a study of effect of air in-leakage on performance of steam surface

International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, http://iraj.in

Volume- 5, Issue-8, Aug.-2017

A STUDY OF EFFECT OF AIR IN-LEAKAGE ON PERFORMANCE OF STEAM SURFACE CONDENSER 1

SWAPNIL M.MULE, 2S. H. KULKARNI, 3NITIN P. GULHANE

1

Student, 2, 3Associate Professor, Mechanical Engineering Department, Veermata Jijabai Technological Institute, Mumbai, India

Abstract - A steam surface condenser is a specially designed heat exchanger of the shell and tube type which receives steam from the exhaust of a low-pressure turbine; this steam is condensed to a liquid by removing the predominately latent heat of vaporization through condenser tubes in which cooling water is circulating. Steam surface condensers are commonly used in thermal power plants, refineries, chemical processing plants, Fertilizer, petrochemical, food processing applications. In response to the problem occurred about air in-leakage in steam surface condensers while designing and maintenance stages, we carried out the detailed study emphasizing on sources of air in-leakage, impact air in-leakage on condenser back pressure, shell side resistance heat to transfer and overall heat transfer coefficient of condenser unit and is provided in the present work. The paper describes method adopted to establish numerical relationship between these parameters and air ingress inside steam surface condensers. This document attempts to selectively provide the information on condenser performance enhancement. Keywords - Overall Heat Transfer Coefficient, Air In-leakage, Hydraulic Pressure Drop, HEI Standards, Suction Pressure, Vacuum Pump, Non-condensable.

NOMENCLATURE

I. INTRODUCTION

T = Temperature of Condenser Shell (Deg.C) Ta= Temperature Air present in Condenser (Deg.C) Ts = Steam Temperature in Condenser (Deg.C) ACFM = Actual Cubic Feet per Minute SCFM = Standard Cubic Feet per Minute M = Mass Flow Rate of Vacuum Pump (Kg/s) M = Mass Flow Rate of Steam (Kg/s) M = Mass Flow Rate of Air (Kg/s) V = Volume Flow Rate of Vacuum Pump (m3/s) V = Volume Flow Rate of Steam (m3/s) V = Volume Flow Rate of Air (m3/s) V = Volume of Condenser unit(m3) P = Total Condenser Back Pressure (Pa) P = Partial Pressure of Steam (Pa) P = Partial Pressure of Air (Pa) R = General Gas Constant for Steam (J /kg K) R = General Gas Constant for Air (J /kg K) A = Air In-Leakage inside Condenser (SCFM) ∆A = Increase inAir In-Leakage in Condenser (SCFM) P = Condenser Back Pressure (mBar) ∆P = Increase in Condenser Back Pressure (mBar) P = Condenser Back Pressure before Air Introduction Test (mBar) A = Air In-Leakage inside Condenser before Air Introduction Test (mBar) Rm = Tube Wall Resistance (hr-ft²-°F/BTU) Rt = Tube Side Resistance (hr-ft²-°F/BTU) Rs = Shell-Side Resistance (hr-ft²-°F/BTU) Rf = Fouling Resistance (hr-ft²-°F/BTU) U* = Overall Heat Transfer Coefficient Adjusted To Design-Reference Condition Udesign = Overall Heat Transfer Coefficient at Design Condition

For maximum thermal efficiency, corresponding to a minimum back pressure, a vacuum is maintained in the condenser. However, this vacuum encourages air in-leakage. Thus, to keep the concentration of noncondensable gases as low as possible, the condenser system must be leak tight, together with any part of the condensate system that is under vacuum. Failure to prevent or remove the non-condensable gases may cause serious corrosion in the system, lower heat transfer properties, and/or increase plant heat rate due to the back pressure rise associated with a high inleakage. The cost of excess back pressure in terms of additional fuel or increased heat rate is discussed by Harpster et al. A very important part of condenser monitoring is the ability to determine the effectiveness of the air removal capabilities of the condenser and the amount of air leakage that the condenser may be experiencing. Because, a decrease in condenser performance is not only due to fouling but also due to either air ingress and/or the inability of the air removal system to maintain the concentration of noncondensable in the shell side of the condenser at an acceptable level. In 1963, Professor R. S. Silver[11] published a stimulating paper dealing With the general theory of surface condensers, wherein it was stated that, “It is Well known to all operators and designers of condensing plants that the presence of a small proportion of air in the vapor can reduce the heat transfer performance in a marked manner.” In a recent publication on the effects of air ingress, Richard Putman stated, “The presence of even small amounts of air or other non-condensable in the shell

A Study of Effect of Air In-Leakage on Performance of Steam Surface Condenser 56



space can cause a significant reduction in the effective heat transfer coefficient.”[2] Thus, an adequate air-removal and monitoring system is essential.

condenser circulating water inlet temperatures. It also permits maintenance to be conducted without taking the unit out of service. See Figure 2 for an illustration of a liquid ring vacuum pump.

AIR REMOVAL EQUIPMENTS: 1. Steam Jet Air Ejectors 2. Vacuum Pumps

II. DESIGN OF VACUUM CONDENSING OPERATION

PUMP

FOR

Sub cooling / Condensate Temperature Depression: Condensate depression is the difference between the condensing steam temperature and the temperature of the condensate in the hot well. Condensate falling from the upper portion of the condenser tube bundle onto the tubes below tends to become sub cooled below the condenser saturation temperature due to longer tube resistance time.The sub-cooling in vent area is very important when determining the suction capability. Larger the air leakage into the condenser or the smaller the throughput of the vacuum pump, the higher the air partial pressure and larger will be the under cooling. We have derived following methodology to determine Vacuum Pump Capacity.

Fig.1. Removal of Non-Condensable Gases

Figure 1 shows how non-condensable gas flows within and out of a condenser. Non-condensable gas has a tendency to flow to the coldest area. This area is typically the circulating water inlet region of the condenser. This tendency occurs because the partial pressure of the condensing steam is lowest in the cold region. Having the air outlet at the circulating water inlet might not be possible with all condenser bundle designs. Steam jet air ejectors and/or vacuum pumps establish a vacuum in the condenser before start-up and pull non-condensable gas with some steam from the condenser during operation. We have selected vacuum pump to remove noncondensate.

According to Dalton’s Law, total pressure is the sum of partial pressure of gases (water vapour and inert gas) existing in a given space.” Each gas expands as if other gas is not present. Each gas of gas mixture would remain at its partial pressure if it was to fill the whole volume by itself. ie. Ptotal = Ps + Pa The general gas equation for a mixture of 2 gases: steam and air is – (P + P ) × V = ( m R + m R ) × T ……………… (1) The specific gas equations for individual gases: steam and air are – PV = m R T And ……………… (2) PV = m R T ………………. (3) Here T = T = T ,V = V =V

Vacuum Pump

Divide Equation (2) by (3) m R = m R R = 287.1 J/kg.K R = 461.5 J/kg.K

Fig.2. Typical Liquid Ring Vacuum Pump

Liquid ring vacuum pumps are the most common form of mechanical pump used in air-removal systems for steam surface condensers. The liquid ring vacuum pump is a rotary, positive displacement pump using a liquid as the principal element in gas compression. It is not unusual for more than one liquid ring vacuum pump system connected in series.This allows the air-removal capacity to be adjusted, especially during low load operation or low

×

P P

287.1 P × 461.5 P P m = m × 0.6221 × P P m = m × 0.6221 × P − P ………………… (4) The Vacuum Pump has to draw off: ∴

m = m ×



Mass Flow of Mixture in Kg/s: m = m = m + m …………………. (5) Volume Flow of Mixture in m3/s: V

= V V

= m

= (m R

R

+ mR

)

(L&T) Ltd. the PERFORMANCE GUARENTEE TESTING (PG Test) was carried out. The Plant Data of PG Test on Condenser was monitored, which is studied by us in developing relationship between various mechanical and thermal parameters as case study. The data of each minute of PG Test is captured and have been used in calculations. Artificial Air in-leakage Test: This test is conducted by artificially introducing external air near different tube bundle locations and measures the quantity of air evacuated by different vacuum pumps. This method reflects the condenser’s response to air–in leakage. Artificial Air In-Leak Test: Source Location Throughout the Performance Guarantee Test, Cooling Water Inlet Temp., Mass Flow Rate of Cooling Water, Mass Flow Rate of Steam was kept constant intentionally. The air isintroduced inside the condenser.

T P T

P ………………….. (6) With Dalton’s Law: V = V = V Hence, Volume Flow Rate of Vacuum Pump: T V = m R P T V = mR P ………………….. (7) III. IMPACT OF AIR IN-LEAKAGE CONDENSER PERFORMANCE


ON

We have derived capacity of vacuum pump based on design conditions. In that, we have kept total pressure of condenser shell as constant. But in actual practice, the total pressure of condenser increases due to successive air in-leak over the period of time, thus making vacuum pump incapable to remove all the air leaked inside. In this case partial pressure of steam remains constant as steam flow rate is not changed. But partial pressure of air increases as quantity of air inside the condenser increases. This increases total shell pressure. For detection of amount of air leak, manufacturer has to perform whole Air In-Leakage Test whose cost is very huge. So, we have derived relationships to detect rise in air in-leakage, simply by knowing rise in Shell Pressure of Steam Surface Condenser. Some air will always be present in the condenser shell operating under vacuum and is allowed or in the design. The principle source of the air present is due to in-leakage through openings that develop in joints and components of the condenser system operating under vacuum allowing ambient air to enter. Air causes the shell side ﬁlm heat transfer co efficient to decrease. Silver [5] noted that as little as 1% of air could have a very substantial effect even though the steam pressure is still 99% of the total pressure under these conditions. This is phenomenon called Air Binding or Blanketing. Air gets coated around the outer region of cooling water tubes and restricts the heat transfer to condensing steam. We studied Power Plant Performance Guarantee Testing, where artificial air was introduced inside condenser. We studied all the recorded parameters and their behavior with air addition. 3.1Case study: Test Plant Condenser Performance Guarantee Test: One of the power plant equipment- Condenser was designed and manufactured by Larsen and Toubro

Fig.3. Artificial Air Ingress Source Locations in Condenser Shells: A and B

Artificial Air In-Leak Test: Plan The Artificial Air-Introduction Test took place between 4:00PM To 6:20PM. For the system to become steady and avoid error due to instability, we have considered the Plant Data from 3:40Pm To 6:40PM. Condenser is single pressure dual shell rectangular steam surface condenser with equalizing duct. 3.2 Effect of Air Ingress on Heat Transfer: For maximum thermal efficiency, corresponding to a minimum back pressure, a vacuum is maintained in the condenser. However, this vacuum encourages air in-leakage. Thus, to keep the concentration of noncondensable gases as low as possible, the condenser system must be leak tight, together with any part of the condensate system that is under vacuum. Failure to prevent or remove the non-condensable gases may cause serious corrosion in the system, lower heat transfer properties, and/or increase plant heat rate due to the back pressure rise associated with a high in-



leakage. The cost of excess back pressure in terms of additional fuel or increased heat rate is considerable. Thus, an adequate air-removal and monitoring system is essential. Most new condensers are now guaranteed to reduce oxygen levels to below 7.0 ppb at full load. However, air in-leakage can occur in any part of a condenser system that is operating at sub-atmospheric pressures so achieving and maintaining these guaranteed figures largely depends on proper equipment maintenance. Condensers are provided with air-removal equipment to evacuate the vapor/air mixture that accumulates in the area of those tube bundles termed the air-removal section. Presence of even small amounts of air or other non-condensable gases in the shell space can cause a significant reduction in the effective heat transfer coefficient. It should also be noted that a reduction in the Performance of Condenser can be due to either fouling or air ingress, but it is difficult to quantify how much each of these two effects is contributing individually to an observed change in heat transfer without examining other operating factors. In our Project, we established relationship between 1.% Rise in Shell Pressure of Condenser (Pcond) 2.% Rise in Shell Side Resistance to Heat Transfer (Rshell-side) 3.% Reduction in Overall Heat Transfer Coefficient (Uactual)


average of the values of Plant Pressure Shell A&B (mBar) and Air In-Leak in Shell A&B (SCFM).

Plant Data Prior to AIL Test Now, to take a reference value of Total Pressure and Air In-leakage as datum, we studied the Plant Data prior to the test. We studied data of half an hour from 3:00 PM To 3:30 PM, 18th January 2016, when no artificial air was introduced. We calculated average Plant Shell Pressure and average Air In-leakage. Pdatum = 62.01 mBar Adatum = 43.03 SCFM

3.3 Impact of Air Ingress on Condenser Shell Pressure: From the graph, we can say that, as the air is introduced in Shell B, the rise in SCFM in shell B is clearly seen while the SCFM in Shell A is almost constant. The Condenser Shells are connected to each other by Equalizing Duct. Equalizing Duct neutralizes the differential pressure between the shells. Hence, although air is introduced in Shell B only, Rise in Pressure in both Shells is observed. So, we combine the data of Air Ingress in Shell A & B to study its effect on Total Pressure Increase (considering both the shells). Total Shell Pressure (mBar) and Air In-leakage (SCFM) The Air Introduction is carried out in 5 successive steps. With each step, sufficient time is provided to the system to become stable. We can observe the 5 steps of Air Ingress along with rise in Shell Pressure in graph. For establishing the relationship between Air Ingress and Pressure Rise, we have considered 5 Time Slots where the system behavior was stable and we neglected the transition period having steep slope between successive steps of Air Introduction. For every step, Air In-leakage is kept constant .From these graphs, we can say that, for every step, Pressure corresponding to fixed air In-leakage is remains constant(as slope of lines is near to zero, lines are almost horizontal). So, we can assume a constant pressure for step of air ingress. Thus we calculated

Table 1:Back Pressure and Air In-leakage

We calculated differential Pressure and differential Air In-leakage and % Rise in Pressure due to air ingress. We plot the graph of % Rise in Pressure v/s %Increase in Air In-leakage Rate. % Rise in Pressure v/s % Increase Air In-leakage




Where P is in mBar and A is % increase in SCFM Air In-leak. Hence, with this above formulation, by knowing the values of increase in pressure, we can calculate the Amount of Air In-leakage taking place in the condenser provided that reference values of pressure and air ingress are known. 3.4 Impact of Air Ingress on Shell Side Resistance to Heat Transfer The shell side condensing heat transfer is the most complex component in the evaluation of steam surface condenser. On the similar basis, we calculated different types of resistances to Heat Transfer and studied their behavior with respect to increase in air ingress.Based on ASME Performance Test Codes 12.0, calculations of thermal resistances is done for every minute data of Artificial Air Introduction Test. The thermal resistance to heat transfer consists of four major components: i. Tube wall resistance ii. Tube side resistance iii. Shell-side resistance iv. Fouling Resistance

Table 2:Shell Side Resistance Rs and Air In-leakage Rate

% Rise in Shell Side Resistance Rs v/s %Increase in Air In-leakage Rate: From the graph, we can say that %Increase in Rs is function of % Rise in Air In-leakage. The relationship can be given as:

The Air Introduction is carried out in 5 successive steps. With each step, sufficient time is provided to the system to become stable. We can observe the 5 steps of Air Ingress along with rise in Shell Pressure.When air is introduced, resistances values increases. Thus, we obtained 5 points of 5 steps, one for each, representing air ingress and corresponding thermal resistance. With these 5 points, we generate a graph of thermal resistances v/s air ingress. We draw a curve passing though these points and found the equation of that curve using regression equations joining maximum of 5 points. The graphs show that shell side resistance to heat transfer has the major deflection with air introduction as compared to other 3 resistances to heat transfer.The maximum rise in resistance value of Tube Wall Resistance, Tube Side Resistance and Fouling Resistance is less than 12% of rise in Shell Side resistance. Resistances Rm, Rt and Rf are almost constant throughout the air addition process.Hence we can neglect the other 3 resistances and focus on Shell side resistance alone, as shell side resistance changes drastically with air introduction. We studied the behavior of Shell Side Resistance with respect to Air in-leakage rate.

Where Rs is in hr-ft²-°F/BTU and A is % increase in SCFM Air In-leak. 3.5 Impact of Air Ingress on Overall Heat Transfer Coefficient: Similarly, we evaluated impact of air ingress and corresponding Overall Heat Transfer Coefficients of every minute of Plant Data. . Following is the data of calculation of U-coefficient as per ASME PTC 12.2

Plant Data Prior to AIL Test Rsdatum = 0.000691 hr-ft²-°F/BTU



provided that Values are corrected to Design Reference Conditions. When air is artificially introduced, Overall Heat Transfer Coefficient value decreases. Thus, we obtained 5 points of 5 steps, one for each, representing air ingress and corresponding Ucoefficient. With these 5 points, we generate a graph of Overall Heat Transfer Coefficient v/s air ingress. We draw a curve passing though these points and found the equation of that curve using regression equations joining maximum of 5 points.


IV. RESULTS AND DISCUSSION As the Air Ingress ↑, Condenser Pressure ↑ Linearly, Rs ↑, U*↓ .By knowing the values of increase in pressure, we can calculate the Amount of Air Inleakage taking place in the condenser provided that reference values of pressure and air ingress are known. With these calculations, we can define whether installed vacuum pump is sufficient enough to remove excess air or not. If not, we recommend installation of new Vacuum Pump in addition to old one. Capacity of new vacuum pump can be calculated as discussed before.

We also studied Pre-test plant data to calculate Heat Transfer Coefficient Prior to Test: Udatum = 629.85 Btu/hr.ft².°F. The same data as earlier mentioned i.e. Half an hour before the test was studied.

For a given MW load, possible causes for the condenser back pressure to rise are: a) An increase in circulating water inlet temperature b) A reduction in circulating water flow c) Fouling of the inside or outside surfaces of the condenser tubes d) An increase in concentration of non-condensable including air in-leakage in the shell side of the condenser. Throughout the artificial air in-leakage testing, circulating water inlet temperature and circulating water flow are kept constant. In our experimental analysis, when air in-leakage rate is constant, shell pressure remains constant. This proves that other factors apart from air in-leak (point d) such as fouling (point c) do not influence pressure. Hence equation of pressure rise developed from experimental data is accurate as it considers only air ingress responsible for rise in pressure during test.

Table 3:Overall Heat Transfer Coefficient and Air In-leakage Rate

CONCLUSIONS a) Methodology is developed to determine vacuum pump capacity and amount of sub-cooling of condensate. b) Empirical Relationship is established between amounts of air ingress and condenser backpressure as well as between amounts of air ingress and condenser overall heat transfer coefficient. c) In actual practice, simple installation of real time pressure sensor inside condenser shell can give us idea about air ingress rate inside condenser provided that cooling water inlet temp. and cooling water flow rate are constant and fouling does not play any role. d) We can calculate increase in Shell Side Resistance and decrease in the value of Overall Heat Transfer Coefficient numerically by knowing value of Air In-leakage in SCFM and datum values of unknowns. Provided that all other parameters are constant. e) As calculating Shell Side Resistance is difficult task, our method of calculation provides

% Drop in Ratio of Overall Heat Transfer Coefficient Adjusted to Design-Reference Condition To Overall Heat Transfer Coefficient at Design Condition v/s % Increase in Air Inleakage Rate:

From the graph, we can say that U*/Udesign is function of %Rise in Air In-leakage. The relationship can be given as: U∗ = 2 × 10 Udesign ×

× (∆A/Adatum) − 0.0014 ∆A + 0.4814 Adatum


International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 5, Issue-8, Aug.-2017 http://iraj.in [5] Joseph W. C. Harpster, “Condensers And Their convenient way for evaluation of various Monitoring”, United States Patent, Patent No: US parameters and Performance Testing of Surface 7,926,277 B2 Condensers time-to-time. [6] A. Grunsky, “Condenser Application And Maintenance Guide”, EPRI, August 2001 [7] “Guide To The Design Of Secondary Systems And REFERENCES Their Components To Minimize Oxygen Induced Corrosion”, EPRI NP-2294 [1] Robert Bartholomew and Sheppard T. [8] “Steam Suace Condensers: Performance Test Codes”, Powell,“Controlling the Condensate andFeed water ASME PTC 12.2-2010 Revision of ASME PTC 12.2 Dissolved Oxygen and Air In-leakage”. 1998 (R2007). [2] Dave Leissner and Richard Putman, “Recommendation [9] Richard E. Putman and Dr. Joseph W. Harpster, “The for Design and Operation of Vacuum Pumps at Steam Measurement of Condenser Losses Due To Fouling and Turbine Condensers”. Those Due To Air Ingress”. [3] “Design And Operating Guidelines For Nuclear Power [10] “Heat Exchange Institute: Standards For Steam Surface Plant Condensers”, EPRI NP-7382 Condensers”, 11th Edition, 2012 [4] Shengqi Zhou and Alan Turnbull, “Steam Turbine [11] R. S. Silver, “An Approach to a General Theory of Operating Conditions, Chemistry Of Condensates, And Surface Condensers”, Proceedings of institution of Environment Assisted Cracking- A Critical Review”. Mechanical Engineers, Vol. 178 Pt 1, No. 14, London, pp. 339-376, 1963-64.

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a study of effect of air in-leakage on performance of steam surface

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