COMPRESSOR STATION ANCILLARY EQUIPMENT: KNOW WHAT YOU ARE BUYING PART 1 Frederick J. Mueller Mueller Environmental Designs Michael A. Smith, P.E. Texas Gas Transmission LLC ABSTRACT Compressor station design engineers and other design professionals are usually "systems" designers. These professionals design, specify and procure equipment that is engineered and fabricated by others. This course will address the basics of some of the ancillary equipment required to create a natural gas compressor station. This will help the designer to make more informed decisions. The equipment covered in Part 1 will be Natural Gas Separation / Filtration Equipment, Air Cooled Heat Exchangers (ACHE) and Intake Air Filters. The following are the basic areas covered for each type of equipment: ¨ What (is in the pipeline; are you cooling; is in the air) ¨ Types and/or designs available and their application; ¨ Construction considerations; ¨ Specifying your equipment; and ¨ Analyzing your options Successful completion of the course will allow design personnel to create more detailed specification sheets and to better analyze the proposals received from vendors. INTRODUCTION An introduction to a paper should normally begin with telling the reader what they are going to be told. This introduction, however, will start by telling you what you are not going to be told. This paper is not intended to teach all of the requirements for gas separator, air-cooled heat exchanger and air filter design and fabrication. You will not be able to completely design this equipment and you certainly will not be able to go into business for yourself making this equipment, at least, not because of this paper. In today's gas pipeline industry, companies are striving to do more work with less people. As a result of that trend, gas pipeline design professionals are less able to target a specific area of expertise and less "expert" help is available as backup within the Company. As a group that designs, specifies and purchases equipment for all areas of a compressor station, it is only practical that we adopt the "jack-of-all-trades, master of none" mentality. What you will gain from this paper is the ability to write a specification for this equipment to insure that it meets your specific needs and, as a result, you will be able to analyze the bids you receive to be sure that the equipment offered meets your needs. 1 Most of the information in this paper is "industry standard" and some might even be termed "common sense", at least to the technically minded. Much of the information is based on the experiences of the authors who have 43 years of combined experience in the gas pipeline industry. If you have several years of experience in this industry, your experiences may vary from some of those presented in this paper since design requirements and philosophies can vary greatly in different parts of the country. If you are relatively new to the gas pipeline industry, you will certainly have many experiences ahead of you that can build upon what you learn here. Remember, we usually learn much more from our mistakes than from our successes. NATURAL GAS SEPARATION / FILTRATION EQUIPMENT Separation/filtration is the removal of any unwanted liquid or solid that may cause damage to equipment or contaminate the end product. Some of the separation/filtration applications in the transmission of natural gas include: ¨Compressor station suction to protect the compressor from liquid slugs and prevent cylinder wear from solids. ¨Removal of oil from reciprocating compressors to improve pipeline efficiency. ¨Removal of liquid hydrocarbons, water, sand, and pipe scale from the gas at metering stations and city gates. ¨Protection of desicant beds, to keep liquids and solids from fouling desicant. ¨Gas storage to prevent injection or withdrawal of liquids and solids. ¨Removal of solids and liquids in fuel lines to power plants, industrial plants and engines. The equipment used in the removal of liquid and/or solids from the gas stream are commonly referred to as slug catchers, scrubbers, filter separators, and coalescers. Each of these devices have unique separation/filtration techniques. Problems encountered in the gas transmission industry are as varied as the contaminants in the pipeline. WHAT IS IN THE GAS PIPELINE BESIDES NATURAL GAS? The most common contaminants in natural gas pipelines include water, lubricating fluids, amine, glycol, drilling fluids, liquid hydrocarbon, salts, chlorides, sand, dirt and black powder. Black powder is a catchall term that describes a material that can be wet, with a tar-like appearance, or dry and be a fine powder. Components of black powder can be chemically broken down into several forms of iron oxide and iron sulfide. It can be pyrophoric when exposed to air. Black powder mixes with most or all of the above stated contaminants. Images 1 & 2 are examples 2 Dry Black Powder Image 1 Wet Black Powder Image 2 3 TYPICAL SEPARATION/FILTRATION EQUIPMENT (See Table 1 for performance) Separator Type Slugs Solids Liquids Effluent Slug Catchers Yes 100%>50 microns and larger 100%>50 microns and larger Dependent of Shell Velocity Gas Scrubbers Impingement Type Separation Mesh Pad No No 100%>8 microns and larger <0.10 Gallons per MMSCF Vane Packs No No 98%>10 microns and larger <0.10 Gallons per MMSCF Centrifugal No 98%>10 microns and larger 99%>10 microns and larger <0.10 Gallons per MMSCF Multi-Cyclone Yes-Intermittent 99%>5 microns and larger 100%>5 microns and larger <0.10 Gallons per MMSCF Helical Coil Yes-Continuous 99.95%>5 microns and larger 100%>5 microns and larger <0.10 Gallons per MMSCF Filter Separator No 99.5%>5 microns and larger 100%>3 microns and larger <0.01 Gallons per MMSCF Coalescing Separator No 99.98%>.03 microns and larger 99.98%>.03 microns and larger <0.01 Gallons per MMSCF Table 1 Slug catchers - This particular separator design is able to absorb sustained inline flow of large liquid volumes at irregular intervals (slug and annular flow regimes). LLC The design typically is horizontal with inlet and outlet connections located on the upper portion of vessel heads. The Gravity or Knock Out Drum primary method of separation is gravity. Some designs incorporate impingement plates, mesh pads, and vane packs to facilitate separation. Figure 1 depicts a LLC typical knock out drum or slug catcher design. Figure 1 Applications include withdrawal out of storage before compression, sections of pipeline prone to liquid slugging, gathering systems and two phase pipeline systems. Advantages ¨ Ability to absorb large volumes of fluid ¨ Lower initial cost when compared to other separator designs Disadvantages ¨ Not very efficient, particularly if concerned with mist flow regime Gas Scrubbers - The term gas scrubber typically is applied to a class of separators that utilize centrifugal and impingement separation techniques to remove liquid from gas in a mixed phase gas stream. They utilize mesh pads, centrifugal elements, and vane packs for separation. Figure 2, depicts centrifugal, mesh pad, and vane type separators. 4 LLC LLC Centrifugal Separator Wire Mesh Separator Vane Type Separator Figure 2 Applications include removal of liquid hydrocarbons, water, lubricating and other fluids ahead of compressor suction, absorption plants, gathering lines, metering stations and gas storage. Mesh pad type separators employ impingement wire mesh pads, (Image 3) rely on downward force of gravity to overcome the upward gas velocity and surface tension of liquid for separation. Advantages ¨ Removes gross liquid flow and mist particles ¨ Lower initial cost when compared to other scrubber designs ¨ Can be used in slug catchers when configured properly Disadvantages ¨ Sensitive to design flow conditions; at low velocity, efficiency is reduced due to drift and at high velocity due to flooding. ¨ Applicable for liquids only. Dirt, solids, and sticky viscous liquids will plug mesh pad. Image 3 Centrifugal type separators impart a change in the direction of flow, moving liquids and solids to the periphery of the shell. Gravity will separate these particles from the gas flow. Centrifugal separators are velocity dependent for separation performance. Illustration 1 is a centrifugal type separator. 5 Illustration 1 Advantages ¨ Removes mist particles ¨ Removes small amounts of solids ¨ Lower initial cost when compared to vane type scrubber design Disadvantages ¨ Sensitive to design flow conditions, poor turn down rate when compared to vane type scrubbers ¨ Particle removal rate and size is dependent on inlet velocity and the number of turns the particulate makes within the separator Vane type separators make use of a labyrinth formed from sinusoidal parallel plates with pockets. Flowing gas and liquids change direction a number of times causing liquids to become captured in side pockets. Once captured, liquids drain downward to a liquid holding sump due to gravity. Figure 3 is vane type separator elements. Pocket Hook Figure 3 Advantages ¨ Removes both gross liquids flow and small mist particles. ¨ Good down turn ratio (not velocity sensitive) ¨ Can be used in slug catchers when configured properly Disadvantages ¨ Applicable for liquids only. Dirt, solids, and sticky viscous liquids will plug drain pockets. Filter Separators - Filter separators are designed to provide removal of both solids and liquids from the gas stream. The filter separator employs filter elements and vane packs to achieve impingement, diffusion and interception separation/filtration methods to reach its efficiency. Illustration 2, is representative of a filter/separator design. 6 Illustration 2 Advantages ¨ Removes both liquids and solid particles to one micron in size. ¨ Good turn down ratio (not velocity sensitive) Disadvantages ¨ Can not handle intermittent slugs ¨ High maintenance requirements ¨ Replacement filter costs Gas Liquid/Solid Separator - Gas liquid/solid separator refers to a type of separator that makes use of cyclone tube and helical coil separators to remove both solids and liquid from gas in a mixed phase gas stream.