Vacuum tightness is critical for air-cooled condensers operating at low absolute pressure. Low vacuum is aimed for because vacuum dominates the power plant efficiency. To verify vacuum tightness usually a vacuum drop test is made with the system empty at normal atmospheric temperature and free of any liquids. This test is done before commissioning of the power unit and generally follows the recommendations of the Heat Exchanger Institute (HEI) as outlined in §6.1.1 of “Standards for Steam Jet Vacuum Systems”.
However, over time of operation the power plant may develop leakages, which were not present at the time of the original drop test. This calls for a tightness test at operating conditions where pre-conditions for the standard vacuum drop test are not fulfilled. The report describes a vacuum drop test without interfering too much into normal power plant operation. The test is suitable for stationary operating conditions using standard operation readings.
The assessment of leakage flow is based on the measured vacuum decay rate. It is shown that vacuum decay rates taken from tests before and after commissioning are different. Contractual fixing of acceptable vacuum decay rates should therefore be treated with care. Example graphs for easy evaluation are given.
Table of Contents
1 Introduction
2 Standard Vacuum Drop Test
3 Leakage Test with ACC in Operation
3.1 Inert Gas Accumulation in ACC during Operation
3.2 Loss of Condensing Capacity Caused by Surface Blanketing
3.3 Effective Pressure Effect
4 Comparison of Static and Steady State Results
5 Conclusion
Objectives and Research Themes
This report investigates the challenge of assessing vacuum tightness in air-cooled condensers (ACCs) during active operation, as the standard vacuum drop test is only applicable when the system is out of operation. The research focuses on developing a simplified, reliable method for monitoring leakage using standard operational data, such as turbine exhaust pressure and condensate temperature, by modeling the effects of inert gas accumulation and surface blanketing.
- Analysis of standard vacuum drop test procedures versus operational requirements.
- Evaluation of vacuum decay sources, specifically inert gas accumulation and the loss of condensing capacity.
- Development of mathematical models for estimating leakage rates during steady-state conditions.
- Impact assessment of operational parameters like sub-cooling levels and exchanger volume fractions on system pressure.
- Comparison between static (HEI-based) and steady-state leakage assessment results.
Extract from the Book
3 Leakage Test with ACC in Operation
With the ACC in operation the pre-conditions for using the HEI vacuum test are no longer applicable [3]. Therefore, §3.3.5 of VGB ACC acceptance test standard [1] recommends an air leakage test following VGB Guideline R-126 L (“Recommendations for Design and Operation of Vacuum Pumps for Steam Turbine Condensers”) [5]. However, this test requires a multitude of single measurements and the modification of the vacuum piping by different standardized leakage nozzles. So, normal operation of the ACC is somehow affected during the test. Therefore, at time of erection the HEI test is accepted by clients and suppliers of ACC equipment as confirmation of vacuum tightness in deviation from the VGB test code [1].
None-the-less a situation may arise after commissioning of the power unit to check the vacuum tightness of the system because of loss of vacuum. A leakage test avoiding extra time and effort by using operational data would be extremely desirable. For the proposed simplified method we need to consider at least steady state operation in order to come close to meaningful results.
For the following procedure the steam saturation line is needed. A good approximation is ts(p) = a / [b - ln(p)] + c or ps(t) = exp [b - a / (t-c)] with a = 3888,11; b = 23,3084; c = 43,2 (SI units).
For the ACC operation decay test the inert gas evacuation is blocked for some time (by closing the evacuation valve), while all operational parameters (steam flow, fan settings, ambient conditions) are kept constant. The loss of vacuum due to inert gas accumulation is visible by the change of turbine exhaust pressure over time. This suggests that the HEI method may apply because again, the ACC volume is filled over time with inert gas. However, accumulation of inert gas impedes also the heat exchange process which has an effect on condensation pressure level as well.
Summary of Chapters
1 Introduction: Provides an overview of the necessity for hermetic sealing in vacuum systems and describes the general features of air-cooled condensers.
2 Standard Vacuum Drop Test: Explains the conventional HEI-recommended procedure for assessing leakage in static conditions with the system out of operation.
3 Leakage Test with ACC in Operation: Details the proposed method for assessing vacuum decay during active operation by modeling inert gas accumulation and condensing capacity loss.
4 Comparison of Static and Steady State Results: Compares the results of traditional static testing with the proposed steady-state operational model to illustrate the magnitude of difference in leakage assessment.
5 Conclusion: Summarizes the feasibility of using control room readings to assess leakage and highlights the limitations of the static test for operational monitoring.
Keywords
Air-cooled Condenser, ACC, Air Leakage, Vacuum decay, HEI, VGB-R126L, VGB-R131, Condensing capacity, Surface blanketing, Inert gas, Operational data, Vacuum tightness, Steam saturation, Heat exchanger effectiveness.
Frequently Asked Questions
What is the primary purpose of this work?
The work aims to provide a practical method for monitoring vacuum tightness in air-cooled condensers while they are in operation, addressing the limitations of traditional static testing.
What are the main research themes covered?
The research covers vacuum system integrity, modeling of inert gas accumulation, thermodynamics of condensation in ACCs, and the development of simplified leakage assessment metrics.
What is the core objective of the proposed method?
The objective is to enable operators to estimate leakage rates using existing control room data without requiring complex modifications to the vacuum piping or taking the system out of service.
Which scientific methods are employed?
The author uses thermohydraulic modeling, applying the ideal gas law for inert gas accumulation and heat balance equations to determine the impact of surface blanketing on condensation efficiency.
What does the main body of the work focus on?
It focuses on the derivation of mathematical models to calculate vacuum decay caused by both gas accumulation and the loss of active surface area due to surface blanketing.
What key terms characterize the study?
Key terms include ACC, vacuum decay, surface blanketing, inert gas accumulation, and operating pressure factors.
Why is the standard HEI vacuum test not suitable for operating ACCs?
The HEI test assumes a static system at atmospheric temperature and requires the system to be out of operation, which does not reflect the complex thermohydraulic state of an active power plant ACC.
What role does "surface blanketing" play in vacuum decay?
Surface blanketing occurs when inert gas-steam mixtures accumulate in the condenser tubes, rendering parts of the surface ineffective for condensation and causing a rise in exhaust pressure.
How is the "operation pressure factor" used?
It is used as a tool to bridge the gap between measurable vacuum decay rates and actual leakage rates by subsuming various steam-side operational parameters into a single factor.
Does the author conclude that static tests are redundant?
Not entirely, but the author concludes that static tests cannot represent the conditions of steady-state operation, and that leakage rates in operation are typically much higher than those determined by static testing.
- Citar trabajo
- Dipl.-Ing. Hans Georg Schrey (Autor), 2018, Vacuum Drop Test of Air-Cooled Condensers in Operation, Múnich, GRIN Verlag, https://www.grin.com/document/421324
