Deaerator Design Aspects and Troubleshooting PDF

1 VOLUME 9 NUMBER 37 MARCH 2023 WWW.IACPE.COM  2 In This Issue Volume 9 | Number 37 EDITOR SPECIAL FEATURES...

1 VOLUME 9 NUMBER 37 MARCH 2023 WWW.IACPE.COM  2 In This Issue Volume 9 | Number 37 EDITOR SPECIAL FEATURES Karl Kolmetz Deaerator Design Aspects and DIGITAL EDITOR 04 Troubleshooting Abdullah Al Bin Saad, Shauna Tysor Prashant Sankarankandath REFINING CONTRIBUTING AUTHOR Dr. Marcio Wagner da Silva Flash Steam and Steam 14 Condensates in Return Lines Jayanthi Vijay Sarathy PROCESS ENGINEERING CONTRIBUTING AUTHOR Jayanthi Vijay Sarathy Importance of Production Cost SAFETY CONTRIBUTING EDITOR 20 Models for Renewable Integration Studies in Developing countries Chris Palmisano Mousumi Guha CONTRIBUTING AUTHOR Ronald J. Cormier Burn is not the Only Way! Non- 26 Energetic Derivatives as Route to Add Value to the Crude Oil Dr. Marcio Wagner da Silva Rock Bottom View: 38 Isn’t Denial of Climate Change Really a Nod to Continued Inflation? Ron Cormier 3  4 Deaerator Design Aspects and Troubleshooting Abdullah Al Bin Saad, Prashant Sankarankandath BACKGROUND: Can steam be produced straight from raw wa- ter? is a possible query. Steam is one of the most useful and vital utili- ties in many chemical process industries. It is Of course, the response would be negative! the favored heating medium in process plants We all know that the source of the water af- for several reasons: fects its quality. Water typically contains the following contaminants, for example · High content of latent heat associated with condensation. · Sand and silt. · It is non-explosive. · Iron · Given that it is generated from water, it is · Copper cheap. · Silica Steam applications can be divided into the following categories: · Aluminum · Recoverable usage: · Calcium · Heating medium in heat exchangers to · Magnesium heat process fluid to desired tempera- · Hardness ture. · Total dissolved solids · To spin turbines to generate power or drive rotating equipment. · Suspended solids and organic material · Steam tracing to maintain the process · Dissolved gasses the temperature at desired temperature Some contaminants, such silicates, which or prevent freezing have a tendency to deposit on the steam tur- · Non-recoverable usage: bines' rotor blades, can lead to scaling. Such deposits will throw off the rotor's balance, · Steam purging to avoid plugging in which will create vibration, which will eventu- some services in relief valves ally harm seals and shaft bearings. Addition- · Steam stripping, such as in a distillation ally, additional pollutants like oxygen and car- column, to reduce the partial pressure bon dioxide, which are present, promote cor- of the hydrocarbons as per Dalton’s rosion. So generally, the raw water first needs Law. to have primary treatment through filters to remove sand and silt. Then, this water can be · Addition of steam to the relief valve out- sent to secondary treatment such as demin- let piping to improve dispersion and eralization or hot lime softening to remove reduce the flammability of certain dissolved solids. chemicals, such as ethylene oxide. · Steam injection in flare for smokeless operation. · Steam ejectors · Steam reforming to produce Syngas.  5 Here are some common treatment processes: · Reverse Osmosis (RO) · Softening. · Chemical precipitation · Deaeration Before steam is produced in the boiler, dis- solved gases (such as oxygen and carbon di- oxide) must also be eliminated. This process is termed ((deaeration)) and it happens in an in- tegral devise called a deaerator. (For a typical Steam plant, see Figure 1) The second scientific concept that explain deaeration further is the relationship between gas solubility and temperature. Simply, gas solubility in a solution decreases as the tem- perature of the solution increases and ap- proaches saturation temperature. (See Figure 3) This article will focus on deaeration principles, design aspects of deaerator system and com- mon deaerator malfunction. DEAERATION PRINCIPLE Industrial steam boilers are a huge investment and are considered as long lead items. As such, it is required to extend their life and keep their efficiency for as long as possible. The ex- istence of oxygen in the boiler system can be a substantial problem due to its corrosivity at high temperature. Also, carbon dioxide which Figure 3- Solubility of oxygen in water (from is produced from dissolved solids could lead to Reference #3) serious corrosion into downstream heat ex- A deaerator uses both concepts to take away changers. That happens as carbon dioxide dis- dissolved oxygen, carbon dioxide and other solves in condensed steam and produces for- non-condensable gases from the boiler feed mic acid. water. The feedwater is sprayed in thin films The purpose of the deaerator is to remove dis- though spray nozzles into a steam allowing it solved gases from boiler feed water oxygen to become quickly heated to saturation. and carbon dioxide). Deaeration is The pur- Spraying feedwater in thin films rises the sur- pose of the deaerator is to remove dissolved face area of the liquid in contact with the gases from boiler feed water oxygen and car- steam, which enhance the mass transfer and bon dioxide). Deaeration is built on two scien- results in more quick oxygen elimination and tific concepts. The first scientific concept can lower gas concentrations. The liberated gas- be defined by Henry’s Law. es are then vented from the deaeration sec- tion. 6  7 DESIGN ASPECTS OF DEAERATOR upward through the perforations. Combined action of spray valves & trays guarantees A typical deaerator consists of two sections, very high performance because of longer stripping section for deaerating and heating contact time between steam and water. and another section provides storage. Types Generally, Deaerators are classified into two types as following: Spray-Type A spray-type deaerator is classically a single horizontal vessel which consists of two sec- tions, deaeration section and a preheating section. The two sections are separated by a baffle. Low pressure steam passes into the deaerator through a sparger in the bottom of the deaerator. (See Figure 4) Figure 5- Tray-type deaerator (Taken from Stork Vendor) The steam strips the dissolved gas from the feedwater and leaves through the vent valve. (See figure #5) Spray Type Vs Tray Type Figure 4- Spray-type deaerator (Taken from Stork Vendor) The feedwater is sprayed in thin films over spray nozzles into the preheating section (1st section in above photo), where it is heated to saturated temperature to enable stripping out 1. Scaling will lead to operating issues in the dissolved gases in the subsequent deaera- tray type. tion section. 2. Tray-type copes well with load variation Then, the heated feedwater moves to the de- such as night, summer and with turbines aeration section where it is deaerated by the that have extensive range of load. Howev- steam rising from the bottom sparger. The er, some vendors guarantees higher turn- stripped-out gases of the water exit via the down in special spray type such as vent that is mounted on the top of deaerator. STORK Spray Deaerator. Tray-Type Deaerator Performance The typical tray-type deaerator has a vertical Deaerator shall be designed to reduce oxy- domed deaeration section, that may be hori- gen to 0.007ppm or less. Deaerators will zontal in some cases (i.e big deaerator), eliminate free carbon dioxide. Chemical scav- mounted above a horizontal feedwater hold-up enging of oxygen is still enormously signifi- storage vessel. cant despite the apparent low levels achieved Make-up feedwater enters the domed deaera- by physical means. tion section over spray valves above the perfo- rated trays and then flows downward through the holes in thetrays. Low-pressure steam joins below the perforated trays and flows  8 Steam Consumption: Please be noted that this vent stream is minor and its value rises with the selected operating Steam Consumption could be calculated by pressure and temperature of the deaerator means of applying basic form of the first law of (e.g. 50-200 kg/h at 105 C, 1000kg/h at 150 thermodynamics: C). However, Process engineers sometimes uses other approach to estimate vent steam same as rule of thumb: Equation can be simplified with following: 1- Ws = shaft work =0 (as no moving part in Deaerator system). 2- Kinetic and potential energy (∆EK,∆EP )=0 3- Q =0 as deaerator is insulated. Therefore, Equation will be as following: For simplicity in this example, vent steam is assumed as 200 kg/h. Boiler feed water and Steam flow rates shall be calculated. We have two equations (mass H: Specific enthalpy (KJ /Kg) balance and energy balance) and two un- knowns so degree of freedom is zero. M: Mass flow rate (Kg/h) Hence, this example can be solved with fol- Let’s take quick example to estimate Steam lowing equations: requirement for new deaerator with following design data: M1+M2+M3=M4+M5 (1) Operating Pressure/Temperature: 0.42barg /110℃ M1 X H1 +M2 X H2 +M3 X H3 =M4 X H4 +M5 X Deaerator Sketch H5 (2) M1, M2 and M3 are knowns from Deaerator Streams Table, so equation (1) would be: M5=689319 + M3 (3) Then, if we substitute equation (3) in in equa- tion (2), equation (2) would be: (4) Now, M3 and M5 can be calculated simply from equation (4). M3=25380.17 Kg/h Figure5- Deaerator Sketch M5=714699.17 Kg/h Deaerator Streams Table: M1, H1: Mass flowrate, Enthalpy of Make-up Water M2, H2: Mass flowrate, Enthalpy of recovered condensate M3, H3: Mass flowrate, Enthalpy of LP steam M4, H4: Mass flowrate, Enthalpy of vent steam M5, H5: Mass flowrate, Enthalpy of boiler feed Water TBC: To be calculated  9 CASE STUDY The results are summarized in below table: The intention of this case study is to show im- pact of recovered condensate temperature on low-pressure steam requirement. Before diving into case study, lets have quick overview of typical condensate system in chemical process industry (CPI). Recovered condensate usually represent 80% to 90% of total inlet stream to deaerator. Hence, Temperature have more impact on low -pressure steam requirement than make up water. Recovered condensate can be classified into two into following categories: Suspected Condensate: Condensate comes from process heat ex- changer where hydrocarbon can leak to con- The below plot may demonstrate results in densate system. Such condensate shall be better way than previous table: \ sent to condensate polishing unit where hydro- carbon is adsorbed on activated carbon. Clean Condensate: Condensate comes from condensing steam turbine that use water-cooled shell and tube heat exchanger to condense its exhaust steam. Cooling water leak to condensate system can increase silica that has bad impact on rotor blades of steam turbines as mentioned previ- ously. Such condensate shall be sent to con- densate polishing unit where cations /anions limits are restored by means of mixed bed col- umn. It is typically necessary to cool the condensate to nearby 50 °C to inhibit temperature degra- Figure 6- Steam requirements Vs Tempera- dation of the anion resins. ture of Condensate In such case, process engineer shall find a The below tables show steam requirement way to raise condensate temperature by reduction % based on temperature rise by 5 ° means of heat recovery such routing conden- C: sate stream through Condensate in waste heat boiler. Such heat recovery will reduce steam requirement, fuel gas and boiler feed water power consumption. Back to case study, HYSYS was utilized to realize steam requirement to deaerator at vari- ous temperatures.  10 If condensate is heated from 50 °C to 95 °C, malfunctions encountered in typical deaera- steam requirement will reduce by 58%, which tors and their possible causes will decrease fuel gas and pump power con- sumption accordingly. Malfunction Possible cause COMMON DEAERATOR MALFUNCTION Deaerators play a key role in ensuring a long and efficient equipment life. Mechanical de- aeration is the primary means of removing dis- High dissolved Inadequate venting solved gases from various condensate Oxygen in BFW Very high delta T be- streams and make-up water streams. tween deaerator tem- perature and BFW Some of the essential factors required for de- outlet aerators to work efficiently are as outlined be- low: Internal channeling or Temperature: damaged spray noz- zles Ensuring the entire water flow to the reaches the saturation temperature is vital to the per- Large variations in formance of the deaerator. Dissolved gases incoming water are released from water at the saturation tem- flowrates and temper- perature. atures Spraying/Mixing: Air leakage through Good spraying or turbulence increases the surface condenser for contact area and promotes the release of gas- turbine es. Pressure Heating steam control Venting: fluctuations valve hunting or incor- rectly sized Adequate venting capacity plays a major role in the deaerator performance. The released Excessive vibration Blocked internals or gases should be continuously vented from the of deaerator or damaged condenser system. tank tubes Stable operating conditions: Low Outlet Incorrect Thermome- Temperature ter reading Wide range of fluctuations in feed water flow rate and temperatures can affect the deaerator Insufficient steam performance adversely. By creating, addition- flow al demands on steam control systems. The following table lists some of the common Spray valves or inter- nals malfunction PT. Dinamika Teknik Persada is an Engineering Consultants focused on providing engineering and technical services to the oil and gas industry. We develop innovative and cost effective solu- tions and helping our clients to achieve high per- formance from their assets by providing exper- tise, novel methods and appropriate tools -FEED to Detailed engineering PT Dinamika Teknik Design Persada provide Engi- -Independent Design Verifica- neering Design to the tion upstream and down- -Risk Assessments -Asset Integrity Management stream sectors of oil & -Risk Based Inspection gas industry: -Reliability Centered Mainte- nance - Processing plants - Pressure vessels Address : Ruko Golden Boulevard Blok K No. 1-2 - Heat exchangers - Piping systems Jl. Pahlawan Seribu, BSD City, Serpong - Onshore pipelines Tangerang 15322 – Indonesia - Offshore pipelines Phone / Fax : +62 21 53150601 Email : [email protected]  11 STORK DEAERATOR ACKNOWLEDGMENT Last but not least, we would like to present a We are grateful to STORK Company, world- technical comparison between conventional leading designer and manufacturer specialty tray-type and STORK- Spray type. The below equipment (e.g. boilers, deaerators and burn- comparison table is handed by STORK ven- ers, along with complex pipe spools), for dor. providing some pictures and sharing some practical information about deaerators.  12 REFERENCES Lieberman, Norman P., and Elizabeth T. Lieberman. A Working Guide to Process Equipment: How Process Equipment Works. McGraw-Hill, 1997. GPSA Engineering Data Book. Gas Proces- sors Assoc, 2013. Patel, M. M.. The Handbook Of Chemical Pro- cess Engineering, 2017. “Deaerator Troubleshooting.” Kansas City De- aerator, 24 Apr. 2019, deaerator.com/technical -papers/deaerator-troubleshooting/. AUTHORS Prashant Sankarankandath holds a bachelor’s degree in chemical engineering from Universi- ty of Calicut, India. His professional experi- ence of Over 17 years covers plant operations, process engineering, process control, feasibil- ity study, design as well commissioning Abdullah Hussain holds a bachelor’s degree in chemical engineering from King Fahd universi- ty of petroleum & minerals with honor degree. His professional experience of Over 9 years covers various design stages such as feasibil- ity study, pre-feed for grass root project, front- end engineering, detail design as well commis- sioning 13  14 Flash Steam and Steam Condensates in Return Lines Jayanthi Vijay Sarathy In power plants, boiler feed water is subjected conditions, with time, flash steam in the to heat thereby producing steam which acts as lines condense leaving behind conden- a motive force for a steam turbine. The steam sates due to natural cooling. upon doing work loses energy to form conden- 5. Condensate return line design must also sate and is recycled/returned back to reduce consider the effects of water hammering. the required make up boiler feed water (BFW). When multiple steam return lines are con- Recycling steam condensate poses its own nected to a header pipe that is routed to a challenges. Flash Steam is defined as steam flash drum, flash steam in the presence of generated from steam condensate due to a cooler liquid from other streams would drop in pressure. When high pressure and condense rapidly to cause a water ham- temperature condensate passes through pro- mer. cess elements such as steam traps or pressure Fraction of Flash Steam reducing valves to lose pressure, the conden- Taking an example case, condensate flows sate flashes to form steam. Greater the drop in across a control valve from an upstream pressure, greater is the flash steam generated. pressure of 5 bara to 2 bara downstream. This results in a two phase flow in the conden- The saturation temperature at 5 bara is sate return lines. 151.84 0C & 120.20C at 2 bara. The specific General Notes volume of water at 5 bara is 0.001093 m3/kg 1. To size condensate return lines, the prima- & 0.00106 m3/kg at 2 bara. The latent heat of ry input data required to be estimated is A. saturated steam upon reaching 2 bara is Fraction of Flash Steam and condensate, 2201.56 kJ/kg. The % flash steam generated B. Flow Rates of Flash Steam & conden- is estimated as, sate, C. Specific volume of flash steam & condensates, D. Velocity limits across the condensate return lines. 2. Sizing condensate return lines also require lower velocity limits for wet steam since liq- Where, uid droplets at higher velocities cause inter- nal erosion in pipes and excessive piping hf,1 = Upstream specific enthalpy [kJ/kg] vibration. A rule of thumb, for saturated wet hf,2 = Downstream specific enthalpy [kJ/kg] steam is 25 – 40 m/s for short lines of the order of a few tens of metres and 15 - 20 hf,g = Latent Heat of Saturated Steam [kJ/kg] m/s for longer lines of the order of a few The upstream specific enthalpy, hf1 of saturat- hundred metres. ed water at 5 bara is 640.185 kJ/kg and hf2 of 3. Condensate return lines work on the princi- 504.684 kJ/kg at 2 bara. The steam specific ple of gravity draining. To effectuate this, volume at 2 bara is 0.8858 m3/kg. drain lines are to be sloped downward at a ratio of atleast 1:100. The fraction of flash steam is calculated as, 4. Proper sizing of stem condensate return lines requires consideration of all operating scenarios, chiefly start up, shutdown and (2) during normal running conditions. During Therefore the condensate fraction is, plant start up, steam is not generated in- stantly. As a result, the condensate lines (3) would be filled with liquids which gradually turn two-phase until reaching normal run- ning conditions. During shutdown  15 The steam volume is calculated as, The homogenous model for gravitational pressure drop is applicable for large drop in (4) pressures and mass velocities < 2000 kg/ The condensate volume is calculated as, m2.s, such that sufficient turbulence exists to cause both phases to mix properly and en- sure the slip ratio (uv/uL) between the vapour (5) and liquid phase is ~1.0. For more precise estimates capturing slip ratios and varying Condensate Return Pipe Sizing void fraction, correlations such as Friedal To size the condensate return line, the bulk (1979), Chisholm (1973) or Muller- properties and mixture properties can be used Steinhagen & Heck (1986) can be used. to estimate the pipe size. It must be remem- bered that as the two-phase mixture travels The total pressure drop is the sum of the stat- through the pipe, there is a pressure profile ic head, frictional pressure drop & pressure that causes the flash % to change along the drop due to momentum pressure gradient. pipe length. Additionally due to the pipe incli- nation, a certain amount of static head is add- (9) ed to the total pressure drop. To estimate the pipe pressure drop across the The Static Head [DPstatic] is computed as, pipe length, a homogenous model for model- ling the two phase pressure drop can be adopted. The homogenous mixture acts as a (10) pseudo-fluid, that obeys conventional design Where, based on single phase fluids characterized by the fluid’s average properties. H = Pipe Elevation [m] The mixture properties can be estimated as, q = Pipe inclination w.r.t horizontal [degrees] The pressure drop due to momentum pres- (6) sure gradient [DPmom] is, Where, rL = Condensate Density [kg/m3] (11) 3 rv = Steam Density [kg/m ] If the vapour fraction remains constant across the piping, the pressure drop due to momen- eh = Homogenous void fraction for a given tum pressure gradient is negligible. steam quality [x] [-] The frictional pressure drop is calculated as, The homogenous void fraction [eh] for a given steam quality [x] can be estimated as, (12) Where, DP = Pressure drop [bar] (7) f =Darcy Friction Factor [-] The dynamic viscosity for calculating the L = Pipe Length [m] Reynolds number can be chosen as the vis- rh = Mixture Density [kg/m3] cosity of the liquid phase or a quality averaged viscosity, µh. V = Bulk fluid Velocity [m/s] D = Pipe Inner Diameter, ID [m] (8)  16 suitable accuracy for almost all industrial ap- plications will be achieved in less than 10 iter- (13) ations. For Reynolds number up greater than Where, µh = Dynamic Viscosity [kg.m/s] ~4000, rh = Homogenous Density [kg/m3] (18) The Darcy Friction Factor [f] depends on the Reynolds number follows the following criteria, Homogenous Property Calculations The two phase mixture flows through the con- If Re