Troubleshooting Vacuum Systems E-Book

Norman P. Lieberman

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 305

Veröffentlichungsjahr: 2012

Beschreibung

Vacuum systems are in wide spread use in the petrochemical plants, petroleum refineries and power generation plants. The existing texts on this subject are theoretical in nature and only deal with how the equipment functions when in good mechanical conditions, from the viewpoint of the equipment vendor. Also, the existing texts fail to consider the interaction of the vacuum system with the process equipment it serves and the variability of the motive steam conditions, change in cooling water temperature condenser fouling and erosion of the ejectors. Here are some of the many questions answered in this groundbreaking volume: * Why does my first stage jet make a surging sound during hot weather? * Why does the vacuum suddenly break? * I've seen moisture condensing on the jet's body! What's causing that? * Why do I have to steam-out the drain legs from our condensers? * Superheated steam is making our vacuum worse. Is this normal? * How can I locate and measure air leaks? * Reducing the steam pressure to my jets improves vacuum. But why? * I can't pull the pre-condenser bundle. The shell side is fouling. What should I do? * We're not getting our normal horsepower from our steam turbine. Could this be a jet problem? * Raising the seal drum level improves vacuum! Is there an explanation for this? * Our turbine exhaust steam pressure to our surface condenser has doubled in the last two years. What should we do? * Restricting cooling water flow from our elevated condensers improves vacuum! Is this possible? * What's a converging-diverging ejector all about? * What's the difference between a barometric condenser and a surface condenser? Which is better?

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Dedication

Preface

Introduction

Definition of Terms

Other Books by Author

Chapter 1: How Jets Work

1.1 The Converging-Diverging Ejector

1.2 Interaction of Steam Nozzle with Converging-Diverging Diffuser

1.3 Compression Ratio

1.4 Converging-Diverging Ejector

1.5 Velocity Boost

1.6 Surging

1.7 Critical Discharge Pressure

1.8 Observing the Conversion of Heat to Velocity

1.9 Jet Discharge Pressure

1.10 Reducing Primary-Jet Discharge Pressure

1.11 Bypassing First Stage Ejectors

Chapter 2: Making Field Measurements

2.1 Getting Started

2.2 How to Unscrew Steel Plugs

2.3 Effect of Barometric Pressure on Indicated Vacuum

2.4 Use of Piccolo

2.5 Measuring Deep Vacuums using an Hg Manometer

2.6 Measurement of a Deep Vacuum without Mercury

2.7 Measuring Condensibles in Feed to First Stage Ejector

2.8 Identifying Loss of Sonic Boost by Sound

2.9 Identifying Air Leaks

2.10 Air Leaks in Flanges

2.11 Vacuum Measurement Units

Chapter 3: Tabulation of Vacuum System Malfunctions

3.1 Tidal Flop in Delaware

3.2 Critical Discharge Pressure

3.3 Fouling in Final Condenser

3.4 Reduction in Back Pressure

3.5 Loss of LVGO Pan Level

3.6 Variations in Cooling Water Temperature

3.7 Multi-Component Malfunctions

3.8 Partial Tabulation of Vacuum System Malfunctions

Chapter 4: Effect of Water Partial Pressure on Jet Efficiency

4.1 Vapor Pressure of Water Limits Vacuum

4.2 Reminder about Water Partial Pressure

4.3 Air Leaks in Steam Turbine Surface Condensers

4.4 Variable Cooling Water Temperature

4.5 Loss of Sonic Boost

4.6 Relative Jet Efficiency

4.7 Definition of “Vacuum Breaking”

4.8 Critical Discharge Pressure Exceeded

Chapter 5: Air Leaks

5.1 Upper Explosive Limits

5.2 How to Find Air Leaks

5.3 Diffuser Air Leaks

5.4 Air Leaks on Vacuum Towers

5.5 Air Leaks in Heater Transfer Lines

5.6 Air Leaks – Turbine Mechanical Seal

Chapter 6: Sources and Disposal of Hydrocarbon Off-Gas

6.1 Evolution of Cracked Gas

6.2 Sources of Cracked Gas

6.3 Cracked Gas Evolution from Boot

6.4 Air Equivalent

6.5 Overloading Vacuum Jets

6.6 Excess Cracked Gas Flow

6.7 Field Checking Gas Flow Meter in Vacuum Service

6.8 Surging 3rd Stage Jet Bogs Down Primary Jet

6.9 Exchanger Leaks Overloads Jets

6.10 Poor Vacuum Tower Feed Stripping

6.11 Level Connection Purges and Pump Mechanical Seal Gas

6.12 Effect of Heater Outlet Temperature

6.13 Extracting H2S from Vacuum Tower Off-Gas Upstream of Ejectors

6.14 Disposal of Seal Drum Off-Gas

6.15 Fouling of Waste Gas Burner

Chapter 7: Motive Steam Conditions

7.1 Effect of Wet Steam

7.2 Water in Motive Steam

7.3 The Tale of Weak Steam

7.4 Internal Freezing of Steam Nozzle

7.5 High Pressure, Superheated Motive Steam

7.6 Effect of Moisture Content of Saturated Steam on Temperature

7.7 Steam Pressure Affects Vacuum

7.8 Effect of Superheated Steam

Chapter 8: Mechanical Defects of Ejectors

8.1 Steam Nozzle Testing

8.2 Other Mechanical Defects of Jets

8.3 Fouled Steam Nozzles

8.4 Diffuser Erosion

8.5 Repair of Ejector Body

8.6 Changing Worn Steam Nozzles

8.7 Restoring Critical Flow

Chapter 9: Condenser Fouling and Cleaning

9.1 Fouling Mechanism in Condensers for Refinery Vacuum Towers

9.2 Fouling Due to Chemical Additives

9.3 Minimizing Condenser Fouling in Vacuum Towers

9.4 Fouled Pre-condenser

9.5 Fixed Tube Sheet Condensers

9.6 Cleaning Condensers On-Stream

9.7 Optimum Condenser Bundle Configuration

9.8 Chemically Cleaning Condensers (Contributed by Gerry Obluda)

9.9 Ball Cleaning Condenser Tubes (Contributed by an Operator at the Syncrude Plant in Canada)

9.10 Corrosion Control by Better Desalting

Chapter 10: Pressure Control of Vacuum Towers

10.1 Positive Feedback Loop

Chapter 11: Condenser Cooling Water Flow

11.1 Cooling Water Flow Configuration

11.2 Air Evolving from Cooling Water Reduces Cooling Water Flow

11.3 Cooling Water Pressure to Surface Condesers

11.4 Tube Leaks

Chapter 12: Condensate Back-Up in Condensers

12.1 Undersized Condenser Drain Nozzle

12.2 Seal Drum Level Indication

12.3 Leaking Gauge Glass on Surface Condenser Boot

12.4 Condensate Pump Cavitation Due to Air Leaks

12.5 Condensate Back-Up in Surface Condenser Boot

12.6 Experiment with Condensate Back-Up

12.7 Condensate Back-Up

Chapter 13: Seal Leg Drainage

13.1 Sludge Accumulation in Seal Drum

13.2 Seal Leg Leak Inside Seal Drum

13.3 Seal Leg Flange Leak Outside Seal Drum

13.4 Seal Leg Design

13.5 Inadequate Seal Leg Length for Hydrocarbons

13.6 Inadequate Seal Leg Capacity

13.7 High Back-Pressure from Seal Drum

13.8 Detecting Condensate Back-Up in Seal Legs

13.9 Condensate Back-Up Due to Air Leak in Barometric Drain Line

13.10 Seal Drum Design

13.11 Seal Drum Fills with Corrosive Deposits

13.12 Seal Drum Design Tips

13.13 An Unfortunate Incident

Chapter 14: Other Types of Vacuum Equipment

14.1 Hogging Jets

14.2 Use of Hogging Jet on Surface Condenser

14.3 Liquid Seal Ring Compressors

14.4 Gas Ejectors

14.5 Liquid Ejectors

14.6 Ejector Compression Efficiency

Chapter 15: Air Baffle and Impingement Plate in Surface Condensers

15.1 Mechanical Configuration of Seal Strips

15.2 Corroded Brass Seal Strips

15.3 Air or Vapor Baffle Leak

15.4 Identifying Defective Seal Strips

15.5 Air Baffle Clearance

15.6 Fouling Mechanism in Vacuum Tower Surface Condensers

15.7 Surface Condenser Impingement Plate

15.8 Oversized Impingement Plate

15.9 Impingement Plates as Vapor Distributors

Chapter 16: Optimizing Vacuum Tower Operation

16.1 Steam to Heater Passes

16.2 LVGO Pan Level Loss Causes a Loss in Vacuum

16.3 Carry-Over of LVGO Pumparound Spray

16.4 Optimizing Vacuum Tower Top Temperature

16.5 Plugged Vacuum Tower Top Demister

16.6 Bypassing Primary Ejector

Chapter 17: Frequently Asked Questions

17.1 Vacuum Systems

The Norm Lieberman DVD/Video Library of Troubleshooting Process Operations

Index

Troubleshooting Vacuum Systems

Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106

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Library of Congress Cataloging-in-Publication Data:

ISBN 978-1-118-29034-7

To Liz – Wife, friend, and partner in life. She has climbed many a vacuum tower structure with me. Infrared temperature sensing gun in hand, Liz has provided innumerable surveys for steam ejector and inter-stage condenser surface temperatures for vacuum systems at the most remote corners and hostile environments of our little home planet.

Preface

Challenging Human Intellect

You might think that the author of a book pertaining to troubleshooting vacuum systems would be super successful in resolving such problems. Especially considering I’ve had 47 years of experience and practice on literally a thousand such systems, and considering that I am, at least in my own mind, really smart. But you would be very much mistaken.

I rarely resolve all the problems of vacuum systems on my first try. Often, I’ll require several retrofit attempts to eliminate all the malfunctions. Not infrequently, I’ll miss the point completely and only years later will someone else discover the underlying problem.

Process engineering is one of the most difficult activities that mankind has selected in our desire to dominate the universe. And, the performance of multi-stage ejector-condenser systems is clearly one of the most complex features of process engineering. The performance of such systems lies at the outer edge of average human comprehension. I guess that if one could engage the services of the very top intellects who have fanatically dedicated and devoted 100% of their time to vacuum systems, then surface condensers and vacuum towers could consistently be operated at design pressures.

But in reality, the vast majority of such systems are operated at some fraction of their design efficiency. Thus, there is, and will continue to be, a huge economic incentive in refineries, petrochemical plants, and power generation plants to correct vacuum system malfunctions.

So many of my clients operate with inefficient vacuum systems for so long, that they consider their current operations normal. They have no conception of the increased product yields or the energy savings that would result if they could only develop the design vacuum.

Often, I find my inability to identify a vacuum problem to be depressing. I’ll be all alone, on the 8th landing, in the cold and damp of a dark night, listening to a great jet surging. The loneliness and isolation of the situation just makes me colder.

“Now what?!” I’ll think. I’ll be at the limit of my mental capacity to grasp the malfunction. But then, the defining characteristic of humankind comes to my aid. Not intelligence, but determination.

Need Help?

It is my intention in writing this book to contribute to the reader’s ability to improve the performance of their vacuum systems. This has got to be a “Hand’s-on” activity. However, often it may help you to discuss your observations with me. I never charge for such consultations. You can reach me at:

1-504-887-7714 (phone)1-504-456-1835 (fax)[email protected]

Introduction

Time and Determination

Vacuum system malfunctions are treated in the process industry, and especially in my universe of petroleum refineries, differently than other sorts of process equipment malfunctions. Problems with pumps, compressors, fired heaters, and distillation towers are eventually resolved and corrected. Problems with vacuum systems are only too often never fixed or even recognized, meaning my clients just become accustomed to running with a bad vacuum.

A young engineer assigned to improve the vacuum in a lube oil asphalt, or a coker feed vacuum tower, will typically tread down the following path of frustration:

Consult texts on vacuum equipment. Usually a waste of time. These texts describe how the vacuum equipment is supposed to work and not how it reacts to malfunctions such as wet steam, defective condenser seal strips, and eroded steam nozzles.

Consult the equipment vendor. Totally a waste of time. These fellows do not understand how their jets react with the process itself and the associated equipment.

Consult with a senior engineer. No help here. These gentlemen and ladies are always on their way to an important meeting and are too busy to work on such a humble task as ejector malfunctions.

So, the first step in troubleshooting ejectors and vacuum systems is to accept the fact that you’ll have to do it by yourself. And how about reading my book that you’ve just purchased—the very text that you have in your hands right now? Well, if you will really read this book in its entirety, you will have a growing sense of desperation and depression. I know. I’ve been there a hundred times. It’s all so terribly complex.

After all, if no one has solved this problem in the past 20 years, why would anyone expect you to resolve it? Perhaps it’s best to make some superficial, long term recommendations and get on with your life. Here are a few such recommendations that you might suggest to your upper management and supervisors:

1. During the next unit turnaround, disassemble the ejector and ship the internals back to the manufacturer for testing.

2. Install a knock-out drum to provide dryer motive steam.

3. Purchase an on-stream analyzer to monitor the seal drum off-gas composition.

4. Re-tube the surface condenser bundle with 12 gauge Titanium tubes.

I’m often tempted to slip away from reality with these sorts of suggestions. But in my heart and soul, I know what’s really required—steely determination. If I make enough field measurements, spend countless hours talking to the operators on all the shifts, run tests at various vapor loads, steam conditions, and cooling water flows, examine all the operating data and devote hours and days analyzing the problem, eventually I’ll come to a profound conclusion. Which is then typically proven wrong by a definitive plant test. And then I’ll have to start all over again.

It’s that very willingness to discard all my theories and begin anew, with an open mind that is the key to troubleshooting vacuum systems. I must never give up. Better death than defeat.

However, to be entirely honest, sometimes I have given up and recommended to my clients that they replace an outmoded steam ejector with a modern model. Then, I’ll look back on the project years and decades later with a feeling of sadness and longing for a squandered opportunity.

“If only I had lowered the boot level in the pre-condenser, perhaps I could have unloaded the primary ejector,” I’ll think. But it will be too late. That was 20 years ago in a refinery that still exists only in my memories.

Sometimes, I’ll start out my troubleshooting assignment to improve vacuum performance, surrounded by an enthusiastic group of young tech service engineers, older operators, and supervisors—all watching and waiting for the famous expert to solve their vacuum system problem. Three days later, I’m sitting all alone on the stairs leading to the 5th landing of the vacuum tower. It’s starting to rain, and the evening chill is settling down across the refinery.

Now what? I’m almost out of ideas. I’ve tried almost everything I know without success. But there’s always one more concept to explore. Did I remember to check the motive steam pressure to the third stage jet? Maybe it’s a lot lower than it’s supposed to be. It’s always that one final measurement or observation that leads to the correct solution and on to that ultimate victory.

You just have to have faith that the correct solution is within your grasp if only you try long enough and with sufficient fortitude.

As an old Stillman, Leroy Wilkes, once said to me in a Texas City refinery in 1974, “Son, it’s only a matter of time and determination.”

Definition of Terms

Air Baffle –

Means the same as vapor baffle.