the l iquid level (especially during emergency conditions) and operator intervention because any uncontrolled/unpredicted l iquid level rise can risk the plant production ( i f there is a plant shutdown at H H L L ) .

It should be noted that irrespective o f l iqu id handling system type, the selected control configuration and the level o f automation, no credit is usually given to the amount o f the l iquid that can be drained during emergency conditions to reduce the size o f flare K O D because availabil i ty and functionality o f l iquid handling system can also be adversely affected at this condit ion. However, i f the draining the l iqu id is essential, the re l iabi l i ty o f power supply and the availabil i ty o f destination system to receive the l iquid especially during emergency condit ion (when SDVs are closed) should be ensured.

K O D I n t e r n a l s

The fo l lowing section reviews the possible internals for the flare K O D , related design considerations and their effects on the size o f K O D :

*^

• Inlet Device

In general, anything that can block the rel ief path is not acceptable on the flare system. In line wi th this, the K O D inlet nozzle is usually equipped wi th only simple and robust (properly designed or attached) inlet devices such delleclor baffle, 90° elbow or ha l f open pipe to direct the incoming stream towards the dish end and make the vessel length available for the l iquid droplets to settle. Refer to the paper "Three Phase Separators - Inlet Devices" for further details about different inlet devices.

There are some debates about the suitabil i ty o f deflector baffle/plate on the inlet nozzle but it is believed that the failure o f the deflector baftle/plate (which is a rare case anyway) w i l l not obstruct the f low path into and out o f the K O D . Some companies accept using the multi-vane distributor on the inlet nozzle w i th particular design, fabrication and installation considerations.

The difference between a K O D w i t h and without inlet device is that in the second one the effective length for l iquid droplet separation is the distance between the inlet and outlet nozzles centerlines. This can be considerably less than the K O D actual length considering Ihe large nozzles (which are quite common in this application) and the nozzle welding requirement (the distance between nozzle neck at tangent line weld lines). I n a K O D wi th the 90 elbows on both inlet and outlet nozzles the full vessel length is practically available for l iquid droplet separation. Therefore installing a suitable device on inlet and outlet nozzles can result in substantial reduction in K O D length. On the other hand, adding an inlet device can add some limitations on the max imum l iquid fill as discussed in the paper "Three Phase Separators-L i q u i d Levels",

• Ou t l e t Device

The choice o f outlet device is much more restricted than the inlet nozzle so that most o f companies don' t even accept the deflector baffle on the outlet nozzle as it can potentially block the flow path once it is dislodged. Therefore, the outlet nozzle is either wi thout any device or wi th 90° elbow (this is not possible in split ent iy and split exit designs).

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• G a s - L i q u i d Separation Device

Though separation o f l iqu id droplets in the range o f 300-600 microns through gravity settling is relatively easy, using any gas-liquid separation device such as mist eliminators to reduce the K O D size shall be avoided because o f the potential for blockage from scale or waxy deposits.

• L i q u i d - L i q u i d Separation Device

K O D s are usually treated as two phase separators, however, provisions can be made for water boot i f it is really cri t ical to separate the water from hydrocarbon before further processing. Considering the fact that there is an ample o f residence time in K O D for the gravity l iqu id- l iqu id separation, it is hard to believe that a l iqu id- l iqu id separation device such as plate pack or coalescing mat w i l l be ever needed in the K O D .

From K O D sizing viewpoint , using the boot for an absolutely dry K O D , in one hand, may enable the designer to accommodate the l iqu id levels up to the H L L w i t h i n a small volume o f the boot and free up the horizontal vessel ful l cross section area for the l iqu id droplet separation which w i l l reduce the vessel diameter and subsequently the total weight and cost o f the K O D . On the other hand, having boot (especially for the wet K O D s ) can increase the overall elevation o f K O D inlet nozzle, the entire plant pipe racks carrying the Hare headers and shoot up the plant capital cost.

L i q u i d L e v e l Set t ing

Apart from the l iqu id hold up t ime and l iquid droplet separation requirements which determine the overall size o f flare K O D , the fo l lowing considerations shall be taken into account in order to address operation and control aspects in K O D size:

• L o w - L o w L iqu id level ( L L L L ) is the m i n i m u m allowable l iquid level inside the K O D . This is the level at which ESD system stops the l iquid draining by tripping/closing the pumps/heater/drain shutdown valve. It is normally specified based on the pump N P S l l , heater submergence depth or the size o f l iqu id outlet nozzle (size/height o f vortex breaker). This varies from 150mm in gravity drain arrangement to lOOOmm in presence o f an electrical heater (exact value is set based on the heater bundle diameter).

• Low Liquid Level ( L L L ) is where the low level alarm is triggered to warn operator that l iquid level is still fall ing and there is a risk o f gas b low-by. L L L should be 1-2 minutes above L L L L based on the max imum draining rate, wi th m i n i m u m o f I OOmm.

• L L L - 1 is the level at which pumps/heater/drain control valve is stopped/switched off/closed by the control system. L L L -I level should be at least 100 mm above L L L .

• H L L - 1 is the level at which the first pump/heater/drain control valve is started/switched on/opened by the control system. H L L - 1 level should be at least 100 m m below H L L .

• H igh L i q u i d Level ( H L L ) is where high level alarm is triggered to warn operator that despite o f control system action at H L L - 1 the l iquid level is s t i l l r is ing and there is a risk o f plant trip. To accommodate control system actions ( L L L - 1 and H L L - 1 ) , H L L should be at least 300mm above L L L i f the l iquid is pumped out and 200mm above L L L in other arrangements. In order to meet the A P I requirement, the volume between L L L to H L L should be larger than Miscellaneous Volume too. .

• H L L - 2 is the level at wh ich the second pump is started by the control system to increase the l iquid discharge rate and prevent high-high level t r ip. H L L - 2 should be at least 100 m m above H L L . This level does not exist in K O D s wi th healer (vaporizer) or gravity draining.

• H igh -High L i q u i d Level ( H H L L ) is where the upstream process plant is usually tripped, especially i f tr ipping o f facilities is practical and the risks associated wi th l iquid carryover to the stack are high. It is usually al lowed to depressurize the plant when the K O D level is at H H L L . In line wi th APl -521 standard, the distance between H L L and H H L L is specified to accommodate the max imum incoming l iquid flow rate for 20-30 minutes. The area above H H L L , vapor space, is available for l iquid di'oplet separation.

Dead Volume

Although the representation o f K O D volumes in APl-521 (Figure 1) indicates that the entire bottom section o f K O D is available for holding the maintenance/miscellaneous drains, in reality part o f this volume which is below L L L - 1 w i l l remain always l iquid filled. I call this idle volume the Dead Volume hoping that the new term w i l l help the designer make distinction between miscellaneous and dead volumes and correctly specify the volume inside the vessel that can be dedicated to the drain system.

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Post E S D Volume

As mentioned above some companies have hnked the high-high hquid level in the Hare K O D with the entire plant shutdown. Then they realized that some o f the equipment (such as compressors or reactors) cannot be held in pressurized condit ion for a long time and needs to be depressurized after shutdown. Moreover, since depressuring is often associated w i th l iqu id condensation on the flare side, the need for new additional l iquid level emerged; Emergency High -High L iqu id Level ( E H H L ) ,

E H H L L is specified based on the total volume o f l iquid that can reach the t laie K O D after plant shutdown. The l iquid is mainly due to the condensation o f vapor released from a particular equipment or part o f the plant that cannot be maintained under pressure due to process, operation or safety' considerations.

releases tof

/ minimum vapour space for dropout velocity

• / Post ESD volume • liquid liold-up from safety relief valves

and other emergency releases (emergency volume)

\ slop and drain liquid imiscelianeous volume) dead volume

pumpout

Figure 3 - Flare Knock out Drum Sizing Basis

According to the philosophy established fo l lowing this design development:

• E H H L L is where the upstream plant depressurization is inhibited. In order to specify the level, further study needs to be conducted to identify the section o f plant that potentially requires to be depressurized after overall shutdown and calculate the amount o f l iqu id that enters the K O D (Post ESD volume). In case the distance between H H L L and E H H L L is less than lOOmm, 100 m m shall be used.

Figure 3 depicts a revised version o f A P I representation o f flare K O D showing both Dead and Post ESD volumes.

Contact / « ,

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