Review of the vacuum decay test in air-cooled steam condensers


Polemic Paper, 2018
13 Pages

Excerpt

Table of contents

1. Introduction

2. VGB Air Leakage and Evacuation Test

3. Evacuation Flow

4. Cold Volume Evacuation

5. Summary

6. Glossary

7. Bibliography

Abstract: Condensing capacity of air-cooled condensers is critical for proper functionality of steam cycles in power plants. Condensers are operating 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 air-cooled condenser (VGB-S-131) proposes a simple procedure to verify ample vacuum tightness and evacuation capacity. However, the following report demonstrates that the informational value of test results in connection with the proposed procedure are to some extent dubious. They cannot stand for themselves but need to be accompanied by further information gathered from other tests - e.g. thermal performance tests. This puts the usefulness of the test itself into question.

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].

In practice, vacuum tightness test procedures should be as simple and practicable as possible without noticeable impediment of power plant operation. Over time a simple tightness test procedure has come into general use. It was adopted by the latest revision of VGB test code [9] and is described in §3.3.5. of the standard. The main advantage is the simplicity of the proposed procedure because it avoids the measurement of effective leakage flow rates which is a rather complex task.

At this point – a word of caution may be justified. It is a real question if a simple procedure is capable to verify the effectiveness of ACC vacuum tightness and appropriate design of the evacuation. Connected to this issue is the question of adequate thermal design of the ACC. The following paper shows that the expectation of proper verification of vacuum tightness by this test goes a bit too far.

2. VGB Air Leakage and Evacuation Test

VGB standard S-131 [9] proposes a leakage and evacuation test in §3.5.5. of the code. After reaching stationary state conditions of the steam cycle the test shall be made in two steps:

(1) Stop evacuation system for some period of time and monitor the exhaust pressure increase (loss of vacuum)
(2) Re-start evacuation and monitor the elapsed time until the initial exhaust pressure is recovered

Stationary operation is characterized by permanence of the test operation parameters [cf. §3.5.5. (a) to (d)]. The claim is that if the elapsed time to recover the vacuum pressure is less than the time of pressure increase the evacuation capacity is adequate – i.e. the system is vacuum tight as guaranteed. The test must be made at “no-wind” conditions (wind speed < 3 m/s). Furthermore, “ample functionality of the evacuation system” is required in order to avoid leakage flow affecting the condenser steam pressure. Increasing steam pressure before start of test does not fulfil this latter condition. On the other hand, sinking pressure will always recover the initial pressure after test – and even goes beyond.

Abbildung in dieser Leseprobe nicht enthalten

Summarizing, the vacuum test conditions act on the assumption of stationary operation – not only thermally but also of the evacuator (although not mentioned in [9]).

Figure 1: VGB Tightness Test (source: own sketch)

To check if this test really proves ample functionality of the evacuation system we need a closer look on the physics involved. The example shown in figure 5 of the VGB standard [9] will be used as basis. Figure 1 shows a qualitative re-print.

Changes of steam pressure during the test - shown on the upper part of the graph - are approximately linear. This applies to vacuum loss as well as the vacuum recovery curve. For better overview the qualitative variation of the mixture subcooling at evacuation point is indicated in the graph as well. This is important because the composition of steam and inert gas varies with the subcooling level. Formally, the unknown evacuation subcooling profile is tagged in fig. 1 by question marks.

Mixture subcooling is defined as the difference between steam saturation temperature and mixture temperature. Generally, the colder the mixture temperature the larger the air content in the mixture. Thus, the subcooling level is a measure of inert gas content.

Wherever needed we keep changes of the operation parameters – as stipulated in the VGB example - approximately linear. This is justified because the absolute change of steam pressure must be kept small, e.g. to avoid a turbine trip [9]. Within the test range we do not consider a possible surge of capacity of the evacuation system but keep the extracted volume flow constant.

The lower part of figure 1 is an extension to the VGB graph. It shows the flowrates of inert gas and attached steam during the test at evacuation point. After evacuation stop the ingress of air carries on as before which leads to additional air fill in the ACC. Steam flow into the vacuum system however, is not impeded by the stop of evacuation. Consequently, all inerts entering the ACC during stop of evacuation are collected and concentrated at the end of the condensation section where the local subcooling level increases. Thus, after time the evacuation system faces colder mixture as compared to steady state operation at start of test.

At restart of evacuation some time is needed to reach stationary conditions again. Normal operation pressure is recovered when the excess air fill has been completely removed from the ACC, i.e.

Abbildung in dieser Leseprobe nicht enthalten

Form factor is defined by the profile of the excess air flow during time step . Most probably, the form factor is expected somewhere between linear and maximum (or, box type) profile, (cf. fig. 1). Generally, the form factor is defined as

Abbildung in dieser Leseprobe nicht enthalten

Defining the ratio of the elapsed times as the VGB criterion [9] reduces to . Therefore,

Abbildung in dieser Leseprobe nicht enthalten

In case of it takes longer to recover the original stationary state operation. Note that the ratio of inert air flows is only dependent on different subcooling levels at stationary and test condition and form factor but not on absolute leakage flowrates.

Defining the relative initial inert excess flow ratio at restart after we get to the general form of VGB criterion (2-3):

Abbildung in dieser Leseprobe nicht enthalten

with . Note that can be interpreted as minimum elapsed time ratio which is at .

3. Evacuation Flow

Abbildung in dieser Leseprobe nicht enthalten

Figure 2: General ACC System (source: own sketch)

To start the general discussion look at figure 2 showing a typical ACC layout with one street of exchanger modules. The most important feature is the separation of the condenser into two sections to enable successful inert extraction from the steam cycle. The evacuator is connected to the exit of the secondary condenser which is of reflux type. Thus, steam/condensate is effectively separated from inert gas carrying residual steam.

Sources of vacuum leakage may be found in the vacuum part of the turbine and cover the whole ACC volume including auxiliary systems – primarily, at bolted connections and ineffective pump shaft seals.

The design of the evacuation system (steam ejector, water-ring pump) is based on three factors:

(a) Subcooling of steam-air mixture at evacuation point
(b) Expected ingress of air, HEI [7]
(c) Absolute steam pressure at evacuation point

Subcooling (a) controls the composition of residual steam and inert air flow at evacuation point. A typical design value is . Subcooling is defined as difference of saturation temperatures at system pressure and partial steam pressure:

Abbildung in dieser Leseprobe nicht enthalten

The expected ingress of air into a vacuum system (b) depends on total design steam flow to the ACC. HEI standard [7] gives general recommendations.

The absolute steam pressure at evacuation point (c) is defined by the turbine exhaust pressure and losses in steam duct, tube bundles and evacuation piping. Figure 3 shows a typical steam pressure profile between turbine exhaust and evacuation point. As long as only traces of inert gas are contained in the steam flow the temperature follows the steam saturation line. This is changed to a large extent close to the termination point of condensation. In this region the local condensation process is impeded by the presence of inert gas and the mixture undergoes noticeable subcooling.

Abbildung in dieser Leseprobe nicht enthalten

Figure 3: Steam Pressure and Temperature Profile (source: own sketch)

In contemporary designs the total pressure drop between turbine exhaust and evacuation point is normally in the range of . It is affected not only by thermal design data (steam flowrate, turbine exhaust pressure) but also by bundle geometry and module layout.

In all phases of the test procedure the extracted air is assumed to be saturated. This is justified by gravitational draining of all formed condensate back to the condensate collection line (fig. 2).

For saturated air the following balance holds [10]:

Abbildung in dieser Leseprobe nicht enthalten

In stationary operation, and , the evacuator extracts volume flow carrying inert mass with it. With constant volume extraction flow the relative excess flow rate at is

Abbildung in dieser Leseprobe nicht enthalten

[...]

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Details

Title
Review of the vacuum decay test in air-cooled steam condensers
Author
Year
2018
Pages
13
Catalog Number
V455056
ISBN (eBook)
9783668892743
ISBN (Book)
9783668892750
Language
English
Tags
VGB-S-131
Quote paper
Hans Georg Schrey (Author), 2018, Review of the vacuum decay test in air-cooled steam condensers, Munich, GRIN Verlag, https://www.grin.com/document/455056

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