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The Lease Pumper's Handbook Chapter 10 The Tank Battery Section B PRESSURIZED VESSELS This section focuses on the pressurized vessels and related equipment commonly used for tank batteries: · The flow line. From the well to the tank battery. · The header. A manifold of all flow lines to the first pressurized vessel. · The separator. Typically, the first pressurized vessel in the system. · The heater/treater. Typically, the second pressurized vessel in the system. The information presented in this section is general in nature due to the many variations in equipment configurations and specific uses by different companies. B-1. The Flow Line. A flow line is laid from the well to the tank battery. Where there is a long distance to the tank battery and the production is low, the lines may be joined at some convenient spot so that one line is laid from that point to the tank battery. This does not present a problem except when one of the wells needs to be tested. To test wells individually, a well tester, mounted on a small double-axle trailer, can be brought to each location. The second option is to shut one well in while the other is being tested. Limiting one flow line to several wells is never an ideal situation but may be the cheapest alternative. The material selected for the flow lines depends on many factors. Options include: · Steel is usually preferred for highpressure and flowing wells. It can be standard line pipe, heavy-duty line pipe, or new or used upset tubing. It can also be welded, use pipeline couplings, or have groove clamps. Steel can be plasticcoated to combat corrosion and scale accumulation. · Polyethylene may be selected for medium- to low-pressure lines and can be practical also when paraffin or scale is present. It is especially satisfactory when steel lines deteriorate rapidly. · Fiberglass can also be considered when extremely corrosive conditions are encountered.

Figure 1. Polyethylene line to be laid as a flow line.

10B-2 Laying new flow lines. When laying new flow lines, the weather must be considered. If a line is laid straight from one point to another in hot weather, when it gets cool it will shrink several feet in length. When it gets extremely cold it will shrink many feet in length. This will cause it to pull so hard that it will part, or in extremely hot situations it may buckle. Placing correct slow bends in flow lines can accommodate this problem, with plastic pipe add a few slow curves to provide extra line near the destination points and road crossings. Road crossings. When crossing a lease with a flow line, it is always best to lay a joint of casing across the road and run the line through it (Figure 2). If the ends of the casing are to be sealed or buried, a vent is welded to the casing to remove gasses in event of a line leak. A riser is welded into the conduit line at each end before it is buried to allow this to occur. All steel lines that go through a conduit should be coated and wrapped. each other. As they come up through a riser, a check valve is installed. In the event a hole should develop in a flow line and this check valve does not seal, all of the production entering the header can flow back through this line and result in a large oil spill. If the check valve on the casing of the well should become locked open, not only will the well begin to circulate by losing the produced oil back down the well, but produced oil from the header can flow back down the flow line and also be lost down the well. A tee will be installed in multiple well batteries, so that the flow may be diverted into the production or the test separator. The line to the heater/treater will have a connection in it to allow the injection of treating chemical. When injecting the chemical at the tank battery instead of at the well, it is always injected into the header and usually after the header but before the first vessel. This is standard procedure. Quarter-round opening valves--either plug or ball--are more appropriate for the tank battery than multiple round opening valves because the lease pumper can determine if they are open or closed at a glance. The header should also have a line installed to permit the flow to the separator to be diverted through a bypass around the vessel. Most vessels must be bypassed while repairs or changes are made. B-3. Pressurized Vessels. Tank battery typical operating pressures. Figure 2. A road crossing for flow line. B-2. The Header. As the flow lines approach the tank battery, they are lined up about 18 inches apart and enter the tank battery parallel to · Separators. Separators will have a maximum operating pressure of about 100 pounds with a test pressure of 150 pounds. Normal operating pressure is from 15-50 pounds, 25-35 pounds is about average. The pressure must be

10B-3 great enough to push the liquid from the separator into the heater/treater with a small safety margin. · Heater/treaters. These vessels are larger around than separators, so it takes a thicker shell to hold the same pressure. Thicker shells raise the cost of the vessel to a much higher level. Fifty pounds operating pressure is about average with a test pressure of less than 100 pounds for a heater/treater. A higher pressure separator is usually located ahead of it to lower the operating pressures to save money during construction. Vertical heater/treaters are taller than the stock tank and the oil outlet is about the same height. One pound of pressure will lift oil about three feet, so 10-15 pounds of pressure will usually be satisfactory at the heater/treater. There are several openings and lines that all pressurized vessels have in common (Figure 3). There are also a few openings and lines that are normally limited in use and are installed on specific purpose vessels. A good understanding of the purpose and locations of specific special purpose lines for pressurized vessels is important to the lease pumper. The emulsion inlet. The emulsion inlet is located on the side of the vessel near the top and above the fluid level in the vessel. Some pressurized vessels, such as the separator, will have a diverter plate on the inside to give the fluids a swirling motion upon entry. This allows the gas to break out of the liquid phase and reduce liquid carryover into the gas sales. Emulsion inlet lines are usually above the operating liquid level to prevent loss of liquids from the vessel in the event of line leaks before the line gets to the vessel and siphoning effects. The gas outlet. The gas outlet is always located at the center of the top. A mist extractor will be installed inside the vessel to further limit liquid carryover. The drain outlet. The drain outlet is located in the center of the bottom. The high oil outlet (optional). If the oil outlet is located high on the upper side of the pressurized vessel as it is in the heater/treater, this will be the oil outlet and also the height of the fluid in the vessel.

Figure 3. Typical line Openings in a pressurized vessel. The lower liquid outlet or water leg. If a liquid outlet is located near the bottom on the side of the vessel and is utilized as a water leg, this will indicate that the vessel is a three-stage separator. The fluid will be separated into the base contents, gas, oil, and water. If a fire tube is also added for heat, then it will become a heater/treater. Heater/treaters are also three stage separators. During the summer months, most companies try to use the heater/treater as a

10B-4 three stage separator, operating it without heat with only chemical treating in order to save and sell the gas. This procedure works well for lighter (higher API gravity crude) oils. If the crude oil is heavy and has a high paraffin and water content, the oil may not treat without heat. Floats. Floats in vessels may be discriminate or indiscriminate, according to need and design. The purpose of a float is to control the level of liquid inside a vessel by means of an outside arm or pneumatic mechanism. · Indiscriminate floats. Indiscriminate floats are made to float on both oil or water indiscriminately. The size of the float depends upon the power that it takes to operate the control arm, the turbulence of the liquid, the volume moving through the vessel, the length of the arm, the weight of the ball, and several other factors. The ball float is the most common indiscriminate float. Discriminate floats. Discriminate floats in the oilfield are floats that have a density or weight that will allow them to sink in oil but float on water. The float will operate on this interface. Since the weight per gallon of oil will vary according to the viscosity of the oil, and the weight of water will vary according to how much salt is contained in the water, discriminate floats are available with several densities. Oil will weigh over 7 pounds per gallon and water will weigh 8.3-9.6 pounds per gallon. Fresh water weighs 8.3 pounds per gallon and at about 9.6 pounds it begins to reach maximum natural salt saturation. B-4. The Separator. There are several styles of separators, which are classified by shape and by separation method. The basic shapes of separators are: · Vertical separators · Spherical separators · Horizontal separators The basic separation methods are: · Two-stage · Three-stage · Metering Operation of the two-stage vertical separator. Two-stage vertical separators have historically been manufactured in three styles: right-hand, left-hand, and the combination. The combination vessel has two identical emulsion inlet openings, located opposite of each other about twothirds of the way up the sides of the vessel. All of the other openings are standard and this is the only difference between them. One of the inlet openings will be selected as the most appropriate to face the inlet manifold, and the other will be plugged, usually with a four inch bull plug. The right and the left hand separators will only have one inlet opening, and these are opposite from the other. The design of a two-stage vertical separator is shown as a cutaway drawing in Figure 4 and as a photograph in Figure 5 on the next page. The gas will rise to the top, go through a mist extractor which is built into the top of the separator, and enter the gas line. This gas pressure is not really high pressure in a literal sense, because it is usually no more than 20-50 pounds.


Ball floats are usually weighted with sand to allow them to sink through the oil phase but float in the water phase.


Figure 4. Cutaway drawing of a vertical separator showing the operation of the vessel. Since the atmospheric vessels will have no more than a few ounces of backpressure-- usually 2-4 ounces--the pressure is high in relation to the atmospheric vessels, and this pressure is never directed toward an atmospheric pressured vessel. The dumping of oil and water is regulated by a float-controlled dump valve. In newer designs, this valve draws the liquid from

Figure 5. A typical two-stage vertical separator with a high liquid level alarm. much closer to the bottom than previous styles, reducing corrosion problems. This was achieved by adding a short line inside the separator that ends near the bottom, reducing the amount of corrosive water that is present below the dump valve outlet. A pressure gauge is added above the dump valve and a sight gauge glass is mounted to the left of the dump valve. The

10B-6 pressure gauge line turns upward inside a second short section of pipe that has a cap on top. If the liquid level rises all the way to the gas outlet, none will enter the gauge. A hail guard is added to protect the sight gauge glass from being accidentally broken by people, animals, or hail. An O-ring is placed on a new sight glass at the fluid level. A glance at this periodically gives an instant reference to several operating problems. A string may be tied on the glass to replace the O-ring when it becomes weather cracked and drops off. The emulsion inlet has a device that forces the oil to swirl as it enters. This circular action creates turbulence in the liquid phase and accelerates gas breakout from the liquid. This lowers the amount of mist lost into the gas line. As the fluid is dumped out of the separator, it will be directed toward the heater/treater, gun barrel, or a stock tank. The dump valve should be gently tested by hand periodically to be sure that it is still working freely and is not becoming frozen in one position. Care must be exercised, because a harsh push may break the float off, necessitating repairs. The two valves on the sight glass have a built-in reamer on the valve seat to keep the hole into the tank open and to remove paraffin or scale that might accumulate. These should be carefully closed and opened periodically to ream them clean and to keep the fluid level in the gauge accurate. The lease pumper can close the bottom gauge valve and leave the top one open, then open the drain cock that is screwed into the bottom opening of the bottom valve to flush out the gauge glass. Experience at each separator will help determine how often this may be required. It may be necessary to occasionally adjust a valve stem packing if leakage or oil staining should occur. Some separators have pneumatic instead of mechanical controls (Figure 6). Note the gas line coming off the top of the vessel to supply gas pressure to operate the diaphragm operated dump valve. Since the diaphragm is a medium- to low-pressure automation device, a pressure regulator is installed in the line to reduce the pressure and protect the valve.

Figure 6. A separator with pneumatic controls. Pressure safety devices. Two types of safety devices are placed on top of pressurized vessels: the pop-off or relief valve and the rupture disc or safety head (see Figure 7 on the next page). The pop-off valve is set near the limit or maximum pressure that might cause the vessel to become dangerous. The pop-off valve has stainless steel springs, ball and

10B-7 seat, and other parts to prevent rust from changing the setting or making them fail to operate correctly. The pop-off valve can release the excessive pressure and has the ability to automatically shut off when the pressure returns to a safe level. It is an automatic valve requiring little or no maintenance. The safety rupture disc is a thin, domeshaped disc made of stainless steel, aluminum, or, in the case of some older separators, brass. The rupture pressure of the disc is several pounds higher than the safety release valve. When the safety release valve fails to open and the pressure continues to rise above a safe limit, the rupture disc will burst, releasing the pressure on the vessel. When the rupture disc bursts, the pressure will continue to be released into the atmosphere until the lease pumper comes by and shuts in the well or switches from the vessel. For this reason a line from the rupture disc may be run to the pit to prevent ground contamination. But if a line is plumbed from the rupture disc to a pit, it must be run so that there are no low places where water can collect and freeze during cold weather. B-5. The Heater/Treater. The heater/treater (Figure 8) is a threestage, pressurized vessel with heating capabilities, although they are also manufactured as atmospheric vessels. To be atmospheric, the stock tanks must be low enough to allow the oil and water to gravity flow to the stock tank and the water disposal system.

Figure7. Examples of safety release valves and pressure rupture discs.

Figure 8. A typical vertical heater/treater. The lines can be seen entering the vessel. The site gauges and firebox are on the opposite side.

10B-8 Three stage means the fluid is removed through three lines. In the heater/treater pictured nearby, the oil, water, gas emulsion coming into the vessel enters through the highest line on the right side of the vessel. This should not be confused with the gas line that comes off the center of the top. The natural gas flows out through the gas line coming off the center of the top of the vessel through the line to the right. A backpressure valve is installed nearby to control the pressure inside the vessel. The produced oil flows out the lower line on the right side of the vessel. about eight feet from the top. This is the fluid level inside the vessel. Note that a liquid valve with a back pressure weight is installed approximately four feet from the ground. This valve controls the amount of oil standing in the down comer line and does not control the height of the liquid in the tank. The water comes out of a line on the lower left side of the vessel at 90 degrees from the other lines, and the water goes up and spills over the water leg a small distance lower than the oil outlet. The height of the water in the water leg is controlled by a valve identical to the oil outlet control valve. The heater/treater is usually the only pressurized vessel in the tank battery system where the heights of the fluids in the vessel are controlled by line height. The fluid levels in the gun barrel are also controlled by line height. The water level in the bottom of the heater/treater is maintained at approximately one foot above the fire tube. Oil does, however, come into contact with the fire tube. Most operators try to heat the produced crude oil only in the winter. The use of appropriate chemicals and the heat from the sun is usually satisfactory for treating oil in the summer months. The firebox can consume a lot of gas. Selling the gas provides income toward the purchase of the chemicals. It is a give-and-take situation in determining when heat is necessary. Controlling the height of the water. Earlier, the use of the beam balance scale in weighing water and oil was reviewed. Calculations for computing the height of the water leg is reviewed in Appendix F, Mathematics. The height of the water in the heater/treater should be approximately one foot above the top of the fire tube. The amount of water in the heater/treater is controlled by raising or lowering the height of the side boot on the water leg. If the weir nipple is raised in the water leg boot, the amount of water retained inside the heater/treater will be higher. If the weir nipple is lowered, the water retained will be lower. The total fluid column height will remain constant, controlled by the height of the oil outlet opening. The fluid sight glasses. The lower fluid level sight glass should display water in the lower half and oil in the top half. If the lower valve is occasionally closed, it is a good practice to close and re-open the top valve, and then bleed the lower petcock into a container. This will effectively clean the glass to allow the liquid level to correct itself. Tie a small rag at this level to indicate future level changes. The top sight glass will have oil in the bottom and gas in the top. It can be cleaned in the same manner. Backpressure valves for pressurized vessels. The Kimray diaphragm-operated backpressure valves are widely accepted by the petroleum industry as a dependable automatic operating valve (Figure 9). It is a

10B-9 very popular selection for controlling the back pressure for the separator, the heater/treater, the emergency vent line, and the gas purchasing company, and on several other vessels where a back pressure valve is installed. The term backpressure refers to the pressure in back of the valve. The valve that controls pressure after the backpressure valve or downstream is usually referred to as a regulator.

Figure 10. Treater oil and water valves. (courtesy Kimray, Inc.)

Figure 9. A diaphragm-operated backpressure valve. (courtesy Kimray, Inc.) The treater oil and water valves (Figure 10) are used to control the dumping of oil toward the stock tanks and the water toward the water disposal and injection system. These are basically liquid dump valves. The installation of the valve is shown in Figure 11. The drawing shows one method of securing the necessary gas pressure to the oil or water valve that prevents fluctuations in gas pressure from affecting the operation of the valve.

Figure 11. How the treater liquid valve is installed. (courtesy Kimray, Inc.) As the oil spills out of the heater/treater into the oil line, or water spills over the equalizer riser on the water outlet line, these

10B-10 liquids create a downward pressure against the valve. When the column of liquid builds up to about four or five feet in the line, the valve opens, allowing part of it to flow toward the next vessel. The height of the line, not the valve, controls the liquid level in the vessel. As the pressure gets lower, the valve closes again. A gas line comes off the top of the vessel and connects to this valve. The purpose of this line is to equalize the gas pressure inside the vessel across the liquid dump valve. If a flowing well opens up or someone opens a valve and a surge of additional pressure comes into the vessel through the inlet line, this change in pressure will also occur on both sides of the diaphragm. This change in pressure will have no effect upon the operation of the valve. The height of the water leg is determined by the weight per square inch of the column of salt water and the weight of the oil on top of the water. The height of this column of liquid is from the oil overflow outlet to the bottom of the vessel. In Appendix F, Mathematics, the procedure for calculating the height of water legs is reviewed. The float-controlled separator dump valve pictured in Figure 12 is a popular valve. The pressure below the seat of the valve is transferred to the top mechanism of the valve. This removes any stress or unusual pressures from being exerted against the diaphragms in the valve and results in smooth and dependable valve action. If the dump valve arm needs to operate in the opposite direction, remove the bolts in the top, rotate the top one-half turn, and replace the bolts. Now the mechanism will operate in the opposite direction. The direction of the arm can also be turned in the opposite direction according to need.

Figure 12. A float-controlled separator dump valve. (courtesy Kimray, Inc.) B-6. Interior Design of the Vertical Heater/Treater. In the past, two types of heater/treaters have been constructed. During the past few years, several innovative changes have made the basic design more efficient, so only one type is illustrated as Figure 13 on the next page. This style is designed for high water production. Inlet line. The inlet line enters above the fluid level. The gas, being lighter, flash separates and goes up through the mist extractor and into the gas line. The liquid falls to the bottom of the vessel through a tube.

10B-11 At the bottom. At the bottom, the free water that has flash separated continues on down and out of the vessel through the water leg, absorbing very little of the vessel heat. The oil migrates up through the water section, past the fire tube, and is heated as it continues to migrate up to the oil section of the heater/treater. The oil trip upward. As the oil travels upward, it moves from one side to the other passing upward through openings, while any suspended water contacts the plates, separates, and moves back downward toward the water leg and on to the disposal system. When the oil reaches the top, it falls into the oil outlet and on to the gun barrel or the stock tank. Any new gas that breaks out passes upward through a provided tube up to the mist extractor and into the gas system. This tube also equalizes pressure between the upper and the lower section of the vessel. The water leg. The operation of the water leg is also visible, and the weight of the water in the water leg is exactly the weight of the water and oil column inside this vessel. This three-stage separation is fully controlled by line heights instead of a discriminate float as it is in the free water knockout. This vessel is one of the most useful vessels in oilfields. Figure 13. Interior view of a heater/treater. (courtesy of Sivalls, Inc.)



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