Condensing the capacity of air-cooled condensers is critical for proper functionality of steam cycles in power plants. Condensers operate in the low vacuum regime to promote the efficiency of power generation. Leakages enable ingress of ambient air into the steam cycle and thus impede condensation and power generation capacity. Therefore, vacuum tightness is essential for effective plant operation and must be verified by means of an acceptance test.
The latest revision of the VGB acceptance test code for the air-cooled condenser (VGB-S-131) proposes a simple procedure to verify ample vacuum tightness and evacuation capacity. However, the following report will demonstrate that the informational value of test results in connection with the proposed procedure are to some extent questionable. They cannot stand for themselves but need to be accompanied by further information gathered from other tests, as for example thermal performance tests. This puts the usefulness of the test itself into question.
Table of Contents
1. Introduction
2. VGB Air Leakage and Evacuation Test
3. Evacuation Flow
4. Cold Volume Evacuation
5. Summary
Research Objectives and Topics
The primary objective of this work is to critically evaluate the informational value and reliability of the vacuum leakage and evacuation test as proposed in the VGB standard S-131 for air-cooled steam condensers (ACCs), assessing whether this simplified test procedure is capable of effectively verifying vacuum tightness and evacuation system performance.
- Physics of inert gas extraction in steam cycles
- Evaluation of the VGB standard S-131 test criteria
- Impact of test boundary conditions on verification results
- Correlation between evacuation capacity and subcooling levels
- Critical limitations of simplified vacuum tightness testing
Excerpt from the Book
1. Introduction
Dry air-cooled steam condensers (ACCs) form a core element of contemporary power generating units. The importance of ACCs was especially stimulated by the introduction of combined-cycle power plants (CCPPs) in the 1990s. As the steam cycle in CCPPs accounts generally only for 40% of plant power generation the investment has been drastically reduced as compared to classical thermal power plants. Water saving has been the primary factor to promote ACC technology - specifically in arid regions of the globe. Last not least the introduction of single row condenser designs enabled a considerable investment cost reduction. Over the years, ACCs have replaced conventional wet cooling units even in large power stations.
Although dry cooling technology is established and well-proven over the years some everyday issues remain up to now. Reliable extraction of inert gas from the vacuum system is one of these issues. Reasons for inert gas pocket formation and ways to counteract have been discussed over a long period of time - cf. [1], [2], [3], [4], [5], [6]. With the large size of ACC units ingress of ambient air into the vacuum part of the steam cycle is un-avoidable. At design stage expected leakage flow rates obtained from long-term experience serve as guidance for sizing evacuation units. To this end the HEI standard [7] is one of the most referred to data collections.
Inert gas contained in vacuum steam impedes the condensation capacity of ACC units. Therefore, vacuum tightness must be ensured under all circumstances. Tightness tests under various conditions have been proposed over time. At standstill, an over-pressure test is the easiest way to identify leakage flow rates. In operation however, things are different. The simple static procedure does no longer supply reliable data as regards to vacuum leakage flowrates [8].
Summary of Chapters
1. Introduction: This chapter provides an overview of the role of air-cooled steam condensers in modern power plants and highlights the challenges regarding vacuum tightness and inert gas extraction.
2. VGB Air Leakage and Evacuation Test: This chapter outlines the specific procedures defined in the VGB standard S-131 and examines the assumptions of stationary operation under which the test is conducted.
3. Evacuation Flow: This chapter analyzes the physical factors influencing the evacuation system design, including subcooling levels, air ingress, and steam pressure profiles.
4. Cold Volume Evacuation: This chapter introduces a model to simulate reality during the test, specifically addressing how mixture composition behaves after the restart of the evacuation system.
5. Summary: This chapter concludes that the VGB vacuum test is not a sufficient standalone metric and suggests that thermal capacity must be considered for a comprehensive evaluation.
Keywords
Air-cooled Condenser, ACC, Air Leakage, Vacuum Decay, VGB-S131, HEI, Steam Cycle, Inert Gas, Evacuation System, Vacuum Tightness, Steam Condensation, Power Plant Operation, Thermal Design, Subcooling, Acceptance Test
Frequently Asked Questions
What is the core subject of this publication?
The work focuses on the technical assessment of vacuum leakage and evacuation tests for air-cooled steam condensers (ACCs) as defined by the VGB standard S-131.
What are the primary thematic fields covered?
The publication covers power plant engineering, thermal condensation physics, inert gas extraction processes, and the validation of industrial acceptance test standards.
What is the central research question?
The research questions whether the simplified procedure proposed in the VGB standard S-131 is actually capable of verifying the effectiveness of ACC vacuum tightness and proper evacuation design.
Which scientific methodology is applied?
The author applies a theoretical-analytical approach, utilizing mathematical models for mass balance and inert gas behavior to challenge the VGB test assumptions.
What topics are discussed in the main part of the document?
The main part analyzes the VGB test procedure, evaluates evacuation flow physics, examines the impact of subcooling, and presents graphical scenarios of inert mass flow ratios under different design conditions.
Which keywords characterize this work?
The work is characterized by terms such as Air-cooled Condenser, Vacuum Decay, VGB-S131, Inert Gas, and Evacuation System performance.
Why does the author consider the VGB test "dubious"?
The author argues that the test results are heavily dependent on test boundary conditions and the test engineer, making the test a "nice to have" measurement rather than a standalone proof of quality.
What role does the "form factor" play in the author's analysis?
The form factor is used to identify the evacuation flow profile, which is critical for understanding the VGB criterion for vacuum recovery.
How does ambient wind affect the validity of the vacuum test?
Since the VGB test is performed under "no-wind" conditions, it may lead to an over-design of the ACC, causing the system to fail the vacuum test criterion even when the design is appropriate.
- Arbeit zitieren
- Hans Georg Schrey (Autor:in), 2018, Review of the vacuum decay test in air-cooled steam condensers, München, GRIN Verlag, https://www.grin.com/document/455056
